Transport and gas exchange in plants, absorption of nutrients

See also

Introduction to diffusion, osmosis, transport, active transport in animals

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will help you answer questions such as ...

 How does gas exchange take place in plants?

 How does a plant absorb mineral nutrients?

 How does a plant transport minerals around in itself?



Flowering plants have two separate transport systems which must reach all parts of the plant

Plants have two networks of 'fine tubes' to transport molecules and ions (xylem tubes and phloem tubes).

The xylem vessels transport water and minerals from the root hairs to all the rest of plant i.e. to very tips of the leaves.

The phloem vessels transport sugars from the leaves (where they are made from photosynthesis) to all parts of the plant e.g. for growth of new cells or to storage tissue where they are converted to starch.

The xylem tissue transports water and mineral ions from the roots to the stem and leaves.

Xylem tubes are made of dead cells joined together 'end to end' in such a way they form a complete fine tube through which water and minerals are freely transported from the roots, up through the stem to the leaves. The xylem cells have no end wall but a hole down the middle allows the free movement of fluid. The xylem cell walls are strengthened by a material lignin.

The movement of water from the roots through the xylem and out of the leaves is called the transpiration stream and is caused by the diffusion of water and its subsequent evaporation. The transpiration stream only flows in one direction - up through the plant.

Water that evaporating through the stomata causes it to be replaced by water absorbed by the roots and this water moves up via the xylem tube system. If the stomata pores are open, evaporation of water will always take place because the concentration of water in the air is less than the concentration off water in outer layers of a leaf.

The diffusion and evaporation of water from the leaves produces a water deficiency in the plant, so water is automatically (if available) drawn up through the xylem tube system, so the transpiration stream is driven by this evaporation of moisture from the leaves.

Water is essential to the plant for both transportation and photosynthesis.

Phloem cells are living cells and the phloem tube tissues carry dissolved sugars (food) from the leaves to the rest of the plant, including the growing regions and the storage organs.

This process is called translocation.

Phloem cells are elongated with end walls that have pores in them to allow fluids to flow through and the phloem system allows transportation in both directions..

This allows the transport of water and dissolved substances to all part of the plants where nutrients are needed for immediate use in growth or converted to starch for storage

The phloem tubes mainly carry the sugars made in the leaves from photosynthesis to all parts of the plant e.g. for new growth or to the food storage organs.

 


The function of roots

In plants most of the water and mineral ions are absorbed by roots.

The surface area of the roots is increased by root hairs and the surface area of leaves is increased by the flattened shape and internal air spaces.

The cells on the surface of a plant's roots grow into long cells that shape into 'hairs' that stick out into the soil.

The fine root hairs considerably increase the surface area of the roots for absorbing water and minerals and each branch of the root is covered millions of these microscopic hairs.

Root hair cells are very elongated combining into fine hair-like structures, which greatly increases the surface area of contact with the soil through which most of the plant's water and mineral intake are absorbed.

As long as the water concentration is higher in the soil, the root hairs will naturally absorb water by osmosis.

However, the concentration of minerals in the root hair cells is higher than in the moisture surrounding the roots, and therefore an absorption problem. This because the plants cells would naturally lose essential mineral ions by osmotic diffusion back into the soil moisture, not good!

Therefore, active transport systems must be used by the plant to counteract the natural direction of diffusion from a high mineral concentration in the plant cells to a low mineral concentration in the soil moisture.

Energy from respiration is usually required to absorb minerals into the roots from the soil moisture by working against the concentration gradient - active transport mechanism.

Simple demonstration of osmosis.

Water diffuses out of the potato cells into the sugar solution.

If you wanted to reverse the process, and make water go back into the potato cells, you would have to use an energy 'input' system to do so. In living organisms this is known as 'active transport'.


Leaf adaptations to aid photosynthesis and more on transpiration

See detailed notes on Photosynthesis, importance explained, limiting factors affecting rate  gcse biology revision notes

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.

In daylight more carbon dioxide will be taken in than given out 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 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.

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 surface area.

Plants have stomata (tiny pores or holes), mainly on the underside of leaves, to obtain carbon dioxide gas from the atmosphere and to give out the oxygen gas produced in photosynthesis.

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.

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

Water, and the oxygen produced in photosynthesis diffuse out.

 


More on transpiration

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.

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

The loss of water from leaves creates a small shortage of water in the leaves and so more water is drawn up 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 too

 


Factors affecting the rate of transpiration and the function of the stomata and guard cells

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.

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

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

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

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

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 guard cells are swollen with water (turgid) the stomata are open to increase the rate of water loss.

When the guard cells are low on water (flaccid, 'limp') the stomata are closed to decrease the rate of water loss.

More on the environmental factors affecting the rate of water loss

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 by a greater air flow, the steeper the diffusion gradient is - the concentration of water vapour is much greater in the stomata than in the air surrounding the leaf - which is much lower because the water vapour is being constantly carried away in the air current.

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.

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. The stomata must be open to let 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.

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.


An experiment to investigate the rate of transpiration - using a potometer

The potometer - a means of measuring the uptake of water by a plant

A potometer consists of a vertical tube with a plant shoot sealed in it.

The tube is connected to a reservoir of water, controlled by a tap, used to replace water lost by transpiration.

The horizontal capillary tubing, connected to the plant tube and reservoir, has a scale set up beside it.

Inside the capillary tube, adjacent to the scale is an air bubble which will move along to the left as water evaporates from the leaves.

After a measurement of transpiration has been made, the reservoir tap is opened to allow water to flow in and move the air bubble to the right near the start of the scale.

Excess water runs out into the beaker which also acts as a reservoir of water itself during an experimental run, rather than sucking in air.

You note the starting position of the air bubble on the right. You then measure the time it takes for the air bubble to move from right to left and note the total distance moved.

To repeat the experiment you let water in from the reservoir to bring the bubble back to near the start of the scale.

Estimated transpiration rate = distance air bubble travels / time taken

So your rate of transpiration units might be mm or cm/min (an arbitrary scale based on the experimental setup).

BUT, note that the experiment assumes the water uptake by the plant through its roots is directly related to the water loss by evaporation from the leaves.

The sorts of investigations you can do

Remember - wherever possible keep everything constant except the one factor you are investigating.

1. Varying light intensity: You can use a reasonably powerful light to increase light intensity, placing it quite close to the plant, with the bulb at the same height as the centre of the plant shoot.

You measure the distance of the centre of the lamp bulb to the centre of the plant shoot (d).

Run the experiment and measure the rate of transpiration.

You can then move the same lamp back, measuring the new longer distance and re-measure the transpiration rate. You should repeat all measurements for varying lamp distances.

The intensity of light on the plant is proportional to1/d2 or you can measure the intensity with a light meter.

You can then plot a graph of transpiration rate against light intensity.

You must make sure the temperature is constant and that there is no air flow over the plant (still air).

2. Varying air flow: You could use a hair dryer to blow cold air (room temperature) at different speeds over the plant to see if increased air flow increases transpiration rate. Difficult to get data to produce a meaningful graph, but no air blowing, low blowing setting and a high blowing setting should give you the trend.

You must make sure the temperature is constant and the light intensity stays the same.

3. Varying temperature: This is also tricky to get good quantitative data.

You can easily compare blowing cold air and warm air over the plant, and that should give a difference in transpiration rate. You can measure the temperature of the air near the plant during the experiment.

You must make sure the air flow is constant (still air best) and the light intensity stays the same.

 


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Practical investigations you might have encountered

  • investigating potato slices in different concentrations of liquid in terms of mass gain and mass loss - this is to illustrate the process of osmosis.

  • designing an investigation to measure the mass change of potato when placed in a series of molarities of sucrose solution

  • investigating the relationship between concentrations of sugar solution and change in length of potato strips

  • placing shelled eggs in different concentrations of liquid to observe the effect

  • placing slices of fresh beetroot in different concentrations of liquid to observe the effect, and then taking thin slices to observe the cells

  • observing guard cells and stomata using nail varnish

  • observing water loss from plants by placing in a plastic bag with cobalt chloride paper.

  • investigating flow rate in xylem using celery, which can include calculation of flow rate

  • investigate the content of artificial phloem and xylem given knowledge of the appropriate tests

  • planning an investigation using a potometer to measure the effect of temperature or wind speed on the transpiration rate.


 General PLANT BIOLOGY revision notes

See also cell biology section

Photosynthesis, importance explained, limiting factors affecting rate, leaf adaptations  gcse biology revision notes

Transport and gas exchange in plants, transpiration, absorption of nutrients, leaf and root structure gcse biology revision notes

See also Diffusion, osmosis, active transport, exchange of substances - examples fully explained

Respiration - aerobic and anaerobic in plants  gcse biology revision notes

Hormone control in plants and uses of plant hormones  gcse biology revision notes

Plant diseases and defences against pathogens and pests  gcse biology revision notes

See also Adaptations, lots explained including plant examples  gcse biology revision notes


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