Cell Structure and Organisation

The cell as the unit of life

The cell is the basic structural and functional unit of all living things. In the Kenyan Senior School CBC (KICD) Biology curriculum, you use a light microscope to observe cells from onion peel, Elodea and cheek smears.

Cell theory

• All living things are made of one or more cells.
• The cell is the basic unit of structure and function.
• New cells arise from existing cells.

Organelles and their functions

• Cell membrane - partially permeable boundary; controls entry and exit of materials.
• Cytoplasm - jelly-like fluid where chemical reactions occur.
• Nucleus - controls activities; contains chromosomes (DNA).
• Mitochondria - sites of aerobic respiration, releasing energy as ATP.
• Ribosomes - make proteins.
• Endoplasmic reticulum - transports materials; rough ER carries ribosomes.
• Golgi apparatus - packages and secretes substances such as enzymes.
• Lysosomes - contain enzymes that digest worn-out parts.
• Chloroplasts (plants) - contain chlorophyll for photosynthesis.
• Cell wall (plants) - rigid cellulose layer giving support.
• Vacuole - stores cell sap; keeps plant cells turgid.

Plant and animal cells compared

Specialised cells

Cells are adapted to their jobs. Red blood cells are biconcave and have no nucleus so they carry more oxygen. Root hair cells have long projections that increase surface area for absorbing water. Palisade cells contain many chloroplasts for photosynthesis.

Levels of organisation

Cell, tissue, organ, organ system, organism. For example, muscle cells form muscle tissue; the heart is an organ; the circulatory system moves blood; and the human is the organism.

Using the microscope

Total magnification = eyepiece magnification x objective magnification.

Worked example 1

An eyepiece lens is x10 and the objective lens is x40. Find the total magnification. Answer: 10 x 40 = x400.

Worked example 2

A cell appears 2 mm long in a drawing magnified x400. Find its real length. Answer: Real size = image size / magnification = 2 / 400 = 0.005 mm = 5 micrometres.

Try it

• Prepare a slide of onion epidermis, stain it with iodine and draw what you see.
• Make a table linking five organelles to their functions.
• Explain why red blood cells lack a nucleus.
• Calculate the magnification of a drawing that is 60 mm long for a real cell of 0.3 mm.