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B2 Q) The Structure of Leaves
B2 Q) The Structure of Leaves
Plants are made out of many different types of cells that are organised into tissue. The tissue is then organised into organs; examples of organs in plants are stems, roots and leaves. The organs work together to form organ systems that perform a variety of different activities to ensure that the plant has everything that it needs in order to grow, survive and reproduce.
Plants are made out of a variety of different types of tissue and we are going to look at some of the different types of tissue in this section. Here are the types of tissue that you are required to know:
Plants are made out of a variety of different types of tissue and we are going to look at some of the different types of tissue in this section. Here are the types of tissue that you are required to know:
- Epidermal tissue – this covers the whole of the outside of the plant
- Palisade mesophyll tissue – this is where the majority of photosynthesis takes place in a plant (the equation for photosynthesis is: carbon dioxide + water –> glucose + oxygen). Palisade mesophyll tissue is found in the leaves.
- Spongy mesophyll tissue – this is also found in the leaves underneath the palisade mesophyll tissue. The spongy mesophyll tissue contains big air pockets, which allows gases to diffuse around the leaves. The gases that diffuse are carbon dioxide (needed for photosynthesis) and oxygen (produced by photosynthesis).
- Xylem and phloem tubes – these are responsible for transporting stuff around plants. The xylem tubes are responsible for transporting water and dissolved mineral ions from the roots to the stems and then the leaves of plants; this is referred to as transpiration. The phloem tubes are responsible for transporting cell sap around the plant (cell sap is a solution that contains sugars, amino acids and other soluble food molecules that have been produced during photosynthesis in the leaves); this is referred to as translocation.
- Meristem tissue – this is found where the plant is growing; the shoots and the roots. Meristem tissue contains undifferentiated plant cells that can differentiate into lots of different types of plant cells. The type of cells that the meristem tissue differentiates into depends on the type of tissue that the plant needs to grow. Meristem tissue is essentially the plant version of human stem cells.
Structure of a Leaf
Leaves are highly adapted organs that maximise photosynthesis. The leaves of plants are made out of many different types of tissue. The diagram below is a side view of a leaf.
Leaves are highly adapted organs that maximise photosynthesis. The leaves of plants are made out of many different types of tissue. The diagram below is a side view of a leaf.
The epidermal tissue covers the top and bottom of a leaf. The epidermal tissue is covered in waxy cuticles, which helps to reduce water loss by evaporation, prevents infection and acts as a barrier against mechanical injury. The epidermal tissue is transparent so that light can pass through the epidermal tissue to the palisade mesophyll tissue where the light is used for photosynthesis.
The palisade mesophyll tissue is where the majority of photosynthesis takes place in the plant. The palisade mesophyll cells contain lots of chloroplasts because the chloroplasts are the subcellular structure responsible for photosynthesis. The palisade mesophyll cells are column shaped and arranged closely together. They are also near the top of the leaf because the top part of the leaf receives the most light; this means that there is plenty of light for photosynthesis reactions to take place in the palisade mesophyll tissue.
The bottom of the leaf is covered in epidermal tissue. There are small holes called stomata in the epidermal tissue on the bottom of leaves. The stomata facilitate gas exchange in leaves. Carbon dioxide diffuses through the stomata into a leaf where it is used for photosynthesis. Photosynthesis reactions produce oxygen, which diffuses out of a leaf through the stomata. The opening and closing of the stomata is controlled by guard cells (we will be looking at stomata and guard cells in more detail in a few sections time).
The carbon dioxide that diffuses into the leaves enters air pockets in the spongy mesophyll tissue. The cells in the spongy mesophyll tissue are packed quite loosely with lots of gaps for efficient gas exchange. The spongy mesophyll cells are also covered in a thin layer of water, which the gases dissolve in as they move in and out of cells; the thin layer of water helps the efficient exchange of gases.
Leaves also have xylem tubes and phloem tubes. The xylem tubes bring water and dissolved mineral ions from the roots through the stems to the leaves; the water is used in photosynthesis. The phloem tubes take cell sap around the plant; the products from photosynthesis (glucose) will be transported away from the leaves through the phloem tubes.
The palisade mesophyll tissue is where the majority of photosynthesis takes place in the plant. The palisade mesophyll cells contain lots of chloroplasts because the chloroplasts are the subcellular structure responsible for photosynthesis. The palisade mesophyll cells are column shaped and arranged closely together. They are also near the top of the leaf because the top part of the leaf receives the most light; this means that there is plenty of light for photosynthesis reactions to take place in the palisade mesophyll tissue.
The bottom of the leaf is covered in epidermal tissue. There are small holes called stomata in the epidermal tissue on the bottom of leaves. The stomata facilitate gas exchange in leaves. Carbon dioxide diffuses through the stomata into a leaf where it is used for photosynthesis. Photosynthesis reactions produce oxygen, which diffuses out of a leaf through the stomata. The opening and closing of the stomata is controlled by guard cells (we will be looking at stomata and guard cells in more detail in a few sections time).
The carbon dioxide that diffuses into the leaves enters air pockets in the spongy mesophyll tissue. The cells in the spongy mesophyll tissue are packed quite loosely with lots of gaps for efficient gas exchange. The spongy mesophyll cells are also covered in a thin layer of water, which the gases dissolve in as they move in and out of cells; the thin layer of water helps the efficient exchange of gases.
Leaves also have xylem tubes and phloem tubes. The xylem tubes bring water and dissolved mineral ions from the roots through the stems to the leaves; the water is used in photosynthesis. The phloem tubes take cell sap around the plant; the products from photosynthesis (glucose) will be transported away from the leaves through the phloem tubes.