OCR GCSE Biology: Cell Level Systems (J247)

This guide covers Topic B1 Cell level systems, the first of six teaching topics in OCR GCSE (9-1) Gateway Science Suite Biology A (J247). The qualification is tiered (Foundation, papers 1-2, grades 5-1; Higher, papers 3-4, grades 9-4), and a seventh, practical-skills topic (B7) runs alongside the six content topics.

Where this fits in J247

Cell level systems establishes what cells are made of, what they need to survive, and how they release and capture energy – the foundation for later topics such as Scaling up (multicellular organisation) and the topics covering exchange, transport and disease.

Syllabus coverage

OCR GCSE BIOLOGY (J247) — TOPIC 1 CELL LEVEL SYSTEMS

• B1.1 Cell structures — the structures found in animal, plant and bacterial cells and how they relate to function
• B1.2 What happens in cells (and what do cells need)? — DNA structure, protein synthesis (transcription and translation), and enzyme mechanism, including the effect of temperature and pH
• B1.3 Respiration — how cells release energy through aerobic and anaerobic respiration
• B1.4 Photosynthesis — how plant cells capture and use light energy

How to approach it

Cell structures (B1.1) rewards direct comparison: build a clear picture of what animal, plant and bacterial cells have in common and what distinguishes each, since exam questions frequently test this directly. Respiration and photosynthesis (B1.3-B1.4) are natural counterparts – one releases energy from glucose, the other captures energy to build it – so revising them side by side, including their word and (where required) balanced equations, helps avoid confusing the two processes. Because this topic sits at the start of the specification, treat its vocabulary (diffusion, cell membrane structure, energy transfer) as a foundation that later topics on exchange and transport will assume without re-explanation. What happens in cells (B1.2) is easy to underestimate – it links the structural detail of B1.1 to the processes of B1.3 and B1.4, so use it to check that you can explain, not just name, why a cell needs the substances it does.

Official syllabus

OCR GCSE (9-1) Gateway Science Suite Biology A (J247) specification, for first assessment 2018 — ocr.org.uk.

Cell structures and their functions

Structure	Function	Found in
Nucleus	Contains DNA, controls the cell	Animal, plant
Cytoplasm	Site of many reactions	All
Cell membrane	Controls entry and exit	All
Mitochondria	Aerobic respiration	Animal, plant
Ribosomes	Protein synthesis	All, including bacteria
Chloroplasts	Photosynthesis	Plant
Cell wall	Support and rigidity	Plant, bacteria
Permanent vacuole	Stores cell sap, maintains turgor	Plant

Prokaryotes — bacteria — have no nucleus and no membrane-bound organelles. Their DNA is a single circular loop, and many bacteria also carry one or more plasmids; their ribosomes are smaller than those of eukaryotes.

Microscopy

Light microscopes magnify to roughly x2000 and resolve to about 200 nm. Electron microscopes reach far higher magnification and resolution, revealing internal structure. Resolution — the ability to distinguish two close points as separate — matters as much as magnification, since magnifying beyond the resolving power produces a bigger but no clearer image.

magnification = image size / actual size
1 mm = 1000 micrometres = 1 000 000 nanometres

DNA and protein synthesis

DNA is a double helix of two strands held by complementary base pairing: A with T, C with G. A gene is a section of DNA coding for one protein, and the sequence of bases determines the sequence of amino acids.

Protein synthesis has two stages: transcription, where the DNA of one gene is copied into mRNA in the nucleus, and translation, where ribosomes read the mRNA in triplets and assemble the corresponding amino acids into a polypeptide.

Because the folded shape of a protein depends on its amino acid sequence, a change in the DNA base sequence can change the shape and destroy the function — which is how mutations cause disease.

Enzymes and respiration

Enzymes are proteins with an active site complementary to a specific substrate. Rate rises with temperature until the optimum, then falls sharply as the enzyme denatures — the tertiary structure breaks and the active site changes shape irreversibly. pH behaves the same way.

Aerobic:    glucose + oxygen -> carbon dioxide + water    (large ATP yield)
Anaerobic in muscle: glucose -> lactic acid               (small yield, oxygen debt)
Anaerobic in yeast:  glucose -> ethanol + carbon dioxide  (fermentation)

Respiration happens in every living cell, continuously, and is not the same as breathing.

Photosynthesis

Green plants and algae are the main producers of food, and therefore of biomass, for life on Earth. They trap light energy from the Sun and use it to fix carbon dioxide, combining it with hydrogen from water to make glucose, an organic compound.

carbon dioxide + water --light energy--> glucose + oxygen

6CO2 + 6H2O --light energy--> C6H12O6 + 6O2

Photosynthesis is an endothermic reaction — energy is transferred from the environment (as light) to the chloroplast, rather than released. It happens in two stages: light energy absorbed by chlorophyll is first used to split water, and that energy is then used to combine carbon dioxide with hydrogen to build glucose. Both stages take place in the chloroplasts.

Three factors can each limit the rate of photosynthesis: light intensity, carbon dioxide concentration and temperature. A limiting factor is whichever of these is in shortest supply at a given moment — it alone determines the rate, however much of the other two factors is available. On a rate graph, increasing one factor raises the rate until it plateaus, at which point a different factor has become limiting. Light intensity follows an inverse square law with distance from a light source, so doubling the distance from a lamp quarters the intensity reaching the plant.

Worked example

A cell measures 60 micrometres. Its image is 12 mm wide. Find the magnification.

Convert to the same unit: 12 mm = 12 000 micrometres

magnification = 12 000 / 60 = x200

Converting before dividing is where the marks are won or lost.

Common mistakes

Saying bacteria have no DNA rather than no nucleus. Confusing magnification with resolution. Writing that enzymes are “killed” rather than denatured, or that denaturing is reversible. Equating respiration with breathing. Forgetting that anaerobic respiration in humans and in yeast give different products. Omitting unit conversion in magnification calculations. Writing the photosynthesis equation unbalanced, or describing it as exothermic rather than endothermic. Assuming that whichever factor was deliberately varied in an experiment is automatically the limiting factor, rather than checking which one is genuinely in shortest supply.

Quick revision checklist

• Give the function of each cell structure and say which cell types contain it.
• Compare prokaryotic and eukaryotic cells.
• Distinguish magnification from resolution and calculate magnification with unit conversion.
• State the base-pairing rules and describe transcription and translation.
• Explain enzyme action and the effects of temperature and pH, using denature correctly.
• Write word equations for aerobic and both types of anaerobic respiration.
• Give the word and balanced symbol equation for photosynthesis, and explain why it is endothermic.
• Describe the two-stage process of photosynthesis and identify light intensity, carbon dioxide concentration and temperature as limiting factors.