TOPIC 1.CLASSIFICATION KINGDOM FUNGI

FACTORS AFFECTING RATE OF DIFFUTION

• Concentration gradient: high diffusion rate with higher concentration and vice versa
• Surface area to volume ratio: the higher it faster the diffusion rate.
• Distance over which diffusion takes place: example a thin layer of cells increases diffusion rate

Osmosis

Osmosis is a form of passive transport considered as a special form of diffusion involves movement of water molecules through semi-permeable membrane.

Osmosis defined as the process by which water move from a weak solution into a strong through a semi-permeable membrane. The semi permeable membrane is only permeable to some solutes (dissolved substances).

For osmosis to take place there must be two separated solution by a semi-permeable membrane. One solution should have greater water and a lesser quantity of solute than other solution. This solution is hypotonic, it has a lower water potential. The second should have a lesser volume of water andvolume of solute than the other solution. This solution is hypertonic, meaning it has greater water potential.

Two solutions have the same water potential are said to be isotonic
 

Effects of osmosis in living organisms

Osmosis and animal cells

When an animal cell is put in a hypotonic solution, it absorbs water. If it remains in the solution for a long time, it absorbs excess amounts of water. A cell that does not have a mechanism for removing the excess water bursts due to the excessive internal pressure.

When an animal cell is placed in a hypertonic solution, it loses water. If it remains in the solution for a long time, it loses a lot of water, shrinks and shrivels.

These effects of osmosis on animal cells can be observed in red blood cells. Under normal conditions, the osmotic pressure of red blood cells is equal to that of the blood plasma, i.e. they are isotonic. Thus, there is equal movement of water in and out of the cells. This helps to maintain the disc shape of these cells.

When red blood cells are put in a hypotonic solution, they absorb water, causing the cell volume to increase. Excessive amounts of water cause haemolysis (bursting).

When red blood cells are put in a hypertonic solution, they lose water, leading to shriveling of the cell. This is referred tocrenation

 

Osmosis is important for the reabsorption water in the colon and the kidneys. This help to maintain the body's water balance.

Osmosis and plant cells

In an isotonic solution, plant cells neither lose nor gain water. In a hypotonic solution cells absorb water, causing the cell membrane to push against the cell wall. The cell is to be turgid. It does not burst because membrane exerts pressure on the cell wall restricts additional intake of water. Turgid plants to maintain their shape.

In a hypertonic solution, plant cells lose water this causes the vacuole to shrink and their cell membrane to pull away from wall, making the cell flaccid. Such a cell is to be plasmolyzedand the process plasmolysis.

If a plasmolyzed cell is placed in a hypotonic solution, it absorbs water and becomes turgid.

 

Osmosis is importantforthe absorption of water by plant roots. Opening and closing of stomata also depend on osmosis. When guard cells absorb water the stomata open and when they lose water the stomata close.

Osmosis and unicellular organisms

Unicellular organisms that live in fresh water, for example amoeba and euglena, are hypertonic to surrounding so water enters the organisms by osmosis. These organisms have a contractile vacuole. The contractile vacuole collects the excess water and removes it from the cell. This prevents the cells from bursting

Mass flow

Mass flow is the bulk movement of substances from one region to another due to the difference in pressure between the two regions. Mass flow occurs within a cell or along a vessel.

This mode of transport is important in large complex organisms where substances are required in large amounts and also have to be transported over large distances.

Examples of systems where mass flow occurs are:

• The circulatory system (flow of blood) in animals.
• The lymphatic system (flow of lymph) in animals.
• Transport of manufactured food material in plants from the site of manufacture (mostly leaves) to the point of use (all plant parts) through the phloem. This process is called translocation

Differences between diffusion, osmosis and mass flow

The following table gives a summary of the differences between diffusion, osmosis and mass flow.

Differences between diffusion, osmosis and mass flow

Characteristics	Diffusion	Osmosis	Mass flow
Substance transported	liquids and gases	Water molecules	Solids and liquids
Transportation	None structure	Semi permeable membrane	Cytoplasm and vessel
Causes of movement	Diffusion gradient	Osmotic pressure	Different in pressure

Chapter summary

1. Transport is necessary for the movement of substances within, into and out of cells so as to enable vital life processes to occur.
2. Transport can be carried out through diffusion, osmosis or mass flow.
3. Diffusion is the movement of particles from a region of high concentration to a region of low concentration.
4. Osmosis is the movement of water molecules from a weak solution to a strong solution through a semi-permeable membrane.
5. A hypotonic solution has a lower water potential.
6. A hypertonic solution has a higher potential.
7. A red blood cell haemolysis in a hypotonic solution and crenates in a hypertonic solution.
8. A plant cell becomes turgid in a hypotonic solution   and plasmolyzed in a hypertonic solution.
9. Mass flow is the bulk movement of substance due to pressure differences in two regions.

TRANSPORTATION IN MAMMALS

Introduction

Mammals are complex multicellular organisms. Their bodies are made up of numerous cells and tissues. Hence, diffusion alone is not enough to ensure efficient carrying out of life processes. Mammals therefore have an elaborate transport system called the circulatory system. The circulatory system is made up of the heart, the blood and the blood vessels.

The mammalian heart

An example of the mammalian heart is the human heart. The human heart is approximately the size of a clenched fist. It is located in the chest cavity between the two lungs.

The external structure of the mammalian heart

The mammalian heart is broader at the top and narrower at the bottom. It is enclosed by a double layer of tough inelastic membranes called the pericardium. The membranes prevent the heart from over-expanding when it is beating very fast. The pericardium also secretes a fluid called pericardial fluid. This fluid enables the membranes to move smoothly against each other.