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3.3.4.2 Mass Transport in Plants - Coggle Diagram
3.3.4.2 Mass Transport in Plants
Specialised Cells
Root hair cells
Increase rate of absorption of water and minerals from soil
Large surface area
No chloroplasts due to no light for photosynthesis
Xylem Vessels
Carry water and dissolved mineral ions up the stem from roots
No living components or end walls
Water flows without obstruction as continuous column
Lignin rings thicken walls to withstand negative pressures caused by water movement (cohesion-tension theory)
Palisade Mesophyll cells
On upper surface of leaf so less light is blocked before reaching cells
Single continuous layer to maximise light absorption
Many chloroplasts for high rate of photosynthesis
Vacuole pushes the chloroplasts to edges of cell to maximise light absorption
Phloem Vessels
Transport organic molecules in the stem
Have perforated individual sieve-tube elements to allow passage of cell sap
Thin layer of cytoplasm and lack of large organelles to reduce resistance of flow of sap
Has companion cells connected with plasmodesmata to allow passage of large molecules to the sieve tube elements as a result of lack of large organelles
Transpiration
Evaporation of water from the leaves of a plant to the atmosphere
Main way plants loose water
Water enters the xylem by osmosis at the bottom (from the endodermis). This forces water up the stem by a few cms. This is root pressure.
Water leaves the leaf through the stomata by evpouration from the air spaces.
Water in the spongy mesophyll cells evaporates into the air spaces.
The evaporation of the water lowers the water potential in the spongy mesophyll cells causing water to enter from the xylem via osmosis, down a water potential gradient.
Water molecules cohere together and adhere to the sides of the xylem vessels. Removal of water at the top of the xylem into the mesophyll puts the xylem vessels under tension. This cohesion and tension draws water up the xylem vessels. This is the cohesion tension theory.
Measuring the rate
Factors effecting
Increased temperature:
Molecules have more kinetic energy. This increases the holding capacity of the air outside the cell.
Increased light intensity:
More photosynthesis so stomata open more for gas exchange. Therefore more water is lost.
Increased wind speed:
Reduces water potential outside of leaf larger gradient for osmosis.
Increased humidity:
Increases water potential outside of leaf. Smaller gradient for osmosis.
Potometer experiment:
Evaporation of water from the leaves of the shoot pulls water up from the stem of the shoot, and up from the apparatus. This causes the air bubble to move along the capillary tube.
The distance moved by the air bubble in a given time is measured a number of times and a mean is calculated.
Using the mean value for distance moved, the volume of water ‘lost’ is measured. (Calculate the area of the cross section of ten capillary tube X by distance moved)
When the air bubble reaches the junction near the reservoir tube, the tap is opened until the bubble is pushed back to the start of the scale.
Structure of a leaf
Upper epidermis
Waterproof layer to prevent excess water loss, and protect from UV radiation
Palisade mesophyll
Contains many chloplasts for maximum rate of photosynthesis
Spongy mesophyll
Has air spaces between cells for gas exchange
Lower epidermis
Waterproof layer to prevent excess water loss
Guard cell
Fills with water to open the stomata
Stomata
Allows for gas exchange in and out of leaf
Translocation
The transportation of organic substances inside the phloem
Glucose is produced by photosynthesis and converted to sucrose in the source cell.
Sucrose diffuses from photosynthesising cell into companion cells by facilitated diffusion.
H+ ions (coupled with sucrose) are actively transported from the companion cells into their cells walls.
The H^+ ions (coupled with sucrose) diffuse through co-transport proteins into sieve tube elements (sieve cells)
Sucrose in the sieve cells lowers their water potential, lower than the xylem. Water moves from the xylem into the sieve cells by osmosis. This creates a high hydrostatic pressure.
Sucrose is actively transported by companion cells out of the sieve cells.
It is then activity transported into the sink tissues. At the respiring cells (sink) sucrose is used up or stored, so the concentration of sucrose stays low.
At the sink, sucrose moves into the cells. This lowers the water potential of the sink. Water moves into the sink from the sieve cells by osmosis, lowering the hydrostatic pressure in the sieve tubes.
This creates a mass flow of sucrose down the hystrostatic gradient in sieve tubes.