Cells and Tissues
The cell and what is inside it
Name the parts of a plant and an animal cell and say what each does, tell a prokaryotic cell from a eukaryotic one, and use osmosis to explain what happens to a cell in salty water.
In 1665 Robert Hooke cut a thin slice of cork, put it under a microscope he had built himself, and saw rows of little box-like compartments. He called them cells. Almost nothing about a cell can be seen with the unaided eye — two points closer together than about 0.1 mm already blur into one — so the whole of this chapter rests on lenses.
Three parts, and then a dividing line
Most cells have a selectively permeable plasma membrane, a jelly-like cytoplasm and a nucleus. The dividing line across the living world is what happens to that third part: a bacterial cell keeps its DNA loose in a region called the nucleoid and has no membrane-bound organelles, which makes it prokaryotic. A plant or animal cell wraps its DNA in a nuclear membrane and runs a set of organelles, which makes it eukaryotic.
Have a play
The parts of a cell, each one named.
Tap any part of the picture.
Worked example
Two matching pieces of potato are weighed. One goes into plain water, the other into a 20 per cent salt solution. An hour later the first has gained weight and the second has lost it. What has moved, and which way?
Rule out the salt: the cell membrane lets water through and holds the salt back, so whatever moved was water.
That is what selectively permeable means. The membrane is a filter, not a door.
Try it together
Now work out how big an onion cell is, the way the microscope activity does it.
The circular field of view measures 5 mm across, and 25 onion cells lie end to end along that diameter. Answer with a bare number each time.
1.First put the field of view into micrometres. What is 5 mm in micrometres?
Have a go
Have a go on your own. A field of view 8 mm across holds 20 cells end to end. How wide is one cell, in micrometres? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.