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 plantsWater 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.)

Osmosis 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.

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 CO2 in when photosynthesis is taking place

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

However, 2 reasons stomata may close by 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.

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

HO

H

HO

H

HO

H

HO

Attraction between molecules

Adhesion: different molecules sticking together e.g. water sticking to xylem walls

H

HO

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

H2O

H2O

H2O

H2O

H2O1.Cohesion between water molecules in the narrow xylem tubes causes the water to form into a continuous column or stream in the xylem.

2.

H2O

H2O

H2O

H2O

H2O

H2O

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 CO2 is obtained by the leaf

Learn how carbohydrates and oxygen produced byphotosynthesis 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 CO2

True Rate of = Rate of CO2 + Rate of CO2 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 PlantsNeed to know:

One example of a root, stem and leaf modified as a food storage organ

Food Storage Organs in PlantsPlants 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:

CarrotExample

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

LenticelLateral 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 CO2 diffuses from the atmosphere into leaves.

• Large number of stomata increases the rate of gas exchange:

CO2 enters O2 + H2O leave

• Inside the leaf CO2 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:

How do gases [oxygen and carbon dioxide] get through the waterproof, corky surface of bark?

They do so through lenticels, which are areas where the packing of bark cells is loosened up a bit.

Lenticels often look like tiny raised blisters on a branch or twig.

• Normally in lenticels :

O2 enters CO2 + H2O leave

Stomatal Opening and Closing• Each stoma is a gap between two specialised cells, called guard cells.

• The guard cells have a thicker wall on the sides that face each other.

• 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.

• When guard cells lose water they shrink in size causing the gap (stoma) between them to close.

Whether a stoma opens or closes depends on how much CO2 is in the air spaces of the leaf.

Higher Level OnlyControl of Stomatal Opening and Closing

Low levels of CO2 cause stomata to open: Photosynthesis starts when the sun rises in the morning:

• CO2 starts to get used up in photosynthesis and so there is a drop in CO2 levels in the air spaces.

• This causes stomata to open.

High levels of CO2 in the air spaces of the leaf cause stomata to close:At night, the sun goes down and photosynthesis stops:

• The combination of less CO2 being absorbed by mesophyll cells and continuing respiration raises the CO2 levels in the air spaces.

• As a result the stomata close.

Other factors affecting stomatal opening and closing:- Uptake and loss of potassium ions in guard cells- Internal Clock