3.3 - C - Transport In Plants Flashcards
What are dicotyledonous plants?
Plants with 2 seed leaves and a branching pattern of veins in the leaf
What is a meristem?
A layer of dividing cells
Define phloem
Transports dissolved assimilates, such as sugars, up and down a plant
Define vascular tissue
Consists of cells specialised for transporting fluids by mass flow (xylem and phloem)
Define xylem
Transports water and mineral ions upwards in a plant
Explain the cross-section (incl. vascular bundles) of a root of a plant
Xylem in x/+ shape
Phloem in 4 quadrants
Epidermis surrounds them (vascular tissue)
Around that is the thick cortex with an epidermis at the edge
Inside endodermis is the pericycle
Define pericycle
A ring of meristem cells - found inside the endodermis in roots
Explain the cross-section (incl. vascular bundles) of a stem of a plant
Vascular bundles found near outer cells of the stem
The xylem is near the inside of the bundle (closer to centre of stem), phloem on outside
In between the xylem and phloem is cambrium
What is cambium?
A layer of meristematic cells capable of differentiating into new xylem and phloem vessels
Explain the formation of vascular bundles in leaves
They form the veins of the leaf
The xylem is above the phloem
Define what companion cells are
The cells that help load sucrose into sieve tubes
Define what sieve tube elements are
They make up the tubes in phloem tissue that carry sap up and down the plant. They are separated by sieve plates
List 5 structural aspects of xylem vessels and explain their functions
Continuous, hollow tubes with no end walls or contents - less resistance to flow of water and more space
Walls impregnated with lignin - strengthens wall, waterproofs wall, improves adhesion of water molecules - increases capillarity
Lignification in spiral pattern - flexibility and stretching of stem
Narrow lumen - more effective capillary action
Bordered pits (pores) in walls - can move around and avoid blockages
List 5 structural aspects of sieve tube elements and explain their functions
Little cytoplasm, lots of absent organelles - less resistance and more space for transport
Siege plates - connects S.T.E to allow sucrose (as sap) through
Joined end to end to for tube - allow continuous transport
Bi-directional flow - allows sucrose to go both up and down the plant
Living - allows active processes
List 3 structural aspects of companion cells and explain their functions
Many mitochondria - a lot of respiration is needed to provide lots of ATP for active processes
Nucleus - controls the functions of both the companion cell and S.T.E
Plasmodesmata - allows continuation of cytoplasm between companion cell and sieve tube element
What is plasmodesmata?
Gaps in the cell wall containing cytoplasm that connects 2 cells
List the 3 pathways
The apoplast pathway
The symplast pathway
The vacuolar pathway
Explain the apoplast pathway
Water passes through the spaces in the cell walls and between cells. Doesn’t pass through any plasma membranes into cells. It moves by mass flow, not osmosis. This means dissolved mineral ions and salts can be carried with water
Explain the symplast pathway
Water enters the cell cytoplasm through the plasma membrane. It passes through the plasmodesmata from one cell to the next
Explain the vacuolar pathway
This is similar to the symplast pathway, but the water is not confined to the cytoplasm of the cells. It is able to enter and pass through vacuoles as well
Define water potential
A measure of the tendency of water molecules to move from one place to another
Define turgid
When a cell is full/saturated with water
Define transpiration
The loss of water by evaporation out of plant’s leaves via the stomata
What is a potometer?
A device that can measure the rate of water uptake as a leafy stem transpires
List 8 factors of transpiration
Number of leaves Number and size of stomata Presence of a waxy cuticle Light Temperature Humidity Wind Availability of water
Explain how and why the number of leaves affects water loss in transpiration
More leaves = more water loss
It’s a larger SA for water to evaporate out of
Explain how and why the number and size of stomata affects water loss in transpiration
More/bigger stomata = more water loss
A larger SA allows more water to evaporate out of them
Explain how and why the presence of a waxy cuticle affects water loss in transpiration
If it’s present - less water loss
It reduces water evaporating from stomata as its hydrophobic
Explain how and why light affects water loss in transpiration
The lighter it is = the more water lost
The stomata open wider in light to allow gas exchange for photosynthesis. If they are open there is a larger SA for water to evaporate out of
Explain how and why temperature affects water loss in transpiration
Higher the temp = more water loss
More kinetic energy, water evaporates faster and water vapour diffuses out of leaves faster
Explain how and why humidity affects water loss in transpiration
The higher the humidity = the less water lost
When humid, air is more saturated with water. Whilst saturation in the air spaces in the leaf is still higher, there is a shallower water potential gradient for diffusion if water vapour
Explain how and why wind affects water loss in transpiration
More wind = more water loss
It carries water vapour that has just diffused from the leaf away, making the air immediately surrounding the leaf less saturated and maintaining a steeper water potential gradient
Explain how and why water availability affects water loss in transpiration
More water in the soil = more water loss
The more water available, the more water it can lose
List 5 things to do when setting up a potometer as to ensure the results are valid
Set it up under water so no air bubbles are inside it
Ensure the shoot is healthy
Cut the stem underwater to prevent air entering the xylem
Cut the stem at an angle so a large SA is in contact with the water
Dry the leaves
How do you calculate volume in a cylinder?
V=pi x r^2 x length
How do you calculate the rate of transpiration?
Rate = volume / time
Explain why using a potemeter does not give an exact measure for rate of transpiration
Transpiration is the loss of water by evaporation from leaves.
A potometer measures water uptake to replace loss.
Some water may be used e.g. in photosynthesis rather than all evaporating from the leaves.
Also uptake by detached shoots may not be the same as that of the whole plant
Define adhesion
The attraction between water molecules and the walls of the xylem vessel
Define cohesion
The attraction between water molecules caused by hydrogen bonds
Explain water uptake and movement across the root
Root hair cells absorb mineral ions and water from soil via osmosis. It moves across the root cortex down a water-potential gradient to the endodermis of the vascular bundle via the apoplast pathway blocked by caspian strip.
What drives continual osmosis into root hair cells?
Minerals are actively transported into the root hair cell from the soil. This decreases the water potential of the root hair cell
List 6 adaptations of root hair cells
Big SA - water and minerals can be exchanged
Lots of mitochondria - release energy during respiration needed for active transport
Tube-like protrusion - penetrate between soil particles, reducing distance water and mineral ions need to travel
Large vacuoles - have salts to speed up water absorption
No cuticles - would prevent water absorption
Thin walls - thin exchange surface
What is the purpose of the casparian strip?
It blocks the apoplast pathway, ensuring nutrients pass through the cells
Give 3 ways water is helped to move up xylem vessels in the stem
Root pressure - the push from the water entering the xylem vessels in the roots (doesn’t move water far)
Capillary action ‐ adhesion of water molecules to lignin in narrow xylem vessels can pull the water up the sides of the vessel
Transpiration pull ‐ most of the driving force - evaporation causes cohesion and pulls up water in chains. It creates tension and proves the purpose of lignin: to keep the xylem from collapsing.
How does water move across and out of the leaf?
Osmosis
Define water potential
The measure of the tendency of water molecules to move from one place to another
Why is water loss via transpiration unavoidable?
Gas exchange of carbon dioxide and oxygen occur through open stomata.
Stomata open during the day when it is lightest and warmest.
More stomata open means a larger area for water vapour to diffuse through.
What is a xerophyte?
A plant that is adapted to reduce water loss by transpiration so that it can survive in very dry (arid) conditions
List 7 adaptations of xerophytes
Epidermis covered in hair Thicker waxy cuticle Small leaves/needles Sunken stomata (in pits) Curled leaves Small air spaces in mesophyll Stomata shut in day, open in night
What is a hydrophyte?
A plant that is adapted to living in water or where the ground is very wet
List 3 adaptations of hydrophytes
Large air spaces in leaf - keeps leaf afloat so they are in the air and can absorb sunlight
Stomata on the upper epidermis - exposed to the air not the water for gas exchange
Leaf stem has many large air spaces - helps with buoyancy, also allows oxygen to diffuse quickly to the roots for aerobic respiration
Explain how having the epidermis covered in hairs reduces transpiration to help xerophytes survive
Hairs trap water which stops the wind removing water vapour. More humid air around the leaf reduces water potential gradient - less evaporation and therefore transpiration
Explain how a thicker waxy cuticle reduces transpiration to help xerophytes survive
Waxy cuticles are hydrophobic
This prevents even more water passing through the epidermis of a plant
Less evaporation causes less transpiration
Explain how small leaves/needles reduce transpiration to help xerophytes survive
Small surface area and/or fewer stomata causes less evaporation and transpiration
Explain how sunken stomata reduce transpiration to help xerophytes survive
Pits trap water vapour which stops the wind removing it. More humid air around leaves reduce water potential gradient so less evaporation and transpiration occurs
Explain how curled leaves reduce transpiration to help xerophytes survive
Lower epidermis is not exposes to the atmosphere
This traps and stops wind removing water vapour
More humid air around the lead reduces the water potential gradient causing less evaporation and transpiration
Explain how small air spaces in mesophyll reduce transpiration to help xerophytes survive
Less water can evaporate into air spaces and so the leaf quickly becomes saturated
This reduces area for loss of water
Explain how having stomata shut during the day and open at night reduces transpiration to help xerophytes survive
Dry/hot climates cause more evaporation and transpiration during the day whereas it’s cooler during night
Define translocation
The transport of assimilates between the sources and sinks of a plant in the phloem tissue. This requires energy
What are assimilates
Carbon containing compounds produced by a plant using the carbon from carbon dioxide
What is a source?
Where sucrose and other assimilates are loaded into the phloem e.g. leaf
What is a sink?
Where sucrose and other assimilate are unloaded from the phloem e.g. flower
Explain how sucrose enters the phloem from the source by active loading
H+ ions are actively transported (requires ATP) out of the companion cells
This produces a diffusion gradient for the H+ ions
They move back into the companion cell via facilitated diffusion through co‐transporter carrier proteins along with sucrose
Sucrose has been actively loaded into the companion cell
There is a high concentration of sucrose in the companion cell compared to the sieve tube element, so it diffuses into it down the concentration
gradient through the plasmodesmata
Explain how sucrose moves along the phloem at the source
Sucrose is actively loaded into the sieve tube elements at the source
This reduces the water potential in the sieve tube element
Water enters the sieve tube elements by osmosis
This increases the hydrostatic pressure in the sieve tube element near the source
Explain how sucrose moves along the phloem at the sink
Sucrose is unloaded at the sink by diffusion (or active transport) and used in respiration/stored
This increases the water potential in the sieve tube element
Water moves into the sink via osmosis down the water potential gradient
This reduces the hydrostatic pressure in the sieve tube element near the sink
Water in the sieve tube element at the source moves down the hydrostatic gradient from source to sink
This creates a flow which carries the sucrose and other assimilates along the phloem via mass flow either up or down the plant
How are sieve tube elements adapted to allow mass flow to occur? (3)
Elongated elements, joined end to end to form a column
Sieve plates with pores in end walls allow sucrose through
Little cytoplasm and no nucleus ‐ less resistance to transport
Give 3 pieces of evidence which prove phloem is used in translocation
Radioactively labelled carbon dioxide supplied for photosynthesis appears in phloem
Aphids feeding on plant stems insert mouthparts into phloem
Sugars collect above ring when tree is ringed to remove phloem
Give 3 pieces of evidence which prove ATP is needed in translocation
Companion cells have many mitochondria
Translocation is stopped if a poison which stops ATP production is given
Flow of sugars is very high that ATP must be used ‐ much faster than would be possible with diffusion
Give 2 pieces of evidence that prove translocation takes place
pH of companion cells is higher than surrounding cells (H+ ions reduce pH)
Concentration of sucrose is higher in source than sink
Give 3 pieces of evidence against translocation
Not all solutes in phloem move at same rate
Sucrose moved to all parts of plant at same rate and doesn’t go to places with lowest concentration faster
The role of sieve plates is unclear
Give 4 reasons why transpiration is important
It transports usefulness mineral ions up the plant
It maintains cell turgidity
Supplies water for growth, cell elongation and photosynthesis
Supplies water that, as it evaporates, can keep the plant cool on a hot day
