• This is the study of the functions of cell structures. 

Membrane Structure and Properties 

• A membrane is a surface structure which encloses the cell and 

organelles. Membranes regulate the flow of materials into out of 

the cell or organelle. 

• Examples of membranes: cell membrane, tonoplast (membrane 

surrounding the vacuole), nuclear membrane, mitochondrial 

membrane, chloroplast membrane etc. 

The Cell Membrane

• It has three layers, two protein layers and a phos-pholipid layer 

sandwiched in between the two. 

Diagram 

Properties of Cell Membrane 

1. Semi-permeability. – It has small pores allowing for the passage 

of molecules of small size into and out of the cell. Cell Wall

however allows all materials to pass through it hence it is referred 

to as being Permeable.

2. Sensitivity to Changes in Temperature and pH – Extreme 

temperature and pH affects the cell membrane since it has some 

protein layers. Such changes alter the structure of the membrane 

affecting its normal functioning.

3. Possession of Electric Charges – it has both the negative and 

positive charges helping the cell to detect changes in the 

environment. These charges also affect the manner in which 

substances move in and out of the cell

Physiological Processes 

• The ability of the cell to control the movement of substances in 

and out of the cell is achieved through physiological processes 

such as Diffusion, Osmosis and Active Transport.

Diffusion 

• This is a process by which particles move from a region of high 

concentration to a region of low concentration.

Practical Activity 1 

To demonstrate diffusion using potassium permanganate (VII) 

• The difference in concentration of particles between the region of 

high concentration and the region of low concentration is known 

as the diffusion gradient.

Role of Diffusion in Living Organisms 

1. Absorption of Materials 

• Mineral salts in the soil enter the root by diffusion since their 

concentration in the soil is greater than in the root hair cells. 

• Digested food (glucose and amino acids) diffuse across the wall 

of the ileum into the blood for transport to rest of the body. 

2. Gaseous Exchange in Plants and Animals 

• In both plants and animals, respiratory gases (oxygen and Carbon 

(IV) oxide) are exchanged through simple diffusion depending on 

their concentration gradient. 

3. Excretion of Nitrogenous Wastes 

Visit: www.kcse-online.info for thousands of educational Resources 

4. Transport of Manufactured Food form Leaves to other Plant 

Parts. 

5.

Factors Affecting Diffusion 

a) Diffusion Gradient 

• A greater diffusion gradient between two points increases the rate 

of diffusion.

b)Surface Area to Volume Ratio 

• The higher the ratio the greater the rate of diffusion and the lower 

the ratio the lower the rate.

• This means that small organisms expose a large surface area to 

the surrounding compared to large organisms.

• Small organisms therefore depend on diffusion as a means of 

transport of foods, respiratory gases and waste products.

Diagrams 

c) Thickness of Membranes and Tissues 

• The thicker the membrane the lower the rate of diffusion because 

the distance covered by the diffusing molecules is greater. The 

thinner the membrane, the faster the rate. 

• Size of the Molecules 

• Small and light molecules diffuse faster than large and heavy 

molecules.

d)Temperature 

• Increase in temperature increases the energy content in molecules 

causing them to move faster.

Osmosis

• This is the process where solvent molecules (water) move from a 

lowly concentrated solution (dilute) to a highly concentrated 

solution across a semi-permeable membrane. 

Diagram fig 4.6 

• The highly concentrated solution is known as Hypertonic 

Solution. 

• The lowly concentrated solution is called Hypotonic solution. 

• Solution of the same concentration are said to be Isotonic.

• Osmosis is a special type of diffusion because it involves the 

movement of solvent (water) molecules from their region of high 

concentration to region of low concentration across a semi 

permeable membrane.

Practical activity 2 

Practical activity 3 

Osmotic Pressure 

• This is the pressure which needs to be applied to a solution to 

prevent the inward flow of water across a semi permeable 

membrane. This is the pressure needed to nullify osmosis. 

• Osmotic pressure is measured using the osmometer. 

Osmotic Potential 

• This is the measure of the pressure a solution would develop to 

withdraw water molecules from pure water when separated by a 

semi permeable membrane. 

Water Relations in Animals 

• Cell membrane of the animal cell is semi permeable just like the 

dialysis/visking tubing. 

• Cytoplasm contains dissolved sugars and salts in solution form. 

• If an animal cell e.g. a red blood cell is placed in distilled water 

(hypotonic solution), water flows in by osmosis. 

• The cell would swell up and eventually burst because the cell 

membrane is weak. The bursting of the red blood cell when 

placed in hypotonic solution is called Haemolysis. 

• If a similar red blood cell is placed in a hypertonic solution, water 

is drawn out of the cell by osmosis. The cell will shrink by a 

process called Crenation. 

• Body fluids surrounding the cells must therefore have same 

concentration as to that which is found inside the cell. 

Diagrams 

Water Relations in Plants  

• When a plant cell is placed in a hypotonic solution it gains water 

by osmosis and distends outwards. 

• As the cell gains more water, its vacuole enlarges and exerts an 

outward pressure called turgor pressure. As more water is drawn 

in, the cell becomes firm and rigid and is said to be turgid. 

• The cell wall in plant cell is rigid and prevents the cell from 

bursting unlike the case in animal cells. 

• The cell wall develops a resistant pressure that pushes towards the 

inside. This pressure is equal and opposite the turgor pressure and 

is called wall pressure.

Diagrams 

• When a plant cell is placed in hypertonic solution, water 

molecules move out of the cell into the solution by osmosis. The 

cell shrinks and becomes flaccid. 

• If the cell continues to lose more water, plasma membrane pulls 

away from the cell wall towards the center. 

• The process through which plant cells lose water, shrink and 

become flaccid is called plasmolysis.

• Plasmolysis can be reversed by placing a flaccid cell in distilled 

water and this process is called deplasmolysis.

Study Question 5 

Practical Activity 4 

Wilting 

• When plants lose water through evaporation and transpiration, 

cells lose turgidity, shrink and the plant droops. This is called 

wilting.

• If water supply from the soil is inadequate, plants do not recover 

hence permanent wilting.

Study Question 6 

Role of Osmosis in Organisms 

1. Absorption of water from the soil 

• Root hair cells of plants absorb water from the soil by osmosis.

2. Support 

• Cells of herbaceous plants, which are less woody, absorb 

water, become turgid hence support.

3. Opening and closing of the stomata 

• During the day, guard cells synthesize glucose, draw in water, 

become turgid hence open the stomata.

• During the night, they lose turgidity since there is no 

photosynthesis. As a result, they shrink thus closing the 

stomata.

4. Feeding in insectivorous plants 

• These plants are able to change their turgor pressure on the 

leaves which close trapping insects which are digested to 

provide the plant with nitrogen.

5. Osmoregulation

• In the kidney tubules, water is reabsorbed back to the body by 

osmosis. 

Factors Affecting Osmosis

i.)Concentration of Solutions and Concentration Gradient. The 

greater the concentration gradient between two points, the faster 

the rate of osmosis.

ii.) Optimum Temperature as long as it does not destroy the 

semi-permeability of the membrane. 

Active Transport 

• This is the process that moves substances across cell 

membranes against a concentration gradient.

• This process requires energy to move these substances across 

cell membranes and involves carriers.

• Substances such as amino acids, sugar and many ions are taken 

in by living organisms through active transport. 

Role of Active Transport 

i.)Re-absorption of sugars and useful substances by the kidney 

Visit: www.kcse-online.info for thousands of educational Resources 

ii.) Absorption of some mineral salts by plant roots 

iii.) Absorption of digested food from the alimentary canal into 

the blood stream 

iv.) Accumulation of substances in the body to offset osmotic 

imbalance in arid and saline environment 

v.) Excretion of waste products from body cells 

Factors Affecting Active Transport. 

i.) Oxygen concentration. 

ii.) Change in pH. 

iii.) Glucose concentration. 

iv.) Temperature. 

v.) Enzyme inhibitors. 

NB/ Any factor affecting energy production affect the rate of active 

transport. 

Revision Questions. 

Cell Specialization, Tissues, Organs and Organ Systems 

1. Cell specialization 

• This is where cells are modified to perform specific functions. 

Such cells are said to be specialized. 

• Examples include the sperm cell which has tail for swimming and 

the root hair cell which is extended creating large surface area for 

water absorption. 

2. Tissues. 

• These are cells of a particular type that are grouped together to 

perform the same function. 

Animal tissues include; 

- Epithelial tissue – which is a thin continuous layer of cells for 

lining and protection of internal and external surfaces. 

- Skeletal – it is a bundle of elongated cells with fibres that can 

contract. Its contraction and relaxation brings about movement. 

- Blood tissue – this is a fluid containing red blood cells, white 

blood cells and platelets. It transports many substances and 

protects the body against infections. 

Connective tissue – made up of strong fibres that

connect other tissues and organs holding them together.

Epidermal tissue of a plant – this is a single layer of cells

 protecting the inner tissues of the plant.

Palisade tissue – this is a group of cells rich in chloroplasts 

containing chlorophyll. They absorb light energy during photosynthesis. 

Parenchyma tissue – it is made thin walled irregularly shaped cells.

They store water and food.

Vascular bundle – consists of the xylem and phloem.

Xylem conducts water and mineral salts while phloem

conducts food substances

3. Organs 

• Many tissues become specialized and grouped together to 

perform a functional unit called the organ

• Examples of organs in plants include; roots, leaves, flowers 

and stem. 

• In animals they include heart, lungs, kidney, brain, stomach 

and the liver. 

4. Organ systems. 

• This is made of several organs whose functions are coordinated 

and synchronized to realize an effective action is called an 

organ system. Examples include; digestive, circulatory, 

excretory, respiratory, reproductive and nervous system. 

Handling and Care of the Microscope 

 The following rule should be observed: 

1. Use both hand when carrying the microscope. One hand should 

hold the base and the other holds the limb. 

2. Never place the microscope too close to the edge of the bench. 

3. Do not touch the mirror and the lenses with the fingers. 

4. Clean dirty lenses using soft tissue. 

5. Clean other parts using a soft cloth. 

6. Do not wet any part of the microscope. 

7. Make sure the low power clicks into position in line with the eye 

piece before and after use. 

8. Always store the microscope in a safe place free from dust and 

moisture. 

Using the Microscope 

1. Place microscope on the bench with the stage facing away from 

you. 

2. Turn the low power objective lens until it clicks into position. 

3. Ensure the diaphragm is fully open. 

4. Look through the eyepiece with one eye. Adjust the mirror to 

ensure maximum light can pass through. 

5. Place the slide containing the specimen on the stage and clip it 

into position. Make sure the slide is at the centre of the field of 

view. 

6. Again look through the eyepiece while adjusting the mirror to 

ensure maximum light reach the specimen. 

7. Use the coarse adjustment knob to bring the low power objective 

lens to the lowest point. While viewing through the eyepiece, turn 

the coarse adjustment knob gently until the specimen comes into 

focus. 

8. Use the fine adjustment knob to bring the image into sharp focus. 

9. Make a drawing of what you see. 

10. For higher magnification, turn the medium power into 

position and adjust the focus using the coarse knob. Use the fine 

adjustment knob for sharper focus. 

11. For even large magnifications, turn the high power objective 

lens into position. In this case use only the fine adjustment knob 

to bring details into sharper focus. 

Magnification 

• Magnification of the object viewed under the microscope is 

calculated by; 

Magnification = Eye Piece Lens Magnification X Objective 

Lens Magnification. 

• If the eyepiece lens has the magnification of x5 and the low 

power objective lens has a magnification of x10, the total 

magnification is 5x10=50. 

Cell Structures as Seen Under the Light Microscope 

• The following cell organelles can be seen under the light 

microscope. 

- Cell wall. 

- Cell membrane 

- Cytoplasm 

- Nucleus 

- Vacuole. 

- Chloroplasts. 

Diagrams- plant and animal cells 

The Electron Microscope. 

• It is more powerful than the light microscope. 

• It can magnify up to 500,000 times and has high resolving power.

• The high resolving power of the electron microscope enables it to 

separate objects which lie close to one another.

• Electron microscope uses a beam of electrons instead of light to 

illuminate the object.

Study Question 2 

Practical Activity 2 

Cell Structures as Seen Under the Electron Microscope 

Diagrams – Plant and Animal Cells 

The Cell Organelles 

i) Cell membrane (Plasma Membrane). 

• It has three layers i.e. one layer of phospho-lipid layer 

sandwiched between two protein layers. 

• It is flexible with pores and ahs the following main functions. 

a) Encloses all the cell contents. 

b)It allows selective movement of substances into and out of the 

cell since it is semi-permeable. 

Diagram 

ii) Cytoplasm 

• It is s fluid medium in which chemical reactions take place. 

• It has some movement called cytoplasmic streaming. 

• It contains organelles, starch, glycogen, fat droplets and other 

dissolved substances. 

iii) Nucleus 

• It has double membrane called the nuclear membrane.

• The membrane has pores allowing passage of materials into and 

out of the cell.

• Nucleus has a fluid called nucleoplasm in which the nucleolus 

and chromatin are suspended.

• Nucleolus manufactures ribosomes while chromatin contains the 

hereditary material.

iv) Mitochondria(Mitochondrion) 

• They are sausage shaped and are the respiratory sites. 

• Mitochondrion has two membranes. Inner membrane is greatly 

folded into cristae to increase the surface area for respiration. 

• Cells that require a lot of energy have large number of 

mitochondria e.g. muscle cell, sperm cell, kidney cell etc. 

Diagram 

v) Endoplasmic Reticulum (ER) 

• Some endoplasmic reticulums have granules called Ribosomes on 

their surfaces hence referred to as rough endoplasmic reticulum. 

• Others do not contain ribosomes hence the name smooth 

endoplasmic reticulum.

• Rough endoplasmic reticulum transport proteins while the 

smooth endoplasmic reticulum transports lipids. 

Diagrams 

vi) Ribosomes 

• They are spherical in shape and form the site for protein 

synthesis. 

vii) Lysosomes 

• They contain lytic enzymes which break down large molecules, 

destroy worn out organelles or even the entire cell. 

viii) Golgi Bodies (Golgi apparatus) 

• Their function is to package and transport glyco-proteins. 

• They are also associated with secretion of synthesized proteins 

and carbohydrates.

Diagram 

ix) Centrioles 

• They are rod shaped structures that are used in cell division and in 

the formation of cilia and flagella.

• Plant cells lack the Centrioles.

x) Chloroplasts 

• They are egg shaped and contain two membranes. 

• Chloroplast has chlorophyll which traps light energy to be used 

during photosynthesis. 

xi) Vacuoles 

• This are sacs filled with a fluid called cell sap. 

• Animal cells contain small vacuoles while plant cells have large 

vacuoles. 

• Sap vacuoles store sugars and salts. 

• Food vacuole store and digest food while contractile vacuoles 

excrete unwanted materials from the cell. 

xii) Cell wall 

• It is a rigid outer cover of the plant cells made of cellulose. 

• It gives the plant cell a definite shape while providing 

mechanical support and protection. 

• Cell wall also allows water, gases and other materials to pass 

through it.