Mass transport in plants (transpiration, xerophytes, potometer)

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

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

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

What are the factors that affect the rate of transpiration in plants?

• temperature

• wind speed

• humidity

• light intensity

How does temperature impact the rate of transpiration in plants?

• increased temperature

• increased kinetic energy (speed/movement) of water molecules

• increased rate of evaporation of water from leaves AND increased rate of diffusion

THEREFORE: increased rate of transpiration

How does humidity impact the rate of transpiration in plants?

• increased air humidity

• increased number of water molecules in air

• water potential gradient between atmosphere (air outside) and air spaces (air inside leaf) is reduced

• decreased rate of transpiration

How does air movement impact the rate of transpiration in plants?

• water vapour evaporated through stomata (transpiration) and accumulates outside of leaf

• faster air movement quickly moves/ disperses this thick layer of humid / moist air

• Water potential outside stomata decreases

• Water potential gradient between moist air in air spaces in leaf and drier air outside is increased

THEREFORE: increased rate of transpiration

How does light intensity impact the rate of transpiration in plants?

• Increased light intensity

• Increased photosynthesis

• More stomata (in leaves) open to let CO2 diffuse in for photosynthesis

• more movement out of leaf into atmosphere

THEREFORE: rate of transpiration increases

What is the equation for photosynthesis?

6CO2 + 6H2O —> C6H12O6 + O2

What is source?

Parts of a plant where glucose is generated by photosynthesis, stores of starch that are broken down eg root vegetables

What is a sink?

Parts of a plant where glucose is used for respiration in all cells to make ATP, fruit/root vegetable when it stores glucose in the form of starch

Define translocation.

Translocation is the movement of minerals and substances are put down the flowing from the source to the sink due to differences in pressure

What are the steps of translocation (transport through the phloem) according to the ‘mass flow theory’?

1. At the source, active transport is used to move sugars from the source cells into the companion cells and then into the sieve tube element of the phloem (=loading)

2. This lowers the water potential of the sieve tube element

3. Water enters the sieve tube element (from the xylem) by osmosis

4. So hydrostatic pressure increases

5. Increased pressure causes the contents of the cell to be pushed through the pores in the sieve end plates —> Mass movement towards sink cells (eg in the root)

6. Organic molecules are unloaded from the sieve tube element into sink cells

7. Sugars used for respiration or used for storage in sink cells (eg root)

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

• at the source cell, Sucrose is actively transported from the source cell into sieve Tube element of phloem (loading) By companion cells

• Lower the water potential of the sieve Tube element and water enters the sieve Tube From the asylum via osmosis

• So hydrostatic pressure is increased

• Increased hydrostatic pressure causes cell contents to be pushed through sieve pores in end plates —> Mass flow toward root sink cells

• Biological molecules are unloaded from the sieve tube

• Sugars are used for respiration in sink cells

What is the phloem made up of?