GCSE Basic Physiology and water transport.
"I have chosen to relate to the following text book because it is written by a person who like myself is not entirely satisfied by the explanations put forward in the relevant subjects".
Figure C’s results raise the questions; What is osmosis and how are its qualities explained in the text books.
For the currently accepted view of osmosis and all other views on water transport I will refer to one of the standard GCSE text books entitled GCSE BIOLOGY, D.G. Mackean. ISBN 0-7195-4281-2 first published in 1986.
Page 34 fig 3 Diffusion gradient
Page 36 OSMOSIS
Osmosis is the special name used to describe the diffusion of water across a membrane, from a dilute solution to a more concentrated solution. In biology this usually means the diffusion of water into or out of cells Osmosis is just one special kind of because it is only water molecules and their movement we are considering. Figure 3 showed that molecules will diffuse from a region where there are a lot of them to a region where they are fewer in number; that is from a region of highly concentrated molecules to a region of lower concentration. Pure water has the highest possible concentration of water molecules; it is 100% water molecules, all of them free to move.
Figure 9 shows a concentrated sugar solution, separated from a dilute solution by a membrane, which allows water molecules to pass through. The dilute solution, in effect contains more water molecules than the concentrated solution. As a result of this difference in concentration, water molecules will diffuse from the dilute to the concentrated solution. The level of the concentrated solution will rise or, if it is confined to an enclosed space, its pressure will increase. The membrane separating the two solutions is often called selectively permeable or semi-permeable because it appears as if water molecules can pass through it more easily than sugar molecules can.
Osmosis then is the passage of water across a selectively permeable membrane from a dilute solution to a concentrated solution.
This is all you need to know in order to understand the effects of osmosis in living organisms, But a more complete explanation is given below.
ALTERNATIVE EXPLANATION FOR OSMOSIS
The current text book explanation for osmosis appears to have ignored the effects of gravity on liquids. The constant pull of gravity acts differently on concentrated solutions than dilute solutions i.e. The concentrated solution is heavier than the dilute solution and will always settle at the bottom of a reservoir or in this case a vessel.
To see this clearly, picture Fig 9 without the membrane; the result would be that the concentrated solution would sink and the dilute solution would rise. This effect will not stop because of the membrane. The concentrated solution will still cause the dilute solution to rise as we have seen earlier; and as the concentrated solution moves into the opposite side containing the dilute solution, the dilute solution is dragged through the membrane in a circular motion. For every action there must be a reaction. In order to prove this point add a little dye to the sugar solution and watch the exchange between the liquids.
"When the effect that gravity exerts on concentrated solutions is added to the equation of water transport and osmosis, it gives us a very clear understanding of the driving mechanisms involved".
Chapter 7 Transport in plants
The main force which draws water from the soil and through the plant is caused by a process called transpiration. Water evaporates from the leaves and causes a kind of ‘suction ‘ which pulls water up the stem. The water travels up the vessels in the vascular bundles and this flow of water is called the transpiration stream. The water vapour passes by diffusion through the air spaces in the mesophyll and out of the stomata. It is this loss of water vapour from the leaves which is called transpiration. The cell walls which are losing water in this way replace it by drawing water from the nearest vein. Most of this water travels along the cell walls without actually going inside the cells. Thousands of leaf cells are evaporating water like this and drawing water to replace it from the xylem vessels in the veins. As a result , water is pulled through the xylem vessels and up the stem from the roots. This transpiration pull is strong enough to draw up water 50 metres or more in trees.
Most of this water evaporates from the leaves; only a tiny fraction is retained for photosynthesis and to maintain the turgor of the cells. The advantage to the plant of this excessive evaporation is not clear.
A rapid water flow may be needed to obtain sufficient mineral salts, which are in very dilute solution in the soil. Evaporation may also help to cool the leaf when exposed to intense sunlight.
Against the first possibility it has to be pointed out that, in some cases, an increased transpiration rate does not increase the uptake of minerals.
Many biologists regard transpiration as an inevitable consequence of photosynthesis, in order to photosynthesise, a leaf has to take in carbon dioxide from the air. The pathway that lets carbon dioxide in will also let water vapour out whether the plant needs to lose water or not. In all probability, plants have to maintain a careful balance between the optimum intake of carbon dioxide and a damaging loss of water.
Humidity if the air is very humid, i.e. contains a great deal of water vapour, it can accept very little more from the plants and so transpiration slows down. In dry air, the diffusion of water vapour from the leaf to the atmosphere will be rapid. ( " I will deal with this point later on because it is very important and has implications for human health ") Air Movements: In still air, the region round a transpiring leaf will become saturated with water vapour so that no more can escape from the leaf. In these conditions, transpiration slows down. In moving air the water vapour will be swept away from the leaf as fast as it diffuses out. This will Speed up the transpiration. Furthermore, when the sun shines on the leaves, they will absorb heat as well as light. This warms them up and increases the rate of evaporation.
Page 73 continued Water movement in the xylem
You may have learned in physics that you cannot draw water up by suction to a height of more than about ten metres. Many trees are taller than this yet they can draw up water effectively. The explanation offered is that, in long vertical columns of water in very thin tubes, the attractive forces between the water molecules are greater than the forces trying to separate them. So in effect the transpiration stream is pulling up thin threads of water which resist the tendency to break.
There are still problems however, it is likely that the water columns in some of the vessels do have air breaks in them and yet the total water flow is not affected. The evidence all points to the non-living xylem vessels as the main route by which water passes from the soil to the leaves.
"This statement suggests that the long thin tubes of the tree ,are used for water transport, which are none-living , therefore must represent the tubes used in my experiments at Brixham."
In Experiment 8 on page 79 it is demonstrated that liquid may be forced up a stem by root pressure from the root system. The usual explanation for this is that the cell sap in the root hairs is more concentrated than the
soil water and so water enters by osmosis (see page 36). The water passes from cell to cell by osmosis and is finally forced into the xylem vessels in the centre of the root and up the stem.
This is rather an elaborate model from very little evidence. For example, a gradient of falling osmotic potentials from the outside to the inside of a root has not been demonstrated. However, there is some supporting evidence for the movement of water as a result of root pressure.
root pressures of 1-2 atmospheres have been recorded, and these would support columns of water 10 or 20 metres high. Some workers claim pressures of up to eight atmospheres (i.e. 80 metres of water)
" A column of water 80 metres high would undoubtedly cause water pressures of eight atmospheres at the roots. However It is very difficult to see how a root could generate 8 atmospheres of pressure."
However, root pressure seems to occur mainly in the young herbaceous (i.e. non-woody) plants or in woody plants early in the growing season and though in many species it must contribute to water movements in the stem. The observed rates of flow are too fast to be explained by root pressure alone.
Transport of salts
The liquid which travels in the xylem is not, in fact pure water. It is a very dilute solution, containing from 0.1to1.0% dissolved solids, mostly amino acids, other organic acids and mineral salts. The organic acids are made in the roots; the mineral salts come from the soil. The faster the flow in the transpiration stream, the more dilute is the xylem sap. Experimental evidence suggests that salts are carried from the soil to the leaves mainly in the xylem vessels.
Transport of food
The xylem sap is always a very dilute solution, but the Phloem sap may contain up to 25 per cent of dissolved solids, The bulk of which consists of sucrose and amino acids.
There is a good deal of evidence to support the view that sucrose amino acids and may other substances are transported in the phloem. The movement of water and salts in the xylem is always upwards, from the soil to the leaf. But in the phloem the sap may be travelling up or down the stem. The carbohydrates made in the leaf during photosynthesis are converted to sucrose and carried out of the leaf to the stem. From here the sucrose may pass upwards to growing buds and fruits or downwards to the roots and storage organs. All parts of a plant which cannot photosynthesise will need a supply of nutrients bought by the phloem. It is possible for substances to be travelling upwards and downwards at the same time in the phloem.
"note the dual flow has been observed in experiments with concentrated solution and water filled tubes."
Page 74 continued
There is no doubt that substances travel in the sieve tubes of the phloem But the mechanism by which they are moved is not fully understood.
There are several theories, which attempt to explain how sucrose and other solutes are transported in the phloem but none of them is entirely satisfactory.
Uptake of water and salts
The water tension developed in the vessels by a rapidly transpiring plant is thought to be sufficient to draw water through the root from the soil. The precise pathway taken by the water is the subject of some debate, but the path of least resistance seems to be in or between the cell walls rather than through the cells.
When transpiration is slow, e.g. at night time or just before bud burst in a deciduous tree, then osmosis may play a more important part in the uptake of water.
One problem for this explanation is that it has not been possible to demonstrate that there is an osmotic gradient across the root cortex which could produce this flow of water from cell to cell. Nevertheless, root pressure developed probably by osmosis can be shown to force water up the root system and into the stem
The methods by which roots take up salts from the soil are not fully understood. Some salts may be carried in with the water drawn up by transpiration and pass mainly along the cell walls in the root cortex and into the xylem.
It may be that diffusion from a relatively high concentration in the soil to a lower concentration in the root cells accounts for uptake of some individual salts. But it has been shown (a) that salts can be taken from the soil even when their concentration is below that in the roots and (b) that anything which interferes with respiration impairs the uptake of salts. This suggests that active transport (p.35) plays an important part in the uptake of salts.
The thing that becomes clear from reading the established explanations for water transport is that if it were a bucket, very little water would be transported due to the large number of holes in it !
Death is natures way of telling us to slow down.