Organelle Structure and Function
Spec mapping: AQA 7402 Section 3.2.1.1 — eukaryotic cell ultrastructure (refer to the official AQA specification document for exact wording).
A thorough understanding of organelle ultrastructure and function is the spine of A-Level cell biology. You need to know how each organelle's three-dimensional architecture is related to its biochemical role, how organelles cooperate within an integrated system (the secretory pathway being the canonical example), and how to interpret electron micrographs at the level of resolution that distinguishes a sixth-form candidate from an undergraduate. The structures introduced here recur throughout A-Level: ribosomes link to translation (Course 4), the Golgi reappears in cell-wall biology and glycoprotein synthesis, mitochondria are revisited at length under respiration (Course 5), and the cytoskeleton governs both cell division and muscle contraction. Treat this lesson as a reference layer that you will return to repeatedly.
Key Definition: An organelle is a structurally distinct sub-cellular component, either membrane-bound (e.g., mitochondrion, nucleus) or non-membrane-bound (e.g., ribosome, centriole), that performs a specific function. The term literally means 'little organ'.
The Nucleus
The nucleus is the largest organelle in most eukaryotic cells and serves as the control centre.
Structure
- Surrounded by a nuclear envelope — a double membrane perforated by nuclear pores (approximately 3000 per nucleus).
- Nuclear pores are about 100 nm in diameter and are lined by specific pore proteins that regulate transport of large molecules (mRNA, ribosomal subunits out; histones, DNA polymerase in).
- Contains chromatin — a complex of DNA and histone proteins. During cell division, chromatin condenses into visible chromosomes.
- One or more nucleoli (singular: nucleolus) are present. The nucleolus is a dense region where ribosomal RNA (rRNA) is transcribed and ribosomal subunits are assembled before being exported through nuclear pores.
- The nucleoplasm (nuclear sap) is the fluid matrix inside the nucleus, containing dissolved enzymes, nucleotides, and ions.
Functions
- Contains the cell's genetic information (DNA) which directs protein synthesis.
- DNA is transcribed into mRNA in the nucleus, which then passes through nuclear pores to ribosomes for translation.
- The nucleolus produces rRNA, essential for ribosome assembly.
- Retains the chromosomes within a protected environment, separated from the cytoplasmic enzymes that might damage DNA.
Exam Tip: Be precise about the nuclear envelope — it is a double membrane, not a single membrane. Each nuclear pore is a complex structure, not simply a hole.
Mitochondria
Mitochondria are the sites of aerobic respiration — the process that produces the majority of a cell's ATP.
Structure
- Double membrane: the outer membrane is smooth; the inner membrane is folded into cristae (singular: crista).
- The cristae greatly increase the surface area of the inner membrane, providing more space for the electron transport chain and ATP synthase — the enzymes involved in oxidative phosphorylation.
- The fluid-filled interior is called the matrix. It contains enzymes for the Krebs cycle (also known as the citric acid cycle or tricarboxylic acid cycle), as well as the link reaction.
- Mitochondria contain their own circular DNA and 70S ribosomes, allowing them to synthesise some of their own proteins.
- Typically 1–10 µm in length and 0.5–1 µm in diameter. The number per cell varies: metabolically active cells such as muscle cells and liver hepatocytes contain many thousands, whereas red blood cells contain none.
Functions
- Carry out the link reaction and Krebs cycle in the matrix.
- Perform oxidative phosphorylation on the inner membrane (cristae), producing the majority of the cell's ATP via chemiosmosis.
- Involved in apoptosis (programmed cell death) through the release of cytochrome c.
Structure–Function Link: Cells with high energy demands (e.g., sperm cells, cardiac muscle fibres, active transport epithelial cells) contain large numbers of mitochondria. The extensive cristae maximise ATP production.
Endoplasmic Reticulum (ER)
The endoplasmic reticulum is an extensive system of membrane-bound flattened sacs (cisternae) and tubules continuous with the nuclear envelope.
Rough Endoplasmic Reticulum (RER)
- Studded with 80S ribosomes on its cytoplasmic surface.
- Synthesises proteins destined for secretion (e.g., digestive enzymes, antibodies, hormones such as insulin), for the cell surface membrane, or for lysosomes.
- Proteins are threaded into the RER lumen during translation, where they may be folded and undergo initial modifications (e.g., glycosylation — the addition of carbohydrate groups).
- Transport vesicles bud off from the RER and carry proteins to the Golgi apparatus for further processing.
- Cells specialised for protein secretion (e.g., B lymphocytes producing antibodies, pancreatic acinar cells producing digestive enzymes) have extensive RER.
Smooth Endoplasmic Reticulum (SER)
- Lacks ribosomes on its surface.
- Synthesises lipids, including phospholipids and steroids (e.g., cholesterol, oestrogen, testosterone).
- Involved in detoxification of drugs and alcohol in liver cells (hepatocytes).
- Stores and releases calcium ions in muscle cells (sarcoplasmic reticulum), which is essential for muscle contraction.
- Cells in the adrenal cortex and ovaries/testes have extensive SER due to steroid hormone production.
Golgi Apparatus (Golgi Complex)
Structure
- A stack of flattened, membrane-bound cisternae (typically 5–8 per stack), with a distinct cis face (receiving side, nearest the ER) and trans face (shipping side, nearest the cell surface membrane).
- Transport vesicles from the RER fuse with the cis face; modified products bud off from the trans face in secretory vesicles.
Functions
- Modifies proteins and lipids received from the ER (e.g., further glycosylation, phosphorylation, sulphation).
- Sorts and packages molecules into vesicles for their correct destination: secretory vesicles (exocytosis), lysosomes, or the cell surface membrane.
- Produces lysosomes containing hydrolytic enzymes.
- Synthesises polysaccharides for plant cell walls (e.g., pectins and hemicelluloses).
- Produces glycoproteins by adding carbohydrate chains to proteins.
Exam Tip: A common exam question is to describe the pathway of a secreted protein from gene to extracellular space: DNA → mRNA (transcription in nucleus) → mRNA exits via nuclear pore → translation on 80S ribosomes on RER → protein enters RER lumen → transport vesicle to Golgi → modification in Golgi → secretory vesicle buds off trans face → vesicle fuses with cell surface membrane → protein released by exocytosis.
Lysosomes
Structure
- Single-membrane vesicles, typically 0.1–0.5 µm in diameter.
- Contain a range of hydrolytic (digestive) enzymes (e.g., proteases, lipases, nucleases, glycosidases) that function optimally at an acidic pH (around pH 4.5–5.0). The lysosome membrane contains proton pumps that maintain this low internal pH.
Functions
- Intracellular digestion: break down materials engulfed by phagocytosis (e.g., white blood cells digesting bacteria).
- Autophagy: digest worn-out or damaged organelles so their components can be recycled.
- Autolysis: self-destruction of the cell (e.g., the regression of the tadpole tail during metamorphosis, or the destruction of cells during apoptosis).
- Release enzymes by exocytosis for extracellular digestion (e.g., breakdown of bone tissue by osteoclasts, or digestion of the head of a sperm cell by acrosomal enzymes — the acrosome is a modified lysosome).
Ribosomes
Structure
- Non-membrane-bound organelles composed of rRNA and protein.
- 80S ribosomes in eukaryotic cytoplasm (60S + 40S subunits).
- 70S ribosomes in prokaryotes, mitochondria, and chloroplasts (50S + 30S subunits).
- Each ribosome has two subunits that come together during translation.
Function
- The site of translation — the synthesis of polypeptides from mRNA.
- Free ribosomes in the cytoplasm produce proteins for use within the cell.
- Ribosomes bound to RER produce proteins for secretion, membranes, or lysosomes.
Centrioles and the Cytoskeleton
Centrioles
- Found in animal cells (and some lower plant cells), typically in pairs within the centrosome near the nucleus.
- Each centriole is a cylindrical structure composed of nine triplets of microtubules arranged in a ring (9 + 0 arrangement).
- During cell division, centrioles move to opposite poles of the cell and organise the spindle fibres (made of tubulin microtubules) that separate chromosomes.
- Centrioles also form the basal bodies from which cilia and flagella grow.
The Cytoskeleton
The cytoskeleton is a dynamic network of protein filaments that provides structural support and enables movement within the cell.
- Microtubules — hollow tubes of the protein tubulin (25 nm diameter). Form the spindle during cell division, provide tracks for organelle transport (via motor proteins dynein and kinesin), and form the structural core of cilia and flagella (9+2 arrangement).
- Microfilaments (actin filaments) — solid rods of the protein actin (7 nm diameter). Involved in cell movement (e.g., amoeboid movement), cytokinesis (cleavage furrow formation), and muscle contraction (with myosin).
- Intermediate filaments — rope-like filaments of various proteins (e.g., keratin, vimentin) approximately 10 nm in diameter. Provide mechanical strength and maintain cell shape.
Exam Tip: The cytoskeleton is often overlooked in revision, but questions about it do appear. Remember: microtubules = tubulin = spindle fibres; microfilaments = actin = cell movement.
Vesicles and Vacuoles
- Vesicles are small, membrane-bound sacs used for transport within the cell. They include secretory vesicles, transport vesicles (ER to Golgi), and endocytic vesicles.
- Vacuoles in plant cells are large, permanent, and surrounded by a membrane called the tonoplast. They contain cell sap (water, sugars, ions, pigments) and are vital for maintaining turgor pressure — the hydrostatic pressure exerted by the cell contents against the cell wall, which provides support in non-woody plants.
Peroxisomes
Peroxisomes are small (0.2–1 µm), single-membrane-bound organelles found in most eukaryotic cells. Although not heavily examined by AQA, they are part of the modern organelle picture and worth knowing.
Structure and function
- A single membrane encloses a granular matrix of oxidative enzymes.
- The defining enzymes are oxidases (which use O₂ to oxidise substrates, producing H₂O₂) and catalase (which decomposes H₂O₂ into H₂O and O₂, preventing oxidative damage).
- In animal cells, peroxisomes oxidise very-long-chain fatty acids and metabolise some toxins (e.g., the oxidation of ethanol in liver hepatocytes).
- In plant cells, specialised peroxisomes called glyoxysomes in germinating seeds convert stored fats to sugars via the glyoxylate cycle, supporting seedling growth before photosynthesis begins.
Peroxisomes illustrate the broader principle that compartmentalisation sequesters dangerous chemistry: confining hydrogen peroxide to peroxisomes prevents it from damaging the wider cytoplasm.
Chloroplasts (Plant Cells)
Although the AQA spec for this course focuses on animal and bacterial cells, plant-cell organelles are examinable and chloroplasts appear later under photosynthesis.
Structure
- Double membrane bounding a fluid stroma.
- The stroma contains a system of internal membranes called thylakoids, stacked into grana (singular granum) and interconnected by lamellae.
- Thylakoid membranes carry the photosynthetic pigments (chlorophyll a and b, carotenoids) organised into photosystems with electron transport chains.
- The stroma contains enzymes for the Calvin cycle, 70S ribosomes, and circular chloroplast DNA — providing endosymbiotic evidence parallel to mitochondria.
Function
- The thylakoid membranes carry out the light-dependent reactions: photolysis of water, generation of ATP and reduced NADP.
- The stroma carries out the light-independent (Calvin) reactions: fixation of CO₂ into organic compounds.
The folded thylakoid architecture maximises the surface area available for light absorption and the electron transport chain, in much the same way that mitochondrial cristae maximise the area for the respiratory electron transport chain — a recurring structure–function principle.
How Organelles Cooperate: The Secretory Pathway
A protein destined for secretion (for example, the digestive enzyme amylase, the antibody IgG, or the peptide hormone insulin) passes through a coordinated sequence of organelles. This is the canonical AQA worked example for structure–function integration.