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How do cells function?
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
A focused answer to the VCE Biology Unit 1 dot point on cell organelles. Covers the structure and function of the nucleus, ribosomes, ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuole, cytoskeleton and cell wall, and the endosymbiotic theory for the origin of mitochondria and chloroplasts.
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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:
- A large ancestral anaerobic prokaryote (host) engulfed a smaller aerobic prokaryote (the ancestor of mitochondria), probably by phagocytosis.
- Instead of being digested, the aerobic prokaryote survived inside the host. The host gained ATP via aerobic respiration; the symbiont gained nutrients and protection.
- 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.
- 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.
Worked example
A student observes a cell under an electron microscope. They see a nucleus, mitochondria with cristae, rough ER studded with ribosomes, a Golgi stack, lysosomes, a small vacuole, and a cytoskeleton, but no chloroplasts and no cell wall. This is an animal cell. If they instead saw chloroplasts with stacked thylakoids, a large central vacuole, and a cellulose cell wall, it would be a plant cell.
Common traps
Saying plants do not have mitochondria. They do. Plants photosynthesise during the day but respire 24 hours a day, in mitochondria.
Saying lysosomes are in plants. Plants use the vacuole and other vesicles for similar digestion; lysosomes are an animal-cell organelle.
Saying mitochondria "make energy". Mitochondria convert chemical energy from glucose into chemical energy in ATP. Energy is not created.
Forgetting the second membrane. Mitochondria and chloroplasts are double-membraned. The endosymbiotic theory needs this detail.
Saying endosymbiotic theory is "just a hypothesis". It is the consensus model, supported by DNA, ribosome, membrane and division evidence.
In one sentence
Eukaryotic cells are divided into specialised compartments (nucleus, ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuole, peroxisomes) supported by the cytoskeleton, with mitochondria and chloroplasts originating, according to the endosymbiotic theory, as free-living aerobic and photosynthetic prokaryotes engulfed by an ancestral eukaryote and retained as permanent organelles.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2022 VCE4 marksOutline the endosymbiotic theory and describe two pieces of evidence that support it.Show worked answer →
A 4-mark answer needs the theory plus two distinct evidence points.
Theory. Mitochondria and chloroplasts originated as free-living prokaryotes (an aerobic bacterium and a photosynthetic cyanobacterium respectively) that were engulfed by a larger ancestral host cell. Instead of being digested, they survived inside the host as endosymbionts, gradually becoming permanent organelles. This is how the eukaryotic cell acquired oxidative phosphorylation and photosynthesis.
Evidence (any two):
- Mitochondria and chloroplasts have their own circular DNA, separate from the nuclear genome, and resembling prokaryotic DNA.
- They have their own 70S ribosomes, like prokaryotes, rather than the 80S cytosolic ribosomes of eukaryotes.
- They are surrounded by a double membrane: the inner membrane resembles a prokaryote, the outer membrane resembles the host's engulfing vesicle.
- They replicate independently of the host cell, by a process similar to binary fission.
- Their inner membranes contain prokaryote-like lipids (cardiolipin in mitochondria).
2024 VCE3 marksDescribe the path of a secreted protein from synthesis to release at the plasma membrane.Show worked answer →
A 3-mark answer needs the four organelles in order.
- Ribosomes on the rough endoplasmic reticulum (RER) translate the protein; the growing polypeptide enters the RER lumen.
- The protein is packaged into a transport vesicle that buds from the RER and fuses with the Golgi apparatus (cis face), where it is modified (glycosylation, sorting tags added).
- The protein leaves the trans face of the Golgi in a secretory vesicle that fuses with the plasma membrane, releasing the protein outside the cell by exocytosis.
Markers reward the correct sequence (RER, vesicle, Golgi, secretory vesicle, plasma membrane) and identification of exocytosis.
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