AQA GCSE Biology: Cell Biology — Revision Notes

Condensed for the final weeks. For the full explanation, use the Cell Biology study guide.

Cell types

	Prokaryotic (bacteria)	Eukaryotic (plant, animal)
Nucleus	No — single DNA loop	Yes
Plasmids	Often	No
Size	~1 μm	10–100 μm
Organelles	Ribosomes only	Mitochondria, chloroplasts, etc.

Plant cells have three things animal cells lack: cell wall (cellulose), chloroplasts, and a permanent vacuole. Both have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.

Specialised cells

Cell	Adaptation	Purpose
Sperm	Tail, many mitochondria, acrosome enzymes	Swim to and penetrate the egg
Nerve	Long axon, myelin sheath, branched ends	Fast transmission over distance
Muscle	Many mitochondria, protein fibres that shorten	Contraction needs energy
Root hair	Long projection — large surface area, no chloroplasts	Absorb water and minerals
Xylem	Dead, hollow, no end walls, lignified	Water transport and support
Phloem	Sieve plates, companion cells	Transport of dissolved sugars

The pattern in every answer: name the feature, then say what it allows. A feature alone scores nothing.

Microscopy

magnification = image size / actual size
1 mm = 1000 um       1 um = 1000 nm

Electron microscopes have higher magnification and higher resolution than light microscopes, so sub-cellular structures can be seen in far more detail. Resolution is the ability to distinguish two points as separate.

Worked example: an organelle has a real width of 20 um and appears 40 000 um wide in a micrograph. Rearranging the formula gives magnification = image size / actual size = 40 000 / 20 = x2000. Light microscopes typically reach a maximum useful magnification of only about x2000, so a result at this figure is right at the upper limit of what a light microscope can achieve, while a result clearly above it signals that an electron micrograph, not a light micrograph, must have been used – exam answers that ask you to justify the choice of microscope should quote this approximate ceiling rather than just repeating “higher magnification.”

Required practical: thin specimen, stain (iodine for plant cells, methylene blue for cheek cells), lower the coverslip at an angle to avoid air bubbles, start with the lowest power objective, focus with the coarse then fine adjustment. Drawings must be in pencil, with clean unbroken lines, no shading, and a magnification stated.

Mitosis and the cell cycle

Three stages:

1. Growth and DNA replication — the cell grows, organelles duplicate, each chromosome is copied.
2. Mitosis — chromosomes line up at the equator, and one copy of each is pulled to each pole; the nucleus divides.
3. Cytoplasm and membrane divide, producing two genetically identical daughter cells with the same number of chromosomes as the parent.

Mitosis is used for growth, repair and asexual reproduction. Do not confuse it with meiosis, which halves the chromosome number and produces four genetically different gametes.

Stem cells

An undifferentiated cell that can divide to produce cells of many different types.

Source	Can become
Embryonic	Almost any cell type
Adult (bone marrow)	Limited range — mainly blood cells
Plant meristem	Any plant cell, throughout life

Therapeutic cloning takes an embryo with the same genes as the patient, so the cells are not rejected. Uses include treating diabetes and paralysis.

Arguments against: destruction of embryos, risk of viral transmission, ethical and religious objections. Arguments for: relief of serious suffering, and unused embryos from fertility clinics would be destroyed anyway. A good exam answer gives both sides, then a conclusion.

Transport

Process	Moves	Down/against gradient	Energy
Diffusion	Any particle	Down	None
Osmosis	Water only, through a partially permeable membrane	Down (dilute → concentrated solution)	None
Active transport	Dissolved substances	Against	Yes — ATP from respiration

Rate of diffusion increases with a steeper concentration gradient, higher temperature, and larger surface area.

Osmosis is about water, so define it as the movement of water from a dilute to a concentrated solution across a partially permeable membrane. Saying “particles move” loses the mark.

Active transport examples: root hairs absorbing minerals from soil where the concentration is lower than in the cell, and the gut absorbing glucose when its concentration is higher in the blood. Both require energy because they work against the gradient.

Surface area to volume ratio falls as an organism gets larger, which is why large organisms need exchange surfaces — lungs, gills, villi — that are large, thin, well-supplied with blood, and ventilated.

Exam traps

• Listing an adaptation without saying what it achieves.
• Saying plant cells have no mitochondria — they do.
• Confusing mitosis with meiosis.
• Defining osmosis as movement of particles rather than water.
• Forgetting active transport needs energy from respiration.
• Not converting units in magnification calculations.

Self-test

1. Give three structures found in plant but not animal cells.
2. Why do sperm cells have many mitochondria?
3. Describe the three stages of the cell cycle.
4. Define osmosis precisely.
5. Give two examples of active transport and say why energy is needed.

Answers: 1. Cell wall, chloroplasts, permanent vacuole. 2. To release energy by respiration for the tail to move, so the sperm can swim to the egg. 3. Growth with DNA replication and duplication of organelles; mitosis, in which chromosomes separate to opposite poles and the nucleus divides; then division of the cytoplasm and membrane to give two identical cells. 4. The movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane. 5. Mineral uptake by root hairs and glucose absorption in the gut; both move substances against the concentration gradient, which requires energy from respiration.