IGCSE NOTES : Biology - Leaf structure
The relationship between a leaf and the rest of the plant is described in other topics. A typical leaf of a broad-leaved plant is needed as well as a transverse section through the leaf.) It is attached to the stem by a leaf stalk, which continues into the leaf as a midrib. Branching from the midrib is a network of veins that deliver water and salts to the leaf cells and carry away the food made by them. As well as carrying food and water, the network of veins forms a kind of skeleton that supports the softer tissues of the leaf blade. The leaf blade (or lamina) is broad.
The epidermis is a single layer of cells on the upper and lower surfaces of the leaf. There is a thin waxy layer called the cuticle over the epidermis.
In the leaf epidermis there are structures called stomata (singular = stoma). A stoma consists of a pair of guard cells surrounding an opening or stomatal pore. In most dicotyledons (i.e. the broad-leaved plants; see ‘Features of organisms’ in other topics), the stomata occur only in the lower epidermis. In monocotyledons (i.e. narrowleaved plants such as grasses) the stomata are equally distributed on both sides of the leaf.
The tissue between the upper and lower epidermis is called mesophyll. It consists of two zones: the upper palisade mesophyll and the lower spongy mesophyll. The palisade cells are usually long and contain many chloroplasts. Chloroplasts are green organelles, due to the presence of the pigment chlorophyll, found in the cytoplasm of the photosynthesising cells. The spongy mesophyll cells vary in shape and fit loosely together, leaving many air spaces between them. They also contain chloroplasts. Veins (vascular bundles) The main vein of the leaf is called the midrib. Other veins branch off from this and form a network throughout the leaf. Vascular bundles consist of two different types of tissues, called xylem and phloem. The xylem vessels are long thin tubes with no cell contents when mature. They have thickened cell walls, impregnated with a material called lignin, which can form distinct patterns in the vessel walls, e.g. spirals. Xylem carries water and salts to cells in the leaf. The phloem is in the form of sieve tubes. The ends of each elongated cell are perforated to form sieve plates and the cells retain their contents. Phloem transports food substances such as sugars away from the leaf to other parts of the plant.
Functions of parts of the leaf
The epidermis helps to keep the leaf’s shape. The closely fi tting cells (Figure 6.18(c)) reduce evaporation from the leaf and prevent bacteria and fungi from getting in. The cuticle is a waxy layer lying over the epidermis, which helps to reduce water loss. It is produced by the epidermal cells.
Changes in the turgor and shape of the guard cells can open or close the stomatal pore. In very general terms, stomata are open during the hours of daylight but closed during the evening and most of the night. This pattern, however, varies greatly with the plant species. A satisfactory explanation of stomatal rhythm has not been worked out, but when the stomata are open (i.e. mostly during daylight), they allow carbon dioxide to diffuse into the leaf where it is used for photosynthesis. If the stomata close, the carbon dioxide supply to the leaf cells is virtually cut off and photosynthesis stops. However, in many species, the stomata are closed during the hours of darkness, when photosynthesis is not taking place anyway. It seems, therefore, that stomata allow carbon dioxide into the leaf when photosynthesis is taking place and prevent excessive loss of water vapour (see ‘Transpiration’ other topics) when photosynthesis stops, but the story is likely to be more complicated than this.
The detailed mechanism by which stomata open and close is not fully understood, but it is known that in the light, the potassium concentration in the guard cell vacuoles increases. This lowers the water potential (see ‘Osmosis’ in other topics) of the cell sap and water enters the guard cells by osmosis from their neighbouring epidermal cells. This infl ow of water raises the turgor pressure inside the guard cells. The cell wall next to the stomatal pore is thicker than elsewhere in the cell and is less able to stretch. So, although the increased turgor tends to expand the whole guard cell, the thick inner wall cannot expand. This causes the guard cells to curve in such a way that the stomatal pore between them is opened.
When potassium ions leave the guard cell, the water potential rises, water passes out of the cells by osmosis, the turgor pressure falls and the guard cells straighten up and close the stoma. Where the potassium ions come from and what triggers their movement into or out of the guard cells is still under active investigation. You will notice that the guard cells are the only epidermal cells containing chloroplasts. At one time it was thought that the chloroplasts built up sugar by photosynthesis during daylight, that the sugars made the cell sap more concentrated and so caused the increase in turgor. In fact, little or no photosynthesis
takes place in these chloroplasts and their function has not been explained, though it is known that starch accumulates in them during the hours of darkness. In some species of plants, the guard cells have no chloroplasts.
The function of the palisade cells and – to a lesser extent – of the spongy mesophyll cells is to make food by photosynthesis. Their chloroplasts absorb sunlight and use its energy to join carbon dioxide and water molecules to make sugar molecules as described earlier in this chapter. In daylight, when photosynthesis is rapid, the mesophyll cells are using up carbon dioxide. As a result, the concentration of carbon dioxide in the air spaces falls to a low level and more carbon dioxide diffuses in from the outside air, through the stomata. This diffusion continues through the air spaces, up to the cells which are using carbon dioxide. These cells are also producing oxygen as a by-product of photosynthesis. When the concentration of oxygen in the air spaces rises, it diffuses out through the stomata.
The water needed for making sugar by photosynthesis is brought to the mesophyll cells by the veins. The mesophyll cells take in the water by osmosis because the concentration of free water molecules in a leaf cell, which contains sugars, will be less than the concentration of water in the water vessels of a vein. The branching network of leaf veins means that no cell is very far from a water supply. The sugars made in the mesophyll cells are passed to the phloem cells of the veins, and these cells carry the sugars away from the leaf into the stem. The ways in which a leaf is thought to be well adapted to its function of photosynthesis are listed in the next paragraph.
Adaptation of leaves for photosynthesis
When biologists say that something is adapted, they mean that its structure is well suited to its function. The detailed structure of the leaf is described in the fi rst section of this chapter and although there are wide variations in leaf shape, the following general statements apply to a great many leaves.
- Their broad, fl at shape offers a large surface area for absorption of sunlight and carbon dioxide.
- Most leaves are thin and the carbon dioxide only has to diffuse across short distances to reach the inner cells.
- The large spaces between cells inside the leaf provide an easy passage through which carbon dioxide can diffuse.
- There are many stomata (pores) in the lower surface of the leaf. These allow the exchange of carbon dioxide and oxygen with the air outside.
- There are more chloroplasts in the upper (palisade) cells than in the lower (spongy mesophyll) cells. The palisade cells, being on the upper surface, will receive most sunlight and this will reach the chloroplasts without being absorbed by too many cell walls.
- The branching network of veins provides a good water supply to the photosynthesising cells. No cell is very far from a water-conducting vessel in one of these veins. Although photosynthesis takes place mainly in the leaves, any part of the plant that contains chlorophyll will photosynthesise. Many plants have green stems in which photosynthesis takes place.
Plants need a source of nitrate ions (NO3−) for making amino acids. Amino acids are important because they are joined together to make proteins, needed to form the enzymes and cytoplasm of the cell. Nitrates are absorbed from the soil by the roots. Magnesium ions (Mg2+) are needed to form chlorophyll, the photosynthetic pigment in chloroplasts. This metallic element is also obtained in salts from the soil.
Sources of mineral elements and effects of their deficiency
The substances mentioned previously (nitrates, magnesium) are often referred to as ‘mineral salts’ or ‘mineral elements’. If any mineral element is lacking, or deficient, in the soil then the plants may show visible deficiency symptoms. Many slow-growing wild plants will show no deficiency symptoms even on poor soils. Fast-growing crop plants, on the other hand, will show distinct deficiency symptoms though these will vary according to the species of plant. If nitrate ions are in short supply, the plant will show stunted growth. The stem becomes weak. The lower leaves become yellow and die, while the upper leaves turn pale green. If the plant is deficient in magnesium, it will not be able to make magnesium. The leaves turn yellow from the bottom of the stem upwards (a process called chlorosis). Farmers and gardeners can recognise these symptoms and take steps to replace the missing minerals. The mineral elements needed by plants are absorbed from the soil in the form of salts. For example, a plant’s needs for potassium (K) and nitrogen (N)
might be met by absorbing the ions of the salt potassium nitrate (KNO3). Salts like this come originally from rocks, which have been broken down to form the soil. They are continually being taken up from the soil by plants or washed out of the soil by rain. They are replaced partly from the dead remains of plants and animals. When these organisms die and their bodies decay, the salts they contain are released back into the soil. This process is explained in some detail, for nitrates. In arable farming, the ground is ploughed and whatever is grown is removed. There are no dead plants left to decay and replace the mineral salts. The farmer must replace them by spreading animal manure, sewage sludge or artificial fertilisers in measured quantities over the land. Three manufactured fertilisers in common use are ammonium nitrate, superphosphate and compound NPK.
Ammonium nitrate (NH4NO3)
The formula shows that ammonium nitrate is a rich source of nitrogen but no other plant nutrients. It is sometimes mixed with calcium carbonate to form a compound fertiliser such as ‘Nitro-chalk’.
These fertilisers are mixtures of minerals. They all contain calcium and phosphate and some have sulfate as well.
Compound NPK fertiliser
‘N’ is the chemical symbol for nitrogen, ‘P’ for phosphorus and ‘K’ for potassium. NPK fertilisers are made by mixing ammonium sulfate, ammonium phosphate and potassium chloride in varying proportions. They provide the ions of nitrate, phosphate and potassium, which are the ones most likely to be below the optimum level in an agricultural soil.
It is possible to demonstrate the importance of the various mineral elements by growing plants in water cultures. A full water culture is a solution containing the salts that provide all the necessary elements for healthy growth, such as
• potassium nitrate for potassium and nitrogen
• magnesium sulfate for magnesium and sulfur
• potassium phosphate for potassium and phosphorus
• calcium nitrate for calcium and nitrogen.
From these elements, plus the carbon dioxide, water and sunlight needed for photosynthesis, a green plant can make all the substances it needs for a healthy existence. Some branches of horticulture, e.g. growing of glasshouse crops, make use of water cultures on a large scale. Sage plants may be grown with their roots in flat polythene tubes. The appropriate water culture solution is pumped along these tubes. This method has the advantage that the yield is increased and the need to sterilise the soil each year, to destroy pests, is eliminated. This kind of technique is sometimes described as hydroponics or soil-less culture.