UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 22/05/2023 [SEARCH]

Transport in plants: 5. More on evolutionary leaf adaptations to aid photosynthesis and gas exchanges and environmental factors affecting rate of water loss - rate of transpiration

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There are various sections to work through,

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INDEX of biology notes on transport in plants

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(5A) More on leaf adaptations to aid photosynthesis and gas exchange

See also detailed notes on

Photosynthesis, importance explained, limiting factors affecting rate 

Gas exchange in plants

In plants carbon dioxide enters leaves by diffusion and then diffuses into cells where photosynthesis takes place.

Oxygen will diffuse out from the leaf surface.

Reminder: Photosynthesis takes place inside the subcellular structures called chloroplasts in the palisade cells.

 carbon dioxide + water == light + chlorophyll  ==> glucose + oxygen

 6H2O(l) + 6CO2(g)  == sunlight/chlorophyll ==> C6H12O6(aq) + 6O2(g)

In daylight more carbon dioxide will be taken in for photosynthesis in than given out from respiration and more oxygen given out than taken in - the effect of more photosynthesis than respiration - the surplus glucose is converted into starch.

At night-time the opposite will happen, more carbon dioxide from respiration will be given out than taken in, and more oxygen taken in than given out - respiration increases and food stored as starch becomes the source of energy in the dark.

Beneath the apparently flat surface of a leaf is quite a porous layer of air spaces between the outer layers of cells - particularly on the underside of leaves - quite often the lower surface of a leaves feel rougher and 'roughness' means a more disrupted surface of a larger gas exchange surface area.

Photosynthesis and diffusion (with reference to the above diagram of leaf structure)

Plants have stomata (tiny pores or holes), mainly on the underside of leaves in the spongy mesophyll, to obtain carbon dioxide gas from the atmosphere for photosynthesis and to give out the 'waste' oxygen gas produced as a by-product in photosynthesis.

Carbon dioxide is absorbed from air and water from the roots for photosynthesis.

Carbon dioxide diffuses into the leaves through the stomata and is depleted through photosynthesis.

Therefore as photosynthesis proceeds, the internal carbon dioxide concentration in the leaf is much lower than in the surrounding air, so carbon dioxide will diffuse into the leaf down this concentration gradient.

The rate of diffusion of the carbon dioxide (and any other gas) is increased by:

Increasing the surface area of the leaf - always the broadest part of any plant.

The smaller the distance the molecules have to travel as they diffuse - thin leaves with an even thinner mesophyll layer.

An increase in the carbon dioxide concentration gradient - always be there while photosynthesis is taking place.

As the CO2 is absorbed, wind blows by fresh supplies of carbon dioxide to maintain a high inward concentration gradient.

Oxygen from photosynthesis diffuses out through the stomata, and most water is lost in the same way (transpiration).

The air spaces in the leaf structure create a larger surface area to allow this diffusion to take place efficiently.

Leaves are also thin, so distance and diffusion times are short, further increasing the efficiency of gas exchange.

Carbon dioxide can diffuse in through the stomata and oxygen can diffuse out and stomata also allow water vapour to escape as part of the process of transpiration (details in later section).

Since carbon dioxide is being used up in photosynthesis, the concentration gradient enables more carbon dioxide to diffuse in through the stomata.

The size of the stomata are controlled by guard cells (more on this in the next two sections on transpiration).

The flattened shape of leaves increases the surface area over which efficient gas exchange can take place - greater chance of carbon dioxide to diffuse into the leaves (see photographs below).

Inside the leaf the cell walls form another exchange surface and the air spaces between these cells further increase the surface are for gas exchange - carbon dioxide in, oxygen and water vapour out.

Water vapour evaporates from the surfaces of the leaf cells.

The higher concentration of water vapour in the leaves means there is a natural diffusion gradient to the outside air so the it can exit the leaves in the process of transpiration.

A high humidity reduces the concentration gradient of water vapour between the interior and exterior of the leaf, slowing down the diffusion of water vapour - slowing down transpiration.

Conversely, very dry air (low humidity) will increase the concentration gradient and increase the rate of water loss from leaves.

Estimating leaf surface area

If a surface has a regular shape like a square or rectangle it is easy to measure and calculate the surface area (length x breadth).

However, in the case of a leaf, you have an irregular shaped surface, even if there is a line of symmetry down the middle!

If the leaf is laid out on a marked out grid, like the ones illustrated above, you can count the squares to arrive at an estimate of the leaf's surface area. Make sure the leaf is fully flattened out.

If a square is mainly filled with leaf (over filled) you count it towards the total.

If a square is not well filled (less than filled), it should not be counted in the total area.

Using this method I estimate that the left leaf has an area of 52 cm2. (1, do you agree?)

In this case I found it easier to count the blank squares.

On the right, I estimate the smaller leaf has an area of 32 cm2. ( do you agree?)

In this case I found it easier to count the green squares.

 

The upper side of a leaf is smoother and greener - richer in chloroplasts to capture the sunlight The under side of a leaf is rougher - more 'porous' for efficient gas exchange and the veins more prominent

A summary of adaptations for the effective functions of leaves - some very important for transport

Layer Adaptation Function
Upper epidermal layer thin and transparent waxy cuticle allows light through and protect leaf from excess water loss
Palisade mesophyll regular shaped cells arranged in end-on, near the upper surface, maximises chloroplasts at the top of the cells enables the maximum amount of light to be absorbed
Spongy mesophyll irregular shaped cells creating air spaces increases the surface are for gas exchange - CO2 in, O2 out - increases efficiency
Lower epidermal tissue many stomata (pores) for gas exchange surrounded by pairs of guard cells the guard cells open and close each stoma (pore) to control the diffusion of the gas exchange
Vascular bundles contain xylem and phloem tubes in the veins transport substances around the plant including to, and from, the leaves

See also Plant adaptations and controlling water loss

More on the environmental factors (ambient conditions) affecting the rate of water loss - the rate of transpiration

1. Air flow

The more air that flows over the leaves, e.g. stronger wind, the greater the rate of transpiration. Conversely, the lower the wind speed, the slower the rate of transpiration.

The more quickly the water vapour is removed and swept away by a greater air flow, the steeper the water diffusion gradient out of the leaf is - the concentration of water vapour is much greater in the stomata than in the air in and surrounding the leaf - which is much lower because the water vapour is being constantly carried away in the air current across the surface of the leaves.

If the air is quite still, the water vapour accumulates around the leaf, considerably reducing the diffusion gradient because the water vapour concentrations become similar. The concentration of water vapour is high in the stomata and BUT only a bit less in the surrounding air.

A particle model of diffusion in gases and liquids:

Reminder that the net flow of a substance in diffusion is from a higher concentration to a lower concentration e.g. the movement of the 'green' water particles in the diagram sequence below.

2. Humidity

The less humid (more drier) the air surrounding the leaves, the greater the rate of transpiration.

The lower the water concentration on the outside of leaves, the steeper the water diffusion gradient from the leaves to the external air, the faster the rate of water loss by transpiration.

When the air is very humid with a high water concentration, there is a smaller difference in the higher (in leaf) and lower (outside leaf) concentrations - so a smaller water diffusion gradient resulting in smaller rate of water loss by transpiration.

3. Light intensity

The greater the intensity of light (e.g. the brighter the sunlight) the greater the rate of transpiration because the rate of photosynthesis increases. This stimulates the guard cells to open up the stomata more to let more carbon dioxide in and water vapour and oxygen out.

As it gets darker, photosynthesis rate decreases and the stomata begin to close up - they don't need to be open to allow carbon dioxide to diffuse in. When the stomata are closed, little water can escape - I presume enough oxygen can get in for the plant's respiration at night.

4. Temperature

The warmer the surrounding air the greater the rate of transpiration.

This is because the water molecules have more kinetic energy to escape the intermolecular forces at the surface of the liquid water in the stomata. So, the water molecules can evaporate more quickly and diffuse out of the stomata.

Also, the rate of photosynthesis increases with increase in temperature - just like any other chemical reaction - therefore more water is required to be drawn up through the roots.

 

The daily cycle of the rate of transpiration

daily transpiration cycle factors affecting it

The graphs above show how the rate of transpiration is likely to vary through the day for two different plants.

The rates of both photosynthesis and transpiration increase and decrease with change in light intensity over a 24 hour daily cycle.

On average, at midday (noon) the sun is at its maximum height, the sunlight intensity is at a maximum, so photosynthesis can be at a maximum, but only if the transpiration rate maximises too, to supply the water for photosynthesis.

The peak heights will vary depending on the effect of the factors that control transpiration (discussed above) and photosynthesis.

The light intensity has a greater effect on the rate of transpiration of plant B compared to plant A, even though it starts from a lower base at midnight.

In the night time, when photosynthesis is at a minimum, the water uptake through the roots is greater than the rate of transpiration.

Through the day, and peaking at midday, the transpiration rate exceeds the rate of water uptake.

In daylight the rate of transpiration cannot be the same as the rate of water uptake because some of the water is used in photosynthesis and the rest of the plant's metabolic processes and the rate of evaporation increases too.

That rate of transpiration exceeds the rate of water uptake as the rate of photosynthesis increases.

As the light intensity increases, the stomata open to allow in more carbon dioxide for photosynthesis.

BUT, this also allows more water vapour to evaporate.

See also Plant adaptations and controlling water loss


(5B) More on the factors affecting the rate of transpiration and function of the stomata and guard cells

Transpiration is defined as the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells, followed by diffusion of water vapour through the stomata.

Plants are constantly losing water, but cannot be healthy without a balancing water intake. The water is needed for transportation and photosynthesis - in fact most of the water is used in the transport of materials through the plant, only a few% is used in photosynthesis.

Transpiration is result of the way leaves have become adapted to facilitate photosynthesis - the stomata aiding the transport system by allowing gas exchange - carbon dioxide, oxygen and water vapour.

The process of water movement from the roots through the xylem and out of the leaves is called transpiration and is essential for a plant's transport system.

Water is absorbed through the root hairs, passes up the root, continues up the stem and spreads into all the leaves

Transpiration is caused by the evaporation and diffusion of water from a plant's surface - mostly from the leaves.

Most of the loss of water vapour takes place through the stomata on the surface of leaves.

Water on the spongy surface of the mesophyll evaporates and diffuses out of the leaves.

Plants continually lose water because the concentration of water in the plant fluids is greater than the concentration of water in the air outside - the concentration gradient is in the 'outward' direction.

The cell surface of a leaf is large area punctuated by the interconnecting air spaces and stomata.

Since plants need water all the time, water is continually transported through the xylem in the veins.

The loss of water from leaves by evaporation, creates a small shortage of water in the leaves and so a column of water molecules is drawn up by cohesion in the xylem, from the rest of the plant through the xylem tubes to replace the water loss.

Therefore, this causes in turn, more water to be absorbed and drawn up from the roots.

So, there is a constant flow of water up through the plant - the transpiration stream - which carries the mineral ions dissolved from the soil up into the whole of the plant.

So, important functions of transpiration

Water is needed for photosynthesis.

Water carries dissolved substances around the plant.

Evaporation from leaves cools the plant.

Cells filled with water give the plant physical support.

 

Evaporation is more rapid in hot, dry and windy conditions.

If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.

If too much water is lost through the stomata, plants will wilt ('flop') and die.

As we have said, plants are constantly losing water by evaporation, but cannot be healthy without a balancing water intake. If plants lose water too fast they will wilt - the leaves droop and hang down. This reduces the surface area available for evaporation through the stomata. The stomata close and photosynthesis stops to prevent water loss. There is a danger the plant will overheat. Plants will stay wilted until they can absorb water and the temperature falls and no longer in sunlight.

The size of stomata is controlled by pairs guard cells, which surround them.

Therefore stomata and guard cells control the rate of evaporation from leaves.

(Note: stoma is singular, stomata is plural).

Two guard cells surround each stoma.

The size of the opening of the stomata (diagram on left) must be controlled by the guard cells or a plant might lose too much water and wilt.

It is the guard cells that regulate the rate of transpiration.

It is the guard cells that control the rates of water loss and gain AND the rate of gas exchanges.

The 'kidney shaped' guard cells can change shape to control the size of the pore.

Water diffuses out of the spongy mesophyll producing a film of water on the surface of the cells. Water evaporates into air spaces between the cells and the water vapour diffuses down the concentration gradient to the stomata and escapes from the leaf into the surrounding air.

Water will diffuse out and evaporate away much faster in less humid-drier, hotter or windier weather conditions.

Stomata close automatically if the water supply begins to 'dry up' to reduce water loss.

The guard cells will respond to the ambient conditions ie close up the stomata if the rate of water loss is to great for water to be replenished from the roots.

When the plant has lots of water, the guard cells become swollen with water (turgid) and the stomata are open to increase the rate of water loss, but also increase the intake of carbon dioxide for photosynthesis (and oxygen diffusing out).

When the plant is short of water, guard cells lose water (flaccid, 'limp') and the stomata are closed to decrease the rate of water loss.

If the plant is very short of water the cytoplasm inside the cells shrinks and the cell membrane comes away from the rigid cell wall. This process is called plasmolysis and the cell is said to plasmolysed.

Three more points

(i) Adaptations of guard cells:

Apart from their shape, guard cells have other adaptations which help them in their function to aid in controlling gas exchange and water loss.

They have thin outer walls and thickened inner walls which allow the opening and closing mechanism to work efficiently.

The guard cells also respond to light levels - they close at night to save water - conserved for photosynthesis and open up again when daylight returns to allow the exchange of gases.

(ii) You usually find more stomata on the underside of leaves compared to the top.

The lower leaf surface is more shaded and cooler, this reduces water loss, compared to the water loss that would happen on the upper surface.

Plants growing hot climates need to conserve water and so they have fewer and smaller stomata on the underside of the leaves and no stomata on the upper epidermal surface.

See also plant adaptations - examples in extreme environments

turgid guard cell flaccid guard cell controlling stomata pores(iii) It is changes in the concentration of ions inside the guard cells that facilitate the opening and closing of stomata.

When guard cells lose water, it causes the cells to become flaccid and the stomata openings to close - reducing water loss. This occurs when plants has lost an excessive amount of water OR if light levels drop and the use of carbon dioxide in photosynthesis decreases.

Guard cells respond to light, if light levels increase, potassium ions (K+) are pumped into them by active transport. (diagram below)

movement of potaasium ions K+ into guard cells to control opening and closing of stomata poresThis increases the concentration of dissolved particles in the guard cells fluid and decreases the concentration of water molecules (decreases the cell's water potential).

Therefore water diffuses into the guard cells by spontaneous osmosis making the guard cells turgid (diagram above) and the stoma opens allowing carbon dioxide to enter for photosynthesis.

The reverse happens light levels or water levels are low.

When potassium ions exit the guard cells, the concentration of water molecules increases (increasing the cell's water potential).

Water will then move out of the guard cells by osmosis, they become flaccid and the stoma closes reducing the loss of water, and not as much carbon dioxide is available for photosynthesis - not needed at all at night.

Extra note on plant cells and water potential

(i) When you water a plant it increases the water potential of the soil around it.

Therefore the plant cells will draw water in by osmosis until they become turgid - fatter and swollen.

The cell fluids (contents of the cell) will push against the cell wall, known as turgor pressure, and this helps support the plant tissues (therefore the plant as a whole).

(ii) If the soil is very dry, lacking in water, the plant starts to wilt and the water potential of the plant is greater than the surrounding soil.

The result is the plant cells become flaccid and begin to lose water.

The plant doesn't droop (flop) completely and retains much of its shape because the strong cellulose cell wall is relatively inelastic and helps the plant retain its shape.

An experiment to measure the rate of transpiration

Place some damp soil in a plastic bag.

Plant a small plant into the soil and tie the bag tight around the stem, ensuring the leaves stick out of the bag - otherwise transpiration can't happen!

Weigh the 'bagged' plant and record the mass in grams.

You then leave the plant in a well-lit place for 24 hours - you set up it in the lab and leave a lit table lamp in front of the plant at night - but the light intensity will vary from daylight time to nighttime..

Ideally the plant is placed in a dark room, at the same temperature, and illuminated artificially for the 24 hours, to standardise experimental conditions.

After 24 hours, re-weigh the bag and plant and record the mass in grams.

The mass should have decreased and the mass loss equals the amount of water lost by transpiration.

You can then do some calculations.

Suppose the initial bag and plant weighed 400 g.

After 24 hours it weighed 350 g.

mass loss = 400 - 350 = 50 g of water.

rate of transpiration = 50 / 24 = 2.1 g/hour (water loss rate to 2 s.f.).

If you weighed the plant before placing it in the plastic bag, you can then calculate the percentage change in the mass of the plant.

e.g. suppose the initial plant weighed 380 g before planting, and using the 50 g loss from above.

the plant weighed 380 - 50 = 330 g after transpiration.

the % loss = 100 x mass loss of water / initial mass of plant

% loss = 100 x 50 / 380 = 13% (to 2 s.f.)

Sources of error

The plant will lose a tiny amount of mass as oxygen is produced by photosynthesis, but their is small gain in weight as carbon dioxide is absorbed for photosynthesis.

Variations on the experiment

You can vary the light intensity level, from intense to dark.


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