> CAMBRIDGE IGCSEM* BIOLOGY: COURSEBOOK Making nectar, to attract pollinators Many plants reproduce sexually, producing male and female gametes in flowers, Unlike animal gametes, the ‘male gametes of flowers cannot move themselves from Place to place. Instead, they rely on insects, bats or birds to carry them, inside pollen grains, from one flower to Another (Figure 6.6), No animal is going to do this for a plant uniess it gets a reward, so flowers produce nectar ‘that animals can feed on. Nectar contains different kinds of sugar, all made from the glucose that the plant has made by photosynthesis. Figure 6.6: Insects are attracted to flowers to collect the carbohydrate-ich nectar, and protein-ich pollen, Unknown to the insects, they help the flower by pollinating it. | vectar: a sweet liquid secreted by many insect: | . pollinated flowers, to attract their pollinators Making amino acids, to make proteins Plants use some of the glucose made in photosynthesis to make amino acids These amino re can fen be used to make proteins forgowth, A cel You may remember that proteins contain not only carbon, hydrogen and oxygen, but also nitrogen. So, in order to make amino acids from glucose, plants need 4 souree of nitrogen, They get this from the soi, in the form of nitrate ions. Usually, these ions are taken in by active transport, through the root hairs. The ions can be transported to all parts of the plant, where they can be combined with ghicose to make amino acids If a plant cannot get enough nitrate ions, it will not be able to synthesise proteins effectively, and so will not ‘grow quickly or strongly Making other substances, e.g. chlorophyll Glucose can also be used to make chlor Chlorophyll is nota protein, but it does ntti It also contains magnesium. Plants therefore nc take in magnesium ions, as well as nitrate ions. | chlorophyll, Without these ions, the plant’s leas. Took yellow rather than the green we would exp there is plenty of chlorophyll present (Figure 6. |! plant cannot make lots of chlorophyll, it will nc to photosynthesise well, and therefore will not © re 6.7: These leaves are on an orange tree, growing in soil that does not contain enough magnesium ions. The tree cannot make enough chlorophyll, so the leaves are not completely green Questions 4. Starch can easily be broken down to glucose by the enzyme amylase, which is found in plaitts. Cellulose is much more difficult to break down to glucose, Suggest how these differences between starch and cellulose relate to their functions in a plant. 5 Animals do not make or store starch. What is the substance that animals store, which is made of chains of glucose molecules? Explain why some parts of a plant must have sucrose delivered to them.ee 7 Copy and complete this table. Element Arkroden | magnesium Mineral salt | nitrate ions magnesium ions ‘Why needed | t Cnr ‘eA Sochyi! and then oF proteins onino Deficiency | weak growth, | yellowing of yellow leaves | leaves, often especially between the veins 8 Making nectar costs a plant energy because it uses up glucose that the plant has made. Explain why the expense is worthwhile 6.2 Leaves Photosynthesis happens inside cl gplasts, This is Itis therefore not surprising that most plants have leaves that are perfectly adapted to hely synthesis to take place as gffickly and efficiently as possibld. The structure of a leaf A leaf consists of a broad, flat part (Figure 6.8), which js joined to the rest of the plant by ak . Insid leaf stalk are collections of Gascula ‘uwadles, which also form the yeins i es in the vascular bundles carry substances to and from the leaf (Chapter 8). ining side by sid the veins inaleaf bya — cross-section transverse section of leaf of vein Figure 6.8: The structure of a leaf You will remember that the raw materials for photosynthesis are carbon dieickeane-wtter-tnd-that sunlit needa a provide energy, Most mt he larg€ surface area allows large amounts of sunlight to fall onto the leaf. The large surface area also increases the rate at which carbon dioxide can diffuse into the lea from the air. Only 0.04% of the air is carbon dioxide, so the structure of a leaf must ensure that Ie move in really easily: yyy the feor® js Aria 2 Being thin means that sunlight can pass right through the leaf, allowing many cells inside it to photosynthesise The thinness also helps carbon dioxide to reach all the cells quite quickly by diffusion, Water is brought to the leat from the soil, in tubes called xylem (pronounced éi-lgh) vessels. These run in the vascular bundles, which form the veins in the leat You, can often see many tiny veins in a leaf if you look closely You may remember that dicol plants have-vemsarcanged in Anetsark, while monocots havessinsanranged — parallel to one another (Figures 6.9 and 6.10). ee ee Figure 6.9: The network of veins in this dicot leaf carries water to all parts of it.> CAMBRIDGE IGCSE™ BIOLOGY: COURSEBOOK Making nectar, to attract pollinators Many plants reproduce sexually, producing male and female gametes in flowers. Unlike animal gametes, the male gametes of flowers cannot move themselves from place to place. Instead, they rely on insects, bats or birds to carry them, inside pollen grains, from one flower to another (Figure 6.6). No animal is going to do this for a plant unless it gets a reward, so flowers produce nectar that animals can feed on. Nectar contains different kinds of sugar, all made from the glucose that the plant has made by photosynthesis. Figure 6.6: Insects are attracted to flowers to collect the caibohydrate-rich nectar, and protein-rich pollen. Unknown to the insects, they help the flowar by pollinating it. vectar: a sweet liquid secreted by many insect- pollinated flowers, to attract their pollinators Making amino acids, to make proteins Plants use some of the glucose made in photosynthesis to make amino acids. These amino acids can then be used to make proteins, for growth man 4 Bays You may remember that proteins contain not only carbon, hydrogen and oxygen, but also nitrogen. So, in order to make amino acids from glucose, plants need a source of nitrogen. They get this from the soil, in the form of nitrate ions. Usually, these ions are taken in by active transport, through the root hairs. The ions can be transported to all parts of the plant, where they can be combined with glucose to make amino acids Ifa plant cannot get enough nitrate ions, it will not be able to synthesise proteins effectively, and so will not grow quickly or strongly. Making other substances, e.g. chlorophyll Glucose can also be used to make chlorophyll. Chlorophyll is not a protein, but it does conta It also contains magnesium. Plants therefore need take in magnesium ions, as well as nitrate ions. t | chlorophyll. Without these ions, the plant’s leaves ' look yellow rather than the green we would expect there is plenty of chlorophyll present (Figure 6.7). plant cannot make lots of chlorophyll, it will not b ble to photosynthesise well, and therefore will not gro Figure 6.7: These leaves are on an orange tree, growing in soil that does not contain enough magnesium ions. The tree cannot make enough chlorophyll, so the leaves are "ot completely green. completely gree ——— Questions 4 Starch can easily be broken down to glucose by the enzyme amylase, which is found in plahts. Cellulose is much more difficult to break down to glucose. Suggest how these differences between starch and cellulose relate to their functions in a plant. 5 Animals do not make or store starch. What is the substance that animals store, which is made of chains of glucose molecules? 6 Explain why some parts of a plant must have sucrose delivered to them.een 7 Copy and complete this table. Element | _Avkcoaen [magnesium Mineral salt | nitrate ions magnesium rr - it ions Why needed] to make cniotocin\ = on and then oF a proteins arnino dc'ch reve Le Deficiency | weak growth, _ | yellowing of a ee section of lea yellow leaves | leaves, often especially Figure 6.8: The structure of a leaf 7 between the Figure 6.8: The structure of lee —— You will remember that the raw materials for photosynthesis are carbon 8 Making nectar costs a plant energy bi cause it use up glucose that the plant has made. Explain why the sunlight is needs t piensa Mont uot al ea lar; e expense is worthwhile. " larg€sarface area allows large amounts of sunlight to fall onto the leaf. The large surface area also increases aaeticen diffuse into the leaf 6.2 Leaves the rate at which carbon -©| from the air. Only 0.04% of the air is carbon dioxide, so Photosynthesis happens inside chloreplasts, This is the structure of a leaf must ensure that ipo move in wire theGarjnsoand (clo seh that cata really easily: yslyy the feor® js Anion AY and supply i ees Ate ‘Being thin means that sunlight can pass right through he cells in t i cane. the leaf, allowing many cells inside it to photosynthesise. = The thinness also helps carbon dioxide to reach all the Its therefore not surprising that most plants have leaves | cells quite quickly by diffusion. that are perfectly adapted to help photosynthesis to take : , - place as qffickly and efficiently as possibl). Water is brought to the leaf from the soil, in tub The structure of a leaf can often see many tiny veins ina leaf if you look closely. You may remember that dicot planis have-rermsseange_ A leaf consists of a broad. flat part (Figure 6.8), which is in q network, while monocots have-v joined to the rest of the plant by a leaf stalk. In panel o cae anotbert igures 6.9 and 6.10). llections of Gascula) Se es y Vly called xylem (pronounced Gi-lenh) vessels. These run in the vascular bundles, which form the veins in the leaf. leaf stalk are ‘usadles, which also form the in the vascular bundles carry substances to and from the leaf ici 8), llorophyllis found CASTE Spreaouneia SmERETEES se that sunlightcan reach, KEY WORDS. vascular bundles: collections of xylem tubes and plo veel Is running side by side, which form theveinsinaleaf i Figure 6.9: The network of veins in this dicot leaf carries water to all parts of it. ———————— & Fess we a 103 ) ory Sat"> _ CAMBRIDGE IGCSE™ BIOLOGY: COURSEBOOK Figure 6.10: in monocots, th : In menocots, the veins usually run par: one anther, rather than forming anetwork. ame cuticle cell wall vacuole Palisade mesophyll mesophyll layer spongy mesophyll lower epidermis guard cell Figure 6.11: A diagram of a transverse section (TS) through a leaf, cytoplasm Tissues in a leaf Although a leaf is thin, it is made up of several ys of cells. You can see these if you look at a transv. section (TS) of a leaf under a microscope (Figure II, 6.12 and 6.13). Each tissue in a Jeaf has its own function. We will from the top down, considering each one in turn. ork, On the top of the leaf is the upper epidermis. The Ils in this layer are packed tightly together, to reduce quantity of water vapour escaping from the leaf47 ey do not contain chloroplasts, so they cannot photosyn .csise ‘These cells secrete (make and release) a waxy subs\.nce, which forms a thin, transparent, waterproof covering called the cuticle, The next layer down is the palisade mesop! i. This is made up of tall, narrow cells containing very large numbers of chloroplasts. Their main function is photosynthesis. As they are close to the top of the leaf, they get plenty of sunlight, The transparent epidermis cells above them let the light through easily. nucleus chloroplast vascular bundle stoma air spaceupper epidermis, made of tightly packed cells vascular bundle, containing xylem vessels (top) and phloem tubes (below) lower epidermis, made of tightly packed cells ] Palisade mesophyll layer, made of tall, narrow cells containing many chloroplasts spongy mesophyll layer, made of more rounded cells, with fewer chloroplasts air spaces between the spongy mesophyll cells Figure 6.12: A scanning electron micrograph of a section through a leaf from a taro plant (400) Tennniateate He a nat Whe 0 A Figure 6.13: A photomicrograph of a small part of a leaf from a tea plant (x400). Can you identify some of the tissues labelled in Figure 6.117 Beneath the palisade mesophyll is the spongy mesophyll. The cells in this tissue also contain chloroplasts, but not as many as in the palisade cells. They are not as tightly packed, either. There are many air spaces between them. These spaces allow carbon dioxide and oxygen to diffuse between the air and the cells inside the leaf. The spac also allow vapour to move from the surface of the cells to the outside of the leaf. You will learn more about this in Chapter 8. aaa iio); epidermis: the outer layer of tissue on a plant; also the outer layer of an animal's skin secrete: make a useful substance and then send it out of the cell where it is made, to be used in another part of the body cuticle: a thin layer of wax that covers the upper surface of a leaf palisade mesophyll: the layer of cells immediately beneath the upper epidermis, where most photosynthesis happens spongy mesophyll: the layer of cells immediately beneath the.palisade mesophyll, where some photosynthesis happens; this tissue contains a lot of air spaces between the cells 105)> CAMBRIDGE IGCSE™ BIOLOGY: COURSEBOOK The bottom of the leaf is covered by a tissue similar to the upper epidermis, called the lower epidermis. On some leaves, this tissue makesia cuticle, but usually it does not, This is because the underside of the leaf does not often have sunlight falling onto it, so it does not get as hot and therefore does not lose as much water vapour, ‘There are openings in the lower epidermis called stomata (singular stoma), Each stoma i surounded by 2 pair of gua cells (Figure 6.14). The guard cells, unlike the other celsin the epidermis, contain chloroplasts. The ‘guard calls can change their shape, which can open and close the stomata. The stomata allow diffusion of carbon dioxide and oxygen in and out of the leaf: Water vapour also diffuses out of the leaf through the stomata, cell walls epidermal. | cells ‘guard cells stomata \ surface ofa leaf, mo 9st commonly in the lower y are surrounded by pairs of guard surface; they cells, which orclosed guard cells! a pair of ces that surrounds @ stoma and controls its opening; guard cells are the only cells in the epidermis that contain chloroplasts the stomata are open Figure 6.15 summarises how the raw materials for photosynthesis - carbon dioxide and water ~ travel chloroplast in a palisade cell, Sunlight passes through the tran) eitaciepe noms Cer 4 ai ac Carbon dioxide f 4 diffuses through |} airspaces, dioxide ao ion Wateptravel: a ess, camugss 0 ‘stomata from the here, Figure 6.15: How the a Paterils for photosynthesis go into apalisade cell. Chloroplasts You have sen thatthe palisade mesophyll cells, Spongy mesophyll cells and guard cells in a leaf all contain chloroplasts. It is inside chloroplasts that Photosynthesis happens Figure 6 16shows the structure of a chloroplast, Man, Chloroplasts are found in a cell, and the palisad ce.” onan most of them. Chloroplasts ean move aroun y inside the palisade cells, to ensure that they get the Beg, quantity of sunlight, Wepleg rn —— membranes around chloroplast fach grain Stack of membranes @ containing chlorophy| Figure 6.16: One ofthe chloroplasts in a pales, rrQuestions 9 Explain briefly how each of these features of a leaf are adaptations for photosynthesis: a havinga large surface area b being thin 10 Explain how a palisade mesophyll cell is adapted for its function, 41 Suggest why the cells in the lower epidermis of a leaf (apart from the guard cells) do not contain chloroplasts, 12 Look at the photographs in Figure 6.12 and Figure 6.13. One is taken using a scanning electron microscope (and then artificially coloured), and one with a light microscope. The magnification is almost the same for each one, ‘What differences can you see in the type of image these two kinds of microscope produce? 0 dot edd Testing a leaf for starch Leaves turn some of the glucose that they make in photosynthesis into starch. If we find starch in a leaf, that tells us if it has been photosynthesising. In this experiment, you will practise working safely, making careful observations and drawing conclusions. 6.3 Fact photosynth The rate at which photosynthesis h several environmental factors. These include: © the supply of the raw materials carbon dioxide and water the quantity of sunlight, which provides energy for the reactions «the température, because this affects the activity of enzymes. ‘As well as these environmental factors, the quantity of the chlorophyll in the leaf also helps to determine how fast photosynthesis can take place. fected by In this section, you will investigate how each of these factors affects the rate of photosynthesis. If you are the Supplement, you will also find out about ing factors. Safety: Alcohol is flammable (burns easily). Do not, collect the alcohol until you have turned out your burner. fyour alcohol (or someone else's) does start to burn, put a damp cloth over it immediately, Method 1 Boil some water in a beaker (Figure 6.17). t Take @ green leaf from a healthy plant and drop it into the boiling water. This breaks down the cell membranes in the leaf. Leave the leaf in the boiling water for about 30s. 2 Turn out the flame. 3 Put some alcohol into a boiling tube. Stand the tube of alcohol in the hot water. 4 Use the forceps to remove the softened leaf from the hot water. Drop it into the tube of alcohol. Chlorophyll is soluble in alcohol, so it will come out of the leaf. Leave the leaf in the alcohol until all the chlorophyll has come out of the leaf. 107)If a plant is given plenty of sunlight, carbon dioxide and water, the limit on the rate at which it can photosynthesise is its own ability to absorb these materials and make them react. However, quite often plants do not have unlimited supplies of these materials, and so their rate of photosynthesis is not as high as it might be, A factor that is in short supply, and that therefore limits how quickly the plant can photosynthesise, is called a limiting factor, hotosynthesise slowly. As light intensity > rate of photosynthesis increases, until the a plant cannot photosynthesise at all. In dim esising as fast as it can. At this point, Between B and C, however, light is ‘You can see this because, even whe ensity increases, the rate of photosynthesis stays the same, The graph shows that increasing the light intensity beyond B does not increase the rate of photosynthesis. The plant already has as much light as it can use. SA li (Figure 6.24). The more carbon dioxide a plant is given, the faster it can photosynthesise. But once the carbon dioxide concentration reaches a certain level, there is no further increase in the rate of photosynthesis. photosynthesis Rate of O 004 0.08 012 0.16 0.20 Concentration of carbon dioxide /% Figure 6.24: The effect of carbon dioxide concentration on the rate of photosynthesis, ictions involved in photosynthesis by enzymes, and therefore temperature 2 of ‘osynthesis in the same way as n. A graph of rate ofES ‘__ > COURSEBOOK eae Carbon dioxide diffuses into the leaf through the stomata. If the ‘stomata are closed, photosynthesis Cannot take place because the plant lacks one of its raw materials. Stomata often close if the weather is Very hot and sunny, to prevent too much water being lost. This ‘Means that on a really hot or bright day photosynthesis may slow down. Questions 13 Compare Figure 6.23 with the graph that you drew for your investigation into the effect of light intensity on the rate of photosynthesis. Are they similar in any way? What differences are there? Can you explain these differences? Look at Figure 6.24. Between which values of carbon dioxide concentration is carbon dioxide a limiting factor? Explain how you can tell. Sometimes, you may be asked to think about two limiting factors at once, The graph in Figure 6.25 shows how light intensity affects the rate of photosynthesis, at two different carbon dioxide concentrations, 30 First, look at the curve labelled A. As light in increases, up to a value of 25 a.u. ( rate of photosynthesis increases. So. intensity is a limiting factor over this intensity increases above 25 a.u, there isn rate of photosynthesis. So light intensity is no: factor now —it is not lack of light that is stopp plant from photosynthesising faster. But the graph does show a way of helping the p photosynthesise faster, at these high light intens We can give it more carbon dioxide. Curve B sh esults for a plant that was given a higher conce of carbon dioxide. Clearly, it is photosynthesis than the one with a low concentration, So, for high light intensities, carbon dioxide is a limiti these are sometimes used o; graph scale to represent quantitative differen: | between values, instead of ‘real’ units such as seconds or centimetres; this is usually because the real units would be very complicated to use oar /carbondioxide Rate of photosynthesis / bubbles per minute tration a 15 Figure 6.25: The effect of light intensity and carbon dioxide 20 2B Light intensity / arbitrary units 30 35 40, concentration on the rate of photosynthesis, ee