Transport in Plants Flashcards
How is the xylem structurally adapted for its functions?
- Conducts water and dissolved mineral salts from the roots to other parts of the plant.
ADAPTATION: Xylem vessels are long hollow tubes without any cross walls or protoplasm (empty lumen), which provides a continuous lumen and hence, reduces water resistance to the flow of water. - Provides mechanical support for the plant.
ADAPTATION: Inner walls are strengthened by deposits of lignin to prevent collapse of vessels.1
How is the phloem structurally adapted for its functions?
- Transports manufactured food (sucrose and amino acids) from the leaves to the other parts of the plant.
ADAPTATION: Phloem sieve tube cell has only a thin layer of cytoplasm and has lost its central vacuole, nucleus and most organelles (AKA little protoplasm). This reduces the resistance to the flow of substances within the phloem. Also the ‘end walls’ called sieve plates have pores, allowing rapid flow of manufactured food substances.
ADAPTATION: Companion cells have numerous mitochondria to release energy for the active loading of sugars into the sieve tubes by active transport.
How are the vascular tissues organized in the stem VS in the root VS in the leaf?
In stem: Xylem in, Phloem out (XIPO)
In root: Xylem and phloem alternate in the center.
In leaf: Xylem top, Phloem bottom
Explain the movement of water between plant cells, and between them and the environment in terms of water potential.
- Root hairs are in close contact with the surrounding soil particles.
- Each soil particle has a thin film of liquid surrounding it. The soil solution is a dilute solution of mineral salts.
- The sap in the root hair cell is more concentrated due to the presence of sugars and mineral salts; sap has a lower water potential than the soil solution. Hence, water enters the root hair by osmosis.
Outline the pathway by which water is transported from the roots to the leaves through the xylem vessels.
(Continuation to environment to roots)
- The entry of water dilutes the root hair’s cell sap. The sap of the root hair cell now has a higher water potential than that of the next cell (cell B).
- Hence, water moves by osmosis from the root hair cell into the inner cell.
- Similarly, water passes from cell B into the next cell (cell C). This process continues until the water enters the xylem vessels and moves up the plant.
Define transpiration.
NOT IN SYLLABUS MS TAY SAID !!!!!!!!!!!!!!!!!!!!! Transpiration is the loss of water from the aerial parts of the plant, especially through the stomata of the leaves.
Describe and explain the effects of variation of air movement, temperature, humidity and light intensity on transpiration rate.
- Humidity of air:
- Increasing humidity of air will decrease water vapour concentration gradient (conc. gradient is less steep) between the leaf and the atmosphere. Thus, the rate of transpiration decreases. - Wind or air movement:
- Wind blows away the water vapour that accumulates outside the stomata. This maintains the water vapour concentration gradient between the leaf and the atmosphere. Thus, the stronger the wind, the higher the rate of transpiration.
- In still air, the water vapour that diffuses out of the leaf makes the air around the leaf more humid. This decreases the rate of transpiration. - Temperature of air:
- A rise in the temperature of the surroundings increases the rate of evaporation of water from cell surfaces. Thus, the rate of transpiration is greater at higher temperatures. - Light:
- Light affects the size of the stomata on the leaf. It will therefore affect the rate of transpiration. In sunlight, the stomata open and become wider. This increases the rate of transpiration. In darkness, the stomata close, so less water is lost from the leaf.
Describe and explain how wilting occurs.
- Wilting occurs when the rate of transpiration exceeds the rate of absorption of water by the roots.
- The cells of the plants therefore lose their turgor and become flaccid.
Define translocation.
Translocation is the transport of manufactured food substances in plants.
Illustrate the process of translocation through a translocation study using aphids.
- The fluid that exudes from the cut end of the proboscis of a feeding aphid contains sucrose and amino acids.
- If the stem is sectioned and examined under a microscope, you will see that the feeding stylet of the aphid is inserted into the sieve tube.
- This shows that translocation occurs in the phloem tissue.
Illustrate the process of translocation through a translocation study using the ‘ringing experiment’.
- Removing a ring of bark from the stem of a woody branch will result in the removal of the phloem tissue in that region.
- Swelling will be observed in the region above the ring.
- The swelling above the ring where the phloem is caused by an accumulation of sugars in the region above the ring. Furthermore, the accumulation of sugars in the cells in that region results in the lowering of water potential of the cells. Water molecules from the xylem enter the cells in the region, adding onto the swelling.
Illustrate the process of translocation through a translocation study using radioactive carbon isotope.
- Radioactivity would be detected in the phloem (as shown by darken X-ray photographic film) if radioactive CO2 is supplied to the leaf of a plant and photosynthesis of the leaf occurs.
- This shows that sugars containing radioactive carbon have translocated in the phloem.
How is the root hair cell adapted to its function of absorption?
- Long and narrow protrusion
- Increases SA:VR of root hair cell, increasing rate of absorption of water and mineral salts. - Presence of cell membrane
- Prevents cell sap from leaking out to maintain a lower water potential than soil solution. Water molecules can enter the root hair cell by osmosis. - Presence of many mitochondria
- Mitochondria release energy from aerobic respiration for active transport of ions into cell.
Describe the movement of water through a leaf.
- Water that moves out of the mesophyll cells form a thin film of moisture around the cells.
- Water from the thin film of moisture evaporates to form water vapour in the air spaces. The water vapour accumulates in the air spaces near the stomata.
- Water vapour diffuses out of the stomata into the dried air outside the leaf. This is transpiration.
- Movement of water out of the mesophyll cells to replace the thin film of moisture that has evaporated decreases the cell sap’s water potential.
- The mesophyll cells absorb water via osmosis from the cells deeper in the leaf.
- These cells, in turn, absorb water from the xylem vessels.
- This results in transpirational pull, a suction force that pulls the column of water in the xylem vessels up.
What are the advantages of wilting?
- The surface area exposed to sunlight is reduced. Excessive loss of water causes the guard cells to become flaccid and the stomata to close.
- This reduces water lost through transpiration.
