Difficulty: Introductory | Prerequisites: Sections 6.1–6.4 (microscopy, cell types, nucleus, endomembrane system).
Sections 6.5 and 6.6 cover the organelles that power the cell (mitochondria and chloroplasts), the structures that give it shape and movement (the cytoskeleton), and the junctions that connect cells to their neighbours. Mitochondria and chloroplasts are especially important because they sit at the centre of cellular respiration and photosynthesis, the two major energy pathways you will study later. The cytoskeleton underpins cell division, intracellular transport, and muscle contraction. Cell junctions come back in tissue biology and physiology. If you are not yet comfortable with the endomembrane system (Sections 6.3–6.4), review that first.
Mitochondria produce ATP through cellular respiration; chloroplasts convert light energy to chemical energy through photosynthesis. Both have double membranes, their own DNA, and their own ribosomes, which supports the endosymbiont theory. The cytoskeleton (microtubules, intermediate filaments, microfilaments) provides structural support, enables cell movement, and organises the interior. Cells connect to neighbours via plasmodesmata (plants), gap junctions, desmosomes, and tight junctions (animals).
Endosymbiont theory
The theory that mitochondria and chloroplasts originated as prokaryotic cells engulfed by an ancient eukaryotic cell. Over time, the engulfed cell became an endosymbiont, living and reproducing within the host. This explains why both organelles have double membranes, their own circular DNA, and their own ribosomes.
Mitochondria (singular: mitochondrion)
Double-membraned organelles found in all eukaryotic cells. Their primary function is producing ATP through cellular respiration. Think of them as the cell's power stations.
Cristae
The infoldings of the mitochondrial inner membrane. They increase the surface area available for the enzymes and proteins that drive ATP production.
Mitochondrial matrix
The fluid-filled space enclosed by the inner membrane. Contains enzymes for cellular respiration, along with mitochondrial DNA and ribosomes.
Intermembrane space (mitochondria)
The narrow region between the outer and inner mitochondrial membranes.
Chloroplast
A double-membraned organelle found in photosynthetic eukaryotes (plants, algae). Contains chlorophyll and enzymes for photosynthesis. A member of the plastid family.
Thylakoid
Flattened, interconnected membranous sacs inside the chloroplast. The light-dependent reactions of photosynthesis occur in and across the thylakoid membrane.
Granum (plural: grana)
A stack of thylakoids.
Stroma
The fluid outside the thylakoids but inside the inner membrane of the chloroplast. Contains DNA, ribosomes, and enzymes for the Calvin cycle.
Peroxisome
A single-membraned metabolic compartment. Contains enzymes that transfer hydrogen atoms to oxygen, producing hydrogen peroxide (H₂O₂) as a by-product. Other enzymes then convert H₂O₂ to water. Some peroxisomes break down fatty acids; others detoxify alcohol in liver cells.
Glyoxysome
A specialised peroxisome found in the fat-storing tissues of plant seeds. Converts fatty acids to sugars the seedling uses as an energy source before it can photosynthesise.
Cytoskeleton
A network of protein fibres inside the cell that provides structural support, enables cell movement, and positions organelles. Made of three types of filaments.
Microtubule
The thickest cytoskeletal fibre, made from alpha- and beta-tubulin proteins. Provides compressional support, guides cell division, and forms flagella and cilia.
Flagellum (plural: flagella)
A long, whip-like projection built from microtubules that propels a cell through its environment.
Cilium (plural: cilia)
A shorter, hair-like projection built from microtubules that beats rhythmically to move fluid across the cell surface.
Intermediate filament
Rope-like protein fibres of intermediate diameter. Provide tensile strength, stabilise overall cell structure, and make up the nuclear lamina.
Microfilament (actin filament)
The thinnest cytoskeletal fibre, composed of actin protein. Provides tensional support, drives muscle contraction, shapes the cell, and enables cell movement.
Centrosome / centriole
The centrosome is the main microtubule-organising centre of the cell. It contains a pair of centrioles and is important in positioning structures within the cell and organising the mitotic spindle during cell division.
Plasmodesmata (singular: plasmodesma)
Narrow, membrane-lined channels that span plant cell walls and allow the flow of water, nutrients, signalling molecules, and some proteins and RNA between adjacent plant cells.
Gap junction
The animal-cell equivalent of plasmodesmata. Cylinders of membrane proteins forming a pore between two cells, allowing ions, amino acids, sugars, and other small molecules to pass.
Desmosome
A localised spot-weld junction that holds adjacent animal cells together. Provides adhesion, particularly in tissues under mechanical stress.
Tight junction
A junction where the outer surfaces of adjacent cell membranes fuse in a continuous seal. Nothing can pass between cells through a tight junction. Found where a leak-proof barrier is needed.
Mitochondria and chloroplasts share features that suggest they were once free-living prokaryotes:
Double membrane (the inner membrane may derive from the original prokaryote, the outer from the host's engulfing vesicle)
Their own circular DNA
Their own ribosomes
They grow and divide semi-autonomously within the cell
They change shape and position unpredictably
Present in all eukaryotic cells. The number per cell correlates with metabolic activity.
Function: produce ATP via cellular respiration.
Outer membrane: smooth.
Inner membrane: highly folded into cristae, which increase surface area for the enzymes of the electron transport chain.
Intermembrane space: the region between the two membranes.
Mitochondrial matrix: the interior space enclosed by the inner membrane. Contains enzymes that catalyse steps of cellular respiration, plus mitochondrial DNA and ribosomes.
Found in photosynthetic eukaryotes (plants, algae). Members of the plastid family.
Function: convert light energy to chemical energy (photosynthesis), producing sugar.
Contain chlorophyll and the enzymes needed for photosynthesis.
Internal structure:
Thylakoids: flattened, interconnected membranous sacs where the light reactions occur
Grana: stacks of thylakoids
Stroma: the fluid surrounding the thylakoids, containing DNA, ribosomes, and enzymes for the Calvin cycle
Three compartments: intermembrane space, stroma, thylakoid space.
Single-membrane compartments (not part of the endomembrane system).
Contain enzymes that transfer hydrogen atoms from substrates to oxygen, producing H₂O₂.
Other peroxisomal enzymes convert the toxic H₂O₂ to water.
Some peroxisomes break fatty acids into smaller molecules.
In liver cells, some detoxify alcohol by transferring hydrogen from poisonous compounds to oxygen.
Glyoxysomes (in plant seeds) convert stored fatty acids to sugars, providing energy until the seedling can photosynthesise.
Peroxisomes can grow and split into two.
Provides structural support and enables movement within and of the cell.
Microtubules (thickest)
Made from tubulin protein (alpha and beta subunits).
Provide compressional (weight-bearing) support.
Guide chromosome separation during cell division.
Form flagella (whip-like, propel the cell) and cilia (shorter, beat to move fluid).
Intermediate filaments (middle diameter)
Rope-like, made of various fibrous proteins.
Provide tensile strength to stabilise overall cell structure.
Make up the nuclear lamina (the cytoskeleton of the nucleus).
Microfilaments (thinnest)
Made of actin protein.
Provide tensional support.
Drive muscle contraction (interacting with myosin).
Shape the cell and enable cell movement (e.g. amoeboid crawling).
Centrosome and centrioles
The centrosome is the main microtubule-organising centre.
Contains a pair of centrioles.
Important for positioning structures in the cell and organising the spindle during cell division.
Junction | Found in | Structure | Function |
|---|---|---|---|
Plasmodesmata | Plants | Membrane-lined channels through cell walls | Allow flow of water, nutrients, signalling molecules, some proteins and RNA |
Gap junction | Animals | Cylinders of membrane proteins forming a pore | Allow flow of ions, amino acids, sugars, small molecules |
Desmosome | Animals | Localised spot-welds of adhesion proteins | Hold adjacent cells together, especially in tissues under stress |
Tight junction | Animals | Fused outer membranes forming a continuous seal | Prevent anything from passing between cells |
Mitochondrial dysfunction is linked to a range of diseases, from neurodegenerative disorders to metabolic syndromes, because cells that cannot produce enough ATP fail. Chloroplasts are the basis of all food chains that start with photosynthesis. Drugs that target microtubules (e.g. colchicine, taxol) are used in cancer treatment because they disrupt the mitotic spindle and prevent cell division. Tight junctions in the gut lining prevent stomach acid and digestive enzymes from leaking between cells into the body.
Students sometimes think only plant cells have mitochondria. All eukaryotic cells have mitochondria. Plant cells have both mitochondria and chloroplasts.
Students confuse the three compartments of the chloroplast (intermembrane space, stroma, thylakoid space) with the two compartments of the mitochondrion (intermembrane space, matrix). The chloroplast has an extra compartment because of its thylakoid system.
A common error is thinking cilia and flagella are made of different proteins. Both are built from microtubules (the "9+2" arrangement of tubulin). They differ in length and beating pattern, not composition.
Students often mix up desmosomes and tight junctions. Desmosomes hold cells together (adhesion). Tight junctions seal the space between cells so nothing can pass through. They do different jobs.
⚠️ The endosymbiont theory is a favourite exam topic. Know the evidence: double membrane, own DNA, own ribosomes, semi-autonomous replication.
⚠️ Be able to label the parts of a mitochondrion (outer membrane, inner membrane/cristae, intermembrane space, matrix) and a chloroplast (outer membrane, inner membrane, thylakoid, granum, stroma).
⚠️ Know the three cytoskeletal elements, what each is made of, and what each does. A comparison table is a likely exam format.
⚠️ Cell junctions: be able to match each type to its function and whether it is found in plants or animals.
True or false: Chloroplasts are found in all eukaryotic cells. (False, only in photosynthetic eukaryotes such as plants and algae.)
The infoldings of the mitochondrial inner membrane are called ______. (cristae)
True or false: Intermediate filaments are made of actin. (False, microfilaments are made of actin. Intermediate filaments are made of various fibrous proteins.)
The plant-cell equivalent of the animal gap junction is the ______. (plasmodesma / plasmodesmata)
Peroxisomes produce ______ as a by-product, which is then converted to water. (hydrogen peroxide / H₂O₂)
Q: What evidence supports the endosymbiont theory for the origin of mitochondria and chloroplasts?
A: Both organelles have a double membrane, contain their own circular DNA, have their own ribosomes, and can grow and divide semi-autonomously within the cell. These features are consistent with them having once been free-living prokaryotes engulfed by an ancestral eukaryotic cell.
Q: Compare the structure of a mitochondrion with the structure of a chloroplast.
A: Both have an outer membrane and an inner membrane with an intermembrane space between them. The mitochondrion's inner membrane folds into cristae, enclosing the matrix where respiratory enzymes reside. The chloroplast has an additional internal membrane system (thylakoids, which stack into grana) within the stroma, creating a third compartment (thylakoid space) where light reactions take place.
Q: Name the three components of the cytoskeleton, state what each is made of, and give one function of each.
A: Microtubules are made of tubulin and guide cell division. Intermediate filaments are made of various fibrous proteins and provide tensile strength. Microfilaments are made of actin and drive muscle contraction.
Q: What is the difference between a desmosome and a tight junction?
A: A desmosome is a spot-weld that holds adjacent cells together (adhesion). A tight junction fuses the outer surfaces of adjacent cell membranes into a continuous seal, preventing any material from passing between the cells.
Q: What is the function of glyoxysomes, and where are they found?
A: Glyoxysomes are found in the fat-storing tissues of plant seeds. They contain enzymes that convert fatty acids to sugars, providing the seedling with an energy source before it can photosynthesise.
Mitochondria are central to the chapters on cellular respiration (Chapter 9) and chloroplasts to photosynthesis (Chapter 10). The endosymbiont theory connects to evolutionary biology. The cytoskeleton reappears in cell division (Chapter 12, mitotic spindle), muscle physiology, and intracellular transport. Cell junctions are foundational for tissue biology and organ physiology, where barrier function (tight junctions) and cell communication (gap junctions, plasmodesmata) are recurring themes.
endosymbiont theory, endosymbiosis, mitochondria, mitochondrion, cristae, mitochondrial matrix, intermembrane space, ATP, cellular respiration, chloroplast, chlorophyll, thylakoid, granum, grana, stroma, plastid, photosynthesis, peroxisome, hydrogen peroxide, glyoxysome, cytoskeleton, microtubule, tubulin, flagella, cilia, 9+2 arrangement, intermediate filament, microfilament, actin, myosin, muscle contraction, centrosome, centriole, cell junction, plasmodesmata, gap junction, desmosome, tight junction, cell wall, cell communication