1

CELL PORCESSES
OBJECTIVES
- identify different parts of plant and animal cells.
- identify from fresh preparations and diagrams or on photomicrographs cell membrane, cytoplasm, cell
organelles: nucleus, mitochondrion and ribosome of an animal cell.
- identify from fresh preparations and diagrams or on photomicrographs the cell wall, cell membrane, sap
vacuole, cytoplasm, nucleus and chloroplasts of a plant cell.
- state the functions of the nucleus.
- state the functions mitochondrion, ribosome, cell wall and cell membrane.
- describe the relationship between cell structure and function using a red blood cell and nerve cell in animals and
root hair cell and xylem in plant cells.
- define tissue, organ, system and organism.
- define diffusion
- define osmosis
- describe the effect of concentration gradient in the uptake of water by plants.
- describe the effect of osmosis on plant tissues, in terms of flaccid cells, turgid cells and plasmolysis.
- describe the effect of osmosis on animal tissues. [ refer to bursting and shrinking ].
- conduct experiments using solutions of varying concentration to demonstrate the process of osmosis

NOTES

What is a cell?
It is the basic unit of life
Types of cells
There are two types
 Animal
 Plant
Structure of animal cell
Draw and label

Structure of plant cell

Draw and label

STRUCTURE PROPERTIES FUNCTIONS

Cell membrane 2 lipid layer with one Selects substances entering
protein layer sandwiched in & leaving the cell
between
Nucleus Contains genetic material Regulates cellular activities
(chromosomes)
Cytoplasm Contains water & dissolved Where biochemical reaction
substances e.g. glucose, take place
minerals, vitamins
Ribosome Small grains found Responsible for protein
scattered in the cytoplasm, formation
some may be attached to
endoplasmic reticulum
mitochondrion Surrounded by 2 Site for aerobic respiration
membranes, inner
membrane has folds
Cell wall Made carbohydrate called Protects cell against
 2

cellulose, its freely mechanical damage & gives

permeable plant a permanent shape
vacuole Has concentrated solution Stores the cell sap (vital
made of salts, glucose chemicals for the cell)

SPECIALIZED CELLS
These are cells which have stopped growing, have a definite shape and perform one specific
function.
Examples of specialized cells.

Cell Function Relationship between structure & Function.

1. Red Blood Cells Responsible for transporting - Has a biconcave structure that helps it move in
(Erythrocytes) O2 the blood vessels
- Has a spongy cytoplasm that helps it to
compress itself in blood vessels with small lumen
- Absence of nucleus increases surface area for
transportation of O2
- Presence of haemoglobin that binds to O2 and
carry it as an unstable compound called
oxyhaemoglobin
2. Muscle cell - Long & have numerous protein fibres in the
It moves bones in different cytoplasm. (fibre helps to shorten the cell when
directions (contract / relax to there is energy)
help move structures) - Can of contract & relax in order to move parts

3. Sperm cell - Has streamlined head to enable it to move

Fuse with ovum to from a towards ovum with less friction
zygote - Has a tail to help it swim towards the ovum.

4. Ovum - It has a large food store to provide the zygote

Fuse with sperm to from a with nourishment before implantation
zygote

5. White Blood Cells Phagocytes – engulf & digest - Phagocytes are polymorphs; can change shape
(Leucocytes are two bacteria (pathogens) of cytoplasm like amoeba.
types; Phagocytes & - Have Granular cytoplasm with lobed nucleus.
Lymphocytes) Lymphocytes – produce
chemicals called antibodies
that kill bacteria

6. Root hair cell - Absorb water from the soil - Large number of mitochondria to generate
- Absorb mineral ions (salts) energy needed fro active transport of ions
from the soil - Has a finger-like projection that enables to
- anchor the plants firmly to penetrate tightly packed soil particles
the ground. - Have Large concentrated sap vacuole which
ensures water uptake through osmosis

7. Xylem cells - Transport water & dissolved - Has dead hollow tube without cytoplasm to allow
 3

minerals from the roots to top flow of water

part of a plant - Walls are lignified (coated with lignin) to
- Supports the plant against withstand water pressure
mechanical damage - cylindrical cells are attached end to end form a
xylem vessel.

8. Nerve Cell Transmits nerve impulses - Have very long fibres which connect distant
(neuron) (messages) around the body parts of the body
- Fibres are coated with sheath to prevent loss &
distortion of messages.
- Chemical reactions cause the impulses to travel
along the fibre

9. Palisade cell To make sugar (starch) - Has lots of chloroplasts to produce chlorophyll
through photosynthesis which traps sunlight energy for the process
- Elongated so that they can pack tightly
- Located close to the upper epidermis to receive
more sunlight

10. Guard cells To regulate the opening & - Crescent shaped

closing of stomata - Always paired to form a stomata
- Are able to change shape to close / open
depending on whether its day or night.

Cell Organization

Cells are organized into tissues, organs, systems and ultimately an organism (plant / animal)

 A Cell; is the basic unit of life

 A Tissue; is a group of similar cells carrying out the same function. E.g. xylems cells make
xylem tissue
 An Organ; is a group of tissues which work together to perform a particular function. E.g. the
heart is organ made of nerve tissue, muscle tissue, epithelial tissue etc.
 A System; is a group of organs which work together to perform a particular function. E.g. the
digestive system is made of; stomach, mouth, intestine, liver etc
 An Organism; Is a group of different systems which come together to form a living thing e.g.
Human beings are made of; nervous, skeletal, circulatory, digestive, muscle systems etc.

Movement of Substances in & out of cells

There are three processes that are involved in the movement of substances in & out cells
 Diffusion
 Osmosis
 Active transport
 4

1. Diffusion;
It is the random movement of particles from their region of their higher concentration to their
region low concentration down a concentration gradient until they are evenly distributed.

N.b. Diffusion occurs in all states of matter.

a) Diffusion in gases
The spread of perfume particles (or air freshener) from the nozzle of the spray can to fill up the
room occurs through diffusion.

b) Diffusion in liquids
The spread of potassium permanganate from the bottom of a beaker to the top is by diffusion.

Diffusion is affected by;

I. Changes in temperature;
An increase in tesmperature increases the kinetic energy of particles, hence particles will
move faster leading to an increase in the rate of diffusion
II. Particle size
Small particles dissolve faster because they are less dense & have a large surface area
hence can diffuse rapidly from one end to the other.
III. Concentration gradient
This is the difference in the amount of particles between two adjacent areas. Particles
move faster when the difference in the amount of particles is large. The rate of diffusion
diminishes as the concentration gradient reduces.

2. Osmosis;
It is the random movement of water molecules from their region of higher concentration to their
region of lower concentration along the concentration gradient through a partially / selectively
permeable membrane.
Similarities between Osmosis & Diffusion
 Particles are moving down the concentration gradient
 Both processes are passive, no energy is expended (used)

Differences between Osmosis & Diffusion

OSMOSIS DIFFUSION
Occurs mainly in liquids (only water molecules Occurs in all states of matter (any particle is
are moved) moved)
Materials are moved across a barrier (a Does not require a barrier (partially membrane)
partially permeable membrane required)

3. Active Transport
This is the movement of materials from their region of low concentration to their region of high
concentration against the concentration gradient using energy from respiration. Substances are
moved through the aid of protein carrier molecules and a barrier is not important.

Active transport is different from the other two modes of movement in that;
 Unlike osmosis & diffusion, it occurs against the concentration gradient
 It requires energy
 Only mineral ions are moved

Osmosis and cells;

A cell can be exposed to the following conditions;
 Immersed in pure (distilled) water / dilute solution
 Immersed in a concentrated solution
 5

??? Explain what happens when a plant cell is immersed in pure water / dilute solution

Ans; There will be an influx of water from the container into the cell by osmosis leading to an
increase in the contents of the protoplasm. As a result turgor pressure builds inside the cell
membrane causing it to stretch until it touches the cell wall. When this happens the cell is said to
be Turgid.

??? Explain what happens when a plant cell is immersed in a concentrated solution.

Ans; the water molecules will move out of the cell by osmosis. The outflux of water molecules will
lead to a decrease in the contents of the protoplasm hence a decrease in turgor pressure. The
cell membrane will become flaccid and pull away from the cell wall. When this happens the cell is
said to be plasmolysed.

??? Explain what happens when an animal cell is immersed in pure water / dilute solution

Ans; Water molecules will move into the cell by osmosis and due to build up of turgor pressure
the cell membrane will burst (since the animal cell has no cell wall to limit the stretching of the
membrane).

??? Explain what happens when a animal cell is immersed in a concentrated solution.
 6

Ans; Due to outflux of water by osmosis, the cell will decrease in size and when this happens the
cell is said to be shrunken.

ACTIVITY; The Effect of various sugar concentrations on potato tissue

Apparatus & materials

 5 different solns; 0%, 5%, 15%, 25% and 60%
 5 petri dishes
 Cork borer
 White tile
 Scalpel
 30cm ruler
 Large potato
 25ml measuring cylinder

Procedure
- Label the petri dishes A to E.
- Place equal volumes (10ml) of the sugar concentrations in the petri dishes as; 0% in petri dish A
5% in dish B, 15% in dish C etc.
- Prepare 5 equal sized e.g. (40mm) potato slices by boring the potato using a cork borer
- Place one potato slice in each petri dish.
- Leave the potato slices in the solutions for 30 – 60 minutes. The remove the slices & blot them with
a paper tissue.
- Measure the final lengths of the slices and record the results as follows

Sugar soln Initial length Final length Change in length

0%
5%
15%
25%
60%

Practice Questions on the activity above;

 Plot a graph of sugar concentration against change in length
 From the graph, work out the concentration of the potato tubes
 At what concentration would the concentration of the sap vacuole of the potato equal that
of the surrounding sugar soln?
 What process is responsible for the change in length of the potato tubes?
 What is the formula for calculating the change in length of the potato tubes?
 What else can be measured to find out the effect of concentration on the potato slices?