Transport in plants adaptations OCR A level biology

Xerophyte

A plant adapated to reduce water loss in arid conditions

What is transpiration

How does light intensity affect transpiration?

• This increases transpiration

• Light stimulates stomata to open in order to take in CO₂ gas exchange - for photosynhtesis

• Larger surface area for evaporation of water

How does temperature affect transpiration?

• Water moelcues gain kinetic energy

• Move faster increasing evaporation - water gas

Humidity

• This decreases water potential

How does wind affect transpiration?

• This increase transpiration

• Wind will blow away water molecules away near stomata

• Steep conc gradient - for increased diffusion

Thick waxy cuticle

Waterproof reduce water loss by evaporation

Longitudinally rolled leaves

• Protects lower epidermis.

• traps moisture to increase humidity - more water vapour inside tube

• Decreases water vapour conc gradient - reducing - water loss by diffusion

• Reduces diffusion of water out stomata

Stomata

Fewer stomata to reduce water loss

Stomata location

• Stomata are in pits trapping moist air - increase humidity

• Reduces water potential gradient

• Reduce loss of water vapour

Spongey mesophyll

• Very dense with few air spaces

• Less surface for evaporation of water

Trichomes

• Trap moist air near leaf surface

• Reduces water potential gradient

• Less water loss via transpiration

Stomata opening

• Stomata only open at night to absorb carbon dioxide

• Reduces water loss in day time

Roots of xerophyctes

• Roots are very extensive

• To abosrb rainfall

Cacti

• Cacti are succulents

• Store water in their stems which are ribbed and can expand when water is available

Why do large multi cellular plants need a transport system

• Diffusion is not fast enough

• Small surface area to volume rattio

• Transport of named substance. e.g. sucrose

• Long distance from external surface to cells

Explain the benefit of internal transports systems

• Surface area to volume ratio is too small

• Diffusion from outer surface is not sufficient'

• Ensures molecules like sucrose, and water reach all tissues

PLANT ROOT

• Xylem vessels are mechanicly strong and are always closer to center to provide support

PLANT STEM

• Vascular bundles arrange in a ring around edge to withstand bending

• Pith made of parenchyma cells

• Phloem edge

• Xylem closer to center

What type of cell is the cambium made of?

• Meristem cells

For what molecule does a plant need nitrogen/phosphorus

DNA

How does the casparian strip prevent ions

• The strip is impermeable to water

• Forces water to pass through cell membrane

• Phospholipid bilayer repels ions

Why can water from hydrogen bonds with nitrate?

• Water is polar

• Nitrate is a charged ion

Process in cell surface membrane of root

• Mineral ions are absorbed through active transport using ATP

• This decreases water potential in roots

• Water diffuses by osmosis from high to low water potential

How to observe leaf stalks

• Cut a transverse cross section

• Dye the leaf stalk

Walls of xylem vessel in hydrophyte

less lignin in walls

Walls of xylem vessels in xerophyte

More lignified walls

Similarities between xylem and phloem

• Both lack nuclei

• Both made up of cells joined end to end

• Both complex tissues made of multiple cells

Differences between xylem and phloem

• Phloem has companion cells xylem doesn’t

• Xylem is lignified phloem is not

• Xylem has a wider lumen than phloem

• Phloem has sieve plates xylem does not

• Xylem has vessels phloem does not

• Xylem has pits phloem doesn’t

• Xylem has not cytoplasm or organelles phloem does

Why do some plants like mosses not need a xylem

High surface area to volume ratio

Function of pits

Lateral movement of water

Companion cell

Mitochondria - respire - energy for active loading

Translocation

Movement of assimilates around plants

Translocation process

• Companion cell uses ATP to pump hydrogen ions to source cell

• Increases Hydrogen concentration is source cell - diffuses down conc gradient

• However must also be transported with sucrose through cotransporter protein

• Higher sucrose concentration in companion cell - moves down gradient through plasmodesmata into sieve tube

• Decreases water potential in sieve tube

• Water moves via osmosis into sieve tube from xylem to phloem high to low water potential increasing hydrostatic pressure

• Mass flow - moves assimilates down the sieve tubes to sinks

• Sucrose diffuses out the phloem to companion cells down conc gradient

• Water potential increases moves via osmosis back to xylem

• Sucrose diffuses into sieve cells through plasmodesmata

Roots of hydrocytes

• Very short - not extensive

• Water does not need to be absorbed as frequently

How does water move against gravity?

• Cohesion - tension theory

• Water is cohesive due to partially negative O₂ and partially positive H

• This creates H bonds making water stick together

• This creates a continuous column of water

• Water is adhesive to the lignin in xylem walls

• narrower xylem larger SA of water in contact - faster rate of transpiration

How does root pressure affect movement of water?

• As water

Transpiration

1. Water evaporates out of stomata - reducing volume of liquid - decreasing pressure

2. Water moves in to fill space pulling water up xylem

3. Water stuck in a column - H bonds cohesive - water is replaced as it transpires

4. Water adheres to lignin in xylem this pulls walls inwards as water rises upwards

5. Narrower more adhesion + root pressure —> pushes water up