Chpt. 25: Transport, Food Storage and Gas Exchange in Flowering Plants

You need to…. Know the definition of Autotroph Learn how water is taken up by plant roots and the path taken by the water through the root, stem and leaf Understand the role of osmosis, diffusion, root pressure, transpiration and stomata in plant transport

Why do plants need a Transport System?? Plants are Autotrophic they make their own food by photosynthesis Some processes of metabolism in plants include Photosynthesis, Respiration, Growth and Reproduction. For all these processes to occur plants need to get and transport water, carbon dioxide, oxygen, carbohydrates (produced in photosynthesis) and certain minerals.

Water transport in plants Water uptake by roots: The region of the root with root hairs is called the Piliferous layer Water enters the root hair cells by osmosis Root hairs are adapted to this process by having: 1. Thin walls 2. A large surface area

Osmosis: The water around soil particles is relatively pure and is called capillary water (high water conc.) The cytoplasm in the root hairs is full of solutes and is more concentrated than the water outside in the soil (low water conc.) OsmosisOsmosis describes the way water will move from an area of high water concentration to an area of low water concentration

Water movement through the root : Water moves by diffusion from the root hairs into the ground tissue and eventually reaches the xylem Root Hair Ground tissue Xylem Xylem form continuous hollow pipes from roots to leaf Water

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!!

Upward Movement of Water Two mechanisms combine to cause upward movement of water through the stem in the xylem: 1.Root Pressure 2.Transpiration (this is the loss of water by evaporation through the leaves and other aerial parts of a plant)

1. Root pressure As water moves into the root by osmosis it builds up a pressure that pushes water up the xylem. 2. Transpiration As water evaporates from the leaf by transpiration, more water is pulled upwards through the xylem into the leaf

More detail on role of Transpiration *Water evaporates from the leaf cells in transpiration.* This creates an Osmotic Gradient The ground tissue cells in the leaf become less turgid Causing water to move out of the Xylem to the ground tissue cells in the leaf.

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

Transverse section of a leaf

The control of Transpiration: Why???? 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

3 methods of controlling transpiration: 1.Leaves have a waxy cuticle through which water cannot pass – this is thicker 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.guard cells Stomata closing reduces water loss Stomata opening increases water loss

More Detail on Stomata: Normally stomata open by day this allows water vapour out and CO 2 in when photosynthesis is taking place Stomata close at night reducing water loss and CO 2 intake as photosynthesis is not occurring However, 2 reasons stomata may close by day: 1.If the plant has lost too much water 2.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.

Cohesion-Tension Model of Water Transport in Xylem (Higher Level Only) Need to Know how plants move water up to great heights against the force of gravity Know the contribution of Irish scientists Dixon and Joly to plant biology Understand the terms transpiration, cohesion, adhesion, tension, osmosis and use them to explain water movement up through xylem

Cohesion-Tension Model of Xylem Transport explains how water is transported in plants to extreme heights against the force of gravity

Theory proposed by two Irish scientists: Henry Dixon John Joly Working in Trinity College 1894:

Important Terms: Cohesion: similar molecules sticking together e.g. water molecules sticking together = high cohesion H H O H H O H H O H H O Attraction between molecules

Adhesion: different molecules sticking together e.g. water sticking to xylem walls H H O Attraction Xylem Wall * Note: the force of attraction between water molecules and xylem wall is not as great as the cohesive forces of water

The Cohesion – Tension model H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O 1. Cohesion between water molecules in the narrow xylem tubes causes the water to form into a continuous column or stream in the xylem.

2. H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O As water molecules evaporate in transpiration at the leaf, cohesion between the water molecules replaces the water by pulling the next water molecule up the xylem. As the column of water is hard to break this pull is felt down the entire column of water to the root

3. Transpiration from the leaf puts the column of water under tension This tension causes the column of water to be stretched but the cohesive forces between the water molecules are strong enough to prevent the column of water breaking and lignin in the walls prevents the xylem from collapsing in. The tension in the xylem due to transpiration is great enough to pull the column of water to great heights in plants 4.

5. Stomata open in daylight and transpiration occurs The tension produced in the water column causes the xylem to become narrower and as a result the stem becomes slightly narrower by day. Stomata close at night and transpiration stops. The lack of tension allows the xylem to return to its original wider shape and subsequently the stem becomes wider.

Learning Check Explain the terms Cohesion, Adhesion, Transpiration, Tension, Osmosis Name the Irish scientists who proposed the tension cohesion model of water movement. Explain how plants move water to great heights against the force of gravity

Mineral Uptake and Transport Need to Learn how minerals are absorbed and transported Learn how CO 2 is obtained by the leaf Learn how carbohydrates and oxygen produced by photosynthesis are transported away from the leaf

Mineral uptake and transport Plants require numerous minerals e.g. calcium, magnesium. Minerals exist in soil dissolved in water. Minerals along with other dissolved substances (nitrates, phosphates, potassium ions) enter the root hairs from the soil dissolved in water. Minerals enter root hairs by a process called active transport which requires energy. Once inside the root minerals etc. are transported throughout the plant via xylem, dissolved in water

Uptake and Transport of Carbon Dioxide Sources of Carbon Dioxide: 1.Diffuses from the air in through the stomata. - the rate of absorption of carbon dioxide is a measure of the apparent rate of photosynthesis 2.Produced in the leaf cells during respiration. High = increased = increased temperature respiration production of CO 2

True Rate of=Rate of CO 2 + Rate of CO 2 Photosynthesis absorbed by produced in stomata respiration

The Fate of the Products of Photosynthesis Oxygen: - is transported through the air spaces and out through the stomata by diffusion. - some of the oxygen produced can be used in leaf cells for respiration. Glucose: - used for respiration - converted to starch for storage Note: - starch stored in leafs important in diet of leaf eating animals. - Glucose sucrose phloem sap

Food Storage Organs in Plants Need to know: One example of a root, stem and leaf modified as a food storage organ

Food Storage Organs in Plants Plants can alter their roots stems and leaves to act as food storage organs Root modified to store food: some dicots produce a large V-shaped root, tap root, that penetrates deep into the soil Generally tap root – anchors plant - absorbs water In plants such as carrots, turnips and sugar beet the tap root becomes stored food later used for the production of flowers, seeds and fruit. Tap root crops are harvested before they can produce flowers.

Root modified to store food: Carrot Example Dicot plants, e.g. carrots produce one main tap root This root becomes fleshy and swollen with stored starch

Stem modified to store food: Example Potatoe Potatoes have an underground stem system The tips of these stems become swollen with stored starch The swollen tips are called Tubers

Swollen stem tip is a potato tuber Apical Bud Lenticel Lateral Bud

Leaf modified to store food: Example Onion bulb Is a reduced underground stem With fleshy leaves swollen with stored starch surrounding a central apical bud

Dry Scaly leaves Fleshy leaves Apical Bud Reduced stem Adventitious roots

Learning Check Give an example of a stem modified to store food. Give an example of a root modified to store food. In what form is the food stored ? What is a bulb ?

Gas Exchange in the Leaf and Stem Need to Know: Explain the role of the leaf (stomata) & stem (lenticel) in gas exchange.

Gas Exchange in the Leaf: Function of stomata – gas exchange in leaves Photosynthesis requires carbon dioxide and it is via the stomata that CO 2 diffuses from the atmosphere into leaves. Large number of stomata increases the rate of gas exchange: CO 2 entersO 2 + H 2 O leave

Inside the leaf CO 2 diffuses to the mesophyll cells through the air spaces between these cells

Similarly oxygen, produced during photosynthesis, diffuses from the mesophyll cells, into the intercellular air spaces and out of the leaf through the stomata. Water vapour also diffuses out of the leaf via the stomata.

Gas Exchange in Stems: Cells on the inside of stems or trunks of trees and shrubs require oxygen for respiration and subsequently produce carbon dioxide, so: bark  How do gases [oxygen and carbon dioxide] get through the waterproof, corky surface of bark? lenticels  They do so through lenticels, which are areas where the packing of bark cells is loosened up a bit. blisters  Lenticels often look like tiny raised blisters on a branch or twig.

Normally in lenticels : O 2 entersCO 2 + H 2 O leave

Stomatal Opening and Closing stoma Each stoma is a gap between two specialised cells, called guard cells. guard cells The guard cells have a thicker wall on the sides that face each other.

enters When water enters the guard cells, via osmosis, they expand into a curved shape and a gap (the stoma) opens up between the two cells. lose When guard cells lose water they shrink in size causing the gap (stoma) between them to close.

CO 2 Whether a stoma opens or closes depends on how much CO 2 is in the air spaces of the leaf. Higher Level Only Control of Stomatal Opening and Closing Low levels of CO 2 cause stomata to open: Photosynthesis starts when the sun rises in the morning: CO 2 starts to get used up in photosynthesis and so drop in CO 2 there is a drop in CO 2 levels in the air spaces. open. This causes stomata to open.

High levels of CO 2 in the air spaces of the leaf cause stomata to close: At night, the sun goes down and photosynthesis stops: The combination of less CO 2 being absorbed by raises the mesophyll cells and continuing respiration raises the CO 2 levels CO 2 levels in the air spaces. close. As a result the stomata close. Other factors affecting stomatal opening and closing: - Uptake and loss of potassium ions in guard cells - Internal Clock