3.3.4.2 Mass Transport in Plants Flashcards
How does water out through the stomata?
Humidity of the atmosphere is less than that of the air spaces next to the stomata
When stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
Water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of surrounding mesophyll cells
By changing the size of the stomatal pores, plants can control their rate of transpiration
What is the function of xylem tissue?
It transports water and soluble materials from the roots to the leaves
How does water move across the cells of a leaf?
Mesophyll cells lose water to the air spaces by evaporation due to heat
Cells have a lower water potential so water enters via osmosis from neighbouring cells
Loss of water from neighbouring cells lowers water potential
They take water from neighbouring cells
This creates a water potential gradient where water moves to the mesophyll then into the atmosphere
How does water move up the xylem stem?
Water evaporates from mesophyll cells due to heat
Water molecules form hydrogen bonds and stick together- cohesion
Water forms a continuous column across mesophyll cells and down the xylem
As water evaporates, more molecules are drawn up via cohesion (transpiration pull)
Transpiration puts the xylem under tension which creates negative pressure (cohesion tension theory)
What is the structure of the xylem?
Long cells
Have thick cells containing lignin
Lignin waterproofs walls of cells and strengthens them
Cells die from lignin which forms a long tube
Lignification is not complete and pores (called pits or bordered pits) allow water to move between vessels or into living parts
Lignin forms rings or spirals around the vessel and strengthen the tube
What is the apoplastic pathway?
Movement between walls of neighbouring cells
What is the symplastic pathway?
Movement through plasma membranes and plasmodesmata to cytoplasms from cell to cell
What is the vacuolar pathway?
Same as the symplastic pathway but also through vacuoles
What are the effects of the cohesion tension theory?
In daytime there is greater transpiration which creates a higher tension thats pulls the xylem walls inward and shrinks the trunk Therefore the trunk is wider at night
Xylem vessel is broken which allows air to enter it. No water is drawn up because the continuous column of water is broken and cohesion is stopped
Or the xylem vessel is broken so water does not leak because air is drawn in instead which creates tension
Is transpiration pull an active/passive process?
Passive process and xylem tissue is dead so cannot actively move water
Is transpiration an active/passive process?
An active process as heat from the sun evaporates water from the leaves
How does xylem tissue withstand the pressure of cohesion tension?
It is strengthened with lignin
What is transpiration?
The loss of water vapour from upper parts of the plant
What is the endodermis?
A layer of cells surrounding the xylem
Cells move minerals by active transport into the xylem
What is capillary action in plants?
Adhesion of water to xylem vessels as they are narrow
What affects the rate of transpiration?
Leaf number, number/size/position of stomata, cuticle, light, temperature, humidity, wind, water availability
What is translocation?
The process by which organic molecules and some mineral ions are transported from one part of a plant to another
What is the function of phloem tissue?
It transports biological sugars from sites of production, sources, to the places where they are stored or used, sinks
What is the structure of the phloem?
Sieve tube elements
Long thin structures arranged end to end
Walls are perforated to form sieve plates
Associated with the sieve tube elements are cells called companion cells
What molecules does the phloem transport?
Organic: sucrose and amino acids
Inorganic: phosphate, potassium, chloride, magnesium ions
Sucrose is the most common translocated process
In which direction do phloem tissues transport material?
As sinks can be above or below the source, translocation occurs in either direction
What is the function of companion cells?
They transport products of photosynthesis to sieve tube elements through plasmodesmata
What is the tissue which transports biological molecules?
Phloem
What is a source or sink in the phloem transport system?
A source is the site where sugars are produced, e.g. the leaf
A sink is where they are used directly or stored for future use. Sinks can be anywhere in the plant
In which direction are substances moved in the phloem?
Both ways as the sink can be above or below the source
Which kinds of substances does the phloem transport?
Organic molecules including sucrose and amino acids
Inorganic ions such as potassium, chloride, phosphate and magnesium ions
Is mass flow an active or passive process in the phloem system?
Passive process but occurs as a result of the active transport of sugars do it classed as active
This means it is affected by temperature and metabolic poisons
How does sucrose move into companion cells?
Sucrose is manufactured in chloroplasts (source)
Sucrose diffuses down a concentration gradient by facilitated diffusion into companion cells
How do hydrogen ions move into companion cells?
They are actively transported
How does hydrogen move into sieve tube elements?
Hydrogen ions use carrier proteins to move into sieve tube elements
How is sucrose moved into sieve tube elements?
Sucrose is transported along with hydrogen ions via co-transport proteins
How does water move into the sieve tubes?
Sucrose produced by chloroplasts is actively transported into sieve tubes in step 1
This decreases water potential in the phloem
Water moves into the phloem by osmosis from a high water potential to a low water potential
How does water move into the sink?
At the sink, sucrose is used up or converted to starch for storage
This decreases sucrose content so sucrose is actively transported into the sink
This reduces the water potential at the sink
The low water potential causes water to move into the sink from sieve tubes by osmosis
This lowers hydrostatic pressure in the sieve tubes
What is the mass flow theory?
High hydrostatic pressure at the source and low at the sink
Mass flow of sucrose solution down this hydrostatic gradient
What are the stages of transport or organic substances in the phloem? (No explanation)
Sucrose moves into companion cells Hydrogen ions move into companion cells Hydrogen moves into sieve tube elements Sucrose moves into sieve tube elements Water moves into the sieve tubes Water moves into the sink
What is the evidence supporting the mass flow hypothesis?
Concentration of sucrose is higher at source than sink
Downward movement of phloem occurs in light but ceases in darkness
Increases in leaf sucrose levels is linked to an increase in phloem sucrose
Companion cells contain many mitochondria
What evidence contradicts the mass flow hypothesis?
Function of sieve plates is unclear as they hinder mass flow
Not all solutes move at the same speed
Sucrose is delivered at roughly the same rate to all regions
What are the steps of a ringing experiment?
Section of the outer layers of the plant are removed (protective layers and phloem)
After time, the region above will swell
The swelling is caused by the sugars of the phloem accumulating above the ring
Some tissues below the ring wither and die
This is due to the interruption of the flow of sugars below the ring
This proves that the phloem is responsible for transporting sugars
What are the steps of a tracer experiment?
Radioactive isotope used for tracing: e.g 14C
14C is used by the plant to make carbon dioxide
14C isotope is used to make sugars
Radioactive sugars can be traced as they move through the plant by x-rays
What is the evidence of translocation of organic molecules occurs in the phloem?
When the phloem is cut, a solution of organic molcules flow out
Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in phloem after a short time
The removal of a ring of phloem from around the whole circumference of a stem leads to the accumulation of sugars above the ring and their disappearance below it
What are the xerophyte adaptations in the leaves?
Hairs so ‘trap’ water vapour and water potential gradient decreased; Stomata in pits/grooves so ‘trap’ water vapour and water potential gradient decreased; Thick (cuticle/waxy) layer so increases diffusion distance; Waxy layer/cuticle so reduces evaporation/transpiration. Rolled/folded/curled leaves so ‘trap’ water vapour and water potential gradient decreased; Spines/needles so reduces surface area to volume ratio
