🌿 Animal and Plant Cells

All animal cells contain these five essential structures:

NUCLEUS: contains the cell's DNA, organised into chromosomes. The nucleus controls all cell activity by determining which proteins are made. It has a double membrane called the nuclear envelope with pores that allow molecules to pass in and out.

CELL MEMBRANE: a thin, flexible layer made of phospholipids and proteins. It controls what enters and leaves the cell — it is selectively permeable. It also plays a role in communication between cells.

CYTOPLASM: a watery, gel-like substance that fills the cell. Most chemical reactions happen here. It contains dissolved enzymes, salts, sugars and other molecules, as well as all the organelles.

MITOCHONDRIA: oval-shaped organelles with a folded inner membrane (cristae). This is where aerobic respiration occurs — glucose is broken down using oxygen to release energy as ATP. Cells that are very active (muscle cells, sperm cells, liver cells) have many more mitochondria than less active cells.

RIBOSOMES: tiny structures found either floating in the cytoplasm or attached to the endoplasmic reticulum. This is where PROTEIN SYNTHESIS takes place — amino acids are joined in a specific sequence to build proteins. Every cell needs proteins, so all cells have ribosomes.

Plant cells have all five animal cell structures PLUS up to three additional ones:

CELL WALL: a rigid outer layer made of cellulose fibres. It surrounds the cell membrane and gives the cell a fixed shape. It prevents the cell from bursting when it absorbs water by osmosis — the wall resists the pressure. This is what makes plants structurally rigid (along with turgor pressure).

CHLOROPLASTS: oval-shaped organelles containing the green pigment chlorophyll. Chlorophyll absorbs light energy, which is used to drive photosynthesis — converting CO₂ and water into glucose. IMPORTANT: only cells that are exposed to light have chloroplasts. Root cells, storage cells (e.g. in potato tubers) and cells deep inside stems do NOT have chloroplasts.

PERMANENT VACUOLE: a large central compartment in mature plant cells, filled with cell sap (a solution of sugars, salts and pigments). The vacuole pushes against the cell wall and contributes to TURGOR PRESSURE — the pressure that keeps plant cells firm and gives the plant structural support. Without water in the vacuole, the plant wilts.

The key principle to understand is that every structural feature exists because it serves a specific function.

More mitochondria = more aerobic respiration = more ATP energy. Cells with high energy demands (muscle, sperm, liver) have the most mitochondria.

More chloroplasts = more photosynthesis. Palisade mesophyll cells in leaves have up to 70 chloroplasts — they are the main photosynthetic cells of the plant.

Large vacuole = more turgor pressure = firmer cell. This is why plants wilt when they lose water — the vacuoles deflate and cells lose their rigidity.

Cell wall + vacuole together = turgid plant cell that holds its shape and provides structural support to the whole plant.

To observe cells under a light microscope:

1. Prepare a thin section of tissue (e.g. onion epidermis, cheek cells).

2. Place on a glass slide with a drop of water or mounting fluid.

3. Add a coverslip carefully to avoid air bubbles.

4. Apply a stain if needed — iodine solution stains starch and nuclei blue/purple; methylene blue stains nuclei of animal cells.

5. Focus using the coarse adjustment knob first, then the fine adjustment.

You can observe with a light microscope: nucleus, cytoplasm, cell wall, vacuole, chloroplasts.

You CANNOT see clearly: individual ribosomes (too small), cell membrane detail, internal mitochondria structure.

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