Transport, food storage and gas exchange in flowering

plants

Why do plants need a transport system??

• Plants are Autotrophic they make their own food by photosynthesis

• Plants store this food and use it for energy in respiration

• Other processes of metabolism in plants include cell division, enzyme action and hormone action

• For all these processes to occur plants need to get and transport water, carbon dioxide, oxygen and certain minerals

Water transport in plants

Water uptake by roots

• The region of the root with root hairs is called the Piliferous layer

• These are extensions of epidermis cells but don’t have a waxy surface (cuticle) and have thin walls

• How do you think this makes them suitable for absorption??

• The more root hairs the more absorption can take place….. Do you think roots have many or few root hairs??

Osmosis

• The water around soil particles is relatively pure and is called capillary water

• The cytoplasm in the root hairs is full of solutes and is more concentrated than the water outside in the soil

• Osmosis describes the way water will move from low concentration to high concentration

How does water from root hair cells get to the xylem?

• Where are root hairs in relation to xylem?

• What type of tissue does water need to get across to reach the xylem vessels?

• Water moves across here by diffusion

Transverse section of root to show movement of water

• The xylem form a hollow pipeline throughout the plant going from the roots up the stem into the petiole and from here to the leaves

Giant Redwoods

• The largest and oldest trees in the world

• A single mature giant redwood can draw 650,000 litres of water up through it in one season!!

• How is this possible?

Upward movement of water

• 2 mechanisms combine to cause upward movement of water

1. Root pressure

2. Transpiration

Root pressure

• When water is drawn into roots by osmosis the extra volume causes pressure to build up

• This pressure pushes water up through the xylem

• However root pressure isn’t strong enough to push water through very tall plants

Transpiration

• The loss of water by evaporation through the leaves + other aerial parts of a plant

• The water vapour escapes through tiny holes on the undersides of leaves called STOMATA

Trans section of a leaf

• Excess water in the ground tissue evaporates into air spaces in the leaf and diffuses into the atmosphere

• When the cells in the ground tissue lose water they become less swollen and turgid

• For this reason the cytoplasm becomes more concentrated and water from the xylem passes into the ground tissue by osmosis

• As each water molecule is “pulled” from the xylem another water molecule is “pulled” up from the root

Water molecules

• This pulling force is passed from water molecule to water molecule all the way down the plant

• This is how water is pulled up through the plant by transpiration

The control of Transpiration

• Leaves need to replace the water they lose in transpiration or they may wilt and die

• At certain times particularly dry weather and drought it is difficult for plants to absorb water from the soil

• To prevent wilting plants need to control how much water they lose in transpiration

3 methods of controlling transpiration

1. Leaves have a waxy cuticle through which water cannot pass – this is on the upper side of a leaf as this side is more exposed and more water can evaporate here

2. Stomata are normally found on the lower surface of a leaf as less evaporation occurs here

3. Each stoma has two guard cells that can open or close the stoma by changing shape

When are stomata open and closed??

• Normally stomata open at day and close at night

• This allows water vapour out and CO2 in when photosynthesis is taking place

• Stomata close at night reducing water loss and CO2 intake as photosynthesis is not occurring

Conditions when stomata close at day

2 reasons stomata may close at day

1. If the plant has lost too much water2. If temperatures are too high

By closing stomata the plant reduces water loss

In dry conditions stomata remain closed for long periods, photosynthesis cannot occur and food crops are reduced

The cohesion-tension model of water transport in xylem

• Two Irish Scientists working in Trinity College Henry Dixon and John Joly put forward this model in 1894

• Cohesion – the sticking of similar molecules to each other , water molecules stick to each other

• Adhesion – when different molecules stick together, water adheres to the walls of xylem but this force is not as great as the cohesive forces of water

Experiment to show the cohesion of water molecules

• Capillary action is the process which causes liquid in the soil to rise up through roots and stems of plants or water to seep through a sponge. Pick a flower such as a carnation. Carefully slice the stem in half length ways. Place one half of the stem in dark blue coloured water and the other half of the stem in  dark red coloured water. You will soon notice the flower becoming half blue, half red. The flower 'sucks up' the water through narrow tubes in its stem. The capillary action overcomes the pull of gravity.

Outline of cohesion tension model

Summary points• Water evaporates from the leaf,as each water molecule

evaporates another is pulled up through the thin xylem column

• This pulling of water molecules puts all the water in the xylem vessels under tension

• This tension is great enough to pull water to a height of 150m

• Stomata open in daylight and transpiration occurs causing xylem vessels to become narrow, stems therefore are narrower at day

• The strong lignin prevents xylem vessels collapsing inward

• When transpiartion stops (at night) the tension is released and xylem vessels return to their normal width

Water movement in the leaf by day and night

Mineral uptake and transport

• Plants require minerals to function normally• Calcium, magnesium, phosphates, potassium

etc. enter root hairs dissolved in water• This entry requires energy so root hair cells have

lots of mitochondria• Because they need energy to be transported the

process is called active transport• Once inside the plant mineral get around

dissolved in water that moves in the xylem

Uptake and transport of Carbon dioxide

• Photosynthesis takes place mostly in the mesophyll cells in the leaf

• Most of the Carbon dioxide comes in through the stomata from the atmosphere

• It diffuses into the air spaces and then into the photosynthesising cells in the ground tissue

The Rate of Photosynthesis

• A measure of how much CO2 is being absorbed tells us the apparent rate of photosynthesis

• However CO2 is also produced by respiration in the plant and this may be used in photosynthesis

• The true rate of photosynthesis is calculated by adding the carbon dioxide taken in through the stomata and the carbon dioxide formed in respiration

What happens to the products of photosynthesis?

• Oxygen produced in photosynthesis diffuses into air spaces in the leaf and out through stomata into the atmosphere

• However some of the oxygen produced is needed for respiration

• Glucose is the carbohydrate made in photosynthesis

• This may be used immediately for energy for the plant in respiration or it may be stored as starch

• Some of this starch is stored in the spongy mesophyll cells in leaves

• Starch stored in leaves is important in the diet of leaf eating animals such as horses, monkeys and cattle

Glucose converted to sucrose

• When glucose get converted to sucrose it enters phloem sieve tubes and is transported through the plant

• This sugary water is called phloem sap

An insect extracting sugary phloem sap from a plant

Food transport

• The precise mechanism of food transport in phloem isn’t fully known

• Phloem carries food to all parts of the plant, some food is sent to growth areas such as buds, flowers and roots

• Food is used to form new plant structures, for respiration or it is stored as starch

Food storage organs in plants

Modified Root

• In some plants with Tap roots (mainly dicots) the root becomes swollen and fleshy with stored food

• This food will be used by the plant to produce flowers, seeds and fruits

• Humans harvest these before this happens!

• Eg Carrots, turnips, sugar beet

Modified stem

• Potato plants develop an underground stem system

• The tips become swollen with stored starch

• These swollen tips are called stem tubers

• In nature a potato tuber would remain dormant in the soil over winter and in spring the buds on the tuber (better known to us as the “eye”) would grow into new plants

Modified Leaves

• Plants such as onions, daffodils and tulips produce bulbs

• A bulb contains a tiny underground stem which have swollen fleshy leaves attached

• This stem has an apical bud and lateral buds can be seen where the leaves meet the reduced stem see fig 25.11

• The entire bulb is protected by dry scaly leaves

• Some bulbs are edible (onion, garlic) while others are poisonous (daffodil, tulip) this prevents organisms in the soil from eating them!

Modified Petioles

• Celery and Rhubarb are modified petioles

Gas Exchange in the leaf

Stomata

• Function of Stomata is Gas exchange

• Plants need carbon dioxide for photosynthesis

• Carbon dioxide diffuses in through the stomata from the atmosphere

Huge number of stomata on a leaf

• About 50,000 per cm2 !

• That’s the size of this

• This increases the rate of gas exchange

• Carbon dioxide diffuses through the intercellular air spaces to the mesophyll cells

Intercellular air spaces

Mesophyll cells

How do air spaces affect gas exchange??

• Air spaces increase the internal surface area

• The inner surface are is about 20 times more than the outer surface area of a leaf

• Think of a sponge

What gas exits the leaves??

• Photosynthesis produces oxygen which diffuses out through the mesophyll cells and air spaces then out through the stomata

• When the earth was first formed there was no oxygen plants are responsible for releasing oxygen into the atmosphere (it now makes up about 20% of air!)

What else exits the leaf through stomata??

• Water vapour in transpiration

Recap – when are stomata open and closed??

• Open at day for gas exchange

• Closed at night to reduce water loss

Gas exchange in stems

• Cells on the inside of stem and trunks need oxygen for respiration

• When they respire they produce carbon dioxide these gases need to get in + out

• The dermis and bark are thick and do not allow substances in and out easily

Lenticles are openings in the barks of trees and shrubs that allow gas

exchange

Stomatal opening and closing

• Each stomata has a pair of kidney shaped guard cells

• These guard cells open and close stomata by changing shape

• Remember this animation

• Wall of guard cells are thick on the insides this causes them to curve when they absorb water

• When water gets into guard cells by osmosis they curve and open to reveal a wide stomata

• When guard cells lose water they shrink and the gap between them (the stomata closes)

Control of Stomata opening and closing

• The concentration of carbon dioxide in the air spaces of the leaf plays a huge role in controlling the opening and closing of stomata

• High levels of CO2 cause the stomata to close

• Low levels of CO2 cause the stomata to open

Evening/Night time

• Photosynthesis stops when there is no light

• The mesophyll cells are no longer absorbing CO2 so it builds up in the air spaces and the stomata close

Day time

• When it starts getting light the mesophyll cells absorb CO2

• This means the levels of CO2 in the air spaces drops so the stomata open again

Proof!

• If a leaf is placed in a dark chamber with no CO2 the stomata will open

• What does this tell us?

• The exact mechanism of how CO2

controls stomatal opening is not fully understood

• Other factors are also involved1. The uptake and loss of patassium ions

by guard cells 2. An internal clock that tells guard cells to

open and close at regular intervals