Transport in Plants - Biology - Maryam, Naba, Zainab
Transport in Plants
8.1 Xylem and Phloem
Plant transport system
Organisms obtain substances from their environment and for plants, they obtain carbon dioxide for photosynthesis and minerals from the soil.
Plants have shapes that give them more surface area and this means that most cells are close to the surface. Water comes from further away when plants absorb it through their roots and it is transported up the leaves. The tissue that helps do this is called xylem.
Plants also have a transport system that is made up of a tissue called phloem that transports sucrose from amino acids from the leaves to its roots and flowers.
A xylem vessel is a long pipe like structure that is made of dead cells joined from each end which have been hollow, causing them to be empty and open tubes.
Xylem vessels run from roots to the stem and branch at every stem.
Xylem vessels don’t contain cytoplasm or nuclei, rather they are made up of cellulose and lignin (a waterproof, hard substance that makes the walls of xylem) which helps it stay strong, sturdy and upright.
This is how xylem vessels are related to their functions:
For the tree’s support, the walls of the vessels contain lignin which is very strong and supports weight of very heavy trees.
For leaves, xylem vessels in vascular bundles keep the leaf flat for better surface area to absorb light.
For transport, the cells are dead with no contents and water can flow easily
There are no walls between the dead cells and rather there is a tube for water to flow the way from roots to the leaves.
The walls contain lignin that make sure that the vessels stay open and do not collapse and hence water can flow easily.
The diameter of the vessels are about 15 micrometers - 200 micrometers
Vascular bundles
Xylem vessels and phloem tubes are close together and together they are called a vascular bundle.
In a root, vascular bundles are found in the centre and in stems they are found, neatly organised as a ring near the outer edge. They are also found in the leaves to support them, laying them flat to absorb sunlight.
8.2 Transport of water
Plants take water from the soil through the root hairs. These root hairs are present at the end of the root. The tip is a protective cap, while the rest of the root is covered by a layer of epidermis(cells) from which the root hairs are formed.
The function of the root hair is to absorb water and minerals from the soil.
Transport by osmosis:
Water moves into a root hair by osmosis
The cytoplasm and cell sap inside the root hair are concentrated
Water in the soil is diluted
Water diffuses into the root hair down the concentration gradient through a partially permeable membrane
Pathway of water through a plant:
Root hairs are at the edge of a root, and xylem vessels are in the centre. Which means, water must pass through the root to reach the xylem vessels before reaching the rest of the plant.
Water travels through the cortex cell by cell, then it reaches the xylem vessels in the middle
The water is then transported to the rest of the plant
When water reaches the xylem, the pressure at the top of the vessels is lowered, while the pressure at the bottom remains high. (Similar to how liquids travel through a straw when we suck it)
Transpiration:
There are usually more stomata on the underneath part of the leaf(lower epidermis)
Mesophyll cells inside the leaf are covered with a thin layer of moisture
Some of this film evaporates from the cells
Water vapour is diffused through the stomata
Water reaches the leaf then travels through the mesophyll cells from the xylem by osmosis. (To replace it)
Water is constantly taken from the xylem vessels in order to supply the cells in the leaves. This reduces pressure at the top of the xylem vessels, allowing water to flow up them. This process is called the transpiration stream.
Water is taken from the top of the xylem vessels to supply cells in the leaves.
This reduces the pressure at the top, letting water flow up.
This is called the transpiration stream.
Water moves up the xylem vessels to the mesophyll cells through osmosis
Water evaporates from the surface of the mesophyll cell walls.
The air spaces contain water vapour
Water vapour diffuses out of the air spaces, through the stomata.
Transpirational pull:
The spongy mesophyll tissue in a leaf helps to keep water moving through the plant
The large combined surface area of the mesophyll cells is in contact with the air spaces in the leaf
A lot of water evaporates from the cell walls, which then ends up getting stored as water vapour in the interconnecting air spaces
The rate at which water vapour diffuses out of the lead is greater when there are a lot of open stomata
This process of water entering through the roots and diffusing out the stomata means that water is removed from the upper ends of the xylem vessels, therefore reducing pressure in the vessels
This means that the pressure at the top of the vessels is reduced, but the pressure at the bottom is high
This pressure difference is called transpiration pull. (similar to how water moves when drinking through a straw)
Measuring transpiration rates:
The faster a plant transpires, the faster it takes up water
Potometer is used to measure the rate of transpiration
You can compare how fast the plant takes water by recording the speed of the air or water moving up the tube
Conditions that affect transpiration rate:
The rate at which water vapour diffuses out of a leaf is affected by the environment.
Transpiration is faster when the temperature is hot and when it is windy
There is always a diffusion gradient for the water vapour
Humidity also affects the transpiration rate.
High humidity means less water vapour diffuses out of the leaves
This is because there isn’t really a gradient for the water inside the leaf and the wet air outside it
Transpiration decreases as humidity increases
The leaves of a plant start wilting when there is less transpiration due to windy and hot temperatures. This is when the leaves turn soft and floppy
8.3 Translocation of sucrose and amino acids
Translocation
Leaves make carbohydrates by photosynthesis
They also use some of the carbohydrates to make amino acids, proteins, fats, oils, etc.
Sucrose and amino acids (made in leaves) are transported to other parts of the plant through phloem tubes
This movement is called translocation
Source and Sink
Source : Part of the plant where sucrose/amino acids are transported
Sink: Part that receives them
Example (normal growing time):
Leaves are the source (they make sucrose by photosynthesis)
Roots, flowers, fruits are the sink
Uses of Translocated Materials
Roots convert sucrose to starch (for storage)
Flowers use sucrose to make fructose (for nectar)
Fruits use sucrose to become sweet and juicy (to attract animals)
Wet Season (Good growing time)
The tree has leaves
Leaves do photosynthesis and make sucrose
This sucrose is moved through the phloem to other parts of the plant.
So in this season:
Leaves = source (they make sucrose)
Roots/trunk/flowers = sinks (they receive it)
Dry Season (Bad growing time)
No photosynthesis happens.. the plant can’t make new sucrose.
Instead, the starch stored in the roots and trunk is converted back into sucrose.
So now:
Roots/trunk = source (they supply)
Leaves = sinks (they receive)
Phloem and Xylem
Phloem: Transports sucrose and amino acid
Which can move up or down the plant
Xylem: Transports water
Which can only move upwards
This is because water is pulled up by transpiration
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