Surface Area : Volume Ratio…

The cell is the basic unit, or building block, of all known forms of life. It has a membrane containing cytoplasm in which all of the essential reactions for life take place. There are two types of cell, prokaryotic cells and eukaryotic cells. Eukaryotic cells have discrete structures within them that perform different jobs. The most important of these is the nucleus which contains chromosomes (strands of DNA). Prokaryotic cells are much smaller (up to a 1000 times smaller) and do not have so many specialist structures within them.

Plant and animal cells differ in three important ways. Plant cells have cellulose cell walls around them, they contain a large water sac called a vacuole and they may contain chloroplasts. You can read more about animal cells and plant cells by clicking on those two links.

Many cells have specific shapes to help them perform their particular function. Examples of specialised animal cells would include muscle cells, nerve cells and sperm cells. A good example of a specialised plant cell is a root hair cell. The links will take you to specific posts about specialised cells. You really should have some good examples at your fingertips.

The cell membrane is a selectively permeable layer that protects the contents of the cell from its surroundings. It allows some substances in and out but blocks others (if you want to know the more advanced version click here). Water moves freely in and out of a cell, as do other small molecules such as oxygen, carbon dioxide and glucose. Oxygen diffuses in when there is a higher concentration of oxygen outside the cell than there is inside the cell.

This process takes time, so if a cell is too large then the oxygen will not be able to make it to the middle before the cell dies! The following video explains this in terms of surface area to volume ratio. It uses cubes of jelly to represent the cells. The cubes have an indicator called phenolphthalein mixed into them. Phenolphthalein is pink in alkaline conditions and colourless in acidic solutions.

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Questions…

1. Name, and give the function of, one cell organelle not mentioned in the passage above.
2. Which food group, essential for a healthy human diet, is mostly composed of plant cell walls?
3. What might be the pH of the hydrochloric acid used in the video?
4. What is a gene?
5. Name a chemical other than carbon dioxide probably excreted by an animal cell.