Describe conditions for gas exchange in a leaf.
-No living cell is far from external air, therefore source of O2 and CO2
-Diffusion takes place in air, which makes it more rapid.
-Short, fast diffusion pathway.
-Also plant leaf has large SA compared to volume of living tissue.
-NO specialised transport system is needed for gases- move in and through plant by diffusion..
-Most gas exchange occurs in leaves.
How is a leaf adapted for rapid diffusion? (3)
-thin, flat shape provides large SA
-many small pores, called STOMATA, mostly in lower epidermis
-numerous interconnecting air spaces that occur throughout mesophyll.
When does photosynthesis occur and when does it not and what happens?
-Usually takes place during day
- some CO2 comes from respiring cells however most has to come from external air. O2 from photosynthesis is used in respiration but most of it diffuses out of plant.
-Doesn't occur in dark
-O2 diffuses into leaf because constantly being used by cells during respiration. CO2 produced during respiration diffuses out.
What happens during photosynthesis?
Plant cells take in CO2 and produce O2. At times gas produced from one process can be used for the other. reduces need for gas exchange with external air.
What are the stomata and what are guard cells?
Minute pores which occur mainly, but not exclusively, on the leaves, especially the underside.
Each stoma is surrounded by a pair of special cells (guard cells). These cells can open and close the stomatal pore.
So they can control rate of gaseous exchange.
IMPORTANT BECAUSE TERRESTRIAL ORGANISMS LOSE WATER BY EVAPORATION. Stomata close to prevent water loss.
What are root hairs and what do they do?
EXCHANGE SURFACES IN PLANTS that are responsible for absorption of water and mineral ions. Water lost through transpiration replaced by water absorbed through root hairs. Each root hair is a long, thin extension of a root epidermal cell.
Why are root hairs efficient surfaces for the exchange of water and mineral ions?
-Provide large SA as v. long extensions and occur in thousands on each of the branches of a root.
-Have a thin surface layer (cell surface membrane and cellulose cell wall)
How does water move into root hair cells?
Surrounded by a soil solution which contains small quantities of mineral ions - is mostly water so has high water potential.
Root hairs and other cells of the root, have sugars, amino acids and mineral ions dissolved inside them.
MUCH LOWER WATER POTENTIAL.
Water moves by osmosis from soil solution into root hair cells down water potential gradient.
What are the two ways water continues its journey across the root?
Apoplastic pathway Symplastic pathway
Attraction between molecules of the same type.
Describe the apoplastic pathway.
As water is drawn into endodermal cells, it pulls water along behind it, due to cohesive properties of water molecules.
This creates a TENSION that draws water along the cell walls of the cells of the root cortex.
The mesh like structure of the cellulose cell walls of these cells has many water filled spaces and so there is little or no resistance to this pull of water along the cell walls.
Describe the symplastic pathway.
Takes place across the cytoplasm of the cells of the cortex as a result of osmosis.
Water passes through the cell walls along tiny openings called plasmodesmata.
Each plasmodesma is filled with a thin strand of cytoplasm.
So continuous column of cytoplasm extending from root hair cell to the xylem at the centre of the root.
Water moves along column as follows:
1. Water entering by osmosis increases the water potential of root hair cell.
2. Root hair cell now has a higher water potential than the first cell in the cortex.
3.Water moves from root hair cell to first cell in the cortex by osmosis down water potential gradient.
4.This first cells has higher water potential than its neighbour to the inside of the cell.
5.SO water moves by osmosis into neighbouring cell.
6.Water moves by same process to next cell along, at the same time water leaves previous cell and lowers its water potential , causing more water to enter it by osmosis.
7.Water potential gradient is set up across all the cells of the cortex, which carries water along the cytoplasm from root hair cell to endodermis.
How does water from the apoplastic pathway meet water from the symplastic pathway?
Water reaches endodermis by apoplastic pathway, the waterproof band that makes up the casparian strip in endodermal cells prevents it progressing further along cell wall.
Water is forced into the living protoplast of the cell, where it joins water that has arrived there by the symplastic pathway.
how does water enter xylem through active transport?
Endodermal cells actively transport salts into the xylem.
Process requires energy and can therefore only occur within living tissue.
Takes place along carrier proteins in the cell surface membrane.
If water is to enter the xylem, it must first enter the cytoplasm of endodermal cells which is why the water from the apoplastic pathway is forced into the cytoplasm of the endodermal cells by the caparian strip.
Active transport of mineral ions into the xylem by endodermal cells creates a lower wp in xylem. Water moves into xylem by osmosis. This creates a force that helps to move water up the plant called root pressure.
What is the evidence for the existence of root pressure? (4)
-The pressure increases with a rise in temperature an decreases at lower temp. (active transport rate)
-metabolic inhibitors prevent most energy release by respiration and also cause root pressure to cease.
-A decrease in the availability of O2 or respiratory substrates causes reduction in root pressure.
-Xylem sap exudes from the cut stems of certain plants at certain times.
How does water move out through the stomata?
HUmidity of atmosphere usually less than that of air spaces next to stomata. Provided stomata are open, water molecules diffuse out of the air spaces into surrounding air. Water lost from air spces is replaced by water evaporating from the cell walls of surrounding mesophyll cells.
How does water move across the cells of a leaf?
water lost from mesophyll cells by evaporation from their surface to air spaces of the leaf. This is replaced by water reaching the mesophyll cells from the xylem by either apoplastic or symplastic pathway. In symplastic pathway water movement occurs because:
-mesophyll cells lose water to air spaces
-cells now have a lower wp so water enters by osmosis from neighbouring cells.
-the loss of water from these neighbouring cells lowers their wp.
-they, in turn take in water from their neighbours by osmosis/ WP gradient established that pulls water from the xylem, across leaf mesophyll, and finally out into atmosphere.
What are the two main factors responsible for the movement of water up the xylem from from the roots to the leaves?
Describe how the cohesion tension theory works.
-water evaporates from leaves as a result of transpiration.
-Water molecules form hydrogen bonds between one another and hence tend to stick together.
KNOWN AS COHENSION.
-Water forms a continuous, unbroken pathway across the mesophyll cells and down the xylem.
-As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up.
-Water is hence pulled up xylem as a result of transpiration.
Called TRANSPIRATIONAL PULL.
-Transpirational pull puts xylem under tension i.e. negative pressure within xylem.
Evaporation of water from a plant.
What evidence is there to support cohesion-tension theory. (3)
1.Change in diameter of tree trunks due to transpiration. During day, transp. at its highest, more tension in the xylem. Causes trunk to shrink in diameter.
At night, transp. at its lowest, less tension so diameter of trunk increases.
2.If a xylem vessel is broken and air enters it, tree can no longer draw up water. Because continuous flow of water is broken.
3. When a xylem vessel is broken, water does not leak out, as it would be if it were not under pressure. Instead air is drawn in.
Where does the energy come from for transpiration to occur?
Describe the xylem
Series of continuous, unbroken tubes from roots to leaves.
Is transpirational pull passive or does it require energy?
What are the benefits of transpirration?
Materials such as mineral ions, augars and hormones are moved around the plant dissolved in water. This water is carried up the plant by the transpirational pull. allows rapid transport of materials.
what are the 4 factors affecting transpiration?
-Light -Temperature -Humidity -Air Movement
How does light affect the rate of transpiration?
Photosynthesis only occurs in the light so stomata of most plants open in the light and close in the dark. When stomata are open, water moves out of the leaf into the atmosphere. INCREASE IN LIGHT INTENISTY CAUSES AN INCREAS IN THE RATE TRANSPIRATION
How does temperature affect the rate of transpiration?
Changes 2 factors:
-How much water the air can hold (humidity)
-The speed at which water molecules move.
A rise in temperature:
-increases the kinetic energy of water molecules. Increased movement increases the rate of evaporation of water. water evaporates more rapidly from leaves and so rate of transpiration increases.
-decreases humidity of the air outside the leaf i.e. decreases water potential. Reduction in temp has reverse effect.
How does humidity affect the rate of transpiration?
HUMIDITY IS A MEASURE OF THE NUMBER OF WATER MOLECULES IN THE AIR. Affect the water potential gradient between the air and the inside of the leaf. when high humidity rate of transpiration is lower, low humidity increases it.
How does air movement affect the rate of transpiration?
As water diffuses though the stomata, it accumulates as vapour around the stomata on the outside of the leaf. WP around stomata increased. reduces WP gradient gradient between inside and outside leaf. Transpiration therefore lower. Air movement around leaf with dispersse the humid layer at the leaf surface so decrease WP of the air. INCREASE WP GRADIENT. Faster rate of transp. afster the air movement the more rapidly the humid air is removed and greater rate of transp.
how can the rate of water loss from a plant be measured?
How is the experiment for measuring water loss carried out?
-A leafy shoot is cut under water. Care taken not to get water on leaves.
-Potometer filled completely with water, no air bubbles.
-Using rubber tube, the leafy shoot is fitted to potometer UNDER WATER.
-potometer removed from under water and all joints sealed with waterproof jelly.
-Air bubble introduced to capillary tube.
-distance moved by air bubble in a given time is measured a number of times and a mean calculated.
-using mean value water lost is calculated.
-Vol of water lost can be plot against time on a graph.
-Once air bubble nears the junction of the reservoir tube and the capillary tube, the tap on the reservoir is opened and the syringe is pushed down until bubble is pushed back to start.
-Experiment can be repeated in different conditions e.g humidity, temp., light intensity, or different water uptake of different species.
A plant adapted to living in dry conditions. Water loss due to tranp. way exceed uptake. Without adaptations would become dessicated and die. MOdifications designed to increase water uptake, to store water and reduce transp.
What are the 5 modifications of xerophytic plants?
-rolling up of leaves
-stomata in pits or grooves.
-reduced SA to volume ratio of leaves.
How does a thick cuticle reduce water loss?
Despite waxy cuticle, transp. can still occur through leaves. The thicker the cuticle, the less water can escape by this means.
Why is water difficult to absorb in winter, cold places?
How does rolling up of leaves reduce water loss?
Most leaves have stomata on lower epidermis. Rolling helps protect from outside- traps region of still air within rolled leaf. Region becomes saturated with water vapour and so there is no water potential gradient between inside and outside of leaf. Transp. reduced.
How do hairy leaves prevent water loss?
Traps moist air next to leaf surface. Water potential gradient between inside and outside reduced so less water lost by transp.
How does stomata being in pits or grooves prevent water loss?
Trap moist air next to leaf and reduce water potential gradient.
How does a reduced SA to volume ratio reduce water loss?
Slower rate of diffusion. rate of water loss reduced. Must be balanced with need for photosynthesis.
What other conditions may lead to plants not getting enough water? why?
Sand dunes- water drains quickly through sand and away from roots. Salt marshes- water potential. Coastal regions with high wind speeds. Cold- frozen soil.