Ch16 Flashcards
Why do plants have root hair cells
Majority are terrestrial organism so have adaptions like waterproof covering which stops them from being able to absorb water over general body surface
Adaptations of roof hair cells
Provide large SA as are very long extensions for absorption of water
Have this surface layer so shorter diffusion distances where materials can move rapidly
How water moves into root hair cells
Soil solution mostly consist of water so higher WP
Root hair cells and other cells of root have sugars amino acids and minerals dissolved so lower WP
Water moves in my osmosis down WP gradient
Water continues its way through apo or symp pathway
Apoplastic pathway
Most water travels via
Series of spaces running through cellulose cell wall
Moves by diffusion and pulled due to cohesive properties creating tension that draws water along the cells walls of root cortex
Casparian strip form barrier and forces water into symplastic pathway through cell membrane so plant can control entry and exit of products
Symplastic pathway
Takes place across cytoplasm of cells of cortex due to osmosis
Mesophyll cells lose water to air spaces
Water moves into root hair cells by osmosis and increases its WP
Root hair has higher WP than first cell in cortex due to
Water moves from root hair to neighboring cell
By osmosis down a water potential gradient
WP gradient set up across all cells of the cortex which carries water along the cytoplasm from root hair to endodermis
Root pressure
Active transport of salts by endodermal cells into xylem
Takes place through carrier protein
Creates lower WP in xylem making water move in down WP gradient
Evidence for AT in root pressure
Pressure increases with rise of temp and decreases with decrease of temp
Inhibitors like cyanide prevent energy release = stops root pressure
Lack of oxygen causes reduction in root pressure
Movement of water through stomata
Humidity of atmosphere usually less
Water vapor molecules diffuse out of the air down WP gradient
By changing stomata size plants can control rate of transpiration
Movement of water across cells of a leaf in symplastic
Water lost from mesophyll cells by evaporation
Cells now have lower WP so water enters by osmosis from neighboring cells
Loss of water from neighboring cells lower their WP and therefore creates WP gradient
Replaced by water reaching the mesophyll cells from the xylem by symp or apo
Cohesion tension
Water evaporates from leaves as result of transpiration
Water molecules form hydrogen bonds between one another and stick tgt cohesion
Water forms a continuous unbroken column
As water evaporates more molecules of water are drawn up behind as a result of cohesion and transpiration pull
Evidence supporting cohesion theory
Change in diameter of tree trunks ;
During day when transpiration is at greatest and negative pressure tree trunks shrinks
If xylem vessel broken and air enters water can not be drawn up as continuous column broken
Role of transportation
Helps materials such as mineral ions sugars and hormones to be moved around the plant as dissolved in water and drawn up by transpiration pull so more rapid transport of materials
Factors affecting transpiration
Light intensity
Temperature
Humidity
Wind speed
Light intensity
Photosynthesis causes stomata to open as exchange of gases happens
Stomata opening water moves out the leaf
Increases transpiration
Temperature
Increase KE of water molecules
Increases the rate of evaporation
Decreases humidity around leaf
Humidity
Air in atmosphere has a high humidity which means higher WP
WP gradient is decreased
Transpiration rate decreases
Wind speed
Water vapor accumulates around the stoma leaf surface
WP increases
Reduce WP gradients between atmosphere and air spaces
Rate of transpiration decreases
Disperse or humid layer
Decreases WP gradient of air
Increases rate of transpiration
Translocation
Process by which organic molecules and mineral ions transported around the plant using the phloem
What’s phloem made up of
Sieve tube elements : end walls perforated to form sieve plates
Companion cells
Sources and sinks
Source where sites of production of sugars produced in photosynthesis
Sinks where sugars are directly used or stored
Movement happens in all direction
Mass flow theory 3 parts
Transfer of sucrose into sieve elements from photosynthesising tissue
Mass flow of sucrose through sieve tube elements
Transfer of sucrose from sieve tube elements into storage or other sink cells
Part 1
Sucrose manufactured by P
Diffuses down Conc gradient by FD from p cells to companion cells
Hydrogen ions Actively transported from companion cells into spaces within cell walls using ATP
Hydrogen ions then diffuse down conc gradient through carrier proteins into sieve tub elements
Sucrose molecules transported along with hydrogen ions by cotransport
Part 2
Sucrose transported by p cells is actively transported into the sieve tubes
This lowers water potential in sieve tubes
As xylem has higher WP water moves from Xylem into sieve tubes by osmosis creating hydrostatic pressure
At respiring cells sucrose either used up or converted to starch for storage
Cells have lower sucrose content so sucrose actively transported into them from sieve tubes lowering WP
Due to lowered WP water also moves into respiring cells from sieve tubes by osmosis
Hydrostatic pressure of sieve tubes in this region is lowered
Water entering the STE at source and leaving at sink creates high HP air source and low at sink
Therefore mass flow of sucrose down this hydrostatic gradient in sieve tubes
Part 3
Sucrose actively transported by companion cells out of sieve tubes and into sink cells
Removing the phloem of stem in ringing
The sugars of the phloem accumulating above the ring which leads to swelling as cells will take up more water by osmosis in response to the sugar they contain to lower WP
Interruption of flow of sugars to the region below the ring and death of tissues
Conclusion drawn from ringing experiment
Phloem rather than xylem is the tissue responsible for translocating sugars in plants
Tracer experiment
Radioactive isotopes trace movement of substances in plants
Isotope of C used to make radioactive co2 which will be incorporated into the sugars produced during P
Film becomes blackened in regions that correspond to to where the phloem tissue is
Other tissues do not blacken
Evidence that translocation of organic molecules occur in phloem
When phloem cut solution of organic molecules flows out
Plants provided with radioactive co2 can be shown to be radioactively labeled in phloem
Removal of a ring of phloem from around the whole circumference of a stem leads to accumulation go sugars above the ring
Aphids lifecycle
Aphids overwinter as eggs Which are laid in autumn
Hatch into wingless females
Females feed on the plant they hatch on and once they mature they give birth to daughter aphids without fertilization (parthenogenesis)
Some aphids can have wings to migrate to another plant for when there’s food shortage
In autumn Winged male aphids and female produced which can then mate to lay eggs
Aphids feeding
Inserting mouthparts into phloem and extracting phloem sap
Sap in the sieve tube is under pressure which results in some sap being exuded from the rear of aphid
sticky and sugary which provides substrate for moulds to grow on leaves
Reducing photosynthesis and transmits viruses
Reduce yield of crops
Plant viruses
Consist of nucleic acid and a protein coat
Can only reproduce inside host cell but cannot penetrate plant cuticles or cell wall
Rely on entering through damaged sites or vectors like aphids and fungi
Viruses are systemic so can spread throughout plant and be transmitted to the next generation through seeds
Symptoms of plant viral disease and effects
Produce yellowing in leaves known as chlorosis
Distortion of parts of the plant
Reduces market value as well as yield
