BIOLOGY FOURTH PACKAGE S.

Cell Physiology

For the cell to function properly, there should be net movement of materials in and out of the
cells.

Materials move in and out of the cells across the cell membrane also known as the plasma
membrane.

Cell physiology is the study of the functioning of cells.

Structure of the cell membrane

A cell membrane is made of three layers.

Two outer protein layers and middle phospholipid layer.

The cell membrane

Properties of cell membrane

1. The cell membrane is semi-permeable.

It allows substance with small molecules to pass through but prevents passage of substance
with large molecules (selective movement of substance)
2. Sensitive to changes in temperature and PH

The protein layers are affected by changes in temperature and PH.

3. Has electric charges.

The electrical charges enable the cell to defect changes in the environment.

Physiological processes
Movement of materials in and out of the cell occurs through three main physiological processes
mainly:

(a) Diffusion

(B) Osmosis

(C) Active Transport

(A) Diffusion
This is the process by which solute molecules move from a region of high concentration to a
region of a low concentration until there is uniform distribution of molecules.

Experiment to demonstrate diffusion using potassium magnate (vii)

A crystal of potassium magnate (vii) is placed at the bottom of a beaker

containing water.
Observation

After sometime the water turns purple in colour.

Explanation

Particles of potassium magnate (vii) move from the crystal where they are highly concentrated
and spread in water where their concentration is low until there isw uniform distribution of
particles in water.

Roles of diffusion in living organisms.

 Gaseous exchange in plant and animals
 Excretion of nitrogenous wastes
 Absorption of salts from oil.
 Absorption of digested food into blood stream.
 Transport of manufactured food from leaves to other parts of the plant.

Factors affecting the rate of diffusion

Temperature

When temperature is high there is a high rate of diffusion, when low there is low rate of
diffusion.

Surface area to volume ratio

When high, there is a high rate, when low, diffusion is also low.

Size of the molecules diffuse at a higher rate where as large molecules diffuse at a low rate.

Thickness of the membranes

When thin, particles diffuse across it at a high rate, when thick diffusionrate is low.

(B) Osmosis

Osmosis is the movement of water molecules from a hypotonic solution to hypertonic solution
through a semi permeable membrane.

Osmosis is a special type of diffusion, as it involves movement of water molecules from a region
where they are highly concentrated to region of low water molecules concentration.

Note:

Movement of water molecules continues until the two solutions become isotonic.

Types of solution
(i) Hypertonic solution

This is a highly concentrated solution it contains more solute molecules than solvent molecules.

It has a high osmotic pressure.

(ii) Hypotonic solution

This is a lowly concentrated solution. It has more solvent molecules than solute molecules.

It has low osmotic pressure.

(ii) Isotonic solution

Two solutions are isotonic if they have same concentration.

Therefore there is no movement of molecules from one region to another.

Experiment to demonstrate osmosis using visking tubing.

Observation:

At the beginning of the experiment the level of water in the beaker is high and the visking tubing
is half full of concentrated sugar solution.

At the end of the experiment level of water in the beaker is lower and the visking tubing and
increased in size and contents.

Explanation

Water molecules moved from the beaker when they are highly concentrated (hypotonic) to the
visking tubing where they are lowly concentrated (hypertonic)

The visking tubing is a semi permeable membrane. It allows water molecules which are small to
pass through but does not allow sugar molecules which are large to pass through.

Animal cells and osmosis

When an animal cell is placed in distilled water or hypotonic solution, it gains water by osmosis,
swells and finally bursts, a condition known as hemolysis.

An animal cell placed in a hypotonic solution.

When an animal cell is placed in concentrated solution (hypertonic). It loss water by osmosis and
shrinks a condition known as crenation.

An animal cell placed in a hypertonic solution

When a plant cell is placed in a distilled water or hypotonic solution it gains water by osmosis.
Its vacuole enlarges and outer ward pressure known as turgor pressure. Turgor pressure pushes
the cytoplasm against the wall until the cell becomes turgid.

When turgid, the cell walls exerts wall pressure which is equal and opposite turgor pressure
preventing the cell from bursting.

A plant cell placed in a hypotonic solution

When a plant cell is placed in a concentrated solution or hypertonic it loses water by osmosis and
shrinks.

The cell becomes flaccid.

The plasma membrane pulls always from the cell wall. The cell is said to plasmolysed. The
process through which a cell shrinks after losing water is known as plasmolysis.

A plant cell placed in a hypertonic solution

OTHER TERMS ASSOCIATED WITH OSMOSIS

Osmotic pressure

It is the tendency of a solution to take up water separated from pure water by a semi permeable
membrane.

Osmotic potential

It is the tendency of water molecules to diffuse out of solution

Wilting

Occurs during hot and dry weather. Plant cells lose water by osmosis and become flaccid making
the plant to drop.

Most plants recover from wilting when water becomes available.

Wilting enables plants to reduce the rate of transpiration thus conserve water.

Permanent wilting

It is the point at which a plant cannot recover from wilting even when water is available.

Roles of osmosis in organisms

 Uptake of water from the soil into the root hairs of plant roots.
 Transport of water from the roots to the other parts ofr the plant.
 Support in plants.
 Opening and closing of stomata.
 Feeding in insectivorous plants.
 Reabsorption of water into the bloodstream from kidney tubules.

Factors affecting osmosis.

(i) Concentration Gradient

High concentration gradient leads to a high rate of osmosis where as a low concentration
gradient leads to a low rate of osmosis.

(ii) Temperature

(C) Active Transport

The process by which substances move across the cell membrane from a region of low
concentration to a region of high concentration by use of energy.

Carriers transport substances from one part of the membrane to another.

Roles of active transport

(i) Absorption of salts from the sol by root hair cell.

(ii) Reabsorption of glucose and salts back into bloodstream in kidney tubules.

(iii) Excretion of waste products from the body cells.

(iv) Absorption of digested food from the gut into the bloodstream.
(v) Process by which marine organisms offset osmotic imbalance by accumulating substance into
the bodies.

Factors affecting active transport

(i) Oxygen concentration

(ii) Concentration of sugar

(iii) PH

(v) Enzyme inhibitors

OBJECTIVES QUESTIONS

The process through a cell shrinks after losing water is known as.

A. Crenation
B. Wall pressure
C. Plasmolysis
D. Hypotonic solution

2. Movement of water molecules continues until the two solutions become.

A. Hypertonic
B. Isotonic
C. Hypotonic
D. Concentrated

3. Materials move in and out of the cells across the.

A. Chlorophyll
B. Cell membrane
C. Cell solution
D. Cell physiology.

4. ________________ is the study of the functioning of cells.

A. Cytology
B. Cell physiology
C. Cell size
D. Cell membrane

5. Which one of the following is not physiological process?

A. Active transport
 B. Osmosis
C. Hypertonic solution
D. Diffusion

Questions

1. What is cell physiology?

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2. Define the following terms.

(a) Hypotonic solution.

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(b) Hypertonic solution.

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(c) Isotonic solution

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3. Give two roles of osmosis in:

(a) Plants

(i) ___________________________________________________________________________

(ii) ___________________________________________________________________________

(b) Animals

(i) ___________________________________________________________________________

(ii) ___________________________________________________________________________

4. Explain the following factors that affects the rate of diffusion.

(a) Temperature.
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(b) Surface area to volume ration.

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(c) Thickness of membranes

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5. Distinguish between osmotic pressure and osmotic potential.

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6. Mention three properties of the cell membrane.

(i) ___________________________________________________________________________

(ii) ___________________________________________________________________________

(iii) __________________________________________________________________________

7. Outline the three physiological processes.

(i) ___________________________________________________________________________

(ii) ___________________________________________________________________________

(iii) __________________________________________________________________________

8. State three role of active transport in living organisms.

(i) ___________________________________________________________________________

(ii) ___________________________________________________________________________

(iii) __________________________________________________________________________

9. Explain what happens to an animal cell when placed in:

(a) Distilled water.

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(b) Concentrated salt solution.

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