Transport, food storage and gas exchange in flowering plants

Xylem Phloem Osmosis Root hairs

Root pressure Transpiration

Active transport Stomata

Lenticel KEYWORDS Cohesion

Tension Adhesion

Dixon and Joly Mesophyll

Tubers Bulb Guard cells Diffusion

Why plants need transport?

Plants need to be able to transport water, carbon dioxide, oxygen and minerals.

Water uptake by roots

• Root hairs are found at the tip of

roots.

• Root hairs have thin walls (helps

them to absorb water)

• Root hairs occur in large numbers

(large surface area)

NOTE: root hairs absorb water by

osmosis.

Movement of water into xylem • Water diffuses from the root hair into the ground tissue.

• It then diffuses from ground tissue into the xylem in centre of root.

• Water moves up through the stem to leaves and flower.

Movement of water through roots

Upward movement of water

Water moves up through the plant by two methods:

1) Root pressure

2) Transpiration

Root pressure

• As water enters a root by osmosis, the build up of water in the roots causes root pressure.

• Root pressure pushes water up through small plants.

NOTE: root pressure does not explain how water can reach great heights.

Transpiration

Transpiration is the loss of water vapour due to evaporation from the leaves of a plant.

Transpiration takes place through the stomata (openings on the under side of a leaf).

As each water molecule is pulled through the xylem cells by

osmosis, the next water molecule is pulled also.

Water is pulled through the plant by transpiration

Transpiration

Movement of water up a plant

What allows water to reach great heights in trees?

Cohesion Tension model • Allows large amounts of

water to move quickly

from roots to leaves in warm

conditions.

• Can move 220 litres in one hour

up through a plant.

• This model was put forward by

Henry Dixon and John Joly (Irish

Scientist in Trinity College)

Cohesion Tension Model

Cohesion: the sticking of water molecules to each other.

Adhesion: when different molecules stick together (water sticks to the wall of the xylem)

Cohesion Tension Model

Leaf

Root

1) Water evaporates from the xylem out through the stomata into the air.

2) During transpiration each water molecule is pulled through the xylem.

3) Due to cohesion the next water molecule is pulled along by the one in front of it.

4) Xylem are adapted for movement of water because they are narrow.

5) When the water column in the xylem is stretched it is said to be under tension.

Cohesion Tension Model

• The tension between the water molecules allow the water to be pulled up to a height of 145 metres.

• Stomata open in daylight hours allowing for transpiration to occur, the xylem vessel narrows.

• Stomata close in night time hours which causes the xylem to return to normal size.

NOTE: lignin prevents the xylem vessel from collapsing.

Transpiration - YouTube

Control of transpiration in leaves • If leaves lose to much water, it will wilt and die.

They control the rate of transpiration by the following:

1) Leaves have cuticle which does not allow water to pass through (cuticle thicker on upper surface as this side is exposed to the sun)

How to control rate of transpiration:

2) Stomata are located on lower surface of leaf to reduce water loss (more evaporation would occur on upper surface)

How to control rate of transpiration:

3) Each stomata is surrounded by two guard cells. The stomata can open or close by the guard cells changing shape.

Factors impacting stomata

The following environmental conditions affect stomata opening or closing:

1) Plants lose to much water

2) High temperatures

3) High wind

4) Carbon dioxide concentration

NOTE: Plants do not grow well in dry conditions because the stomata will remain closed for long periods.

How does CO2 concentration control stomata opening or closing?

High concentration of CO2:

• High levels of CO2 cause stomata to close.

• Photosynthesis rate decreases in the evening causing the build up of CO2.

sachin joshi _ botany student - YouTube

• Low levels of CO2 cause the stomata to open.

• When photosynthesis begins in the morning CO2 is absorbed by the mesophyll cells (ground tissue).

SUMMARY:

High concentration of CO2 Stomata close

Low concentration of CO2 Stomata open

How do stomata open or close:

• Guard cells open and close the stoma by changing shape

• When water enters the guard cell by osmosis, they swell and become turgid.

• This causes the guard cells to buckle outwards creating a gap between the guard cells.

• When the guard cells lose water they shrink, which causes the stoma to close.

Gas exchange in the leaf

Gas exchange in the leaf:

Carbon dioxide:

Stomata allows for gas exchange to be carried out.

Once in side the leaf the CO2 diffuses from the air spaces into the mesophyll.

NOTE: the air spaces increase the internal surface area which allows more CO2 to diffuse quickly into the mesophyll for photosynthesis

NOTE: there can be 50000 stomata per cm2 which increases the rate of gas exchange.

Gas exchange in the leaf

Oxygen:

Photosynthesis produces oxygen.

Oxygen diffuses from the mesophyll into the air spaces and out through the stomata.

Water vapour

Water vapour diffuses out from the leaf through the stomata (transpiration)

Gas exchange in stems

Lenticels are openings in the stems of plants that allow gas exchange.

• Oxygen diffuses inwards through the lenticel.

• Carbon dioxide and water vapour diffuses outwards through the lenticel

NOTE: Cells inside the stem need oxygen for respiration to occur.

Gas exchange in the lenticels

Oxygen Carbon dioxide + Water

Mineral uptake in leaves

• Plants require minerals to

function (calcium, magnesium)

• Minerals are absorbed from

the soil by root hairs in a

process called active transport

(requires energy)

• Root hairs have lots of

mitochondria to supply energy.

Food storage organs in plants

Modified roots:

• Dicot plants can have large tap roots to store food (starch).

• This food is used to produce flowers, seeds and fruits.

Example:

Carrots

Turnips

Modified stem

• Potato plants produce an underground stem to store food (starch).

• These swollen stems are called tubers.

Modified leaves

Onions, daffodils and tulips all produce a bulb.

What is a bulb? • A bulb is an underground stem that has swollen fleshy leaves

to store food. • The bulb is protected by a dry scaly leaf on the outside.

Dry scaly leaves

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