Edexcel Biology Gcse - Plant Organisation Part 3
Transpiration decreases when temperature is low, humidity is high, air movement is limited, or light intensity is low. These conditions reduce water evaporation and the diffusion of water vapour from the leaf surface.
Root hair cells
Plants absorb water from the soil by osmosis. They absorb mineral ions by active transport, against the concentration gradient. Root hair cells are adapted for taking up water and mineral ions by having a large surface area to increase the rate of absorption. They also contain lots of mitochondria, which release energy from glucose during respiration in order to provide the energy needed for active transport.
Key Terms
Root hair cells
Plants absorb water from the soil by osmosis. They absorb mineral ions by active transport, against the concentration gradient. Root hair cells are...
Root hair cell
| 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
...What are stomata?
Stomata are tiny holes found in the underside of leaves. They control water loss and gas exchange by opening and closing. They allow water vapour a...
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...
Size of stomata
The size of the stomatal opening is used by the plant to control the rate of transpiration and therefore limit the levels of water loss from the le...
Xylem consists of dead cells. The cells that make up the xylem are adapted to their function:
They lose their end walls so the xylem forms a continuous, hollow tube.
They become strengthened by a substance called lignin. Lignin gives s...
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| Term | Definition |
|---|---|
Root hair cells | Plants absorb water from the soil by osmosis. They absorb mineral ions by active transport, against the concentration gradient. Root hair cells are adapted for taking up water and mineral ions by having a large surface area to increase the rate of absorption. They also contain lots of mitochondria, which release energy from glucose during respiration in order to provide the energy needed for active transport. |
Root hair cell | 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 |
What are stomata? | Stomata are tiny holes found in the underside of leaves. They control water loss and gas exchange by opening and closing. They allow water vapour and oxygen out of the leaf and carbon dioxide into the leaf. |
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 bright light the guard cells take in water by osmosis and become plump and turgid. 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. Guard cells are adapted to their function by allowing gas exchange and controlling water loss within the leaf. |
Size of stomata | The size of the stomatal opening is used by the plant to control the rate of transpiration and therefore limit the levels of water loss from the leaf. This helps to stop the plant from wilting. |
Xylem consists of dead cells. The cells that make up the xylem are adapted to their function: | They lose their end walls so the xylem forms a continuous, hollow tube. They become strengthened by a substance called lignin. Lignin gives strength and support to the plant. We call lignified cells wood. |
The phloem moves food substances that the plant has produced by photosynthesis to where they are needed for processes such as: | growing parts of the plant for immediate use storage organs such as bulbs and tubers developing seeds |
Transport in the phloem | Transport in the phloem is therefore both up and down the stem. Sucrose is the transport sugar in the phloem. Transport of substances in the phloem is called translocation. Translocation requires energy as it is an active process. |
Phloem consists of living cells. The cells that make up the phloem are adapted to their function: | Sieve tubes - specialised for transport and have no nuclei. Each sieve tube has a perforated end so its cytoplasm connects one cell to the next. Sucrose and amino acids are translocated within the living cytoplasm of the sieve tubes. Companion cells - transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell(s). |
Type of transport in xylem | Physical process |
Type of transport in phloem | Requires energy |
Substances transported in xylem | Water and minerals |
Substances transported in phloem | Products of photosynthesis; includes sucrose and amino acids dissolved in water |
Direction of transport in xylem | Upwards from roots to leaves |
Direction of transport in phloem. | Upwards and downwards |
Transpiration | When the plant opens its stomata to let in carbon dioxide, water on the surface of the cells of the spongy mesophyll and palisade mesophyll evaporates and diffuses out of the leaf. This process is called transpiration. |
Xylem water molecules hydrogen bonding | Water molecules inside the xylem cells are strongly attracted to other water molecules. There is strong cohesion between the molecules because of hydrogen bonding. A continuous column of water is therefore pulled up the stem in the transpiration stream by evaporation from the leaves. |
Transpiration is an unavoidable consequence of photosynthesis - only five per cent of the water taken up by the plant is used for photosynthesis - but does have its purposes: | provides the water for photosynthesis transports mineral ions cools the leaf as water evaporates provides water that keeps the cells turgid, which supports herbaceous plants |
Water uptake and transport across the root | Root hairs are single-celled extensions of epidermal cells in the root. They grow between soil particles and absorb water and minerals from the soil. Water enters the root hair cells by osmosis. This happens because soil water has a higher water potential than the cytoplasm of the root hair cell. Minerals enter by active transport. |
The rate of transpiration is affected by several factors. These include: | temperature humidity air movement light intensity |
What is the change in factor that increases transpiration rate for the factor temperature | increase |
What is the change in factor that increases transpiration rate for the factor humidity | decrease |
What is the change in factor that increases transpiration rate for the factor air movement | increase |
What is the change in factor that increases transpiration rate for the factor light intensity | increase |
Explain how the factor temperature increases the transpiration rate | Increases molecular movement so that: more water molecules evaporate from cell surfaces; the rate of diffusion of water molecules from the leaf is increased |