2.1 Cell structure

Why this matters

Robert Hooke coined the word 'cell' in 1665 because the small rectangular spaces he saw in cork tissue under his microscope reminded him of the cells where monks lived. Three and a half centuries later we know every living organism — every plant, every animal, every fungus, every bacterium — is built from cells, and that how each cell is organised determines what it can do. A cell that secretes a lot of protein has hundreds of ribosomes and lots of rough endoplasmic reticulum; a cell that contracts a lot has many mitochondria packed against the membranes that need ATP. These structural choices are why a sperm cell looks nothing like a nerve cell, even though both are eukaryotic. The OCR specification asks you to know both the structure (what each organelle looks like, where it sits in the cell) and the function (what it actually does, and how it works with the others). Questions on microscopy, ultrastructure and prokaryote vs eukaryote appear in almost every H420 paper — they're among the most reliably-examined topics in the whole A-Level.

How to learn this topic

Build on what you already know

• GCSE: cells are the basic units of life; plant cells have cell walls, chloroplasts and a permanent vacuole; animal cells don't.
• GCSE: ribosomes make proteins; mitochondria release energy by aerobic respiration.
• GCSE: bacteria are single-celled and have no nucleus.
• AS-Level: basic understanding of phospholipid bilayers (covered in detail in 2.5 Biological membranes).

1. Start with WHY cells matter: cells are the unit of life — every function you'll learn later (respiration, photosynthesis, the nerve impulse, the immune response) ultimately happens inside or across a cell membrane.
2. Tools next: without microscopes we'd know nothing of ultrastructure. Cover light vs TEM vs SEM, then nail down the difference between magnification and resolution.
3. Build the eukaryotic cell organelle-by-organelle, grouping them by function. The protein-secretion route (ribosome → rough ER → vesicle → Golgi → vesicle → plasma membrane) is the most powerful single revision aid.
4. Strip the eukaryotic cell down to what a prokaryotic cell has and hasn't — note carefully which features are 'membrane-bound' and which aren't.
5. Finish with the cytoskeleton — the part most students under-revise, and the part examiners love to test.

Key terms

magnification

How many times bigger the image is than the real object. Calculated as image size ÷ real object size. A scalar with no unit. (Examiners want you to distinguish this clearly from resolution — magnification is bigger-ness, not detail.)

resolution

The smallest distance between two points at which they can still be distinguished as separate. Determined by the wavelength of the radiation used: shorter wavelength = greater resolution. (Marks awarded for linking shorter wavelength to greater resolution. 'Higher magnification = better resolution' is wrong and is not rewarded.)

eukaryotic cell

A cell with a membrane-bound nucleus and other membrane-bound organelles. Found in animals, plants, fungi and protists.

prokaryotic cell

A cell with no membrane-bound nucleus and no membrane-bound organelles. DNA is a single circular molecule in the cytoplasm. Found in bacteria and archaea.

organelle

A specialised structure within a cell with a specific function, such as a nucleus or mitochondrion. (In comparison questions the marking point is usually 'membrane-bound organelle', not just 'organelle' — bacteria do have ribosomes.)

ultrastructure

The fine structure of a cell as seen under the electron microscope, including individual organelles and membrane systems. (Use 'ultrastructure' (not just 'structure') when describing electron-microscope-level detail — it signals you understand the technical distinction.)

mitochondrion

Double-membrane-bound organelle, site of aerobic respiration and ATP production. The inner membrane is folded into cristae to maximise surface area.

ribosome

Site of protein synthesis. Made of ribosomal RNA and protein, in two subunits. Free in the cytoplasm or attached to rough ER. 80S in eukaryotes, 70S in prokaryotes. (Ribosomes are NOT membrane-bound, so they are found in both prokaryotic and eukaryotic cells. Critical for any 'differences between' question.)

rough endoplasmic reticulum

A network of flattened membrane-bound sacs studded with ribosomes. The site of protein synthesis for proteins that will be secreted or inserted into membranes.

Golgi apparatus

A stack of curved, flattened membrane sacs (cisternae) that modifies, sorts and packages proteins received from the rough ER into vesicles for secretion.

cytoskeleton

A 3D network of protein filaments (microfilaments, intermediate filaments, microtubules) in the cytoplasm that gives the cell shape, transports vesicles and organelles, and enables cell movement. (Examiners want all three functions named: shape/support, intracellular transport, and cell movement (e.g. flagella, cilia, muscle contraction).)

cristae

Folds of the inner mitochondrial membrane that increase its surface area, allowing more ATP synthase and electron carriers to be embedded. (The marking point is 'surface area', not 'surface-area-to-volume ratio'. Saying SVR is a common drop-mark answer.)

Notes

Why cells matter

A cell is the smallest unit of structure that can carry out all the processes of life — taking in nutrients, releasing energy, responding to its environment, reproducing. Every plant, animal, fungus and bacterium is built from cells. Understanding how a cell is organised is therefore the foundation of every later topic in A-Level Biology: respiration happens in mitochondria, photosynthesis in chloroplasts, protein synthesis on ribosomes, gene expression in the nucleus. Get cell structure right and the rest of the course slots into place.

Microscopy — three instruments, three jobs

The light microscope uses visible light focused through glass lenses. Its maximum useful magnification is about ×1500 and its resolution limit is roughly 200 nm — set by the wavelength of visible light. It can show whole cells, large organelles like the nucleus, and the outline of mitochondria, but it cannot resolve individual ribosomes or membrane bilayers.

The transmission electron microscope (TEM) fires a beam of electrons through a thin specimen onto a screen. Electrons have a wavelength roughly 100,000 times shorter than visible light, so the TEM can resolve down to about 0.2 nm — sharp enough to show individual organelles in cross-section, including cristae inside a mitochondrion and ribosomes on the rough ER. This is what revealed cell ultrastructure in the 1940s and 50s.

The scanning electron microscope (SEM) scans an electron beam over the surface of a specimen and detects scattered electrons to build a 3D image. Resolution is lower than TEM (about 10 nm), but the image is 3D and shows surface texture — useful for cells in their living shape (e.g. red blood cells, pollen grains, the surface of an insect's eye).

Magnification vs resolution — examiners love this distinction

Magnification is how many times bigger the image is than the real object. It's calculated as image size ÷ real object size. It has no unit. You can magnify a blurry image and it just gets bigger and stays blurry.

Resolution is the smallest distance between two points at which they can still be distinguished as separate. It is determined by the wavelength of the radiation used to view the specimen: shorter wavelength gives greater resolution. The electron microscope wins on resolution because the electron beam has a much shorter wavelength than visible light — that's the marking point examiners want.

The magnification formula on the spec is the obvious one: magnification = image size / actual size. Always convert to the same units first; the most common error in calculations is leaving the image in mm and the object in μm.

The eukaryotic cell — every organelle the spec names

The OCR spec names a fixed list of organelles in 2.1.1(g) that you must be able to identify and describe the function of:

• Nucleus: large membrane-bound organelle containing the cell's DNA as linear chromosomes. The double membrane is called the nuclear envelope and is studded with nuclear pores that let ribosomal subunits and messenger RNA out.
• Nucleolus: a dense region inside the nucleus where ribosomal RNA is transcribed and ribosomal subunits assembled.
• Rough endoplasmic reticulum (RER): flattened membrane-bound sacs studded with ribosomes. Site of protein synthesis for proteins that will be secreted or inserted into membranes.
• Smooth endoplasmic reticulum (SER): similar membrane sacs without ribosomes. Site of lipid and steroid synthesis.
• Ribosomes: tiny structures made of ribosomal RNA and protein. Free in the cytoplasm (making proteins that stay inside the cell) or attached to rough ER (making proteins for export). They are not membrane-bound.
• Golgi apparatus: a stack of curved, flattened membrane sacs called cisternae. Modifies proteins received from the rough ER (e.g. adds carbohydrates to make glycoproteins, cleaves segments), then packages them into vesicles for secretion.
• Mitochondria: double-membrane-bound organelles where aerobic respiration happens. The inner membrane is folded into cristae to maximise surface area for ATP synthase and electron carriers. The fluid inside the inner membrane is the matrix and contains the enzymes of the Krebs cycle, mitochondrial DNA and 70S ribosomes.
• Lysosomes: small membrane-bound sacs full of digestive enzymes (acid hydrolases). They break down old organelles and engulfed material.
• Chloroplasts (plant cells only): double-membrane-bound organelles where photosynthesis happens. Internal stacked thylakoids contain chlorophyll; the stroma is the fluid where the Calvin cycle runs.
• Plasma membrane: phospholipid bilayer with embedded proteins that surrounds every cell. Partially permeable.
• Centrioles: short hollow cylinders of microtubules. Organise spindle fibres in cell division.
• Cell wall (plant + fungal + prokaryotic cells, not animal): rigid outer layer outside the plasma membrane. In plants it's made of cellulose.
• Flagella and cilia: hair-like or whip-like extensions of the plasma membrane containing the 9+2 microtubule arrangement. Flagella are long and few (e.g. sperm tail); cilia are short and many (e.g. trachea lining).

The protein secretion pathway — a single revision-aid for half the spec

Most questions about organelle interaction can be answered with one diagram:

1. A free or rough-ER-bound ribosome synthesises the protein.
2. The protein enters the lumen of the rough ER, where it folds.
3. A transport vesicle buds off the rough ER carrying the protein.
4. The vesicle fuses with the cis face of the Golgi apparatus.
5. The Golgi modifies the protein (carbohydrate, phosphate or lipid groups added; some peptide segments cleaved).
6. A secretory vesicle buds off the trans face of the Golgi.
7. The secretory vesicle moves to the plasma membrane and fuses with it, releasing the protein outside the cell (exocytosis).

This is exactly the route a goblet cell uses to secrete mucus or a beta cell uses to secrete insulin. Examiners love it because every step has a named organelle doing a named job — easy to mark, easy to mis-spell.

The cytoskeleton — the part students forget

The cytoskeleton is a 3D network of protein filaments throughout the cytoplasm. It does three things examiners want named:

1. Mechanical support: maintains cell shape; stops the cell collapsing.
2. Intracellular transport: motor proteins walk along the filaments carrying vesicles and organelles.
3. Cell movement: drives flagella, cilia, muscle contraction and the movement of phagocytes.

The filaments are of three types: microfilaments (actin), intermediate filaments, and microtubules (tubulin). You don't need to recall the chemistry, but you do need to name all three functions to score full marks on cytoskeleton questions.

Prokaryotic vs eukaryotic — say 'membrane-bound'

A prokaryotic cell (bacterium or archaeon) is typically 0.5–5 μm across — about a tenth the linear dimension of a typical eukaryotic cell (10–100 μm), and roughly a thousand times smaller in volume. The differences examiners reward:

• No membrane-bound nucleus — DNA sits free in the cytoplasm as a single circular molecule.
• No membrane-bound organelles — no mitochondria, no Golgi, no ER, no chloroplasts, no lysosomes.
• 70S ribosomes (smaller) instead of 80S.
• A cell wall made of peptidoglycan (not cellulose).
• Often has a flagellum (different structure from the eukaryotic flagellum — solid rod, rotates), pili (attachment), and sometimes a capsule outside the wall.

The critical phrasing is 'no membrane-bound nucleus' and 'no membrane-bound organelles'. Saying 'no nucleus' or 'no organelles' loses marks because bacteria do have DNA and they do have ribosomes (which aren't membrane-bound).

Exam tips

• When asked about microscopy, always pair resolution with wavelength. The phrase examiners want: 'the electron beam has a shorter wavelength than light, so the electron microscope has greater resolution.'
• If the question is 'prokaryotic vs eukaryotic', use 'membrane-bound' before nucleus and organelles. Saying 'no nucleus' won't score full marks — bacteria have DNA, just not in a membrane-bound nucleus.
• On mitochondrial cristae questions, name ATP synthase or electron carriers explicitly. 'Allows more reactions' is too vague to score; 'allows more ATP synthase to be embedded in the inner membrane' scores.
• For protein secretion, draw the route: ribosome → rough ER → vesicle → Golgi → vesicle → plasma membrane. Labelled flow diagrams pick up marks long-answer questions otherwise miss.
• Surface-area-to-volume ratio: always state that volume increases proportionally faster than surface area as a cell gets bigger — that's the marking point, not the conclusion.
• Don't conflate cilia with flagella. Cilia are short and many; flagella are long and few. Both share the 9+2 microtubule arrangement (which examiners DO want named).
• On cytoskeleton questions, give all three functions: structural support, intracellular transport, and cell movement. Naming only one rarely scores full marks.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions