Plant Nutrition and Transport 3.3.1 Nutrition in the Flowering Plant 3.3.2 Modified Plant Storage Organs H 3.3.7 Cohesion – Tension Model of Xylem Transport

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

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

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

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

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

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

Cohesion – Tension Model of Xylem Transport

- 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

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

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.

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.

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.

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

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

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

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

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

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

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.

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