Photosynthesis

Introduction• Leaves enable photosynthesis to occur. Photosynthesis

is the process by which leaves absorb light and carbon dioxide to produce carbohydrate (food) for plants to grow. Leaves are adapted to perform their function, e.g. they have a large surface area to absorb sunlight.

• Plants have two different types of 'transport' tissue, xylem and phloem, that move substances in and around the plant. When water evaporates from the leaves, resulting in more water being drawn up from the roots, it is called transpiration.

Structure of a leaf

• The function of a leaf is photosynthesis – to absorb light and carbon dioxide to produce carbohydrates. The equation for photosynthesis is:

• Carbon dioxide and water → glucose and oxygen• Did you know:• Leaves are the source of all of food on the planet• Leaves recycle all of the world's carbon dioxide in the

air• Leaves contain the world's most abundant enzyme

Adaption Purpose

Large surface area To absorb more light

Thin Short distance for carbon dioxide to diffuse into leaf cells

Chlorophyll Absorbs sunlight to transfer energy into chemicals

Network of veins To support the leaf and transport water and carbohydrates

Stomata Allow carbon dioxide to diffuse into the leaf

Leaves are adapted in several ways to help them perform their function.Features of leaves

Inside a leaf

• The internal structure of the leaf is also adapted to promote efficient photosynthesis:

Adaption Purpose

Epidermis is thin and transparent To allow more light to reach the palisade cells

Thin cuticle made of wax To protect the leaf without blocking out light

Palisade cell layer at top of leaf To absorb more light

Spongy layer Air spaces allow carbon dioxide to diffuse through the leaf, and increase the surface area

Palisade cells contain many chloroplasts To absorb all the available light

Factors affecting photosynthesis

• Three factors can limit the speed of photosynthesis: light intensity, carbon dioxide concentration and temperature.

• Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.

• Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.

• If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.

• If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.

• In practice, any one of these factors could limit the rate of photosynthesis.

Maximising growth• Farmers can use their knowledge of these

limiting factors to increase crop growth in greenhouses. They may use artificial light so that photosynthesis can continue beyond daylight hours, or in a higher-than-normal light intensity. The use of paraffin lamps inside a greenhouse increases the rate of photosynthesis because the burning paraffin produces carbon dioxide, and heat too.

Plant Transport

• No heart, no blood and no circulation, but plants do need a transport system to move food, water and minerals around. They use two different systems – xylem moves water and solutes from the roots to the leaves – phloem moves food substances from leaves to the rest of the plant. Both of these systems are rows of cells that make continuous tubes running the full length of the plant.

Xylem

• Xylem cells have extra reinforcement in their cell walls, and this helps to support the weight of the plant. For this reason, the transport systems are arranged differently in root and stem – in the root it has to resist forces that could pull the plant out of the ground. In the stem it has to resist compression and bending forces caused by the weight of the plant and the wind.

Plant Stem• Stem – the xylem and phloem are

arranged in bundles near the edge of the stem to resist compression and bending forces.

Plant Root• Root - xylem and phloem in the centre

of the root to withstand stretching forces.

Comparison of xylem and phloem

Tissue Process What is moved Structure

Xylem Transpiration Moves water and minerals from roots to leaves

Columns of hollow, dead reinforced cells

Phloem Translocation Moves food substances from leaves to rest of plant

Columns of living cells

Root hair cells• Plants absorb water from the soil by osmosis. Root hair

cells are adapted for this by having a large surface area to speed up osmosis.

• The absorbed water is transported through the roots to the rest of the plant where it is used for different purposes:

• It is a reactant used in photosynthesis• It supports leaves and shoots by keeping the cells rigid• It cools the leaves by evaporation• It transports dissolved minerals around the plant

Leaves• Leaves are adapted for photosynthesis by having a

large surface area, and contain openings, called stomata to allow carbon dioxide into the leaf. Although these design features are good for photosynthesis, they can result in the leaf losing a lot of water. The cells inside the leaf have water on their surface. Some of this water evaporates, and the water vapour can then escape from inside the leaf by diffusion.

• To reduce loss the leaf is coated in a wax cuticle to stop the water vapour escaping through the epidermis. Leaves usually have fewer stomata on their top surface to reduce this water loss.

Stomata

• Plants growing in drier conditions tend to have small numbers of tiny stomata and only on their lower leaf surface, to save water loss. Most plants regulate the size of stomata with guard cells. Each stoma is surrounded by a pair of sausage-shaped guard cells. In low light the guard cells lose water and become flaccid, causing the stomata to close. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.

Turgidity• Most plant cells are turgid at all times. This

supports the weight of the plant, which is especially important where there is no woody tissue, such as leaves, shoot and root tip. If the plant loses water faster than it can be absorbed the cells lose turgor pressure and become flaccid. This causes the plant to wilt.

• You should be able to explain why most plants will wilt if they get flooded by sea water. (Hint: sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.)

Osmosis

• Osmosis is the movement of water molecules from an area of high concentration of water to an area of lower concentration of water through a partially permeable membrane. This can be the cell membrane. An example is the flooding of plants by sea water. Sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.