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2. Diffusion and levels of organisation GapFill
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Movement of substances in and out of cells
Chemical reactions happen in our cells all the time, to keep us alive.
In order for these reactions to happen, molecules of chemical substances must be able to get in and out of the cell.
All cells have a cell , which is partially permeable. This means that it has small holes in it, and it will let small molecules go in and out through it.
Substances like oxygen and carbon move into and out of cells by a process called . It happens when the molecules are free to move. This is true in gases and solutions.
Molecules diffuse down a concentration gradient, from an area of high concentration to an area of concentration. This is how the smell of cooking travels around the house from the kitchen, for example.
Molecules spread out from a place where there’s a lot of them to a place where there’s not so many, until it’s on both sides. This does not need any energy, it just happens.
- The greater the difference in concentration, the faster diffusion will happen.
- It will also happen faster if the cell has a large surface .
- Only small molecules can diffuse through cell membranes – that’s why the nutrients in our food have to be broken down by enzymes before we can absorb them into our bloodstream.
Examples of diffusion in living organisms include:
- Gas exchange in the human .
- Absorption of nutrients from food into the bloodstream in the small .
- Diffusion of water into the roots of plants.
Levels of organisation
Unicellular organisms can get everything they need through their cell membranes. However, organisms such as humans are made up of many millions of cells.
The cells of these organisms differentiate and become adapted for specific functions in the body. A group of cells of the same type is a tissue.
A tissue is a group of specialised cells that have a similar structure and function. Examples of tissues include:
- tissue, which can contract to bring about movement
- nerve tissue, which sends electrical signals around the body
- epithelial tissue, which covers some parts of the body
are made up of lots of tissues working together. One of these may contain several tissues; for example, the heart is mainly made up of muscle tissue, but it also contains nerve tissue and blood vessels.
are groups of body parts that work together to carry out a specific function.
For example, in the digestive system the stomach and intestines work together to obtain nutrition from the food we eat.
The human body has several of these groups. Each one carries out a particular purpose and together they make up the whole body, or organism.