Biology - Mass Transport

What is transpiration?

The loss of water vapour from the stomata by evaporation in xylem

What are they key conditions for transpiration?

• ↑ Light intensity → more stomata open → higher SA for evaporation

• ↑ Temperature → more kinetic energy, faster moving molecules so ↑ ψ gradient so more evaporation

• ↑ Humid air → decreases evaporation → ψ is higher outside plant which reduces ψ gradient

• ↑ Wind → ↑ evaporation→ blows away humid air which maintains ψ gradient

What makes up the Cohesion Tension Theory?

Cohesion + Adhesion (Capillarity) + Root pressure

What is cohesion

• sticking together of water molecules

• water is a dipole molecule so hydrogen bonds form between hydrogen and oxygen molecules which creates cohesion ∴ water travels up xylem as a continuous water column

What is adhesion (capillarity)

• water molecules can stick to xylem walls by forming hydrogen bonds which helps hold up water column against gravity

• the narrower the xylem, the bigger the impact of capillarity

What is root pressure

• as water moves in by osmosis, it increases the volume of liquid inside the root which increases the pressure which forces water upwards due to positive pressure

How does water move up xylem?

(cohesion tension theory)

1. Water evaporates out stomata, which decreases the pressure & water potential

2. Water lost in transpiration is replaced as water moves up xylem

3. This is due to hydrogen bonds between water molecules which makes them cohesive which creates a water column within xylem

4. Water molecules also adhere to xylem walls which helps push water column upwards

5. This creates tension, pulling in the xylem to make it narrower

Why do plants need transport systems?

• They are multicellular with a low surface area to volume ratio

• Diffusion is too slow to meet their metabolic needs

Adaptations of xylem?

• They are elongated, hollow tubes with no end walls

• They lack organelles

• They are thickened with lignin for support

What is translocation?

Active process by which organic molecules and mineral ions are transported from one part of the plant to another

Adaptations of phloem

• Sieve cell → no organelles to maximize space for transport & it has strong walls to resist pressure

• Companion cells → many mitochondria release ATP for active transport

Mass Flow Hypothesis

1. High conc. of sucrose is created during photosynthesis so it enters companion cell via facilitated diffusion from source cell

2. Active transport of H⁺ ions occurs from companion cells into cell walls using energy

3. This creates a conc. gradient so co-transport of H⁺ ions with sucrose occur into sieve tube element

4. This decreases ψ so water enters phloem from xylem via osmosis which increases hydrostatic pressure so liquid is forced down to sink

5. sucrose is used in respiration & is actively transported into sink so ψ decreases so water moves out of phloem via osmosis into xylem which decreases hydrostatic pressure

How to investigate translocation?

• tracers

• ringing experiment

What is tracing experiment?

• plants are only provided with ¹⁴C which means they create sugars with radioactively labelled carbon during photosynthesis

• stem is cut & placed on x-ray film that turns black

• this shows where sugars are transported in phloem and where phloem are

What is ringing experiment?

• a ring of bark and phloem is removed off tree

• so trunk above phloem swells because sugars cant be transported

• analysis of liquid in swelling shows phloem transports sugars

What is a potometer?

• It measures the uptake of water & the effect of a variable on transpiration

• (It can’t measure transpiration directly so water uptake is said to be directional to transpiration)

• (but it might not be directional because water can leak from apparatus and some water is used in photosynthesis during transpiration)

How do you use a potometer?

1. Sample of plant must be cut underwater to prevent air entering xylem and breaking water column

2. Potometer is filled with water and all air bubbles are removed

3. The cut plant is attached to the potometer using petroleum jelly so equipment is air tight (to prevent water leaking out and affecting uptake)

4. One air bubble is introduced & the distance it has moved towards the plant is recorded

5. The distance the air bubble moved can be used to work out volume of water which is divided by time it took

Explain why the plant shoot must be cut underwater

So air doesn’t draw into xylem and the water column doesn’t break which would prevent transpiration

Using knowledge of transpiration, why is rate of water flow in xylem highest at midday?

• light intensity is highest

• so stomata opens and more water is drawn up due to cohesion and tension