Transport in Plants Flashcards
Function of the xylem
Transport of water and mineral ions from roots to other parts of the plant
Provide mechanical support to the plant
Structure of the xylem
Hollow and dead (no cell organelles): space available in the xylem for transport of water and mineral ions
Continuous !from roots to stem to leaves!: allows water absorbed at roots —> leaves
Lignified cell walls: strong and waterproof, provides mechanical support
Function of the phloem
The phloem transports manufactured food (dissolved sucrose and amino acids) from leaves to other parts of the plant (translocation)
Structure of phloem
Sieve plates found in sieve tube elements, facilitates the flow of substances from cell to cell along the sieve tube element
Companion cell is connected to sieve tube element and keeps it alive, to provide the energy required for translocation (transport of manufactured food)
Sieve tube element is hallow (lacks cell organelles), space available for substances to be transported in the plant
Describe what the phloem is made out of
Sieve tube elements separated by sieve plates. Each element has a companion cell
Similarities between xylem and phloem tissues
Both are tissues that are involved in the transport of substances
Contents in both tissues are dissolved in solution
Differences between xylem and phloem tissues
Substances transported: xylem (water and mineral ions), manufactured food (dissolved amino acids and sucrose)
Direction of transport: xylem (from the roots to leaves, upwards), phloem (from the source to sink, upwards and downwards)
Living or non-living tissues: xylem (non-living), phloem (living, sieve tube element and companion cell, not sieve plate)
Walls of the vessel: xylem (lignified, provides mechanical support), phloem (no lignin no support)
What are vascular bundles?
Structures where xylem and phloem tissues are found together
What is the cambium?
The cambium of vascular bundles is a group of actively dividing cells that will differentiate eventually into phloem or xylem tissues
It is found in roots, stems and leaf vascular bundles
What are root hair cells?
They are specialised epidermal cells found in roots that function to absorb water and mineral ions
Structure of a root hair cell
Concentrated cell sap: increases steepness of water potential gradient between cell and soil
Elongated extension (root hair): increases the SA:V ratio, allowing efficient absorption fo water and mineral ions from the soil
Large vacuole: allows more water and mineral salts to enter the cell
Many mitochondria: energy released during aerobic respiration is used for active transport of mineral salts into the cell
Absence of chloroplasts: unnecessary bc underground
Describe the uptake of water and mineral ions into the root hair cell
Water is taken into the root hair cell via osmosis
Water molecules move from a region of less negative water potential in the soil to an area of more negative water potential in the cell sap, across a partially permeable membrane (the cell membrane)
Mineral ions are taken into the root hair cell via active transport
Mineral ions move from an area of higher concentration in the soil to an area of lower concentration in the cell sap against a concentration gradient, requiring energy
What are the routes water molecules absorbed at the root hair cells can take to the xylem in the centre of the roots?
Apoplast pathway
Water and mineral ions move along the continuum of the interconnected porous cell walls (without entering the cells)
Symplast pathway
Water and mineral ions move from cell to cell along the continuum of the living cytoplasm where cells are connected by cytoplasmic bridges called plasmodesmata
Vacuolar pathway (transmembrane) Water and mineral ions move in and out of cells passing across cell membranes and cell walls
What is the endodermis and casparian strip, and what is its function?
The endodermis is a layer of cells located just outside the vascular bundle, and it contains a waterproof substance known as the casparian strip
The casparian strip forces water and mineral ions to pass through the cell membrane and enter the xylem to be transported to the rest of the plant, preventing the leaking back of mineral ions into the cells in the roots and the soil
Water and mineral ions that enter the xylem will be transported up the stem towards the leaves, by a force known as transpiration pull
How is xylem sap transported up the plant?
Transpiration pull: main suction force that is generated by transpiration
Capillary action: due to the adhesive and cohesive nature of water
Root pressure: pushing force caused by water entering the xylem at the roots
Define transpiration
Loss of water vapour from the aerial parts of the plant, through the stomata of the leaves
Describe the process of transpiration
Stomata are open during the daytime, to allow CO2 to enter for photosynthesis
Water molecules move out of the xylem vessel to the surrounding mesophyll cells
Water molecules evaporate from the surface of the mesophyll cells forming water vapour in the intercellular air spaces of the spongy mesophyll
Diffusion: water molecules moved from the intercellular air spaces (area of higher concentration) to the surrounding atmosphere (area of lower concentration), due to the concentration gradient
What are the functions of transpiration?
To create a suction force called transpiration pull responsible for transporting water and mineral ions up the plant in the xylem
Cooling down the plant, evaporation of water from the stomata of the plant removes the latent heat of vaporisation
Summary of the route of water up the plant
Water molecules enter the plant at the roots, absorbed by root hair cells via osmosis
Water molecules move from the root hair cells to the xylem via the apoplast, symplast and vacuolar pathways
Water molecules are transported up the xylem by transpiration pull, suction force and adhesion and cohesion
Water molecules move out of the xylem to the surrounding mesophyll cells in the leaves and is lost in the form of water vapour to the surroundings via transpiration
How does light intensity affect transpiration rate?
As light intensity increases, transpiration rate increases
Since light is a condition required for photosynthesis, stomata open larger under high light intensities to provide more CO2 for more photosynthesis, so increased gaseous exchange occurs and there is more evaporation of water through the stomata of the leaves
How does air movement affect transpiration rate?
As air movement increases, transpiration rate increases
More wind sweeps water vapour away, resulting in a lowered concentration of water vapour on the leaf surface, so the concentration gradient between the intercellular air spaces and surroundings remain high, speeding up diffusion of water molecules
How does humidity affect transpiration rate?
As humidity increases, transpiration rate decreases
Since there is more humidity and a higher concentration of water vapour in the atmosphere, the concentration gradient between the surroundings and the intercellular air spaces of the leaf decreases, so the rate of diffusion of water vapour decreases
How does temperature affect transpiration rate?
As temperature increases, transpiration rate increases
Higher temperatures result in a higher rate of evaporation of water from the surface of the mesophyll cells to the intercellular air spaces in the spongy mesophyll layer, resulting in a steeper concentration gradient between intercellular air spaces and the surroundings, so the rate of diffusion is faster
Why do plants wilt and under what conditions?
Wilting occurs when the amount of water lost via transpiration is higher than the rate of water absorption at the roots
High transpiration: high light intensity, more air movement, lower air humidity, higher temperatures
Low water absorption rate: more negative water potential of soil caused by infrequency watering, use of excessive fertiliser, or soil with high salinities
What are plant cells and stomata in wilted plants like?
Plant cells: plasmolysed
Stomata: may close to decrease transpiration rate as a protective function, decreasing photosynthesis rate
Adaptations to prevent wilting
- Thick cuticle: barrier to evaporation and shiny surface reflects heat
- Sunken stomata: harder for wind to blow water away
- Rolled leaf: trap water vapour, smaller SA of leaf exposed to atmosphere
- Trichomes/hairs on leaf surface: trap water vapour and reduce water loss
- Small, needle-like leaves: less SA, smaller evaporating surface
- Thick, fleshy stems to store water
- Fewer stomata
- Stomata opening in cooler evening
Function, components, and units of a potometer
Function: to measure the rate of water absorption/uptake of a plant
Components: water reservoir (supplies water for plant), air bubble (indicates how fast water is being moved), scale (quantifier), oil (prevents evaporation)
Units: mm/s, cm/s (speed of air bubble’s movement)
mm^3/s, cm^3/s (rate of volume of uptake of water by plant)
Define translocation
Transport of manufactured food substances such as sucrose and amino acids in the phloem tissue of plants
Translocated from the source (leaf the producer, or root the storage) to the sink (plant organ that consumes or stores sugar)
Upwards or downwards
Describe translocation
At the source, sucrose is actively transported into the sieve tube element using energy from the companion cell
The high concentration of sucrose in the sieve tube element at the source is a region of more negative water potential, so water molecules from the surrounding xylem will enter the sieve tube element via osmosis
The entry of water at the sieve tube element at the source creates an area of high hydrostatic pressure, and forces manufactured food in the sieve tube element to flow along the phloem
At the sink, sucrose is unloaded and removed from the sieve tube element, resulting in a region of less negative water potential. Water molecules in the sieve tube element will leave via osmosis back into the xylem.
Explain tissue ringing experiments
Part of the stem is removed, only the phloem not the xylem
Leaves are unaffected, but stem area above the ring became swollen and the stem area below the ring had reduced growth, since sucrose cannot be transported from the leaves to the other parts of the plant and accumulates above the ring
Describe and explain the aphid studies
Aphids feed on plant sap from the sieve tube element using their stylets
CO2 gas is used to knock out the aphids while they feed, and they are decapitated so the stylets are still embedded in the phloem and the plant sap will leak out the stylets
Based on the rate of flow of plant sap out of the stylets
Do food tests on the plant sap to determine its composition
Explain radioactivity experiments
Using carbon isotopes and radioactive 14CO2 to supply to leaves, so 14C glucose synthesised and transported in the phloem, so X-ray film identifies it as shaded/grey regions, proving products of photosynthesis are transported via the phloem
What are root pressure and capillary action?
Root pressure is upward push of xylem sap driven by the positive pressure as a result of active transport of ions
Capillary action is the upward movement of water as a consequence of its adhesive and cohesive nature
