Cell Organelles

• To see the cell as the basic unit of life — and organelles as specialised compartments, each with a job.
• To trace how structure links to function: nucleus stores instructions, mitochondria release energy, ribosomes build proteins.
• To compare the cell membrane (flexible boundary in all cells) with the cell wall (rigid — plants only).
• To prepare and interpret simple microscope slides (onion peel, cheek cells) with correct staining.

1. The cell — one unit, many parts

Every living organism is built from cells. A cell has three main regions:

• Cell membrane — thin, flexible outer boundary (all cells).
• Cytoplasm — jelly-like fluid filling the cell; organelles float here.
• Nucleus — usually the largest organelle; control centre.

2. Organelles and their functions

3. Building the picture step by step

1. Observe cells under a microscope (onion peel or cheek scrape).
2. Stain with iodine or methylene blue — nucleus appears darker.
3. Label membrane, cytoplasm, nucleus on your diagram.
4. Ask 'why': Why does the nucleus stain darker? (It contains dense chromatin/DNA.)
5. Connect: Without a nucleus, most cells cannot divide or make new proteins for long.

Identifying the control centre in a stained cheek cell

Step 1 — Gently scrape inside cheek; smear on slide with saline drop.
Step 2 — Add methylene blue stain; cover with cover slip.
Step 3 — Under microscope: irregular cell outline (no cell wall), granular cytoplasm.
Step 4 — Dark round **nucleus** near centre — this directs all cell activities.
Step 5 — Compare with onion peel: rectangular cells, cell wall visible, nucleus also present.

• Calling cell wall present in all cells (only plants, fungi, bacteria — not animal).
• Misconception: nucleus exists only in plant cells (both plant and animal cells have a nucleus).
• Confusing cell membrane (thin, inside wall in plants) with cell wall (thick, cellulose).
• Thinking cells are visible to the naked eye (most need a microscope — 100× or more).
• Believing mitochondria make energy from sunlight (chloroplasts do that — in plants only).

• What is an organelle? Give two examples with functions.
• Why is the nucleus called the control centre?
• What is the difference between cell membrane and cell wall?
• Stretch: Why do muscle cells have more mitochondria than skin cells?

Open the Practice tab for graded questions on Cell Organelles.