Flashcards in transport In Plants Deck (27):
Why do plants need transport systems?
Small SA : V ratio
Direct diffusion would be too slow to meet metabolic demands
What’s the difference between herbaceous dicots and arborescent dicots?
Herbaceous dicots tend to die down at the end of the growing season to ground level
Arborescent dicots have hard lignified tissues and have a long life cycle (up to hundreds of years)
How are the vascular bundles laid out in the stem?
The vascular bundles are around the edge to give strength and support.
How are vascular bundles layer out in the roots?
The vascular bundles are in the middle to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind.
How are vascular bundles layer out in the leaves?
The midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ. Many small branching veins spread through the leaf functioning both in transport and support.
What is the structure and function of the phloem?
Transports food in the form of organic solutes around the plant.
Made of sieve tube elements — many cells joined end to end to form a long, hollow structure. With no nucleus and pores in the sieve plates.
Closely linked with companion cells by plasmodesmata. It functions as a life support system.
What is the structure and function of the xylem?
Transports water and mineral ions, and support.
A non-living structure. Hollow structures made by several columns of cells fusing together end to end as it has been lignified. So there is also a spiral of lignin to prevent it collapsing.
How are root hair cells adapted as exchange surfaces?
Large SA : V ratio
Thin surface layer for diffusion and osmosis
Concentration of solutes maintains a water potential gradient between soil water and the cell.
What is the symplast pathway?
Water moves through the symplast - the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata. Diffusion along a water potential gradient (high to low). And leaves the root hair cell via osmosis.
What is the apoplast pathway?
Water moves through the apoplast - the cell walls and the intercellular spaces due to cohesive forces between the water molecules.
Describe the movement of water into the xylem.
Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis - cells surrounding the vascular tissue containing the casparian strip. Stopping the apoplast pathway (water forced into cytoplasm) then the endodermal cells move mineral ions into the xylem by active transport, creating a water potential gradient. So water goes into the xylem via osmosis.
What is the casparian strip?
A band of waxy material called Suberin that runs around each of the endodermal cells forming a waterproof layer.
What is transpiration?
The loss of water vapour from leaves and stems of plants.
Describe the process of gas exchange.
Carbon dioxide diffuses into the leaf down a concentration gradient from the air spaces within the leaf. Oxygen also moves out of the cell into the air down a concentration gradient. At the same time water vapour evaporates from the surfaces of the leaf cells into the air spaces, and is lost through the stomata.
What is the transpiration stream?
Water moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem through the cells of the leaf where it evaporates from the freely permeable cellulose cell walls of the mesophyll cells into the air spaces. The water vapour then moves into the external air through the stomata along a diffusion gradient.
What is the cohesion-tension theory?
Water is drawn up since transpiration causes a state of tension or negative pressure in the xylem vessels. The replacement water molecules form a continuous stream due to cohesion (attractive forces between water molecules due to hydrogen bonding) and adhesion (attractive between water molecules and the inside surfaces of the lignified xylem vessels.
Evidence for the cohesion-tension theory.
Changes in the diameter of trees - high = diameter shrinks, low = diameter increases.
When a xylem is broken - air is drawn in rather than water leaking out.
How do you measure transpiration?
A potometer is a device for measuring the rate of water uptake by an isolated leafy shoot. The water in the xylem elements in the shoot is continuous with the water filling a capillary tube attached to its cut end. The rate at which a bubble of air moves along the capillary indicates the rate of water uptake, which is equivalent to the rate of transpiration.
What factors affect the rate of transpiration?
Light intensity - high = increase in open stomata, low = most will close
Relative humidity - high = decrease rate, low = increase rate
Temperature - increase = increase in kinetic energy and concentration of water vapour the external air can hold
Soil-water availability - dry = reduced
What is translocation?
Plants transport organic compounds in the phloem from sources to sinks (the tissues the need them)
What are the main sources and sinks?
Green leaves + green stems
Storage organs that are unloading their stores at the start of the growth period
Food stores in seeds when they germinate
Roots that are growing/ or actively absorbing mineral ions
Meristems that are actively dividing
Any parts of the plant that are laying down food stores
Evidence for translocation in the phloem
Describe phloem loading
Hydrogen ions are actively pumped out of the companion cell into the surrounding tissue by ATP
Then hydrogen ions return to the companion cell down a concentration gradient with sucrose (the co-transporter)
This increases the sucrose concentration so there is a build up of turf or pressure due to rigid cell walls
Therefore water molecules move in by osmosis and carries the assimilates to a lower pressure/ concentration
What are xerophytes?
Xerophytes are plants in dry habitats that have evolved a wide range of adaptations that enable them to live and reproduce in areas of low water availability
Hot or cold climate
Thick waxy cuticle
Reduced numbers of stomata
Tap roots + wide spread shallow roots
Avoiding the problem
What are hydrophytes?
Plants that grow in water or in permanently saturated soil, evolved to increase gas exchange for photosynthesis