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CHAPTER 6 NUTRITION
Types of nutrition
• All living organisms-nutrients for growth, maintenance and repair of damaged tissues.
• Nutrition is the entire process by which organisms obtain energy and nutrients from food, for growth, maintenance and repair of damaged tissues.
Autotrophic nutrition
• The process by which autotrophs (autos: self; trophos: feed) synthesise complex organic compounds from raw, simple inorganic substances using light or chemical energy.
Autotrophic nutrition
• Photosynthesis (photos: light) - the process through which green plants (called photoautotrophs) produce organic molecules from CO2 and H2O using light as a source of energy.
• Chemosynthesis (chemo: chemical) - the process by which chemoautotrophs synthesise organic compounds from CO2 and H2O without the help of light, but obtain energy by oxiding inorganic substances such as hydrogen sulphide and ammonia.
Heterotrophic nutrition
• A type of nutrition in which an organism (cannot synthesise their own nutrients)
Heterotrophic nutrition
• Holozoic nutrition (holo: like; zoon: animal) - ingesting solid organic matter
• Saprophytism - dead and decaying organic matter.
• Parasitism - absorbs readily digested food from its host.
Balanced diet
• Balanced diet = a diet consisting of all the nutrients in the correct proportiond to meet the requirements of the body.
The necessity for a balanced diet
• Nutrients provide the body with its basic needs:– source of energy,– chemical building blocks for growth and repair of
damaged body tissues– metabolic reactions.
• Every individual requires an adequate daily supply of– Energy-providing foods (carbohydrates and lipids),– Growth-providing foods (proteins), and – Sufficient amounts of minerals, vitamins, water
and roughage to maintain health.
Daily energy requirement
• A balanced diet -daily energy requirement• The sum of all chemical reactions in the body is
called metabolism. • The energy consumed by the body in a day while
carrying out all the basic processes known as the basic metabolic rate (BMR).
• The minimum daily energy requirement varies for different individuals and is dependent upon various factors
Bomb calorimeter
• Burning a known mass of the food completely in the presence of oxygen
• To calculate the energy value of various types of food samples.
• Energy value = the amount if heat generated from the combustion of one gram of food. The unit used to describe energy values in food is joule per gram (Jg-1).
• The amount of energy in food can also be expressed in terms of calories.
• 1calorie (cal) = 4.2 joules (J). 4.2 joules of energy are needed to raise the temperature of water by 10C.
Food class Energy value (kJg-1)
Carbohydrates 16.7
Proteins 22.2
Fats 37.6
• The energy value of food can be calculated as follows:
Vitamins
• Essential for the maintenance of good health and efficient metabolism.
• Fat-soluble vitamins - are vitamins A, D, E and K- can be stored in body fat.
• Water-soluble vitamins - include vitamins B and C- cannot be stored in the body, and have to be constantly supplied in the daily diet
Sources, functions and effects of vitamin deficiency
Vitamin Dietary sources Functions Symptoms of deficiencyA
(retinol)Egg yolk, butter, fish liver oil, dairy products, green vegetables
Needed for the formation of light-sensitive pigment in the retina.
Acts as an antioxidant by combining with free radicals to minimize its damaging effects. Free radicals are produced when body cells generate and use energy. Free radicals react with DNA, leading to serious damage; and in some cases, causing cancer.
Maintenance of epithelial tissues. Delays the ageing process.
Vision problems (night blindness)
Cornea becomes dry and opaque (xeropthalmia)
Scaling skin
D Dairy products, egg yolk, cod liver oil, milk, (also made in human skin in the presence of sunlight)
Aids in the absorption of calcium and phosphorus ions in the small intestine and the use of these ions in the formation of bones and teeth.
Promotes bone growth.
Rickets – a disease characterized by poor teeth and bone formation in children
Deformities such as bowed legs and knock knees develop
Stunted growth in childrenOsteomalacia – softening of bones in adults
E(tocopherol)
Wheat germ, nuts, green vegetables, grains, olive oil, milk
Produces red blood cells. Acts as an antioxidant, as it combines easily with
unstable metabolic products (free radicals). Thus, vitamin E protects unsaturated fatty acids with vitamins A and C.
Prevents damage to phospholipids in cell membranes, so maintaining their structure.
A range of disorders in different species, including muscular dystrophy, liver damage and infertility
May also cause anaemia
Vitamin Dietary sources Functions Symptoms of deficiency
K (phylloquino
ne)
Egg yolk, cabbage, spinach, a form of the vitamin is synthesized by intestinal bacteria
Important in blood cotting.
Defective blood clotting which leads to extensive bleeding
B1 (thiamine)
Yeast extract, whole grains, nuts, milk, liver, wheat germ, legumes
Precursor of a coenzyme which functions in carbohydrate metabolism.
Beri-beri (muscle weakness, nerve disorder, heart disorder, swollen feet and loss of skin sensitivity)
B2 (riboflavin)
Wheat germ, dairy products, yeast extract, milk, liver, eggs
Component of coenzymes in energy metabolism.
Sore eyes and swollen tongues Skin lesions at the corner of mouth,
nose and ears
B3 (niacin)
Liver, lean meat, legumes, unpolished rice, fish, yeast extract
Component of coenzymes in energy metabolism.
Pellagra (skin and gastrointestinal lesions, nervous, mental disorders and loss of appetite)
Vitamin Dietary sources
Functions Symptoms of deficiency
B5
(pantothenic acid)
Egg yolk, green vegetables, liver, fish, meat
Component of coenzyme A, with a role in energy metabolism.
Muscle cramps, fatigue, impaired motor coordination
B6 (pyridoxine)
Potatoes, meat, fish, whole grains, nuts, yeast extract, liver, milk
Coenzymes in amino acid metabolism.
Irritability Muscular twitching Convulsions Dermatitis Retarded growth Kidney stones
B12
(cobalamin)Milk, meat, eggs and cheese
A coenzyme in nucleic acid metabolism.
Synthesis of red blood cells.
Pernicious anaemia Neurological disorders Weight loss
Folic acid Green vegetables, oranges, nuts, legumes, whole grains
Acts as a coenzyme in nucleic acid and amino acid metabolism.
Anaemia Gastrointestinal disturbances Diarrhoea Birth defects
Vitamin Dietary sources
Functions Symptoms of deficiency
Biotin Legumes, vegetables, meat
A coenzyme in the synthesis of fat, glycogen and amino acids.
Fatigue Depression Nausea Dermatitis Muscular pains
C (ascorbic
acid)
Citrus fruits, tomatoes, green peppers
Required in the synthesis of collagen.
Maintenance of cartilage, bone and dentin.
A strong antioxidant. Aids in detoxification. Improves ferum absorption.
Poor collagen formation
Scaly akin Scurvy – symptoms
include swollen, bleeding gums and tooth loss
Degeneration of blood vessels, muscles and cartilage
Epithelial haemorrhages
Delayed wound healing
Impaired immunity
Minerals
• Minerals are simple inorganic nutrients which must be obtained through the diet, either from food or dissolved in drinking water, cause the body cannot manufacture them.
• Minerals do not provide energy, • Major minerals, called macrominerals, are
required in relatively large quantities. Examples of macrominerals are calcium, magnesium, phosphorus and sodium
• Some of the macrominerals are recommended in amounts more than 100mg per day.
• Microminerals are required in trace amounts of less than 20mg per day.
• Examples of microminerals are cobalt, fluorine, iodine, manganese, zinc and molybdenumj. These minerals have very specific functions.
• If a particular mineral is deficient in a person’s diet, the normal health and metabolism of that person can be affected.
• This will result in a deficiency disease with characteristic symptoms.
• A deficiency disease can be avoided by supplementing the diet with the necessary nutrients.
Sources, functions and effects of mineral deficiency
Mineral Sources Functions Symptoms of deficiencyCalcium Milk, cheese,
vegetables, anchovies, grains
Bone and tooth formation. Aids in blood clotting. Needed in muscle and nerve
coordination. Transmission of nerve
impulses.
Rickets in children Stunted growth Delayed blood clotting Osteoporosis among
the elderly due to loss of bone mass
Magnesium Green leafy vegetables, whole grains, meat
Activates most types of enzymes in protein synthesis.
Maintains normal function of muscles and nerves.
Bone and tooth formation.
Retarded function if muscles and nerves
Iron(ferum)
Green leafy vegetables, liver, egg yolk, meat, legumes
Component of haemoglobin needed for oxygen transport in the blood.
Component of enzymes involved in cellular respiration.
Iron-deficiency anaemia
Reduced reistance to infection
Mineral Sources Functions Symptoms of deficiency
Sodium Table salt An important component in blood plasma.
Maintains the acid-base balance and water balance.
Normal muscle and nerve function.
Involved in nerve impulse transmission.
Muscle cramps Loss of appetite
Chlorine Table salt Acid-base balance Osmotic balance Nerve function
Muscle cramps Loss of appetite
Potassium Meat, dairy products, fruits, vegetables and grains
Needed for the correct functioning of the heart.
Maintenance of acid-base balance and water balance.
Muscle and nerve function.
Heart failure Muscular weakness Paralysis
Mineral Sources Functions Symptoms of deficiency
Iodine Seafood, seaweed, iodised salt
Component of the thyroxine hormone.
Goitre (enlarged thyroid gland)
Sulphur Meat, fish, nuts, milk, dairy products
Component of certain amino acids.
Needed for muscle growth.
Symptoms of protein deficiency
Phosphorus
Milk, cheese, egg, grains
Bone and tooth formation. Acid-base balance Nucleotide synthesis
(component of DNA and RNA).
Involved in the transfer of energy in ATP.
Rickets in children Demineralisation of
bones Weakness Loss of calcium and
minerals
fluorine Drinking water, fish, tea
Maintenance of strong teeth and probably bone structure.
Helps resist tooth decay.
High frequency of tooth decay
Dietary Fibre
• Dietary fibre -indigestible part of plant food which consists mainly of cellulose.
• Not a nutrient, dieticians -25-50g of fibre should be eaten day to ensure good health.
• Foods high in fibre content include fruits, vegetables, nuts and whole meal grains
• Foods that contain dietary fibre satisfy the appetite and delay hunger.
• Dietary fibre has no nutritional value.
Water
• Water is very essential to the survival of humans as all metabolic reactions in the human body tale place in solutions.
• Water males up about 70% of the total body weight.
Functions of Dietary fibre
• High water holding capacity• Dietary fibre aids in peristalsis. • Aids in bowel movement. • Deficiency constipation• Dietary fibre lowers the cholesterol level in
the blood.• It also reduces the risk of heart disease and
colon cancer
Water
• Is the medium for all cellular biochemical reactions.
• Is the medium of transportation for respiratory gases and nutrients.
• Regulates body temperature.• Removes• Excretory waste such as lactic acid, urea and
excess mineral salts through perspiration and urination.
Water
• Maintains osmotic pressure in the tissue fluid and blood plasma. Osmotic pressure is determined by the amount of water and mineral salts in the blood plasma or tissue fluid.
• Aids peristalsis movement.• Dissolves most chemical substances.• Enables hydrolysis of food substances during
digestion.
• For our bodies to function efficiently, we need to maintain a proper water balance in the body.
• A normal and healthy adult requires about 2 to 2.5 litres of water daily to replace water lost– through the skin during perspiration,– through evaporation from the lungs during breathing,– in the form of urine from the kidneys and also in the
faeces.
• Water loss can have an adverse effect on the physiological processes and physical performance of the body.
• Failure to replace the water lost will result in dehydration. A severe loss of water can be fatal.
Selection of an Appropriate Balanced Diet
Choosing an appropriate diet menu for different target groups
• The balanced diets of different people vary according to their age, lifestyle, health conditions and specific nutritional needs.
• Each target group needs to choose a diet appropriate to its needs.
• The various target groups are as follows:
Pregnant mothers
• Calcium and phosphorus for the formation of strong bones in the growing fetuses
• Folic acid and ferum for the formation of red blood cells.
• Proteins for the formation of new tissues.• Green vegetables and whole grains as a source of
fibre to prevent constipation.• Fats and sugar, and caffeinated drinks should be
reduced or avoided.
Infants and children
• Proteins, carbohydrates, lipids, vitamins and minerals to help them grow.
• Calcium and phosphorous-formation of strong bones and teeth.
Teenagers
• Proteins and vitamins• Fruits and vegetables to supply the important
nutrients needed, including vitamins and minerals.
• Ferum to prevent iron-deficiency anaemia.
Vegetarians
• Plant proteins. Plant protein sources are grains, soya beans, tofu and nuts.
Athletes
• Weight lifting -high protein foods to build new tissues and strong muscles.
• A runner -proteins, vitamins, minerals and carbohydrates to provide constant energy during training and competition.
• Athletes -calcium, sodium and potassium to prevent muscle cramps.
The aged
• Proteins, vitamins such as D, B6, folic acid and minerals such s calcium and phosphorus to prevent osteoporosis.
• Vitamin d help prevent osteomalacia and folic acid helps synthesise red blood cells.
• Reduce their intake of salt, carbohydrates and fats to -high blood pressure, diabetes and coronary heart disease.
People with specific diseases
• Diabetes should avoid sugary and high carbohydrate food to maintain a normal level of sugar in his blood.
• High risk of coronary heart disease should reduce the consumption of saturated fats and cholesterol.
Malnutrition
• Malnutrition results from an unbalanced diet, in which certain nutrients are deficient, in excess, or are in the wrong proportions.
• If this condition persists over a long period of time, the person’s health will be adversely affected.
Protein Deficiency
• Marasmus– Frequently occur in children aged between 9 and
12.– Marasmus is the general wasting of the body due
to protein deficiency combined with a lack of energy-providing nutrients.
– The child becomes very thin with wrinkled skin.
Protein Deficiency
• Kwashiorkor – A child suffering from kwashiorkor does not
receive sufficient proteins in his diet.– The child has flaky skin, thin muscles, thin hair and
a swelling of the body due to retention of fluid in tissues. The child normally experiences stunted growth.
– In both cases, the mental and physical development of the child is severely impaired
.Effects of Calcium, Phosphorus and Vitamin D Deficiencies
• Osteoporosis-calcium-Women• Bones to become brittle, porous and crack easily.• Not treated, bones will fracture easily, especially
at the hips, backbone and wrists. The backbone may shorten until a person becomes severely bent.
• Calcium and phosphorus supplements and having regular exercise can counteract the development of osteoporosis.
• Osteomalacia (soft bones)- pregnant women.• Teenagers and adolescents whose diets lack
calcium, phosphorus and vitamin D are at a higher risk of getting this disease at old age. This is because bone growth reaches its maximum density before the age of 35.
• women need to consume food rich in calcium, phosphorus and vitamin D, such as milk, eggs and liver.
• Vitamin D is also required in calcium absorption.• Vitamin D deficiency results in reduced calcium
absorption from digested food. This will encourage calcium intake from bones.
• Exercise is also important for children and teenagers as this will enable them to achieve maximum bone density. Examples of exercises that help strengthen bones are walking and running.
Excessive intake of lipids
• Body fat- diet rich in saturated fats can result in
• cardiovascular diseases• Arteriosclerosis• Hypertension
• Arteries become clogged by plaques, the threat of heart attacks and strokes becomes even greater.
• If the coronary artery is partially blocked, the person may feel chest pains (angina pectoris).
• heart is not receiving sufficient oxygen. • strenuous activity or when feeling intense emotion• A fully blocked coronary artery will result in a
heart attack or myocardial infarction.
• Hypertension or high blood pressure.• Arteriosclerosis raises the blood pressure by
narrowing the lumen of blood vessels and reducing their elasticity.– The normal blood pressure for adults is 120/80. – Blood pressure can cause the small arteries to burst and
can lead to strokes if these occur in the brain.– Strokes occur as a result of the death of nerve tissue in
the brain, usually caused by a blockage of the arteries in the brain. In some cases, strokes can be fatal.
A normal artery
An artery partially clogged by arteriosclerotic plague
Arteriosclerosis
Excessive intake of minerals • An excessive intake of vitamins and minerals can also bring about adverse
effects to our health.• An excess of minerals will cause an imbalance in the osmotic pressure of body
fluids.• An excess of sodium is associated with high blood pressure, a major factor
that contributes to heart disease and strokes in some people.– Excessive salt and a lack of water can lead to the formation of kidney stones. Kidney
stones can develop when crystalds form in the urine and build up in the inner surfaces of the kidney.
– Most stones are made of calcium and oxalate. Oxalate is a substance found in nuts, leafy greens, chocolates and vitamin C.
– The stones may either remain in the kidney or travel down the ureter. This will complicate kidney function and urinating will be painful.
• The kidneys of people who lack exercise and do not perspire work hard to remove the excess salt. Eventually, this will lead to kidney failure.
• Excess calcium can also increase the risk of kidney stone formation.
• Excess ferum can lead to liver and kidney damage, kidney toxicity, and death, especially among children.
Figure 6.5 Kidney stone in a kidney
Excessive intake of vitamins
• Generally, an excessive intake of water-soluble vitamins is not harmful to the body, as excess vitamins are excreted in the urine.
• However, excess fat-soluble vitamins are not excreted from the body but are deposited in body fat over time.
• Therefore, overdoses of fat-soluble vitamins will result in an accumulation of these organic compounds to toxic levelos in the body. Effects of an overdose of these vitamins are given in Table 6.3.
Effects of overdoses of vitamins Vitamin Effects
C Gastrointestinal upset
A Hair loss, vomiting, bone ache, joint pain, liver and bone damage
E Kidney damage
D Too much calcium in the blood and widespread calcification of soft tissues which interferes with the functions of muscles and heart tissue.
K Liver damage and anaemia
B6 Numb feet and poor coordination
B3
(niacin)~ Flushed face and hands ~ Liver damage
Excessive intake of proteins
• increase the uric content in the blood.• Uric acid forms crystals in the soft tissues of the
joints. This condition leads to gout.• Uric acid can also crystallize and form stones in
the kidney and cause kidney damage.• To prevent gout and formation of kidney stones,
avoid foods high in purine-contaning nucleic acids such as liver, kidneys and sardines.
Food Digestion
• complex organic molecules which are too large to pass through plasma membranes and enter body cells.
• form that can be readily absorbed by the body cells.
• The process that breaks down complex food substances into simpler, soluble molecules that are small enough for the body to absorb is called digestion.
• Digestion breaks down – starch into glucose molecules,– proteins into amino acids, and – lipids into glycerol and fatty acids.
• These essential substances are required by the body cells to carry out metabolic processes. For example,
• glucose is oxidized to generate energy,• amino acids are used to synthesise new proteins, such
as enzymes and hormones,• lipids form a major component of plasma membranes
Digestion in the mouth
• The chewing• Secretion of saliva by three pairs of salivary
glands• The tongue• Amylase which begins the hydrolysis of starch
to maltose.• Bolus in preparation for swallowing.
• Swallowing, the bolus enters the throat. • Pharynx, • When swallowing, a cartilage flap called the
epiglottis temporarily closes the airway to prevent food from entering the trachea.
• Oesophagus, a muscular tube lined with epithelium and mucous glands.
• The mucus lubricates the movement of the bolus along the oesophagus by peristalsis, a series of wave-like muscular contractions along the oesophagus wall
• When the cardium sphincter relaxes, the bolus enters the stomach.
Digestion in the stomach– The stomach is a thick-walled, sausage-shaped organ situated below the diaphragm.– It is a muscular sac with a highly folded inner wall.– The epithelial lining of the stomach contains gastric glands.– Food stays in the stomach for a number of hours.– During this period, the food is thoroughly churned and mixed the gastric juice by the peristaltic contraction
of the stomach wall.– Eventually, the contents of the stomach become a semi-fluid called chyme.– Relaxation of the pyloric sphincter allows the chime to gradually enter the duodenum.– Health Watch: Excessive secretions of hydrochloric acid and pepsin are thought to cause peptic ulcers.
Recently, this condition has been linked to the presence of Helicobacter pylori (a type of bacteria) in the stomach. There is also evidence which links H. pylori to certain types of stomach cancer.
– Hydrochloric acid• Creates an acidic condition (pH 1.5-2.0) which is optimal for the action of the enzymes in the stomach.• Stops the activity of salivary amylase.• Helps to kill bacteria in food.
• - pepsin and rennin• Pepsin starts the hydrolysis of large protein molecules into smaller chains of polypeptides by breaking specific peptide
bonds.• Rennin coagulates milk by converting the soluble milk protein, caseinogen, into insoluble casein.
•
– Digestion in the small intestine
– The small intestine consists of the duodenum, jejunum and the highly coiled ileum.– The duodenum, the first part of the small intestine, receive chime from the stomach and secretions
from the gall bladder and pancreas. – Liver
» Liver secretes bile, an alkaline greenish-yellow liquid produced by the liver and stored in the gall bladder.» Bile does not contain any digestive enzymes.» Bile creates an alkaline environment (pH 4.6-8.6) for the enzyme action in the duodenum.» It helps to reduce the acidity of chime and optimizes the pH for enzyme action.» Bile salts emulsify lipids, transforming large lumps of lipids into tiny droplets, thus providing a greater surface area
for digestion by enzymes. This allows lipids digestion to procedd more rapidly.
– Pancreas• Pancreas secretes pancreas juice which contains the enzymes pancreatic amylase, trypsin
and lipase.• The optimum pH required for the action of enzymes in the pancreatic juice is between 7.14
and 8.2.• ** Bile enters the duodenum via the bile duct.• ** Pancreatic juice is secreted into the duodenum by the pancreas via the pancreatic
duct.
– Duodenum• The digestion of the starch, proteins and lipids takes place in the duodenum.• Pancreatic amylase
• Pancreatic amylase completes the digestion of starch and maltose. Starch + water maltose
• trypsin
• Trypsin digests polypeptides into shorter chains of peptides. Polypeptides + water peptides
• lipase
• Lipase completes the digestion of lipids into fatty acids and glycerol, which are small enough to be absorbed by the epithelial lining of the small intestine. Lipids droplets + water glycerol + fatty acids
• Hydrolysis of lipids is especially difficult because lipids are insoluble in water. Bile acts as an emulsifier which lowers the surface tension of the lipids and coats tiny fat droplets to form a stable emulsion.
• Self-digestion occurs when digestive juices secreted into the alimentary canal destroy the epithelial lining.
– Ileum • Glands in the wall of the ileum secrete intestinal juice which contains digestive enzymes
needed to complete the digestion of peptides and disaccharides.• The intestinal enzymes require an alkaline medium to act at an optimal rate.• At the end of the digestive process, all carbohydratesare digested into monosaccharides
such as glucose, fructose and galactose.• Proteins are digested into amino acids and lipids into fatty acids and glycerol.• Vitamins and the minerals are extremely small and soluble and need not be digested.• Dietary fibre cannot be digested in the human body as the enzyme cellulose is not
produced in the alimentary canal.• The appendix is a small tubular appendage that extends outwards from the caecum of
large intestine. It does not play a vital role in the human body. When this organ becomes inflamed, a condition called appendicitis develops.
• Protein digestion • erepsin
• Peptides are digested by erepsin (a peptidase) into amino acids.
Digestive system in ruminants and rodents
• The digestive system of ruminants• > Herbivores like ruminants and rodents feed on plants which contain a high percentage of cellulose, a
polysaccharide which is extremely insoluble.• > Therefore, much of the energy in their diets is stored in this complex carbohydrate. The breakdown of
cellulose requires the enzyme cellulase.• > Ruminants obtain most of their energy from the breakdown of cellulose of plant cell walls by cellulose.• > Although ruminants do not produce cellulose, their digestive system are specially adapted to carry out
cellulose digestion.• > Ruminants like cows and goats have stomachs which are divided into four chambers, namely rumen,
reticulum, omasum and abomasums. • > This adaptation enables ruminants to carry out rumination, the process of regurgitating food and
rechewing it.• > The first two chambers, the rumen and reticulum, are specialized compartments which have large
communities of bacteria and protozoa.• > These microorganisms are able to secrete cellulose to digest cellulose.• > In many cases, the microorganisms also use the sugars and other products of cellulose digestion along
with minerals to synthesise certain nutrients, such as vitamins and amino acids, which are essential to the ruminants.
• > Figure 6.13 shows the processes involved in cellulose digestion in the digestive system of a cow.
• The digestive system of rodents • > In rodents like rabbits and rats, the caecum and appendix are enlarged
to store the cellulose-producing bacteria (Figure 6.14).• > Unlike ruminants, the breakdown products pass through the
alimentary canal of rodents twice.• > The faeces in the first batch are usually produced at night, and are soft
and watery.• > These are eaten again to enable the animals to absorb the products of
bacterial breakdown as they pass through the alimentary canal for the second time.
• > The second batch of faeces become drier and harder.• > This adaptation allows rodents to recover the nutrients initially lost
with the faeces.
The differences between the processes of cellulose digestion in
Humans Ruminants Rodents
-humans do not produce enzymes that can digest cellulose.-cellulose does not provide any nutrients for humans.-the cellulose fibrils pass through the digestive tract and stimulate the intestinal lining to secrete mucus, which aids in the movement of food through the intestinal tract.
-cellulose digestion by symbiotic microorganisms (for example, cellulose-producing bacteria) occurs in the rumen and reticulum of the stomach.-these microorganisms not only digest the cellulose into simple sugars but also convert the sugars into a variety of nutrients essential to the ruminants.
-cellulose digestion by symbiotic bacteria occurs in the large intestine as well as in the caecum.-nourishing by-products of fermentation by bacteria are obtained when rodents eat some of their faeces and the partially digested food passes through the alimentary canal the second time.
Problems Associated with Food Digestion
– The function of the digestive system depends largely on proper nutrition.
– Proper nutrition can help the digestive system function at its best.
– There are many probles associated with the digestion of food.
– Incomplete digestion of food• Incomplete digestion of food may cause severe pain in the
abdomen followed by nausea, vomiting and a bloated stomach.
• Incomplete digestion of food is caused by excessive intake of food, eating too much oily food or eating too fast.
• If the food is not chewed properly before swallowing the stomach cannot properly digest the food and this decreases the effectiveness of the digestive enzymes.
• Eating moderately and chewing food properly help prevent incomplete digestion of food.
– Reduced production of specific digestive enzymes• Reduced production of specific digestive enzymes
can cause digestive problems. Adults usually find it difficult to digest lactose (milk sugar) compared to a baby or a child because of the lack of lactase.
• Damage to organs such as the pancreas causes reduced production of digestive enzymes for the digestion of starch, proteins and lipids. As a result, digestion of these foods will be disrupted and the body will not be able to obtain sufficient nutrients.
– Gallstone preventing the flow of bile• A person who often eats fatty food encourages the formation of
gallstones in the bile duct and gall bladder.• Gallstones are caused by the hardening of cholesterol. It is also caused by
the excessive secretion of bilirubin and bile salts.• The size of the gallstones may be as a grain of sand or as big as a golf ball.• When the gallstones block the bile duct, bile cannot be channeled out. As
a result, lipids cannot be emulsified and are difficult to digest.• Symptoms associated with the presence of gallstones include fever,
vomiting, jaundice and continuous pain in the upper abdomen.• If the gallstones are found in the pancreas and this results in severe pain
and inflammation of the pancreas.• Formation of gallstones usually occurs in obese people.
Absorption and Assimilation of Digested Food
• To enter the body cells, nutrients in the lumen of the small intestine must be transported across the intestinal lining into the bloodstream.
• Iluem is the major site of nutrient absorption.
– Adaptive Characteristics of the Digestive System• The wall of the small intestine is covered with epithelial cells that are
specialized to complete the digestive process and absorb the resulting nutrient molecules.
• The small intestine, with a length of about 6m, is the longest section of the alimentary canal.
• The intestinal lining is highly folded and covered entirely by tiny, finger-like projections called villi.
• The epithelial cells of a villus has a fringe of microscopic projections called microvilli.
• The total area exposed to the lumen of the small intestine has been estimated to be 300m2 . Each villus is only about 1mm long and less than 0.25mm thick. There are about 40 villi on 1mm3 of the intestinal lining. Estimate the total number of villi present in the human intestine.
• If the small intestine is simply a smooth tube, without villi on its inner lining, it would have to be 500 to 600m long to achieve a surface area of 300m2.
• HOW ARE THE VILLI ADAPTED FOR THE PROCESS OF FOOD ABSORPTION?
• The villi– are numerous, thus increasing the internal surface area of the
ileum for absorption.– are very thin-walled (only one cell thick); thus, undigested
food can be absorbed rapidly.– contain a network of blood capillaries for the efficient
transport of digested food. – contain special structure (lacteals) for absorbing fatty acids
and glycerol.
• It is important for the body to retain as much water and minerals as possible. This is to prevent the body from becoming severely dehydrated and to maintain a balanced osmotic pressure.
• Health Watch : Excessive consumption of alcohol can lead to cirrhosis, a condition where the liver is severely damaged. The liver cells are forced to break down excessive amounts of alcohol which is the converted to fatty acids. This in part contributes to severe liver tissue damage due to the formation of fibrous scar tissue.
• The liver also acts as a storage place for minerals like copper, potassium and iron. Iron is needed for making haemoglobin. It also stores fat-soluble vitamins such as vitamins A and K.
– Assimilation of Digested Food» Some of the products of digestion are brought directly to
the liver for processing and prepared for metabolic processes or assimilation.
» Assimilation takes place in the cells where the nutrients are used to form complex compounds or structural components. For example, the transformation of amino acids to proteins in the protoplasm.
» The liver acts as a checkpoint which controls the amount of nutrients released into the blood circulatory system.
» From the liver, glucose and amino acids are transported by the blood circulatory system to the heart to be pumped to all body cells.
Amino acids Glucose Lipids
The process of assimilation in the liver
~Amino acids have to pass through the liver before they reach the blood circulatory system.~The liver synthesizes plasma proteins from amino acids.~Plasma proteins have various functions, for example, blood clotting and osmoregulation.~When there is a short supply of glucose and glycogen, the liver converts amino acids into glucose.~Excess amino acids cannot be stored in the body and are broken down in the liver by a process called deamination.~Urea, the common nitrogenous waste produced and transported to the kidneys to be excreted.
~Glucose in the liver is used for respiration. According to the needs of the body, excess glucose is converted into glycogen and stored in the liver.~When the blood sugar level falls and the body needs energy, the stored glycogen is converted back into glucose.~Once the glycogen store in the liver is full, excess glucose is converted into lipids by the liver.
~Lipids which enter the heart through the subclavian veins are transported in the bloodstream to body cells.
The process of assimilation in the cells
~Amino acids which enter the cells are used for the synthesis of new protoplasm and the repair of damaged tissues.~They are also important building blocks in the synthesis of enzymes and hormones.~Amino acids are also used in the synthesis of proteins of plasma membranes.
~When the glucose molecules reach the body cells, they are oxidized to release energy during cellular respiration.~Energy is required for the various chemical processes which take place in the cell, for example, in muscle contraction and synthesis of proteins.~Excess glucose is also stored as glycogen in the muscles.~Glycogen is a long-chained molecule that is insoluble.
~Lipids like phospholipids and cholesterol are major components of plasma membranes.~Fats that are stored around organs act as cushions that protect organs from injuries.~Excess fats are stored in the adipose tissue underneath the skin as reserve energy.~When the body lacks glucose, fats are oxidized to release energy.
WHAT ARE THE FUNCTIONS OF THE LIVER?
– Secretes bile • >Bile is a mixture of bile pigments and bile salts which are
delivered to the duodenum. Bile is greenish in colour due to the presence of the pigment bilirubin.
• >Bile pigments do not participate in digestion. They are waste products from the liver’s destruction of old red blood cells.
• >Bile pigments are eliminated together with faeces. An accumulation of bile pigments in the body results in a condition called jaundice.
• >Bile salts disperse the lipid droplets present in chime into as emulsion of tiny droplets. This increases the surface area for the action of lipase. Bile is stored and concentrated in the gall bladder.
• 2. Site for the synthesis of blood plasma proteins• >Fibrinogen, prothrombin and many other proteins, which are vital
clotting agents, are synthesized in the liver. The liver also maintains blood protein concentration within a narrow range.
• 3. Regulation of blood glucose concentration• >The liver removes glucose from the blood converting it into
glycogen. The process is stimulated by the pancreatic hormone, insulin.
• >If the blood glucose level is low, the liver releases glucose into the blood.
• >The liver also converts amino acids into glucose during fasting periods.
• 4. Detoxification• >The liver detoxifies blood by removing and metabolizing poisonous substances.• >The liver absorbs or chemically modifies substances before these substances reach
the rest of the body.• >Ingested alcohol and drugs are metabolized by liver cells.• >Toxins, pesticides, carcinogens and poisons are detoxified and eliminated from the
body.• 5. Storage of nutrients• >Excess glucose is transformed into fats in the liver and later stored in other parts of
the body. The liver also stores fat-soluble vitamins, A, D, E and K, vitamin B12 and ferum from the haemoglobin of disintegrated red blood cells.
• >Storage of ferum• The red blood cells of the body become worn out after some time. These cells are
destroyed in the spleen. The haemoglobin is broken down in the liver. Ferum is released during the breakdown of haemoglobin and stored in the liver.
• 6. Deamination of amino acids• >Excess amino acids cannot be stored in the
body and are broken down in the liver into erea, through a process called deamination. In this process, the amino group (-NH2) is removed and the remaining part of the molecule is respired or converted into glycogen.
6.6 Formation of Faeces and Dfaecation
• The Process of Defaecation• -After the absorption of nutrients has taken place in
the small intestine, the intestinal contents enter the colon.
• -The intestinal contents consist of a mixture of water, undigested food substances and indigestible fibre, most of which is cellulose from plant cell walls.
• -The movement of this undigested materials along the colon is slow and helped by peristalsis.
•
• Colon – Reabsorption of water and minerals • # The colon reabsorbs almost 90% of water and minerals into
the bloodstream.• # Absorption of water from the undigested remains in the
colon results in the formation of faeces, which are s semi-solid waste.
• # Faeces also contain dead cells shed by the intestinal lining as well as waste products like bile pigments and toxic substances which to be eliminated from the body.
• # The wall of the colon secretes mucus which helps to bind the faeces and lubricates the movement of faeces along the colon.
• Rectum • # After 12-24 hours in the colon, the faeces pass
to the rectum for temporary storage.• # After water is absorbed, the undigested residue
hardens to form faeces.• # As the faeces accumulate, pressure in the
rectum increase, causing a desire to expel the faeces from the body.
•
• The process of defacation• # The elimination of faeces is known as
defaecation. • # This process is controlled by muscles around
the anus, the opening of the rectum.• # When the rectum is full, the muscles of the
wall of the rectum contract to eject the faeces via the anus.
• Microorganisms in the colon • # The most common microorganisms found in the colon are
Escherichia coli. E. coli is not harmful to us but lives symbiotically in the intestines by digesting organic substances in the colon. E. coli synthesizes vitamins B and K as by-products of their metabolism.
• # The presence of useful microorganisms is important because – they secrete antibiotics that inhibit the life cycle of harmful
microorganisms, for example, Lactobacillus acidophilus secretes acidophilin.
– the microbial population maintains a stable environment in the alimentary canal. However, the overuse of antibiotics can reduce the microbial population. As a result, food digestion and absorption of nutrients in the intestine will be affected.
Problems Related to Defaecation
• -One of the most common intestinal bacteria is Escherichia coli which feeds on unabsorbed nutrients. Some of these bacteria synthesise several types of vitamins B and K as by-products of their metabolism. The synthesis of vitamins by the colonic bacteria provides a valuable supplement to our dietary intake, especially vitamin K, which is often deficient in a normal diet.
• -Constipation is caused by faeces moving too slowly through the colon.• -As a result, a greater amount of water is reabsorbed in the colon, making the faeces hard.• -This will lead to painful defaecation.• -Constipation can be avoided by drinking a lot of water.• -The daily diet should contain sufficient amounts of fibre.• -Intake of food rich in fibre can help in the movement of undigested food substances along
the colon and during defaecation.• -The water absorbed softens the fibre and increases its bulk. This stimulates the muscles to
push out the residue, thereby preventing constipation.• -A person needs at least 30g of fibre every day. Dietary fibre is found only in plant foods.
The best sources include whole grain and bran cereals, fruits and vegetables.
• -Chronic constipation is associated with haemorrhoids and some cases can lead to colon cancer.
• Haemorrhoids are abnormally swollen veins in the rectum and anus.• When bulging haemorrhoidal veins are irritated, they cause the surrounding
membranes to swell, burn, itch, become very painful and bleed.• Haemorrhoids are caused by too much pressure in the rectum, forcing blood
veins to stretch, bulge and sometimes rupturing them.– Tumours of the colon and rectum are growths arising from the inner wall of the large
intestine. Malignant tumours of the large intestine are called colon cancer or colorectal cancer.
– Diets high in fats are believed to cause colon cancer. It is believed that the breakdown of products from fat metabolism leads to the formation of cancer-causing chemicals (carcinogens).
– Diets high in vegetables and high fibre foods such as wholemeal bread and cereals may rid the bowel of these carcinogens and help reduce the risk of cancer.
• -Some hints to help increase fibre intake • Eat wholemeal or high fibre white bread.• Eat brown or unpolished rice which contains more fibre than polished rice.• Eat more fruits and vegetables (at least 2 servings each).• Use wholemeal flour whenever possible in baking or cooking.• Eat breakfast cereals like oats and bran.• Eat wholemeal biscuits, for example, digestives, wholemeal crackers, oatmeal and
bran biscuits. • OTHER IMPORTANT BENEFITS OF A HIGH FIBRE DIET INCLUDE:• Better blood sugar control in diabetics. Fibre such as guar gum reduces the rate of
glucose absorption, thus, preventing high swing in blood sugar level.• Helps control overeating or snacking. Foods rich in fibre are usually more filling. This
discourages over-consumption of other foods rich in calories which can be fattening. •
– Evaluating Eating Habits – It is important to practice good eating habits.– Good eating habits mean taking food in the correct quantity at the correct time.– Good eating habits include : ~ taking meals at the appropriate time. We have
to eat breakfast, lunch and dinner at the correct times.• ~ refraining from overeating or eating too little during a meal.• ~ eating a variety of foods to satisfy the body’s nutrient requirements.• ~ eating a balanced diet.• ~ eating sufficient amounts of fibre from fruits and vegetables.• ~ drinking at least 2 to 3 litres of water daily. This will help to flush out
toxins from the body.• ~ avoiding excessive fatty food and food rich in sugar.
– Poor eating habits are associated with various health problems which affect many people.
– In most cases, the symptoms of gastritis are relatively mild and short-lived. The most common symptoms include abdominal pain, nausea, belching, vomiting, a burning sensation in the upper region of the abdomen and loss of appetite. Blood in vomit or faeces may be a sign of bleeding in the stomach.
– The Body Mass Index (BMI) can be used to determine whether a person is underweight, overweight or obese. BMI is calculated as follows:
• BMI = body mass (kg) / height2 (m2)• Compared your BMI with the values of the standard chart
bellow:
BMI Category
<20 Underweight
20-24 Acceptable
25-30 Overweight
>30 Obese
– Choosing a Healthy Menu• Choose plain rice and eat it with dishes that contain less fats but more vegetables.• Choose wholemeal bread.• Remove all visible fat from meat.• Grill, steam or poach vegetables, meat and fish.• Buy vegetable oils and polyunsaturated spreads instead of animal fats and butter.• Replace full fat cheese with low fat alternatives.• Take cereals for breakfast instead of fried food.• Replace full cream milk with semi-skimmed milk.• Cut down on sugar foods and sweets.• Use only a little bit of margarine, butter and jam with toasted bread and biscuits.• Eat fruits every day. Make it a habit to eat fruits as a snack and as a desert.• Drink fruit juices instead of cordial and carbonated drinks.• Eat lots of fresh vegetables and anything that can be eaten raw, for example, cucumbers, tomatoes and carrots.• Eat proteins from various animal and plant sources.• Eat more fish and grains.• Reduce intake of fried food.• Eat low fat, low sugar snacks without monosodium glutamate (MSG).
– Health Problems Related to Eating Habits• Gastritis
– Gastritis is a condition where the epithelial lining of the stomach becomes inflamed.– The stomach lining is usually covered with a layer of mucus which protects it from hydrochloric acid and digestive enzymes.– Gastric juice is secreted into the lumen of the stomach whenever one feels hungry.– If food is not taken at regular times, the absence of food in the stomach will result in the acidic gastric juice acting on the
epithelial lining of the stomach wall.– Gastritis occurs when the protective mechanism in the stomach is disrupted and the stomach lining is damaged.– When the breach in the stomach lining develops into a hole, a gastric ulcer results. – Inflammation and damage to the stomach lining can also be caused by
• Excessive alcohol consumption and stress.• Taking aspirins and other pain relievers regularly.• Susceptibility to ulcer is also increased by the presence of Helicobacter pylori. Infection by
these bacteria weakens the mucosal barriers and damages the mucus layer, causing the stomach to be exposed to the action of acid and digestive enzymes. Treatment with antibiotics reduces the symptoms and may also cure the ulcers.
• For most types of gastritis, treatment involves taking medication such as antacids (sodium bicarbonate and magnesium hydroxide). Antacids can neutralize the hydrochloric acid in gastric juice.
– Obesity• Obesity is defined as the excessive storage of energy in the
form of fats which results from as imbalance between food intake and energy expenditure.
• An obese person is predisposed to a number of diseases, including cardiovascular diseases, hypertension and diabetes mellitus.
• Factors which lead to obesity include eating excessively, eating too much fats and oily foods as well as a lack of exercise.
• Obesity can be overcome by practicing a balanced diet and eating not more than what is required by the body.
– Anorexia nervosa» People with anorexia nervosa experience an intense fear of gaining weight.» They are persistently concerned about their body shape and weight.» Individuals with this disorder have a distorted body image which convinces them that
they are fat.» This eating disorder occurs frequently among female adults and teenagers.» People with anorexia nervosa lose their appetite to eat or do not want to eat at all.» They intentionally deprive themselves of food to achieve severe loss of body weight,
often 15% or more below their normal body weight.» This condition is potentially fatal and is recognized as a psychological disorder.» In as attempt to boost their self-esteem, they refrain from eating to the extent that
they become extremely emaciated.» They lose both fat and muscle and this eventually leads to a disruption of the
functions of the heart, endocrine system and reproductive system. » Early treatment through nutrition and gradual restoration of body mass can correct
some of the physical symptoms.» Counseling is also needed to help the patients to correct any distorted belief the
patients may have about their body shape and weight, as well as to help the patients overcome their emotional distress.
• Bulimia• Unlike anorexics, victims of bulimia may have a normal body mass.• Bulimia is characterized by sequences of excessive food intake and
purging to counteract the effects of the binge.• The victims consume huge amounts of food in a short period of
time.• The victims feel out of control and are unable to stop eating during a
binge. They also feel guilty, ashamed, disgusted or depressed following a binge.
• This is followed immediately by purging through self-induced vomiting or misuse of laxatives or diuretics.
• Repeated purging results in serious injury to the digestive tract and can cause an imbalance of mineral salts in the blood. The victims will experience dehydration, irregular periods or the periods may stop completely. Symptoms associated with bulimia include malnutrition, hormonal imbalance, increased risk of diseases such as influenza, kidney and cardiovascular problems and liver disease.
• Bulimia nervosa can be overcome by observing the patient to ensure that the person eats correctly, obtains counseling and medication.
• Warning Signs• Anxiety • Mood swings, depression• Frequent trips to the bathroom after eating • Hiding food in unusual places • Highly self-critical• Difficulties in breathing or swallowing • Fainting spells • Irregular menstrual periods • Frequent weight fluctuations• Pronounced jaw swelling from frequent vomiting
• Nutrient Content in Food – Good eating habits also involve wisely choosing the type of food we eat.– The nutrient content of commercially packaged foods is provided on the
nutritional labels.– The labels on food packets and containers provide complete information
about the total calories per serving and the contents of various nutrients. – By studying the food labels, we can make informed choices amount the type
of food we consume.– When choosing food, we must consider the
• Nutritional contents of food• Freshness of food (check the expiry date)• Presence of food additives such as food flavouring, coluring and preservaties.
– The nutritional information obtained from these sources will help reduce health problems related to poor eating habits. Consumers should practice a critical attitude when choosing commercial food. Take note of the way the food is cooked and the ingredients used in preparing them
The Importance of a Healthy Digestive System
• Effects of A Defective Digestive System• If the alimentary canal is not healthy, the rest of the body
will be affected because it is the point of nutrient entry.• For example, a defective pancreas or stomach may disrupt
food digestion because the secretion of gastric and pancreatic juices may be halted.
• If the gut lining becomes inflamed or damaged, food absorption will be disrupted.
• It is important to take care of our digestive system by practicing good eating habits.
• Taking Care of the Digestive System• Our general health and well-being depend to a large extent on a healthy
digestive system.• We need to be concerned about what we eat so that we can lead
healthy and active lives.• One way to take good care of the digestive system is to take proper
meals at regular times of the day.• We should avoid taking junk food which includes foods that are high in
salt, sugar, fats and low in nutritional value.• Junk food offers little in terms of proteins, vitamins or minerals and
contains lots of calories from sugar or fat.• The term ‘empty calories’ refers to the lack of nutrients in these types of
food.
– The Importance of Macronutrients and Micronutrients in Plants • Elements Required by Plants• Plants need water, carbon dioxide and sunlight to synthesise carbohydrates during photosynthesis.• To synthesise nutrients and other organic substances, plants need additional elements.• Mineral elements are essential chemical elements requires by plants to achieve optimal growth and development.• Minerals that are needed by plants can be divided into macronutrients and micronutrients.• Macronutrient are elements required by plants in relatively large amounts. These macronutrients are carbon (C ), hydrogen (H), oxygen (O),
nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S).• In the water culture method, macronutrients are needed in a few hundred parts per million.• Carbon (C ), hydrogen (H) and oxygen (O) are macronutrients that can be easily obtained from carbon dioxide in the atmosphere and water from
the soil. Therefore, deficiency in these nutrients rarely occurs. • They are the most abundant elements found in a plant and form the major ingredients of organic compounds, most of which are carbohydrates.• The remaining mineral elements are obtained in the form of inorganic ions from the soil.• Micronutrients are elements that are required by plants in small quantities. These micronutrients are boron (B), copper (Cu), ferum (Fe),
manganese (Mn), molybdenum (Mo) and zinc (Zn).• In the water culture method, the quantities of micronutrients that are needed are as little as one part per million.• To determine which elements are required for normal growth, plant seedings are growth in complete culture solutions, also known as Knop’s
solution. The solution contains the following ingredients:• Calcium (Ca(NO3)2) 0.8g• Potassium nitrate (KNO3) 0.2g• Potassium dihydrogen phosphate (KH2PO4) 0.2g• Magnesium sulphate (MgSO4) 0.2g• Ferum (III) phosphate (FePO4) trace• Distilled water 1000cm3
• Experiments can be carried out to examine the importance and effects of the lack of a certain element to the plant by eliminating it from Knop’s solution. For example, to examine the effects of the lack of calcium, the calcium nitrate in Knop’s solution is replaced with sodium nitrate. The growth of this plant can be compared to the growth of another plant that is grown in a complete Knop’s solution.
• Macronutrients and micronutrients are involved in the synthesis of chemical substances essential for the healthy growth of plants.
• They are also required for the various metabolic processes which take place in plants.• The absence of one or more of these nutrients can lead to mineral deficiencies in
plants.• The symptoms of a mineral deficiency depend on the function or functions of the
mineral in the plants.• The functions and effects of deficiencies in macronutrients and micronutrients are
given in Table 6.11 and Table 6.12 respectively.•
The functions and effects of macronutrient deficiencies
Macronutrient Functions Effects of deficiency
Nitrogen A major component of proteins, nucleic acids, chlorophyll and enzymes for photosynthesis and respiration.
Important for rapid stem and leaf growth. Increases seed and fruit yields.
Stunted growth. Chlorosis, in which the synthesis of chlorophyll is inhibited, results in pale yellow
leaves.
Phosphorus Synthesis of nucleic acids, adenosine triphosphates (ATP), and phospholipids of plasma membranes.
Acts as a coenzyme in photosynthesis and respiration.
Poor root growth. Formation of dull, dark green leaves. Red purple spots on old leaves.
Potassium Protein synthesis. Carbohydrate metabolism. A cofactor for many enzymes. Maintains turgidity in plants.
Reduced proteins synthesis. Yellow-edged leaves. Premature death of plants.
Calcium A major constituent of the middle lamella of cell walls. Formation of spindle fibres during cell division.
Stunted growth. Leaves become distorted and cupped. Areas between leaf veins become yellow.
Magnesium The main structural component of the pigment chlorophyll. Activates many plant enzymes. Involved in carbohydrate metabolism.
Yellowing of the regions between the veins of mature leaves. Red spots on leaf surfaces. Leaves become cupped.
Sulphur A component of certain amino acids. A constituent of vitamin B and some coenzymes.
General yellowing of the effected leaves or the entire plant.
Micronutrient Functions Effects of deficiency
Boron Aids in calcium ion uptake by roots and translocation of sugars. Involved in carbohydrate metabolism. Aids in the germination of pollen grains. Required for normal mitotic cell division in the meristems. Acts as a cofactor for chlorophyll synthesis.
Death of terminal buds. Abnormal plant growth. Leaves become thick, curled and brittle.
Copper An important component of enzymes. Involved in nitrogen metabolism and photosynthesis. Important for reproductive growth and flower formation in plants.
Death of tips of young shoots. Brown spots appear on terminal leaves. Plants are stunted.
Ferum A cofactor in the synthesis of chlorophyll. Essential for young growing plants.
Yellowing of young leaves.
Manganese An activator of enzymes in photosynthesis, respiration and also nitrogen metabolism.
A network of green veins on a light green background. Brown or grey spots between the veins.
Molybdenum Involved in nitrogen fixation. Reduction of nitrates during protein synthesis.
Chlorosis in the areas between the veins of mature leaves. Pale green leaves. Reduction in crop yields.
Zinc Formation of leaves. Synthesis of auxin (a type of growth hormone in plants). Acts as a cofactor in carbohydrate metabolism.
Mottled leaves with irregular areas of chlorosis. Retarded growth.
• ~ Two German botanists, Julius Sachs and Wilhelm Knop, grew plants in culture solutions to determine the roles of macronutrients in plant growth.
• ~ Sachs showed that a plant would grow well in a culture solution containing potassium nitrate, sodium chloride, calcium sulphate, magnesium sulphate and ferum (II) chloride.
• ~ Knop discovered that sodium chloride could be omitted without affecting the plant’s growth. He also altered the solution’s components by supplying the same elements in different compounds.
•
Photosynthesis
Scientist Discovery Aristotle Soil had the ability to convert dead organic matter into useful nutrients which can be absorbed by plants.
Jean Baptiste van Helmont (1640; a Belgian physician)
Conducted an experiment to investigate how plants grow. He planted a seeding weighing 2.3kg in a pot that contained 90.7kg of soil. The rim of the pot was kept covered with an iron plate pierced with tiny holes. This prevented dust from mixing with the soil but allowed air and water to enter. After five years, the seeding had grown into a tree weighing 76.9kg, but only 57g of the soil was lost from the pot. He concluded that plant growth was mainly due to the water which was regularly added and not the result of the soil.
Joseph Priestly (1772) Demonstrated that green plants can restore air and make it capable of supporting combustion and respiration. He placed a burning candle in an upturned glass jar and lat the candle burn until the flame went out. He placed a sprig of mint plant in the jar for ten days. Another burning candle was then placed inside the jar. The candle burned perfectly well in it. Not only that, the air in the jar which contained the plant was able to support a mouse placed in it. Although Priestly did not know about oxygen, his work showed that plants release oxygen into the atmosphere.
Scientist Discovery Jan Ingenhousz Discovered that plants only release oxygen in the presence of sunlight and that only the green parts of plants could release oxygen.
Recognized the importance of sunlight and chlorophyll in photosynthesis and that carbon dioxide is the source of carbon for green plants.
Jean Senebier (1780s) Discovered that carbon dioxide is used by plants during photosynthesis.
de Saussure (1804) Showed that water is required for photosynthesis.
Robert Mayer (1845) Recognized that plants convert solar energy into chemical energy during photosynthesis.
Blackman (1905) Discovered that photosynthesis involves two reactions. He showed that photosynthesis involves a photochemical reaction which is light dependent and a biochemical reaction which is light independent. The photochemical reaction is now referred to as the light reaction whilst the biochemical reaction is referred to as the dark reaction.
Robert Hill (1937) Showed that isolated chloroplasts placed in water in the presence of a suitable oxidizing agent were able to release oxygen. He proved that chloroplasts are able to produce oxygen by splitting water molecules in the absence of carbon dioxide.
• A Brief History of Discovery of Photosynthesis• In conclusion, scientists have shown that plants:
– require carbon dioxide (from the air) and water (from the soil) to synthesise food in the presence of light energy.
– synthesise carbohydrates (glucose) and release oxygen during photosynthesis.
• carry out photosynthesis in the green parts of plants which contain chloroplasts
Leaf Structure and Function
• Leaves are the main photosynthetic organs in a plant.
• They are adapted to carry out photosynthesis efficiently.
• How are the physical structures of leaves adapted for photosynthesis?– A leaf consists of a flat, thin lamina which is joined to the stem by a petiole. From the petiole, a
main vein leads down the leaf and branches out into side veins which support the lamina. This petiole holds the leaves in the best position to receive the maximum amount of sunlight.
– The flattened shape of the lamina has a large surface to trap sunlight and it is thin so that light can penetrate.
– The thinness of the lamina also allows diffusion of gases involved in photosynthesis to take place efficiently in the leaf.
– The veins that connect the leaf to the rest of the plant contain xylem and phloem tissues. Xylem transports water absorbed by the roots to the leaf while phloem transports products of photosynthesis away from the leaf.
– The leaves of a whole plant are arranged so that they shade each other as little as possible. This arrangement, called a leaf mosaic (Photograph 6.1), allows hardly any light to pass through. In this arrangement, the leaves of a plant are in an optimal position for photosynthesis.
– Many plants can also detect the direction of light, so that their leaves are always in the best position to absorb light.
•
Adaptation of the structure of a leaf for photosynthesis
Structure Features Functional adaptationsCuticle >A waxy covering which protects the leaf. >The cuticle is waterproof to help prevent excessive water loss.
>It is transparent to allow light to enter the leaf.
Upper epidermis Figure 6.21 Epidermis cells
>Coated with cuticle>Consists of a single layer >Does not contain chloroplasts
>The upper epidermis of a leaf is thin and transparent. This allows light to penetrate the leaf and reach the light-trapping chloroplasts inside.>Sunlight can penetrate easily because the epidermal cells do not contain chloroplasts.
Palisade mesophyllFigure 6.22 Palisade mesophyll and spongy mesophyll
>Palisade cells are packed tightly together in an upright arrangement near the upper surface of the leaf.>These cells have a high density of chloroplasts. >The cell walls of palisades are coated with a film of water.
>Receive the maximum amount of light. >These cells are the most active cells in photosynthesis. The chloroplasts within these cells are able to move about and arrange themselves to carry out maximum light absorption.>Respiratory gases can dissolve in the film of water before diffusing into the cells.
Spongy mesophyll (Figure 6.22) >The spongy mesophyll consists of cells which have an irregular shape.>The cells have fewer chloroplasts than palisade cells.>The cells are loosely arranged and between each of them are air spaces that connect the mesophyll to the stomata.>The cell walls of spongy mesophyll are coated with a film of water.
>The irregular shape of these cells increase the internal surface area for gaseous exchange.>Less important for photosynthesis than the palisade mesophyll layer.>The large air spaces allow for easy diffusion of water and carbon dioxide through the interior of the leaf to the palisade cells.>The moist surfaces allow gaseous exchange to take place efficiently in the cells.
Vascular bundle Figure 6.23 Vascular bundle
>Consists of xylem and phloem >Xylem transports mineral ions and water to the leaf.>Phloem transports products of photosynthesis away from the leaf.
Lower epidermis Figure 6.24 Guard cells and a stoma
>A layer of epidermal cells that forms the lower protective boundary to the leaf.>Does not contain chloroplasts except for guard cells which are specialized epidermal cells.>Each stoma is flanked by two guard cells which regulate the size of the pore.>Stomata are normally more abundant on the lower epidermis of the leaf than the upper epidermis.
>Guard cells enable the opening and closing of stomata.>Stomata support photosynthesis by allowing the exchange of gases between the inside of the leaf and its surroundings.>Carbon dioxide from the atmosphere diffuses into the leaf through the stomata and oxygen (a by-product of photosynthesis), diffuses out of the leaf via the same route.
• • Adaptation of Plants from Different Habitats
to Carry Out Photosynthesis» Plants from different habitats show a variety of
adaptations to carry out photosynthesis optimally.» The distribution of stomata and chloroplasts within plants
varies according to their environmental conditions.
Habitat Distribution of stomata Distribution of chloroplastsLand plants that live in a tropical area, for example, hibiscus. >Large numbers of stomata on the lower epidermis of the leaf which allow maximum carbon
dioxide absorption.>The upper epidermis does not have many stomata since direct exposure to sunlight would lead to excessive evaporation and water loss.
>Most of the chloroplasts are found in the palisade mesophyll cells and the spongy mesophyll cells. This facilitates maximum absorption of sunlight for photosynthesis.
Habitat Distribution of stomata Distribution of chloroplastsPlants that float on the surface of the water. Floating plants are of two types: those that are rooted with floating leaves (for example, water lilies) and those that are not rooted in the sediment, but just float on the surface (for example, duckweeds)
>The stomata are mostly distributed on the upper epidermis of the leaves.>This upper epidermis is often covered by a thick, waxy cuticle to repel water and to keep the stomata open.>Air-filled internal cavities are also present.
>Chloroplasts are found mainly in the upper epidermis. This maximizes the absorption of sunlight.>Weak stems produce a massive floating canopy of leaves which allow maximum absorption of sunlight for photosynthesis.
Desert plants, for example, cacti. >The leaves contain very few stomata and this helps to prevent excessive loss of water through transpiration.>The stomata are normally located in the grooves along the stem.>Some cacti such as the Dessert Trumpet (Eriognum inflatum) open their stomata at night when it is cooler, rather than during the day when it is hotter. They absorb and store carbon dioxide during the night. the carbon dioxide is used during the day when the stomata are forced to close to reduce the loss of water through transpiration.>Some shrubs in the dessert, such as Hakea sp., have embedded or sunken stomata in the leaves to reduce water loss through transpiration.
>In a cactus plant, the stem performs virtually all of the photosynthesis since cacti have reduced leaves and most of the leaves are modified to become thorns.>Hence, chloroplasts are found all over the plant, that is, in the thorns and stems that are green in colour.
Aquatic plants that are submerged, for example, Hydrilla sp. >The epidermal layer of aquatic plants does not have cuticles.>The cells on the surface are able to absorb water, nutrients and dissolved gases directly from the surroundings.>Hence, stomata are not found on the leaves.>Air-filled cavities often extend through-out the leaves and stems of aquatic plants, providing an internal atmosphere where gaseous exchange can take place.>Aquatic plants have thin-textured, feathery and highly dissected or divided leaves.>This adaptation has the advantage of creating a very large surface area for absorption and photosynthesis.
>The leaves and stems are green in colour. >Hence, chloroplasts are found all over the surface of the plant to miximise the absorption of sunlight.>This is an important adaptation because of the low intensity of sunlight in water.
The Mechanism of Photosynthesis
• The parts of Chloroplasts in Relation to Photosynthesis– There are two main stages in photosynthesis the light reaction and dark reaction.– The light reaction occurs only in the presence of light while the dark reaction happens during
the day and night.– The two reactions of photosynthesis occur at different sites in the chloroplasts. – Chloroplast
• Chloroplasts contain membranous structure which is piled in stacks called grana (singular: granum).
• Grana contain the light-trapping pigment chlorophyll. Chlorophyll is the most abundant photosynthetic pigment in plants.
• Grana are embedded in a gel-like matrix called stroma.• The dark reaction of photosynthesis takes place in the stroma.• The enzymes responsible for the dark reaction of photosynthesis are also found here. • Starch grains which act as a temporary storage place for the products of
photosynthesis are also found in the stroma.
Light reaction Differences Dark reaction
Day time Time of reaction Day & night
Grana Site of reaction Stroma
Water Substances required for reaction Carbon dioxide
Oxygen & water Products of reaction Glucose & water
Occur Photolysis of water Does not occur
Needed Light energy Not needed
Factors Affecting Photosynthesis
• - The rate of photosynthesis which takes place in plants is affected by factors such as
• (a) light intensity• (b) concentration of carbon dioxide• (c) temperature• (d) water supply
Light intensity
• Light is essential during the light reaction of photosynthesis• When the concentration of carbon dioxide & temperature are controlled
at constant levels, the rate of photosynthesis is directly proportional to light intensity up to a certain point– As the light intensity increases, the rate of photosynthesis is increases up to
saturation point at P– A further increase in light intensity does not increase the rate of photosynthesis
because of limiting factors such as the concentration of carbon dioxide & temperature which stop the rate of reaction from increasing further along PQ
– When the carbon dioxide concentration of the environment is raised up to 0.13%, the rate of photosynthesis at each light intensity is higher than the rate of photosynthesis of the corresponding light intensity
– At every high light intensity, the rate of photosynthesis slow down because the pigment chlorophyll is damaged by ultraviolet rays
Concentration of carbon dioxide
• Carbon dioxide is needed in the dark reaction as a raw material used in the synthesis of glucose
• The concentration of carbon dioxide in the atmosphere varies between 0.03% & 0.4%
• If there is no other factor limiting photosynthesis, an increase in the concentration of carbon dioxide results in an increase in the rate of photosynthesis
• As in the case of light intensity, the rate of photosynthesis level off at a certain point (saturation point)
• Although the concentration of carbon dioxide increases, the rate of carbon dioxide will not increase further because light intensity acts as a limiting factor
Temperature
• The dark reaction of photosynthesis catalysed by the photosynthetic enzymes & therefore changes in temperature will affect the rate of photosynthesis
• Generally, an increase of 10˚C in the surrounding temperature will double the rate of photosynthesis
• The optimum temperature varies for the different species of plants, but most plants have an optimum temperature of between 25˚C & 30˚C
• However, when the temperature is too high, the photosynthetic enzymes are destroyed (denaturation) & photosynthesis stops altogether
Water supply
• Water is needed for photosynthesis but water also is rarely the limiting factor in photosynthesis because the amount of water required is small
• If water is not supplied, wilting occurs & the stomata will close. This prevent the diffusion of carbon dioxide into the leaves
• As a result, the rate pf photosynthesis decreases as the lower concentration of carbon dioxide becomes the limiting factor
