← Unit 1: Cells and multicellular organisms
Topic 1: Cells as the basis of life
Describe the structure and function of cellular components, including the plasma membrane (fluid mosaic model), cytosol, nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, vacuoles, cell wall and cytoskeleton
A focused answer to the QCE Biology Unit 1 dot point on cellular components. Describes the plasma membrane using the fluid mosaic model, then names the structure and function of each membrane-bound organelle (nucleus, mitochondrion, chloroplast, ER, Golgi, lysosome, vesicle, vacuole) plus the cytoskeleton and cell wall.
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What this dot point is asking
QCAA expects you to describe the plasma membrane using the fluid mosaic model and to give a structure-plus-function pairing for each named organelle. Stimulus questions often present an electron micrograph, ask you to identify two or three organelles, and then ask how their structures relate to their functions.
The answer
A eukaryotic cell is a compartmentalised system. The plasma membrane bounds the cell; internal membranes partition reactions into specialised organelles.
The plasma membrane and the fluid mosaic model
The plasma membrane is a phospholipid bilayer studded with proteins, cholesterol, glycoproteins and glycolipids.
- Phospholipid bilayer. Two layers of phospholipids, hydrophilic phosphate heads facing the aqueous extracellular and cytosolic environments, hydrophobic fatty acid tails facing inward. Small non-polar molecules (O2, CO2) cross by simple diffusion; charged and large polar molecules cannot.
- Cholesterol. Buffers fluidity. Stiffens the membrane at high temperatures and prevents tight packing at low temperatures.
- Integral (transmembrane) proteins. Channels, carriers and pumps that move solutes across. Many are receptors for hormones or neurotransmitters.
- Peripheral proteins. Attached to the membrane surface; often enzymes or structural anchors.
- Glycoproteins and glycolipids. Carbohydrate chains on the extracellular face; cell recognition and adhesion.
The "fluid mosaic" name captures two properties: lateral movement of components within the bilayer (fluid), and the diverse mixture of molecules studded across the surface (mosaic).
The cytosol and cytoskeleton
Cytosol. Aqueous gel-like solution filling the cell. Site of glycolysis and many biosynthetic reactions.
Cytoskeleton. Three protein filament systems.
- Microfilaments (actin). Cell shape, cytoplasmic streaming, muscle contraction.
- Intermediate filaments. Mechanical strength, anchoring of organelles.
- Microtubules (tubulin). Tracks for organelle transport, spindle fibres in mitosis, cores of cilia and flagella.
Organelles of information
Nucleus. Bounded by a double membrane (nuclear envelope) studded with nuclear pores. Contains the linear chromosomes and the nucleolus (where ribosomal subunits are assembled). Site of DNA replication and transcription.
Ribosomes. Not membrane-bound. Translate mRNA into polypeptides. Free in the cytosol or bound to the rough ER.
Organelles of energy
Mitochondrion. Double membrane; the inner membrane is folded into cristae that increase surface area. Matrix contains the enzymes of the Krebs cycle and mitochondrial DNA. Site of aerobic respiration (Krebs cycle, electron transport chain, ATP synthesis).
Chloroplast (plants and algae). Double membrane enclosing the stroma; thylakoid membranes stacked into grana. Site of photosynthesis (light reactions on thylakoids, Calvin cycle in stroma).
The endomembrane system
Endoplasmic reticulum (ER). A network of flattened sacs continuous with the nuclear envelope.
- Rough ER carries ribosomes; folds and modifies proteins destined for secretion or for membranes.
- Smooth ER lacks ribosomes; lipid synthesis, detoxification (liver cells), Ca2+ storage (muscle cells).
Golgi apparatus. A stack of flattened cisternae. Modifies, sorts and packages proteins and lipids arriving from the ER. Cis face receives, trans face dispatches.
Vesicles. Small membrane-bound sacs that ferry cargo between organelles and to the plasma membrane.
Lysosomes. Membrane-bound sacs of hydrolytic enzymes (acid hydrolases, optimal pH around 5). Digest worn organelles, phagocytosed material and (programmed) the cell itself in apoptosis.
Vacuoles. Membrane-bound storage sacs. Plant cells have a single large central vacuole that stores water, sugars, pigments and waste, and supports turgor pressure.
Boundaries and walls
Cell wall. External to the plasma membrane.
- Plants. Cellulose; provides structural support and limits cell expansion.
- Fungi. Chitin.
- Bacteria. Peptidoglycan.
Mapping organelle to function
| Function | Key organelle |
|---|---|
| Genetic control | Nucleus |
| Protein synthesis | Ribosomes, rough ER |
| Lipid synthesis, detoxification | Smooth ER |
| Modification, sorting | Golgi |
| Digestion, recycling | Lysosome |
| ATP synthesis | Mitochondrion |
| Photosynthesis | Chloroplast |
| Storage, turgor | Vacuole |
| Shape, transport, division | Cytoskeleton |
| Boundary, signalling | Plasma membrane |
Common traps
Forgetting cholesterol. It is a named component of the fluid mosaic model on QCAA mark schemes.
Confusing ER types. Rough = ribosomes = proteins. Smooth = no ribosomes = lipids and detoxification.
Treating Golgi as a generic "modifier". Specify a modification (glycosylation, phosphorylation, cleavage) where the question allows.
Calling the cell wall a membrane. The cell wall sits external to the plasma membrane and is structurally different (carbohydrate-based, not phospholipid).
Cross-link to Year 12 assessment
This dot point underlies the cellular biology assumed in IA3 research investigations on biotechnology applications (Unit 4) such as recombinant protein production, where the secretory pathway (ER to Golgi to plasma membrane) is the production line being engineered.
In one sentence
The plasma membrane is a fluid mosaic of phospholipids, cholesterol and proteins that bounds a eukaryotic cell, inside which the nucleus controls gene expression, mitochondria and chloroplasts power metabolism, the ER and Golgi process and dispatch proteins, lysosomes and vacuoles store and digest, and the cytoskeleton organises it all.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2023 QCAA style5 marksDraw and label the fluid mosaic model of the plasma membrane. Explain the role of cholesterol and membrane proteins.Show worked answer →
A 5-mark answer needs the labelled diagram and explained roles.
The diagram. A phospholipid bilayer with hydrophilic phosphate heads facing outward (extracellular and cytosolic) and hydrophobic fatty acid tails facing inward. Embedded in the bilayer are integral (transmembrane) proteins, peripheral proteins, glycoproteins, glycolipids and cholesterol molecules.
Why "fluid mosaic". Phospholipids and most proteins drift laterally in the plane of the membrane (fluid). Many different molecule types stud the bilayer like a mosaic.
Role of cholesterol. Wedges between phospholipids. At high temperatures cholesterol reduces fluidity by restraining phospholipid movement; at low temperatures it prevents tight packing and maintains fluidity. It is a thermal buffer.
Role of membrane proteins. Integral transport proteins (channels, carriers, pumps) move polar and charged solutes across the bilayer. Receptor proteins bind hormones and signalling molecules. Recognition glycoproteins identify the cell to the immune system. Enzyme-linked proteins catalyse reactions at the membrane surface.
Markers reward a correctly oriented bilayer with at least four labelled molecule types and a function for cholesterol and proteins.
2022 QCAA style4 marksDescribe how the endoplasmic reticulum, Golgi apparatus and vesicles cooperate to secrete a protein.Show worked answer →
A 4-mark answer needs the four-step pathway and the role of each organelle.
Step 1 (rough ER). Ribosomes attached to the rough ER translate the mRNA. The polypeptide is threaded into the ER lumen where it folds and acquires initial glycosylation.
Step 2 (transport vesicle). A vesicle buds from the ER and carries the protein to the Golgi apparatus.
Step 3 (Golgi). The Golgi modifies the protein (further glycosylation, phosphorylation, proteolytic cleavage) and sorts it. Modified proteins move cis to medial to trans through the Golgi stacks.
Step 4 (secretory vesicle). A secretory vesicle buds from the trans Golgi, travels along the cytoskeleton to the plasma membrane and fuses with it, releasing the protein by exocytosis.
Markers reward an ordered ER to Golgi to plasma-membrane pathway with at least one specific modification.
Related dot points
- Describe the cell theory and distinguish between prokaryotic and eukaryotic cells, recalling that prokaryotes include bacteria and archaea
A focused answer to the QCE Biology Unit 1 dot point on cell theory and cell types. States the three postulates of cell theory, contrasts prokaryotic and eukaryotic cells across membrane-bound organelles, genetic material, ribosomes and size, and groups bacteria and archaea as the two prokaryotic domains.
- Describe passive and active transport processes that move materials across cell membranes, including diffusion, osmosis (hypertonic, hypotonic, isotonic solutions), facilitated diffusion, protein pumps, endocytosis (phagocytosis and pinocytosis) and exocytosis
A focused answer to the QCE Biology Unit 1 dot point on membrane transport. Defines diffusion, osmosis (with tonicity), facilitated diffusion and active transport including protein pumps, endocytosis and exocytosis, and predicts the direction and energy requirements for each.
- Summarise the inputs, outputs and locations of photosynthesis and of aerobic and anaerobic cellular respiration
A focused answer to the QCE Biology Unit 1 dot point on photosynthesis and respiration. Writes the balanced word and chemical equations for photosynthesis and aerobic respiration, locates each in chloroplasts and mitochondria, and compares anaerobic respiration in animals (lactic acid) and yeast (ethanol).