CB1 · Key concepts in biologyTopic 1 — Key concepts in biology
Microscopes, cells, enzymes and transport.
Revise the key ideas
Microscopes and magnification
Magnification describes how many times larger an image is than the real object. It has no unit: write it as, for example, ×400.
Image size = magnification × actual object size. Rearrange this to magnification = image size ÷ actual size, or actual size = image size ÷ magnification.
For a light microscope, total magnification = eyepiece magnification × objective magnification. A ×10 eyepiece with a ×40 objective gives ×400.
Use the same units for image and actual size before calculating: 1 mm = 1,000 µm; 1 µm = 1,000 nm. An image 20 mm long of a cell 50 µm long has magnification 20,000 ÷ 50 = ×400.
Resolution is the smallest distance between two points that can still be seen as separate points. Higher resolution shows finer detail. Making an image bigger (increasing magnification) does not necessarily improve its resolution.
Electron microscopes have greater magnification and resolving power than light microscopes. They reveal smaller cell structures, including fine internal details.
Microscopy practical: place a thin specimen on a slide, add an appropriate stain if needed, and lower a coverslip gently to reduce trapped air. Handle slides and stains safely.
Start with the lowest-power objective. Use coarse focus to find the specimen, then fine focus to sharpen it. At high power, use fine focus and keep the lens clear of the slide.
Make a large, clear biological drawing with single lines, no shading and labelled structures. Include a scale or magnification; calculate sizes using matching units.
Animal and plant cells
Animal and plant cells are eukaryotic: their DNA is enclosed in a nucleus. Both contain cytoplasm, a cell membrane, ribosomes and mitochondria.
The nucleus contains genetic material and controls the cell’s activities. The cytoplasm is where many chemical reactions occur.
The cell membrane controls which substances move into and out of the cell. Ribosomes are where proteins are made: this is called protein synthesis.
Mitochondria are the site of most reactions of aerobic respiration, which transfers energy for cell processes.
A typical photosynthetic plant cell also contains chloroplasts, a permanent vacuole and a cell wall. Some plant cells, such as root cells, do not contain chloroplasts.Cell structures: simplified overview. The surrounding notes explain each structure’s function.
Chloroplasts contain chlorophyll, which absorbs light for photosynthesis. The vacuole contains cell sap and helps maintain pressure inside the cell.
The plant cell wall is made of cellulose and supports the cell. It lies outside the cell membrane; plant cells have both a wall and a membrane.
Specialised cells and bacteria
Specialised cells have structures suited to their functions. Explain an adaptation by linking a structure to the job it helps the cell perform.
A sperm cell has a tail for movement, many mitochondria to supply energy, and an acrosome containing enzymes that help it penetrate the egg’s outer layers.
An egg cell contains plenty of cytoplasm with nutrients for early development. Its outer layer changes after fertilisation to help prevent more sperm entering.
Cells lining the small intestine have microvilli: tiny folds that provide a large surface area for absorbing digested nutrients.
Ciliated epithelial cells have hair-like cilia that move substances. In airways, cilia move mucus containing trapped particles towards the throat.
Bacteria are prokaryotic cells. They have cytoplasm, a cell membrane, ribosomes and a cell wall, but no nucleus or mitochondria.
Bacterial genetic material includes a large circular loop of DNA in the cytoplasm; some bacteria also contain small DNA rings called plasmids.
Some bacteria have a flagellum for movement and a slime capsule for protection. Their cell walls are not made of cellulose.
Enzymes and reaction rates
Enzymes are biological catalysts, usually proteins. They speed up reactions without being used up, including reactions that build molecules (synthesis) and break them down.
A substrate is the substance an enzyme acts on. It fits into the enzyme’s active site because their shapes match (are complementary). This makes the enzyme specific: it usually catalyses one particular reaction.The active site is complementary to its substrate; denaturation changes that fit.
An enzyme–substrate complex forms, the reaction produces products, and the products leave. The enzyme can then catalyse another reaction.
Proteases break proteins down into amino acids. Carbohydrases break carbohydrates down into sugars; amylase breaks starch down into maltose, rather than directly producing glucose.
Lipases break lipids down into fatty acids and glycerol. Synthesis reactions join smaller molecules to make larger biological molecules.
Increasing temperature initially increases reaction rate because particles have more kinetic energy and collide more often. The optimum temperature gives the fastest rate for that enzyme under those conditions.
At high temperatures an enzyme may denature: its active site changes shape so the substrate no longer fits. Low temperature usually slows activity rather than denaturing the enzyme.
Each enzyme has an optimum pH. A pH far from the optimum can change the active site and reduce activity or denature the enzyme.
Increasing substrate concentration raises the rate until all active sites are occupied. Beyond this point, enzyme availability limits the rate.
Reaction rate = amount of product formed ÷ time, or amount of substrate used ÷ time. For a fixed endpoint, relative rate can be estimated as 1 ÷ time; with time in seconds its unit is s⁻¹.
Enzyme practical: investigate pH using amylase and starch. Use buffer solutions to change pH and sample at regular intervals onto iodine on a spotting tile.
Iodine turns blue-black if starch remains. When a sample stays orange-brown, the starch has been broken down. Record the time to reach this endpoint.
For a fair pH investigation, keep temperature, enzyme and starch concentrations, volumes and sampling intervals constant. Repeat measurements, calculate a mean and compare rates; use eye protection and follow safe handling guidance.
Diffusion, osmosis and active transport
Diffusion is the overall (net) movement of particles from a region of higher concentration to a region of lower concentration. This is movement down a concentration gradient. It happens because particles move randomly.
Particles move randomly in both directions, but more move from high to low concentration than the other way round. Once the concentrations are equal (at equilibrium), particles still move, but there is no overall movement in either direction.
Diffusion is faster with a steeper concentration gradient, higher temperature, larger surface area or shorter diffusion distance. It does not require energy from respiration.
Osmosis is the net movement of water through a partially permeable membrane from a more dilute solution (higher water concentration) to a more concentrated solution (lower water concentration).Small aqua dots represent water; large navy dots represent solute. Water moves both ways, with a net movement to the right.
A partially permeable membrane lets some particles through but not others. In osmosis, water crosses the membrane; it is not the net movement of solute.
In a dilute solution, water enters a plant cell by osmosis and it becomes firm (turgid). The cell wall stops it bursting. In a concentrated solution, water leaves and the cell becomes limp (flaccid). If it loses enough water, the cell membrane pulls away from the wall: this is plasmolysis.
An animal cell may swell and burst when too much water enters, or shrink when water leaves. It has no cell wall to resist swelling.
Active transport moves substances against their concentration gradient, from lower to higher concentration, using carrier proteins in the cell membrane.
Active transport requires energy transferred by respiration. For example, root hair cells can absorb mineral ions from dilute soil solutions against a concentration gradient.
Osmosis practical and calculations
Cut potato cylinders of equal size. Measure their initial masses, place them in a range of sucrose concentrations, and leave them for the same amount of time.
Remove the cylinders, gently blot off surface solution in the same way, and measure final mass. Keep temperature, solution volume and potato size/type controlled; take care with cutting equipment.
Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100. A positive result means mass gain; a negative result means mass loss.
A potato increasing from 2.0 g to 2.3 g has a percentage mass change of +15%. Water entered its cells by osmosis. A decrease from 2.0 g to 1.8 g gives −10%.
Repeat each concentration and calculate a mean. Plot percentage mass change against sucrose concentration; where the graph crosses 0% estimates the concentration giving no net water movement.
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