WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.786 Unit S B 8: Exchange and transport in animals. 00:00:10.467 --> 00:00:17.462 Cells need oxygen and glucose for aerobic respiration, water for reactions, and nutrients for growth and repair. 00:00:17.612 --> 00:00:22.296 Dissolved food molecules and mineral ions must reach the cells that need them. 00:00:24.967 --> 00:00:32.431 Digestion breaks proteins into amino acids and lipids into fatty acids and glycerol so these products can be absorbed. 00:00:32.581 --> 00:00:38.191 Respiration transfers energy from nutrients; it is not the same process as digestion. 00:00:40.867 --> 00:00:45.911 Carbon dioxide is a waste product of respiration and is removed through the lungs. 00:00:46.061 --> 00:00:52.481 Urea is made in the liver during breakdown of excess amino acids and is carried to the kidneys for excretion. 00:00:55.133 --> 00:00:59.197 Diffusion is net movement from a higher to a lower concentration. 00:00:59.347 --> 00:01:05.757 A large exchange area, a short diffusion distance and a steep concentration gradient increase the rate. 00:01:08.433 --> 00:01:18.063 Small organisms can exchange substances across their surface because they have a high surface area relative to volume and short distances to internal cells. 00:01:20.733 --> 00:01:27.793 Large multicellular organisms have a smaller surface area relative to volume and longer internal distances. 00:01:27.943 --> 00:01:34.618 Specialised exchange surfaces and a transport system meet the demands that diffusion alone cannot meet. 00:01:37.300 --> 00:01:43.411 For a cube of side length L, surface area equals six times L squared and volume equals L cubed. 00:01:43.561 --> 00:01:49.075 Calculate both using the same length unit before finding the surface area to volume ratio. 00:01:51.733 --> 00:02:00.757 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. 00:02:03.433 --> 00:02:12.851 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. 00:02:13.001 --> 00:02:17.633 Its total area is greater, but its area relative to volume is smaller. 00:02:20.300 --> 00:02:27.883 A cube with 3 centimetres sides has surface area to volume ratio equals 54:27 equals 2:1. 00:02:28.033 --> 00:02:31.929 As size increases, volume grows faster than surface area. 00:02:32.079 --> 00:02:35.975 As size increases, volume grows faster than surface area. 00:02:40.633 --> 00:02:46.201 Lower surface area to volume ratio means less surface for exchange per unit of tissue. 00:02:46.351 --> 00:02:51.411 Larger organisms also need transport to overcome long diffusion distances. 00:02:54.067 --> 00:02:58.156 Report a simplified ratio in the format requested by a question. 00:02:58.306 --> 00:03:04.267 Surface area and volume have different units, so the numerical ratio depends on the length unit used. 00:03:06.933 --> 00:03:09.515 Alveoli are tiny air sacs in the lungs. 00:03:09.665 --> 00:03:14.976 Their large combined surface area allows rapid gas exchange between air and blood. 00:03:17.633 --> 00:03:24.202 The alveolar wall and the adjacent capillary wall are each one cell thick, creating a short diffusion distance. 00:03:26.867 --> 00:03:34.492 Oxygen dissolves in the moist surface lining and diffuses from alveolar air into the blood down its concentration gradient. 00:03:37.167 --> 00:03:44.247 Carbon dioxide diffuses from the blood into alveolar air down its own concentration gradient, then is breathed out. 00:03:46.900 --> 00:03:53.982 Ventilation replaces air in the alveoli, while a rich blood supply carries oxygen away and brings carbon dioxide. 00:03:54.132 --> 00:03:57.319 Together they maintain steep concentration gradients. 00:03:57.469 --> 00:04:02.876 Oxygen and carbon dioxide diffuse in opposite directions down their own gradients. 00:04:07.533 --> 00:04:13.569 Breathing is movement of air into and out of the lungs; gas exchange is diffusion between air and blood. 00:04:13.719 --> 00:04:19.137 Cellular respiration is a chemical process in cells, rather than another name for either. 00:04:21.800 --> 00:04:26.323 Blood contains plasma, red blood cells, white blood cells and platelets. 00:04:26.473 --> 00:04:30.002 Each component has a different role in transport or defence. 00:04:32.667 --> 00:04:41.237 Plasma is the liquid carrying cells and dissolved substances, including glucose, amino acids, carbon dioxide, urea and hormones. 00:04:41.387 --> 00:04:43.349 It also distributes heat. 00:04:46.000 --> 00:04:55.375 Red blood cells, or erythrocytes, contain haemoglobin, which binds oxygen in the lungs and releases it in tissues where oxygen concentration is lower. 00:04:58.033 --> 00:05:02.193 Red blood cells are biconcave discs: both sides dip inwards. 00:05:02.343 --> 00:05:07.419 This gives a large surface area compared with their volume and a short diffusion distance. 00:05:07.569 --> 00:05:12.572 Mature human red blood cells have no nucleus, leaving more room for haemoglobin. 00:05:15.233 --> 00:05:17.974 White blood cells defend against pathogens. 00:05:18.124 --> 00:05:26.197 Phagocytes engulf pathogens; lymphocytes are involved in specific responses, including antibody production and immune memory. 00:05:28.867 --> 00:05:31.821 Platelets are cell fragments involved in clotting. 00:05:31.971 --> 00:05:36.643 A clot reduces blood loss and helps stop pathogens entering through a wound. 00:05:39.300 --> 00:05:45.347 Most oxygen is carried bound to haemoglobin in red cells, rather than simply dissolved in plasma. 00:05:45.497 --> 00:05:49.323 Plasma carries much of the carbon dioxide in dissolved forms. 00:05:52.000 --> 00:05:54.378 Arteries carry blood away from the heart. 00:05:54.528 --> 00:06:00.020 Thick muscular and elastic walls withstand pressure and stretch and recoil as blood is pumped. 00:06:02.700 --> 00:06:06.654 A pulse is the pressure wave associated with heartbeats in arteries. 00:06:06.804 --> 00:06:10.011 It is not blood flowing alternately forwards and backwards. 00:06:12.667 --> 00:06:15.481 Capillaries connect small arteries and veins. 00:06:15.631 --> 00:06:22.318 Their walls are one cell thick so dissolved substances can exchange over a short distance with surrounding tissues. 00:06:25.000 --> 00:06:31.260 Many narrow capillaries form networks with a large combined exchange area and bring blood close to cells. 00:06:31.410 --> 00:06:34.971 Substances move according to their concentration gradients. 00:06:37.633 --> 00:06:40.671 Veins carry blood towards the heart at lower pressure. 00:06:40.821 --> 00:06:46.666 Compared with similar-sized arteries, they generally have thinner walls and a wider central space (lumen). 00:06:49.333 --> 00:06:53.727 Many veins, especially in the limbs, contain valves preventing backflow. 00:06:53.877 --> 00:06:57.617 Skeletal-muscle contractions help push blood towards the heart. 00:06:57.767 --> 00:07:01.513 Compare wall thickness and lumen; sketches are not to scale. 00:07:01.663 --> 00:07:03.839 Vein valve shown schematically. 00:07:08.500 --> 00:07:13.342 Artery and vein are defined by direction of flow, not oxygen content. 00:07:13.492 --> 00:07:20.199 The pulmonary artery carries deoxygenated blood to the lungs; pulmonary veins carry oxygenated blood back. 00:07:22.867 --> 00:07:28.754 The heart is a muscular pump with four chambers: right and left atria above right and left ventricles. 00:07:28.904 --> 00:07:32.301 The atria receive blood; the ventricles pump it out. 00:07:34.967 --> 00:07:43.286 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. 00:07:43.436 --> 00:07:46.689 Blood passes through the heart twice in one complete circuit. 00:07:49.367 --> 00:07:54.325 Deoxygenated blood returns from the body in the venae cavae to the right atrium. 00:07:54.475 --> 00:08:00.617 It passes through a valve to the right ventricle, then through another valve into the pulmonary artery to the lungs. 00:08:03.300 --> 00:08:08.367 Oxygenated blood returns from the lungs through pulmonary veins to the left atrium. 00:08:08.517 --> 00:08:14.021 It passes through a valve to the left ventricle, then through another valve into the aorta to the body. 00:08:16.700 --> 00:08:21.399 Valves prevent backflow when pressure changes during contraction and relaxation. 00:08:21.549 --> 00:08:26.892 Atria contract to move blood into ventricles; ventricular contraction pumps blood out. 00:08:29.567 --> 00:08:36.381 The left ventricle has a thicker muscular wall than the right because it must generate higher pressure to pump around the whole body. 00:08:36.531 --> 00:08:39.262 The right ventricle pumps to the nearby lungs. 00:08:41.933 --> 00:08:49.249 The septum separates the right and left sides, preventing oxygenated and deoxygenated blood mixing within a healthy heart. 00:08:49.399 --> 00:08:55.178 Right and left are the person's anatomical sides, not necessarily the viewer's sides on a diagram. 00:08:55.328 --> 00:08:58.653 Follow the arrows through pulmonary and systemic circuits. 00:08:58.803 --> 00:09:05.224 Chamber boxes show routes rather than anatomical shape; additional outflow valves are described in the notes. 00:09:09.900 --> 00:09:14.416 Coronary arteries supply the heart muscle itself with oxygen and glucose. 00:09:14.566 --> 00:09:23.993 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. 00:09:26.667 --> 00:09:29.227 Heart rate is the number of beats per minute. 00:09:29.377 --> 00:09:37.055 Stroke volume is the volume pumped by one ventricle in each beat; cardiac output is the volume pumped by one ventricle per minute. 00:09:39.733 --> 00:09:43.272 Cardiac output equals stroke volume times heart rate. 00:09:43.422 --> 00:09:51.154 If stroke volume is in cubic centimetres per beat and heart rate in beats per minute, output is in cubic centimetres per minute. 00:09:53.833 --> 00:09:59.389 For a stroke volume of 70 cubic centimetres and a heart rate of 75 beats per minute, 00:09:59.539 --> 00:10:06.472 cardiac output equals 70 times 75 equals 5,250 cubic centimetres per minute, 00:10:06.622 --> 00:10:09.361 or 5.25 litres per minute. 00:10:12.033 --> 00:10:21.461 Rearrange when needed: stroke volume equals cardiac output divided by heart rate, and heart rate equals cardiac output divided by stroke volume. 00:10:21.611 --> 00:10:23.787 Keep volume units consistent. 00:10:26.467 --> 00:10:30.905 During exercise, muscles need more energy transferred by respiration. 00:10:31.055 --> 00:10:38.124 Heart rate and often stroke volume increase, delivering more oxygen and glucose and removing more carbon dioxide. 00:10:40.800 --> 00:10:43.434 Regular training can increase stroke volume. 00:10:43.584 --> 00:10:52.606 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. 00:10:55.267 --> 00:10:58.911 Measure pulse over a known time and convert to beats per minute. 00:10:59.061 --> 00:11:05.373 Repeated readings improve comparisons; keep exercise intensity and measurement conditions consistent. 00:11:08.033 --> 00:11:15.413 Cellular respiration consists of reactions that transfer energy from glucose and other fuels for cell processes. 00:11:15.563 --> 00:11:19.958 It occurs continuously in living cells and is exothermic overall. 00:11:22.633 --> 00:11:29.161 Energy is transferred for movement, active transport, building larger molecules and maintaining body temperature. 00:11:29.311 --> 00:11:34.568 Energy is not a substance made from nothing or an extra atom in a balanced equation. 00:11:37.233 --> 00:11:43.846 The word equation for aerobic respiration is glucose plus oxygen to carbon dioxide plus water. 00:11:43.996 --> 00:11:47.859 Energy is transferred to useful processes and the surroundings. 00:11:50.533 --> 00:11:57.615 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. 00:11:57.765 --> 00:12:00.921 Oxygen is required for aerobic respiration. 00:12:03.600 --> 00:12:09.389 Most reactions of aerobic respiration take place in mitochondria in eukaryotic cells. 00:12:09.539 --> 00:12:15.916 The first stage takes place in the cytoplasm, so not every respiration reaction happens in mitochondria. 00:12:18.567 --> 00:12:23.665 Do not confuse respiration with breathing, or say that plants only respire at night. 00:12:23.815 --> 00:12:29.712 Plants and animals respire continuously; photosynthesis is a separate light-dependent process. 00:12:32.367 --> 00:12:41.165 If muscles cannot get enough oxygen during vigorous exercise, anaerobic respiration transfers energy from glucose without using oxygen. 00:12:41.315 --> 00:12:43.534 It takes place in the cytoplasm. 00:12:46.200 --> 00:12:50.109 In animal muscle, the word equation is glucose to lactic acid. 00:12:50.259 --> 00:12:58.545 Lactic acid is commonly called lactate in the body; energy is transferred, but less per glucose molecule than in aerobic respiration. 00:13:01.200 --> 00:13:09.049 Anaerobic activity can support intense exercise for a short time, but cannot sustain the same energy demand indefinitely. 00:13:09.199 --> 00:13:14.393 Fatigue has several causes and should not be explained only as lactic acid poisoning. 00:13:17.067 --> 00:13:20.635 Breathing and heart rate stay higher for a while after exercise. 00:13:20.785 --> 00:13:25.915 The extra oxygen helps break down lactate and restore energy and oxygen stores. 00:13:26.065 --> 00:13:31.481 At GCSE, this extra oxygen needed for recovery is called the oxygen debt. 00:13:34.133 --> 00:13:37.501 Lactate travels in the blood to organs including the liver. 00:13:37.651 --> 00:13:42.782 It can be broken down using oxygen (oxidised) or converted back into glucose. 00:13:42.932 --> 00:13:49.021 Carbon dioxide produced during its breakdown leaves through the lungs; lactate itself is not breathed out. 00:13:51.700 --> 00:13:59.261 In yeast, anaerobic respiration produces ethanol and carbon dioxide: glucose to ethanol plus carbon dioxide. 00:13:59.411 --> 00:14:02.930 This differs from anaerobic respiration in human muscle. 00:14:05.600 --> 00:14:11.912 Compare the processes: aerobic respiration requires oxygen and releases more energy per glucose; 00:14:12.062 --> 00:14:16.050 anaerobic respiration uses no oxygen and releases less, 00:14:16.200 --> 00:14:18.675 with different products in muscle and yeast. 00:14:18.825 --> 00:14:25.181 Word summaries compare products and energy transfer; aerobic respiration also consumes oxygen. 00:14:29.833 --> 00:14:34.633 A respirometer estimates respiration rate by measuring oxygen consumption. 00:14:34.783 --> 00:14:42.759 Investigate the effect of temperature on small organisms using a simple tube apparatus in water baths at different temperatures. 00:14:45.433 --> 00:14:53.315 Place soda lime or another suitable carbon dioxide absorbent in the tube, separated from the organisms by cotton wool. 00:14:53.465 --> 00:15:00.258 It absorbs carbon dioxide released during respiration, so the gas-volume decrease reflects oxygen uptake. 00:15:02.933 --> 00:15:07.041 Seal the apparatus and connect it to a capillary with coloured liquid. 00:15:07.191 --> 00:15:15.189 As oxygen is consumed and carbon dioxide is absorbed, pressure inside falls and the liquid moves towards the organism chamber. 00:15:15.339 --> 00:15:20.789 Carbon dioxide is absorbed, so the fall in gas volume estimates oxygen consumed. 00:15:25.467 --> 00:15:30.533 Allow the apparatus and organisms to reach the water-bath temperature before timing. 00:15:30.683 --> 00:15:36.740 Record liquid displacement over a fixed interval; use the same capillary cross-section for comparisons. 00:15:39.400 --> 00:15:46.459 Control organism species and mass or number, measurement time, apparatus volume and other relevant conditions. 00:15:46.609 --> 00:15:53.179 Compare with a suitable non-respiring control to account for pressure changes unrelated to respiration. 00:15:55.833 --> 00:16:00.647 Rate can be expressed as distance moved divided by time for the same capillary. 00:16:00.797 --> 00:16:09.873 If area is known, oxygen volume equals capillary cross-sectional area times distance moved; volume rate equals volume divided by time. 00:16:12.533 --> 00:16:24.957 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. 00:16:25.107 --> 00:16:32.647 To compare organisms of different masses, divide the volume rate by mass to get the rate per gram (a mass-normalised rate). 00:16:35.300 --> 00:16:38.381 Repeat at each temperature and calculate a mean. 00:16:38.531 --> 00:16:47.797 Interpret increases over a safe temperature range using enzyme activity; avoid assuming that the rate increases indefinitely at damaging temperatures. 00:16:50.467 --> 00:16:56.044 Keep apparatus airtight, use eye protection and handle the absorbent under school instructions. 00:16:56.194 --> 00:17:05.697 Protect organisms from absorbent contact, extremes of temperature and prolonged oxygen depletion; minimise stress and return them to suitable conditions. 00:17:08.367 --> 00:17:14.072 An airtight seal matters because air leaking in can hide the decrease caused by oxygen uptake. 00:17:14.222 --> 00:17:17.038 Gas volume also changes with temperature. 00:17:17.188 --> 00:17:24.973 Allow the apparatus to reach the water-bath temperature and use a non-respiring control so these changes do not distort the result. 00:17:27.633 --> 00:17:34.565 Use consistent units and state whether a result measures displacement, gas volume per time or volume per time per gram. 00:17:34.715 --> 00:17:40.697 Do not describe oxygen uptake and carbon dioxide production as the same measured quantity. 00:17:43.367 --> 00:17:50.821 A yeast investigation can compare carbon dioxide production under controlled glucose concentrations and temperatures. 00:17:50.971 --> 00:17:56.571 Under anaerobic conditions it measures fermentation output, rather than oxygen consumption. 00:17:59.233 --> 00:18:04.501 Compare means from repeat readings and identify anomalies before drawing conclusions. 00:18:04.651 --> 00:18:08.938 Change one independent variable at a time and control the others. 00:18:11.600 --> 00:18:18.756 When comparing exercise data, distinguish an immediate response to activity from longer-term training adaptations. 00:18:18.906 --> 00:18:24.537 A higher exercise heart rate does not mean the person necessarily has a higher resting rate. 00:18:27.200 --> 00:18:31.511 A larger exchange surface lets more particles cross in a given time. 00:18:31.661 --> 00:18:38.519 A steeper concentration gradient increases net diffusion; a thinner exchange barrier shortens the diffusion path. 00:18:41.200 --> 00:18:50.681 Fick’s law shows how diffusion rate depends on three factors: rate is proportional to surface area times concentration difference divided by membrane thickness. 00:18:50.831 --> 00:18:54.227 The is proportional to symbol means “is proportional to”. 00:18:54.377 --> 00:19:00.893 The relationship lets you compare rates; calculating an actual rate also needs a proportionality constant. 00:19:01.043 --> 00:19:05.301 Ventilation and circulation help maintain the concentration difference. 00:19:09.967 --> 00:19:15.245 For the same substance and other conditions, doubling surface area doubles diffusion rate. 00:19:15.395 --> 00:19:18.403 Doubling the concentration difference also doubles it. 00:19:18.553 --> 00:19:20.879 Doubling membrane thickness halves it. 00:19:21.029 --> 00:19:26.094 Use these changes to compare rates without needing to know the proportionality constant. 00:19:28.767 --> 00:19:32.328 Alveoli combine a very large surface area with thin walls. 00:19:32.478 --> 00:19:41.799 Ventilation refreshes air and blood flow removes absorbed oxygen, maintaining the gradient; a thickened alveolar wall can reduce oxygen transfer. 00:19:44.467 --> 00:19:48.384 Villi and their microvilli increase intestinal exchange area. 00:19:48.534 --> 00:19:56.107 Good blood flow maintains concentration differences; efficient exchange relies on area, barrier thickness and gradient together. 00:19:58.767 --> 00:20:05.571 If area doubles while thickness doubles and the gradient stays unchanged, the effects cancel and the rate stays the same. 00:20:05.721 --> 00:20:10.233 If area triples and thickness halves, the rate becomes six times as large. 00:20:12.900 --> 00:20:15.874 That completes Exchange and transport in animals. 00:20:16.024 --> 00:20:19.369 Revisit the notes and test yourself on the revision website.