WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.331 Unit C B 1: Key concepts in biology. 00:00:10.000 --> 00:00:14.960 Magnification describes how many times larger an image is than the real object. 00:00:15.110 --> 00:00:16.603 It has no unit. 00:00:16.753 --> 00:00:19.480 Write it as, for example, times four hundred. 00:00:22.133 --> 00:00:26.085 Image size equals magnification times actual object size. 00:00:26.235 --> 00:00:35.460 Rearrange this to magnification equals image size divided by actual size, or actual size equals image size divided by magnification. 00:00:38.133 --> 00:00:45.193 For a light microscope, total magnification equals eyepiece magnification times objective magnification. 00:00:45.343 --> 00:00:49.897 A times ten eyepiece with a times forty objective gives times four hundred. 00:00:52.567 --> 00:00:56.637 Use the same units for image and actual size before calculating. 00:00:56.787 --> 00:00:59.985 One millimetre equals one thousand micrometres. 00:01:00.135 --> 00:01:03.513 One micrometre equals one thousand nanometres. 00:01:03.663 --> 00:01:12.643 An image twenty millimetres long of a cell fifty micrometres long has magnification twenty thousand divided by fifty, which equals times four hundred. 00:01:15.300 --> 00:01:20.654 Resolution is the smallest distance between two points that can still be seen as separate points. 00:01:20.804 --> 00:01:23.279 Higher resolution shows finer detail. 00:01:23.429 --> 00:01:29.285 Making an image bigger (increasing magnification) does not necessarily improve its resolution. 00:01:31.967 --> 00:01:37.608 Electron microscopes have greater magnification and resolving power than light microscopes. 00:01:37.758 --> 00:01:42.239 They reveal smaller cell structures, including fine internal details. 00:01:44.900 --> 00:01:53.571 Microscopy practical: place a thin specimen on a slide, add an appropriate stain if needed, and lower a coverslip gently to reduce trapped air. 00:01:53.721 --> 00:01:56.067 Handle slides and stains safely. 00:01:58.733 --> 00:02:01.229 Start with the lowest-power objective. 00:02:01.379 --> 00:02:05.923 Use coarse focus to find the specimen, then fine focus to sharpen it. 00:02:06.073 --> 00:02:10.159 At high power, use fine focus and keep the lens clear of the slide. 00:02:12.833 --> 00:02:18.493 Make a large, clear biological drawing with single lines, no shading and labelled structures. 00:02:18.643 --> 00:02:23.507 Include a scale or magnification; calculate sizes using matching units. 00:02:26.167 --> 00:02:30.999 Animal and plant cells are eukaryotic: their D N A is enclosed in a nucleus. 00:02:31.149 --> 00:02:35.944 Both contain cytoplasm, a cell membrane, ribosomes and mitochondria. 00:02:38.600 --> 00:02:43.345 The nucleus contains genetic material and controls the cell’s activities. 00:02:43.495 --> 00:02:46.832 The cytoplasm is where many chemical reactions occur. 00:02:49.500 --> 00:02:53.443 The cell membrane controls which substances move into and out of the cell. 00:02:53.593 --> 00:02:57.903 Ribosomes are where proteins are made: this is called protein synthesis. 00:03:00.567 --> 00:03:07.339 Mitochondria are the site of most reactions of aerobic respiration, which transfers energy for cell processes. 00:03:10.000 --> 00:03:16.537 A typical photosynthetic plant cell also contains chloroplasts, a permanent vacuole and a cell wall. 00:03:16.687 --> 00:03:21.011 Some plant cells, such as root cells, do not contain chloroplasts. 00:03:21.161 --> 00:03:23.741 Cell structures: simplified overview. 00:03:23.891 --> 00:03:26.963 The surrounding notes explain each structure’s function. 00:03:31.633 --> 00:03:36.188 Chloroplasts contain chlorophyll, which absorbs light for photosynthesis. 00:03:36.338 --> 00:03:40.702 The vacuole contains cell sap and helps maintain pressure inside the cell. 00:03:43.367 --> 00:03:46.884 The plant cell wall is made of cellulose and supports the cell. 00:03:47.034 --> 00:03:51.749 It lies outside the cell membrane; plant cells have both a wall and a membrane. 00:03:54.400 --> 00:03:57.823 Specialised cells have structures suited to their functions. 00:03:57.973 --> 00:04:02.902 Explain an adaptation by linking a structure to the job it helps the cell perform. 00:04:05.567 --> 00:04:14.920 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. 00:04:17.600 --> 00:04:22.506 An egg cell contains plenty of cytoplasm with nutrients for early development. 00:04:22.656 --> 00:04:27.871 Its outer layer changes after fertilisation to help prevent more sperm entering. 00:04:30.533 --> 00:04:38.425 Cells lining the small intestine have microvilli: tiny folds that provide a large surface area for absorbing digested nutrients. 00:04:41.100 --> 00:04:45.498 Ciliated epithelial cells have hair-like cilia that move substances. 00:04:45.648 --> 00:04:50.199 In airways, cilia move mucus containing trapped particles towards the throat. 00:04:52.867 --> 00:04:55.235 Bacteria are prokaryotic cells. 00:04:55.385 --> 00:05:01.770 They have cytoplasm, a cell membrane, ribosomes and a cell wall, but no nucleus or mitochondria. 00:05:04.433 --> 00:05:14.395 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. 00:05:17.067 --> 00:05:21.749 Some bacteria have a flagellum for movement and a slime capsule for protection. 00:05:21.899 --> 00:05:24.395 Their cell walls are not made of cellulose. 00:05:27.067 --> 00:05:30.879 Enzymes are biological catalysts, usually proteins. 00:05:31.029 --> 00:05:37.747 They speed up reactions without being used up, including reactions that build molecules (synthesis) and break them down. 00:05:40.400 --> 00:05:43.575 A substrate is the substance an enzyme acts on. 00:05:43.725 --> 00:05:48.558 It fits into the enzyme’s active site because their shapes match (are complementary). 00:05:48.708 --> 00:05:53.722 This makes the enzyme specific: it usually catalyses one particular reaction. 00:05:53.872 --> 00:05:59.014 The active site is complementary to its substrate; denaturation changes that fit. 00:06:03.667 --> 00:06:09.271 An enzyme, substrate complex forms, the reaction produces products, and the products leave. 00:06:09.421 --> 00:06:12.290 The enzyme can then catalyse another reaction. 00:06:14.967 --> 00:06:18.239 Proteases break proteins down into amino acids. 00:06:18.389 --> 00:06:26.889 Carbohydrases break carbohydrates down into sugars; amylase breaks starch down into maltose, rather than directly producing glucose. 00:06:29.567 --> 00:06:33.232 Lipases break lipids down into fatty acids and glycerol. 00:06:33.382 --> 00:06:38.905 Synthesis reactions join smaller molecules to make larger biological molecules. 00:06:41.567 --> 00:06:49.181 Increasing temperature initially increases reaction rate because particles have more kinetic energy and collide more often. 00:06:49.331 --> 00:06:54.268 The optimum temperature gives the fastest rate for that enzyme under those conditions. 00:06:56.933 --> 00:07:03.395 At high temperatures an enzyme may denature: its active site changes shape so the substrate no longer fits. 00:07:03.545 --> 00:07:08.217 Low temperature usually slows activity rather than denaturing the enzyme. 00:07:10.867 --> 00:07:13.404 Each enzyme has an optimum P H. 00:07:13.554 --> 00:07:20.080 A P H far from the optimum can change the active site and reduce activity or denature the enzyme. 00:07:22.733 --> 00:07:28.321 Increasing substrate concentration raises the rate until all active sites are occupied. 00:07:28.471 --> 00:07:31.765 Beyond this point, enzyme availability limits the rate. 00:07:34.433 --> 00:07:41.313 Reaction rate equals amount of product formed divided by time, or amount of substrate used divided by time. 00:07:41.463 --> 00:07:46.550 For a fixed endpoint, relative rate can be estimated as one divided by time. 00:07:46.700 --> 00:07:49.537 With time in seconds, its unit is per second. 00:07:52.200 --> 00:07:56.701 Enzyme practical: investigate P H using amylase and starch. 00:07:56.851 --> 00:08:02.961 Use buffer solutions to change P H and sample at regular intervals onto iodine on a spotting tile. 00:08:05.633 --> 00:08:08.479 Iodine turns blue-black if starch remains. 00:08:08.629 --> 00:08:12.652 When a sample stays orange-brown, the starch has been broken down. 00:08:12.802 --> 00:08:15.255 Record the time to reach this endpoint. 00:08:17.933 --> 00:08:25.697 For a fair P H investigation, keep temperature, enzyme and starch concentrations, volumes and sampling intervals constant. 00:08:25.847 --> 00:08:32.235 Repeat measurements, calculate a mean and compare rates; use eye protection and follow safe handling guidance. 00:08:34.900 --> 00:08:42.855 Diffusion is the overall (net) movement of particles from a region of higher concentration to a region of lower concentration. 00:08:43.005 --> 00:08:45.928 This is movement down a concentration gradient. 00:08:46.078 --> 00:08:48.851 It happens because particles move randomly. 00:08:51.533 --> 00:08:58.241 Particles move randomly in both directions, but more move from high to low concentration than the other way round. 00:08:58.391 --> 00:09:06.166 Once the concentrations are equal (at equilibrium), particles still move, but there is no overall movement in either direction. 00:09:08.833 --> 00:09:16.769 Diffusion is faster with a steeper concentration gradient, higher temperature, larger surface area or shorter diffusion distance. 00:09:16.919 --> 00:09:19.829 It does not require energy from respiration. 00:09:22.500 --> 00:09:33.977 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). 00:09:34.127 --> 00:09:38.862 Small aqua dots represent water; large navy dots represent solute. 00:09:39.012 --> 00:09:42.301 Water moves both ways, with a net movement to the right. 00:09:46.967 --> 00:09:51.223 A partially permeable membrane lets some particles through but not others. 00:09:51.373 --> 00:09:56.241 In osmosis, water crosses the membrane; it is not the net movement of solute. 00:09:58.900 --> 00:10:04.820 In a dilute solution, water enters a plant cell by osmosis and it becomes firm (turgid). 00:10:04.970 --> 00:10:07.048 The cell wall stops it bursting. 00:10:07.198 --> 00:10:11.861 In a concentrated solution, water leaves and the cell becomes limp (flaccid). 00:10:12.011 --> 00:10:17.386 If it loses enough water, the cell membrane pulls away from the wall: this is plasmolysis. 00:10:20.067 --> 00:10:25.452 An animal cell may swell and burst when too much water enters, or shrink when water leaves. 00:10:25.602 --> 00:10:28.013 It has no cell wall to resist swelling. 00:10:30.667 --> 00:10:39.882 Active transport moves substances against their concentration gradient, from lower to higher concentration, using carrier proteins in the cell membrane. 00:10:42.533 --> 00:10:46.422 Active transport requires energy transferred by respiration. 00:10:46.572 --> 00:10:53.845 For example, root hair cells can absorb mineral ions from dilute soil solutions against a concentration gradient. 00:10:56.500 --> 00:10:59.252 Cut potato cylinders of equal size. 00:10:59.402 --> 00:11:06.335 Measure their initial masses, place them in a range of sucrose concentrations, and leave them for the same amount of time. 00:11:09.000 --> 00:11:14.855 Remove the cylinders, gently blot off surface solution in the same way, and measure final mass. 00:11:15.005 --> 00:11:21.581 Keep temperature, solution volume and potato size/type controlled; take care with cutting equipment. 00:11:24.233 --> 00:11:31.465 Percentage change in mass equals final mass minus initial mass, divided by initial mass, times one hundred. 00:11:31.615 --> 00:11:33.620 Subtract the masses first. 00:11:33.770 --> 00:11:36.245 A positive result means mass gain. 00:11:36.395 --> 00:11:38.837 A negative result means mass loss. 00:11:41.500 --> 00:11:49.370 A potato increasing from two point zero grams to two point three grams has a percentage mass change of plus fifteen percent. 00:11:49.520 --> 00:11:52.111 Water entered its cells by osmosis. 00:11:52.261 --> 00:11:58.009 A decrease from two point zero grams to one point eight grams gives minus ten percent. 00:12:00.667 --> 00:12:03.904 Repeat each concentration and calculate a mean. 00:12:04.054 --> 00:12:12.884 Plot percentage mass change against sucrose concentration; where the graph crosses 0 percent estimates the concentration giving no net water movement. 00:12:15.567 --> 00:12:18.233 That completes Key concepts in biology. 00:12:18.383 --> 00:12:21.729 Revisit the notes and test yourself on the revision website.