The importance of Osmosis  
Osmosis is an important process in both plants and animals.  
(i) Osmosis aids the absorption of water by the roots form the soil.  
(ii) It aids the opening and closing of stomata. When guard cells absorb water the  
stomata open and when they lose water, the stomata close.  
(iii) It facilitates seed germination. It enables the absorption of water that softens the  
seed coat, hence, increasing seed permeability for dissolved mineral salts and  
oxygen. This promotes the growth of seed embryo during germination.  
(iv) Osmosis is important for reabsorption of water in the alimentary canal and kidneys.  
This process helps to maintain water balance in the body.  
(v) It is used to preserve food by salting. This causes food such as meat to lose water  
and become dry.  
(vi) In lower organisms such as Amoeba, osmosis helps to remove water and excess  
water from their bodies.  
EFFECTS OF OSMOSIS IN LIVING ORGANISMS  
1. Osmosis in animal cells. The effect of osmosis can be observed in red blood cell when  
placed in different solutions.  
(a) When an animal cell is placed a hypotonic solution, the cell absorb water by  
osmosis and swell. If it remains in the solution for a long time, it absorbs excess  
amounts of water and the cell finally bursts because the cell membrane is weak.  
The bursting of the red blood cell is known as haemolysis (bursting).  
Figure 3.3: Effect of osmosis on red blood cell placed in hypotonic solution  
(b) When an animal cell is placed in a hypertonic solution, the cell loses water by  
osmosis. If it remains in the solution for a long time, it loses a lot of water and the  
cell shrinks. The process by which an animal cell loses water and shrinks is called  
crenation  
.
Figure 3.4: Effect of osmosis on red blood cell placed in hypertonic solution  
(c) When animal cell is place in isotonic solution (under normal conditions), the  
osmotic pressure of red blood cells is equal to that of the blood plasma. Thus, there  
is equal movement of water in and out of the cells. This helps to maintain the disc  
shape of these cells.  
Figure 3.5: Effect of osmosis on red blood cell placed in isotonic solution  
2. Osmosis in plant cells.  
(a) When a plant cell is place in isotonic solution (under normal conditions), the cell  
neither loses nor gains water.  
(b) When a plant cell is place in a hypotonic solution, the cell gains water by osmosis  
causing the cell membrane to push against the cell wall. The cell does not burst  
because of the presence of cellulose cell wall which resists pressure exerted from  
the cell membrane and restricts additional intake of water. The cell said to turgid  
.
Turgidity helps the plant cells to maintain their shape.  
(c) When a plant cell is placed in a hypertonic solution, the cell loses water by osmosis  
causing the vacuole to shrink and the cell membrane pull away from the cell wall,  
making the cell flaccid. The flaccid cell is said to be plasmolyzed and the process  
is called plasmolysis  
.
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If a flaccid cell is put in distilled water it gains water by osmosis and regaains its  
orinal shape. This is called deplasmolysis. If a plant loses more water than it gains  
wilting occurs and finally death.  
Figure 3.6: Effect of osmosis on a plant cell  
3. Osmosis in unicellular organisms. Unicellular organisms that live in fresh water such  
as Amoeba and Euglena, are hypertonic to surrounding. Hence, water enters the  
organisms by osmosis. These organisms have a contractile vacuole which collects the  
excess water and removes it from the cell. This prevents the cell from bursting. Osmosis  
also is used for regulating their body mineral contents.  
Figure 3.7: Water balance in amoeba through contractile vacuole  
MASS FLOW  
Mass flow is the bulk movement of substances from one region to another due to differences  
in pressure between the two regions. Or  
Mass flow is the movement of materials in large quantities and across a long distance in  
the body of an organism due to differences in pressure between the two regions.  
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Mass flow occurs within a cell or in a long vessel or when materials need to be  
transported over long distance. It is important in large and complex organisms  
where substances are required in large amounts.  
Diffusion and osmosis occurs very slowly and cover short distances. In animals and  
plants, materials are usually transported a long distance and in large quantities.  
Therefore, an efficient and fast mechanism is required to facilitate this movement. That is  
when mass flow comes in.  
Examples of systems where mass flow occurs include:  
(i) The blood circulatory system in animals.  
(ii) The lymphatic system in animals (flow of lymph).  
(iii) Transport of manufactured food material in plants from the site of manufacture  
(leaves) to the site of utilization through phloem. (This process is called  
translocation).  
(iv) Transport of water from the roots to different parts of the plant through the xylem.  
Importance of Mass Flow  
In plants  
(i) Mass flow enables transportation of water and minerals salts from the roots to  
different parts of the plant through xylem vessels.  
(ii) It enables transportation of manufactured food from the leaves to the area where it  
can be used or stored through phloem vessels.  
In animals  
(i) Mass flow enables transportation of food and water within the blood vessel to  
various parts of the body.  
(ii) It enables transportation of blood in the circulatory systems and lymph in lymphatic  
system.  
(iii) It enables transportation of waste products from cells to regions of excretion or  
disposal.  
Table 3.1: Differences between diffusion, osmosis and mass flow:  
Characteristics  
Diffusion  
Osmosis  
Mass flow  
Substance transported  
Gases, liquids Water  
and ions  
Solids, liquids and gases  
Transport structures  
Cause of movement  
None  
Semi-permeable  
Cytoplasm or vessels  
Difference in pressure  
membrane  
Osmotic  
Diffusion  
gradient  
pressure