Transport, Food Storage and Gas Exchange in Flowering Plants

Transport, Food Storage and Gas
Exchange in Flowering Plants
Chapter 25
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
5. To investigate the fate of the products of
6. To consider how food is stored in plants
7. To study the cohesion-tension model of water
transport in xylem
8. 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
• 1. Grow down towards
gravity (Geotropism)
• 2. Anchor the plant in the
ground to give support
• 3. Take in water and
• 4. Store food, e.g. Carrots
Water uptake by the Roots
Root hairs:
1. Found along the root
2. 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 Roots
The cytoplasm of root
• Contains many
dissolved solutes.
• Is more
concentrated than
the water in the soil.
Therefore water
enters root hairs by
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 plant
Two mechanisms:
1. Root Pressure
When 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
2. Transpiration
Transpiration 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
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 leaves
Leaves 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.
Structure of leaf
Large surface area
To absorb more light
Short distance for carbon dioxide
to diffuse into leaf cells
Absorbs sunlight to transfer
energy into chemicals
Network of veins
To support the leaf and transport
water and carbohydrates
Allow carbon dioxide to diffuse
into the leaf
Internal Structure of Leaf
How plants reduce their rate of
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
Mineral uptake and transport
Plants require minerals for healthy growth.
Minerals are found in the soil dissolved in
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
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 photosynthesis
Leaf 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 stomata
2. Glucose: Used immediately in respiration or
converted to starch for storage.
Glucose is transported around the plant in the
Phloem can carry food (glucose) to all parts of
the plant for growth, storage, respiration etc.
Food Storage Organs in Plants
Plants 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 stem
Potato plants produce underground stem system.
Tips of some underground stems become swollen
with stored starch.
Swollen underground stems are called tubers.
3. Modified Leaves
Plants 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.
Cohesion – the sticking of similar molecules to
each other. Water molecules tend to stick
Adhesion – when different molecules stick
together. Water has great adhesion.
Outline of Cohesion – Tension Model
Water 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
2. The water in the xylem is under tension
The cohesive forces between water molecules in
Xylem are strong enough to hold water in a
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
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 &
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