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Plants, Water and Nutrients
Transcript of Plants, Water and Nutrients
Water is used as a transport medium in plants as it can dissolve both the organic molecules created by the plant and the ions it absorbs from the soil.
Minerals ions are absorbed through the roots and transported via the xylem to the other tissues.
Organic molecules such as sugars and amino acids are produced in the leaves and are transported to those areas that need them via the phloem.
A plant that has not had enough water it will wilt.
When a plant has had enough water the vacuoles inside the cell are full. The result is that the cell is pressing against the cell wall surrounding it. The cell is turgid.
If there isn't enough water in the cell it cannot press against the cell walls (which don't change shape) and the structure is not as well supported.
However if a cell gets too much water the cells might burst. This is known as plasmolysis. You are left with just the cell walls.
Water is used in photosynthesis and without water this cannot take place.
Water is also used to break up starch into glucose molecules.
Factors affecting transpiration
Water is also used as a solvent within cells allowing the diffusion and transport of materials throughout the cell
Transpiration is the loss of water through the stomata in the leaves by evaporation.
Stomata are pores in the bottom of leaves that allow gas exchange to occur. They are controlled by guard cells which can open and close the stomata depending on the situation.
Temperature, light intensity, wind speed and the structure of a leaf all affect the rate of transpiration. How?
To measure transpiration you need to measure the amount of water that is evaporated from the leaves or is taken up by the roots.
A bubble potometer measures the uptake of water by the plant. As water is taken up by the plant the water within the tube is drawn up. The rate of transpiration is determined by the amount the water moves in a certain time.
In a mass potometer you measure the evaporation of water from the plant. The plant roots are submerged in water and the beaker placed on a balance. Over time the total volume will drop. This shows the evaporation of water.
What are some of the issues with either method?
Transpiration occurs due to 3 factors
Water moves up the xylem of the plant through capillary action. The water adheres to the side of the xylem vessels and is pulled up the plant.
The narrower the vessel the further the water will travel vertically up the tube.
Water molecules "stick" together. This is called cohesion. Inside the xylem vessels they form a continuous column.
As water molecules evaporate from the top they "pull" the molecules behind them up the xylem.
This is known as transpirational pull.
Water moves into the root by osmosis. The concentration of solutes within the cell is much greater than the concentration within the soil.
Because more water is moving into the root cells the concentration of solutes in the root cells in comparison to the other cells in the plant drops.
The water will move into the neighbouring cells and this process repeats itself until the water reaches the xylem vessels. This is root pressure.
Leaves are organs that adapted to carry out photosynthesis. They consist of a number of tissues working together.
Using your knowledge and available resources describe how the arrangement of tissues within the leaf allow it to maximise its rate of photosynthesis.
The stomata allow the plants to control the rate of transpiration as well as the rate of gas exchange.
Explain how these two rates can cause conflicts when the plant is trying to preserve its water but maintain its rate of photosynthesis.
Plants in dry environments will have fewer, smaller stomata. Why?
Plants use two different transport vessels to transport materials through the plant.
Cross-section of a root
The xylem transports water and dissolved minerals from the roots up to the leaves (passing through the other tissues in between) by way of the transpirational flow we saw earlier.
The phloem moves organic molecules such as sucrose and amino acids (dissolved in water, forming sap) from areas where they are in high concentration to areas where they are in a lower concentration.
This can be from photosynthesising leaves to the stem or roots or from a bulb to a growing shoot.
Sugars can also be moved to storage organs such as potatoes or carrots where they are converted to insoluble starch.
Plant nutrient requirements
Plants need a variety of mineral ions to allow them to grow and survive.
These ions are moved into the roots through active transport and transported throughout the plant by the xylem.
Nitrates are used as a source of nitrogen which is a key component of amino and nucleic acids.
Potassium is used for many cell processes and a deficiency results in poor flowers, few seeds, leaves that look scorched with yellow veins.
Phosphate is essential for root development.
Calcium is essential for a strong stalk.
Magnesium is used to create chlorophyll.
Sulfate is another essential component for healthy leaves.
In a natural ecosystem, minerals get recycled back into the soil through decomposition. Farming however depletes the soil of nutrients.
To replace lost mineral ions you can add fertiliser to the soil. This can be natural (e.g. manure) or artificial (made from minerals).
Artificial fertilisers will contain NPK plus trace amounts of the other nutrients depending on the plants you are growing and their needs.
Roots have a number of adaptations to maximise water uptake:
A large surface area due to its branching structure and the presence of root hair cells.
The projection of the root hair cell is water permeable
The root hair cells have a lower water concentration allowing osmosis to move water into the cell
The concentration of water in the soil surrounding the water is higher than that inside the cytoplasm of the root hair cell. As a result water will move down the concentration gradient from the soil into the root hair cell.
As the water moves into the root hair cell, the water concentration inside the cell starts to rise and becomes higher than that of the neighbouring cells.
Water will move by osmosis from one cell to the next until it reaches the xylem vessels in the vascular bundle in the center of the root.
Much of the water does not move from cell to cell, but rather moves through the permeable cell walls towards the xylem.
Roots must carry out active transport to move mineral ions into the cells.
The concentration of ions dissolved in the water in the soil is much lower than that in the root hair cell.
To move these ions into the cell the root hair cell uses active transport. This uses energy to move the ions up the concentration gradient from a low concentration to a high concentration.