Transport in plants Flashcards
Why do plants require a transport system?
Metabolic demand - carbondioxide and water need transporting to parts of the plant which do not photosynthesise and the products of photosynthesis need removing.
Size - Effective transport systems are needed to transport substances up and down the plant from the roots to the leaves.
Surface area: volume - overall plants have a relatively small one which means they cannot rely on diffusion alone to supply their cells with everything they need
Relate the structure of xylem to its function.
- Long continuous columns made of dead tissue allow for the transport of water up the plant
- Contain pits allowing water to move sideways between vessels
- Thickened with lignin providing structural support
Relate the structure of the phloem to its function.
- Contain sieve tube elements - these are made up of many cells joined end to end to form a long hollow tube which transports the assimilates
- Contains sieve plates - in areas between the cells the walls become perforated to form sieve plates which let the phloem contents through
- Companion cells are closely linked to the sieve tube elements by many plasmodesmata support the sieve tube elements by carrying out cell metabolism and regulation
Describe the structure and function of the vascular system in the roots.
Consists of xylem and phloem. Xylem arranged in an x shape to provide resistance against force. Surrounded by endodermis.Cells within the endodermis use active transport to move minerals into the xylem which lowers the water potential causing more water to move via osmosis into the xylem.
Describe the structure and function of the vascular system in the stem
Consists of xylem and phloem. Xylem on the inside of the bundle to provide support and flexibility, phloem on the outside. Layer of meristem cells that produce new xylem and phloem tissue when required.
Describe the structure and function of the vascular system in the leaves.
Consists of xylem and phloem which forms the midrib and a network of veins. Involved in transport and support.
What is transpiration?
The process of water movement through a plant and its evaporation from aerial parts such as leaves and flowers. It is a consequence of gaseous exchange when the plant opens the stomata to exchange oxygen and CO2.
Name factors that affect the rate of transpiration and explain their effects.
- Increased light increases transpiration because when its light the stomata open for gaseous exchange so more water can be lost.
- Increased temperature increases transpiration because the warmer water molecules have more energy so evaporate from the cells inside the leaf faster. This increases the water potential gradient between inside and outside of the cell so water diffuses out of the leaf faster.
- Decreased humidity increases transpiration because if the air around the plant is dry the water potential gradient between leaf and air increases which increases transpiration
- Increased air movement (wind) increases transpiration because lots of air movement blows away water molecules from around the stomata which increases water potential gradient and increases rate of transpiration.
Practical investigation: Estimating transpiration rate.
1) Cut a shoot at a slant underwater to pervent air from entering xylem
2) Assemble the potometer in water and insert the shoot underwater
3) Remove apparatus from water but keep capillary tube submerged in a beaker of water
4) Check apparatus is watertight and airtight
5) Dry the leaves allow time for the shoot to acclimatise and then shut the tap
6) Remove the end of the capillary tube from the beaker of water until one air bubble has formed then but the end of the tube back in the water
7) Record the starting position of the air bubble
8) Start a stopwatch and record the distance moved by the bubble per unit of time
What is meant by the apoplastic pathway?
A method of osmosis through the root hair cells where water moves through the cell walls and intercellular spaces. The water can carry solutes and move from an area of high hydrostatic pressure to an area of low hydrostatic pressure
Explain what is meant by the symplastic pathway.
A method of osmosis through the root haur cell where water moves through the cytoplasm via the plasmodesmata. To begin this pathway water must be actively transported into cells.
Explain the cohesion-tension theory.
Water molceules form hydrogen bonds with each other causing them to ‘stick’ together (cohesion). The surface tension of the water also creates this sticking effect therefore as water is lost through transpiration more can be drawn up the stem from the roots.
Explain how adhesion helps the movement of water.
Water molecules are attracted to the walls of the xylem vessels which helps water to rise up through the xylem vessels.
Give adaptatios of the xerophytes that allow them to live in dry conditions.
- Hairs on the surface - trap a layer of air saturated with water vapour reducing the water vapour potential gradient.
- Sunken stomata in pits - trap a pocket of air saturated with water vapour reducing the water vapour potential gradient
- Rolling leaves - the air saturated with water vapour is trapped within the leaf so the water vapour potential gradient is reduced.
- Maintain a high salt concentration in cells which lowers water potential - this is useful in roots as it enables more water to enter by osmosis.
Give ways hydrophytes are adapted to survive in/ on water.
- Air spaces in the leaves helps the leaves to float increasing the amount of light they receive allowing them to photosynthesise.
- Air spaces in the roots and stems so oxygen can move from the leaves to the parts of the plant that are underwater
- Stomata on the upper surface to maximise gas exchange
- Flexible leaves and stems to prevent damage by water currents
