OCR GCSE Biology: Cell Level Systems — Revision Notes

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

Cell structure

	Prokaryotic	Eukaryotic
Nucleus	No — DNA free in cytoplasm	Yes
Size	~1 μm	10–100 μm
Organelles	Ribosomes only	Many membrane-bound
Plasmids	Often	No

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

Organelle functions worth stating precisely: mitochondria — aerobic respiration; ribosomes — protein synthesis; chloroplasts — photosynthesis; the permanent vacuole stores cell sap and maintains turgor (cell rigidity from internal pressure).

Cell walls are not exclusive to plants — bacteria have one too, though built from a different material. Bacterial ribosomes are also smaller than the ribosomes found in eukaryotic cells, and bacterial DNA is a single circular loop plus plasmids, with no nucleus to contain it.

Microscopy

magnification = image size / actual size
1 mm = 1000 um = 1000000 nm

Always convert to the same unit before dividing — this is where most calculation marks go.

Electron microscopes have higher magnification and higher resolution than light microscopes, so sub-cellular structures can be seen. Resolution is the ability to distinguish two points as separate, and it is limited by the wavelength used — electrons have a far shorter wavelength than light.

Magnification and resolution are not the same. Magnifying beyond the resolving limit gives a bigger blur, not more detail.

Worked example. A cell’s actual width is 20 μm; its width in a photograph is 60 mm. Converting to the same unit first: 60 mm = 60,000 μm. Magnification = 60,000 ÷ 20 = ×3,000. Converting before dividing is where the marks are won or lost.

Enzymes

Biological catalysts that lower activation energy. The active site is complementary to the substrate, so each enzyme is specific.

Factor	Effect
Temperature	Rate rises to an optimum, then falls sharply as the enzyme denatures
pH	Optimum varies; away from it the active site changes shape
Substrate concentration	Rises, then plateaus when all active sites are occupied

Denaturation is a change in the shape of the active site, so the substrate no longer fits. Enzymes are not alive and are not “killed”.

Calculating rate: rate = 1 ÷ time. Time is often scaled (e.g. 1000 ÷ time in seconds) to give a more convenient number; a scaled value like this is in arbitrary units, not a defined unit of rate, so state that explicitly if you use it.

Respiration

aerobic:    glucose + oxygen -> carbon dioxide + water  (+ much ATP)
anaerobic (animals):  glucose -> lactic acid  (little ATP)
anaerobic (yeast):    glucose -> ethanol + carbon dioxide

Aerobic respiration releases far more energy per glucose because glucose is completely oxidised; anaerobic respiration only partly breaks it down.

Respiration happens in all living cells, all the time — it is not the same as breathing.

Oxygen debt is the extra oxygen needed afterwards to oxidise the accumulated lactic acid.

Photosynthesis

carbon dioxide + water --light, chlorophyll--> glucose + oxygen

6CO2 + 6H2O --light, chlorophyll--> C6H12O6 + 6O2

Endothermic — it takes in energy, transferred from light.

Limiting factors: light intensity, carbon dioxide concentration, temperature.

A limiting factor is the one in shortest supply, and it alone determines the rate. On a graph, the rate rises then plateaus — the plateau means something else has become limiting. Identifying which factor is limiting at each part of the curve is the standard question.

Light intensity follows an inverse square law, so doubling the distance from a lamp quarters the intensity.

Measuring rate with pondweed bubbles is not fully reliable: bubbles vary in size, so counting them assumes each is the same volume of gas, which is rarely true — measuring the volume of gas collected over a fixed time is a more reliable alternative.

DNA and protein synthesis

DNA is a double helix of two strands. Bases pair A–T and C–G.

A gene is a section of DNA coding for a protein. Three bases (a triplet or codon) code for one amino acid.

Transcription — DNA is copied into mRNA in the nucleus. Translation — the ribosome reads the mRNA and assembles amino acids into a protein.

Why the base sequence matters: it determines the amino acid sequence, which determines the protein’s folding and shape — and for an enzyme, shape determines the active site and therefore function. A mutation changing one base can therefore change the protein’s shape and stop it working.

Exam traps

• Failing to convert units in magnification calculations.
• Confusing magnification with resolution.
• Saying enzymes are killed by heat.
• Confusing respiration with breathing.
• Not identifying which factor is limiting on a graph.
• Saying plant cells do not respire — they do, constantly.

Self-test

1. Give three structures in plant but not animal cells.
2. Why do electron microscopes resolve more detail?
3. What does denaturation actually change?
4. What does a plateau on a photosynthesis rate graph indicate?
5. How does a change in DNA base sequence affect an enzyme?

Answers: 1. Cell wall, chloroplasts, permanent vacuole. 2. They use electrons, which have a much shorter wavelength than visible light, and resolution is limited by wavelength. 3. The shape of the active site, so the substrate no longer fits. 4. That the factor being varied is no longer limiting — some other factor has become the limiting one. 5. It changes the amino acid sequence, which changes how the protein folds and therefore the shape of the active site, so the substrate may no longer bind.