WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.627 Unit C B 6: Plant structures and their functions. 00:00:10.300 --> 00:00:15.035 Plants and algae use photosynthesis to produce food and therefore biomass. 00:00:15.185 --> 00:00:19.157 They are producers, forming the starting point of many food chains. 00:00:21.833 --> 00:00:27.028 Photosynthesis transfers light energy into chemical energy stored in glucose. 00:00:27.178 --> 00:00:30.926 It is an endothermic process because it takes in energy. 00:00:33.600 --> 00:00:38.485 The word equation is carbon dioxide + water to glucose + oxygen. 00:00:38.635 --> 00:00:43.765 Light energy and chlorophyll are needed, but are not substances consumed in the equation. 00:00:46.433 --> 00:00:53.568 The balanced symbol equation is six C O two plus six H two O produces C six H twelve O six plus six O two. 00:00:53.718 --> 00:00:58.688 The carbon in glucose comes from carbon dioxide, rather than from soil minerals. 00:01:01.367 --> 00:01:04.341 Photosynthesis takes place in chloroplasts. 00:01:04.491 --> 00:01:09.505 Chlorophyll is a pigment that absorbs light energy; it is not the name of the organelle. 00:01:12.167 --> 00:01:14.981 Plants respire as well as photosynthesise. 00:01:15.131 --> 00:01:20.251 Respiration occurs in both light and dark, while photosynthesis requires light. 00:01:22.933 --> 00:01:29.718 Glucose can be used in respiration to transfer energy for cell processes, including active transport and growth. 00:01:32.400 --> 00:01:36.270 Plants join glucose molecules together to make starch for storage. 00:01:36.420 --> 00:01:43.074 Starch is insoluble, so it does not build up as dissolved sugar in the cell and draw in extra water by osmosis. 00:01:45.733 --> 00:01:50.204 Plants convert sugars into sucrose for transport to other parts of the plant. 00:01:50.354 --> 00:01:55.153 Stored starch can be broken down and the sugars used or converted for transport. 00:01:57.833 --> 00:02:04.496 Sugars transported to storage organs, such as potato tubers, can be converted into starch and used later. 00:02:07.167 --> 00:02:12.105 Glucose can supply material for cellulose in cell walls and for lipid production. 00:02:12.255 --> 00:02:19.976 Plants also combine carbon-containing substances with nitrogen from nitrate ions to make amino acids and proteins. 00:02:22.633 --> 00:02:31.832 Glucose, starch and sucrose have different roles: glucose is useful in respiration, starch is a storage carbohydrate, and sucrose is a transported sugar. 00:02:34.500 --> 00:02:39.662 Palisade cells near the upper surface of many leaves contain numerous chloroplasts. 00:02:39.812 --> 00:02:44.240 Their position and chloroplasts help them absorb light for photosynthesis. 00:02:46.900 --> 00:02:50.807 A stoma is a pore in the leaf surface; stomata is the plural. 00:02:50.957 --> 00:02:55.320 In many land plants, numerous stomata occur on the lower leaf surface. 00:02:58.000 --> 00:03:00.645 A pair of guard cells controls each stoma. 00:03:00.795 --> 00:03:07.001 When guard cells take up water by osmosis and become turgid, their shape changes and the pore opens. 00:03:07.151 --> 00:03:12.003 Guard-cell water content changes the pore opening; schematic surface views. 00:03:16.667 --> 00:03:20.817 When guard cells lose water and become less turgid, the pore closes. 00:03:20.967 --> 00:03:29.041 Their opening depends on environmental signals and water availability, rather than simply on water flow starting or stopping in the stem. 00:03:31.700 --> 00:03:38.237 During net photosynthesis, carbon dioxide diffuses into the leaf and oxygen diffuses out through stomata. 00:03:38.387 --> 00:03:41.553 These gases move down their concentration gradients. 00:03:44.233 --> 00:03:47.621 Water vapour can also diffuse out through an open stoma. 00:03:47.771 --> 00:03:51.794 Opening pores allows gas exchange but can increase water loss. 00:03:54.467 --> 00:04:01.357 Stomata commonly open in the light and close in darkness, but this is not an absolute rule for every plant or condition. 00:04:01.507 --> 00:04:04.279 Water transport does not simply stop at night. 00:04:06.933 --> 00:04:11.404 A limiting factor is the factor restricting the rate under the current conditions. 00:04:11.554 --> 00:04:17.803 The main factors considered here are light intensity, carbon dioxide concentration and temperature. 00:04:20.467 --> 00:04:26.717 Increasing light intensity increases the rate of photosynthesis while light is the limiting factor. 00:04:26.867 --> 00:04:34.003 When the graph levels off (forms a plateau), another factor, such as carbon dioxide or temperature, is limiting the rate. 00:04:34.153 --> 00:04:37.894 Increasing light raises the rate only while light is limiting. 00:04:42.567 --> 00:04:48.753 Increasing carbon dioxide concentration can increase the rate when carbon dioxide is limiting. 00:04:48.903 --> 00:04:53.255 Increasing a factor already in sufficient supply may have little effect. 00:04:55.933 --> 00:04:59.493 Photosynthesis involves enzyme-controlled reactions. 00:04:59.643 --> 00:05:07.364 Increasing temperature initially speeds reactions, but above an optimum the rate may fall as enzymes lose activity or denature. 00:05:10.033 --> 00:05:12.134 The limiting factors work together. 00:05:12.284 --> 00:05:19.504 For example, adding carbon dioxide may let the rate rise above a previous plateau, until another factor becomes limiting. 00:05:19.654 --> 00:05:23.773 Use the evidence in the question to identify the limiting factor. 00:05:26.433 --> 00:05:32.373 For a suitable small light source, light intensity is proportional to 1 divided by distance squared. 00:05:32.523 --> 00:05:38.610 If the distance doubles, intensity becomes one quarter; if it triples, intensity becomes one ninth. 00:05:41.267 --> 00:05:49.309 The second light intensity divided by the first light intensity equals the first distance divided by the second distance, all squared. 00:05:49.459 --> 00:05:59.644 Moving a lamp from ten centimetres to twenty centimetres gives the intensity ratio of ten divided by twenty, all squared, which equals zero point two five. 00:06:02.300 --> 00:06:08.583 Photosynthesis rate is approximately proportional to light intensity only while light is limiting. 00:06:08.733 --> 00:06:13.853 Do not assume that halving distance always quadruples the rate at an existing plateau. 00:06:16.533 --> 00:06:24.681 Place a suitable piece of pondweed in water or a dilute sodium hydrogencarbonate solution, which supplies carbon dioxide. 00:06:24.831 --> 00:06:28.674 Use a lamp at measured distances to vary light intensity. 00:06:28.824 --> 00:06:34.189 Vary lamp distance while controlling other conditions; measure oxygen production over time. 00:06:38.867 --> 00:06:46.407 Estimate photosynthesis rate by counting bubbles released in a fixed time, or collect the gas and measure its volume per unit time. 00:06:46.557 --> 00:06:50.701 Oxygen production provides an indicator of photosynthesis rate. 00:06:53.367 --> 00:06:56.629 Allow the plant to adjust at each distance before measuring. 00:06:56.779 --> 00:07:01.621 Repeat readings and calculate a mean to reduce the effect of random variation. 00:07:04.300 --> 00:07:10.741 Keep temperature, carbon dioxide supply, pondweed species and size, and measurement time controlled. 00:07:10.891 --> 00:07:16.863 A lamp can also heat the water; monitor temperature and use an appropriate heat-control arrangement. 00:07:19.533 --> 00:07:23.231 Bubble counts are an estimate because bubbles can differ in size. 00:07:23.381 --> 00:07:30.281 Measuring gas volume is usually a better comparison than assuming that every bubble contains the same amount of gas. 00:07:32.933 --> 00:07:35.897 Rate equals gas volume divided by time. 00:07:36.047 --> 00:07:43.599 For example, 3 cubic centimetres of oxygen collected in 2 minutes gives a rate of 1.5 cubic centimetres per minute. 00:07:46.267 --> 00:07:54.393 Plot rate against light intensity, or use 1 divided by distance squared as an estimate of relative intensity when appropriate. 00:07:54.543 --> 00:07:57.337 Interpret any plateau using limiting factors. 00:08:00.000 --> 00:08:06.857 Follow school practical instructions for handling glass, cutting pondweed and using electrical equipment near water. 00:08:07.007 --> 00:08:10.623 Keep the apparatus and procedure consistent between readings. 00:08:13.300 --> 00:08:20.957 A root hair cell has a long projection that provides a large surface area for absorbing water and mineral ions from the soil. 00:08:23.633 --> 00:08:26.449 A thin cell wall gives a short movement distance. 00:08:26.599 --> 00:08:33.499 A large surface area relative to volume helps exchange, but the cell membrane still controls what enters the cell. 00:08:36.167 --> 00:08:45.295 Water enters across the partially permeable cell membrane by osmosis when the surrounding solution has a higher water concentration than the cell contents. 00:08:47.967 --> 00:08:54.888 Mineral ions can be absorbed by active transport when their concentration is lower in the soil than inside the cell. 00:08:55.038 --> 00:08:58.567 Carrier proteins use energy transferred by respiration. 00:09:01.233 --> 00:09:05.805 Nitrate ions supply nitrogen for making amino acids and proteins. 00:09:05.955 --> 00:09:11.447 Water and mineral ions pass through root tissues into the xylem for transport through the plant. 00:09:14.100 --> 00:09:20.157 Water can move through cell-wall spaces as well as across cell membranes on its way through root tissues. 00:09:20.307 --> 00:09:26.652 Do not describe every step as osmosis; osmosis specifically involves a partially permeable membrane. 00:09:29.333 --> 00:09:34.413 Xylem carries water and mineral ions mainly upwards from roots to stems and leaves. 00:09:34.563 --> 00:09:40.033 Mature xylem vessel elements are dead and have no cytoplasm obstructing the passage. 00:09:42.700 --> 00:09:49.323 The vessel elements join end to end with their end walls lost or broken down, making a continuous hollow tube. 00:09:52.000 --> 00:09:55.614 Lignin strengthens xylem vessel walls and supports the plant. 00:09:55.764 --> 00:10:00.425 It also stops the vessels collapsing as water is pulled upwards under tension. 00:10:03.100 --> 00:10:10.437 Water evaporates from moist cell surfaces inside the leaf and water vapour diffuses through stomata into the surrounding air. 00:10:10.587 --> 00:10:13.339 This loss of water vapour is transpiration. 00:10:16.000 --> 00:10:21.111 Evaporation from leaves creates a pull on the continuous water column in the xylem. 00:10:21.261 --> 00:10:26.242 Water is drawn upwards to replace what is lost, creating the transpiration stream. 00:10:28.900 --> 00:10:34.914 Water arriving at leaves supplies photosynthesis, carries mineral ions and helps keep cells turgid. 00:10:35.064 --> 00:10:37.880 Evaporation can also contribute to cooling. 00:10:40.533 --> 00:10:46.975 If a plant loses water faster than it takes it up, its cells become less firm (lose turgor) and it may wilt. 00:10:47.125 --> 00:10:52.585 Closing stomata reduces water loss but also lets less carbon dioxide enter. 00:10:55.267 --> 00:11:03.243 Higher temperature usually increases evaporation and the rate of water loss, provided the plant has enough water and stomata remain open. 00:11:05.900 --> 00:11:11.999 Air movement removes humid air near the leaf, maintaining a steep water-vapour concentration gradient. 00:11:12.149 --> 00:11:14.836 Wind can therefore increase transpiration. 00:11:17.500 --> 00:11:23.473 Low humidity gives a steeper gradient between moist air inside the leaf and drier external air. 00:11:23.623 --> 00:11:26.639 High humidity generally reduces water loss. 00:11:29.300 --> 00:11:34.323 Light often promotes stomatal opening, increasing gas exchange and water loss. 00:11:34.473 --> 00:11:38.868 Drought responses or other limiting conditions can change this pattern. 00:11:41.533 --> 00:11:44.624 Compare conditions while controlling other variables. 00:11:44.774 --> 00:11:53.123 Explain each effect using evaporation, stomatal opening or the concentration gradient, rather than just stating that a plant drinks more. 00:11:55.800 --> 00:12:02.583 Phloem transports sucrose from sources (where sugar is made or released) to sinks (where sugar is used or stored). 00:12:02.733 --> 00:12:05.780 This movement of sugar is called translocation. 00:12:08.433 --> 00:12:11.420 A leaf carrying out photosynthesis can be a source. 00:12:11.570 --> 00:12:14.819 Growing roots, fruits and storage organs can be sinks. 00:12:14.969 --> 00:12:20.963 A storage organ becomes a source when its stored carbohydrate is broken down and released as sugar. 00:12:23.633 --> 00:12:27.023 Phloem contains living sieve-tube elements joined together. 00:12:27.173 --> 00:12:33.091 Sieve plates have pores allowing sap to pass between elements; companion cells support their activity. 00:12:35.767 --> 00:12:41.175 Companion cells contain mitochondria and use energy in sugar loading and unloading. 00:12:41.325 --> 00:12:46.774 Phloem transport depends on living cells, unlike the water-conducting elements of xylem. 00:12:49.433 --> 00:12:55.257 Different phloem tubes can carry sugars upwards or downwards according to source and sink locations. 00:12:55.407 --> 00:12:58.571 Do not treat phloem as an exclusively downward flow. 00:13:01.233 --> 00:13:10.608 Compare tissues clearly: xylem transports water and mineral ions through lignified dead vessels; phloem transports sucrose using living tissue and energy. 00:13:10.758 --> 00:13:19.727 Phloem direction varies between tubes and source–sink arrangements; arrows compare possible directions, not simultaneous opposite flow in one tube. 00:13:24.400 --> 00:13:27.661 A potometer measures water uptake by a leafy shoot. 00:13:27.811 --> 00:13:35.340 Uptake can estimate transpiration because much of the water is lost through leaves, but some is used or retained by the plant. 00:13:38.000 --> 00:13:43.375 In a bubble potometer, measure how far an air bubble moves along a capillary tube in a known time. 00:13:43.525 --> 00:13:48.399 Keep the apparatus airtight and watertight and allow conditions to stabilise. 00:13:51.067 --> 00:13:54.233 Movement rate equals distance divided by time. 00:13:54.383 --> 00:13:59.229 A bubble moving 30 millimetres in 5 minutes has a rate of 6 millimetres per minute. 00:14:01.900 --> 00:14:08.267 If the capillary cross-sectional area is known, water volume taken up equals area times distance moved. 00:14:08.417 --> 00:14:11.978 Use consistent units before dividing the volume by time. 00:14:14.633 --> 00:14:22.643 For a capillary area of 0.5 square millimetres and a bubble movement of 20 mm, uptake is 10 cubic millimetres. 00:14:22.793 --> 00:14:27.235 Over 2 minutes, the volume uptake rate is 5 cubic millimetres per minute. 00:14:29.900 --> 00:14:33.487 Repeat readings under each condition and calculate a mean. 00:14:33.637 --> 00:14:43.865 Reset the bubble consistently and compare one environmental variable at a time; use results as uptake estimates, not exact direct measurements of evaporation. 00:14:46.533 --> 00:14:49.328 That completes Plant structures and their functions. 00:14:49.478 --> 00:14:52.823 Revisit the notes and test yourself on the revision website.