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 - adaptations of leaves

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 lots of 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 sheathed bundles of cells that are the backbone veins of the plant's transport system.

The details of these are described in detail in the next section.

The leaf tissues are adapted for efficient gas exchange.

The broad green leaves of plants exposed to light provide a large surface area for the light absorbing sites of photosynthesis - more than the thinner stem.

The leaves are thin so the absorbed carbon dioxide has only a short distance to diffuse to the photosynthesising cells.

Leaves have veins (vascular bundles) that support the leaf and transport water and minerals to the leaves and glucose away from the leaves.

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).

 

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

The structure and function of plant xylem and phloem cells - the transport tissues

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

The xylem vessel tissue transports water and minerals from the root hairs to all the rest of plant i.e. through the roots and stem to the very tips of all the leaves.

This xylem vessel process is driven by transpiration.

The phloem vessel tissue transports dissolved 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.

This function of the phloem to move sugar molecules (mainly dissolve sucrose), amino acid molecules and mineral ions around the plant is called translocation.

These systems are essential for a plant to be healthy.

In some trees the transport systems run through the bark.

Unfortunately, some animals like to chew this bark.

The transport systems are disrupted and the tree sadly dies!

Consider the xylem first

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

Xylem tubes are made of lignified dead cells joined together 'end to end' in such a way they form a complete fine tube through which water and mineral ions 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 evaporating through the stomata in the leaves, causes it to be replaced by water absorbed by the roots and this water moves up via the xylem tube system through the stem and to all the leaves - from the roots to ALL of the plant and carrying essential mineral ions too.

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.

Evolution adaptation notes:

Transpiration is a necessary adaptation to work in conjunction with photosynthesising leaves - the stomata allow the gas exchange - carbon dioxide in, and water vapour and oxygen out.

Even the narrow roots of plants are further covered in tiny root hairs that greatly increase the surface area even more, and hence increase the efficiency of water absorption.

This adaptation means the water has only got to move a short distance to the xylem to be transported up through the whole of the plant.

Now consider the phloem cells and compare their structure and function with xylem cells

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

The phloem cells also transport other important materials like amino acids for protein synthesis and mineral ions for the function of chlorophyll and enzymes.

This process is called translocation and can operate fluid flow in both directions - another useful adaptation.

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, but has nucleus) that controls the living functions for both cells.

The companion cell, with a nucleus, has lots of mitochondria to provide energy for the phloem's transportation function.

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 contain a little cytoplasm, but no nucleus and have very few sub-cellular structures, this adaptations increase the efficiency of transportation - also aided by the sieve plates of adjoining cells.

 

Vascular bundles

Throughout the plant the chains of xylem and phloem cells are held together in vascular bundles.

You find them in the roots, stem and in the leaf - where you see them as the veins in leaves.

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The structure and function of roots, absorption and necessity of nutrients

Structure and function of roots - absorption of minerals - diffusion and active transport

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.

I'm concentrating on minerals here, but remember plants need lots of water for their internal transport systems (xylem and phloem cells), photosynthesis and to keep cells turgid and support the physical structure of the plant as a whole.

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.

The root hair cells are connected to root cortex cells, which in turn, are closely connected to xylem vessels which transfer minerals and nutrients around the plant efficiently.

Root hair cells are well adapted for their function:

large surface area for absorbing water - which will also contain minerals,

they have no cuticle, but a thin membrane for efficient absorption - short diffusion distance,

they have a thin cell wall, again reduces the distance for water transfer in osmosis,

a large permanent vacuole to absorb and store (temporarily) as much water as possible,

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.

 

Osmosis and active transport

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 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.

It has been shown experimentally that when there is an increase in the uptake of minerals by plants, there is an increase in active transport, accompanied by an increase in respiration.

Examples of moving substances by active transport from a low concentration to a higher concentration:

The concentration of nitrate ions (NO₃^-) is usually greater in the root hairs cells than the surrounding soil water. Therefore the natural diffusion gradient for the movement of nitrate ions is in the direction of root to soil, but pant need the nitrogen in nitrate ions to synthesise proteins. Therefore active transport is used to move nitrate ions from the soil water into the root fluids against their natural diffusion gradient.

The same argument applies to magnesium ions which chlorophyll molecules need in photosynthesis.

Active transport is much more complicated than 'simple' osmotic water exchange in plant cells.  See comparison of diffusion, osmosis and active transport

 

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The importance of minerals to plants - need for absorption of nutrients through the roots

Mineral ions can only be absorbed by plants through the hairs of the root cells.

The concentration of minerals is quite low in soil compared to the concentration in the fluids of root cell hairs - hence the need for active transport for the plant to absorb them - and why there is a high concentration of mitochondria in root hair cells to provide the energy needed.

Several ions found in soil are particularly important.

The health of any plant is harmed if it is deficient in access to these mineral ions.

Just look at their life-supporting roles in the biochemistry of plant cells listed below.

Nitrogen for amino acids and protein synthesis (including enzymes) is obtained from the nitrate ion (NO₃^-).

Lack of nitrogen gives stunted growth.

The magnesium ion (Mg²⁺) is required by the chlorophyll molecule to function in photosynthesis and is also essential for the metabolism of carbohydrates.

 If a plant is deficient in magnesium not enough chlorophyll can be made and the plant suffers from chlorosis - a yellowing of the leaves, and photosynthesis is much reduced - as is the supply of food and energy for the plant.

The potassium ion (K⁺) is involved in the mechanism for opening and closing stomata, the activation of some enzymes, involved in photosynthesis, and the production of ATP in respiration.

Phosphorus, from phosphate ions. A phosphate structure is part of the ATP molecule from respiration - energy source, and part of the structure of cellular DNA and RNA.

Iron ions (e.g. Fe²⁺ or Fe³⁺) are important in the synthesis of chlorophyll and some enzyme functions.

You can also get chlorosis in plants from iron deficiency. (see magnesium above)

Borate ions supply the element boron which plays a key role in many plant functions including cell wall formation and stability, maintenance of structural and functional integrity of biological membranes, the movement of sugar for energy into growing parts of plants.

Soil that is deficient in any of these minerals can be improved by using synthetic artificial fertilisers (NPK types for nitrogen, phosphorus and potassium) often supplemented with lots of additives to supply other trace elements like iron or magnesium that plants need. You must compost or manure if you are a true organic gardener!

For more see Plant diseases and defences against pathogens and pests

 

Investigating mineral requirements of plants and the effect of mineral deficiencies

All plants need a variety of minerals to grow and maintain themselves in a healthy state.

In the previous section, I've described some of the uses plants make of some nutrients.

If plants do not receive any of their essential mineral nutrients ('mineral deficiency'), symptoms will be observed as the plant declines in health e.g.

lack of nitrates - poor growth and yellowing of leaves,

lack of potassium - poor fruits and flowers and discolouring of leaves,

lack of phosphorus - poor root growth and discolouring of leaves

lack of magnesium - yellowing of leaves,

lack of iron - loss of green colour in stems and leaves - known as chlorosis,

lack of boron - the growing tips of the root or shoot show stunting and distortion of the growing tip that can lead to tip death, brittle foliage, and yellowing of lower leaf tips.

How do mineral deficiencies arise?

(i) Naturally poor soil

Some soils are naturally very low in, or devoid of, some essential minerals.

Lime-rich soils are deficient in iron.

This land is not productive unless it is treated with a synthetic inorganic fertiliser or an organic fertiliser to increase the mineral content of the soil.

(ii) Farming methods

From the prehistoric times, throughout the world, woodland has been cut down and burnt to provide land to grow crops - known as 'slash and burn'.

Repeated planting of the same annual crops on the cleared land uses up the minerals in the soil and so the soil becomes deficient in essential minerals.

More woodland is then cut down, a process that continues to this day in many forested areas e.g. the rainforests of Brazil.

Again, use of fertilisers, enables repeated growing of crops.

Modern synthetic inorganic fertilisers have become more complex in their formulation and may contain a dozen different elements that plants need for healthy growth and development.

How can we investigate which minerals are essential for healthy plant growth

You can grow plants without soil if you suspend them in a solution of the minerals they need - the culture solution of a hydroponic technique - no soil used - the experiment is illustrated in the diagram above (I guess I'm no artist, but you should get the 'picture').

The plant is suspended in a culture solution of nutrients, so that the roots can absorb what they need for healthy growth and development - water for transpiration, mineral ions (for reasons discussed above) and oxygen (for respiration).

The culture vessel is covered to simulate the darkness that roots would experience in soil.

A tube supplies air, containing oxygen, to keep the culture solution aerated.

You grow control plants with a culture solution containing all the nutrient minerals they need.

You can then grow the same plants in culture solutions that do NOT contain any of a specific mineral - to artificially create a deficiency of a plant nutrient.

The growth of the plant can be monitored to see the effect of a particularly deficiency - observed symptoms resulting from the specific deficiency

You must only omit one mineral at a time in the investigation, otherwise you could not be sure which one caused the symptoms you are observing.

Using this relatively simple technique, you can investigate the effects of deficiencies in nitrate ions (for nitrogen), phosphate ions (for phosphorus), magnesium ions (Mg²⁺), potassium ions (K⁺), iron ions (Fe²⁺/Fe³⁺) and borate ions (for boron).

The relevance of nutrient knowledge to food production

If you know what minerals are soil is deficient in, you can add a fertiliser to provide the missing nutrients.

Farming, agriculture in general, uses two types of fertiliser.

'Natural' organic fertilisers

Organic fertilisers come from animal ('smelly muck') or plant ('compost') waste material and are considered environmentally superior to inorganic fertilisers.

They produce better quality soil that is more resistant to erosion and better at retaining water.

Organic fertilisers only break down slowly in the soil to release their nutrient minerals - 'intensive farming' farmers would see this as an economic disadvantage.

Good quality organic fertiliser should supply all the essential minerals a plant needs to grow and develop.

'Man-made' synthetic inorganic fertilisers

These are concentrated formulations of particular essential elements plants need for healthy growth.

They can be made to any composition to suit particular soil deficiencies in any soil.

The inorganic fertiliser formulation can also be made to promote the growth of a particular crop to increase the yield.

They do not smell, easily stored (chemically stable), easily and evenly spread on the soil as small pellets and the chemicals are quickly absorbed by plant roots.

The solid pellets can be designed to break down slowly to minimise the polluting effects from run-off into water courses.

These fertilisers do not improve the quality of the soil, but must be regularly applied-replaced, especially if fields are used to grow several yields of crops each year (same or different).

 

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More on leaf adaptations to aid photosynthesis

See also 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.

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

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

 6H₂O_(l) + 6CO_2(g)  == sunlight/chlorophyll ==> C₆H₁₂O_6(aq) + 6O_2(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 ½ filled) you count it towards the total.

If a square is not well filled (less than ½ 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

See also Plant adaptations and controlling water loss

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

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.

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 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.

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

(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.

 

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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

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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

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

A much more sophisticated experiment to look at the rate of transpiration.

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.

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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

Plant cells, transport, gas exchange in plants, transpiration, absorption of nutrients, leaf & 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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