Lesson 1: Cell Biology

Every living thing on Earth — from the bacteria in your gut to the oak trees of Sherwood Forest — is built from cells. Cell biology is the foundation of the entire GCSE biology course: once you understand what cells are, how they are specialised, and how substances move in and out of them, everything from digestion to disease starts to make sense. This lesson covers the two great cell types (eukaryotic and prokaryotic), the key subcellular structures and their jobs, how microscopes revealed this hidden world, how cells divide by mitosis, how stem cells work, and how substances cross cell membranes by diffusion, osmosis and active transport.

Eukaryotic and Prokaryotic Cells

Animal and plant cells are eukaryotic cells: their DNA is enclosed in a nucleus, and they contain membrane-bound organelles such as mitochondria and (in plants) chloroplasts. Eukaryotic cells are relatively large — typically 10 to 40 micrometres across — and far more complex than bacteria. Fungi, including yeast used in baking and brewing, are also eukaryotes. For GCSE you must be able to label the subcellular structures of animal and plant cells and explain what each does.

Bacterial cells are prokaryotic: much smaller (around 1 to 2 micrometres), with no true nucleus. Instead their genetic material is a single circular loop of DNA floating free in the cytoplasm, often accompanied by small rings called plasmids. Bacterial cells may also have a slime capsule for protection, a rigid cell wall (different in chemistry from plant walls), and flagella — whip-like tails for swimming. Some bacteria carry out photosynthesis using pigments in their cytoplasm rather than chloroplasts.

The key comparison to master: eukaryotic cells have a nucleus and membrane-bound organelles; prokaryotic cells do not. Bacteria are prokaryotes; animals, plants and fungi are eukaryotes. Exam questions often ask you to "compare and contrast" — structure your answer as similarities (both have cytoplasm, cell membrane, DNA) versus differences (nucleus, organelles, size, cell wall type). Viruses are not cells at all — they are smaller still and can only reproduce inside a host cell, which is why antibiotics do not work on them.

Subcellular Structures and Their Functions

The nucleus contains the genetic material — DNA organised into chromosomes — and controls all cell activities, including which proteins are made. The cytoplasm is the jelly-like substance where most chemical reactions happen; it is the site of the first stages of respiration. The cell membrane controls what enters and leaves the cell, holding the cell together while remaining partially permeable. Mitochondria are where aerobic respiration releases energy from glucose — cells with high energy demands, like muscle cells, contain many mitochondria.

Plant cells have three extra structures. The rigid cell wall, made of cellulose, strengthens and supports the cell, giving plants their shape without bones. Chloroplasts contain chlorophyll, the green pigment that absorbs light for photosynthesis. The permanent vacuole, filled with cell sap, pushes the cytoplasm against the wall, keeping the cell turgid — this is why a well-watered plant stands upright and a wilted one flops. When learning these, always link structure to function: the exam rewards explanations like "large surface area increases the rate of diffusion" rather than bare facts.

Specialised cells show adaptation beautifully. Sperm cells have a long tail for swimming, many mitochondria for energy, and enzymes in the head to digest through the egg membrane. Root hair cells have a huge surface area to absorb water and minerals. Palisade leaf cells are packed with chloroplasts near the leaf surface to catch light. Red blood cells have no nucleus, maximising space for haemoglobin, and a biconcave disc shape for a large surface area. Phloem and xylem cells transport substances in plants. Learn two or three examples in detail and the general principle of "structure suits function".

Microscopy — Seeing the Invisible

The light microscope was the tool that revealed cells: Robert Hooke coined the word "cell" in 1665 after viewing cork. Modern school light microscopes magnify up to about 1,500 times and can resolve objects roughly 0.2 micrometres apart — enough to see nuclei, chloroplasts and cell walls, but not smaller organelles. Electron microscopes, developed in the 1930s, use beams of electrons instead of light and achieve magnifications of millions of times, revealing mitochondria, ribosomes and membrane detail.

You must master magnification calculations: magnification = image size ÷ real size. If a cell appears 20 mm across in a drawing but is really 0.02 mm, the magnification is ×1,000. Keep units consistent — convert millimetres to micrometres (1 mm = 1,000 µm) before dividing. A common exam trap is mixing units; always write the unit conversion explicitly in your working. Higher-tier questions may ask for standard form answers.

The required practical on microscopy asks you to prepare a stained slide of onion epidermis cells, observe them under the microscope, and draw what you see with accurate labels. Practical tips: use a thin peel of onion skin, add a drop of iodine stain, lower the coverslip at an angle to avoid air bubbles, and start on the lowest magnification to find your sample before zooming in. Your drawing should be large, clear, with ruled label lines — artistic talent is not required, accuracy is.

Cell Division — Mitosis and the Cell Cycle

Growth and repair require new cells, produced by mitosis — nuclear division that creates two genetically identical daughter cells. Before mitosis, during interphase, the cell grows and copies its DNA so each new cell receives a full set of chromosomes. The chromosomes then condense, line up, and are pulled apart into the two new nuclei. The result: two diploid cells, genetically identical to each other and to the parent. This is essential for growth, replacing damaged cells, and asexual reproduction.

The cell cycle has three stages: interphase (growth and DNA replication — by far the longest stage), mitosis (nuclear division), and cytokinesis (the cytoplasm divides). In exam questions, if asked why cells spend most time in interphase, answer: because copying the entire genome accurately takes time, and errors must be avoided. Cancer is essentially mitosis out of control — damaged cells divide without stopping, forming tumours.

Stem cells are undifferentiated cells that can divide to produce more stem cells and differentiate into specialised cells. Human embryos contain embryonic stem cells that can become any cell type — offering enormous medical potential, for example growing replacement nerve cells for spinal injuries or insulin-producing cells for type 1 diabetes. Adult bone marrow stem cells can become blood cells and are already used in treatments. Plant meristems contain stem cells that can produce clones of whole plants quickly and cheaply. The use of embryonic stem cells raises ethical and religious objections, which you should be able to discuss in a balanced 6-mark answer.

Transport Across Membranes

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration, down the concentration gradient. It needs no energy. Examples: oxygen diffusing from the air into the blood in the lungs, and carbon dioxide diffusing out; urea moving from blood into the kidney tubules. The rate of diffusion increases with a larger surface area, a steeper concentration gradient, and a shorter diffusion distance — this explains why lungs have millions of tiny alveoli and why the small intestine has villi.

Osmosis is the diffusion of water across a partially permeable membrane, from a dilute solution (high water concentration) to a concentrated solution (low water concentration). Plant roots absorb water by osmosis; animal cells placed in pure water swell and may burst, while in concentrated solution they shrivel. This is the basis of the required practical investigating osmosis in potato cylinders: you measure the mass change of potato pieces in different sugar concentrations and calculate percentage change.

Active transport moves substances from low to high concentration — against the gradient — and therefore requires energy from respiration. Root hair cells use it to absorb mineral ions from very dilute soil water; gut cells use it to absorb glucose even when its concentration in the gut is lower than in the blood. Exam questions love asking you to compare the three: diffusion and osmosis are passive (no energy), active transport needs energy; osmosis specifically concerns water across a partially permeable membrane. A classic 6-mark question asks you to explain how the small intestine is adapted for absorption — combine large surface area (villi and microvilli), thin walls (short diffusion distance), rich blood supply (maintains gradient), and active transport for glucose.

Worked example — analysing the osmosis required practical

A student placed potato cylinders of initial mass 4.80 g into three sugar solutions for 24 hours, blotted them dry and reweighed them. In distilled water (0.0 mol/dm³) the final mass was 5.28 g; in 0.4 mol/dm³ it was 4.80 g; in 0.8 mol/dm³ it was 4.42 g. Percentage change = (change ÷ initial) × 100, so the results are +10.0%, 0.0% and −7.9%. Plotted against concentration, the line crosses zero at about 0.4 mol/dm³ — that is the concentration effectively equal to the potato cells' own solute concentration, where water enters and leaves at equal rates. Below it, water moves into the cells by osmosis (mass rises); above it, water leaves (mass falls).

Evaluation points examiners reward: cylinders must be blotted the same way each time (surface water adds false mass), the same potato and cylinder size control surface area and tissue type, and repeats allow a mean percentage change to be calculated with anomalies excluded.

Common mistakes

• Magnification units. Mixing mm and µm in magnification = image ÷ real size. Convert first: 1 mm = 1,000 µm.
• Osmosis direction. Water moves from a dilute solution to a more concentrated solution across a partially permeable membrane — describe it as movement from higher water concentration to lower water concentration.
• Active transport energy. It is powered by energy released in respiration, not by "the cell" vaguely — name the source.

Exam Experience

Question 1. State two differences between a bacterial cell and an animal cell. [2 marks]

Question 2. A microscope image of a cell measures 36 mm across. The real cell is 0.012 mm across. Calculate the magnification of the image. Show your working. [3 marks]

Question 3. Explain how a palisade cell and a root hair cell are each adapted to their functions. [6 marks]

Mark scheme

Q1: any two of — bacteria have no (true) nucleus, DNA is a single loop/plasmids; bacteria have a cell wall of a different material / no mitochondria / smaller ribosomes. One mark each.

Q2: magnification = image size ÷ real size = 36 ÷ 0.012 (1 mark for correct substitution with matching units) = ×3,000 (1 mark; a further mark for correct working if the final value slips).

Q3: Level 3 (5–6 marks): a detailed, logically ordered explanation linking at least four adaptations to their effects — palisade cells: many chloroplasts concentrated near the leaf surface for maximum light absorption for photosynthesis; large vacuole pushing chloroplasts to the edges; elongated shape allowing tight packing. Root hair cells: long hair-like extension increasing surface area for absorption of water by osmosis and mineral ions by active transport; thin cell wall shortening the pathway; many mitochondria releasing energy for active transport. Level 2 (3–4 marks): some adaptations linked to functions, but coverage is one-sided or the links are incomplete. Level 1 (1–2 marks): isolated facts naming features or functions without explaining how they help. Indicative content must match the feature to the function — "lots of chloroplasts" alone stays at Level 1–2.