Chapter 8: Transport in plants Flashcards
State the function of xylem vessels.
Xylem transports water to the leaves
List the characteristics of xylem.
- One-way only
- No end walls between cells
- Thick walls stiffened with lignin
- They are made of dead cells
- The cells have no contents, so they form a continuous hollow tube.
State the function of phloem.
The phloem transports sucrose and other molecules to the roots.
List the characteristics of phloem vessels.
- Two-way flow
- Cells have end walls with perforations
How is a xylem vessels formed?
A vessel is made up of a series of long cells joined end to end.
Once a region of the plant has ceased growing, the end walls of these cells are digested away to form a continuous, fine tube.
Why are the xylem cell walls lignified?
To prevent the free passage of water and nutrients.
This causes the cytoplasm to die
This does not affect the passage of water in the vessels.
How do plants take in water?
through the roots.
Describe the structure of the roots.
At the very tip is a root cap.
This is a layer of cells which protects the root as it grows through the soil.
The rest of the root is covered by a layer of cells called the epidermis
The root hairs are a little way up from the root tip.
Each root hair is a long epidermal cell. Root hairs do not live for very long. As the root grows, they are replaced by new ones.
State the adaptations of root hair cells.
Each root hair has a long epidermal cells, which increases the surface area available for absorption.
Root hairs are also long and thin so that they can grow in between even the smallest of soil particles.
Describe the path of water through the plant.
The water travels from the soil into the root hair cell by osmosis, from an area of high to low concentration.
From here, it moves into the root cortex cells and then into the xylem of the root, stem and finally the leaf.
Once inside the leaf, the water is moved to the mesophyll cells.
How do you investigate the path of water?
The path of water can be observed using food dye using the following method:
- Cut the base of a celery stalk (non-leaf end) under water
- Place the stalk in a beaker of water that has been stained with red food dye.
- Leave the celery stalk in the water for 24 hours in order to allow water movement
- Cut the stalk halfway to observe where the xylem vessels are placed.
Describe how water moves up the xylem vessel.
Negative water potential draws water into the root.
Cohesion and adhesion draw water up the xylem
Once in the leaf, the water moves into the mesophyll cells, before forming a thin layer on the surface and evaporating into the air spaces in the spongy mesophyll layer.
Water vapour is then lost via the stomata on the underside of the leaf.
This process is known as transpiration.
Define transpiration.
The loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by the diffusion of water vapour through the stomata.
How does water vapour leave the leaf?
Each leaf contains many air spaces in the spongy mesophyll and the air becomes saturated with water vapour.
There are hundreds of stomata enabling water vapour to diffuse from a high concentration in the air spaces into the atmosphere.
How do cell waters losing water by transpiration replace the water?
By drawing water from the nearest vein.
Most of this water travels along the cell walls without actually going inside the cells.
How does humidity affect rate of transpiration?
The higher the humidity of the air outside the leaf, the lower the rate of transpiration.
This is because water moves down its concentration gradient but because the air is saturated with water, there cannot be any transpiration.
How does temperature affect rate of transpiration?
The higher the temperature, the greater the rate of transpiration.
This is due to the water having a greater amount of kinetic energy, hence the movement will be faster.
This rate however levels off as eventually the rate that water being absorbed into the plant will equal the amount leaving it irrespective of how high the temperature gets.
