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

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

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.

Examples in context

Example 1. Zooxanthellae chloroplasts on the Great Barrier Reef. Symbiodinium algae living inside coral polyps on the Great Barrier Reef are full eukaryotic cells whose chloroplasts (double membrane, thylakoid grana) power photosynthesis. The fixed carbon (glucose, glycerol, amino acids) is exocytosed across the algal plasma membrane and absorbed by the coral host, supplying up to 90 percent of coral nutrition. During the 2024 marine heatwave, prolonged sea temperatures above 30 degrees Celsius damaged thylakoid photosystem II, generating reactive oxygen species that triggered the coral to expel the algae (bleaching). Without chloroplast-derived ATP and sugars, the coral starves within weeks unless the algae return.

Example 2. QIMR Berghofer immune cell organelles. Researchers at QIMR Berghofer Medical Research Institute in Brisbane study natural killer (NK) cells whose lytic granules are specialised secretory lysosomes packed with perforin and granzymes. When an NK cell contacts a tumour or virus-infected target, microtubules of the cytoskeleton rapidly reposition the Golgi apparatus and lytic granules toward the contact site (the immune synapse). The granules then fuse with the plasma membrane by exocytosis, releasing perforin to punch holes in the target. The case shows the endomembrane pathway (ER, Golgi, vesicle, plasma membrane) operating at second-by-second speed in a single cell.

Try this

Q1. Name the organelle responsible for each function and describe one structural feature that suits it to that function: (a) ATP synthesis, (b) modification and packaging of proteins, (c) digestion of worn organelles. [3 marks]

• Cue. (a) Mitochondrion, cristae; (b) Golgi, stacked cisternae; (c) lysosome, hydrolytic enzymes at pH 5.

Q2. An electron micrograph of a pancreatic acinar cell shows extensive rough ER, a prominent Golgi stack and many secretory vesicles near the plasma membrane. Interpret what this organelle profile indicates about the cell's function. [3 marks]

• Cue. Cell is specialised for protein secretion; rough ER synthesises, Golgi modifies, vesicles export digestive enzymes.

Q3. Compare a plant mesophyll cell with an animal liver cell with respect to the named organelles. (a) Identify two organelles found only in the plant cell. (b) Identify two organelles abundant in the liver cell and link each to a metabolic role. (c) Justify why both cell types have many mitochondria. [2+2+2 marks]

• Cue. (a) Chloroplast, large central vacuole, cell wall. (b) Smooth ER (detoxification), peroxisomes (fatty acid oxidation). (c) Both have high ATP demand.