AQA GCSE Biology 4.1 Cell biology: a complete overview of cell structure, division, transport and microscopy

What module 4.1 actually demands

Cell biology is the foundation of AQA GCSE Biology. Everything that follows, from organisation and disease through to inheritance, builds on understanding how cells are structured, how they divide, how they specialise, and how substances move in and out of them. The examiners test precise recall of structures and processes alongside two practical skills: using a microscope and investigating osmosis.

This guide walks through the four areas of the module and ties together the matching dot-point pages, each of which has its own practice questions.

Cell structure

All living things are made of cells. Eukaryotic cells (animals and plants) have a true nucleus and membrane-bound organelles; prokaryotic cells (bacteria) are much smaller, have no nucleus, and carry their DNA as a single loop plus small rings called plasmids.

Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells have all of these plus a cellulose cell wall, chloroplasts and a permanent vacuole. Make sure you can state the function of each part.

Cells become specialised through differentiation: sperm cells, nerve cells, muscle cells, root hair cells, xylem and phloem all have features suited to their function. In animals, most differentiation happens early in development; many plant cells keep the ability to differentiate throughout life.

The cell cycle, mitosis and stem cells

The cell cycle produces two genetically identical daughter cells. During the cycle the cell grows, makes more sub-cellular structures, and copies its DNA so each chromosome forms two identical strands. Mitosis is the stage where the chromosomes line up, separate, and the cell divides. Mitosis is used for growth, repair and asexual reproduction.

Stem cells are undifferentiated. Embryonic stem cells can form any cell type; adult stem cells form a limited range; plant meristem cells can differentiate throughout the plant's life. Therapeutic uses (treating diabetes or paralysis) are weighed against ethical concerns and the risk of rejection or infection.

Transport in and out of cells

Three processes move substances across the cell membrane:

• Diffusion is the passive net movement of particles down a concentration gradient. Rate depends on the concentration gradient, temperature and surface area.
• Osmosis is the diffusion of water across a partially permeable membrane from a dilute to a more concentrated solution.
• Active transport moves substances against the gradient and needs energy from respiration, for example absorbing mineral ions in root hair cells and glucose in the gut.

A larger surface area to volume ratio speeds up exchange, which is why exchange surfaces such as the small intestine and lungs are adapted to be large, thin and well supplied.

Microscopy and magnification

A light microscope magnifies up to about $\times 2000$ with limited resolution; an electron microscope uses electrons for much higher magnification and resolution, revealing fine detail of organelles. Use the magnification equation and convert units carefully:

How module 4.1 is examined

• Recall and labelling. Naming sub-cellular structures, stating functions, and comparing eukaryotic and prokaryotic cells.
• Calculations. Magnification, real size and percentage change in mass for osmosis.
• Practical questions. Microscope technique and biological drawing, and the osmosis investigation (variables, results and conclusions).
• Application. Explaining how specialised cells and exchange surfaces are adapted to their functions.

Check your knowledge

A mix of recall and calculation questions covering module 4.1. Attempt them under timed conditions, then check against the solutions.

1. State two structures found in a plant cell but not in an animal cell. (2 marks)
2. Give one difference between a prokaryotic and a eukaryotic cell. (1 mark)
3. Name the type of cell division that produces two genetically identical cells. (1 mark)
4. Define osmosis. (2 marks)
5. Explain why active transport requires energy. (2 marks)
6. An image is $40\ mm$ wide at a magnification of $\times 2000$. Calculate the real size in micrometres. (2 marks)
7. Give one use of stem cells and one risk of using them. (2 marks)
8. State two factors that increase the rate of diffusion. (2 marks)