Plant systems - Xylem Flashcards
Xylem
Transports water and dissolved mineral ions
Vascular tissue
In roots:
-Xylem is central and star shaped
-Phloem in between the groups of xylem cells
In stems:
- Vascular bundle in a ring
- Phloem outer side
- Xylem inner side
In leaves:
-Vascular tissue in the midrib and in a network of veins
Root structure
- Root hair
- Epidermis (Epi = outer)
- Cortex between epidermis and endodermis
- Endodermis (Endo = inner)
- Star shaped xylem
- Phloem in space between xylem cells
Root tissue description
Epidermis:
- Outer layer of small thin-walled cells with no waxy cuticle
- Root hair cells grow outwards to form root hairs
- Root hairs have large surface area for the uptake of mineral ions, oxygen and water
Cortex:
- Made up of unspecialised parenchyma cells which may store starch grains
- Large spaces between parenchyma cells makes it easier for oxygen to diffuse into the root
Endodermis:
- Single layer of cells which forms a ring around the vascular bundle
- Cell walls of endodermal cells contain waxy waterproof suberin
- Suberin forms the Casparian strip which influences the route by which water enters the vascular bundle from the cortex
Pericycle:
- Layer of lignified sclerenchyma cells
- Provides mechanical support
Absorption of water
- Water potential inside the epidermal cells is lower than surrounding soil due to high concentration of solutes
- Water is absorbed into the epidermal cells of the root by osmosis
Absorption of mineral ions
- Cytoplasm of epidermal cells generally have a much higher solute concentration than the surrounding soil
- Ions are actively transported into the epidermal cells against the concentration gradient, moving along the apoplast pathway until they reach the endodermis
- The Casparian strip blocks the apoplast pathway so ions enter the xylem by diffusion or active transport
- The blocking of the apoplast pathway then allows for ions to be selectively taken up by the endodermis
Pathways of water across the cortex
Water can follow 3 routes across the cortex.
Apoplast pathway:
- The cellulose cell walls are freely permeable so water passes freely from one cell to another
- Water is pulled across the cortex due to forces of cohesion between water molecules
- This is the fastest and most significant pathway
Symplast pathway:
- Water diffuses down its concentration gradient through the cytoplasm of adjacent cells
- Strands of cytoplasm called plasmodesmata pass through pores in the cell walls, forming a continuous pathway for water movement
Vacuolar pathway:
- Water diffuses down the same concentration gradient as in the symplast pathway
- Water moves from the cytoplasm and across the tonoplast of the vacuole and back out into the cytoplasm
Pathway of water from the endodermis to the xylem
- The Casparian strip blocks the apoplast pathway due to the waterproof substance suberin
- Water is forced into the symplast pathway
- Water passes through the pericycle and into the xylem vessels via the symplast pathway down a water potential gradient
Stem Structure
- Epidermis
- Cortex
- Phloem
- Cambium
- Xylem
- Pith
Stem tissue description
Epidermis:
- Single layer of cells
- Helps maintain the shape of stem
Cortex:
-Made up of chlorenchyma cells which have extra cellulose in the corners for added mechanical support
Vascular bundle:
- Arranged in a ring pattern
- Gives the stem strength and flexibility
Cambium:
-Undifferentiated tissue which has the ability to differentiate into either phloem or xylem
Pith:
- The centre of the stem
- Consists of parenchyma cells
Xylem vessel elements
Empty cells joined end to end, lined with lignin, only found in angiosperms (flowering plants)
Formation:
- Begin as normal plant cells arranged end to end
- As the xylem develops, a hard waterproof substance call lignin is laid down within the cell walls
- Lignin is laid down initially in young plants in a circular pattern which allows the young plant to grow
- As the plant matures the cell dies, and the spirals partially join up, providing strength and support
Function:
-Xylem vessels have pits which allow the sideways movement of ions and water into adjacent xylem vessels
Tracheids
- Found in primitive plants
- Like xylem they are dead cells
- Their cell walls also contain lignin and pits
- They are elongated spindle shaped cells with tapering ends
- Water takes a twisting path up the plants
Movement of water from root to leaf
Water movement involves ALL 3 principles:
-Root pressure (Push)
- Cohesion-tension theory (Pull)
- Transpiration stream (Pull)
Root pressure
- Active transport of solutes in the root lower the water potential in the xylem
- The influx of water by osmosis raises the hydrostatic pressure
- This creates a positive root pressure and pushes water up the xylem
Cohesion tension theory
- Capillary action is the movement of water up narrow tubes
- It occurs due to forces of cohesion between water molecules and adhesion between the lining of the tube
- It is a passive process which can occur in dead tissue
Transpiration
The evaporation of water vapour from the leaves or other above ground parts of the plant, out through the stomata into the atmosphere
Transpiration mechanism
- Heat causes water on external mesophyll cells to evaporate
- Water vapour diffuses out of the plant through the stomata
- The loss of water creates a water potential gradient, causing water to flow along a chain of cells to the outermost mesophyll cell via the apoplast, symplast and vacuolar pathways by osmosis
- The transpiration of water molecules through the stomata create tension as water molecules are pulled along due to forces of cohesion (transpiration stream)
- Forces of adhesion between the water molecules and hydrophilic lining of xylem vessels prevents water from falling down due to gravity
Factors affecting transpiration
Temperature:
- Increase in temperature increases kinetic energy of water molecules, so they evaporate and diffuse from the leaf more quickly
- Faster rate of transpiration
- Limited by number of stomata
Humidity:
- Concentration of water molecules outside the leaf increases
- Water potential gradient reduces
- Slower rate of transpiration
Air movement:
- Humid air blown away at leaf surface
- Water potential gradient increases
- Faster rate of transpiration
- Not limited
Light:
- Affects the degree of stomatal opening
- Higher light intensity the wider stomata open
- Faster rate of transpiration
- Limited by number of stomata
Mesophytes
A flowering plant adapted to growing in regions of moderate/adequate water supply
-Shed their leaves in winter to reduce water loss by transpiration when there is little liquid water
Xerophytes
Flowering plants adapted to conditions where water is scarce. They need to reduce transpiration
Marram grass:
- Rolled leaves to reduce surface area exposed to air
- Sunken stomata found only in pits on the upper surface of the leaf trap humid air
- Interlocking hairs on the upper surface to trap water vapour
- Thick waxy cuticle waterproofs the leaf
Hydrophytes
Flowering plants that live in or on water. They have no need to reduce water loss
Water lily:
- Little or no waxy cuticle
- Stomata on upper surface for gaseous exchange to occur
- Stems and leaves have no lignified tissue as water is the support medium
