Nutrition

• This is the process by which organisms obtain and Assimilate 

nutrients. 

• There are two modes of nutrition; Autotrophism and 

Heterotrophism.

 Autotrophism 

• This is where living organism manufacture its own complex food 

substances from simple substances such as carbon (iv) oxide, 

water, light or chemical energy.

• Where sunlight is used as a source of energy, the process is 

referred to as photosynthesis. 

• Photo means light while synthesis means to make. 

• Some none green plants make their own food using energy 

obtained from certain chemicals through a process called

chemosynthesis.

• Organisms that make their own food are referred to as 

autotrophs. 

Heterotrophism

• This is where organisms take in complex food materials such as 

carbohydrates, proteins and fats obtained from bodies of plants 

and animals. 

• Organisms that feed on already manufactured foods are called 

Heterotrophs. 

External Structure of a Leaf

A leaf is a flattened organ which is attached to the stem or a branch of 

a plant. 

Diagrams 

Parts of a leaf

Lamina: This is the flat surface. It is green in colour and contain the 

photosynthetic tissue. 

Midrib: This is a thick structure running through the middle of the leaf 

Veins: They arise from the midrib to forming an extensive network of 

veins. 

Leaf Apex: This is the tip of the leaf and usually it is pointed. 

Petiole: It attaches the leaf to the stem or branch. 

 In some monocotyledonous plants the leaves are attached to the 

stem by the leaf sheath. 

Practical Activity 1: To examine the External Features of a 

Dicotyledonous and Monocotyledonous leaf 

Study Question 1 

Internal Structure of a Leaf

• Internal structure of the leaf is composed of the following parts.

i.)Cuticle. 

• It is a thin waterproof and transparent layer that coats the upper 

and lower surfaces of the leaf. 

• It reduces excess water loss and protects the inner tissue of the 

plant against mechanical injury. 

• It also prevents entry of disease causing micro organisms. 

• Since it is transparent, it allows penetration of light for 

photosynthesis. 

ii.) Epidermis. 

• It is a one cell thick tissue on both the upper and lower leaf 

surfaces. 

• It secretes the cuticle and also protects the inner tissues from 

mechanical damage and prevents entry of pathogens. 

• Epidermal cells have no chloroplast except the guard cells. 

• Guard cells are special bean shaped cells. They have chloroplast 

and are able to carry out photosynthesis hence controlling the 

opening and closing of the stomata. 

• Air moves into and out of the leaf through the stomata. 

iii.) Palisade layer. 

• This is layer of cells located beneath the upper epidermis. 

• It is made of cylindrical shaped cells closely packed together. 

They have numerous chloroplasts containing chlorophyll. 

• Their position and arrangement enables them to receive 

maximum light. 

iv.) Spongy Mesophyll Layer. 

• This is below the palisade layer. The cells are irregularly shaped 

and loosely packed creating large air spaces in between them. 

• The air spaces allow gases to diffuse in between the cells. They 

contain fewer chloroplasts as compared to the palisade cells. 

v.) Leaf Veins. 

• Each vein is a vascular bundle consisting of xylem and phloem. 

• Xylem conducts water and mineral salts from the roots to the 

leaves while the phloem translocates manufactured food from the 

leaves to the rest of the plant.

Adaptations of Leaves to Photosynthesis. 

1. Broad and flat lamina to increase surface area of Carbon (IV) 

oxide and sunlight absorption. 

2. Thin transparent cuticle and upper epidermis; to allow easier 

penetration of light to photosynthetic cells; 

3. Thin; for faster diffusion of gases; 

4. Palisade cells placed next to the upper surface; to trap maximum 

light for photosynthesis; 

5. Palisade cells with numerous chloroplasts; to trap maximum 

amount of light for photosynthesis; 

6. Large/ intercellular air spaces in the spongy mesophyll layer; for 

storage of Carbon (IV) oxide for easier gaseous exchange; 

7. Waxy water proof cuticle; to reduce water loss sand reflect excess 

light; 

8. Leaf mosaic/ non-overlapping leaves; for maximum exposure to 

light; 

9. Guard cells, modified cells to open and close stomata; to control 

amount of water loss from the leaf and allows gaseous exchange;

 10. Leaves have leaf veins; xylem to conduct water to 

photosynthetic cells, Phloem to translocate products of 

photosynthesis to other parts of plant; 

The Chloroplast

• They are disc shaped organelles found in the cytoplasm of plant 

cells. 

• Each chloroplast has a double membrane; the inner and outer 

membrane. 

• Chloroplasts are made of layers of membranes called lamellae

contained in a fluid matrix called stroma. 

• Several lamellae come together to form the granum (grana). 

• Granum contains chlorophyll molecules and other photosynthetic 

pigments. 

• The stroma contains enzymes that speed up the rate of 

photosynthesis.

The Process of Photosynthesis 

• The raw materials for photosynthesis are; water and carbon (IV) 

oxide. The process however requires the presence of sunlight 

energy and chlorophyll pigment. 

• The products of photosynthesis are glucose and oxygen. The 

process can be summarized using an equation as shown below. 

6H2O + 6CO2 ----------> C6H12O6+ 6O2

- Simple sugars formed are used for respiration to provide energy 

or are converted to storable forms e.g lipids, proteins, starch, 

cellulose, etc. 

Study Question 4 

Practical Activity 3: To Investigate the Presence of Starch in a 

Leaf. 

Study Question 5 

Factors Affecting the Rate of Photosynthesis

i.) Light Intensity. 

• Increase in light intensity increase the rate of photosynthesis up to 

a certain level where it slows down and finally levels off. 

• Very bright sunshine may damage the plant tissues due to high 

amount of ultra violet light. 

• Light quality or light wavelength also affects the rate of 

photosynthesis. 

• Red and blue wavelengths of light are required by most plants for 

photosynthesis.

 Light intensity 

ii.) Carbon (IV) oxide concentration 

• Increase in Carbon (IV) oxide concentration increases the rate of 

photosynthesis linearly up to a certain level after which it slows 

down and levels off.

Carbon (IV) oxide concentration 

iii.) Temperature 

• Photosynthesis is an enzyme controlled process, therefore 

increase in temperature increase the rate of photosynthesis up to 

the optimum temperature. 

• Increase in temperature beyond the optimum decreases the rate 

sharply as the enzymes become denatured.

iv.) Water 

• Plants need water for photosynthesis. Hydrogen atoms required 

in the dark stage during Carbon (IV) oxide fixation are derived 

from water during photolysis.

Chemical Compounds Which Constitute Living Organisms

• Cells, tissues and organs are made of chemicals which are 

referred to as chemicals of life. 

• The study of chemical compounds found in living organisms and 

reactions in which they take part is called Biochemistry.

• Chemicals of life include carbohydrates, lipids and proteins.

a) Carbohydrates 

• They are compounds of carbon, hydrogen and oxygen in the ratio 

of 1:2:1 respectively. 

• Carbohydrates have a general formula of (CH2O)n where n 

represents the number of carbon atoms in a molecule of 

carbohydrate. 

• Carbohydrates are divided into three groups; Monosaccharide’s, 

Disaccharides and Polysaccharides. 

i) Monosaccharides 

• They are the simplest carbohydrates and have a general chemical 

formula of (CH2O)n where n = 6. 

• Their chemical formular is therefore C6H12O6. They include; 

glucose, fructose, galactose etc. 

Properties of Monosaccharides 

i) They are soluble in water to form sweet tasting solutions. 

ii) They are crystalissable. 

iii) They have the reducing property where they reduce copper 

sulphate in Benedicts solution to red copper (I) oxide. 

Functions 

i) They are oxidized to release energy during respiration. 

ii) When condensed together, they form polysaccharides such as 

starch, cellulose or glycogen. 

ii) Disaccharides

• They are formed by linking two Monosaccharide molecules 

through the process of condensation where a molecule of water is 

liberated.

ii) They are non reducing sugars. Some such as the maltose can 

reduce copper sulphate in Benedict’s solution when heated 

together and are therefore referred to as complex reducing 

sugars. 

iii) They are readily broken into their constituent

monosaccharide molecules in a process known as Hydrolysis in 

the presence of water. 

• Naturally disaccharides are hydrolyzed by enzymes. In the 

laboratory, hydrolysis is achieved by boiling them in dilute 

Hydrochloric acid. 

Functions 

• They are hydrolyzed by enzymes into monosaccharide’s which 

are then oxidized to produce energy. 

iii) Polysaccharides.They are made of many monosaccharide 

molecules hence are long and more complex.

• They have a general formula of (C6H10O5) n; where the value of n 

is a very large number. 

Examples of polysaccharides 

i) Starch 

• It is present as stored food in plant tissues e.g. maize, wheat, 

potatoes, rice etc.

ii) Cellulose 

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• This is the component of the cell wall in plants. Cellulose gives 

the plant cells their definite shape.

iii) Glycogen 

• This is the form in which carbohydrates are stored in animal 

tissues. Excess glucose is converted into glycogen for storage in 

the liver.

Properties of Polysaccharides

i) All are insoluble in water. 

ii) Do not have a sweet taste hence are referred to as non-sugars. 

Study Question 12 

b)Lipids 

• These are the fats and oils. Fats are found in animals while oils 

are found in plants. 

• Oils are liquid while the fats are solid at room temperature. 

• They contain carbon, hydrogen and oxygen just like the 

carbohydrates. However they contain fewer number of oxygen 

atoms than in carbohydrates. 

• Lipids are made up of three fatty acid molecules and one 

molecule of Glycerol.

• The nature of a lipid formed, depends on the fatty acids it 

contains. Glycerol remains the same in all lipids. 

Diagram 

• Complex lipids are formed through condensation of many lipid 

molecules just like in carbohydrates. 

• Examples of complex lipids include; phospholipids, waxes, 

steroids and cholesterol. 

• Presence of lipids in a food sample is detected using the grease 

spot test or emulsion test. 

Properties of Lipids 

1. When fats are heated they change into liquid while oils solidify 

under low temperature. 

2. Both fats and oils are insoluble in water. They however dissolve 

in organic solvents such as alcohol to form emulsions and 

suspensions. 

3. Lipids are inert hence can be stored in the tissues of organisms. 

Functions of Lipids 

i) Source of energy 

• They give almost twice as much energy as the Monosaccharides. 

ii) Source of metabolic water 

• When oxidized, lipids release more water than Monosaccharides. 

Such water is referred to as metabolic water.

iii) Structural compounds 

• Lipids are constituents of plasma membrane and protoplasm. 

iv) Heat insulation 

• Fats are deposited under the skin of animals forming the adipose 

tissue which acts as a heat insulator.

• Mammals in the temperate regions have thick adipose tissue to 

greatly reduced heat loss.

• Thick adipose tissue in aquatic animals helps them to be buoyant 

in water.

v) Protection 

• Fat is deposited around the major organs such as kidney, heart etc 

where they act as shock absorber. 

• Wax in plant cuticles reduces excessive water loss.

Study Question 13 

Practical Activity 7: testing for the Presence of Lipids 

i) The Grease Spot 

ii) The Emulsion Test 

c) Proteins 

• Like carbohydrates and lipids, proteins are compounds of carbon, 

hydrogen and oxygen. 

• In addition they contain nitrogen and sometimes phosphorous 

and sulphur. 

• Some proteins such as haemoglobin contain other elements such 

as iron. 

• Proteins are made up of small units called amino acids. There are 

about 20 different types of amino acids. 

• All amino acids contain the amino group (-NH2) which consists 

of hydrogen and nitrogen. 

• Two amino acids combine to form a dipeptide molecule through 

the process of condensation. 

• The bond between two amino acids is called peptide Bond. 

Many amino acids join together to form a long protein chain 

called polypeptide chain. 

• The type and sequence of amino acids contained in such a chain 

determine the uniqueness of the protein being formed. 

Properties of Proteins

i.)They dissolve in water to form colloidal suspensions (not true 

solutions) where particles remain suspended in water. 

ii.) They are denatured by temperatures above 40 0C. Heat 

alters the structure of the protein molecule. Chemicals such as 

detergents, acids, bases and organic solvents also denature 

proteins. 

iii.) They are amphoteric whereby they have both acidic and 

basic properties. This property enables them to combine with 

non-protein compounds to form conjugated proteins such as 

mucus, and haemoglobin. In mucus the non protein compound is 

a carbohydrate while in haemoglobin, iron is a non protein. 

Functions of Proteins 

i.) Structural Functions

• Proteins make the framework of living systems e.g. plasma 

membrane, connective tissues, muscle fibres, hair, nails, 

hooves, skeletal materials etc. 

ii.) Metabolic Regulators

• These are divided into two 

a) Enzymes

• Enzymes are organic catalysts which speed up the rate of 

metabolic reactions such as respiration, photosynthesis, 

digestion etc. 

b) Hormones 

• They are chemical messengers which regulate many body 

processes such as growth, reproduction, amount of sugars, salts 

and water in the blood etc. 

iii.) Source of Energy

• Under extreme starvation, proteins are broken down to release 

energy. 

Study question 14 

Practical Activity 8 

To Test for Proteins 

Enzymes 

• They are organic catalysts which are protein in nature. They 

speed up or slow down the rate of chemical reactions in the 

body without themselves being used up. 

• They are divided into two; 

a) Extracellular Enzymes 

• Extracellular enzymes are produced within the cells but are 

used outside the cells which produce them e.g. the digestive 

enzymes. 

b) Intracellular Enzymes

• They are secreted and used within the cells which produce 

them e.g. the respiratory enzymes. 

Naming of the Enzyme

• There are two methods on naming enzymes; 

i) Trivial Naming 

• Enzymes are given names of persons who discovered them. 

• The names end in -in such as pepsin, trypsin ptyalin etc. 

ii) Use of suffix –ase 

• This is the modern method of naming. The suffix –ase is added 

to the substrate (type of food) or the reaction the enzyme 

catalyze

Properties of Enzymes 

1. They are protein in nature hence are affected by changes in 

temperature and pH. 

2. They are substrate specific. 

3. They are efficient in small amounts as they are not affected by the 

reactions they catalyze. They can be used again and again. 

4. They are catalysts that speed up the rate cellular reactions and are 

not used up in the reactions they catalyses. 

5. Most of the enzyme controlled reactions are reversible. 

Factors Affecting the Rate of Enzyme Controlled Reactions

i.) Temperature

• Enzymes are sensitive to changes in temperature and pH since 

they are protein in nature. 

• Enzymes work best within a narrow range of temperature 

called the optimum temperature.  

• Above the optimum temperature, reaction decreases sharply as 

the enzymes are denatured. 

• Most enzymes have optimum temperature between 35-40oC. 

• Very low temperature inactivates the enzymes hence decrease 

rate of reaction. 

Diagrams 

ii.) pH

• Most enzymes have a pH of close to 7. 

• Some however work best in acidic pH e.g. pepsin while others 

work best in alkaline conditions. 

• As pH changes from the optimum, enzyme activity decreases. 

• Extreme acidity or alkalinity denatures most enzymes. 

Diagrams 

iii.) Specificity

• Enzymes are specific in nature where a particular enzyme acts 

on a particular specific substrate. 

• For example, sucrose works on sucrose and not any other 

substrate. 

iv.) Substrate Concentration and Enzyme Concentration.

• When substrate concentration increases, the rate of enzyme 

reaction also increases upto a certain level. 

• Further increase does not increase the rate of reaction as all the 

active sites of an enzyme are occupied. 

• When enzyme molecules are increased, the rate of reaction 

increases proportionally. 

Diagrams 

v.) Enzyme Co-factors and Co-enzymes

• Co-factors are non protein substances which activates enzymes. 

They are required in small quantities and they include metallic 

ions such as those of iron, magnesium, zinc, copper etc. Some 

are vitamins. 

• Co-enzymes are non protein molecules that work in association 

with particular enzymes. Most co-enzymes are derived from 

vitamins. 

vi.) Enzyme Inhibitors

• Inhibitors compete with the normal substrate for the active sites 

and they take up the active site of the enzyme permanently. 

• There are two types of inhibitors; 

a) Competitive Inhibitors

• These are chemicals closely related to normal substrate and they 

compete for active sites with the normal substrate. They slow 

down the rate of reaction. 

b) Non Competitive Inhibitors

• They do not compete with the substrate. They combine 

permanently with enzyme molecules thus blocking the active 

sites. They include poisons such as cyanides, mercury and silver- arsenic compounds. 

Importance of Enzymes 

• Enzymes speed up the rate of cellular reactions and also control 

them. This way, they help prevent violent reactions in the cells.