Travelling organisms
Monacia Williams, Contributor

Hello students! How are you all today? Did you have a very good weekend? Have you started the week on an excellent note? I know you have because all your labs have now been written and you are just waiting for the teacher to ask for them, aren't you? Isn't that a good feeling? Good for you! I hope you found the exercises that we went through beneficial and are now using them to your fullest advantage.

Beginning this week, we will return to more of the theoretical aspects of the subject, starting with transport. What does this topic entail? It covers the movement of substances in living organisms. To fully understand and appreciate the topic, we need to first examine a few concepts. We need to look first at what cause substances to move.

What causes substances to move?

1. The substances that pass in and out of living organisms are made up of tiny particles.

2. These particles are in constant motion, moving not in any set pattern, but randomly.

3. This means that particles will move to fill any space that is available to them. This movement is known as diffusion.

4. In order for the movement to take place and be noticeable, a gradient must be present. What is this gradient? There must be an area where the particles are more abundant and an area where they are less abundant. The area where they are more abundant is said to have a higher concentration of particles than the area where they are less abundant. The gradient is referred to as a concentration gradient. Where the particles are solids, liquids or gases, this concentration gradient is called a diffusion gradient, and where the particles are water molecules, it is called an osmotic gradient.

5. Why is it called an osmotic gradient in the case of the movement of water molecules? This is because a different name is given to the movement of these particles. This name is osmosis. This movement also requires the presence of a membrane which will only allow certain molecules to pass through; this membrane is called a selectively permeable membrane. The membranes of all living cells act as selectively permeable membranes.

Differences and similarities between osmosis and diffusion

• Both require a concentration gradient.
• Diffusion can involve the movement of solids, liquids and gases; osmosis involves the movement of water molecules only.
• Diffusion, while being able to take place across membranes, does not always require one; osmosis must have a selectively permeable membrane in order to take place.

Why does a living organism need a transport system?

Fact is, not all living organisms do! Simple organisms, for example, unicellular ones like the amoeba, do not need a transport system. Simple multicellular ones like jellyfish and flatworms do not need one either! This is because their entire bodies are in close contact with their external surroundings so substances can move in and out by diffusion. These animals have large surface area to volume ratios, meaning, their surface areas are large and their volumes, depth, are not.

The more complex the organism becomes, the smaller the surface area to volume ratio, hence, substances cannot enter and leave by diffusion. Take, for example, food digested in the small intestines. It would be impossible for it to reach the brain by diffusion. How would oxygen from the lungs get to the toes?

Diffusion is only useful over short distances. There is some degree of compensation for this in large organisms, where some areas within the organism become infolded, causing an increase in their surface areas. For example, in animals, these include the alveoli in the lungs and the villi in the small intestines; in plants, these include the root hairs in the root system and the broad, thin leaves with loosely-packed cells in the root system. Did you notice that these areas are near to the sources of certain substances, for example, respiratory gases and digested food? This increase in the surface area is not enough to compensate for the low surface area to volume ratio, therefore both complex animal and plants still require transport systems to move substances around.

The following is a list of the things you will need to know about transport in animals to do well at this topic:

• What type of materials are being transported
• Where the materials come from and where do they go
• How materials move from one part of the body to another
• Why a small organism does not need a transport system
• Why larger organisms, like mammals and trees, have transport systems
• How to draw, label and annotate a simple diagram of the mammalian heart to show structure and function
• Structure and function of capillaries, arteries and veins
• Structure and function of different blood cells and the composition and function of plasma

Some of these we have already covered, the others we will cover as we go along. Have a brilliant week!

Students of John Rollins Success Primary School give an eloquent performance of the spoken word at the school's fifth anniversary celebrations recently. - Sheena Gayle Photo

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com