Topic A: Cells and Molecules of Life

This compulsory part lays the foundation for the whole DSE course. You first meet the biomolecules that make up living things, then the cell as the basic unit of life, the membrane that controls what enters and leaves, and finally the chemistry that powers cells: enzymes, photosynthesis and respiration. Expect questions that ask you to link structure to function and to interpret experimental data on enzyme activity, diffusion and gas exchange.

Biomolecules: carbohydrates, lipids, proteins

Living matter is built mainly from carbohydrates, lipids, proteins and nucleic acids, together with water and inorganic ions. Carbohydrates such as glucose and starch are made of carbon, hydrogen and oxygen and serve as energy sources and stores; monosaccharides join to form disaccharides and polysaccharides by condensation. Lipids (fats and oils) contain the same elements but far less oxygen, giving more energy per gram, and form fat stores and the basis of membranes. Proteins are polymers of amino acids joined by peptide bonds; their specific three-dimensional shape determines their role as enzymes, antibodies, transport molecules or structural fibres. You should be able to state the simple food tests: Benedict's for reducing sugar, iodine for starch, the emulsion or grease-spot test for lipid, and Biuret for protein, and read the colour changes correctly.

Water and inorganic ions

Water is the most abundant molecule in cells and is essential for life. Because it is a good solvent, most metabolic reactions occur in aqueous solution and dissolved substances are transported in blood, lymph and plant sap. Water has a high specific heat capacity, so it resists temperature change and helps organisms maintain a stable internal temperature. Its high latent heat of vaporization means evaporation, such as sweating or transpiration, has a strong cooling effect. Water is also a reactant in hydrolysis and photosynthesis. Inorganic ions have specific roles: nitrate for making amino acids and proteins, magnesium for chlorophyll, calcium for bones, teeth and clotting, and iron for haemoglobin. A shortage of any of these produces recognisable deficiency symptoms.

Cell structure and microscopy

The cell is the basic structural and functional unit of life. Animal cells have a cell membrane, cytoplasm, a nucleus controlling activities, mitochondria for respiration, and ribosomes for protein synthesis. Plant cells have these plus a cellulose cell wall for support, a large permanent vacuole, and chloroplasts for photosynthesis. You should know the function of each organelle and be able to recognise it in a labelled diagram or electron micrograph. The light microscope shows whole cells and large organelles, while the electron microscope reveals fine internal detail because of its much greater resolving power. Magnification equals image size divided by actual size, and you should be able to calculate actual size and use a scale bar correctly.

Movement across membranes

The cell membrane is partially permeable and controls the exchange of materials. Diffusion is the net movement of particles from a region of higher to lower concentration down a concentration gradient, requiring no energy; it moves oxygen, carbon dioxide and small molecules. Osmosis is the diffusion of water molecules across a partially permeable membrane from a more dilute to a more concentrated solution. Active transport moves substances against the gradient and needs energy from respiration, for example mineral uptake by root hair cells. In experiments, a cell placed in a hypertonic solution loses water and the cytoplasm shrinks (plasmolysis in plant cells), while in a hypotonic solution it gains water and may burst if it is an animal cell. Surface area to volume ratio strongly affects the rate of exchange.

Enzymes and metabolism

Enzymes are biological catalysts, made of protein, that speed up reactions without being used up by lowering the activation energy. Each enzyme has an active site with a specific shape that fits only its substrate, explaining the lock-and-key model and enzyme specificity. Enzyme activity rises with temperature up to an optimum, then falls sharply as the enzyme denatures and the active site loses its shape; activity is also highest at an optimum pH. Increasing substrate concentration raises the rate until all active sites are occupied and the rate levels off. When interpreting graphs, always explain the trend in terms of collisions between enzyme and substrate, and use the word denature, not 'killed', because enzymes are not alive.

Photosynthesis

Photosynthesis is the process by which green plants make glucose from carbon dioxide and water using light energy trapped by chlorophyll, releasing oxygen as a by-product. The word equation is carbon dioxide plus water, in the presence of light and chlorophyll, giving glucose plus oxygen. It occurs in chloroplasts and converts light energy into chemical energy stored in glucose, which can be changed into starch for storage, cellulose, proteins or fats. The rate is affected by limiting factors: light intensity, carbon dioxide concentration and temperature, and at any moment the factor in shortest supply limits the rate. Classic experiments test for starch using iodine after destarching, and show that light, chlorophyll and carbon dioxide are each necessary, using variegated leaves and controls.

Cellular respiration

Respiration releases energy from food, mainly glucose, for cell activities, and the energy is carried by ATP. Aerobic respiration uses oxygen and breaks glucose completely into carbon dioxide and water, releasing a large amount of energy; the word equation is glucose plus oxygen giving carbon dioxide plus water. Anaerobic respiration occurs without oxygen and releases far less energy: in muscle it produces lactic acid causing fatigue and an oxygen debt, while in yeast it produces ethanol and carbon dioxide, the basis of brewing and baking. You should be able to compare the two and design experiments measuring oxygen uptake or carbon dioxide output, for example using germinating seeds and a respirometer, including a control with boiled seeds.

The cell cycle and cell division

Cells reproduce by division so organisms can grow, replace worn-out cells and repair damage. The cell cycle includes interphase, when the cell grows and DNA is copied, followed by division of the nucleus and then the cytoplasm. Mitosis produces two genetically identical daughter cells, each with the same chromosome number as the parent, and is used in growth, repair and asexual reproduction. Meiosis produces four cells with half the chromosome number, the haploid gametes used in sexual reproduction, and introduces genetic variation. You should be able to state the differences between mitosis and meiosis in outline, including the number and genetic make-up of daughter cells, without needing the detailed names of every stage.