Xylem Transport (9.1) Flashcards
Describe the structure of a leaf.
Epidermis (single layer of cells) - protects the leaf from physical damage and pathogens. Coated with the cuticle which prevents water loss.
Palisade mesophyll - a layer of cells containing chloroplasts under the upper epidermis. Site of photosynthesis.
Spongy mesophyll - an area of loosely packed parenchyma (cellular tissue) which contains air spaces. Main site of gas exchange. Vascular bundle (including xylem and phloem) are located here.
Stomata are located on the lower epidermis of the leaf. Allows gases and water vapor into and out of the leaf.
Describe the structure of a stem.
Epidermis (single layer of cells) - protects the stem.
Cortex - assists with the transport and storage of materials within the stem.
Cambium - circular layer of undifferentiated cells responsible for lateral growth of the stem. Vascular bundles are arranged around the cambium, xylem in the interior and phloem in the exterior. Remember ‘o’ for outside.
Pith - assists with the transport and storage of materials within the stem.
How is the distribution of vascular bundles in stems different in monocotyledons and dicotyledons?
Monocotyledons have vascular bundles spread through the stem while dicotyledons have vascular bundles in a ring near the outside of the stem.
What are xylem?
Specialised structures that function to facilitate the movement of water throughout the plant. i.e. transports water from the roots to the leaves.
Describe the structure of the xylem.
Tube composed of dead cells that are hollow to allow for the free movement of water. Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only. The cell wall contains numerous pores, which enables water to be transferred between cells. Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension.
What are the different ways in which xylem can be reinforced by lignin?
In spiral vessels, the lignin is present in the form of a helix or coil.
In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other.
Define transpiration.
The loss of water vapour from the stems and leaves of plants.
How is transpiration affected by the level of photosynthesis?
Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis). As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis.
How does water move in transpiration steam?
Water is absorbed from the soil into the roots and then into the xylem by osmosis. The forces of cohesion and adhesion cause the water molecules to form a column in the xylem. Water rises through the xylem vessels to the leaf where it evaporates and leaves the plant via diffusion through the stomata.
How is water loss regulated in leaves?
The opening and closing of the stomata. Transpiration rates will be higher when stomatal pores are open than when they are closed.
When a plant begins to wilt from water stress, dehydrated mesophyll cells release abscisic acid (ABA). This triggers the efflux of potassium ions from the guard cells, decreasing water pressure within the cells and causing them to lose turgor. A loss of turgor makes the stomatal pore close as the guard cells become flaccid and block the opening.
Aside from the level of photosynthesis, what other factors affect transpiration rates?
Humidity, temperature, light intensity and wind.
What properties of water allow it to pass through the xylem?
Cohesion (the attraction between two particles of the same substance eg. two water molecules) and adhesion (the attraction between two particles of different substance eg. a water molecule and the xylem wall).
How can the rate of transpiration be measured?
- Bubble potometer (measuring the distance the water level drops in a graduated tube).
- Mass potometer (measuring the reduction in mass of the potometer).
What is a potometer?
A piece of apparatus used to measure the rate of water loss from a plant.
Describe the transpiration rate practical.
A cut plant stem is sealed into the potometer using a rubber bung or vaseline. An air bubble is introduced to the capillary tube. The distance that the bubble travels shows how much water the stem has taken up.
