Cell Organelles and Function - TCE Biology (Tasmania)
“Identify the main organelles of eukaryotic cells and relate their structure to function.”
Eukaryotic cells contain membrane-bound organelles, each suited to a job. The nucleus stores DNA and controls the cell, mitochondria release energy by respiration, chloroplasts capture light for photosynthesis, ribosomes build proteins, the endoplasmic reticulum and Golgi process and package those proteins, and lysosomes and vacuoles store or digest materials. Compartmentalisation lets many reactions run at once.
A general survey of organelles is Biology Level 2 content (BIO215123) and is not examined in TASC's Biology Level 3 exam (BIO315124, current for 2026). Level 3 examines two organelles in detail as part of criterion 5: the chloroplast in photosynthesis and the mitochondrion in cellular respiration.
What this dot point is asking
Why organelles matter
Organelles are specialised structures inside the cell, most enclosed by their own membrane. By separating the cell into compartments, each with its own conditions and enzymes, the cell can run many different chemical processes at the same time without them interfering. This compartmentalisation is the great advantage of eukaryotic cells over prokaryotic ones.
An organelle is a specialised structure within a cell that carries out a particular function. Most are bounded by one or more membranes that separate their contents from the surrounding cytoplasm.
The nucleus
The nucleus is the control centre. It is surrounded by a double membrane (the nuclear envelope) pierced by pores that allow molecules such as mRNA to pass out. Inside, the DNA is stored as chromatin. The nucleus controls the cell by determining which proteins are made, and it contains the nucleolus, where ribosomes are assembled.
Mitochondria
Mitochondria are the site of aerobic cellular respiration, releasing energy from glucose and storing it as ATP. Each has a double membrane, with the inner membrane folded into cristae to give a large surface area for the reactions that produce ATP. Cells with high energy demands, such as muscle and kidney cells, contain many mitochondria.
The inner membrane of a mitochondrion is folded into cristae, and the membranes of a chloroplast are stacked into structures called grana. In both cases the folding greatly increases the surface area available for the membrane-bound reactions, allowing more energy conversion in a small space.
Chloroplasts
Chloroplasts are found in plant cells and some protists and are the site of photosynthesis. They contain the green pigment chlorophyll, which absorbs light. Internal membranes are arranged in stacks (grana) within a fluid (the stroma), giving a large surface for capturing light. Chloroplasts, like mitochondria, have their own small loop of DNA, evidence that both evolved from once-free-living bacteria.
Ribosomes
Ribosomes are the site of protein synthesis, reading mRNA and joining amino acids into chains. They are not membrane-bound. Some float free in the cytoplasm, making proteins for use inside the cell, and others are attached to the endoplasmic reticulum, making proteins for export.
Endoplasmic reticulum and Golgi apparatus
These two organelles form a production and delivery line for proteins and lipids:
- Rough endoplasmic reticulum is studded with ribosomes and processes the proteins they make, folding them and moving them in vesicles.
- Smooth endoplasmic reticulum has no ribosomes and makes lipids and helps detoxify substances.
- The Golgi apparatus receives proteins and lipids, modifies and packages them, and ships them out in vesicles, either to other parts of the cell or out of it by exocytosis.
A cell makes a digestive enzyme for export. Ribosomes on the rough endoplasmic reticulum build the protein, which is folded and sent in a vesicle to the Golgi apparatus. The Golgi modifies and packages it into a new vesicle that travels to the membrane and releases the enzyme by exocytosis. Each organelle does one step in the sequence, showing how structure and function are linked along the pathway.
Lysosomes and vacuoles
Lysosomes contain digestive enzymes that break down worn-out organelles, waste, and material brought in by endocytosis. Vacuoles store substances; in plant cells a large central vacuole stores water and dissolved substances and helps keep the cell firm by pressing the contents against the cell wall.
Plant cell extras
Plant cells have three features animal cells lack: a rigid cellulose cell wall outside the membrane for support and protection, chloroplasts for photosynthesis, and a large permanent central vacuole. Animal cells instead often contain centrioles, which help organise cell division.
Structure suits function
Across all organelles, structure matches function. Folded membranes maximise reaction surfaces, the nuclear envelope's pores control what leaves the nucleus, and the membranes of the endoplasmic reticulum and Golgi create channels for moving products. Recognising these links is the key skill: given an organelle's structure, you should be able to predict its role, and the reverse.
Students often mix these up because both are involved in energy and both have folded membranes. Chloroplasts capture light energy to make glucose in photosynthesis (only in plants and some protists). Mitochondria release energy from glucose by respiration to make ATP (in nearly all eukaryotic cells, plant and animal). Photosynthesis stores energy; respiration releases it.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksA protein is made in a cell and then secreted to the outside. Describe the pathway the protein takes through the organelles, naming each organelle in order and stating its role in making, modifying or exporting the protein.Show worked answer →
A 6 mark answer traces the secretory pathway with the role of each organelle.
- Ribosome (on rough ER)
- The protein is synthesised by ribosomes on the rough endoplasmic reticulum, which translate the mRNA into a chain of amino acids.
- Rough endoplasmic reticulum
- The protein enters the rough ER, where it begins folding and is packaged into a transport vesicle.
- Golgi apparatus
- The vesicle fuses with the Golgi, which modifies the protein (for example adding sugars) and sorts and repackages it into a secretory vesicle.
- Plasma membrane
- The secretory vesicle moves to the plasma membrane and fuses with it, releasing the protein outside the cell by exocytosis.
Markers reward the correct ordered pathway (ribosome to rough ER to Golgi to plasma membrane) and a correct role for each stage.
Original5 marksExplain how the structure of the mitochondrion and the chloroplast each relates to its function, and state one feature these two organelles share that supports the endosymbiotic theory.Show worked answer →
A 5 mark answer links structure to function for both organelles and gives shared endosymbiotic evidence.
- Mitochondrion
- Has a highly folded inner membrane (cristae) that increases the surface area for the reactions of aerobic respiration, so more ATP can be produced for the cell.
- Chloroplast
- Contains stacked membranes (thylakoids in grana) holding chlorophyll, which increases the surface area to capture light for photosynthesis.
- Shared endosymbiotic feature
- Both have their own small circular DNA and their own ribosomes (and a double membrane), resembling free-living bacteria. This supports the endosymbiotic theory that they were once independent prokaryotes engulfed by a host cell.
Markers reward the surface-area link for each organelle and a valid shared feature (own DNA/ribosomes/double membrane).