3.3.4.2 Mass transport in plants

Define mass transport

The bulk movement off substances over large distances at the same speed and in the same direction due to differences in pressure

Define transpiration

The process of water movement through a plant and it’s evaporation from the leaves

Describe and explain the adaptations of the xylem

Long cells with no end walls between cells-so water can move up in a continuous column during transpiration

Thick cell wall containing lignin-provides structural support and mechanical strength ,waterproofs xylem

No cytoplasm-no Obstruction to water flow so it can move in a continuous column

Describe how water moves (evaporation/osmosis/diffusion)in the leaf for transpiration

The atmosphere is less humid than air in the leaf’s air spaces

Water Vapor in air spaces diffuses out of open stomata into air

Water evaporates from cell walls of surrounding mesophyll cells into air spaces to replace lost water

Water potential in the mesophyll cells decreases

Water moves from the xylem to the mesophyll cells by osmosis

Explain how tension is created in the xylem

Water has left the xylem and causes the pressure to become more negative near the top of the xylem creates t Dion

Water will move from a higher at bottom of xylem to more negative pressure higher up

How is cohesion created

Neighbouring water molecules are pulled upward due to hydrogen bonding between the water molecules,cohesion

This means there’s a continuous column of water up the xylem

Adhesion between xylem walls and water

Why does a plant wilt when not placed in water

If xylem is broken

Air is drawn in or water leaks out

The continuous column of water broken

Water molecules can no longer stick together by cohesion

Plant can no longer draw up water

Why does a tree trunk diameter decrease in the daytime

High rate of transpiration as stomata is open to allow c02 in for photosynthesis

More tension in xylem

More adhesion between water molecules and xylem

Describe the cohesion-tension theory

Water lost from leaf because of transpiration from leaves/stomata open so more water loss

Lower water potential of mesophyll

Creates pull/tension on water in xylem

There is cohesion between water molecules caused by hydrogen bonding

forms a continuous column

This column remains because of adhesion between water and xylem walls

Wall of xylem pulls inward by tension

Explain why the movement of water in the xylem is an example of mass transport

Movement over large distances around the plant

Water is moves in bulk

All move at same speed

In the same direction

Due to pressure differences

Explain how increased temperature affect rate of transpiration

Increasing temperature increases transpiration

Particles have more kinetic energy and faster rate of diffusion and more evaporation

Explain how increasing humidity affects rate of transpiration

Increasing humidity decreases transpiration

Increased number of water molecules in the air

Water potential gradient between atmosphere and air spaces is reduced

Explain how increasing air movement(wind) affects rate of transpiration

Increasing wind increases the rate of transpiration

Water vapour evaporated through stomata and accumulates outside of the leaf

Faster air movement quickly removes moist air

Water potential outside water potential decreases

Water potential gradient between moist air in air spaces and drier air outside is more steep so transpiration increases

Explain how increasing light intensity affects rate of transpiration

Increased light intensity increases rate of photosynthesis

More stomata open to let co2 diffuse in for photosynthesis

More water moves out of leaf into atmosphere so transpiration increases

What are xerophytes

Type of plant that is structured to reduce water los e.g cactus .

Why is the plant cut underneath water water when setting up potometer

To maintain continuous column of water

Why might uptake of water not be equivalent to the rate of transpiration

Water is also used in photosynthesis

Water is used for turgidity

Water is produced in Respiration

Suggest precautions students should take when setting up potometer to obtain reliable measurements

Keep plant under water when setting up apparatus

Dry off leaves

Shut tap

Ensure there are no air bubbles

How do you allow measurements to ckntinue to be taken once experiment completed

Open tap so water comes out resovire pushes the bubble to the start of capillary tube

Explain why the rate of bubble movement/transpiration is greater in worm compared to cold conditions

The more cement of the bubble is greater in warm conditions and at faster rate compared to cold

Rate of transpiration is greater and water molecules have more kinetic energy and higher rated evaporation and diffusion

So more water vapour is lost by diffusion from leaves this creates more tension in xylem so more water is absorbed bubble is moving faster +more

How do you calculate the rate of transpiration

Volume of water/time taken

Volume is calculated by distance traveled by bubble(length) multiplied by cross section area(pie r 2)

What features of xerophytes allow them to reduce water loss

Thick cuticle

Rolling up leaves

Hairy leaves

Sunken stomata

Small leaves/spines-decrease as for water loss

Explain how thick cuticle reduce transpiration

Water proofs lead so there is less evaporation and transpiration.

Explain how sunken stomata decreases transpiration

Water vapour is trapped air next to leaf

Decreases water potential gradient so there is less transpiration

Explain how hairs reduce transpiration

Trap water vapour form a layer of water next to leaf

Decrease water potential gradient and transpiration

Explain how rolling up leaves decreases transpiration

Traps region of still air

Decreased water potential gradient

Decreases transpiration

Define translocation

The movement of minerals and substances up or down the phloem from source to sink due to differences in pressure

Explain why companion cells are usedul

Contains organelles such as ribosomes and mitochondria to provide molecules such as ATP to the sieve tube element

Describe the process of translocation

• The source cell actively transports sugars to companion cells

• Companion cell actively transports sugars to the sieve tube element of the phloem (phloem loading)

• The water potential inside the sieve tube element decreases

• Water moves by osmosis from the xylem to the phloem

• The hydrostatic pressure in the phloem increases this chases mass flow toward the sink down pressure gradient

• Organic molecules are unloaded from sieve tube element into sink cells

• Sugars used for respiration or used for storage in sink cells

• Water moves back into the xylem by osmosis

One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots. Describe how a high pressure is produced in the leaves. (3)

• water potential becomes lower/ becomes more negative (as sugar enters phloem)

• Water enters phloem by osmosis

• Increased volume (of water) causes increased pressure

Describe the mass flow hypothesis for the mechanism of translocation in plants (6)

• In source/leaf sugars are actively transported into phloem

• By companion cells

• Lowers water potential of sieve cell and water enters from the xylem by osmosis

• Increases hydrostatic pressure

• Causes mass flow towards sink/root

• At sink, sugars are unloaded and used for respiration or stored.

Describe and explain how water in the mesophyll cells passes out of the leaf

• water moves through spaces and out through stomata

• By diffusion

• Down concentration gradient

Explain why the transpiration rate increases when the temperature increases (3)

• Molecules have more kinetic energy

• Faster diffusion of water/ more evaporation of water

• More water molecules rise up the column over time due to cohesion

Why does increased air movement increase rate of transpiration? (2)

• Removes water vapour / moisture

• Increases water potential gradient

How does increasing light intensity affect the rate of transpiration?

• when light intensity increases tension in the xylem becomes greater

• Stomata open more

• Increased evaporation / transpiration

• Therefore the water potential of leaf cells becomes more negative/ lower

• Therefore, more water moves from the xylem to surrounding cells

• Down a water potential gradient

• Reference to hydrogen bonds/ cohesion