Cell Structure and Organisation: Revision Notes

Condensed for the final weeks. For the full explanation, use the Cell Structure and Organisation study guide, which covers Topic 1.1 Cell structure and function and 1.2 Specialised cells, tissues and organs in full for Cambridge O Level Biology 5090. For exam-style practice with full worked answers on this topic, see the Cell Structure and Organisation practice questions.

Which cell has what

Structure	Animal	Plant	Bacterial	Function
Cell membrane	Yes	Yes	Yes	Controls what enters and leaves
Cytoplasm	Yes	Yes	Yes	Site of chemical reactions
Nucleus	Yes	Yes	No	Contains DNA, controls the cell
Mitochondria	Yes	Yes	No	Aerobic respiration
Ribosomes	Yes	Yes	Yes	Protein synthesis
Cell wall	No	Cellulose	Yes (not cellulose)	Support, prevents bursting
Chloroplasts	No	Yes	No	Photosynthesis
Permanent vacuole	No	Yes	No	Cell sap, turgor

Bacteria have no nucleus — DNA sits free in the cytoplasm as a circular loop, often with plasmids. Because bacterial cells lack a nucleus, mitochondria and chloroplasts, they are structurally much simpler than either animal or plant cells, and questions asking you to identify a bacterial cell from a diagram usually hinge on spotting exactly which of these membrane-bound structures is missing.

Specialised cells — adaptation to function

Cell	Adaptation	Why
Red blood cell	Biconcave, no nucleus, haemoglobin	Max surface area and space for oxygen
Root hair cell	Long projection	Large surface area for water uptake
Xylem vessel	Hollow, dead, lignified	Uninterrupted water column, support
Palisade cell	Many chloroplasts, near upper surface	Maximum light absorption
Nerve cell	Long axon, branched ends	Carries impulses over distance
Sperm cell	Tail, many mitochondria, acrosome	Swimming energy, penetrating the egg

Always link the feature to the function — one without the other scores half.

Preparing and staining a slide

Cells are usually colourless and transparent under the microscope, so a temporary stain is used to make internal structures visible:

• Iodine solution — stains starch grains and cell walls, commonly used for plant cells (e.g. onion epidermis).
• Methylene blue — stains the nucleus and cytoplasm, commonly used for animal cells (e.g. cheek cells).

A typical method: place the specimen on a slide with a drop of water, add a drop of stain at one edge, then lower a coverslip at an angle rather than dropping it flat, to avoid trapping air bubbles that would otherwise obscure the view.

Magnification

magnification = image size / actual size

Worked example. A photomicrograph shows a cell 40 mm long; its actual length is 0.02 mm.

magnification = 40 / 0.02 = 2000x

The same equation rearranges to find actual size (image size ÷ magnification) or image size (actual size × magnification), given the other two quantities. Always convert image size and actual size to the same units before dividing — mixing mm and µm is the single most common source of a wrong answer here.

Levels of organisation

organelle -> cell -> tissue -> organ -> organ system -> organism

• Tissue — group of similar cells with a shared function (e.g. muscle).
• Organ — several tissues working together (e.g. stomach, leaf).
• Organ system — organs cooperating (e.g. digestive system).

Exam traps

• “Bacteria have no DNA” is wrong — they have no nucleus.
• Plant vacuole must be described as permanent and large.
• Not all plant cells have chloroplasts — root cells do not.
• Cell wall ≠ cell membrane; both are present in plants.
• Give the adaptation and its purpose; “red blood cells are biconcave” alone is incomplete.
• A leaf is an organ, not a tissue.
• Mixing up image size and actual size in the magnification formula — actual size is always the smaller number when a specimen has been magnified.
• Forgetting to convert units before dividing in a magnification calculation, e.g. leaving one length in mm and the other in µm.
• Lowering a coverslip straight down onto a slide instead of at an angle, trapping air bubbles that obscure the specimen under the microscope.

Self-test

1. Give three structures in a plant cell absent from an animal cell.
2. Why do red blood cells lack a nucleus?
3. Name the tissue types in the stomach and their functions.
4. Where is bacterial DNA found?
5. Put in order: organ, cell, organism, tissue, organ system.
6. A cell is drawn 60 mm long and is actually 0.03 mm long. Calculate the magnification.

Answers: 1. Cell wall (cellulose), chloroplasts, permanent vacuole. 2. To create more space for haemoglobin, so more oxygen can be carried. 3. Muscular tissue churns the contents; glandular tissue secretes enzymes and acid; epithelial tissue lines and protects. 4. Free in the cytoplasm as a circular loop, plus plasmids — there is no nucleus. 5. cell → tissue → organ → organ system → organism. 6. magnification = image size ÷ actual size = 60 ÷ 0.03 = ×2000.