3:1:3 Transport in Plants Flashcards
Why are mass flow transport systems needed in plants
- To transport nutrients and waste to the plants exchange sites
- Large transport distance (roots to leaves)
- Surface area: volume decreases as organism size increases
- Mass flow transport systems to help move substances, maintain diffusion gradients, and ensure effective cell activity
Why are transport systems for CO2 and O2 not needed in plants
- Adaptations which give them a high SA:V (branching body, flat and thin leaves, root hair cells)
- Leaves and stems have chloroplasts which produce O2 and use CO2
- Low demand for oxygen due to plants having a low metabolic rate
What are the functions of xylem tissue and how is it distributed
- Carry dissolved minerals and water up the plant
- Structural support (vascular bundle found in centre core of roots to withstand pulling force with xylem inside, on outside of stems for support with xylem on the inside, and form veins of leaves with xylem on the upper side)
- Food storage
What are the functions of the phloem and how is it distributed
- Transport organic compounds (sucrose) from the source (leaves) to the sink (roots), occurring up or down the plant
- In roots vascular bundle is at centre and phloem is on the edge, in stems the vascular bundle and phloem are found on the outside, in leaves vascular bundles form veins and phloem is on the lower side
What are the cells in the xylem tissue
- Tracheids (long narrow tapered cells with pits)
- Vessel elements (large thickened cell walls and no end plates when mature)
- Xylem parenchyma
- Sclerenchyma cells (fibres and sclereids)
What are the structure and function of xylem vessel elements
- Lignified cell walls: strength to withstand hydrostatic pressure, and impermeable to water
- No end plates: allows mass flow, and cohesion and adhesion of substances
- No protoplasm (dead cells): allows transpiration stream
- Pits in wall: allows lateral flow of water to avoid formation of air bubbles
- Small lumen: assists with capillary action, and avoids water column from breaking
How do organic compounds travel in the phloem
Assimilates are dissolved in water to form sap
What cells are in phloem tissue
- Sieve tube elements
- Companion cells
- Parenchyma (storage)
- Strengthening fibres
What are the structures and functions of phloem sieve tube elements
- Sieve plates with sieve pores: allows continuous movement
- Cellulose cell wall: strengthening to withstand hydrostatic pressure
- No organelles (some ER and mitochondria): maximises translocation space
- Thin cytoplasm: reduces friction for increased transport
What are the structures and functions of phloem companion cells
- All organelles present: provide metabolic support to sieve tube element, and helps loading and unloading of assimilates
- Transport proteins in membrane: move assimilates in/out sieve tube elements
- Lots of mitochondria: provide ATP for active transport of assimilates in/out
- Plasmodesmata: link to sieve tube elements allowing movement between them
What are dicotyledonous plants
Plants which have seeds containing two cotyledons (seed leaves), a network of veins, leaves with broad surfaces and stalks, tap roots with lateral branches
What is the plant vascular system
A network of vessels running through the leaves, stems and roots, comprised of the xylem and the phloem arranged in vascular bundles
What are structures E, F and G
E: Cambium tissue
F: Phloem tissue
G: Xylem tissue
What is the process of transpiration
The loss of water vapour from a plant to its environment by evaporation and diffusion, as a consequence of gas exchange at the stomata
What are the advantages of transpiration
- Provides a means of cooling via evaporation
- Transpiration stream helps the uptake of mineral ions
- Tugor pressure of cells provides support to leaves and non-woody stems
What is the transpiration stream
Movement of water from the roots to the leaves via cohesion and adhesion, caused by the gradient in water potential (high in soil, low in atmosphere)
What are the factors affecting transpiration
Concentration gradient of water vapour between outside and inside the plant affected by:
- Air movement
- Temperature
- Light intensity
- Humidity
How does air movement affect transpiration
- Lower concentration of water molecules in air
- When air is still, water molecules accumulate near the leaf surface and create local areas of high humidity, lowing concentration gradient and rate of transpiration
- Moving air sweeps water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration
How does temperature affect transpiration
- Increase in temperature causes increase in kinetic energy of molecules, increasing the rate of transpiration as water, molecules move down the concentration gradient faster
- If temperatures get too high, stomata close to prevent excess water loss, reducing the rate of transpiration
How does light intensity affect transpiration
- Stomata close in the dark, reducing the rate of transpiration
- When light is sufficient, stomata open and transpiration rate increases
How does humidity effect transpiration
- If humidity is high, the concentration of water molecules surrounding the leaf is high, reducing the concentration gradient and causing transpiration to decrease
- At a certain level of humidity an equilibrium is reached and there is no movement of water
What is cohesion
The attraction of water molecules to each other
What is adhesion
The attraction of water molecules to other types of molecules
What is the apoplastic pathway
- Series of spaces in cellulose cell walls, dead cells and hollow tubes of xylem
- Majority of water travels through
- Water moves by diffusion as no membrane crossed
- Can move cell wall to cell wall or in intercellular spaces
- Rapid movement of water
- When water reaches endodermis the apoplastic pathway is blocked by Suberin (waterproof)
- This casparian strip forces the water into the symplastic pathway
- Casparian strip thickens as plant ages, except in passage cells
What is the symplastic pathway
- Pathway through the cytoplasm and plasmodesmata of cells
- Water moves via osmosis into the cell and through plasmodesmata
- Slower than apoplastic pathway
Describe the investigation of the rate of transpiration
- Cut a shoot underwater (prevent air entering xylem)
- Place shoot in a tube
- Set up potometer
- Use Vaseline to seal gaps and ensure it is airtight
- Dry the leaves of the shoot to avoid affecting transpiration rate
- Remove the capillary tube and let a single air bubble form before putting it back
- Set up the factor you are testing (e.g. light intensity)
- Record the start of the air bubble and leave for a set time
- Record the end of the air bubble
- Repeat with different levels of you dependant factor
Describe phloem loading
- Has active and passive aspects depending on the pathway (symplastic/apoplastic) assimilates take
- Sucrose is at high concentration in the source (leaf etc)
- Sucrose diffuses down the concentration gradient into the companion cell walls
- Energy is required to move the sucrose via the apoplastic pathway into the companion cell cytoplasm
- Instead H+ ions move out of the companion cell through proton pumps against the concentration (requires ATP)
- H+ then diffuses back into the companion cell cytoplasm down the concentration gradient through cotransporter proteins, which also take sucrose with it
- Sucrose then diffuses down the concentration gradient into the phloem sieve elements t
Describe how sucrose moves from its source to its sink
- Sucrose diffuses down its concentration gradient into the phloem sieve tube elements
- This lowers the water potential in the phloem causing water from the xylem to move into the phloem
- This increases the turgor pressure, which forces the movement of sucrose to the area of low pressure near the sink
Describe phloem unloading
- Sucrose reaches the low pressure areas of the phloem, and diffuses down the concentration gradient into the companion cells
- It then diffuses into the sink and is converted to other molecules to maintain the concentration gradient
- As sucrose diffuses out of the phloem, this increases the water potential, causing some water to move back into the xylem, and the rest to move to surrounding cells
What is the Mass Flow Hypothesis
- Model to explain the movement of assimilates in the phloem tissue
- Simple model contains two partially permeable membranes holding different solutions connected by a tube
- Scientists now support a modified version of this model: the pressure flow gradient
What is the pressure flow gradient
What are the adaptations of xerophytes
- Fleshy succulent leaves
- ‘Hinge cells’ shrink when flaccid
- Leaves curled/reduced when flaccid
- Stomata closed in light and open in dark
- Sunken stomata
- Reduced number of stomata
- Stomata found only on upper epidermis
- Thick waxy cuticle
What are the common adaptations of hydrophytes
- Floating leaves
- Thin waterproof waxy cuticle
- Stomata located on upper surface of leaves
- Reduced root system
- Reduced veins in leaves
