3.1 transport in plants (transpiration)

define the term transpiration

the loss of water vapour from the leaves and stems of plants through the stomata of the leaves

(x) why is transpiration the consequence of gaseous exchange in plants?

when the stomata open, not only does CO₂ enter and O₂ exit, but water vapour also diffuses out into the atmosphere, causing the loss of water through the stomata.

explain how transpiration can pull xylem sap up a tree (2 properties of water)

• cohesion: the force by which individual molecules stick together. water molecules stick together as a result of hydrogen bonding.

• adhesion: the force by which individual molecules cling to surrounding material and surfaces. water adheres to most surfaces and can be drawn up in long columns without danger of breaking.

describe capillary action

• it is the movement of liquids within narrow spaces (without any assistance and even against gravity)

• created by water molecules sticking together and to surfaces in xylem (cohesion + adhesion)

• occurs when the adhesive forces between the liquid and the walls of the space are stronger than the cohesive forces between the liquid molecules

• responsible for the upward movement of water in plants from the roots through the narrow xylem vessels of plants

describe how capillary action, transpiration and transpiration pull are linked

• while water is taken into plants through their roots (initially) (and moved up by capillary action), it is also evaporating through the stomata of the leaves (loss of water vapour; aka transpiration)

• as transpiration occurs, a suction force is created, drawing water up from the roots to the leaves (transpiration pull)

what is transpiration pull?

the suction force due to transpiration

what is the importance of transpiration

• transpiration pull draws water and mineral salts from the roots to the stem and leaves

• evaporation of water from the surface of the cells in the leaves cools the plant, preventing it from being scorched by the hot sun

• water transported to the leaves can be used in photosynthesis, to keep cells turgid, and to replace water lost by the cells

explain the movement of water between plant cells, and between them and the environment in terms of water potential (CowMMOO)

• soil has a higher water potential than the root hair cell, water enters the root hair cell by osmosis through the selectively permeable cell surface membrane.

• within the root cell, water continues to move cell to cell by osmosis — always from higher to lower water potential.

• water moves out of a cell with a higher water potential than its neighbouring cell and into the next cell

• eventually, water reaches the xylem vessels, which have the lowest water potential, allowing continuous movement inward. 

• when water evaporates from the surface of mesophyll cells into air spaces and diffuses out through stomata, this creates a low water potential in the mesophyll cells, drawing more water up from the xylem.

explain how osmosis facilitates the absorption of water at the cells in the roots (CowMMOO)

• soil has a higher water potential than the root hair cell, water enters the root hair cell via osmosis through the selectively permeable cell surface membrane.

• the water continues to move cell to cell via osmosis until it reaches the xylem vessels, where it is transported upwards through the plant

explain how active transport facilitates the ions / mineral salt uptake at the cells in the roots 

when the concentration of ions/mineral salts in the soil solution is lower than that in the cell sap, ions/mineral salts are absorbed against the concentration gradient via active transport, requiring energy in the form of ATP, which is supplied by the numerous mitochondria in the root hair cells

explain how diffusion facilitates the ions / mineral salt uptake at the cells in the roots 

when the concentration of ions/mineral salts in the soil solution is higher than that in the cell sap, ions/mineral salts diffuse down the concentration gradient

explain the effect of wind/air movement on the rate of transpiration

• effect: wind blows away the water vapour that diffuses out of a leaf. concentration gradient between inside and outside the leaf is steeper

• ROT: increases with more wind

explain the effect of temperature on the rate of transpiration

• effect: when temperature increases, the rate of evaporation of water from the cell surfaces increases

• ROT: increases with increasing temperature

explain the effect of light intensity on the rate of transpiration

• effect: when there is light, the stomata open wider (for photosynthesis)

• ROP: increases with increasing light intensity

explain the effect of humidity on the rate of transpiration 

• effect: the amt of moisture in the surrounding affects the concentration gradient of water vapour between the leaf and the surrounding air

• ROP: decreases with increasing humidity

explain how wilting occurs

it occurs when the rate of transpiration is higher than the rate of absorption of water by the roots

state the advantage of wilting and explain

rate of transpiration decreases

• reduction in leaf surface area exposed to light

• reduced exposure of stomata to atmosphere

• reduced rate of water loss through stomata

• excessive loss of water causes guard cells to become flaccid and stomata to close

state the disadvantage of wilting and explain

rate of photosynthesis decreases

• closure of stomata reduced amount of CO₂ entering the leaf

  • CO₂ becomes a limiting factor

• folding of leaf reduces surface area exposed to light

explain how the structure of a root hair cell is suited for its function of water 

1. long and narrow extension — increases surface area to volume ratio

2. has a cell surface membrane — prevents the cell sap from leaking out

3. has numerous mitochondria — provides energy in the form of ATP for active transport