xylem and cohesion tension theory

What is transpiration and give 4 key conditions that affect it

The loss of water vapour from the stomata by evaporation

4 key conditions-

• Light - positive correlation as more light causes more stomata to open so there’s a larger surface area for evaporation

• Temperature - positive correlation as more heat means more kinetic energy, faster moving molecules and therefore more evaporation

• Humidity - negative correlation as more water vapour in the air will make the water potential more positive outside of the leaf therefore reduces the water potential gradient

• Wind - positive correlation as more wind will blow away humid air containing water vapour, therefore maintaining the water potential gradient so increases rate of transpiration

How is the water potential gradient established that pulls water from the xylem (transpiration)

• the mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun

• These cells now have a lower water potential and so water enters by osmosis from neighbouring cells

• The loss of water from these neighbouring cells lowers their water potential

• They, in turn, take in water from their neighbouring cells by osmosis

What is the xylem tissue and describe the structure

The tissue that transports water in the stem and leaves of plants

• vessels

  • long hollow tubes with no end cell walls forming a continuous column for water transport

  • The wall is strengthened by lignin which prevents collapse and provides structural support when under tension

• Pits

  • Non-lignified regions in the xylem wall which allows lateral movement of water between adjacent xylem vessels

• Lignification patterns

  • Lignin can be arranged in spirals or rings to provide flexibility and prevent breaking under pressure

• No cytoplasm/organelles eg no nuclei in the xylem cells

  • So as to not impede/obstruct flow/ allows easier water flow

Differences between xylem and phloem

Feature	Xylem	Phloem
Function	Transports water & minerals	Transports organic substances (such as sucrose and amino acids)
Direction	One-way (roots to leaves)	Two-way (source to sink)
Cell Type	Dead cells	Living cells
Structure	Hollow tubes with lignin	Sieve tubes & companion cells
Process	Transpiration stream	Translocation

What does water leaving the xylem create

Tension (negative pressure) therefore pulling more water up the stem from the roots

What 2 properties of water allow tension and the movement of water up the column to occur

Cohesion - water molecules are polar and form hydrogen bonds between them. This creates a continuous column of water, preventing breakage

Adhesion - water molecules stick to the xylem walls as the walls are hydrophilic

• this helps the water move against gravity

Adhesion also causes the xylem to change shape in what way?

The xylem walls are pulled slightly inwards- LONGER AND NARROWER- the narrower the xylem, the bigger the impact of capillarity as the narrower the tube the more SURFACE AREA for water molecules to stick to with more ease

How do you describe the water column that moves up the plant

Continuous and unbroken

Summary of water movement

Water evaporates the surface of the mesophyll cells into the air spaces in the leaf then diffuses from a high concentration of water vapour to a lower concentration of water vapour through the stomata- TRANSPIRATION

As water is lost the WATER POTENTIAL OF THE MESOPHYLL DECREASES so water is drawn in from neighbouring cells by osmosis - creating a pull on the water column in the xylem as water molecules are cohesive and adhesive, so tension is created in the xylem- drawing water up from the roots as there is a negative pressure relative to the atmospheric pressure outside the plant

Water molecules stick together due to HYDROGEN BONDING a property known as COHESION which enables them to form a continuous column of water in the xylem vessels

At the same time ADHESION between water molecules and the lignified walls of the xylem helps support this column and prevent it from collapsing under tension

+longer and narrower tubes also provide a higher surface area for water molecules to adhere to