Transport in plants
Terms in this set (103)
Why do plants need a transport system?
Larger plants have a smaller surface area to volume ratio.
Every cell needs a regular supply of oxygen, water, nutrients and minerals. Not very active so don't need much oxygen (respiration) and can be met by diffusion.
Need a transport system to move water and minerals from the roots up to the leaves, and sugars from the leaves to the rest of the plant.
What are the types of specialized vascular tissue found in plants?
What is the function of xylem?
To transport water and soluble mineral ions upwards
What is the function of phloem?
To transport assimilates such as sugars up or down
What is the distribution of vascular tissue in the root of a plant?
Vascular bundle is found in the centre of the root.
Central core of xylem found in an 'X' shape with the phloem found in between each of the arms of the xylem.
Around the vascular bundle is a special sheath of cells called the endodermis. Inside the endodermis is a layer of meristem cells called the pericycle.
Outside of the endodermis the parenchyma (normal cells) are found in the cortex until the epidermis.
Why is phloem and xylem found in the formation it is in the roots?
The arrangement provides strength to withstand pulling forces to which roots are exposed.
What is the key role of the endodermis?
To get water into the xylem.
What are meristem cells?
A meristem is the tissue in most plants containing undifferentiated cells (meristematic cells), found in zones of the plant where growth can take place through mitosis.
What is the pericycle?
This is the cell dividing area of the roots.
When meristem occurs in the stem it is known as...
What is the distribution of vascular tissues in the stem?
Vascular bundles found near the outer edge of the stem. The xylem is found towards the inside of each vascular bundle and the phloem towards the outside. In between the xylem and phloem is a layer of cambrium.
The middle section of the stem is known as the medulla and the cortex is found between the vascular bundle and the epidermis.
What is the cambrium?
A layer of meristem cells that divide to produce new xylem and phloem.
Why is the distribution of vascular tissue in the stem found as it is?
Need strength around edges to keep balance as wind can be in any direction.
What is the distributions of vascular tissue in the leaf?
Vascular bundles form the veins of a leaf. The xylem is located on top of the phloem.
What is the structure of xylem tissue?
Vessels to carry the water and mineral ions
Fibres to help support the plant
Living paranchyma cells which act as packing tissue to separate and support the vessels.
Explain the adaptations of xylem to its function.
Made from dead cells aligned end to end to form a continuous column
Tubes are narrow, so the water column doesn't break easily and capillary action can be effective
Bordered pits in the lignified walls allow water to move sideways from one vessel to another when blockage occurs
Lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows, and enables the stem or branch to bend
No cross walls
No cells contents, nucleus or cytoplasm
Explain why xylem is a dead tissue and the process of lignification
Lignin impregnates the walls of the cells as xylem vessels develop - makes the walls waterproof and kills the cells
The cells walls and contents decay leaving a long column of dead cells with no contents
The lignin thickening forms patterns in the cell walls - spiral, annular or reticularte meaning the vessel isn't too rigid and allows some flexibility of the stem or branch
Explain how bordered pits occur.
When lignification is not complete, leaving gaps in the cell walls
Explain the structure and function of phloem.
Consists of sieve tubes made up of sieve tube elements and companion cells:
Sieve tube elements lined up end to end to form sieve tubes, contain no nucleus and very little cytoplasm leaving space for mass flow of sap to occur.
At the end of each sieve tube element are perforated cross-walls called sieve plates.
Companion cells - in between each sieve tube.
Explain companion cells.
Contain large nucleus and dense cytoplasm. Numerous mitochondria to produce ATP for active processes. Carry out metabolic processes to load assimilates into the sieve tubes.
Plasma desmata pits allow the assimilates to be passed between companion cells and sieve tube elements.
Describe the purpose of sieve plates.
Support the tube, keep lumen open. Block sieve tube after infection or injury by deposition of callose. Prevents loss of sap and inhibits transport of pathogens.
Why does phloem need to be able to transport assimilates up and down the plant?
Food needs to travel to flower and roots as not green, or no light means no photosynthesis.
Food stored in roots in winter as potatoes etc.
Movement of water through plants
What are the three possible pathways taken by water through a plant?
The apoplast pathway
The symplast pathway
The vacuolar pathway
Describe the apoplast pathway.
Water passes through the spaces in the cell walls and between the cells.
Doesn't pass through any plasma membranes into the cells.
Water moves by mass flow rather than osmosis. Dissolved mineral ions and salts can be carried with the water.
Describe the symplast pathway.
Water enters the cell cytoplasm through the plasma membrane. It can then pass through the plasmodesmata from one cell to the next.
Describe the vacuolar pathway.
Water enters the cytoplasm through the plasma membrane. It can pass through the plasmodesmata from one cell to the next.
The water is not confined to the cytoplasm of the cells. It is able to enter and pass through the vacuoles as well.
Describe why the water potential in plant cells is always negative.
Water moves from a region of higher water potential to a region of lower water potential. In a plant cell, the cytoplasm contains mineral ions and sugars that will reduce the water potential because there are fewer 'free' water molecules available than in pure water. As a result, the water potential in plant cells is always negative.
What is the water potential of pure water?
What terminology must you use when describing water potential?
or 'Less negative'
What happens to the water uptake if you place a plant in pure water?
It will take up water molecules by osmosis
Because the water potential in the cell is more negative than the water potential of the water (pure water is 0)
Water molecules move down the water-potential gradient into the cell.
Why won't the plant cell burst if you place it in pure water?
The cell will not continue to absorb water until it bursts because the cell has a strong cellulose cell wall
Once the cell is full of water it is turgid
The water inside the cell starts to exert pressure on the cell wall, called the pressure potential
As the pressure potential builds up, it reduces the influx of water
What happens to the water in plants cells if it is placed in a salt solution?
Salt solution has a very negative water potential therefore it will lose water by osmosis - the solution is hypertonic
The water potential in the plant cell is less negative than the water potential of the solution so water moves down the water potential gradient out of the cell
What happens to the plant cell when water is lost?
The cytoplasm and vacuole shrink
The cytoplasm no longer pushes against the cell wall, and the cell is no longer turgid
If the water continues to leave the cell, then the plasma membrane will lose contact with the wall - plasmolysis
The tissue is not flaccid
What is hypertonic solution?
Means there is a greater concentration of solute molecules outside the cell than inside (therefore a lower concentration of water molecules) meaning the water in the plant cell moves down the water potential gradient and into the solution.
What is hypotonic solution?
Means there is a lower concentration of solute molecules outside the cell than inside (therefore a higher concentration of water molecules) meaning the water in the solute moves down the water potential gradient and into the plant cell.
What is an isotonic solution?
Means there is the same concentration of solute molecules outside the cell as inside.
What is osmosis between plant cells?
When the plant cells are touching, water molecules can pass from one cell to another
The water molecules will move from the cell with the less negative water potential to the cell with the more negative water potential.
Which component of the cell is partially permeable, allowing the movement of water molecules but not mineral ions?
What are plasmodesmata?
Cell junctions at which the cytoplasm of one cell is connected to that of another through a gap in their cells. The junctions are plasmodesmata.
The loss of water vapour from the upper parts of the plant - particularly the leaves.
Where in the plant does transpiration occur?
Some may evaporate through the upper leaf surface
Why does water leave through the stomata?
Because the stomata open to allow oxygen in the plant for photosynthesis which allows water to escape.
Explain the typical pathway taken by most water leaving the leaf.
Water enters leaf by xylem, moves by osmosis into cells of spongy mesophyll.
Water evaporates from spongy mesophyll cell walls.
Water vapour moves by diffusion out of the lead through the stomata - relies on water vapour potential gradient, must be a less negative water vapour potential inside the lead than outside the leaf.
What is the difference between transpiration and transpiration stream?
Transpiration is the loss of water vapour from aerial parts of plant but a transpiration stream is the movement of water up xylem vessels from roots to leaves or air surrounding leaves.
What is the purpose of a transpiration stream?
To transport useful mineral ions up the plant
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
What environmental factors affect the transpiration rate? List them.
Air movement (wind)
Describe how light intensity can affect the rate of transpiration in plants.
During the daylight, the stomata open to allow gaseous exchange for photosynthesis. The stomata is closed by guard cells at night to conserve water. Higher light intensity increases the transpiration rate.
Describe how temperature can affect the rate of transpiration in plants.
Affects it in three ways:
Increases rate of evaporation from cell surface so water-vapour potential in the leaf rises
Increase rate of diffusion through the stomata due to water molecules having more kinetic energy
Decrease relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf
Describe how relative humidity can affect the rate of transpiration in plants.
Higher relative humidity in the air decreases the rate of water loss because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside.
Describe how air movement can affect the rate of transpiration in plants.
Wind will carry away water vapour that has just diffused out of the leaf maintain a high water vapour potential gradient.
The plant will close the stomata by guard cells if it is losing too much water.
Describe how water availability can affect the rate of transpiration in plants.
Little water in the soil, then no water to replace water lost. If insufficient water in the soil, stomata close and the leaves wilt to prevent water loss.
What piece of equipment can be used to measure the rate of transpiration?
What does a photometer measure? Why is it not accurate?
The rate of water uptake by a leafy shoot.
Assuming that the cells are turgid, more than 95% of water taken up is lost by transpiration. This is only an estimate.
What precautions need to be taken to ensure the results are valid when using a potometer?
Set it up under water to make sure there are no air bubbles inside the apparatus
Ensure the shoot is healthy
Cut the stem under water to prevent air entering the xylem
Cut the stem at an angle to provide a large surface area in contact with the water
Dry the leaves
Explain how a potometer works
How can you use this to measure the rate of transpiration?
Water is taken up by the capillary tube as water is lost by the leaves in transpiration then air enters the tube
The movement of the meniscus at the end of the water column can be measured to measure the volume of water lost by transpiration. This can be divided by the time taken to measure the rate of transpiration.
Describe how you would use a potometer to investigate the effect of increasing wind speed on the rate of transpiration.
You would place the plant cutting in a controlled area where the environmental factors affecting transpiration are controlled. You could then change the wind speed to see the change in volume of water by the plant as the wind speed increases.
Define the transpiration stream.
The movement of water from the soil, through the plant, to the air surrounding the leaves. The main driving force is the water potential gradient between the soil and the air in the leaf air spaces.
Describe the water uptake and movement across the root.
The epidermis of a root contains root hair cells which are long extensions that increase the surface area of the root.
These root cells actively absorb mineral ions and make the water potential of the cytoplasm more negative so osmosis occurs.
Moves across the root cortex down a water-potential gradient (by osmosis) in the apoplast pathway to the endodermis of the vascular bundle.
Need to enter the symplast pathway when it hits the endodermis because the apoplast pathway is blocked by the Casparian strip.
Describe the process whereby water and mineral ions travel across the root and into the medulla and xylem.
Movement of water across the root is driven by an active process at the endodermis.
The Casparian strip blocks the apoplast pathway between the cortex and the medulla ensuring that water and dissolved mineral ions have to pass into the cell cytoplasm through the plasma membrane.
The plasma membrane contain transport proteins which actively pump mineral ions from the cytoplasm of the cortex cells in the medulla and xylem.
Makes water potential of the medulla and xylem more negative - water moves from cortex cells by osmosis.
Once the water has entered the medulla, it cannot pass back into the cortex.
What other name is given to the endodermis?
The starch sheath.
Describe the three main factors as to how mass movement of water and mineral ions up the stem occurs.
Describe how root pressure affects the movement of water up the stem.
As minerals move into the medulla and xylem by active transport, water is drawn into the medulla by osmosis. Pressure in the root medulla builds up and forces water into the xylem, pushing it up the xylem.
Pushes it up a stem by a few metres.
Describe how transpiration pull affects the movement of water up the stem.
Why must the xylem vessels be strengthened?
Loss of water from evaporation from leaves must be replaced by water from the xylem. Cohesion forces between water molecules are strong enough to hold the molecules in a long column.
As molecules are lost at the top of the column, the whole column is pulled up as one chain. The pull from above creates tension in the column of water which is why xylem vessels must be strengthened by lignin. It prevents the vessel collapsing under tension.
What is the cohesion-tension theory?
The transpiration pull because this mechanism involves cohesion between the water molecules and tension in the column of water. Relies on the plant maintain an unbroken column of water all the way up the xylem.
What happens if the water column in the stem is broken in one xylem vessel?
The water column can still be maintained through another vessel via the bordered pits.
Describe how capillary action affects the movement of water up the stem.
Water molecules are attracted to the sides of the xylem vessel - called adhesion. Because xylem vessels are narrow, these forces of attraction can pull the water up the sides of the vessel.
Summarize the root of the transpiration stream.
Minerals actively transported into xylem. This lowers the water potential in the xylem and water follows by osmosis.
Cohesion of water molecules, enables water to move by mass flow, pulled upwards by tension from above.
Movement of water out of xylem creates low hydrostatic pressure and thus tension.
Osmosis moves water across the leaf.
Evaporation of water from cell surface.
Diffusion of water vapour out of leaf.
Explain how water vapour leaves the leaf.
Through the stomata.
Through the waxy cuticle.
Evaporation from the cells lining the cavity immediately above the guard cells. This lowers the water potential in these cells, causing osmosis of neighbouring cells. In turn, water is drawn from the xylem in the leaf by osmosis.
Why does the presence of starch in the endodermis suggest that an active process is involved?
Explain the significance of the Casparian Strip.
The Casparian strip blocks the passive flow of materials such as water and solutes from passing into the stele of a plant thereby forcing those to pass through the cell walls of the root where their passage is mediated by gates for each specific nutrient.
Explain how water can be transported up to the leaves of a tall tree.
Transpiration pull and capillary action
What property of water causes cohesion?
List three features of the xylem vessel that enable the easy transport of water.
No cell contents
Impregnated walls with lignin
Explain why it is important that the column of water in the xylem vessel is not broken.
Otherwise the column would not be pulled up therefore transpiration would not occur.
The adaptations of plants to the availability of water
Describe and explain the structural and behavioural adaptations of terrestrial plants.
Waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis
The stomata often found on the under-surface of leaves to reduce evaporation from the sun
Most stomata closed at night when there is no light for photosynthesis
Deciduous plants lose their leaves in winter when ground may be frozen and when temps may be too low for photosynthesis
Where is marram grass found?
Living on sand dunes
What type of plant is marram grass?
Xerophyte - a plant adapted to living in arid conditions
What are the adaptations to marram grass?
Leaf is rolled longtitudinally so air is trapped inside - this air becomes humid, which reduces water loss from the leaf
Thick waxy cuticle on the outer side of the rolled leaf to reduce evaporation
Stomata are on the inner side of the rolled leaf, so they are protected by the enclosed air space
Stomata are in pits in the lower epidermis, which is also folded and covered by hair. These help to reduce air movement therefore loss of water vapour.
Spongy mesophyll is very dense with few air spaces - less surface area for evaporation of water
What type of plant is cacti?
What adaptations do cacti obtain to live in arid conditions?
Cacti are succulents - store water in their stems which become fleshy and swollen. The stem is often ribbed or fluted so that it can expand when water is available.
Leaves are reduced to spines reducing surface area of the leaves
Stem is green for photosynthesis
Roots are very widespread, in order to take advantage of any rain that falls
What are hyrdophytes?
Plants that live in water.
What issues do hydrophytes face?
Getting oxygen to their submerged tissues and keeping afloat (need to keep their leaves in the sunlight for photosynthesis).
What is an example of a hydrophyte?
What are the adaptations of a water lily?
Many large air spaces in the leaf - keeps the leaves afloat for air and sunlight
The stomata are on the upper epidermis, so they are exposed to the air to allow gaseous exchange
The leaf stem has many large air spaces helping with buoyancy and allows oxygen to diffuse quickly to the roots for aerobic respiration.
What is the specialized structure found at the tips or margins of their leaves?
What does a hydathode do?
Release water droplets which may then evaporate from the leaf surface.
Describe two adaptations to the roots that could help plants survive in arid conditions.
A very long tap root that can reach water deep underground
The roots are widespread to take advantage of any rain that falls
Explain why leaves in water cannot transpire.
Transpiration is the loss of water vapour from the surfaces of the leaves - water will not evaporate into water or into air that has a very high humidity. If water cannot leave the plant the transpiration stream stops and the plant cannot transport mineral ions up to the leaves.
Occurs in the phloem, and is the movement of assimilates throughout the plant.
What are assimilates?
Substances made by the plant, using substances absorbed from the environment. These include sugars and amino acids.
What is the part of the plant that loads assimilates into the phloem? Where in the phloem is it loaded?
Where: Sieve tubes
What is the name of the part of the plants that removes assimilates from the phloem sieve tubes?
Describe how sucrose is loaded into the sieve tubes.
By an active process.
ATP energy from the companion cells used to transport hydrogen ions out of the companion cells. Increases concentration outside the cells so they diffuse back over. They can only diffuse back if the hydrogen ions are accompanied by sucrose molecules - this is known as cotransport.
As the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube.
Why is cotransport of sucrose molecules and hydrogen ions also known as secondary active transport?
As it results from the active transport of the hydrogen ions out of the cell and moves the sucrose against its concentration gradient.
What is sap?
Solution of sucrose, amino acids and other assimilates that flows by mass flow either up or down the plant as required.
Describe the movement of sap in the phloem and how it is caused.
Sucrose is actively loaded into the sieve tube element and reduces the water potential
Water follows by osmosis and increases the hydrostatic pressure in the sieve tube element
Sap moves down sieve tube from higher hydrostatic pressure at source to lower hydrostatic pressure at sink
Sucrose is remove from the sieve tube by the surrounding cells and increases the water potential in the sieve tube (active transport)
Water moves out of sieve tube and reduces the hydrostatic pressure
What is a source?
Any part of the plant that loads sucrose into the sieve tube. In early spring, could be the roots where energy is stored as starch is converted to sucrose to enable growth.
Most obvious source is the leaf.
What is the sink?
A sink is anywhere that removes sucrose from the phloem sieve tubes.
What are the different types of plant cells that is formed by mitosis in meristematic tissue for strength?
To add more cellulose to walls, collenchyma is formed which is a cell with more collagen
To add lignin to the xylem, sclerenchyma.
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