WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.860 Unit C B 8: Exchange and transport in animals. 00:00:10.533 --> 00:00:17.529 Cells need oxygen and glucose for aerobic respiration, water for reactions, and nutrients for growth and repair. 00:00:17.679 --> 00:00:22.363 Dissolved food molecules and mineral ions must reach the cells that need them. 00:00:25.033 --> 00:00:32.498 Digestion breaks proteins into amino acids and lipids into fatty acids and glycerol so these products can be absorbed. 00:00:32.648 --> 00:00:38.258 Respiration transfers energy from nutrients; it is not the same process as digestion. 00:00:40.933 --> 00:00:45.977 Carbon dioxide is a waste product of respiration and is removed through the lungs. 00:00:46.127 --> 00:00:52.548 Urea is made in the liver during breakdown of excess amino acids and is carried to the kidneys for excretion. 00:00:55.200 --> 00:00:59.264 Diffusion is net movement from a higher to a lower concentration. 00:00:59.414 --> 00:01:05.823 A large exchange area, a short diffusion distance and a steep concentration gradient increase the rate. 00:01:08.500 --> 00:01:18.129 Small organisms can exchange substances across their surface because they have a high surface area relative to volume and short distances to internal cells. 00:01:20.800 --> 00:01:27.859 Large multicellular organisms have a smaller surface area relative to volume and longer internal distances. 00:01:28.009 --> 00:01:34.685 Specialised exchange surfaces and a transport system meet the demands that diffusion alone cannot meet. 00:01:37.367 --> 00:01:43.477 For a cube of side length L, surface area equals six times L squared and volume equals L cubed. 00:01:43.627 --> 00:01:49.142 Calculate both using the same length unit before finding the surface area to volume ratio. 00:01:51.800 --> 00:02:00.823 A cube with 1 cm sides has an area of 6 square centimetres and a volume of 1 cubic centimetres, giving SA:V equals 6:1. 00:02:03.500 --> 00:02:12.917 A cube with 2 cm sides has an area of 24 square centimetres and a volume of 8 cubic centimetres, giving SA:V equals 3:1. 00:02:13.067 --> 00:02:17.700 Its total area is greater, but its area relative to volume is smaller. 00:02:20.367 --> 00:02:27.949 A cube with 3 centimetres sides has surface area to volume ratio equals 54:27 equals 2:1. 00:02:28.099 --> 00:02:31.995 As size increases, volume grows faster than surface area. 00:02:32.145 --> 00:02:36.041 As size increases, volume grows faster than surface area. 00:02:40.700 --> 00:02:46.267 Lower surface area to volume ratio means less surface for exchange per unit of tissue. 00:02:46.417 --> 00:02:51.478 Larger organisms also need transport to overcome long diffusion distances. 00:02:54.133 --> 00:02:58.223 Report a simplified ratio in the format requested by a question. 00:02:58.373 --> 00:03:04.334 Surface area and volume have different units, so the numerical ratio depends on the length unit used. 00:03:07.000 --> 00:03:09.581 Alveoli are tiny air sacs in the lungs. 00:03:09.731 --> 00:03:15.043 Their large combined surface area allows rapid gas exchange between air and blood. 00:03:17.700 --> 00:03:24.269 The alveolar wall and the adjacent capillary wall are each one cell thick, creating a short diffusion distance. 00:03:26.933 --> 00:03:34.559 Oxygen dissolves in the moist surface lining and diffuses from alveolar air into the blood down its concentration gradient. 00:03:37.233 --> 00:03:44.314 Carbon dioxide diffuses from the blood into alveolar air down its own concentration gradient, then is breathed out. 00:03:46.967 --> 00:03:54.049 Ventilation replaces air in the alveoli, while a rich blood supply carries oxygen away and brings carbon dioxide. 00:03:54.199 --> 00:03:57.386 Together they maintain steep concentration gradients. 00:03:57.536 --> 00:04:02.943 Oxygen and carbon dioxide diffuse in opposite directions down their own gradients. 00:04:07.600 --> 00:04:13.635 Breathing is movement of air into and out of the lungs; gas exchange is diffusion between air and blood. 00:04:13.785 --> 00:04:19.203 Cellular respiration is a chemical process in cells, rather than another name for either. 00:04:21.867 --> 00:04:26.390 Blood contains plasma, red blood cells, white blood cells and platelets. 00:04:26.540 --> 00:04:30.069 Each component has a different role in transport or defence. 00:04:32.733 --> 00:04:41.303 Plasma is the liquid carrying cells and dissolved substances, including glucose, amino acids, carbon dioxide, urea and hormones. 00:04:41.453 --> 00:04:43.416 It also distributes heat. 00:04:46.067 --> 00:04:55.441 Red blood cells, or erythrocytes, contain haemoglobin, which binds oxygen in the lungs and releases it in tissues where oxygen concentration is lower. 00:04:58.100 --> 00:05:02.259 Red blood cells are biconcave discs: both sides dip inwards. 00:05:02.409 --> 00:05:07.486 This gives a large surface area compared with their volume and a short diffusion distance. 00:05:07.636 --> 00:05:12.639 Mature human red blood cells have no nucleus, leaving more room for haemoglobin. 00:05:15.300 --> 00:05:18.041 White blood cells defend against pathogens. 00:05:18.191 --> 00:05:26.264 Phagocytes engulf pathogens; lymphocytes are involved in specific responses, including antibody production and immune memory. 00:05:28.933 --> 00:05:31.887 Platelets are cell fragments involved in clotting. 00:05:32.037 --> 00:05:36.710 A clot reduces blood loss and helps stop pathogens entering through a wound. 00:05:39.367 --> 00:05:45.413 Most oxygen is carried bound to haemoglobin in red cells, rather than simply dissolved in plasma. 00:05:45.563 --> 00:05:49.389 Plasma carries much of the carbon dioxide in dissolved forms. 00:05:52.067 --> 00:05:54.445 Arteries carry blood away from the heart. 00:05:54.595 --> 00:06:00.087 Thick muscular and elastic walls withstand pressure and stretch and recoil as blood is pumped. 00:06:02.767 --> 00:06:06.721 A pulse is the pressure wave associated with heartbeats in arteries. 00:06:06.871 --> 00:06:10.078 It is not blood flowing alternately forwards and backwards. 00:06:12.733 --> 00:06:15.548 Capillaries connect small arteries and veins. 00:06:15.698 --> 00:06:22.385 Their walls are one cell thick so dissolved substances can exchange over a short distance with surrounding tissues. 00:06:25.067 --> 00:06:31.327 Many narrow capillaries form networks with a large combined exchange area and bring blood close to cells. 00:06:31.477 --> 00:06:35.038 Substances move according to their concentration gradients. 00:06:37.700 --> 00:06:40.738 Veins carry blood towards the heart at lower pressure. 00:06:40.888 --> 00:06:46.733 Compared with similar-sized arteries, they generally have thinner walls and a wider central space (lumen). 00:06:49.400 --> 00:06:53.794 Many veins, especially in the limbs, contain valves preventing backflow. 00:06:53.944 --> 00:06:57.683 Skeletal-muscle contractions help push blood towards the heart. 00:06:57.833 --> 00:07:01.580 Compare wall thickness and lumen; sketches are not to scale. 00:07:01.730 --> 00:07:03.906 Vein valve shown schematically. 00:07:08.567 --> 00:07:13.409 Artery and vein are defined by direction of flow, not oxygen content. 00:07:13.559 --> 00:07:20.265 The pulmonary artery carries deoxygenated blood to the lungs; pulmonary veins carry oxygenated blood back. 00:07:22.933 --> 00:07:28.821 The heart is a muscular pump with four chambers: right and left atria above right and left ventricles. 00:07:28.971 --> 00:07:32.367 The atria receive blood; the ventricles pump it out. 00:07:35.033 --> 00:07:43.353 Humans have a double circulatory system: the pulmonary circuit links heart and lungs, and the systemic circuit links heart and the rest of the body. 00:07:43.503 --> 00:07:46.755 Blood passes through the heart twice in one complete circuit. 00:07:49.433 --> 00:07:54.392 Deoxygenated blood returns from the body in the venae cavae to the right atrium. 00:07:54.542 --> 00:08:00.684 It passes through a valve to the right ventricle, then through another valve into the pulmonary artery to the lungs. 00:08:03.367 --> 00:08:08.433 Oxygenated blood returns from the lungs through pulmonary veins to the left atrium. 00:08:08.583 --> 00:08:14.088 It passes through a valve to the left ventricle, then through another valve into the aorta to the body. 00:08:16.767 --> 00:08:21.466 Valves prevent backflow when pressure changes during contraction and relaxation. 00:08:21.616 --> 00:08:26.959 Atria contract to move blood into ventricles; ventricular contraction pumps blood out. 00:08:29.633 --> 00:08:36.448 The left ventricle has a thicker muscular wall than the right because it must generate higher pressure to pump around the whole body. 00:08:36.598 --> 00:08:39.329 The right ventricle pumps to the nearby lungs. 00:08:42.000 --> 00:08:49.315 The septum separates the right and left sides, preventing oxygenated and deoxygenated blood mixing within a healthy heart. 00:08:49.465 --> 00:08:55.245 Right and left are the person's anatomical sides, not necessarily the viewer's sides on a diagram. 00:08:55.395 --> 00:08:58.720 Follow the arrows through pulmonary and systemic circuits. 00:08:58.870 --> 00:09:05.291 Chamber boxes show routes rather than anatomical shape; additional outflow valves are described in the notes. 00:09:09.967 --> 00:09:14.483 Coronary arteries supply the heart muscle itself with oxygen and glucose. 00:09:14.633 --> 00:09:24.060 A blockage can damage or kill part of that muscle in a heart attack; a heart attack is not automatically the same event as the heart stopping in cardiac arrest. 00:09:26.733 --> 00:09:29.293 Heart rate is the number of beats per minute. 00:09:29.443 --> 00:09:37.121 Stroke volume is the volume pumped by one ventricle in each beat; cardiac output is the volume pumped by one ventricle per minute. 00:09:39.800 --> 00:09:43.339 Cardiac output equals stroke volume times heart rate. 00:09:43.489 --> 00:09:51.221 If stroke volume is in cubic centimetres per beat and heart rate in beats per minute, output is in cubic centimetres per minute. 00:09:53.900 --> 00:09:59.455 For a stroke volume of 70 cubic centimetres and a heart rate of 75 beats per minute, 00:09:59.605 --> 00:10:06.539 cardiac output equals 70 times 75 equals 5,250 cubic centimetres per minute, 00:10:06.689 --> 00:10:09.427 or 5.25 litres per minute. 00:10:12.100 --> 00:10:21.527 Rearrange when needed: stroke volume equals cardiac output divided by heart rate, and heart rate equals cardiac output divided by stroke volume. 00:10:21.677 --> 00:10:23.853 Keep volume units consistent. 00:10:26.533 --> 00:10:30.971 During exercise, muscles need more energy transferred by respiration. 00:10:31.121 --> 00:10:38.191 Heart rate and often stroke volume increase, delivering more oxygen and glucose and removing more carbon dioxide. 00:10:40.867 --> 00:10:43.501 Regular training can increase stroke volume. 00:10:43.651 --> 00:10:52.673 A trained person may have a lower resting heart rate while maintaining a similar resting cardiac output; a lower rate alone does not prove greater fitness. 00:10:55.333 --> 00:10:58.977 Measure pulse over a known time and convert to beats per minute. 00:10:59.127 --> 00:11:05.440 Repeated readings improve comparisons; keep exercise intensity and measurement conditions consistent. 00:11:08.100 --> 00:11:15.479 Cellular respiration consists of reactions that transfer energy from glucose and other fuels for cell processes. 00:11:15.629 --> 00:11:20.025 It occurs continuously in living cells and is exothermic overall. 00:11:22.700 --> 00:11:29.227 Energy is transferred for movement, active transport, building larger molecules and maintaining body temperature. 00:11:29.377 --> 00:11:34.635 Energy is not a substance made from nothing or an extra atom in a balanced equation. 00:11:37.300 --> 00:11:43.913 The word equation for aerobic respiration is glucose + oxygen to carbon dioxide + water. 00:11:44.063 --> 00:11:47.926 Energy is transferred to useful processes and the surroundings. 00:11:50.600 --> 00:11:57.682 The balanced symbol equation is C six H twelve O six plus six O two produces six C O two plus six H two O. 00:11:57.832 --> 00:12:00.988 Oxygen is required for aerobic respiration. 00:12:03.667 --> 00:12:09.456 Most reactions of aerobic respiration take place in mitochondria in eukaryotic cells. 00:12:09.606 --> 00:12:15.983 The first stage takes place in the cytoplasm, so not every respiration reaction happens in mitochondria. 00:12:18.633 --> 00:12:23.731 Do not confuse respiration with breathing, or say that plants only respire at night. 00:12:23.881 --> 00:12:29.779 Plants and animals respire continuously; photosynthesis is a separate light-dependent process. 00:12:32.433 --> 00:12:41.232 If muscles cannot get enough oxygen during vigorous exercise, anaerobic respiration transfers energy from glucose without using oxygen. 00:12:41.382 --> 00:12:43.601 It takes place in the cytoplasm. 00:12:46.267 --> 00:12:50.175 In animal muscle, the word equation is glucose to lactic acid. 00:12:50.325 --> 00:12:58.612 Lactic acid is commonly called lactate in the body; energy is transferred, but less per glucose molecule than in aerobic respiration. 00:13:01.267 --> 00:13:09.115 Anaerobic activity can support intense exercise for a short time, but cannot sustain the same energy demand indefinitely. 00:13:09.265 --> 00:13:14.460 Fatigue has several causes and should not be explained only as lactic acid poisoning. 00:13:17.133 --> 00:13:20.702 Breathing and heart rate stay higher for a while after exercise. 00:13:20.852 --> 00:13:25.981 The extra oxygen helps break down lactate and restore energy and oxygen stores. 00:13:26.131 --> 00:13:31.548 At GCSE, this extra oxygen needed for recovery is called the oxygen debt. 00:13:34.200 --> 00:13:37.568 Lactate travels in the blood to organs including the liver. 00:13:37.718 --> 00:13:42.849 It can be broken down using oxygen (oxidised) or converted back into glucose. 00:13:42.999 --> 00:13:49.088 Carbon dioxide produced during its breakdown leaves through the lungs; lactate itself is not breathed out. 00:13:51.767 --> 00:13:59.328 In yeast, anaerobic respiration produces ethanol and carbon dioxide: glucose to ethanol + carbon dioxide. 00:13:59.478 --> 00:14:02.997 This differs from anaerobic respiration in human muscle. 00:14:05.667 --> 00:14:11.979 Compare the processes: aerobic respiration requires oxygen and releases more energy per glucose; 00:14:12.129 --> 00:14:16.117 anaerobic respiration uses no oxygen and releases less, 00:14:16.267 --> 00:14:18.741 with different products in muscle and yeast. 00:14:18.891 --> 00:14:25.248 Word summaries compare products and energy transfer; aerobic respiration also consumes oxygen. 00:14:29.900 --> 00:14:34.699 A respirometer estimates respiration rate by measuring oxygen consumption. 00:14:34.849 --> 00:14:42.826 Investigate the effect of temperature on small organisms using a simple tube apparatus in water baths at different temperatures. 00:14:45.500 --> 00:14:53.381 Place soda lime or another suitable carbon dioxide absorbent in the tube, separated from the organisms by cotton wool. 00:14:53.531 --> 00:15:00.325 It absorbs carbon dioxide released during respiration, so the gas-volume decrease reflects oxygen uptake. 00:15:03.000 --> 00:15:07.108 Seal the apparatus and connect it to a capillary with coloured liquid. 00:15:07.258 --> 00:15:15.256 As oxygen is consumed and carbon dioxide is absorbed, pressure inside falls and the liquid moves towards the organism chamber. 00:15:15.406 --> 00:15:20.855 Carbon dioxide is absorbed, so the fall in gas volume estimates oxygen consumed. 00:15:25.533 --> 00:15:30.600 Allow the apparatus and organisms to reach the water-bath temperature before timing. 00:15:30.750 --> 00:15:36.807 Record liquid displacement over a fixed interval; use the same capillary cross-section for comparisons. 00:15:39.467 --> 00:15:46.526 Control organism species and mass or number, measurement time, apparatus volume and other relevant conditions. 00:15:46.676 --> 00:15:53.246 Compare with a suitable non-respiring control to account for pressure changes unrelated to respiration. 00:15:55.900 --> 00:16:00.713 Rate can be expressed as distance moved divided by time for the same capillary. 00:16:00.863 --> 00:16:09.939 If area is known, oxygen volume equals capillary cross-sectional area times distance moved; volume rate equals volume divided by time. 00:16:12.600 --> 00:16:25.024 For a capillary area of 0.20 square millimetres and movement of 15 mm in 3 minutes, oxygen uptake equals 3 cubic millimetres and rate equals 1 cubic millimetres per minute. 00:16:25.174 --> 00:16:32.714 To compare organisms of different masses, divide the volume rate by mass to get the rate per gram (a mass-normalised rate). 00:16:35.367 --> 00:16:38.447 Repeat at each temperature and calculate a mean. 00:16:38.597 --> 00:16:47.864 Interpret increases over a safe temperature range using enzyme activity; avoid assuming that the rate increases indefinitely at damaging temperatures. 00:16:50.533 --> 00:16:56.111 Keep apparatus airtight, use eye protection and handle the absorbent under school instructions. 00:16:56.261 --> 00:17:05.763 Protect organisms from absorbent contact, extremes of temperature and prolonged oxygen depletion; minimise stress and return them to suitable conditions. 00:17:08.433 --> 00:17:14.139 An airtight seal matters because air leaking in can hide the decrease caused by oxygen uptake. 00:17:14.289 --> 00:17:17.105 Gas volume also changes with temperature. 00:17:17.255 --> 00:17:25.039 Allow the apparatus to reach the water-bath temperature and use a non-respiring control so these changes do not distort the result. 00:17:27.700 --> 00:17:34.631 Use consistent units and state whether a result measures displacement, gas volume per time or volume per time per gram. 00:17:34.781 --> 00:17:40.764 Do not describe oxygen uptake and carbon dioxide production as the same measured quantity. 00:17:43.433 --> 00:17:50.888 A yeast investigation can compare carbon dioxide production under controlled glucose concentrations and temperatures. 00:17:51.038 --> 00:17:56.638 Under anaerobic conditions it measures fermentation output, rather than oxygen consumption. 00:17:59.300 --> 00:18:04.567 Compare means from repeat readings and identify anomalies before drawing conclusions. 00:18:04.717 --> 00:18:09.005 Change one independent variable at a time and control the others. 00:18:11.667 --> 00:18:18.823 When comparing exercise data, distinguish an immediate response to activity from longer-term training adaptations. 00:18:18.973 --> 00:18:24.604 A higher exercise heart rate does not mean the person necessarily has a higher resting rate. 00:18:27.267 --> 00:18:30.241 That completes Exchange and transport in animals. 00:18:30.391 --> 00:18:33.736 Revisit the notes and test yourself on the revision website.