TRANSPORT IN PLANTS Flashcards
What does the vascular system consist of?
Xylem tissue and phloem tissue
Describe the xylem vessels and their function.
Elongated hollow tubes that are made of dead xylem cells linked end to end.
- Transport water and mineral salts from the roots to the leaves
- Provide mechanical support to the plant
How are the xylem cells adapted for their function?
- Absence of protoplasm and cross-walls -> prevent water flow through the lumen from being impeded
- Deposition of lignin on the cell walls strengthens vessel walls, providing support
Describe the phloem tissue and its function.
Consists of sieve tube elements and companion cells.
Conducts sugars and amino acids from the leaves to other parts of the plant.
Describe sieve tube elements.
Elongated, thin-walled living cells that degenerate protoplasm, and hence lack most organelles (e.g. nucleus, vacuole). They are arranged end to end, with porous walls called sieve plates between them. These pores allow food to move through the sieve tubes rapidly.
Describe companion cells.
Companion cells contain nuclei, cytoplasm and numerous mitochondria. One such cell is closely associated with each sieve tube element.
Companion cells perform the metabolic functions of the sieve tube elements.
Describe the arrangement of vascular bundles in dicotyledonous stems.
- Vascular bundles are arranged in a ring around a central pith.
- Between the ring of vascular tissue and the epidermis is the cortex.
- Within the vascular bundles, the phloem tissue is found on the side facing the cortex and the xylem on the side facing the pith.
- Between the xylem and phloem is the cambium
Describe the epidermis and its adaptation.
One-cell thick layer covered by a waterproof cuticle that minimises water loss in the stem.
Describe the cortex and its function.
Consists of a few layers of thick-walled, large and rounded cells with intercellular spaces.
Stores food.
Describe the pith and its function.
Found inside the ring of vascular bundles. Made up of thin-walled cells which serve as storage tissue for food.
What is the function of the cambium?
Cambium cells differentiate into new xylem and phloem tissues that thicken the stem.
Describe the arrangement of vascular tissue in dicotyledonous roots.
- The outermost layer of the root is the epidermis. The outer walls of these cells protrude to form unicellular root hairs. NO cuticle is present
- Layer below the epidermis is the cortex (same as stem)
- The central region of the root contains the xylem and phloem tissues. Xylem tissues radiate from the centre, with phloem tissues alternating between them
How are root hair cells adapted for their function?
- Long and narrow -> large surface area to volume ratio for rapid absorption of water and minerals
- Higher concentration of cell sap -> water enters the roots by osmosis
- Able to carry out active transport -> provide energy for active transport of mineral salts and ions
Describe the process by which water and minerals are taken up by root hair cells.
- Cell sap in the root hair cells contains sugar and ions, causing WP to be lower than that of the soil solution
- Water moves across the partially permeable cell membrane from the soil solution into the cell sap by osmosis
- Cell sap now has a higher WP than the cell sap in the adjacent cell
- Water moves across the cell membranes into the adjacent cell by osmosis
- This continues until the water enters the xylem vessels and up the plant
What mechanisms enable water to move up the stem through xylem vessels?
- Root pressure
- Transpiration
- Capillary action
Explain the root pressure mechanism.
- Root cells pump mineral salts into xylem vessels by active transport -> WP of xylem vessels is lower than WP of root cortex cells
- Water moves up into the xylem vessels by osmosis, causing cells to become turgid. This creates a root pressure that forces water to move upwards
What is the limitation of root pressure?
Only forces water to travel short distances
Explain the capillary action mechanism
Water tends to travel up very thin tubes due to the attractive forces between the water molecules being greater than the forces trying to separate them.
(smaller plants)
Define transpiration
Loss of water vapour from the stomata of the leaves through diffusion
Explain the transpiration mechanism
- During photosynthesis, stomata are open to take in carbon dioxide
- Water vapour from the intercellular air spaces in the leaves diffuses out into the air outside (concentration gradient)
- As water evaporates from the mesophyll cells, the water potential of the sap decreases.
- Mesophyll cells absorb water from adjacent cells closer to the vascular bundles by osmosis. These cells, in turn, absorb water from the xylem vessels
- This pressure suctions water up the xylem from the roots, resulting in transpiration pull
Why is transpiration important?
- Transport water and mineral salts
- Cooling of leaf
What factors affect the rate of transpiration?
- Humidity of surroundings -> higher humidity = higher concentration of water vapour in surrounding air = less steep concentration gradient = less transpiration
- Air movement -> wind removes water vapour that accumulates outside the stomata (transpiration) = maintains steep concentration gradient = more transpiration
- Temperature -> heat increases rate of evaporation = higher rate of movement of water vapour = more transpiration
- Light intensity -> more light = induced stomatal opening = more transpiration
Explain wilting
When a leaf loses water, its turgor pressure will fall -> water can enter the vacuole to restore turgor pressure
- Rate of transpiration exceeds rate of absorption of water by roots
- Cells may get water by osmosis from adjacent cells more readily than from xylem vessels
- Plant cells become flaccid -> WILT
What control mechanisms do plants have to control transpiration?
- Stomata (movement of opening)
- Leaf fall (in wintertime)
- Leaf shape and cuticle
Define translocation
Transport of sugars from the leaves to other parts of the plant by the phloem tissues
Describe the movement of sugars in translocation
- Sugars are supplied by the leaves
- Sugars are transported to the companion cells of the phloem by active transport (requires energy)
- Sugars are transported along the phloem and unloaded at sink (recipient) cells by diffusion or active transport
