Transport mechanisms

Why is water important for plants

• provides turgor/hydrostatic pressure

  • Gives support to stems

  • Provides force for roots to push through ground

• Loss of water helps keep plants cool

• Mineral ions and sugars are transported in aqueous solutions

• Required for photosynthesis

How are mineral ions absorbed by roots

• by root hair cells

• Against concentration gradient

• Requires active transport - protein pumps and ATP

  • Large number of mitochondria found near membrane of root hair cells

What are some mineral ions absorbed by roots

• potassium

• Phosphates

• Nitrates

How is the surface area for absorption of minerals ions increased

• Branching of roots

• Root hair cells

How does water enter root hair cells

Osmosis - higher water potential in soil and lower in cell due to dissolved mineral ions, sugars amino acids

Define plasmodesmata

Continuous cytoplasm channels that link plant cells

What are the two pathways that water uses to move through the root

• symplastic pathway

• Apoplastic pathway

Describe the symplastic pathway

• water moves through cytoplasm

• By osmosis

• Plasmodesmata link adjacent cells

• Water potential gradient maintained by water leaving roots and entering xylem

Describe the apoplastic pathway

• Water moves through the cell walls and intercellular spaces

• Gaps between cellulose fibres filled with water

• Cohesive forces pull water molecules along

  • Creates tension, so continuous flow of water forms

What is the casparian strip

• a band of suberin (waxy material) and lignin

• Lines cells in endodermis

• Provides waterproof layer around xylem

What is the role of the casparian strip

• prevents water moving through the apoplast pathway

• Forces water into symplastic pathway

How does water enter the xylem

• water in apoplastic pathway forced into symplastic pathway by casparian strip

  • Water must pass through cell membranes to enter cytoplasm

  • Cell membranes are selectively permeable - prevents toxic solutes entering cells

• Mineral ions actively transported into xylem

• Water follows through endodermis by osmosis down the water potential gradient

  • Known as root pressure

Define transpiration

• evaporation of water vapour from the surface of mesophyll cells

• Water vapour lost through stomata by diffusion

What is a potometer used to measure

The rate of water uptake

Describe how a potometer is set up

• cut stem at an angle under the water to avoid introducing air bubbles

• Dry the leaves

• Seal joints with petroleum jelly

• Measure the distance moved by the air bubble over a set period of time (eg one minute)

• Repeat/do again the measurements for reliability

Define transpiration stream

• movement of water up xylem vessels

• From roots to leaves

What is the role of a transpiration stream

• cools plants

• Delivers water and mineral ions to leaves

• Provides support

Which properties of water enable it to form a transpiration stream

• hydrogen bonding and polarity of water molecules

• Water molecules are cohesive with others

• Adhesion between water and xylem walls

Describe how a transpiration stream occurs

• cohesion - hydrogen bonding between water molecules

• Adhesion - hydrogen bonding between water and xylem walls

• As water vapour is lost through evaporation, more is drawn up from below

  • Water evaporates from mesophyll cells Mineral ions walls into intracellular air spaces - creates tension

  • Due to cohesive properties of water and lower pressure at top of xylem

Explain how tree diameter provides evidence for the cohesion-tension theory

Tree diameter is smaller during the day

• Rate of transpiration at its highest

• Tension in xylem vessels at its highest

• Pulls stem/trunk inwards

Explain how broken xylem vessels provide evidence for the cohesion-tension theory

When a xylem-vessel breaks, air is drawn in

• Plant can no longer move water up the stem

• Continuous stream of water is broken

How is water carried from the roots to the leaves using cohesion tension theory

• water moves into xylem down a water potential gradient

  • Root pressure at bottom of xylem helps push water up

• Diffusion of water vapour from stomata creates low pressure at top of xylem

  • Creates tension in xylem

• Water molecules are cohesive due to hydrogen bonds

• Water column pulled up by tension

• Adhesion of water molecules to xylem walls help support column

How does water travel from leaves to the stomata

Through apoplastic and symplastic pathways

What controls the opening and closing of stomata

Guard cells

How can plants control water loss

• opening and closing of stomata

• When environmental conditions are favourable, solutes actively pumped into guard cells

• Water follows by osmosis

• Increase in hydrostatic pressure makes guard cells become bean shaped and stomata to open

How are guard cells adapted for their role

• Unevenly thickened cell wall

  • Wall beside pore is thicker, allowing guard cells to bend

• Transport proteins present in plasma membrane

• Chloroplasts and mitochondria to provide ATP

Why is transpiration unavoidable during the day

• stomata are open to allow gas exchange

  • Required for photosynthesis

• Water vapour leaves leaf down water potential gradient

• Higher temperatures during the day cause greater evaporation

What are the factors affecting rate of transpiration

• light intensity

• Temperature

• Humidity

• Wind speed

Explain how a rise in humidity affects the rate of transpiration

Less transpiration

• Air spaces inside leaf are nearly saturated with water vapour

• Smaller concentration gradient with higher atmospheric humidity

Explain how a rise in temperature affects the rate of transpiration

more transpiration

• More kinetic energy of water molecules

• Faster evaporation rate

Explain how a rise in wind speeed affects the rate of transpiration

More transpiration

• water vapour blown away from the leaf

• Increasing the concentration gradient of water vapour

Explain how a rise in light intensity affects the rate of transpiration

More transpiration

• light causes stomata to open

• Low carbon dioxide concentration inside leaf in bright light so stomata open wider

What are xerophytes

Plants adapted to live in very dry conditions (eg. Deserts)

• like cacti or marram grass

Describe and explain how xerophytes are adapted for life in deserts

• thick and waxy cuticle - reduces water loss by transpiration

• Spines instead of leaves (or curled leaves) - reduces water loss by transpiration

• Stomata open only at night - reduces rate of transpiration

• Sunken stomata (in pits) to increase humidity - reduces water potential gradient

• Long, shallow root systems - absorbs more water

• Succulents - stores water in specialised tissue in stems and roots

• Hairs on leaves (trichomes) - reduces air flow near leaf to trap water vapour

  • Reduces water potential gradient

Define translocation

Movement of organic compounds (eg sucrose, amino acids) from sources to sinks

What is a source (+examples)

• A site where loading of sugars and amino acids into sieve tubes of phloem occurs

• Occurs where organic compounds are synthesised

• Eg leaves - where photosynthesis occurs

What is a sink (+examples)

Where assimilated are unloaded for use or storage (eg roots, fruits and seeds)

Explain how sucrose is loaded into the phloem sieve tubes elements

• Protons (H+) are pumped out of companion cells into cell wall by active transport creating a proton gradient

• Protons diffuse back into companion cells down the concentration gradient attached to sucrose molecules

  • Protons and sucrose flow through co-transporter proteins in membrane by facilitated diffusion

  • Sucrose is moved from mesophyll cells against its concentration gradient (polar so can’t cross membrane)

  • Sucrose diffuses into sieve tubes elements via plasmodesmata (and amino acids)

Explain how water moves in the phloem

• High concentration of solutes in phloem sieve tubes at source

• Leads to water uptake from xylem by osmosis

• Hydrostatic pressure increases

• Low concentration of solutes in phloem sieve tubes at sink

• Water moves back into xylem by osmosis

• Hydrostatic pressure decreases

  • Water moves down the hydrostatic pressure gradient due to its incompressibility

Describe in detail the transport of organic compounds/translocation in vascular plants

• phloem transports organic compounds

• From sources to sinks

• Through sieve tubes elements via

• H+ ions actively transported out of companion cells into cell walls

• Sucrose and H+ ions diffuse back into phloem through co-transporter proteins (loading)

• High solute concentration causes water to enter by osmosis at source

• High hydrostatic pressure causes glow from source to sink

• Solutes diffuse out of phloem into sink

• Water moves back out of sieve tubes elements by osmosis

How is glucose produced in photosynthesis transported and stored in plants

• Glucose transformed to sucrose

• Translocation of sucrose by phloem - active process

• Sucrose moves from source (photosynthetic tissue - leaves) to sink (fruits/seeds/roots/storage organs)

• Sucrose converted to starch

• Stored in storage organs/roots/tubers