QCE Biology Unit 1 Cells and Multicellular Organisms: deep-dive 2026 guide

How Unit 1 fits into QCE Biology

Unit 1, Cells and multicellular organisms, is the foundation of the whole QCE Biology course. Every later unit assumes you can reason from cell structure to function. Unit 2 (maintaining the internal environment) builds on membrane transport and specialised cells; Unit 3 (biodiversity) builds on the energy reactions; Unit 4 (heredity) builds on the nucleus and its chromosomes. The QCAA syllabus splits Unit 1 into Topic 1, cells as the basis of life, and Topic 2, multicellular organisms. This guide walks through the high-value subject matter in both, with worked examples and a Check your knowledge section to test recall and application.

Cell theory and cell types

Cell theory has three core statements: all living things are made of one or more cells, the cell is the basic structural and functional unit of life, and all cells arise from pre-existing cells by division. The theory was assembled over the nineteenth century from the microscopy of Hooke, Schleiden, Schwann and Virchow.

Cells fall into two fundamental types.

• Prokaryotic cells (bacteria and archaea) have no membrane-bound nucleus and no membrane-bound organelles. Their single circular chromosome sits in a region called the nucleoid. They are small (typically 1 to 5 micrometres) and have a cell wall (peptidoglycan in bacteria).
• Eukaryotic cells (protists, fungi, plants and animals) have a true membrane-bound nucleus and a set of membrane-bound organelles. They are larger (typically 10 to 100 micrometres) and compartmentalise their chemistry into specialised organelles.

Both cell types share a plasma membrane, cytosol, ribosomes and genetic material made of DNA. The compartmentalisation of the eukaryotic cell is the single most important structural difference, because it lets incompatible reactions run side by side and lets surface area be amplified internally.

Organelles and what they do

A eukaryotic cell is a system of compartments, each suited by its structure to a specific function.

• Nucleus. Double membrane (nuclear envelope) studded with pores; contains the chromosomes and the nucleolus. Site of DNA replication and transcription. The control centre of gene expression.
• Ribosomes. Not membrane-bound. Translate mRNA into polypeptides. Free in the cytosol or bound to the rough endoplasmic reticulum.
• Rough endoplasmic reticulum. Ribosome-studded membrane network; folds and modifies proteins destined for secretion or membranes.
• Smooth endoplasmic reticulum. No ribosomes; synthesises lipids, detoxifies (liver cells), and stores calcium ions (muscle cells).
• Golgi apparatus. Stack of flattened cisternae; modifies, sorts and packages proteins and lipids. The cis face receives, the trans face dispatches.
• Vesicles. Small membrane sacs that ferry cargo between organelles and to the plasma membrane.
• Lysosomes. Sacs of hydrolytic enzymes at acidic pH that digest worn organelles and engulfed material.
• Mitochondrion. Double membrane with the inner membrane folded into cristae; the matrix holds Krebs cycle enzymes. Site of aerobic respiration.
• Chloroplast (plants and algae). Double membrane enclosing the stroma; thylakoid membranes stacked into grana. Site of photosynthesis.
• Vacuole. Storage sac; plant cells have a large central vacuole that maintains turgor pressure.
• Cytoskeleton. Microfilaments, intermediate filaments and microtubules that give shape, anchor organelles and provide transport tracks.

The plasma membrane and the fluid mosaic model

The plasma membrane is described by the fluid mosaic model: a phospholipid bilayer studded with proteins, cholesterol, glycoproteins and glycolipids.

• Phospholipid bilayer. Hydrophilic phosphate heads face the watery extracellular fluid and cytosol; hydrophobic fatty acid tails face inward. Small non-polar molecules such as oxygen and carbon dioxide cross by simple diffusion; charged and large polar molecules cannot.
• Cholesterol. Wedges between phospholipids and buffers fluidity, stiffening the membrane when warm and preventing tight packing when cold.
• Integral (transmembrane) proteins. Channels, carriers and pumps that move solutes; many act as receptors.
• Peripheral proteins. Surface-attached enzymes and structural anchors.
• Glycoproteins and glycolipids. Carbohydrate chains on the outer face used for cell recognition and adhesion.

The model is "fluid" because phospholipids and proteins drift laterally, and a "mosaic" because diverse molecules stud the bilayer.

Transport across membranes

Movement across the membrane is either passive (no energy) or active (uses ATP).

Passive transport.

• Simple diffusion. Net movement of particles from high to low concentration until even. Small non-polar molecules only.
• Facilitated diffusion. Diffusion of polar or charged solutes through channel or carrier proteins, still down the gradient.
• Osmosis. Net movement of water across a semipermeable membrane from a region of high water potential (dilute) to low water potential (concentrated).

Active transport.

• Protein pumps. Move solutes against the gradient using ATP. The sodium potassium pump exports three sodium ions and imports two potassium ions per ATP.
• Bulk transport. Endocytosis (membrane engulfs material inward) and exocytosis (vesicle fuses with the membrane to release contents).

Tonicity describes the effect of an external solution on a cell. In a hypertonic solution (more concentrated outside) water leaves and an animal cell shrinks (crenates); a plant cell loses turgor and the membrane pulls from the wall (plasmolysis). In a hypotonic solution (more dilute outside) water enters and an animal cell may burst (lysis); a plant cell becomes turgid, supported by its wall. In an isotonic solution there is no net movement.

Surface area to volume ratio

A cell exchanges materials across its surface but uses materials throughout its volume. As a cell grows, volume rises faster than surface area, so the surface area to volume ratio falls.

For a cube of side length L, surface area is 6 times L squared and volume is L cubed, so the ratio is 6 divided by L. As L increases, the ratio shrinks.

A low ratio is a problem because the membrane can no longer supply enough oxygen and nutrients or remove enough waste to serve the larger interior. This is why cells stay small, and why exchange surfaces are adapted to maximise area: microvilli on gut cells, flattened alveolar cells, and the folded cristae inside mitochondria all increase surface area without increasing volume much. Large organisms solve the problem by being multicellular (many small cells) rather than one giant cell, and by evolving transport systems to move materials between exchange surfaces and the body interior.

Photosynthesis

Photosynthesis captures light energy and stores it as chemical energy in glucose.

Word equation: carbon dioxide plus water, with light energy, yields glucose plus oxygen.

Balanced equation: 6 carbon dioxide plus 6 water, with light energy, yields one glucose plus 6 oxygen.

It occurs in two stages inside the chloroplast.

1. Light-dependent reactions on the thylakoid membranes. Light excites electrons in chlorophyll; water is split (photolysis), releasing oxygen, electrons and protons; ATP and NADPH are produced.
2. Light-independent reactions (Calvin cycle) in the stroma. ATP and NADPH power the fixation of carbon dioxide into glucose.

QCAA Unit 1 expects the overall equation, the two named stages and the chloroplast location, not the full Calvin cycle detail.

Cellular respiration

Aerobic respiration releases the chemical energy in glucose as ATP, the cell's energy currency.

Word equation: glucose plus oxygen yields carbon dioxide plus water plus energy (ATP).

Balanced equation: one glucose plus 6 oxygen yields 6 carbon dioxide plus 6 water plus ATP (around 36 to 38 ATP per glucose).

It occurs in stages and locations.

• Glycolysis in the cytosol: glucose to two pyruvate, net 2 ATP.
• Link reaction and Krebs cycle in the mitochondrial matrix.
• Electron transport chain on the inner mitochondrial membrane: oxygen is the final electron acceptor, forming water; most ATP is generated here by oxidative phosphorylation.

When oxygen is unavailable, cells switch to anaerobic respiration (fermentation) to regenerate NAD+ so that glycolysis can continue. In animal muscle, glucose becomes lactic acid and 2 ATP; in yeast, glucose becomes ethanol and carbon dioxide and 2 ATP. Both yield far less ATP than aerobic respiration.

Photosynthesis and respiration are reciprocal: the outputs of one are the inputs of the other, cycling carbon and oxygen and linking sunlight to usable energy.

Check your knowledge

A mix of recall, short-response and exam-style application questions covering Unit 1 subject matter. Answer all under timed conditions (about 1 minute per mark), then check against the solutions block.

1. State the three statements of cell theory. (3 marks)
2. Compare a prokaryotic cell and a eukaryotic cell with respect to (a) the nucleus, (b) membrane-bound organelles, and (c) typical size. (3 marks)
3. Describe the fluid mosaic model of the plasma membrane, naming at least four components and giving the role of cholesterol. (4 marks)
4. Distinguish between simple diffusion, facilitated diffusion and active transport, identifying which require ATP and which depend on a concentration gradient. (3 marks)
5. A plant cell with an internal solute concentration of about 0.5 mol per L is placed in a 1.0 mol per L sucrose solution. Predict and explain the direction of water movement, the change in turgor, and the term for the result. (4 marks)
6. A cube-shaped cell has a side length of 5 units. (a) Calculate its surface area to volume ratio. (b) Explain why this ratio matters for the cell, and (c) name one cellular adaptation that increases surface area without greatly increasing volume. (4 marks)
7. Write the balanced equations for photosynthesis and aerobic respiration, state the cellular location of each, and explain the relationship between the two processes. (5 marks)
8. A muscle cell is respiring during a 100 metre sprint when oxygen supply cannot keep pace with demand. (a) Name the pathway used and its products. (b) State the ATP yield per glucose and compare it with the aerobic yield. (c) Explain why this pathway is only a short-term solution. (4 marks)