LIVING THINGS AND THEIR ENVIRONMENT — The Cell

What is a cell?

A cell is the smallest unit of a living thing that can carry out life processes. All plants and animals are made of cells. Cells are too small to see with the naked eye; we use a microscope.

Why cells are important

• They carry out life activities like taking in food, producing energy, growing and reproducing.
• Many cells together form tissues, organs and whole organisms (e.g., a leaf, a liver, a human).

Types of cells (simple)

There are many kinds of cells. For classwork we focus on:

• Animal cells — e.g., human cheek cells, blood cells.
• Plant cells — e.g., onion epidermis, leaf cells (maize, beans).

Basic parts of a typical cell

Most cells have these parts (organelles):

• Cell membrane — thin covering that controls what enters and leaves the cell.
• Cytoplasm — jelly-like substance where cell reactions happen.
• Nucleus — control centre containing genes (DNA).
• Mitochondria — release energy from food (powerhouse).

Parts found in plant cells but not in animal cells

• Cell wall — rigid outer layer made of cellulose; gives shape and support.
• Chloroplasts — contain chlorophyll for photosynthesis (make food using sunlight).
• Large central vacuole — stores water and dissolved substances; helps keep the plant firm.

Simple diagrams: plant cell and animal cell

Cell sizes

Cells are measured in micrometres (µm). Typical sizes:

• Red blood cell ≈ 7 µm
• Plant leaf cell ≈ 20–100 µm

How to observe cells (simple practicals used in school)

1. Onion epidermis (plant cell): peel a thin layer from onion skin, place on slide, add a drop of water and a drop of iodine, cover with cover slip and view under microscope.
2. Cheek cells (animal cell): gently scrape inside of cheek with a clean cotton bud, smear on slide, add a drop of methylene blue stain, cover and view carefully.

Safety: Handle stains and slides carefully. Wash hands after the practical.

The cell theory (simple points)

• All living things are made of one or more cells.
• The cell is the basic unit of life.
• All cells come from pre-existing cells (cells divide to form new cells).

Functions of some organelles (short)

• Nucleus: controls the cell and holds genetic information (DNA).
• Cell membrane: controls movement of substances in and out.
• Chloroplasts (plants): make food by photosynthesis.
• Mitochondria: produce energy by respiration.
• Vacuole (plants): store water and maintain firmness.

Simple classroom activity (10–20 minutes)

Prepare two slides: one onion epidermis and one cheek smear. Compare what you see. Draw labelled diagrams of both at low and high magnification. Note differences such as cell wall, chloroplasts (absent), shape (plant cells regular, animal cells irregular).

Key words (make flashcards)

Cell membrane, Cytoplasm, Nucleus, Cell wall, Chloroplast, Vacuole, Mitochondrion, Microscope

Summary (short)

Cells are the building blocks of life. Plant and animal cells share many parts but plant cells also have a cell wall, chloroplasts and a large vacuole. Use a microscope to observe cells and remember the simple cell theory statements.

Revision questions

1. What is a cell? Give two examples of animal cells and two examples of plant cells.
2. Name three parts of a cell and state the function of each.
3. List two differences between plant and animal cells.
4. Describe how you would prepare an onion epidermis slide for observation.
5. State the three main ideas of the cell theory.

Note: These notes match Kenyan school practicals and are suitable for learners aged about 13. Use your class textbook and laboratory teacher for hands-on help.