Transport in plants🌱 Flashcards
Waxy cuticle function
- Reduces water loss
* Reduces gas exchange in epidermis
What are palisade cells?
- Main photosynthetic cells
- Elongated at right angles to surface
- Densely packed
- Usually one layer so light is able to pass to cells before
- Contains many chloroplasts
What is spongy mesophyll?
- Loosely packed photosynthetic cells
- Contain fewer chloroplasts
- Gas exchange across cell surface
- Cell walls moist to facilitate gas exchange but results in water loss
What are guard cells?
- Change shape to open and close stomata
- Inner wall inelastic compared to outer wall
- Contain chloroplasts
What is the upper epidermis?
- Transparent layer of flattened cells so light can pass through
- No chloroplasts
- Prevents mechanical damage
What are chloroplasts?
- Organelle where photosynthesis occurs
* Able to move towards light
What’s included in the vascular bundle?
Xylem and phloem
This is where mass flow occurs
Xylem function
Transpiration - water and mineral ions to leaves
Phloem function
Translocation - sucrose to other areas
What are stomata?
- Pores in lower surface allow gas exchange with atmosphere
- Water vapour also lost
- Opens in response to light intensity
Intercellular spaces function
Air spaces allow diffusion of gases throughout the leaf
What is symbol equation for aerobic respiration?
6O2 + C6H12O6 -> 6CO2 + 6H2O
What is the symbol equation for photosynthesis?
6CO2 + 6H2O -> C6H12O6 + 6O2
What are the organs of gas exchange and photosynthesis?
Leaves
Leaf adaptations for CO2 absorption
- Thin - short diffusion distance
- Large S.A - air spaces for diffusion
- Air spaces also increase S.A of cells in contact with air
- Cells are moist - gas can dissolve
- Cuticle reduces water loss and gas exchange
- Stomata allow water vapour and gases to diffuse in and out of leaf
- Guard cells close stomata to reduce transpiration if risk of wilting
Leaf adaptations for light absorption
- Thin, flat, large S.A - intercept light
- Able to move to intercept light
- Palisade cells elongated and densely arranged under upper epidermis
- Many chloroplasts which can move towards light
- Light able to pass through spongy mesophyll and leaves below
- Xylem provides water
- Phloem removes products of photosynthesis
What are sclerenchyma fibres?
- Thickened walls due to lignin
- Cells are dead - lignin is impermeable
- Provides mechanical support
What are parenchyma fibres?
- Unspecialised plant cells
- Unthickened walls and provide the packing around other tissues
- Able to store food
- Turgidity provides support
What are collenchyma fibres?
- Living cells reinforced by addition of extra cellulose
- Often Found below epidermis
- Provide extra mechanical support
Function of lignin in xylem walls
- Strength - withstand pressures of water transport
* Impermeable - protoplasm dies, cells become hollow, little resistance for water flow
Function of pits in xylem
Non-lignified areas so water passes sideways between plasmodesmata of xylem vessels
Function of tracheids in xylem
- Elongated cells with tapering ends
- Conduct water, but less well adapted than other vessels
- No open ends - water passes from cell to cell via pits
- Found in finest branches of xylem in the leaves and the roots
Function of fibres in xylem
- Similar to sclerenchyma fibres
* Provides support
Function of parenchyma in xylem
Packing tissue that keeps other xylem elements in place
What is transpiration?
Loss of water from the surface of land plants
How does water exit the leaf?
- Stomata
- Cuticle (small amount)
- Lenticels (small amount)
What is cohesion-tension theory?
- As water molecules are removed from the xylem, more are pulled up to replace them - transpiration pull - creates tension
- Mass flow of water through xylem relies upon cohesion and adhesion of water
- The theory is the drawing of a continuous column of water up the xylem vessel - transpiration stream
What is cohesion?
Molecules stick together by weak hydrogen bonds
What is adhesion theory?
- Xylem vessels are narrow and have a hydrophilic lining
- Water molecules adhere to the walls, causing water to move up the vessel by capillarity
- Adhesion also counters the effect of gravity
Root hair adaptations
- Thin walls - short diffusion pathway
* Large surface area
How does water enter the root?
- Water in soil has a high water potential
- Water in root hair vacuole has a low water potential
- Water enters down a water potential gradient by osmosis
Epidermis function
Outer layer of cells - increased surface area for the uptake of ions and water
Root cortex function
Made of parenchyma cells - provide mechanical support to the root
Endodermis function
- Layer of cells that surround the pericycle
* Endodermal cells have a Casparian strip around them which is made of suberin - waterproof
Pericycle function
Contains the vascular tissues
Why is lignin important?
Provides strength - prevent xylem vessels from collapsing due to the negative pressure inside the vessels during transpiration
Function of sieve tubes in phloem
- Transport organic solutes
- Formed by end-to-end fusion of sieve elements
- Lack a nucleus or lignin
- Cell walls at the ends of each cell are perforated (holes) to form sieve plates - cytoplasm to run into adjacent cells
Function of companion cells in phloem
- Involved in the loading and unloading of sieve tubes with solutes
- Linked to sieve tubes by plasmodesmata
- Contain many organelles, especially mitochondria
Why are the conducting cells of the xylem dead, while the conducting cells of the phloem are alive?
Movement through xylem is passive, do cells do not need to be alive.
Movement through phloem is active, requires ATP from living cells
What is the apoplast pathway?
- Water passes through cell walls of cortex cells and spaces between them (cellulose cell walls)
- As water seeps along micro-spaces, cohesive forces mean more water is pulled along apoplast route
What is the symplast pathway?
Water passes through the cytoplasm of the cortex cells via the plasmodesmata
What is the vacuolar pathway?
Water travels from vacuole to vacuole in adjacent cells
Role of the endodermis in uptake
- Suberin is deposited in cell walls and forms waterproof Casparian strips - block the apoplast pathway
- Ions are actively transported via the symplast pathway - lowers water potential of the cells, causing water to move into symplast by osmosis - this helps generate a water potential gradient, drawing water in from the soil
- The blocking of the apoplast pathway allows the endodermis to selectively uptake ions
What affects mineral uptake?
Oxygen supply
Temperature
Respiratory inhibitors
What is root pressure theory?
- Endodermal cells actively transport ions into xylem vessels
- Lowers water potential - water enters by osmosis, creating hydrostatic pressure
- Pressure forces water up the stem, but not enough to push water to leaves in tall plants
Stomatal opening
- Chloroplasts photosynthesise - energy for respiration - ATP
- K+ ions actively transported into guard cells
- Stimulates breakdown of starch (insoluble) to malate (soluble)
- K+ lowers water potential of guard cells
- Water enters guard cells from epidermal cells via osmosis
- Guard cells become turgid & curve apart (inner walls are thicker and relatively inelastic) so the pore opens
Stomatal closing
- No light - close to conserve water
- No photosynthesis
- K+ leak out
- No breakdown of starch
- Water potential raised
- Water leaves guard cells via osmosis
- Cells become flaccid, closing the pore
How does humidity affect the rate of transpiration? (High)
- Lots of water molecules
- Reduces water potential gradient between air spaces and at atmosphere
- Rate decreases
How does wind speed affect the rate of transpiration? (High)
- Wind disperses water vapour around the stomata
- Increases water potential gradient
- Rate increases
How does temperature affect the rate of transpiration? (High)
- Increases kinetic energy of water molecules
- Increases rate of evaporation from mesophyll cells
- Air is able to hold more water molecules
- Rate increases
How does light affect the rate of transpiration? (High)
- Stomata open
* Rate increases
What is a potometer?
Used to measure the rate of transpiration
•Leafy shoot cut underwater
•Shoot attached by a rubber bung and sealed by vaseline
•Inside of apparatus is flooded with water
•Air bubble is introduced at capillary end
•Distance moved by air bubble per unit time is measured
What is a xerophyte?
Plants that have adapted to conditions of low water availability
E.g. marram grass
Xerophytic adaptations to reduce transpiration
- Thick waxy cuticle reduces evaporation
- Smaller leaves reduces S.A
- Hinge cells cause leaves to roll up - retains humid air, reducing water potential gradient
- Stomata sunken into pits - trap humid air
- Leaf hairs trap humid air
- Rounded shape of plant/leaf - reduces S.A
- Shallow, extensive root system - absorb rainwater
- Close stomata
What are hydrophytes?
Plants adapted to aquatic environments
E.g. water lilies
Hydrophytic adaptations
- Stomata on upper epidermis - gas exchange
- Thin cuticle
- Large air spaces - buoyancy
- Poorly developed xylem - water provides support
- Roots can act as anchors
How may sucrose be loaded at the source?
- Sucrose may travel by apoplast or symplast (or both)
- Specialised donor transfer cells have foldings which increases the S.A of the cell surface membrane - cells have many mitochondria for active transport
- ATP pumps sucrose into the sieve tube element against a concentration gradient
How many sucrose be unloaded at the sinks?
- Low concentration of sucrose at the sinks
* Transfer cells pump sucrose into surrounding tissue via active transport
How is a pressure gradient generated in the phloem?
- As water enters the source, hydrostatic pressure builds up, forcing solution into phloem
- Mass flow of solution occurs along a hydrostatic gradient along the phloem, into the sink - forces water out of the sink, into the xylem
What are mesophytes?
Terrestrial plants that are adapted to neither a dry nor wet environment
Mesophyte adaptations
- Deciduous trees - lose their leaves during autumn to reduce water loss
- Bulbs - dormant in winter and underground, plant emerges in spring and summer
- Herbaceous perennials - growth dies in winter, but roots survive
Evidence for translocation
- Sucrose is transported in the phloem
- Sucrose is transported bi-directionally through the stem
- Rate of translocation is faster than diffusion
Ringing experiments (translocation)
- Cylinders of bark can be removed from the stem, removing the phloem but leaving the xylem
- After a time period, the composition of the phloem above and below is sampled
- Sucrose and other organic solutes are present above the ring but absent below it
- Provides evidence for translocation in phloem
Use of aphids (translocation)
- Aphid stylet penetrates the individual sieve tub elements
- The body is removed, leaving the stylet
- Phloem sap exudes from the stylet and can be analysed
- Shows that organic compounds are translocated
Autoradiography (translocation)
- The leaf is supplied with radioactive CO2 and allowed to photosynthesise
- Carbohydrates made will be radioactively labelled
- Am autoradiograph is an image produced on photographic film - the isotope will fog the film, revealing its location and therefore the location of the phloem
What do the aphid and autoradiography experiments demonstrate? (Translocation)
- Translocation occurs from source to sink
- Translocation may be bi-directional
- Translocation is rapid
What is mass flow hypothesis? (Translocation)
- Suggests that translocation of solutes through the phloem occurs due to differences in hydrostatic pressure - passive movement
- Munch’s experiment demonstrates this
- Water enters A (low w.p) down a water potential gradient by osmosis - increases hydrostatic pressure in A and forced the sucrose solution along the vessel to B (high w.p)
What two sources of evidence don’t support mass flow hypothesis? (Translocation)
- The presence of large numbers of mitochondria in the companion cells
- Translocation has only been observed in living phloem tissue
- Both if these imply translocation involves ATP - active
What is the pressure flow hypothesis? (Translocation)
- Suggests that translocation involves a combination of active transport and mass flow, and takes place in three stages:
- Loading of sucrose at the source
- Translocation of sap from source to sink
- Unloading of sucrose at the sinks
- Account for presence of mitochondria and speed of translocation
What is the cytoplasm streaming hypothesis? (Translocation)
- Within individual sieve tube elements, the cytoplasm circulates around the cell
- Solutes may then be actively transported across the sieve plate, either upwards or downwards in the sieve tube
- This would account for the movement of solutes in opposite directions in the same sieve tube
