Edexcel GCSE Combined Science CB1 Key concepts in biology: a complete overview of cells, microscopy, enzymes and transport

What CB1 actually demands

Key concepts in biology is the foundation of Edexcel Combined Science. Everything that follows, from cells and control through to ecosystems, builds on understanding how cells are structured, how enzymes control reactions, and how substances move in and out of cells. Because it underpins the whole subject, Topic 1 is examined on both biology papers.

This guide walks through the three areas of the topic and ties together the matching dot-point pages, each with its own practice questions.

Cell structure and microscopy

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 circular 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. Cells become specialised for their jobs: sperm cells, egg cells, ciliated cells and root hair cells all have features suited to their function.

A light microscope magnifies up to about $\times 2000$; an electron microscope gives much higher magnification and resolution. Use the magnification equation and convert units carefully:

Enzymes

Enzymes are biological catalysts that speed up specific reactions without being used up. In the lock and key model, only a substrate with a shape complementary to the active site can bind. Enzyme activity is affected by:

• Temperature - rises to an optimum, then falls as the enzyme denatures.
• pH - each enzyme has an optimum; extremes denature it.
• Substrate concentration - rate rises then levels off when all active sites are busy.

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.
• 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.

A larger surface area to volume ratio speeds up exchange, which is why exchange surfaces are large, thin and well supplied.

How CB1 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 the enzyme and osmosis investigations (variables, results and conclusions).
• Application. Explaining how specialised cells and exchange surfaces are adapted to their functions, and predicting the direction of osmosis.

Check your knowledge

A mix of recall and calculation questions covering CB1. 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. Define an enzyme. (2 marks)
4. Explain why an enzyme stops working at high temperatures. (2 marks)
5. Define osmosis. (2 marks)
6. An image is $40\,mm$ wide at a magnification of $\times 200$. Calculate the real size in micrometres. (2 marks)
7. Explain why active transport requires energy. (2 marks)
8. State two factors that increase the rate of diffusion. (2 marks)