WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.627 Unit S 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 plus water to glucose plus 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 --> 00:05:32.363 For a suitable small light source, light intensity is proportional to 1 divided by distance squared. 00:05:32.513 --> 00:05:38.600 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 --> 00:07:43.621 For example, 3 cubic centimetres of oxygen collected in 2 minutes gives a rate of 1.5 cubic centimetres per minute. 00:07:46.300 --> 00:07:54.426 Plot rate against light intensity, or use 1 divided by distance squared as an estimate of relative intensity when appropriate. 00:07:54.576 --> 00:07:57.371 Interpret any plateau using limiting factors. 00:08:00.033 --> 00:08:06.891 Follow school practical instructions for handling glass, cutting pondweed and using electrical equipment near water. 00:08:07.041 --> 00:08:10.656 Keep the apparatus and procedure consistent between readings. 00:08:13.333 --> 00:08:20.990 A root hair cell has a long projection that provides a large surface area for absorbing water and mineral ions from the soil. 00:08:23.667 --> 00:08:26.483 A thin cell wall gives a short movement distance. 00:08:26.633 --> 00:08:33.533 A large surface area relative to volume helps exchange, but the cell membrane still controls what enters the cell. 00:08:36.200 --> 00:08:45.329 Water enters across the partially permeable cell membrane by osmosis when the surrounding solution has a higher water concentration than the cell contents. 00:08:48.000 --> 00:08:54.921 Mineral ions can be absorbed by active transport when their concentration is lower in the soil than inside the cell. 00:08:55.071 --> 00:08:58.600 Carrier proteins use energy transferred by respiration. 00:09:01.267 --> 00:09:05.838 Nitrate ions supply nitrogen for making amino acids and proteins. 00:09:05.988 --> 00:09:11.480 Water and mineral ions pass through root tissues into the xylem for transport through the plant. 00:09:14.133 --> 00:09:20.190 Water can move through cell-wall spaces as well as across cell membranes on its way through root tissues. 00:09:20.340 --> 00:09:26.685 Do not describe every step as osmosis; osmosis specifically involves a partially permeable membrane. 00:09:29.367 --> 00:09:34.446 Xylem carries water and mineral ions mainly upwards from roots to stems and leaves. 00:09:34.596 --> 00:09:40.067 Mature xylem vessel elements are dead and have no cytoplasm obstructing the passage. 00:09:42.733 --> 00:09:49.356 The vessel elements join end to end with their end walls lost or broken down, making a continuous hollow tube. 00:09:52.033 --> 00:09:55.647 Lignin strengthens xylem vessel walls and supports the plant. 00:09:55.797 --> 00:10:00.459 It also stops the vessels collapsing as water is pulled upwards under tension. 00:10:03.133 --> 00:10:10.470 Water evaporates from moist cell surfaces inside the leaf and water vapour diffuses through stomata into the surrounding air. 00:10:10.620 --> 00:10:13.372 This loss of water vapour is transpiration. 00:10:16.033 --> 00:10:21.144 Evaporation from leaves creates a pull on the continuous water column in the xylem. 00:10:21.294 --> 00:10:26.275 Water is drawn upwards to replace what is lost, creating the transpiration stream. 00:10:28.933 --> 00:10:34.947 Water arriving at leaves supplies photosynthesis, carries mineral ions and helps keep cells turgid. 00:10:35.097 --> 00:10:37.913 Evaporation can also contribute to cooling. 00:10:40.567 --> 00:10:47.009 If a plant loses water faster than it takes it up, its cells become less firm (lose turgor) and it may wilt. 00:10:47.159 --> 00:10:52.618 Closing stomata reduces water loss but also lets less carbon dioxide enter. 00:10:55.300 --> 00:11:03.277 Higher temperature usually increases evaporation and the rate of water loss, provided the plant has enough water and stomata remain open. 00:11:05.933 --> 00:11:12.033 Air movement removes humid air near the leaf, maintaining a steep water-vapour concentration gradient. 00:11:12.183 --> 00:11:14.869 Wind can therefore increase transpiration. 00:11:17.533 --> 00:11:23.506 Low humidity gives a steeper gradient between moist air inside the leaf and drier external air. 00:11:23.656 --> 00:11:26.673 High humidity generally reduces water loss. 00:11:29.333 --> 00:11:34.356 Light often promotes stomatal opening, increasing gas exchange and water loss. 00:11:34.506 --> 00:11:38.901 Drought responses or other limiting conditions can change this pattern. 00:11:41.567 --> 00:11:44.657 Compare conditions while controlling other variables. 00:11:44.807 --> 00:11:53.157 Explain each effect using evaporation, stomatal opening or the concentration gradient, rather than just stating that a plant drinks more. 00:11:55.833 --> 00:12:02.617 Phloem transports sucrose from sources (where sugar is made or released) to sinks (where sugar is used or stored). 00:12:02.767 --> 00:12:05.813 This movement of sugar is called translocation. 00:12:08.467 --> 00:12:11.453 A leaf carrying out photosynthesis can be a source. 00:12:11.603 --> 00:12:14.852 Growing roots, fruits and storage organs can be sinks. 00:12:15.002 --> 00:12:20.996 A storage organ becomes a source when its stored carbohydrate is broken down and released as sugar. 00:12:23.667 --> 00:12:27.056 Phloem contains living sieve-tube elements joined together. 00:12:27.206 --> 00:12:33.125 Sieve plates have pores allowing sap to pass between elements; companion cells support their activity. 00:12:35.800 --> 00:12:41.208 Companion cells contain mitochondria and use energy in sugar loading and unloading. 00:12:41.358 --> 00:12:46.807 Phloem transport depends on living cells, unlike the water-conducting elements of xylem. 00:12:49.467 --> 00:12:55.290 Different phloem tubes can carry sugars upwards or downwards according to source and sink locations. 00:12:55.440 --> 00:12:58.605 Do not treat phloem as an exclusively downward flow. 00:13:01.267 --> 00:13:10.641 Compare tissues clearly: xylem transports water and mineral ions through lignified dead vessels; phloem transports sucrose using living tissue and energy. 00:13:10.791 --> 00:13:20.091 Phloem direction varies between tubes and source, sink arrangements; arrows compare possible directions, not simultaneous opposite flow in one tube. 00:13:24.767 --> 00:13:28.028 A potometer measures water uptake by a leafy shoot. 00:13:28.178 --> 00:13:35.707 Uptake can estimate transpiration because much of the water is lost through leaves, but some is used or retained by the plant. 00:13:38.367 --> 00:13:43.742 In a bubble potometer, measure how far an air bubble moves along a capillary tube in a known time. 00:13:43.892 --> 00:13:48.765 Keep the apparatus airtight and watertight and allow conditions to stabilise. 00:13:51.433 --> 00:13:54.599 Movement rate equals distance divided by time. 00:13:54.749 --> 00:13:59.597 A bubble moving 30 millimetres in 5 minutes has a rate of 6 millimetres per minute. 00:14:02.267 --> 00:14:08.633 If the capillary cross-sectional area is known, water volume taken up equals area times distance moved. 00:14:08.783 --> 00:14:12.345 Use consistent units before dividing the volume by time. 00:14:15.000 --> 00:14:23.009 For a capillary area of 0.5 square millimetres and a bubble movement of 20 mm, uptake is 10 cubic millimetres. 00:14:23.159 --> 00:14:27.603 Over 2 minutes, the volume uptake rate is 5 cubic millimetres per minute. 00:14:30.267 --> 00:14:33.854 Repeat readings under each condition and calculate a mean. 00:14:34.004 --> 00:14:44.232 Reset the bubble consistently and compare one environmental variable at a time; use results as uptake estimates, not exact direct measurements of evaporation. 00:14:46.900 --> 00:14:51.611 A broad, thin leaf catches light and gives gases a short diffusion distance. 00:14:51.761 --> 00:14:59.579 Its transparent upper layer (epidermis) lets light reach the palisade mesophyll, the layer of cells containing many chloroplasts. 00:14:59.729 --> 00:15:03.743 Different tissues support light capture, transport and gas exchange. 00:15:08.400 --> 00:15:16.782 Air spaces in the spongy mesophyll connect with stomata, allowing carbon dioxide to reach photosynthesising cells and oxygen to leave. 00:15:16.932 --> 00:15:22.936 Xylem brings water and mineral ions; phloem carries dissolved products of photosynthesis away. 00:15:25.600 --> 00:15:32.426 Plants in dry conditions may have small or rolled leaves, thick waxy cuticles, hairs and sunken stomata. 00:15:32.576 --> 00:15:39.647 These features reduce exposed area or trap moist air, lowering the water-vapour gradient and reducing transpiration. 00:15:42.300 --> 00:15:51.355 Adaptations involve trade-offs: fewer or closed stomata reduce water loss but can also limit carbon-dioxide uptake and photosynthesis. 00:15:51.505 --> 00:15:57.647 Aquatic plants can have air spaces for buoyancy and stomata mainly on the upper surface of floating leaves. 00:16:00.300 --> 00:16:03.658 A tropism is growth towards or away from a stimulus. 00:16:03.808 --> 00:16:10.485 Shoots usually grow towards light (positive phototropism) and away from gravity (negative gravitropism). 00:16:10.635 --> 00:16:14.596 Roots usually grow towards gravity (positive gravitropism). 00:16:17.267 --> 00:16:22.109 Auxin is produced near shoot tips and redistributes when light comes from one side. 00:16:22.259 --> 00:16:28.210 More auxin on the shaded side stimulates cell elongation there, bending the shoot towards the light. 00:16:28.360 --> 00:16:32.970 The diagram describes growth; a shoot does not bend by muscular movement. 00:16:37.633 --> 00:16:41.613 When a plant is horizontal, auxin accumulates on the lower side. 00:16:41.763 --> 00:16:50.359 It promotes elongation in shoots so they bend upwards, but a high auxin concentration inhibits root elongation, so roots bend downwards. 00:16:53.033 --> 00:16:57.255 (Higher tier) Auxins are used in rooting powders and selective weedkillers. 00:16:57.405 --> 00:17:00.636 Suitable doses encourage roots to grow in cuttings. 00:17:00.786 --> 00:17:07.345 Some synthetic auxins disrupt the growth of broad-leaved weeds more than that of grasses, so they act selectively. 00:17:10.000 --> 00:17:17.166 (Higher tier) Gibberellins can stimulate germination, flowering and fruit development, and can help produce some seedless fruits. 00:17:17.316 --> 00:17:20.387 Commercial effects depend on the plant and the treatment. 00:17:23.067 --> 00:17:27.263 (Higher tier) Ethene is a gaseous plant hormone that helps fruit ripen. 00:17:27.413 --> 00:17:34.685 Growers control exposure during storage and transport to delay ripening, or to make fruit ripen at the same time before sale. 00:17:37.367 --> 00:17:42.871 In a tropism investigation, control light direction, plant age, water and temperature. 00:17:43.021 --> 00:17:50.465 Compare with a suitable control and measure bending or growth over time rather than simply deciding whether a plant looks different. 00:17:53.133 --> 00:17:55.928 That completes Plant structures and their functions. 00:17:56.078 --> 00:17:59.423 Revisit the notes and test yourself on the revision website.