By The End Of The Subtopic Learners Should Be Able To;
Identify cells as basic units of living organisms.
Identify cell parts.
Compare plant and animal cells.
Cells
Living organisms are made up of microscopic units called cells.
A cell is defined as the smallest structural and functional unit of an organism.
Cells have the following characteristics:
Respire to obtain energy
Irritability (sensitivity) — response to stimuli
Nutrition to absorb nutrients
Growth and development
Excretion to remove metabolic wastes
Reproduce to make more of same kind
[RINGER]
Most cells cannot be seen by the naked eye but they can be viewed under a microscope.
Fig.1.1.1 below shows the structure of a light microscope, commonly used in school science laboratory.
Table 1.1.1: Parts and functions of a microscope
PART
FUNCTION
Objective lenses
Magnification lenses - usually x4, x10, x40 and x100 magnification
Light source
Projects light up through the diaphragm
Diaphragm
Regulates the amount of light on the specimen
Eye piece
Magnifies the image
Stage
Supports the slide with the specimen to be viewed
Stage clips
Holds the slide in position
Base
Supports the microscope, specimen, and the lenses
Experiment: Examination of onion epidermis
Materials
quarter onion (freshly cut)
iodine solution
distilled water
slide
slide cover
microscope
Procedure
1. Prepare the onion epidermis as illustrated in stages below.
To obtain a clear view of the specimen:
Use a small and thin specimen to allow light penetration
Ensure that the specimen does not dry.
Avoid folding and trapping air between the cover slip and the slide - lower the cover slip slowly.
Do not press the cover slip to avoid damaging the specimen tissue.
Stain the specimen with iodine solution; it improves the contrast between the different structures in the observed image.
2. Observe under low and high magnification of microscope. 3. Select the magnification which provides as much detail as possible for the cells. 4. Calculate the magnification used.
Magnification (M)= Eyepiece magnification x Objective lens magnification, for example:
M = Eyepiece (x10) x Objective lens (x40)
= x400
(NB: magnification should always be expressed with "x" before the digits)
5. Given that the onion specimen is viewed under a total magnification of x1000 and one of the cells measures 15mm in length. Calculate the actual size of the cell using the formula below.magnification =
Expected observations
Diagram below shows the onion cell when viewed under a microscope.
Structure and function of plant and animal cells
The living matter inside a cell is called protoplasm.
Within the protoplasm are structures known as organelles.
Organelles are structures in both plant and animal cells which perform specific functions.
They enable the cells to carry out the basic processes of life.
Organelles common to both plant and animal cells are cytoplasm, nucleus, vacuole, and cell membrane.
In addition to these, plant cells have cell walls, chloroplasts and starch granules.
Structure and function of cell organelles
1. Cytoplasm
This is a gel-like fluid where all organelles are suspended.
Cytoplasm is mostly composed of water and salts.
It is surrounded by a selectively permeable membrane to maintain optimum conditions for chemical reactions.
The chemical reactions include protein synthesis and respiration.
2. Nucleus
It contains the genetic material (DNA) which determines nature of the cell.
The nucleus controls all the activities of the cell.
3. Cell membrane
The cell membrane is a semi-permeable covering which surrounds the cytoplasm separating cell's contents from its surroundings.
It controls the entrance and exit of dissolved substances.
4. Cellulose cell wall
Cellulose is tough carbohydrate which provides structural support to the cell.
The cell wall is freely permeable to water and dissolved substances.
It also protects the cell from damage by osmotic absorption of water.
5. Vacuole
In animal cells, the vacuoles are relatively small and temporary.
Some are involved in digestion (phagocytes) while others are for removing excess water.
In plant cells, the vacuole is large and permanent, and it contains cell sap (solution of salts and sugars).
Cell sap helps to draw water into the cell through osmosis.
6. Chloroplasts
It is the site where photosynthesis takes place.
Chloroplasts contain chlorophyll - a green pigment which absorbs light for photosynthesis.
They also contain enzymes which are needed for photosynthesis.
7. Starch granules
They are located in the cytoplasm or in the chloroplasts.
They serve as storage structures for the insoluble carbohydrates.
