the structure and specialisation of plant and animal cell organelles for distinct functions, including chloroplasts and mitochondria, and the suggested origins of mitochondria and chloroplasts as described by the endosymbiotic theory

What this dot point is asking

VCAA wants the structure and function of the main plant and animal cell organelles, including special attention to mitochondria and chloroplasts, plus the endosymbiotic theory explaining where the latter two came from.

The answer

The major organelles

Nucleus

Double-membrane envelope (nuclear envelope) with pores. Contains linear chromosomes (DNA + histones). The nucleolus inside makes ribosomal RNA. Function: stores the genome, controls transcription, dictates cell function.

Ribosomes

Two subunits made of rRNA and protein. Found free in the cytosol or attached to rough ER. Cytosolic ribosomes are 80S in eukaryotes; ribosomes inside mitochondria and chloroplasts are 70S. Function: protein synthesis (translation).

Rough endoplasmic reticulum (RER)

Folded membrane network studded with ribosomes, continuous with the nuclear envelope. Function: synthesis of membrane proteins and secreted proteins; entry point to the endomembrane system.

Smooth endoplasmic reticulum (SER)

Same network as RER but without ribosomes. Function: lipid and steroid synthesis, detoxification (in liver cells), calcium storage (in muscle cells).

Golgi apparatus

Stack of flattened membrane sacs (cisternae) with a cis face (receiving from RER) and a trans face (releasing vesicles). Function: modifies, sorts and packages proteins and lipids; directs them to lysosomes, the plasma membrane, or secretion.

Mitochondrion

Double membrane: smooth outer, heavily folded inner (cristae) enclosing the matrix. Contains its own circular DNA and 70S ribosomes. Function: aerobic cellular respiration. The Krebs cycle runs in the matrix; the electron transport chain and ATP synthase are embedded in the inner membrane. Often called the "powerhouse" of the cell.

Chloroplast (plant cells only)

Double membrane plus internal thylakoid membranes stacked into grana, surrounded by the stroma. Contains its own circular DNA, 70S ribosomes, and chlorophyll. Function: photosynthesis. Light-dependent reactions occur on thylakoid membranes; the Calvin cycle runs in the stroma.

Lysosome (animal cells only)

Single membrane-bound sac full of hydrolytic (digestive) enzymes at low internal pH. Function: digests damaged organelles (autophagy), pathogens (after phagocytosis), and worn-out macromolecules.

Vacuole

Membrane-bound sac. In plant cells, a large central vacuole stores water, ions, pigments and waste; it provides turgor pressure that keeps the plant rigid. Animal cells have many smaller vacuoles for storage and transport.

Peroxisome

Single-membrane organelle containing oxidative enzymes. Function: breaks down fatty acids and detoxifies hydrogen peroxide.

Cytoskeleton

Network of protein filaments (microfilaments of actin, intermediate filaments, microtubules) through the cytoplasm. Function: cell shape, organelle movement, chromosome separation (spindle in mitosis), cell division.

Plasma membrane

Phospholipid bilayer with proteins, cholesterol and carbohydrates. Function: semi-permeable boundary controlling what enters and leaves; cell signalling; cell recognition. (Covered in detail in the plasma membrane dot point.)

Cell wall

Outside the plasma membrane. Cellulose in plants, chitin in fungi, peptidoglycan in bacteria. Absent in animal cells. Function: structural support, protection, prevents osmotic bursting.

Plant cells vs animal cells

Both have: nucleus, ribosomes, ER, Golgi, mitochondria, cytoskeleton, plasma membrane.

Only plant cells have: chloroplasts, a large central vacuole, a cellulose cell wall, plasmodesmata (channels between cells).

Only animal cells have: lysosomes (in significant numbers), centrioles (organising microtubules for the mitotic spindle), and many small vacuoles instead of one large one.

Endosymbiotic theory

Proposed by Lynn Margulis in the 1960s. It explains the origin of mitochondria and chloroplasts in eukaryotic cells.

Steps:

1. A large ancestral anaerobic prokaryote (host) engulfed a smaller aerobic prokaryote (the ancestor of mitochondria), probably by phagocytosis.
2. Instead of being digested, the aerobic prokaryote survived inside the host. The host gained ATP via aerobic respiration; the symbiont gained nutrients and protection.
3. Over evolutionary time, the symbiont lost the ability to live independently. Most of its genes transferred to the host nucleus; the residual DNA remains in the organelle.
4. Later, a similar engulfment of a photosynthetic cyanobacterium gave rise to chloroplasts in the plant and algal lineage.

Evidence:

• Mitochondria and chloroplasts have their own circular DNA, separate from nuclear DNA, resembling bacterial chromosomes.
• They have 70S ribosomes, the same size as bacterial ribosomes (eukaryotic cytosolic ribosomes are 80S).
• They are bound by two membranes: the inner membrane resembles a prokaryotic plasma membrane (including the unusual lipid cardiolipin); the outer membrane resembles the host's engulfing vesicle.
• They replicate by binary-fission-like division, independent of the host cell cycle.
• Their genomes are most similar to free-living bacteria: mitochondria to alpha-proteobacteria, chloroplasts to cyanobacteria.

This is why eukaryotic cells can do aerobic respiration and photosynthesis: they captured those capabilities from prokaryotic ancestors more than 1.5 billion years ago.

Examples in context

Example 1. Mitochondrial disease research at WEHI. The Walter and Eliza Hall Institute in Parkville hosts mitochondrial biologists who study Leigh syndrome, a rare inherited disease caused by mutations in mitochondrial DNA or in nuclear genes coding for mitochondrial proteins. Because mitochondria carry their own circular DNA (a legacy of their endosymbiotic origin), some mutations are inherited only from the mother. WEHI researchers use patient-derived fibroblasts to measure ATP output and cristae morphology, and have helped trial mitochondrial donation in Australia, recently legalised through Maeve's Law in 2022. The work directly applies the endosymbiotic theory: the organelle is treated as a near-independent genetic entity.

Example 2. Chloroplast biology in Royal Botanic Gardens Cranbourne. Researchers at Royal Botanic Gardens Cranbourne study how the chloroplasts of native Banksia and grevillea species cope with bushfire smoke and reduced light. Chloroplasts, like mitochondria, are double-membraned, carry their own 70S ribosomes and circular DNA, and divide by a binary-fission-like process. By measuring chlorophyll fluorescence in field plots, the team can identify damage to thylakoid membranes within days of a controlled burn. The chloroplast's prokaryotic ancestry helps explain why some herbicides that block bacterial ribosomes (such as streptomycin) also block chloroplast protein synthesis and bleach plants.

Try this

Q1. State two structural features of mitochondria that support the endosymbiotic theory. [2 marks]

• Cue. Circular DNA resembling prokaryotic DNA; 70S ribosomes like bacteria; double membrane; binary-fission-like division (any two).

Q2. A liver cell contains roughly 1000 mitochondria, while a mature red blood cell contains none. Account for this difference in terms of cellular function and energy demand. [3 marks]

• Cue. Hepatocytes do high-ATP metabolism (gluconeogenesis, detoxification) so need many mitochondria; mature erythrocytes lack a nucleus and most organelles to maximise haemoglobin content and oxygen-carrying capacity.

Q3. A plant cell and an animal cell are observed under an electron microscope. (a) List two organelles present in both. (b) List one organelle present only in the plant cell and state its function. (c) Explain why plant cells contain both chloroplasts and mitochondria. [2+2+2 marks]

• Cue. (a) Nucleus, mitochondria, ER, Golgi, ribosomes (any two). (b) Chloroplast for photosynthesis, or large central vacuole for turgor. (c) Photosynthesis runs only in the light; respiration runs continuously to supply ATP for the cell day and night.