The organisation of plant cells into tissues and organs AND the

transport and gas exchange in plants and the absorption of nutrients

See also 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?



Types of plant cells and their organisation into tissues and organs

Plant cells are organised into tissues and these form plant organs such as leaves, roots and stems.

These organs must function together in such way to work as an organ system - to ensure the plant gets all its needs to survive and grow into a mature plant.

Transportation of e.g. nutrients or waste products is one of the most important function of a plant organ system.

Plant organs are made of tissues

(A summary of those you must know are briefly described below and in more detail later where necessary)

Epidermal tissue

Epidermal tissue covers the whole surface of the plant - its the equivalent of our 'skin'!

Meristem tissue

Meristem cells are found in the growing tips of shoots and roots.

They can differentiate into all the different types of plant cell needed for growth and reproduction.

Palisade mesophyll tissue

Most photosynthesis occurs in the palisade mesophyll tissue, part of the leaves.

Spongy mesophyll tissue

Spongy mesophyll tissue forms part of the leaf and contains lots of air spaces to let gases diffuse in and out of the leaf structure.

Xylem and phloem

Xylem and phloem are tubular cell networks that allow the transportation of mineral ions, food e.g. sugars and water around the plant - the leaves, roots and stems must be all connected together.

Waxy cuticle

The cuticle is a water repellent protective layer covering the epidermal cells of leaves and other parts and limits water loss.

and this is how some of the above fit together in the structure of a leaf ...

More on the tissue structures of a leaf and their functions

So, starting from the top layers, and all marked on the above diagram ...

The epidermal tissue on the upper side of the leaf are covered with a waxy cuticle layer which is water repellent - this helps water loss by evaporation.

The upper epidermal layer is transparent to visible light, so light can penetrate to the palisade cell layer where it is needed for photosynthesis.

The palisade mesophyll layer is made of the palisade cells which are packed with chloroplasts - the sites of photosynthesis - note that the palisade cells are near the upper surface to receive the most light.

The xylem and phloem are networks of vascular bundles of cells that are the backbone of the plant's transport system.

These are described in detail in the next section.

The leaf tissues are adapted for efficient gas exchange.

The lower epidermal tissue is full of tiny holes (stomata, pores) which allow carbon dioxide to diffuse into the leaf for photosynthesis.

Reminder:  carbon dioxide + water == light/chlorophyll  ==> glucose + oxygen

The opening and closing of stomata is controlled by guard cells which respond to changes in environmental (ambient) conditions including the movement of water in and out of leaves.

The spongy mesophyll tissues contain air spaces which increase the rate of diffusion gases in (carbon dioxide) and out (oxygen).


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. through the stem to the very tips of all 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 (lumen) allows the free movement of fluid - but in only one direction. The strong xylem cell walls are made from cellulose and are strengthened-stiffened by a material called lignin - these give the plant support.

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 elongated 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.

The sugars from photosynthesis enter the phloem system by active transport and transported around by water which enter the phloem cells by osmosis.

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.

Phloem tubes are sometimes called sieve tube elements and the perforated end-plates allow fluids to pass through.

The phloem cells (sieve tube elements) have no nucleus and can't survive on their own, so each one has a companion cell (not shown) that control the living functions for both cells.

They allow 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 up and down the stem to all parts of the plant for immediate use in respiration, new growth or to the food storage organs to form starch.

Phloem cells have very few sub-cellular structures

 


The function of roots

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

Whereas most of the plant consists of green photosynthetic tissue, the roots are non-photosynthetic and pale in colour.

Therefore sugars would be translocated from photosynthetic tissues like leaves to non-photosynthetic tissues like roots using the phloem tubes.

In plants most of the water and mineral ions (e.g. magnesium ions or nitrate ions) are absorbed by roots, these in turn will be translocated up through the stem to the rest of the plant using the xylem tube system.

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.

Each branch of a root will have millions of root hair cells creating a massive surface area to absorb water and mineral ions.

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.

The water potential of the soil is usually greater than that of the fluid in the root, so water will be naturally absorbed 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 is because the plants cells would naturally lose essential mineral ions by osmotic diffusion back into the soil moisture.

Not good, it means the root hair cells can't use diffusion on its own to absorb minerals from the soil, in fact, without active transport, mineral ions would move out of the root hairs.

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.

The process requires energy from respiration which powers the process by which the plant's root cells can absorb minerals from the soil, even if they are only present in a very dilute solution of the soil's moisture.

So, 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 solution at higher concentrations of sugar.

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'.

For more details on this experiment see Diffusion, osmosis and 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 gas exchange surface area.

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

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

Oxygen diffuses out through the stomata and most water is lost in the same way.

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.

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.


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

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 fluids 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 up into the whole of the plant.

 

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.

(Note: stoma is singular, stomata is plural)

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 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) 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.

(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 ion (K+) are pumped-absorbed into them.

This increases the concentration of particles in the guard cells fluid.

This decreases the concentration of water molecules (decreases the cell's water potential).

Therefore water diffuses into the guard cells by osmosis making the guard cells turgid and the stoma opens - carbon dioxide can enter for photosynthesis.

The reverse happens light levels or water levels are low.

Then, 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.

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

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 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.

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, 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.

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 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.

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.


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

The potometer - a way of measuring the uptake of water by a plant - it measures the rate of transpiration

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

The plant shoot should be cut under water to prevent air entering the xylem and at an angle to increase the surface area for water absorption.

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.

The whole apparatus must be airtight and watertight or the readings will be inaccurate.

You need to let the plant acclimatise ('settle down') to the laboratory conditions before starting the experiment.

You then let one air bubble into the capillary tube which is then back under the water in the beaker.

At the start and 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.

Making a measurement

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 with a potometer

Remember

Wherever possible keep everything constant except the one factor you are investigating e.g. constant temperature, constant air humidity in the laboratory.

You can now investigate various environmental conditions by comparing the relative uptake of water.

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 and humidity are constant and there is no air flow over the plant (still air).

It would be easy to do comparative experiments in a brightly lit room and a darkened room.

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, light intensity and humidity are 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.

BUT, not that easy to keep conditions constant.

You can measure the temperature of the air near the plant during the experiment.

It might be convenient to do the experiment in a cold room and then in a warm room.

You must make sure the air flow is constant (completely still air is the best condition), and the humidity and light intensity stay 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.


Some learning objectives for this page

  • You should know that plant organs include stems, roots and leaves.

    • Details of the internal structure are only needed for the leaf.

    • Know the structure and function of palisade cells and guard cells in plants.

    • Palisade cells contain chlorophyll and are adapted for photosynthesis.

    • Guard cells are adapted to open and close the pores which allow gas exchange and water evaporation.

  • You should know examples of plant tissues including:

    • epidermal tissues, the outer layers which cover the whole plant,

    • mesophyll, between two epidermis layers, where most photosynthesis happens,

    • xylem and phloem, which transport substances around the plant eg sugars like sucrose and glucose, minerals and water.


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 General PLANT BIOLOGY revision notes

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

Plant cells, transport and gas exchange in plants, transpiration, absorption of nutrients, leaf and root structure

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

Respiration - aerobic and anaerobic in plants (and animals)  gcse biology revision notes

Hormone control of plant growth 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

and a section on Stem cells and uses - meristems in plants (at the end of the page!)


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