WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.351 Unit S B 1: Key concepts in biology. 00:00:10.033 --> 00:00:14.993 Magnification describes how many times larger an image is than the real object. 00:00:15.143 --> 00:00:16.637 It has no unit. 00:00:16.787 --> 00:00:19.513 Write it as, for example, times four hundred. 00:00:22.167 --> 00:00:26.118 Image size equals magnification times actual object size. 00:00:26.268 --> 00:00:35.493 Rearrange this to magnification equals image size divided by actual size, or actual size equals image size divided by magnification. 00:00:38.167 --> 00:00:45.226 For a light microscope, total magnification equals eyepiece magnification times objective magnification. 00:00:45.376 --> 00:00:49.931 A times ten eyepiece with a times forty objective gives times four hundred. 00:00:52.600 --> 00:00:56.671 Use the same units for image and actual size before calculating. 00:00:56.821 --> 00:01:00.018 One millimetre equals one thousand micrometres. 00:01:00.168 --> 00:01:03.546 One micrometre equals one thousand nanometres. 00:01:03.696 --> 00:01:12.677 An image twenty millimetres long of a cell fifty micrometres long has magnification twenty thousand divided by fifty, which equals times four hundred. 00:01:15.333 --> 00:01:20.687 Resolution is the smallest distance between two points that can still be seen as separate points. 00:01:20.837 --> 00:01:23.312 Higher resolution shows finer detail. 00:01:23.462 --> 00:01:29.318 Making an image bigger (increasing magnification) does not necessarily improve its resolution. 00:01:32.000 --> 00:01:37.641 Electron microscopes have greater magnification and resolving power than light microscopes. 00:01:37.791 --> 00:01:42.272 They reveal smaller cell structures, including fine internal details. 00:01:44.933 --> 00:01:53.604 Microscopy practical: place a thin specimen on a slide, add an appropriate stain if needed, and lower a coverslip gently to reduce trapped air. 00:01:53.754 --> 00:01:56.101 Handle slides and stains safely. 00:01:58.767 --> 00:02:01.284 Start with the lowest-power objective. 00:02:01.434 --> 00:02:05.977 Use coarse focus to find the specimen, then fine focus to sharpen it. 00:02:06.127 --> 00:02:10.214 At high power, use fine focus and keep the lens clear of the slide. 00:02:12.867 --> 00:02:18.527 Make a large, clear biological drawing with single lines, no shading and labelled structures. 00:02:18.677 --> 00:02:23.540 Include a scale or magnification; calculate sizes using matching units. 00:02:26.200 --> 00:02:31.032 Animal and plant cells are eukaryotic: their D N A is enclosed in a nucleus. 00:02:31.182 --> 00:02:35.977 Both contain cytoplasm, a cell membrane, ribosomes and mitochondria. 00:02:38.633 --> 00:02:43.379 The nucleus contains genetic material and controls the cell’s activities. 00:02:43.529 --> 00:02:46.865 The cytoplasm is where many chemical reactions occur. 00:02:49.533 --> 00:02:53.476 The cell membrane controls which substances move into and out of the cell. 00:02:53.626 --> 00:02:57.936 Ribosomes are where proteins are made: this is called protein synthesis. 00:03:00.600 --> 00:03:07.372 Mitochondria are the site of most reactions of aerobic respiration, which transfers energy for cell processes. 00:03:10.033 --> 00:03:16.571 A typical photosynthetic plant cell also contains chloroplasts, a permanent vacuole and a cell wall. 00:03:16.721 --> 00:03:21.045 Some plant cells, such as root cells, do not contain chloroplasts. 00:03:21.195 --> 00:03:23.775 Cell structures: simplified overview. 00:03:23.925 --> 00:03:26.997 The surrounding notes explain each structure’s function. 00:03:31.667 --> 00:03:36.221 Chloroplasts contain chlorophyll, which absorbs light for photosynthesis. 00:03:36.371 --> 00:03:40.735 The vacuole contains cell sap and helps maintain pressure inside the cell. 00:03:43.400 --> 00:03:46.917 The plant cell wall is made of cellulose and supports the cell. 00:03:47.067 --> 00:03:51.783 It lies outside the cell membrane; plant cells have both a wall and a membrane. 00:03:54.433 --> 00:03:57.857 Specialised cells have structures suited to their functions. 00:03:58.007 --> 00:04:02.935 Explain an adaptation by linking a structure to the job it helps the cell perform. 00:04:05.600 --> 00:04:14.953 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. 00:04:17.633 --> 00:04:22.539 An egg cell contains plenty of cytoplasm with nutrients for early development. 00:04:22.689 --> 00:04:27.904 Its outer layer changes after fertilisation to help prevent more sperm entering. 00:04:30.567 --> 00:04:38.458 Cells lining the small intestine have microvilli: tiny folds that provide a large surface area for absorbing digested nutrients. 00:04:41.133 --> 00:04:45.531 Ciliated epithelial cells have hair-like cilia that move substances. 00:04:45.681 --> 00:04:50.233 In airways, cilia move mucus containing trapped particles towards the throat. 00:04:52.900 --> 00:04:55.268 Bacteria are prokaryotic cells. 00:04:55.418 --> 00:05:01.793 They have cytoplasm, a cell membrane, ribosomes and a cell wall, but no nucleus or mitochondria. 00:05:04.467 --> 00:05:14.429 Bacterial genetic material includes a large circular loop of D N A in the cytoplasm; some bacteria also contain small D N A rings called plasmids. 00:05:17.100 --> 00:05:21.783 Some bacteria have a flagellum for movement and a slime capsule for protection. 00:05:21.933 --> 00:05:24.417 Their cell walls are not made of cellulose. 00:05:27.100 --> 00:05:30.912 Enzymes are biological catalysts, usually proteins. 00:05:31.062 --> 00:05:37.780 They speed up reactions without being used up, including reactions that build molecules (synthesis) and break them down. 00:05:40.433 --> 00:05:43.608 A substrate is the substance an enzyme acts on. 00:05:43.758 --> 00:05:48.591 It fits into the enzyme’s active site because their shapes match (are complementary). 00:05:48.741 --> 00:05:53.755 This makes the enzyme specific: it usually catalyses one particular reaction. 00:05:53.905 --> 00:05:59.047 The active site is complementary to its substrate; denaturation changes that fit. 00:06:03.700 --> 00:06:09.305 An enzyme, substrate complex forms, the reaction produces products, and the products leave. 00:06:09.455 --> 00:06:12.323 The enzyme can then catalyse another reaction. 00:06:15.000 --> 00:06:18.273 Proteases break proteins down into amino acids. 00:06:18.423 --> 00:06:26.923 Carbohydrases break carbohydrates down into sugars; amylase breaks starch down into maltose, rather than directly producing glucose. 00:06:29.600 --> 00:06:33.265 Lipases break lipids down into fatty acids and glycerol. 00:06:33.415 --> 00:06:38.939 Synthesis reactions join smaller molecules to make larger biological molecules. 00:06:41.600 --> 00:06:49.235 Increasing temperature initially increases reaction rate because particles have more kinetic energy and collide more often. 00:06:49.385 --> 00:06:54.323 The optimum temperature gives the fastest rate for that enzyme under those conditions. 00:06:57.000 --> 00:07:03.462 At high temperatures an enzyme may denature: its active site changes shape so the substrate no longer fits. 00:07:03.612 --> 00:07:08.283 Low temperature usually slows activity rather than denaturing the enzyme. 00:07:10.933 --> 00:07:13.471 Each enzyme has an optimum P H. 00:07:13.621 --> 00:07:20.147 A P H far from the optimum can change the active site and reduce activity or denature the enzyme. 00:07:22.800 --> 00:07:28.387 Increasing substrate concentration raises the rate until all active sites are occupied. 00:07:28.537 --> 00:07:31.831 Beyond this point, enzyme availability limits the rate. 00:07:34.500 --> 00:07:41.380 Reaction rate equals amount of product formed divided by time, or amount of substrate used divided by time. 00:07:41.530 --> 00:07:46.617 For a fixed endpoint, relative rate can be estimated as one divided by time. 00:07:46.767 --> 00:07:49.604 With time in seconds, its unit is per second. 00:07:52.267 --> 00:07:56.767 Enzyme practical: investigate P H using amylase and starch. 00:07:56.917 --> 00:08:03.028 Use buffer solutions to change P H and sample at regular intervals onto iodine on a spotting tile. 00:08:05.700 --> 00:08:08.546 Iodine turns blue-black if starch remains. 00:08:08.696 --> 00:08:12.719 When a sample stays orange-brown, the starch has been broken down. 00:08:12.869 --> 00:08:15.322 Record the time to reach this endpoint. 00:08:18.000 --> 00:08:25.763 For a fair P H investigation, keep temperature, enzyme and starch concentrations, volumes and sampling intervals constant. 00:08:25.913 --> 00:08:32.301 Repeat measurements, calculate a mean and compare rates; use eye protection and follow safe handling guidance. 00:08:34.967 --> 00:08:42.922 Diffusion is the overall (net) movement of particles from a region of higher concentration to a region of lower concentration. 00:08:43.072 --> 00:08:45.995 This is movement down a concentration gradient. 00:08:46.145 --> 00:08:48.897 It happens because particles move randomly. 00:08:51.567 --> 00:08:58.275 Particles move randomly in both directions, but more move from high to low concentration than the other way round. 00:08:58.425 --> 00:09:06.199 Once the concentrations are equal (at equilibrium), particles still move, but there is no overall movement in either direction. 00:09:08.867 --> 00:09:16.812 Diffusion is faster with a steeper concentration gradient, higher temperature, larger surface area or shorter diffusion distance. 00:09:16.962 --> 00:09:19.873 It does not require energy from respiration. 00:09:22.533 --> 00:09:34.031 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). 00:09:34.181 --> 00:09:38.917 Small aqua dots represent water; large navy dots represent solute. 00:09:39.067 --> 00:09:42.355 Water moves both ways, with a net movement to the right. 00:09:47.033 --> 00:09:51.289 A partially permeable membrane lets some particles through but not others. 00:09:51.439 --> 00:09:56.307 In osmosis, water crosses the membrane; it is not the net movement of solute. 00:09:58.967 --> 00:10:04.887 In a dilute solution, water enters a plant cell by osmosis and it becomes firm (turgid). 00:10:05.037 --> 00:10:07.115 The cell wall stops it bursting. 00:10:07.265 --> 00:10:11.937 In a concentrated solution, water leaves and the cell becomes limp (flaccid). 00:10:12.087 --> 00:10:17.463 If it loses enough water, the cell membrane pulls away from the wall: this is plasmolysis. 00:10:20.133 --> 00:10:25.519 An animal cell may swell and burst when too much water enters, or shrink when water leaves. 00:10:25.669 --> 00:10:28.079 It has no cell wall to resist swelling. 00:10:30.733 --> 00:10:39.949 Active transport moves substances against their concentration gradient, from lower to higher concentration, using carrier proteins in the cell membrane. 00:10:42.600 --> 00:10:46.489 Active transport requires energy transferred by respiration. 00:10:46.639 --> 00:10:53.911 For example, root hair cells can absorb mineral ions from dilute soil solutions against a concentration gradient. 00:10:56.567 --> 00:10:59.319 Cut potato cylinders of equal size. 00:10:59.469 --> 00:11:06.401 Measure their initial masses, place them in a range of sucrose concentrations, and leave them for the same amount of time. 00:11:09.067 --> 00:11:14.921 Remove the cylinders, gently blot off surface solution in the same way, and measure final mass. 00:11:15.071 --> 00:11:21.647 Keep temperature, solution volume and potato size/type controlled; take care with cutting equipment. 00:11:24.300 --> 00:11:31.531 Percentage change in mass equals final mass minus initial mass, divided by initial mass, times one hundred. 00:11:31.681 --> 00:11:33.687 Subtract the masses first. 00:11:33.837 --> 00:11:36.311 A positive result means mass gain. 00:11:36.461 --> 00:11:38.903 A negative result means mass loss. 00:11:41.567 --> 00:11:49.437 A potato increasing from two point zero grams to two point three grams has a percentage mass change of plus fifteen percent. 00:11:49.587 --> 00:11:52.178 Water entered its cells by osmosis. 00:11:52.328 --> 00:11:58.076 A decrease from two point zero grams to one point eight grams gives minus ten percent. 00:12:00.733 --> 00:12:03.971 Repeat each concentration and calculate a mean. 00:12:04.121 --> 00:12:12.951 Plot percentage mass change against sucrose concentration; where the graph crosses 0 percent estimates the concentration giving no net water movement. 00:12:15.633 --> 00:12:21.243 Prepare a food solution by crushing a sample with distilled water and filtering if necessary. 00:12:21.393 --> 00:12:25.503 Use separate portions for each test so reagents do not interfere. 00:12:28.167 --> 00:12:33.393 Iodine solution changes from orange-brown to blue-black when starch is present. 00:12:33.543 --> 00:12:36.070 A negative result stays orange-brown. 00:12:36.220 --> 00:12:38.695 Use separate samples for the four tests. 00:12:43.367 --> 00:12:47.795 For reducing sugars, add Benedict’s solution and heat in a hot water bath. 00:12:47.945 --> 00:12:54.657 A positive result forms a green, yellow, orange or brick-red precipitate; a negative result remains blue. 00:12:57.333 --> 00:13:04.838 For protein, add the alkaline Biuret reagents: potassium hydroxide, then a small amount of copper sulfate solution. 00:13:04.988 --> 00:13:11.824 A lilac or purple colour is a positive result, showing the peptide bonds that join amino acids in proteins. 00:13:11.974 --> 00:13:14.117 A negative result stays blue. 00:13:16.800 --> 00:13:20.355 For lipids, shake the sample with ethanol, then add water. 00:13:20.505 --> 00:13:25.539 A cloudy white emulsion indicates lipid; a clear mixture is a negative result. 00:13:25.689 --> 00:13:28.841 Ethanol is flammable: keep it away from flames. 00:13:31.500 --> 00:13:39.243 Use known positive samples and distilled-water negative controls, equal sample volumes, clean apparatus and consistent heating. 00:13:39.393 --> 00:13:45.079 Wear eye protection; follow the teacher’s risk assessment for the alkaline reagents and hot water. 00:13:47.733 --> 00:13:52.621 Burn a weighed food sample beneath a boiling tube containing a known mass of water. 00:13:52.771 --> 00:13:58.561 Measure the water’s temperature before and after heating; reweigh the food to find the mass actually burned. 00:14:01.233 --> 00:14:08.037 Energy transferred to water equals mass of water times specific heat capacity times temperature rise. 00:14:08.187 --> 00:14:15.589 If mass is in grams and water’s heat capacity is 4.2 joules per gram per degrees Celsius, the result is joules. 00:14:18.267 --> 00:14:24.036 Divide the energy gained by the water by the mass of food burned to estimate energy per gram. 00:14:24.186 --> 00:14:38.190 For 20 grams water heated by 15 degrees Celsius using 0.5 grams food, energy equals 1260 joules and energy per gram equals 2520 joules per gram. 00:14:40.867 --> 00:14:48.439 The estimate is usually too low: energy heats the air and apparatus, combustion may be incomplete, and water may evaporate. 00:14:48.589 --> 00:14:54.626 A lid, insulation and a draught shield reduce some losses; repeat measurements and compare foods fairly. 00:14:57.300 --> 00:14:59.967 That completes Key concepts in biology. 00:15:00.117 --> 00:15:03.462 Revisit the notes and test yourself on the revision website.