3.1.3 Transport in plants Flashcards
Structure of the roots
- Xylem is found at the centre of the root - resembling a star shape
- Phloem found in between each of the points of the star shape
Structure of the stem
-Xylem found on the inner edge of each bundle, closest to the centre of the stem
- Phloem found on the outer edge of each bundle, closest to the surface of the stem
- Layer of cambium in between the xylem + phloem - meristematic tissue (contains actively dividing pluripotent cells)
Structure of the leaf
-Vascular bundle runs down the centre of the leaf as a vein
- Xylem towards the top of the leaf in each bundle
-Phloem found towards the bottom of the leaf
Phloem tissue
- Sieve tube elements:
-Living cells
-Contain no nucleus
- Contain few organelles - Companion cells:
- Provide ATP required for active transport of organic substances
Xylem
- Xylem cells are dead + hollow cells
- Do not contain any organelles or end walls - stack on top of each other to make a continuous hollow column - ideal for transporting water + mineral ions
- Xylem wall is strengthened with lignin (waterproof chemical)
Transport of water
Into the plant:
- Water absorbed through the root hair cells by osmosis
- RHC adapted to maximise osmosis by having thin walls + large surface area
Into the xylem:
- Water travels to the xylem by symplast or apoplast pathway
Symplast pathway:
-Through cytoplasm of a cell
-Water moves from cell to cell to the xylem by osmosis + through gaps (plasmodesmata)
-Each successive cell’s cytoplasm has lower WP - why the water is able to move via osmosis
Apoplast pathway:
-Through cell walls
-Water can enter cell wall + move due to cohesive force
- Water molecules stick together, forming a continuous stream of water which moves towards the xylem
- Transports the water faster - little resistance to the water
Casparian strip is a ring-like thickening of certain parts of the endodermal cell walls in plant roots, which forces water and dissolved minerals to pass through the semipermeable plasma membrane of these cells, rather than their cell walls (from apoplast to symplast)
Adaptations of plants
-Dicotyledonous plants exchange gases through stomata - tiny pores mainly on leaves - can open of close determined by the guard cells surrounding them
- Helps prevent excessive water loss by evaporation
Xerophytes
-Plants with adaptations to reduce water loss + found in locations with limited water - desert
- Marram grass found on sand dunes - limited water due to sand being porous
- Curled leaves, hairs, sunken stomata to trap moisture to increase local humidity
- Thicker cuticle to reduce evaporation
- Longer root network to reach more water
Hydrophytes
- Plants which live in/on water - require adaptations to survive in an excess of water
- Water lilies - grow on surface of water
- Short roots, thin/no waxy cuticles + stomata being permanently open + top of surface of leaf
- Ensure no additional water is retained in the plant + efficient water loss
- Ensure enough light still absorbed for photosynthesis - large leaves, wide + surface of water
Transpiration
- Loss of water vapour from stomata by evaporation
Factors that affect transpiration
-Light intensity:
Positive correlation
More light causes more stomata to open = large SA for evaporation
- Temperature:
Positive correlation
More heat - more kinetic energy, faster moving molecules + more evaporation
-Humidity:
Negative correlation
More water vapour in air makes the WP more positive outside the leaf - reduces WP gradient
-Wind:
Positive correlation
More wind blows away humid air containing water vapour - maintaining WP gradient
Movement of water up the xylem
Cohesion-tension theory:
Cohesion- water is dipolar, enables hydrogen bonds to form between water molecules - stick together > water travels up the xylem as a continuous column
Adhesion - water sticks to other molecules - adheres to xylem walls. Water molecules adhere to lignin in xylem walls
Root pressure
- As water move into roots by osmosis, volume of liquid + pressure in the root increases - root pressure
- Increase in pressure in roots forces water above it upwards - positive pressure
Process of the movement of water up the xylem
- Water evaporates out of stomata - loss in water volume creates a lower pressure
- More water is pulled up the xylem to replace water lost (by negative pressure)
- Due to hydrogen bonds, water molecules are cohesive - column of water within the xylem
- Also adhere to walls of the xylem - helps pull the water column upwards
- As water column is pulled up, creates tension - pulling xylem in to make it narrower
Translocation
-Mass flow hypothesis
- Transport of organic substances in a plant
- Requires energy - active (co-transport)
- Mass flow from source of production (leaves) to the sink, site where organic substances (glucose + sucrose) used up in respiring tissues
Source to sink explanation
Source cell - photosynthesising cell:
-Sucrose lowers WP of source cell
- Water enters by osmosis
- Increases hydrostatic pressure in cell
-Source cell has higher hydrostatic pressure than sink - solution forced towards sink cell via phloem
Sink cell - respiring cell:
-Using up sucrose - more positive WP
- Water leaves sink cell by osmosis
- Decreases hydrostatic pressure in cell
Translocation 1
Sucrose transports from source to sieve tube element:
1. Active transport of H+ ions - from companion cell onto photosynthesising cells of the source
2. Creates a conc gradient - H+ ions move down conc gradient via carrier proteins back into companion cells
3. Co-transport of sucrose with H+ ions - via protein co-transporters to transport sucrose into companion cells
Sucrose then diffuses through plasmodesmata into sieve tube elements
Translocation 2
Movement of sucrose within phloem sieve tube element:
1. Increase of sucrose in sieve tube element lowers WP
2. Water enters sieve tube elements from surrounding xylem vessels via osmosis
3. Increase in water volume in sieve tube element increase hydrostatic pressure - causing liquid to be forced towards sink
Translocation 3
Transport of sucrose to sink:
1. Sucrose used in respiration at the sink/stored as insoluble starch
2. More sucrose actively transported into sink cell, causes WP to decrease
3. Results in osmosis of water from sieve tube element into sink cell
4. Removal of water decreases volume in sieve tube element + hydrostatic pressure decreases
5. Movement of soluble organic substances is due to the difference in hydrostatic pressure between the source + sink end of the sieve tube element.
