How is a eukaryotic cell organised, and how do microscopes let us see and measure it?
2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
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What this dot point is asking
OCR wants you to recognise every required eukaryotic and prokaryotic structure from an electron micrograph or description, state its function, explain how organelles cooperate to make and export proteins, and use, calibrate and interpret light and electron microscopes including the magnification calculation.
The answer
A eukaryotic cell has a true membrane-bound nucleus and membrane-bound organelles. Compartmentalisation by membranes lets incompatible reactions run side by side, each in its own optimised environment.
The organelles you must know
- Nucleus
- Surrounded by a double membrane (the nuclear envelope) perforated by nuclear pores. Contains chromatin (DNA wound around histone proteins) and a nucleolus that makes ribosomes. Stores genetic information and controls the cell through transcription.
- Rough endoplasmic reticulum (RER)
- A network of membranes studded with ribosomes. Folds and transports proteins destined for secretion or the cell-surface membrane.
- Smooth endoplasmic reticulum (SER)
- Membranes without ribosomes. Synthesises and processes lipids and steroids.
- Golgi apparatus
- A stack of flattened membrane sacs (cisternae). Modifies, sorts and packages proteins and lipids (for example by glycosylation) into vesicles for secretion or to form lysosomes.
- Mitochondria
- A double membrane; the inner membrane is folded into cristae surrounding a fluid matrix. The site of aerobic respiration and ATP synthesis. Cells with high energy demand have many.
- Chloroplasts
- Found in plant and algal cells. A double membrane plus internal thylakoid membranes stacked into grana, surrounded by stroma. The site of photosynthesis; contain their own DNA and 70S ribosomes.
- Ribosomes
- Made of rRNA and protein, free in the cytoplasm or bound to the RER. The site of translation. Eukaryotic ribosomes are 80S; 70S ribosomes occur in prokaryotes, mitochondria and chloroplasts.
- Lysosomes
- Golgi-derived vesicles of hydrolytic enzymes that digest worn-out organelles, ingested material and engulfed pathogens.
- Centrioles
- Two bundles of microtubules at right angles in animal cells; they form the spindle in cell division.
- Cell wall
- Outside the membrane; cellulose in plants, chitin in fungi. Freely permeable; gives mechanical strength and resists turgor.
The cytoskeleton is a network of protein filaments (microfilaments, intermediate filaments and microtubules). It gives the cell shape and support, moves organelles and vesicles, and drives chromosome movement and the beating of cilia and flagella (which contain a 9+2 microtubule arrangement).
Prokaryotic cells
Prokaryotic (bacterial) cells are smaller and simpler. They have no nucleus and no membrane-bound organelles. Their DNA is a single circular loop free in the cytoplasm, not wound around histones, and they may carry plasmids (small DNA rings often bearing antibiotic-resistance genes), a protective capsule, a flagellum for movement, mesosomes, and 70S ribosomes. The cell wall is made of murein (peptidoglycan), not cellulose. Compare prokaryotes and eukaryotes as clear pairs: nucleus versus free circular DNA, membrane-bound organelles present versus absent, 80S versus 70S ribosomes, and the different wall materials.
The protein production and secretion pathway
This is the most heavily examined sequence in the topic.
- The gene is transcribed in the nucleus; mRNA leaves through a nuclear pore.
- Ribosomes on the RER translate the mRNA into a polypeptide, which folds in the RER.
- A vesicle buds from the RER and carries the protein to the Golgi apparatus.
- The Golgi modifies (for example adds carbohydrate) and packages the protein into a secretory vesicle.
- The vesicle moves to the cell-surface membrane, fuses with it, and releases the protein by exocytosis.
ATP from the mitochondria powers vesicle transport and exocytosis throughout.
Microscopy: magnification, resolution and the three microscopes
The first essential distinction is magnification versus resolution. Magnification is how many times larger the image is than the object; resolution is the smallest distance between two points that can still be seen as separate. Resolution is limited by the wavelength of the radiation used, which is why electron microscopes resolve far more than light microscopes.
- Optical (light) microscope. Can view living, coloured specimens but resolves only to about 200 nm; maximum useful magnification about times 1500.
- Transmission electron microscope (TEM). Passes electrons through a thin specimen to reveal internal ultrastructure at about 0.1 nm resolution. The specimen must be dead, dehydrated and in a vacuum, and preparation can create artefacts.
- Scanning electron microscope (SEM). Scans the surface to give a three-dimensional image at lower resolution than the TEM.
To measure, calibrate an eyepiece graticule (a scale with no fixed real size) against a stage micrometer (a slide engraved with an accurate scale). Find the real length of one graticule division, then measure the specimen in graticule units and convert. Recalibrate whenever you change the objective lens. The magnification equation is:
Rearrange it to find any one quantity, and always convert image and object to the same units first (1 mm = 1000 micrometres; 1 micrometre = 1000 nm).
Examples in context
Example 1. Goblet cells. Goblet cells lining the airways are packed with RER and a prominent Golgi apparatus because they continuously make and secrete the glycoprotein mucin: synthesised on the RER, glycosylated in the Golgi, packaged into vesicles and released by exocytosis. They show the whole secretory pathway in one specialised cell.
Example 2. Palisade mesophyll cells. Palisade cells are crammed with chloroplasts and arranged as tall columns near the upper leaf surface to maximise light absorption. Their large vacuole pushes the chloroplasts to the cell edge where light is strongest, showing how organelle number and arrangement match function.
Try this
Q1. Give three structural features of a prokaryotic cell that a eukaryotic cell does not have. [3 marks]
- Cue. Circular DNA free in the cytoplasm (no nucleus), plasmids, a murein (peptidoglycan) cell wall, 70S ribosomes, a capsule or mesosomes (any three).
Q2. Explain why a cell that secretes large amounts of protein has many mitochondria. [2 marks]
- Cue. Mitochondria carry out aerobic respiration to make ATP; ATP is needed to power protein synthesis, vesicle transport and exocytosis.
Q3. An image of a chloroplast measures 40 mm. The actual chloroplast is 8 micrometres long. Calculate the magnification. [2 marks]
- Cue. Convert: 40 mm = 40 000 micrometres. Magnification = 40 000 divided by 8 = times 5000.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20194 marksDescribe the roles of the rough endoplasmic reticulum, the Golgi apparatus and vesicles in the production and secretion of a protein from a cell.Show worked answer →
A 4-mark answer needs the secretory pathway in order, each organelle doing a job.
- Ribosomes on the rough endoplasmic reticulum (RER) synthesise the polypeptide by translation; it folds within the RER.
- A vesicle buds from the RER and transports the protein to the Golgi apparatus.
- The Golgi apparatus modifies the protein (for example by adding carbohydrate to form a glycoprotein) and packages it into a secretory vesicle.
- The secretory vesicle moves to and fuses with the cell-surface membrane, releasing the protein by exocytosis.
Markers reward the correct sequence, naming each organelle, and the word exocytosis.
OCR H420/01 20213 marksAn image of a cell is 75 mm wide at a magnification of times 1500. Calculate the actual width of the cell in micrometres, and explain why an electron microscope can resolve finer detail than a light microscope.Show worked answer →
Convert the image to the same units, then rearrange the magnification equation.
Image width micrometres. Actual size micrometres (1 mark for working, 1 for the answer with units).
For the third mark: an electron microscope uses a beam of electrons, which have a much shorter wavelength than light. Because resolution is limited by the wavelength of the radiation used, the shorter wavelength lets the electron microscope resolve much smaller structures (about 0.1 nm versus about 200 nm for light).
Related dot points
- 2.1.4 Enzymes: the role of enzymes as biological catalysts in metabolic reactions; the mechanism of enzyme action including the lock-and-key and induced-fit models; the effects of temperature, pH, enzyme and substrate concentration on the rate of reaction; the action of competitive and non-competitive inhibitors; the roles of cofactors, coenzymes and prosthetic groups.
A focused answer to the OCR H420 2.1.4 dot point on enzymes. Covers enzymes as catalysts, the lock-and-key and induced-fit models, activation energy, the effects of temperature, pH and concentration, competitive and non-competitive inhibition, and cofactors.
- 2.1.3 Nucleotides and nucleic acids: the semi-conservative replication of DNA and the roles of DNA helicase, DNA polymerase and the complementary base pairing rule; the nature of the genetic code as a triplet code that is degenerate and non-overlapping; the roles of mRNA and tRNA in protein synthesis.
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- 2.1.6 Cell division: the cell cycle and its regulation by checkpoints; the main stages of mitosis (prophase, metaphase, anaphase and telophase) and cytokinesis; the significance of mitosis in growth, repair and asexual reproduction; the calculation and use of the mitotic index.
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- 2.1.6 Cell division, diversity and organisation: how meiosis produces haploid gametes and generates genetic variation through crossing over and independent assortment; the meaning and potential of stem cells (totipotent, pluripotent and multipotent); cell specialisation and the organisation of cells into tissues, organs and organ systems.
A focused answer to the OCR H420 2.1.6 dot point on meiosis, stem cells and cell organisation. Covers meiosis and how it generates variation, the potency of stem cells, their uses and ethics, and the organisation of cells into tissues, organs and systems.
- 2.1.2 Biological molecules: the properties of water and their importance to living organisms; the structure of monosaccharides, the formation of glycosidic bonds by condensation, and the structure and function of starch, glycogen and cellulose; the biochemical tests for reducing and non-reducing sugars and for starch.
A focused answer to the OCR H420 2.1.2 dot point on water and carbohydrates. Covers the biologically important properties of water, monosaccharides and condensation, the structure and function of starch, glycogen and cellulose, and the Benedict's and iodine tests.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)