9.2 Transport in Angiospermophytes
Roots
- Function: absorb water and mineral ions from soil
- Roots increase surface area in contact with soil by: o Develops an extensive, branching root system o Numerous root hair - root hairs are temporary and die away to be replaced by new ones near the growing tip of root.
- Minerals required by plants for growth: o Calcium – constituent of cell wall o Magnesium – needed to make chlorophyll o Iron – required as a cofactor for many enzymes
- These minerals are dissolved in soil water which allows the dissolved ions to move into root cells in different ways:
1. Passive processes – do not require energy in the form of ATP
a. Mass flow: water carrying ions
b. Facilitated diffusion: ions move from soil water to root hairs down their concentration gradient
2. Active transport – require energy in the form of ATP (soil concentration lower than plant)
a. Potassium, nitrate and phosphate absorbed by active transport
b. Root cells have mitochondria that contain ATP for active transport.
c. Roots can only take in minerals if oxygen is available for aerobic transpiration which provides sufficient ATP.
d. Cyanide obstructs enzyme action ∴ prevents active transport
3. Mutualistic relationship between roots and fungus (mycorrhiza). The roots become extensively covered by hyhae which increases surface area for absorption of both water and minerals. Minerals can pass directly from fungal hypae to root cells. Support
- Stem connects roots to leaves and flowers and helps plant support its structure
- Support in plant is provided by: o Cellulose cell walls – contain hemicelluloses and pectin + surround plant cells o Cells may thicken cell walls with lignin (carbohydrates) to provide additional structure around cell
Lignin is a complex substance that is very hard and resistant to decay. o Turgid cells - large vacuole is filled with dissolved minerals. The fluid exerts pressure on cell wall ∴ cell becomes rigid and exerts turgor pressure on surrounding cells.
Turgor pressure isn’t enough to support plant ∴ xylem tissue in vascular bundles carries water + supports plant
• Xylem contains hollow elongated cells and are thickened with lignin, forming a backbone to support stem.
Transpiration
- Transpiration is the loss of water vapour from the leaves and stems of plants o Water is absorbed by roots, travels up the stem in the xylem vessels of the vascular bundles to the leaves and is lost by evaporation through stomata.
- Most plants grow in low humid areas
- During the day, stomata in lower epidermis are open and water vapour (from vascular bundles in leaf and stem) leaves the air spaces in the spongy mesophyll.
- Transpiration stream: the flow of water through a plant, from the roots to the leaves, via the xylem vessels.
- Transpiration allows leaves to cool in warm conditions and carries minerals through plants
Cohesion-adhesion theory
- Explains the movement of water in the xylem
1. Loss of water vapour from the stomata in the leaves results in ‘tension’ or negative pressure in the xylem vessels
2. Water vapour re-enters the air spaces in the leaf from the xylem vessels
3. Continuous columns of water vapour are drawn up the xylem due to cohesion between water molecules and the xylem vessel walls.
a. Cohesion is due to hydrogen bonding between H2O molecules
b. Adhesion is caused by H bonds between water molecules and molecules in the walls of the xylem vessels.
4. The tension in the xylem is strong due to loss of water which may cause the xylem vessels to collapse inwards. The thickening of the lignin prevents this from happening.
5. Water is drawn in from the cortex in the roots to replace water that is lost in transpiration.
6. The tension caused by transpiration also causes water to be drawn into the roots from the soil.
Guard Cells and Stoma
- Guard cells have unevenly shaped cell walls with more cellulose on the side next to the stoma (diagram).
