SB6

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  • Created by: qewsqw1
  • Created on: 29-10-17 14:08

SB6a: Photosynthesis

Photosynthesis is the process by which plants make carbohydrates from raw materials, using energy from light. It’s a series of chemical reactions catalysed (sped up) by enzymes. During photosynthesis:

  • light energy is absorbed by chlorophyll - a green substance found in chloroplasts in green plant cells and algae
  • absorbed light energy is used to convert carbon dioxide (from the air) and water (from the soil) into a sugar called glucose
  • oxygen is released as a by-product

The following equations summarise what happens in photosynthesis:

 

  

Some glucose is used for respiration, while some is converted into insoluble starch for storage. The stored starch can later be turned back into glucose and used in respiration.

From Textbook:

Photosynthesis occurs in chloroplasts, which contain a green substance called chlorophyll that traps energy transferred by light. Since energy enters from the surroundings, the products of photosynthesis have more energy than the reactants and so this is an endothermic reaction.

As glucose molecules are made, they are linked together to form a polymer called starch. This stays in the chloroplasts until photosynthesis stops. The starch is then broken down into simpler substances, which are moved into the cytoplasm and used to make sucrose (another type of sugar molecule). Sucrose transported around the plant and may be used to make:

  • starch (in a storage organ such as a potato)
  • other molecules for the plant (such as cellulose, lipids or proteins)
  • glucose for respiration (to release energy)

Storage organ: A plant organ used to store energy-rich substances such as starch. For example, a potato.

Leaf Structure

The structure of a leaf has adaptations so that it can carry out photosynthesis effectively.

A leaf needs:

a way to transport water to the leaf, and glucose to other parts of the plant

a way to exchange carbon dioxide and oxygen

the ability to absorb light energy efficiently

Transport

Xylem tissue delivers water from the roots to the leaf, and phloem tissue transports glucose away from the leaf. These tissues form vascular bundles in the plant.

 

Gas exchange

Gas exchange happens in the spongy mesophyll tissue of the leaf. Spongy mesophyll cells are covered by a thin layer of water and loosely packed.

When the plant is photosynthesising during the day, these features allow carbon dioxide to diffuse into the spongy mesophyll cells, and oxygen to diffuse out of it.

To get to the spongy mesophyll cells inside the leaf, gases diffuse through small pores called stomata. They also open or close to control the loss of water from leaf by the process of transpiration.

Absorbing light energy

Light absorption happens in the palisade mesophyll tissue of the leaf. Palisade cells are column shaped and packed with many chloroplasts. They are arranged closely together so that a lot of light energy can be absorbed.

 

From Textbook:

Leaves are often broad and flat, giving them a large surface area. The palisade cells near the top of the leaf

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