Cells and organisation, Year 10: cell types and structures

A eukaryotic cell

HintAnimals and plants are both built from this kind.

The whyAnimal and plant cells keep their DNA inside a nucleus, so they are eukaryotic. Bacterial cells have no nucleus and are called prokaryotic. The two words describe the type of cell, not the size of the organism.

A single loop of DNA, free in the cytoplasm

HintPicture a closed ring floating about, with nothing wrapped round it.

The whyBacteria still need genetic instructions, but nothing separates them from the rest of the cell contents. The main DNA is one closed loop lying in the cytoplasm. Many bacteria also carry extra small rings of DNA.

plasmids

HintThe word begins like "plasma" and ends like "orchids".

The whyPlasmids are small circles of DNA that sit in the cytoplasm apart from the main loop. They carry a few extra genes. Animal and plant cells do not normally have them.

It is much smaller

HintOne of them needs a far more powerful lens before you can see it at all.

The whyBacterial cells are roughly a tenth of the length of most animal and plant cells, or less: about one micrometre against tens of micrometres. That is one or two orders of magnitude. It is one reason bacteria were discovered long after larger cells.

wall

HintPlant cells also have one of these, though theirs is made of a different material.

The whyA bacterial cell has cytoplasm and a cell membrane like an animal cell, but the membrane is surrounded by a further rigid layer. Animal cells have nothing outside the membrane.

milli; micro

HintOne is the prefix on a school ruler's smallest division; the other has a Greek letter for its symbol.

The whyCenti means a hundredth, milli a thousandth, micro a millionth and nano a thousand-millionth of the unit. Each step from milli to micro to nano is a factor of 1000. Cells are usually measured in micrometres and the parts inside them in nanometres.

2

HintCount the powers of ten between the two lengths.

The whyAn order of magnitude is a factor of ten. The plant cell is 100 times longer, and 100 is 10 × 10, or 10². Two factors of ten means two orders of magnitude.

7 × 10⁻⁶ m

HintCount how many places the decimal point must move to sit just after the first non-zero digit.

The whyMove the decimal point six places to the right to get 7, so the power of ten is negative six. Small lengths have negative powers. The same length is 7 micrometres, because micro means 10⁻⁶.

0.25 µm

HintA nanometre is the smaller unit, so the number must get smaller. By what factor?

The whyThere are 1000 nanometres in one micrometre, so divide by 1000: 250 ÷ 1000. Going from a smaller unit to a larger one always makes the number smaller.

500

HintDivide the two front numbers, then deal with the powers of ten separately.

The whyDivide the widths: 1 ÷ 2 = 0.5, and 10⁻⁵ ÷ 10⁻⁸ = 10³, because you subtract the powers. Then 0.5 × 1000 gives the answer. Standard form makes it easy to compare things of very different sizes.

Ribosomes

HintThey are tiny dots, far too small to show up under a school microscope.

The whyRibosomes are found in the cytoplasm of animal, plant and bacterial cells. Each one joins amino acids together to build proteins. A cell that makes a lot of protein, such as one that makes enzymes, has a great many of them.

cellulose

HintIt is a carbohydrate, and the main ingredient of cotton and paper.

The whyCellulose forms strong fibres that strengthen the cell and help it keep its shape. Algal cells have cellulose walls too. Bacterial cells also have a wall, but theirs is made of a different substance.

Roots are in the dark, so the cells cannot photosynthesise

HintThink about what those green structures need in order to work, and where the root is.

The whyChloroplasts absorb light for photosynthesis. Underground there is no light to absorb, so making chloroplasts would waste resources. This is why plant cells are said to have chloroplasts "often", not always.

It shows the widest area, so the cells are easier to find and focus on

HintCompare looking for a friend in a crowd with your eyes and then through a telescope.

The whyAt low power you see a large part of the slide, so you can find the specimen and bring it into focus. Only then do you switch to a higher power to see detail. At high power from the start, the lens is close to the slide and you may see nothing at all.

Most cell parts are colourless, and the stain makes them show up

HintImagine trying to see a clear glass marble in a glass of clean tap water.

The whyOnion cells are almost transparent, so light passes through with little contrast. A stain is taken up more by some parts, such as the nucleus, so they stand out against the rest. Different stains pick out different structures.

250

HintShare the distance out among the cells first, then change the unit.

The whyEach cell takes up 2 mm ÷ 8 = 0.25 mm of the width. There are 1000 micrometres in a millimetre, so multiply by 1000. Estimating like this is how you judge the size of something too small to measure directly.