Transport, Food Storage and Gas Exchange in Flowering Plants

Chapter 25

Objectives

1. To investigate water uptake through the roots of plants

2. To define transpiration and discuss how it occurs in plants

3. To examine how minerals uptake and transport in plants

4. To consider how carbon dioxide is absorbed and transported by plants

Objectives

5. To investigate the fate of the products of photosynthesis

6. To consider how food is stored in plants7. To study the cohesion-tension model of water

transport in xylem8. To investigate gas exchange in the leaf and

stem of plants

Structure of Flowering Plants

Flowering plants consist of:

Flowers, leaves, stem and root

Facts about plants

• Plants are autotrophs and make their own food in photosynthesis

• Plants carry out respiration• Plant metabolism refers to reactions such as

photosynthesis, respiration, reactions of cell division, growth and reproduction

• Plants need to be able to acquire and transport water, carbon dioxide, oxygen and some minerals

Roots

• 1. Grow down towards gravity (Geotropism)

• 2. Anchor the plant in the ground to give

support• 3. Take in water and

minerals• 4. Store food, e.g. Carrots

Water uptake by the Roots

Root hairs:1. Found along the root2. Increase the surface

area of the root for water absorption.

3. Do not have a cuticle.Absorption into the roots

takes place by osmosis.

Osmosis into RootsThe cytoplasm of root

hairs:• Contains many

dissolved solutes. • Is more

concentrated than the water in the soil. Therefore water enters root hairs by osmosis.

Movement of Water into Xylem

• Water diffuses from root hair cells in the cortex through the ground tissue into the xylem.

• Xylem vessels form a continuous hollow pipe from roots to all plant parts.

Upward movement of water in plantTwo mechanisms:1. Root PressureWhen water is drawn into roots

by osmosis a pressure is created called root pressure. This pressure pushes water up through the xylem. Root pressure is not strong enough to push water to the top of high trees and root pressure is very low in summer.

2. TranspirationTranspiration is the loss (by evaporation) of water vapour from the leaves and other arial parts of a plant.Most transpiration takes place through small openings on the underside of the leaf called stomata.As each water molecule is pulled from the xylem cells by osmosis it pulls the next water molecule upwards through the stem from the roots

Transpiration exerts a pulling force and the upward pull of water due to transpiration is similar to the way water is sucked up through a straw.

Control of transpiration in the leavesLeaves may lose more than their weight of water each day due to transpiration. If they do not replace this water they will wilt and die.

External Structure of leaf

Adaption Purpose

Large surface area To absorb more light

Thin Short distance for carbon dioxide to diffuse into leaf cells

Chlorophyll Absorbs sunlight to transfer energy into chemicals

Network of veins To support the leaf and transport water and carbohydrates

Stomata Allow carbon dioxide to diffuse into the leaf

Internal Structure of Leaf

How plants reduce their rate of Transpiration

1. Leaves have a waxy cuticle which is thicker on the upper surface and reduces water loss

2. Stomata are usually located on the lower surface because the rate of evaporation is lower there.

3. Each stomata is surrounded by two guard cells which can open or close the stomata. Stomata close at night to reduce transpiration.

Mineral uptake and transport

Plants require minerals for healthy growth.Minerals are found in the soil dissolved in water.Minerals such as Calcium, Magnesium, Nitrogen are needed by the plant.Minerals enter the root hairs dissolved in water by active transport. Minerals are then transported by the xylem to all parts of the plant dissolved in water.

Uptake & transport of Carbon Dioxide

Carbon dioxide is needed by the green parts of the plant (mainly leaves) for photosynthesis.

Two Sources of Carbon dioxide:

1. Most carbon dioxide enters the leaf through the stomata from the atmosphere.

Atmosphere -> stomata -> air spaces in leaves->photosynthesis cells in paliside layer of leaf

2. Carbon dioxide is produced in leaf cells as a waste product of respiration.

True rate of photosynthesis

True rate of photosynthesis =

rate of carbon dioxide absorbed by stomata

+ rate of carbon dioxide produced in respiration.

Fate of products of photosynthesisLeaf is main photosynthetic organ in plant

Photosynthesis takes place in chlorophyll containing cells mainly in the leaf.

Products of photosynthesis:1. Oxygen: diffuses out of leaf through stomata2. Glucose: Used immediately in respiration or

converted to starch for storage.Glucose is transported around the plant in the

phloem.

Phloem can carry food (glucose) to all parts of the plant for growth, storage, respiration etc.

Food Storage Organs in PlantsPlants can alter or modify their roots, stems or

leaves to act as food storage organs.1. Modified root In some plants e.g. carrot and turnip the tap root becomes swollen and fleshy with food.

2. Modified stemPotato plants produce underground stem system.Tips of some underground stems become swollen

with stored starch.Swollen underground stems are called tubers.

3. Modified LeavesPlants such as onions and daffodils produce bulbs. A bulb contains an underground stem that is reduced in size.Swollen fleshy leaves which are modified to store food are attached to this stem.

The cohesion-tension model of water transport in Xylem

First proposed by Dixon and Joly two Irish Scientists in 1894

Thought to be the main model of upward movement of water in plants.

Involves:Cohesion – the sticking of similar molecules to each

other. Water molecules tend to stick together.Adhesion – when different molecules stick

together. Water has great adhesion.

Outline of Cohesion – Tension ModelWater evaporates from xylem into air spaces of

leaf and out of stomata into air.As each water molecule evaporates into the air

the next water molecule is pulled with it due to high cohesion causing water to be sucked up through plant. This will continue as long as there is a continuous column of water in xylem.

2. The water in the xylem is under tensionThe cohesive forces between water molecules in

Xylem are strong enough to hold water in a column.

3. The tension in xylem due to transpiration is strong enough to pull water up through the column to 150m

4. Stomata open in daylight and transpiration occurs. The tension in the water column causes xylem vessels to narrow.

Xylem vessels are strengthened by lignin which prevents them collapsing.

Gas exchange in leaf

Gas Exchange in Stems 

Cells within the stem of plants need oxygen to carry on respiration. As a result, they produce carbon dioxide and water as a waste product of respiration.

The bark of the stems of trees and shrubs have openings called lenticels which allow for the movement of oxygen into and CO2 and water out of them.

Lenticels are openings in the stem of plants that allow gas exchange

Stomata Opening & Closing

Each stomata is enclosed by two guard cells. Guard cells open and close stomata.

Control of stomata opening & closing

Major factor in controlling opening and closing is concentration of Carbon dioxide.

High conc. Carbon Dioxide – cause stomata to close. Carbon dioxide levels can build up in evenings so stomata close in evenings.

Low levels of carbon dioxide – Stomata open