Topic 6.1: Digestion

Assessment Statements 6.1.1: Explain why digestion of large food molecules is

essential 6.1.2: Explain the need for enzymes in digestion 6.1.3: State the source, substrate, products and

optimum pH conditions for one amylase, one protease, and one lipase

6.1.4: Draw and label a diagram of the digestive system

6.1.5: Outline the function of the stomach, small intestine and large intestine

6.1.6: Distinguish between absorption and assimilation 6.1.7: Explain how the structure of the villus is related

to its role in absorption and transport of the products of digestion

Why do we digest?When you eat a snack or a meal,

you begin a set of events that leads to your body cells being provided with needed nutrients◦Ingestion◦Digestion◦Absorption◦Transport

DigestionDigestion solves a problem of

molecular size◦Food is too large to pass through cell

membranes◦Food must be chemically digested to

a suitable sizeMolecule type

Molecular form ingested Molecular form after digestion

Protein Protein Amino acidsLipids Triglycerides Glycerol and fatty

acidsCarbohydrate Polysaccarides, disaccharides,

and monosaccharidesMonosaccharides

Nucleic acids DNA, RNA Nucelotides

DigestionDigestion allows you to turn

molecules into ‘your own’◦All the food that you eat is composed

of plant or animal cells, thus containing molecules characteristics of a living organisms that is not a human being Each type of living organism has its own

set of proteins, nucleic acids, and carboydrates

DigestionWhen we digest food molecules,

we break them down (hydrolyze them) into smallest components. ◦The components can be reassembled

into larger molecules that are useful to you!!

DigestionAs food moves through your

alimentary canal, many digestive enzymes are added along the way◦Each digestive enzyme is specific for

a specific food type◦Examples:

Lipase: is an enzyme is specific for lipid molecules

Amylase: is specific for amylose (starch)

DigestionThe real function of enzyme is to

lower the activation energy of reactions that they catalyze◦Digestive enzymes all help to

catalyze hydrolysis reactions

Examples of digestion enzymes

Salivary Amylase

Pepsin (a Protease)

Pancreatic Lipase

Source Salivary glands Stomach cells Pancreas cellsSubstrate Amylose (starch) Proteins

(polypeptides)Lipids

Products Maltose and glucose

Amino acids Glycerol and fatty acids

Optimum Neutral (pH 7) Acidic (pH 3) Neutral (pH 7)

Human Digestive SystemMuch of the human digestive

system is a tube called the alimentary canal

Alimentary canal consists of:◦ Mouth◦ Esophagus◦ Stomach◦ Small Intestine◦ Large Intestine (colon)◦ Rectum

Any foods that you ingest must either be digested and absorbed for use by the body or remain undigested and be eliminated as solid waste

StomachFood is brought to

your stomach by a muscular tube called the esophagus◦When you swallow,

the food is forced down to your stomach by a sequential series of smooth muscle contractions called peristalsis

StomachOnce in the stomach, the food is

held for a period of time in order to mix it with a variety of secretions collectively known as gastric juice

StomachGastric juice is a mixture of three

secretions form the cells of the stomach inner lining:◦Pepsin- a protease enzyme most active

in acidic pH◦Hydrochloric acid- helps degrade and

break down foods and creates the acidic pH necessary for pepsin to be active

◦Mucus- lines the inside the stomach wall to prevent stomach damage from the hydrochloric acids

StomachThe muscular wall of the stomach

creates a churning motion in order to mix food with the gastric juice◦After a period of time, a valve at the

lower end of the stomach opens and the food enters the small intestine

Small IntestineThe first portion of the

small intestine is called the duodenum◦Three different accessory

organs secrete juices into the small intestine in order to continue the digestive process

◦These secretions include: Bile from the liver and gall

bladder Trypsin (a protease), lipase,

amylase, and bicarbonate from the pancreas

Small IntestineAs the digestive process

continues in the small intestine, molecules are produced that are small enough to be absorbed◦The inner wall of the small intestine

is made up of thousand of finger-like extensions called villi

Small IntestineEach villus contains a capillary bed and a lacteal

◦Lacteal is a small vessel of your lymphatic system

If the inner lining of your small intestine were smooth, you would have a fairly limited membrane surface area for absorption◦The function of the villi is to greatly increase

the surface area for absorption of molecules such as glucose, amino acids, and fatty acids

Small IntestineMost molecules absorbed are

taken into the capillary bed within each villus ◦Except fatty acids which are more

efficiently absorbed into the lactealAll absorbed molecules are taken

to a wide variety of body cells by the circulatory system

Large IntestineThe vast majority of useful

nutrients are absorbed while food is still inside the small intestine◦Much of the water that we drink or

that is naturally contained in many food is also present

Large IntestineThe primary

function of the large intestine is water absorption◦Leaving water in

the alimentary canal as long as possible is beneficial because it keeps the moving food in a fluid environment

Large IntestineThe large intestine is also home

to a very large number of naturally occurring bacteria including Escherichia coli.

Large IntestineThese bacteria are examples of

mutualistic organisms within us.◦We provide nutrients, water, and a

warm environment for them while they synthesize vitamin K and maintain a healthy overall environment for us in our large intestine

Any food undigested by us or the bacteria is eliminated from the body as solid waste or feces