9.1 Transport in the xylem of plants Flashcards
What is transpiration?
Water loss from the stomata in plants. Carbon dioxide and oxygen need to be exchanged in order for photosynthesis to be carried out. Carbon dioxide cannot really be absorbed through the waxy cuticle because it has a very low permeability to it, so pores through the epidermis are needed. These are called stomata and water vapour escapes through them. This is transpiration.
How do plants minimise transpiration?
Using guard cells, these are cells that are found in pairs either side of a stoma. They control the size of the stoma and can adjust from wide open to fully closed.
How can you model water transport?
- Porus pots - What looks like a test tube with holes in it. Water fills the pores within the pot demonstrating adhesion to the clay molecules within the pot. As water is drawn up the pot, cohesion causes water molecules to be drawn up the glass tubing.
- Capillary tubing - If you place capillary tubing into water the levels of water in the tubing will rise higher than the water around it, this is due to the adhesive properties of water. However if you place one in mercury it will not demonstrating water is special.
- Filter paper - Folded paper across two containers will transport water from a full one to an empty one.
What is a potometer?
It is a device used to measure water uptake in plants. It consists of a leafy shoot in a tube, a reservoir (so that you can push the air bubble back and repeat) a graduated capillary tube. A bubble in the capillary tube marks the zero point and as the plant takes water up through the roots the bubble will move along the capillary tubing. This can be timed.
How are xylem vessels adapted?
When transpiration streams pull water upwards it creates negative pressure, as volume is removed. Their walls are thickened and the thickenings are impregnated with a polymer called lignin. This strengthens the walls, so that they can withstand very low pressures without collapsing.
How are xylem vessels formed?
Xylem vessels are formed from files of cells, arranged end to end. In flowering plants, the cell wall material in some areas between adjacent cells in the file is largely removed and the plasma membranes and contents of the cells break down.
State the term for a region of rapid cell division within a plant?
Apical Meristem/ shoot apex
What is directional selection?
Directional selection is when an extreme phenotype/characteristic is favoured.
How does the transpiration stream work?
When water evaporates from the surface of the wall in the leaf, adhesion (attraction between water molecules) causes water to be drawn up through the cell wall from the nearest available supply to replace the water lost by evaporation. The nearest available supply is the xylem vessels in the veins of the leaf.
Even if the pressure in the xylem is already low the force of adhesion between water and the cell walls in the leaf is strong enough to suck water out of the xylem, further reducing its pressure.
The low pressure generates a pulling force that is transmitted through the water in the xylem vessels down the stem and to the ends of the xylem in the roots. This is called transpiration pull and is strong enough to move water upward, against the force of gravity to the top of the tallest tree.
For the plant it is a passive process with all the energy needed for it coming from the thermal energy evaporating the water at the top.
Water is special because it is able to resist these very low pressures, other liquids would break, and although water sometimes does and it is called cavitation it is very rare/
What is cavitation?
When the strings of water molecules break in the xylem.
How are minerals taken in by the roots of plants?
The concentration of mineral ions in the roots is much much higher than in the soil, so protein pumps have to be used in the roots to move mineral ions against the concentration gradient. There are separate pumps for each ion that the cell requires. Mineral ions can only be absorbed if they make contact with the pump, this can occur by diffusion or by mass flow when water carrying the ions drains through the soil. Water then passes by diffusion following the concentration gradient of ions.
However sometimes the uptake of mineral ions is slow because they bind to the soil and do not reach the pumps. To overcome this problem many plants have formed a mutualistic relationship with a type of fungus. The fungus grows on the surface of the root. The thread like hyphae of the fungus grow out into the soil and absorb mineral ions such as phosphate from the surface of the soil particles. These ions are supplied to the roots allowing the plant to grow successfully in mineral deficient soils.
What are xerophytes?
These are plants adapted to growing in deserts and other dry habitats.
How are xerophytes adapted?
- Some are ephemeral, they have a very short life cycle that is completed in the brief period when water is available, so after rainfall. Then they remain dormant as embryos inside seeds until the next rains, sometimes years later.
- Other plants are perennial and rely on storage of water in specialised leaves, stems or roots.
- Stems contain water storage tissue and swell after rainfall. Pleats allow the stem to expand and contact in volume rapidly.
- Thick waxy cuticle,
- ## Stomata only open at night when it is cooler. Carbon dioxide is stored at night and stored in a four carbon compound called malic acid. PLANTS THAT USE THIS ARE CALLED CAM PLANTS. So then they can photosynthesise in the day with their stomata closed.
What are CAM plants?
Plants that are adapted for living in deserts, where there is little water.
The open their stomata at night when there is little heat from the sun so transpiration does not occur as much. Then they store the carbon dioxide as malic acid, and so they can photosynthesise in the day with their stomata closed.
What are saline soils?
Soils that have high concentrations of salts. Plants that live there are called halophytes.
