Lesson 2: Cells

The cell is the fundamental unit of life, and A-level biology demands ultrastructural detail: not just the names of organelles but how their structures enable their functions, how cells divide, and how we study them. This lesson covers eukaryotic ultrastructure and the endomembrane pathway, prokaryotic cells and viruses, the cell cycle and mitosis, binary fission, microscopy (light, TEM, SEM) with magnification calculations, cell fractionation, and stem cells with their therapeutic promise and ethical debates.

Eukaryotic ultrastructure

Animal and plant cells share: the nucleus (DNA in chromosomes, pores for mRNA exit, nucleolus making ribosomal RNA), mitochondria (double membrane, folded cristae — respiration, Lesson 12), rough endoplasmic reticulum (ribosomes, protein folding and transport), smooth ER (lipid synthesis), Golgi apparatus (modifies, packages and secretes proteins in vesicles), lysosomes (hydrolytic enzymes digesting waste and pathogens), ribosomes (80S in eukaryotes, protein synthesis) and the cell-surface membrane (phospholipid bilayer with proteins — selective permeability). Plant cells add: cell wall (cellulose, support), chloroplasts (double membrane, thylakoids with chlorophyll — Lesson 11) and the permanent vacuole (cell sap, turgor, storage).

The protein-secretion pathway links organelles functionally: ribosomes on the RER synthesise proteins → the proteins are transported in vesicles to the Golgi → modified (e.g. glycosylation) → packaged into secretory vesicles → released by exocytosis. This is a favourite extended question: "Describe how a secretory protein (e.g. insulin) is produced and released." Trace the route naming each organelle and its role; marks are awarded per correct step.

Structure–function links for each organelle: mitochondria have folded cristae (large surface area for electron-transport chains) and their own small circular DNA; chloroplasts have stacked thylakoids (large area for light absorption); the nucleus has pores (controlled transport). Mitochondrial and chloroplast DNA, 70S ribosomes and double membranes are evidence for the endosymbiotic theory — know this evaluative point.

Prokaryotic cells and viruses

Prokaryotes (bacteria) are smaller (~1–5 µm) with no membrane-bound organelles: circular DNA free in the cytoplasm (not in a nucleus), 70S ribosomes, a cell wall of murein (peptidoglycan), a capsule (protection), plasmids (small DNA rings carrying, e.g., antibiotic-resistance genes) and flagella in some. They divide by binary fission (below). Compare with eukaryotes in table form: nucleus, organelles, ribosome size, DNA form, cell wall chemistry — comparison tables are the most efficient exam format.

Viruses are acellular: a nucleic acid core (DNA or RNA) in a protein capsid, sometimes with a lipid envelope. They reproduce only inside host cells, hijacking the host's machinery — which is why antibiotics (targeting bacterial walls and ribosomes) do not work on viruses. HIV (an RNA retrovirus) and bacteriophages are key examples. Virus structure questions ask you to relate features to function: e.g. the envelope's glycoproteins bind host-cell receptors.

Pathogenic bacteria damage hosts via toxins or direct cell destruction; their small size and rapid binary fission let populations explode. Antibiotic resistance spreads via plasmids transferred between bacteria (conjugation) — the evolutionary and medical context for Lesson 7. When comparing cell types, always give the feature and the consequence: "no nucleus → DNA accessible for rapid transcription during fast growth."

The cell cycle and mitosis

The cell cycle: interphase (G1 — growth and organelle replication; S — DNA replication, chromosomes duplicated into sister chromatids; G2 — growth and checks), then mitosis and cytokinesis. Interphase is the longest stage — the cell spends most of its time preparing, not dividing. Checkpoints (G1, G2, spindle checkpoint) verify DNA integrity; failure leads to apoptosis or, if evaded, cancer (Lesson 8).

Mitosis stages: prophase — chromosomes condense (become visible), the nuclear envelope breaks down, the spindle forms; metaphase — chromosomes line up on the equator, spindle fibres attach to centromeres; anaphase — sister chromatids are pulled apart to opposite poles (shortening spindle fibres); telophase — nuclear envelopes reform, chromosomes decondense; then cytokinesis splits the cytoplasm (cell plate in plants, cleavage furrow in animals). Result: two genetically identical diploid cells. Learn to identify each stage from micrographs — a staple practical-exam skill.

Mitosis matters for growth (increasing cell number), repair (replacing damaged cells) and asexual reproduction (genetically identical offspring). The root-tip squash (Required Practical 2, Lesson 19) lets you observe mitosis directly: treat root tips with acid, stain with toluidine blue or acetic orcein, squash, and count cells in each stage to calculate the mitotic index (cells in mitosis ÷ total cells × 100). A high mitotic index indicates rapid growth — and uncontrolled mitosis indicates cancer.

Binary fission and viral replication

Binary fission in prokaryotes: the circular DNA replicates, the cell elongates, the DNA copies separate to opposite ends, and the cytoplasm divides — producing two genetically identical daughter cells. It is rapid (E. coli every 20 minutes in ideal conditions), enabling exponential population growth: N = N₀ × 2ⁿ. After just 10 generations one cell becomes 2¹⁰ ≈ 1,024 cells; after 20, over a million. Bacterial growth curves (lag, log, stationary, death phases) are interpreted in terms of nutrient availability and waste accumulation.

Compare binary fission with mitosis: both produce genetically identical cells, but binary fission involves no spindle, no nuclear envelope breakdown (there is no nucleus), and a single circular chromosome rather than linear ones. Exam questions ask for "two differences between binary fission and mitosis" — spindle/nucleus and chromosome form are the expected answers.

Viral replication (the lytic cycle): attachment to host receptors, injection/entry of nucleic acid, hijacking of host ribosomes to make viral components, assembly, and release by cell lysis (bursting) — killing the host cell. The lysogenic cycle integrates viral DNA into the host genome (prophage), replicating silently until triggered. Understanding these cycles explains why antivirals target entry, replication enzymes (reverse transcriptase in HIV) or assembly rather than "killing" the virus directly.

Microscopy — magnification and resolution

Resolution is the minimum distance between two objects that can still be seen as separate; magnification is how much larger the image appears. Light microscopes resolve ~0.2 µm (200 nm) — enough for cells and nuclei but not ribosomes or membranes. Transmission electron microscopes (TEM) resolve ~0.1 nm, revealing ultrastructure, but specimens must be dead, thin-sectioned and stained (artefacts possible). Scanning electron microscopes (SEM) give 3D surface images at lower resolution than TEM. Choose the instrument by the question: internal detail → TEM; surface → SEM; living cells → light.

Worked examples — magnification and mitotic index

Worked example 1: magnification. A chloroplast image measures 20 mm; its real length is 5 µm = 0.005 mm. Magnification = image ÷ real = 20 ÷ 0.005 = ×4,000. Working backwards: a mitochondrion drawn 30 mm long at ×15,000 has a real length of 30 ÷ 15,000 = 0.002 mm = 2 µm. The two classic errors are forgetting to convert units before dividing, and inverting the formula. Also: real size = image size ÷ magnification — and when you use a scale bar, measure the bar, work out its magnification, then apply it to the organelle.

Worked example 2: mitotic index. In a root-tip squash, a student counted 200 cells: 176 in interphase and 24 in stages of mitosis. Mitotic index = 24 ÷ 200 × 100 = 12%. Because cells spend time in each stage in proportion to its duration, interphase occupies ~88% of the cycle — the count itself demonstrates that interphase dominates. A tumour sample with an index of 30%+ would suggest abnormally rapid division (Lesson 8).

Staining increases contrast: iodine for starch, methylene blue for cheek cells, acetic orcein for chromosomes. Electron micrographs are black-and-white (formed by electrons, not light); colour is added artificially. Eyepiece graticules calibrated against stage micrometers measure real sizes under the light microscope — know the calibration method for practical questions.

Cell fractionation and stem cells

Cell fractionation isolates organelles: homogenise tissue (breaking cells while keeping organelles intact — in a cold, buffered, isotonic solution to prevent damage), filter (removing debris), then ultracentrifuge at increasing speeds — nuclei pellet first (largest/densest), then mitochondria, then ribosomes and ER fragments. Each fraction's purity is checked by microscopy or enzyme markers (e.g. succinate dehydrogenase for mitochondria). The cold, isotonic buffer prevents enzyme activity and osmotic bursting — be able to explain why each condition is used.

Stem cells: totipotent (can form all cell types including placenta — the zygote), pluripotent (all body cell types — embryonic stem cells), multipotent (limited lineages — adult bone-marrow stem cells → blood cells), unipotent (one type). Therapeutic uses: bone-marrow transplants for leukaemia, growing skin for burns, potential treatments for Parkinson's and spinal injuries; induced pluripotent stem cells (iPS, reprogrammed adult cells) avoid embryo use. Ethical debate: embryo destruction vs medical benefit, consent, and the risk of uncontrolled division (tumours). Present balanced arguments in "discuss" questions.

Common mistakes

• Confusing magnification with resolution. A blurred image enlarged is still blurred: beyond ~×1,500 a light microscope gives empty magnification. If a question asks why ribosomes are invisible under a light microscope, the answer is resolution (0.2 µm limit), not insufficient magnification.
• Calling viruses cells, or saying they "reproduce" alone. Viruses are acellular particles; they replicate only inside host cells. Antibiotics fail against them because viruses have no cell wall, no ribosomes of their own and no metabolism to poison.
• Stage-identification slips. Chromosomes visible but scattered = prophase; lined up singly on the equator = metaphase; chromatids (not chromosomes) moving apart = anaphase — the moment sister chromatids separate, each is counted as a chromosome again.
• "Mitochondria produce energy." Energy is not produced; mitochondria release energy from glucose and transfer it to ATP (Lesson 12). Examiners penalise creation-of-energy language throughout the course.
• Fractionation order errors. The heaviest organelles pellet at the lowest speeds first: nuclei → mitochondria → ribosomes. If your sequence puts ribosomes first, you have it backwards.

Exam Experience

Q1 (data). In a root-tip squash, 300 cells were counted: 255 interphase, 20 prophase, 10 metaphase, 6 anaphase, 9 telophase. Calculate the mitotic index and explain why metaphase counts are lower than prophase counts. [4 marks] Q2. Describe how a secretory cell produces and releases a protein such as insulin, naming the organelles in sequence. [5 marks]

Mark scheme — Q1: Cells in mitosis = 20 + 10 + 6 + 9 = 45; index = 45 ÷ 300 × 100 = 15% [2]. Stages are occupied in proportion to their duration [1]; metaphase is briefer than prophase (chromosomes only pause to align and be checked), so fewer cells are caught in it at any instant [1].

Mark scheme — Q2: Ribosomes on the rough ER synthesise the protein into the ER lumen [1]; vesicles transport it to the Golgi apparatus [1]; the Golgi modifies (e.g. glycosylation) and packages it into secretory vesicles [1]; vesicles move to the cell-surface membrane and fuse [1]; the protein is released by exocytosis [1]. One mark per correct step in order; generic "the Golgi packages proteins" without the vesicle transport scores once only.