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  • Anne Gunning

R2101 Going Up! The Pathway of Water Uptake in the Plant.

Updated: Aug 27

R2101 4.3 THE MOVEMENT OF WATER IN THE PLANT

Introduction

This is not a little topic to study because it ties together the anatomy of the root, stem and leaf and a cell so you need to be familiar with these cross sectional diagrams before starting to study the detail of water movement. It is also helpful to review photosynthesis. In the syllabus it states that you DO require a diagram of the pathway of water.

You will see that we have mentioned the apoplast and symplast pathway for water movement across cells and this may seem like level 3 RHS study but these terms have been mentioned in examiners' comments so that is why I have included these terms – they are not included in the syllabus so do you need to learn them? I am not sure but I do think it is useful in understanding the two different ways water travels across cells ie osmosis and diffusion.

Please note where I say xylem tissue this means xylem vessels and tracheids.


What is the role of water in the plant?

‘Plump’ cells which is called turgor pressure hold the plant erect by inflating cells.

Water is an ingredient for photosynthesis.

Water is a solvent for nutrients and products of photosynthesis

Water transports hormones and regulates growth

In fleshy fruits water assists seed dispersal

Water cools the leaves as it evaporates (Like us as we sweat)

Water allows storage of waste products in the vacuole in solution.

What processes are involved when water moves through the plant?

Diffusion, Osmosis, Transpiration and Root Pressure.

Diffusion

This is the movement of a substance from an area of high concentration to an area of low concentration (Down the concentration gradient) which continues until the substance is evenly distributed. This process occurs naturally and does not require energy. Think about how a stink bomb permeates a room or better still, a scented candle

Diffusion can happen in air and liquid.

Gaseous diffusion will have been studied when looking at the process of photosynthesis - For example CO2 moving into leaves via stomata and oxygen moving out of leaf via stomata by diffusion.

Water diffusion

In the root water will diffuse from the soil water through the cell walls of the root hairs and across the cell walls of the cortex as long as there is a favourable concentration gradient (High water content towards the outside of the plant root and low concentration of water towards the inside of the root)

Movement of water by diffusion across the root and leaf has a special term – it is called the apoplast route. there is a diagram later.

Osmosis

Osmosis is the movement of water from an area of high concentration to an area of low concentration across a semi- permeable membrane.

In plant cells this semi permeable membrane is the plasma membrane found inside the fully permeable cell wall of the plant cell.

Osmosis is a specific type of diffusion that only ever involves the movement of water. It does not require energy. In the root, water will travel across the cell membrane (which is semi permeable) as long as there is a favourable concentration gradient into the cytoplasm of the cell.

Movement of water by osmosis across the root and leaf has a special term – it is called the symplast route.

Transpiration

This is the loss of water vapour from the plant into the atmosphere by evaporation, through the stomata of the leaves and across the leaf surface. Evaporation is when a liquid turns into a vapour. Transpiration will also take place across the cuticle of the leaf which is why in some plants that can tolerate drought, the waxy cuticle is thick.

Loss of water by transpiration creates a pulling effect known as the transpiration pull which drags columns of water upwards. If plants did not lose water, they would not be able to take it up which sounds strange doesn't it!

Physical Properties of water to help understand how water moves in the plant

This detail is not needed for the exam. Think of a dew drop or a pond skater on the surface of the pond or the meniscus in a glass of water. You can observe that the molecules of water ‘stick’ together as each molecule has positive and negative charges so are attracted to each other just like a magnet. This is cohesion of water molecules.

These adhesive forces also mean that as well as water molecules sticking to each other they also stick to other surfaces. This so called cohesion tension of water means that an unbroken column of water (the transpiration stream) can be pulled up through the xylem vessels as water vapour is lost through the stomata (the transpiration pull) because the water molecules stick together and stick to the inside of the xylem vessels.

Capillarity

This detail is not needed for the exam. This is a phenomenon where water moves up narrow columns (xylem vessels, tracheids or pore spaces in soil) against the force of gravity. It will occur until gravity stops it. It occurs because the cohesion tension between the water molecules sticks them together and the adhesion force between the walls and the water allows the water to climb upwards. If the conducting tubes are thin the water will rise higher compared to wider tubes. Therefore, it will rise higher in tracheids compared to xylem vessels as these are narrower. This is why conifers (gymnosperms) are the tallest trees as they have only tracheids compared to angiosperms that have xylem vessels and tracheids. This is also why clay soils are wetter than sandy soils as the pore spaces which join together are much smaller in a clay soil and thus the 'tubes' are narrower.

The Pathway of Water

1.

Root hairs absorb water from the soil water by osmosis (symplast route). The water travels across the inside of the root hair cell and moves across the internal structure, the plasma membrane (which is semi-permeable) from the soil water towards the xylem tissue.

Root hairs also absorb water from the soil water by diffusion (apoplast route). The water travels through the cell walls of the root hair and moves across root from the outside inwards. The water travels much quicker compared to the symplast route.

Revision tip: how do you remember which is apoplast and symplast?

Sympast begins with S and osmosis has lots of s’s in it. Osmosis = symplast

Adaptation for water uptake - large surface area of tiny root hairs.


Diagram to show apoplast and symplast pathway of water movement across root hair and cortex cells in the root. This also happens in the leaf cells.


2.

Water travels across the root hairs and across the cortex by diffusion and osmosis until it reaches the endodermis of the root. (See diagram above)

The endodermis has an impermeable band surrounding the cells called the Casparian strip which prevents water passing through. When water reaches the Casparian strip, all the water is forced to enter the endodermis by the process of osmosis only as it is forced to follow the symplast route.

This allows the plant to regulate which compounds are entering the xylem vessels.


The 3 D model in the picture is the endodermis and Casparian strip shown in the diagram.


This picture shows that the Casparian strip runs right through the cell walls of the endodermis so water is prevented from following the apoplast pathway.

3.

Root Pressure. There is a build up of water in the cells adjacent to the endodermis as the flow of water is slowed due to the impermeable Casparian strip. This build up of root pressure gives the water column a ‘push’ upwards. This is demonstrated in rising sap in Spring before leaves emerge and there is no transpiration pull. If you cut a stem water continues to exude.

4.

Water passes into the xylem tissue of the root and stems.

Water travels up through the roots and stems in unbroken columns of water called the transpiration stream. The water is dragged up by the transpiration pull. Cohesion tension between water molecules stops the column dropping downwards with gravity. Capillarity and the adhesion force between the water and the inner surface of the vessels helps the column of water to rise also.

5.

Water is transported into the xylem tissue of the veins in the leaf and then osmosis occurs across the leaf cells and diffusion occurs across the leaf cell walls. (apoplast and symplast route) towards the air spaces in the lower area of the leaf, next to the stomata.

6.

Evaporation then occurs into the spongy mesophyll air spaces within the leaf as the liquid water diffuses into the air spaces as water vapour.

Air spaces in leaf become saturated with water vapour. (High concentration of water)

8.

If the stomata are open (Guard cells are turgid and bending outwards to open the pore) water evaporates from the air spaces within the leaf to the external atmosphere. (Diffusion of water vapour from an area of high to an area of low concentration) This is transpiration. The pulling effect is called the transpiration pull.

Water only moves upwards in the plant.

9.

The concentration of water in the open air is (usually) lower than within the leaf air spaces so transpiration occurs through the stomata to the external atmosphere.

The difference in the concentration of water within and outside the leaf is called the concentration gradient. The steeper the gradient the faster the rate of transpiration. So for example if we consider plants which are growing on a windy site on a hot Summer’s day the rate of transpiration is high because the concentration of water outside the plant is low as the wind is constantly taking the water molecules away so water is lost more quickly. Also when temperatures are high the rate of transpiration is high.

Detailed Diagram of pathway of water. This diagram explains the outline above and I would say is more level 3 standard.



This diagram is more simple and with a little more extra text is probably all you need for level 2 exams.


Questions

The following are past questions on this topic. I have provided more answers and detail than are required to give a wider range of correct answers as well as incorporating examiners' comments. The numbers at the end of the question show the number of marks awarded. You will see that the same quesions are repeated and also the examiners comments for the same questions are also slightly different. Don't panic! As long as you get the main points in the answer you will get marks - remember these are 'ideal' answers and they took me a lot of time to compose.


Describe how water moves in the plant under EACH of the following headings:

i) Uptake by the root cells 5

Root hair cells take up water by osmosis across the semi permeable membrane. Soil water moves into the root hair because the concentration of water is higher outside the root compared to the inside. Root hairs have a large surface area.

Water will then move across the roots by 2 paths – symplast pathway which is when water moves by osmosis across the semipermeable membranes of the cortex and apoplast pathway which is when the water travels by diffusion along the cell walls of the cortex cells. Where the water meets the endodermis the Casparian strip controls the diffusion of water into the xylem by diverting it to follow the symplast pathway as it is impermeable to water.


ii) Through and out of leaves 5

Water moves from the veins across the leaves via the symplast and apoplast pathway which is movement by osmosis and diffusion respectively. The water evaporates from the surface of the spongy mesophyll cells into the air spaces to form water vapour. Because the water vapour concentration inside the leaf is higher than outside the leaf ,the water leaves the leaf by diffusion in a process called transpiration. It leaves via openings in the epidermis called stomata. This creates the transpiration pull which drags the columns of water upwards.


a) State what is meant by the term transpiration 1

Loss of water vapour or evaporation of water from the leaves or other plant surfaces via the stomata by diffusion.


b) Name THREE environmental factors that affect the rate of transpiration 3

Light

Temperature

Humidity

Wind speed

Soil water availability

c) Describe THREE ways in which leaves minimise transpiration 6

Rolling – leaves of marram grass will roll inwards into a C shape creating a humid environment outside the stomata so that the rate of transpiration slows as the humidity outside the leaf is made higher than inside so the concentration gradient changes. see picture below:


Hairs on leaf surface – create a humid environment by trapping water vapour near the leaf surface so that the concentration gradient changes


Stomata can respond to water stress and close


Waxy waterproof cuticle prevents water loss through the surface of the leaf.


Stomata concentrated on underside in dicots where humidity is highest so transpiration slows

Palisade mesophyll cells are tightly packed to prevent water escaping between cells.


a) Name ONE environmental factor which affects the rate of water loss from plants

Temperature, heat, wind speed, humidity, light or dark


b) Describe water transport through the plant stem

Only include the stem. Movement is in an upward direction and only takes place in the xylem tissue through xylem vessels and tracheids. Vessels have no end walls and tracheids are long slender vessels connected by pits so a continuous column of water moves upwards held together by cohesion of the water molecules and being pulled by the transpiration pull as water leaves the leaf by transpiration. Root pressure and capillarity also help to move the water upwards.


c) State TWO functions of stems other than water transport

support flowers for pollinators or dispersal of pollen by wind

support leaves in best position for photosynthesis

support fruits

Photosynthesis as are green

Store starch

Store water (cactus)

Modified for defence against herbivores (thorns. prickles)


Describe the pathway of water movement from the soil to the stem with the aid of a fully labelled diagram. (see earlier detailed diagram)

Shows water moving from around soil particles into the root hair by osmosis and diffusion.

Then passes across the cortex of the root to the xylem tissue between cells or from cell to cell (Apoplast and symplast pathways respectively.)

At the endodermis the Casparian strip halts the apoplast route as it is impermeable tro water so that the water has to travel across the endodermis by osmosis across the semi permeable membrane. Water flow is controlled here.

Water enters the pericyle and connects with the xylem tissue of the root which continues into the stem.

a) Describe what is meant by EACH of the following terms in relation to water movement in a plant:

i) Osmosis 4

Osmosis is the movement of water from an area of high concentration (of water molecules) to an area of low concentration across a semi permeable membrane. Water moves across the cortex of the root from the root hair towards the xylem tissue by this method which is also called the symplast route.

ii) Diffusion 2

Diffusion is the movement of water via the apoplast route. Water flows along the cell walls from areas of high to low concentration to an area of low concentyration and it is a passive process which does not require energy. Water also moves by diffusion when water vapour leaves the leaves (transpiration)


b) State FOUR functions of water in a plant 4

Supports the plant because when the cells are turgid leaves and stems are bulkier and remain upright rather than wilting - particularly herbaceous plants.

Controls the opening and closing of guard cells.

Assists the transport of minerals in solution

Assists the transport of sugars in solution

Creates cell turgor which enables cells to expand and plant to grow

Is used in photosynthesis as a starting ingredient.

Is essential in triggering germination

a) Name the process by which water is lost from the leaf 1

Transpiration

b) Name THREE environmental factors which affect the rate of water loss from the leaf 3

High temperature increases the rate of transpiration

High humidity decreases the rate of transpiration

High wind speed increases the rate of transpiration

Light or Dark. In most plants stomata open in light and close in the dark


c) Describe the pathway of water movement through the stem and leaf to the surrounding air. 6

Water moves up the stem due to the transpiration pull. The transpiration pull is the effect of water diffusing out of the leaves (Transpiration) and being replaced by water that is being pulled up the stem as a continuous column, called the transpiration stream. When the water reaches the leaves, it travels through the leaf veins, into the leaf cells (Osmosis) and through the leaf cell walls (Diffusion) and then evaporates into the air spaces in the spongy mesophyll before exiting through the stomata. Cohesion forces between the water molecules contribute to the transpiration pull as the water molecules ‘Stick’ together.

The stomata may be open or closed and this depends on how much water is available and whether it is day or night time. Stomata are usually closed at night as Co2 is not needed for photosynthesis. Water exits through the stomata and some will exit through the leaf cuticle.

(No marks awarded for describing movement of water across root.)


a) State what is meant by each of the following terms

i) Diffusion 3

Diffusion is the movement of a substance/molecules/particles from an area of high concentration to an area of low concentration ie movement down a concentration gradient. Solutes ie substances dissolved in water can also diffuse as can gases.

ii) Osmosis 4

Osmosis is the diffusion specifically of water (Not solutes, minerals or nutrients) across a semi or selectively permeable membrane from an area of high water/Low solute concentration to an area of low water/high solute concentration

b) Describe ONE example of diffusion in a plant 3

The movement of carbon dioxide into the leaf through the stomata as the concentration of carbon dioxide is higher outside the leaf compared to inside. This usually occurs in the day when the stomata are open. (As photosynthesis occurs in light and carbon dioxide is required for photosynthesis)

The movement of water vapour out of the leaves through the stomata to the external atmosphere as the concentration of water inside the leaf air spaces is higher compared to outside the leaf. This type of diffusion by evaporation is called Transpiration.

The movement of oxygen out of the leaves of the plant as a product of photosynthesis. Movement of oxygen out of the leaf through the stomata occurs because the concentration of oxygen inside the leaf is higher than the concentration in the atmosphere outside the leaf. This usually occurs in the day when the stomata are open. (As photosynthesis occurs in light)

The movement of water across/along cell walls (Apoplast route) in the root cortex. The concentration of water towards the outside of the root is higher compared to the inside so water travels across the root from the root hair towards the xylem tissue in the inside of the root.

a) Name the process by which water is lost from plant leaves 1

Transpiration


b) List THREE environmental conditions that increase the rate of water loss from plant leaves 3

Increase in temperature

Increase in wind speed

Decreases in relative humidity outside the leaf

High light levels (as stomata are open)

c) Describe how THREE named leaf adaptations can minimise water loss 6

Water can be lost through the cuticle. The cuticle is a waxy substance which prevents water loss so in plants that are adapted to minimise water loss the cuticle is a lot thicker.

Some leaves roll so that the underside of the leaf forms the inside of a C shape. This means that air is trapped inside the space and so humidity builds up. The relative humidity in the immediate vicinity of the stomata is more saturated with water so transpiration rate decreases.

Some plants have sunken stomata which has the same effect as a rolled leaf. The humidity builds up outside the stomata as the air is trapped. Wind speed is also reduced as there is more shelter.

Some plants have a hairy surface which creates a microclimate close to the leaf surface which increases the relative humidity around the leaf and shelters it from air currents that would decrease the relative humidity. They also create shade which decreases light intensity.

Some leaves are succulent and store water for times when water shortages occur. They can use this reserve to survive drought.

Needles – leaves which are needle shaped as in conifers have a smaller surface area so lose less water by transpiration.


a) State what is meant by the term ‘transpiration’ 1

This is the loss of water from the leaf surface as water vapour diffuses from the air spaces within the leaf into the atmosphere via stomata and through leaf surfaces.


b) Name THREE environmental factors that affect the rate of transpiration 3

Temperature – rate of transpiration rises as temperature rises.

Wind speed – rate of transpiration rises as wind speed increases.

Light/dark Stomata open when light is available to absorb carbon dioxide

Humidity – transpiration rate increases as relative humidity decreases

Soil water availability


c) Describe THREE ways in which leaves minimise transpiration 6

In this question the RHS was looking for ways that ‘mainstream’ leaves minimise water loss rather than special adaptations as in previous questions.

Thick waxy Cuticle – water is also lost through the cuticle, so leaves minimise transpiration by having a waterproof waxy cuticle.

In dicots the stomata are on the underside where humidity is highest, so transpiration rate is slower.

The palisade mesophyll cells are densely packed to prevent water escaping between cells.

When leaves wilt due to water stress the surface area is reduced, a humid environment is created on the underside of the leaf as the leaf droops down and also shelters the stomata from wind.

The stomata can open and close in response to water availability and light levels and so can minimise water loss when conditions are unfavourable.


a) Name ONE environmental factor which effects the rate of water loss from plants 1

Temperature

Light/dark

Relative humidity

Wind speed

b) Describe water transport through the plant stem 5

Water has the physical property of cohesion tension – it ‘sticks’ together. Water flows upwards in the stem as a transpiration stream, an unbroken column of water. The pulling force is the transpiration of water through the stomata which drags a continuous column of water upwards through the xylem tissue which is composed of interconnecting cells or tubes – xylem vessels and tracheids. Water travels higher in tracheid tissue as the tubes are narrower due to capillary action which also assists the movement of water in the stem. Root pressure will also give the water a push upwards.


c) State TWO distinct functions of stems other than water transport 4

Stems support the flowers so that they are in the best position for pollination

Stems support the leaves so that they are in the best position for photosynthesis

Stems hold the fruits in the best position for fruit/seed dispersal

Stems can also photosynthesise and are in fact the main photosynthesising organ in cacti.

Stems can store water as in cacti

Stems can be modified for defence against herbivores

So you have learnt all this theory – how do we relate all this to practical applications? Here are just a few examples to relate theory to gardening practice.

Fertiliser application rates

Do not over apply liquid fertilisers to the soil. This will increase the concentration of solutes in the soil water and so decrease the concentration of water molecules. This means that water could flow in the opposite direction from the roothairs to the soil water as the concentration gradient has changed, which would seriously damage the plant’s health.

High temperature in the greenhouse

The rate of transpiration increases as temperature increases. Using shade paint or a cooling fan or opening all vents and doors will lower the temperature and so lower the rate of water loss. Also in a greenhouse, because it is an enclosed environment, damping down will help as this raises the relative humidity outside the leaves so the rate of transpiration slows.

Cut Flowers

When stems are cut and not put into water immediately the column of water will continue to move upwards but there is no water to pull up at the bottom. This may result in an air bubble in the xylem tisse so it helps to place cut flowers in water immediately or cut the stem at the base again before plunging in water. Keeping cut flowers in a cool place also slows the rate of water loss due to transpiration.

First year of a newly planted plant

When establishing new plants the first year of care is critical. This is because wilting and subsequent death may occur due to lack of water being absorbed by roots. Young plants grown in containers have a limited root hair surface area and so may not be able to survive in their first hot Summer when the rate of transpiration is high. This is why it is often better to plant in the Autumn so the roots can get established before the plant’s first Summer season. If planted in Spring young plants should be watered more frequently than established plants.

Windbreaks

Windbreaks will reduce the rate of transpiration as the relative humidity outside the leaf will be higher and so reduce the amount of water lost to the atmosphere by transpiration. Windbreaks are very important for maximising yield on vegetable plots and orchards.

Further study

http://www.saps.org.uk/animations/plant_biology/index.html?video=1

excellent video summarising water movement and all the plant parts.

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