0610 · 6.2Leaf structure

Leaf adaptations for photosynthesis

0:00Hold a leaf up to the light and you will notice two things. It is very thin, and it is very flat, with a lot of surface for its size. That is no accident. A leaf is the plant's food factory, and almost every part of it is built to make photosynthesis as efficient as possible. In this video, we are going to look inside a leaf, layer by layer, and see how each part helps.

0:21By the end of this video you will be able to explain why leaves are thin and have a large surface area, identify the main structures inside a leaf in a diagram or a photo, and explain how each one adapts the leaf for photosynthesis. You need one idea from the last video. For photosynthesis, a leaf needs light, carbon dioxide and water, and it gives off oxygen. So a good leaf design must let in plenty of light, let carbon dioxide reach the cells and oxygen leave, bring in water, and carry away the sugar it makes.

0:51Start with the outside. The syllabus wants you to state that most leaves have a large surface area and are thin, and to explain how these features are adaptations for photosynthesis. A large surface area means the leaf can absorb more light, and more carbon dioxide can diffuse into it. Being thin means the distance for carbon dioxide to diffuse from the air to the cells inside is very short, so it gets there quickly.

1:15It also means light can reach all the cells inside the leaf, not just the ones at the top.

Upper leaf layers and palisade

1:20Now let us look inside, using a cross section. That is what you would see if you cut across a leaf and looked at the cut edge under a microscope. The syllabus asks you to identify these structures in the leaf of a dicotyledonous plant, the kind with a network of veins, like a bean or a rose. We will go from the top surface down to the bottom. At the very top is the cuticle.

1:41This is a thin layer of wax made by the cells underneath. It is waterproof, so it reduces water loss from the top of the leaf. And it is transparent, so it lets light straight through. Just under the cuticle is the upper epidermis. This is a single layer of flat, closely fitting cells. They have no chloroplasts, so they are transparent too. Their job is to protect the leaf, while letting light pass through to the cells below.

2:06Below that is the palisade mesophyll, which you met in the cell structure video. These are tall, column-shaped cells, packed tightly together in a layer near the top of the leaf. They contain lots of chloroplasts. Being near the top, they get the most light, and being tall and tightly packed, many of them fit into the layer, so as much light as possible is absorbed. This is where most of the photosynthesis in a leaf happens.

2:30The chloroplasts themselves are also an adaptation. They contain chlorophyll, which absorbs the light energy needed for photosynthesis. As an extra fact you do not need for the exam, in palisade cells the chloroplasts can even move towards the top of the cell, to catch more light when it is dim.

Gas exchange and transport tissues

2:47Under the palisade layer is the spongy mesophyll. These cells are rounder and more irregular in shape, and they are loosely packed, with large air spaces between them. They contain fewer chloroplasts than palisade cells, so they carry out less photosynthesis. The air spaces are an important adaptation. They allow carbon dioxide to diffuse through the leaf to reach every mesophyll cell, and oxygen to diffuse out. They also give the inside of the leaf a huge surface area of moist cell walls, where gases can dissolve and cross into the cells.

3:19Running through the mesophyll are the veins, which in a cross section appear as vascular bundles. Each vascular bundle contains two kinds of tissue, xylem and phloem. In a leaf, the xylem is usually on the upper side of the bundle, and the phloem is on the lower side. The xylem brings water and mineral ions to the leaf. Water is a raw material of photosynthesis, so the xylem supplies it to the mesophyll cells.

3:43The phloem carries away the sucrose made from the glucose of photosynthesis, to the rest of the plant. The network of veins also helps support the leaf, holding it out flat to catch the light. At the bottom of the leaf is the lower epidermis, another single layer of protective cells. In most leaves, it contains many tiny pores called stomata. A single pore is called a stoma. Each stoma is surrounded by a pair of guard cells.

4:08Stomata let carbon dioxide diffuse into the leaf from the air, and let oxygen and water vapour diffuse out. The guard cells change shape to open and close the stomata. They usually open them in the light, when the plant needs carbon dioxide for photosynthesis, and close them in the dark, to reduce water loss. Unlike other epidermis cells, guard cells contain chloroplasts. Most stomata are on the lower surface of the leaf.

4:32The lower surface is shaded and cooler, so less water evaporates out through the stomata there than it would from the top.

Following materials through a leaf

4:39Let us put it all together by following the raw materials. Light comes in through the transparent cuticle and upper epidermis, and is absorbed by the chloroplasts, mostly in the palisade mesophyll. Carbon dioxide enters through the stomata, and diffuses through the air spaces to the mesophyll cells. Water arrives through the xylem in the veins. The glucose made is turned into sucrose and carried away by the phloem. And the oxygen diffuses out through the air spaces and the stomata.

5:06Let us try an exam-style question. A diagram of a leaf cross section has labels pointing to different layers. The question asks you to identify the palisade mesophyll and explain how it is adapted for photosynthesis. To identify it, look for the layer of tall, column-shaped, tightly packed cells just below the upper epidermis. Then explain its adaptations, linking each feature to its function. Its cells contain many chloroplasts, to absorb as much light as possible.

5:33It is near the top of the leaf, so it receives the most light. And its cells are tightly packed, so there are many cells and chloroplasts to absorb as much light as possible. Always link the feature to the reason. A feature on its own will not get the mark. Watch out for these common mistakes. The first is mixing up the palisade and spongy mesophyll. The palisade cells are tall and tightly packed, near the top.

5:55The spongy cells are rounded and loosely packed, with air spaces. The second is saying that the epidermis cells photosynthesise. Most of them have no chloroplasts. Only the guard cells do. The third is mixing up xylem and phloem. Xylem brings water and mineral ions in. Phloem carries sucrose away. And the fourth is saying that stomata are for taking in oxygen. In the light, they mainly let carbon dioxide in, and oxygen and water vapour out.

6:22Let us recap. Leaves have a large surface area to absorb more light and take in more carbon dioxide, and they are thin, so carbon dioxide has a short distance to diffuse and light reaches all the cells. The waxy cuticle reduces water loss and lets light through. The upper epidermis is transparent and protective. The palisade mesophyll has tall, tightly packed cells full of chloroplasts near the top, where most photosynthesis happens.

6:47The spongy mesophyll has air spaces that let gases diffuse to every cell. The vascular bundles contain xylem, which brings water and mineral ions, and phloem, which carries sucrose away. And the stomata in the lower epidermis, opened and closed by guard cells, let carbon dioxide in and oxygen and water vapour out. Every part of the leaf, from top to bottom, helps it carry out photosynthesis.