1. Plant Nutrition and Transport
3.3.1 Nutrition in the Flowering Plant
3.3.2 Modified Plant Storage Organs
H Cohesion – Tension Model of Xylem Transport

2. Nutrition in the Flowering Plant
Plants are autotrophic – they make their own food.
Plants need to be able to transport water, carbon dioxide, oxygen and certain minerals around their body as they need them for metabolism, growth and reproduction

3. Water Water moves into the root hair cells from the soil by osmosis
Uptake of water
Water moves into the root hair cells from the soil by osmosis
2.The water then diffuses from cell to cell across the ground tissue
When it reaches the vascular tissue and goes into the xylem cells.
Osmosis is the movement of water from a solution of high concentration to a solution of lower concentration
The root hairs have lots of dissolved minerals so this enables osmosis to happen

4. Xylem vessels – a continuous pipeline through the plant
Xylem cells make up a xylem vessel

5. Water
Transport of Water
root pressure –
the water entering the root cells by osmosis pushes the water up the xylem – a pushing force

6. 2. Water evaporates from the cells in the leaf and they become less turgid.
This creates an osmotic gradient that causes water to move from the xylem cells out to these leaf cells – a pulling force

8. Control of transpiration
The loss of water from the leaves is reduced by:
a waxy cuticle on the leaf
and
the closing of the stomata at night
Stoma – gaps in between two specialised cells called guard cells
when water enters the cell it expands and becomes curved- a gap opens up between two cells – stay closed at night to conserve water

9. Cohesion – Tension Model of Xylem Transport

10. - Henry Dixon and John Joly
A confusing problem…
How water can travel up from the soil to the leaves of a tree, which could be up to 136 m tall, AGAINST THE FORCE OF GRAVITY?
An explanation of this mechanism was first proposed by Two Irish Scientists from Trinity College in 1895.
- Henry Dixon and John Joly
In warm conditions a tree might lose up to 200 litres of water in one day
According to the Guinness Book of Records, the tallest tree ever measured was an Australian gum tree (Eucalyptus regans) which in 1872 was 132.6m high. Vast amounts of water are continuously being evaporated from the surfaces of leaves on trees and the source of this water is the soil surrounding the roots. Water must be moved from the roots to the uppermost leaves, vertical distances of well over 100m. in some cases, but the mechanism by which these trees supply their upper foliage with water posed one of the most intractable problems of plant physiology at the turn of the 20th century . The hypothesis was that evaporation of water from the leaves caused a suction which was transmitted down the plant in the continuous water columns in the xylem, the main conducting tissue, and which drew water through roots from the soil. The energy to drive this process came solely from the sun which evaporated water from the cell walls within the leaves. The theory was greeted with much scepticism in 1895 as the general consensus at the time was that the cells in the xylem somehow pushed the water up through the plant against the forces of gravity using an energy-requiring ‘vital’ process. It was also difficult for many plant physiologists to understand how the evaporation of water from the leaves could produce suction strong enough to pull water to the top of the tallest trees. A mechanical suction pump can only raise water to a height of about 10m before the water column breaks or ‘cavitates’. To raise water to the top of the tallest trees requires a suction ten or more times the maximum achieved by a suction pump

11. Dixon and Joly’s Cohesion – Tension Model
The cohesion – tension model explains how water can be transported in plants to extreme heights against the force of gravity
It is like sucking a straw
Think of the water molecules linked together in a chain

12. 1. Transpiration causes water tension
Transpiration causes water to move from the xylem cells out to leaf cells.
Water molecules stick together (cohesion) and stick to the sides of the thin xylem tubes (adhesion).
As each water molecule is pulled out of the xylem the next water molecule is pulled with it. The water in the xylem is put in a state of tension.

20. 2. Cohesive forces hold water molecules in a column
The water is under great tension BUT the cohesion between the water molecules is strong enough to hold the water molecules in a column without breaking.

21. 3. The transpiration stream
The continuous water tension in the xylem pulls molecules upwards from the roots to replace the water molecules that have been lost at the leaves from transpiration.
The flow of water upwards through the plant is called the transpiration stream.

22. Minerals
Uptake
they are dissolved in the water that comes into the plant at the roots.
Transport
by the xylem cells

23. Carbon dioxide
Uptake
directly from the respiring cells inside the plant or
enter the leaves through open stomata.

24. Oxygen Uptake Oxygen can be obtained from photosynthetic cells or
enter the leaves through open stomata
Can enter the bark through lenticels

25. Photosynthetic products
The glucose product of photosynthesis is :
stored as starch in the leaves or
Transported from the leaves to other plant cells through phloem sieve tubes

26. Modified Plant Storage Organs
Plants can store the food that they make in organs
Modified Plant Storage Organs include roots, stems and leaves

28. Root modification
The first root grows straight down to form the main root of the plant. It may then become fleshy and modified to store food. example: carrot

29. Stem modification stems become modified to store food.
known as “tubers” e.g. Potatoes
Stem tubers – end of stems there are buds which become modified to store food.

30. In some plants leaves are modified to store food e.g. onion
Leaf modification
In some plants leaves are modified to store food e.g. onion