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Biology

Topic: Nutrition in plants and animals.

Introduction to the topic.

Organisms obtain food in various ways to survive and carry out physiological processes

like respiration, growth, and osmo-regulation. Organisms that have chlorophyll (green

pigment) such as plants and protests (i.e. Euglena) make food by combining inorganic

substances namely water and carbon dioxide to make carbohydrates in a process called

photosynthesis. This process involves the use of energy (solar energy) from the sun.

Some use energy from the oxidation of certain substances to make food in a process

called chemosynthesis. Organisms that make their own food are referred to as

autotrophs.

Some organisms obtain complex food substances made by other organisms and break

them down to simple substances while others make their own food from simple inorganic

substances. Some organisms obtain food by breaking down dead organic substances.

These are called saprophytes and the type of feeding is called saprophytism. For

example, fungi feed on a dead decaying wood. Therefore, fungi are saprophytes. Some

organisms obtain food by associating with other organisms in particular kinds of

relationships. Organisms that obtain food by associating with others of not their own

kind, is a relationship called parasitism. In this relationship, the organism that benefits is

called a parasite while the other that does not benefit (may be harmed) is called a host e.g

a tick on a cow. In other relationships, both organisms benefit from the relationship. This

is called mutualism e.g the gut bacteria. In some relationships, one organism may benefit

or not. This is called commensalism and the organisms involved are called commensals.

All relationships where two organisms live together are called symbiosis.

Organisms which obtain food by breaking down complex food substances to simple

absorbable molecules are called heterotrophs.

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Sub-topic: Nutrition in green plants

Time required: 8 periods (320 minutes)

Brief description of unit.

Green plants differ from other organisms because they take in simple materials such as

carbon dioxide, water and mineral salts (i.e. nitrates, phosphates and sulphates) to build

more complex substances including carbohydrates, oils and proteins. The building up

of carbohydrates takes place in the presence of the sunlight in a process called

photosynthesis. Therefore, the raw materials for photosynthesis are water and carbon

dioxide. During photosynthesis the plants give off oxygen according to the equation

below.

Light energy

6H20 +6C02 C6 H 12 06 + 602

Chlorophyll

Some carbohydrates are later changed into lipids and proteins.

This unit therefore deals with: -

The process of photosynthesis.

Rate of photosynthesis.

Factors affecting photosynthesis.

The structure of a leaf and its adaptations to photosynthesis.

Experiments on photosynthesis.

The importance of major plant nutrient elements.

Main content and concepts to emphasize;

Definition of photosynthesis.

Raw materials for photosynthesis namely carbon dioxide and water.

The two main stages of photosynthesis: dark and light stage.

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Factors affecting photosynthesis namely carbon dioxide, light,

chlorophyll.

Products of photosynthesis namely carbohydrates and oxygen gas.

Testing of a leaf for starch.

Adaptations of a leaf to photosynthesis.

Mechanism or process of photosynthesis including equation of

photosynthesis (the word equation and chemical equation)

Experiment to show that:

i) Carbon dioxide is necessary for photosynthesis.

ii) Light is necessary for photosynthesis.

iii) Chlorophyll is necessary for photosynthesis.

iv) Oxygen is given off during the process of photosynthesis

The importance of major plant nutrients and their effects on photosynthesis.

Teaching / Learning Materials, activities and guidance:

Polythene bags, cotton wool, bell jars/ 1 litre mineral water bottles, cut into

a half, petroleum jelly, concentrated sodium hydroxide solution, potted

destarched plants, dark paper, knife, pins, Iodine solution, variegated leaf, water;

submerged leaf, plant, spirogyra, test tubes ,glowing splints, filter funnel ,beaker.

Guidance to the teacher: -

The teacher should set up in advance a number of potted plants and should

destarch these plants by keeping them in darkness for a bout 48 hours.

To avoid experimental errors, take the necessary precautions, for example before

proceeding to test if the plant has been destarched by testing for presence of

starch. If no starch is present proceed with the experiment. On the other hand, if

starch is present destarch the plants further.

Control experiments should be set up in order to be able to make conclusions.

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The process of photosynthesis is very extensive, however, you should not teach

details of the process be guided by the syllabus for depth.

This unit needs a lot of preparation and common seedlings like those of beans or

some other suitable plants with at least 3-5 leaves should be readily available.

Due to experimental work, the topic may take more than 2 weeks.

Glossary:

Photosynthesis: - Is a chemical process by which green plants make organic compounds

from carbon dioxide and water in the presence of sunlight.

Chlorophyll: - refers to a green pigment found in all organisms that carry out

photosynthesis. It absorbs light energy.

Destarch: - Is a process of removing or eliminating starch from the leaves of a plant. It is

done by keeping the plant in the dark for 48 hours.

Variegated leaf: - A leaf that has green and non-green parts in its lamina.

Limiting factor: - Any environmental factor that by its presence or absence, either

decrease or increase limits the growth, metabolic process or distribution of organisms or

populations. In this context, the limiting factor would slow down the rate of

photosynthesis.

Macronutrient: -Is a chemical element needed by plants in relatively large amounts.

Macro elements are sometimes called essential elements or major elements.

Micro- nutrient: - A chemical element needed by plants in relatively small quantities.

They are sometimes called trace elements or minor elements.

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Note that: The division between macro and micro is based on quantities and does not

mean that one is more important than the other.

Autotrophic Nutrition: - Is a type of nutrition in which organisms synthesize organic

materials they need from inorganic sources.

Teacher’s notes:

The process of photosynthesis is summed up in the word and chemical equation below:

Water + carbon dioxide Glucose + Oxygen

(Raw materials) sun light and (Products)

Chlorophyll.

6CO2 + 6H20 Sun light C6 H12 O6 + 6O2

(g) (l) Chlorophyll

Mechanism of photosynthesis.

Photosynthesis occurs in two phases; namely light- dependent stage (light reaction/

stage) and light independent stage (dark reaction/stage.)

Light – dependent stage

Radiant energy is absorbed by chlorophyll. The chlorophyll is activated and it converts

light energy into chemical energy in the form of ATP. [Adinosine Triphosphate]

Water is split into hydrogen ions and hydroxyl ions.

Light

H2O [ H ]+ + [OH]

Chlorophyll

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Oxygen gas is formed from reactions, involving hydroxyl ions.

The products of the light dependent stage are ATP, H+ ions and oxygen.

Light independent stage

The hydrogen ions and chemical energy that were produced during the light dependent

phase are used to reduce carbon dioxide to form glucose.

Adaptations of a leaf for photosynthesis:

Transverse section of a leaf showing some of the adaptations.

INTERNAL STRUCTURE OF A LEAF

Cuticle: - Is non-cellular, water proof and transparent. It allows light to pass

through.

Upper epidermis: - Is a single layer of cells on the upper surface of a leaf. It

allows light to pass to the cells below.

Palisade mesophyll - has cells vertically arranged such that many can fit into a

small space. The cells have large number of chloroplasts.

Chloroplasts: - contain chlorophyll for absorbing sunlight.

Spongy mesophyll: - Have large air spaces for fast diffusion of gases to and from

the photosynthesizing cells. The cells have few chloroplasts.

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Veins (Vascular bundles): - They act as drain pipes distributing raw materials to

the leaves and conducting away manufactured food to other parts of a plant.

Stoma: - A pore that allows gaseous exchange to take place. Carbon dioxide

enters and oxygen leaves the leaf through stomata.

Thin lamina: - Provides a short distance over which diffusion of gases take place.

Broad lamina: - Provides a large surface area for absorbing sunlight energy.

Factors affecting the rate of photosynthesis.

When plenty of carbon dioxide, sunlight and water are provided to a plant,

photosynthesis will be at its maximum rate; sometimes the rate of photosynthesis is

not as high as expected due to inadequacy of factors that include light intensity,

water, temperature and carbon dioxide concentration.

By providing plenty of each of these factors to a plant increases the rate of

photosynthesis. However, if one of these factors is not adequate, the rate of

photosynthesis may become low. A factor that is inadequate is called a limiting

factor.

Light intensity: in case of dim light, the rate of photosynthesis is low. As light

intensity increases, the rate of photosynthesis increases.

Carbon dioxide concentration: the more carbon dioxide a plant is given, the

faster is the rate of photosynthesis, until a maximum is reached.

Temperature: photosynthesis is an enzyme-controlled reaction. Increase in

temperature increases the rate of photosynthesis while a decrease lowers it.

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Stomatal opening and closing regulates the amount of carbon dioxide entering a

plant which it uses for photosynthesis. If the stomata are closed then

photosynthesis cannot take place and when they are open, carbon dioxide enters

and the rate of photosynthesis increases.

Table : Macro and micronutrients and their uses.

Macro nutrients Uses Deficiency

Nitrogen Amino acid and protein

synthesis

- Chlorosis with purple

colourings

Phosphorus Protein formation - Reddish purple leaves

and stunted growth.

Sulphur Formation of certain

amino acids

- Stunted growth

- yellow patches on

leaves

Calcium Formation of cell wall

(middle lamella)

- Poor development of

leaves at shoot apex

Potassium Concerned with

synthesis of

carbohydrates and

protein metabolism in

young leaves.

- Chlorosis, and

yellowing of leaves at

the margins, tips.

- Retarded growth.

Magnesium Chlorophyll formation.

Facilitates enzyme

activity.

- Chlorosis of older

leaves. - Stunted growth.

Iron Required for the

formation of chlorophyll

but not part of the

molecule.

- Chlorosis with pale

leaves

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Micro nutrients

Cobalt, Copper, Zinc,

Manganese, Boron

They are necessary for

the manufacture of

enzymes.

- Stunted growth of

leaves.

Mineral salts are necessary for normal plant growth.

Several elements are known for normal plant growth and development.

Some of these elements are Carbon, Hydrogen and Oxygen.

Other elements are Nitrogen, Sulphur, Phosphorus, Potassium, Calcium and Magnesium.

The last category of important elements includes Iron III, Manganese, Zinc, Boron and

Molybdenum. These elements in last category are called trace elements because they are

needed in small amounts for normal growth. Apart from the first category, the plant

obtains the nutrients it needs from the soil. The effects of these chemical elements can be

discovered by growing plants in water solutions containing balanced amounts of salts

necessary for healthy plant growth. This solution is called a culture solution. If any of

the elements is left out; its effect can be noted. Table II. shows that certain elements are

necessary for normal plant growth.

Table II

Content Culture experiment

result

Function of element

Complete solution Normal healthy growth

__________

Distilled water. Virtually no growth

___________

Component of all

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No Nitrogen. Very little growth proteins

No Phosphorus Thin lanky growth,

poor root development

and reddish leaves

Component of

certain enzymes

and in the nucleus

is substances like

DNA

No Sulphur

Some growth but less

than it would be in

complete solution.

Present in certain

proteins

No Potassium

Little growth. Leaves

turn orange brown.

Needed in cell

formation

No Calcium

Stunted growth.

Needed in cell wall

formation

No Magnesium

Stunted growth and

leaves turn yellow.

Chlorophyll

contains

magnesium

No Iron

Poor growth, leaves

turn yellow.

Needed in

chlorophyll

synthesis

For the teacher and students:

Carbohydrates are the products of photosynthesis. Simple sugars are the first to be

formed but are immediately converted into starch, the first visible products of

photosynthesis. It is therefore possible to find out if a plant has carried out photosynthesis

by testing for starch. Presence of starch implies that photosynthesis has occurred.

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STUDENTS ACTIVITIES.

Worksheet I

Aim: - To test a leaf for starch.

Apparatus and Materials

Beaker, boiling tube, heat source, test tube holder

Iodine solution, ethanol, water and a leaf that has been exposed to light.

Procedure

-Half fill a beaker with water and boil the water. Place the leaf in boiling

water for one or two minutes. This denatures the enzymes and stops chemical

reactions. Put the leaf into a boiling tube and pour enough ethanol to cover the

leaf. Place the boiling tube into the beaker of boiling water. A water bath is

used to prevent accidents that may arise from alcohol being inflammable.

Remove the flame when the ethanol boils.

Re-introduce the flame when the ethanol stops boiling. Repeat until the leaf is

decolourised.

Diagram to illustrate boiling a leaf in alcohol using a water bath.

(Testing a leaf for starch)

- When the chlorophyll has been removed (the leaf looks white) remove the

leaf from ethanol (the leaf will be brittle).

- Dip the leaf in water to soften it and make it permeable.

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- Using a pipette, add dilute iodine solution to the leaf surface.

Student’s discussion:

1. When boiling the leaf in ethanol, what happened to the colour of ethanol? Explain your

observation.

2.What colour did the leaf turn when you added iodine?

3. What do these results show?

Worksheet II To show that carbon dioxide is necessary for photosynthesis.

Aim: To show that carbon dioxide is necessary for photosynthesis.

Apparatus and Materials:

- Two well watered destarched potted plants, polythene bags, cotton wool, 2

bell jars, petroleum jelly, 100cm beaker, concentrated sodium hydroxide

solution, boiling tube, heat source, test tube holder, cork,

Iodine solution, ethanol, water, soda lime.

Procedure / method

Procedure: -

Set up the experiment as shown above and expose to light for 2-6 hours.

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Obtain a leaf from each plant and test it for starch.

Result

- Leaf from jar A stains brown.

- Leaf from jar B turns black or dark blue.

Conclusion.

Carbon dioxide is necessary for photosynthesis.

Worksheet III

Aim: To show that light is necessary for photosynthesis.

Apparatus and Materials: well watered destarched potted plant, dark paper, scissors /

knife / scalpel, pins/clips

Method / procedure:

The set up is as shown below:

- Fix the paper on one of the leaves as shown in the diagram below.

Diagram to show the set up.

- Make a sketch of the leaf showing the covered and uncovered parts.

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- The experimental set up is exposed to sunlight for 2-6 hours.

- The paper is removed and the leaf is tested for starch.

- Make a labelled diagram to show the results.

Results.

Diagram showing results. (the parts that were covered are stained brown, and the rest of

the leaf turns blue or black)

Conclusion.

Starch is produced only in areas that received light, showing that

light is necessary for photosynthesis.

Worksheet IV

Aim: To show that chlorophyll is necessary for photosynthesis.

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Apparatus and Materials

Plant that has variegated leaves, ethanol, beaker, boiling tube, water, heat source

Iodine solution.

Method / procedure:

- Leave the plant with variegated leaves in light for 2-6 hours.

- Remove one leaf and make a labelled diagram to show the green and non-green parts.

- Test the leaf for starch.

Drawing of a leaf to show the distribution of chlorophyll

(A variegated leaf before and after testing for starch with iodine solution)

Conclusion:

Starch is made only in areas of the leaf with chlorophyll, showing that chlorophyll is

necessary for photosynthesis.

Worksheet V

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Aim: To show that oxygen is given off during photosynthesis.

Apparatus and materials:

- Beaker, short stemmed funnel, 2 thick wooden cubes, sodium hydrogen

carbonate powder, test tube, water plant e.g. Elodea. (Spirogyra can be

used too), glowing splint,

Method / procedure:

Set up the experiment as shown below. Expose it to light for about 2 hours.

(The supports ensure circulation of air. Sodium hydrogen carbonate adds carbon dioxide.)

Test any gas collected in a test tube using a glowing splint.

Observations.

A gas collects in the test tube.

The gas re-lights a glowing splint or makes it glow brighter.

Conclusion:

Oxygen is given off during photosynthesis.

Evaluation questions for students.

Section A

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Objectives type questions: -

Which of the following are raw materials for photosynthesis?

a) Water and Glucose

b) Starch and Mineral salts

c) Water and carbon dioxide

d) Glucose and starch.

2. Which carbohydrate does a plant store?

a) Glucose

b) Sucrose

c) Starch

d) Galactose.

3. The light dependant stage of photosynthesis yields the following.

a) Water and oxygen.

b) Hydrogen ions, energy and oxygen

c) Water and energy

d) Oxygen and energy.

4. Why is glucose not very good as storage material in plants?

a) It is insoluble

b) It is soluble

c) It is a disaccharide.

d) It is in short supply.

5. What substance does a plant need to be able to convert glucose into proteins?

a) Nitrogen

b) Oxygen

c) Hydrogen

d) Starch.

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Section B

Structured questions

1. An experiment was performed to find out how fast a plant photosynthesised as the

concentration for carbon dioxide in the air around it was varied. The results were as

follows.

Carbon dioxide

concentration.

% of by volume in air

Rate of photosynthesis in

arbitrary units,

Low intensity of light. High intensity of light

0 0 0

0.02 20 33

0.04 29 53

0.06 35 68

0.08 39 79

0.10 42 86

0.12 45 89

0.14 46 90

0.16 46 90

0.18 46 90

0.20 46 90

a) Plot these results on a graph using same axes.

b) What is the carbon dioxide concentration of normal air?

c) What is the rate of photosynthesis at this carbon dioxide concentration in high

light intensity?

d) Up to what values does carbon dioxide act as a limiting factor at high light

intensity?

2. (a) Distinguish between autotrophic nutrition and holozoic nutrition.

(b) What factors affect the rate of photosynthesis?

(c) How is a leaf adapted to carry out photosynthesis?

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3.Photosynthesis is a plant’s way of making food.

(a) List four things that a leaf must have for photosynthesis to occur.

(b) Which of these are “raw materials”

4. In an experiment to find out that light is necessary for photosynthesis, the set up below

was used.

(a) What should be done to plant before and why?

(b) If you tested the leaf for starch after 24 hours, what results would you expect?

(c) (i). Where is the control in this experiment?

(ii) Is the control satisfactory? If not, can you suggest a better one?

Section C

Essay question.

a) What is a limiting factor?

b) Name three factors that may affect the rate of photosynthesis in a plant.

c) Explain how each of the factors named in (b) above limits the rate of

photosynthesis.

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Answers to evaluation questions:

Section A

Objective type questions

1.C

2.C

3.B

4.B

5.A

SECTION B

Structured questions.

1 (a) In autotrophic nutrition, plants take in simple inorganic molecules that they build up

into complex organic molecules using light energy whereas in holozoic nutrition an

organism obtains nutrients by taking in complex organic food, which it breaks down into

simple absorbable molecules. Holozoic nutrition does not involve use of light energy.

(b) Light intensity, carbon dioxide, water, and temperature.

(c) - a waxy cuticle to minimise water loss.

- vascular tissue to transport water to and manufactured food away from the

leaves.

- stomata that allow carbon dioxide to enter and oxygen to leave

- has chloroplasts, These contain chlorophyll which absorbs sunlight.

- spongy mesophyll cells that have large air spaces where gases can diffuse

easily to and from photosynthesizing cells.

- thin lamina which provides a short distance over which diffusion takes place.

- broad lamina which provides a large area surface for absorbing sunlight.

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Section C

Essay question.

a) A limiting factor is that whose supply decreases the rate of a metabolic reaction

e.g. light intensity limiting the rate of photosynthesis.

b) Carbon dioxide concentration temperature, stomata.

Stomata allow in carbon dioxide, which is used, for photosynthesis. When the stomata

open more carbon dioxide enters and photosynthesis is high. When the stomata close less

carbon dioxide enters and photosynthesis is slow

Students Discussion questions

1. When finding out if a plant needs light, a destarched plant is used.

Why did you do this?

2. Why would it be necessary to set up two plants, one in the dark and the

other in light?

3. When testing a leaf for starch, certain precautions may be taken. Write out

the instructions and suggest the precautions to be taken.

4. Give 2 reasons why photosynthesis is important to humans.

5. Photosynthesis involves a pair of raw materials, a pair of conditions and a

pair of products, Name them.

References: -

1.Mckean D.G (1973), introduction to Biology, Hodder Murry (UK) London.

2. Ian J. Burton (2001), the Cambridge Revision Guide GCE, level Biology,

Cambridge University Press.

3.Maxwell – Ojo.B (1998), Modern Tropical Biology, Evans and Brothers London(UK)

4.Stone R.H and Cozens A. B. (2002), New Tropical Biology (3rd

Edition)

Longmans, London (UK).