Cells and organisation, Year 10: cell types and structures

A eukaryotic cell

提示Animals and plants are both built from this kind.

为什么Animal 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

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

为什么Bacteria 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

提示The word begins like "plasma" and ends like "orchids".

为什么Plasmids 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

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

为什么Bacterial 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

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

为什么A 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

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

为什么Centi 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

提示Count the powers of ten between the two lengths.

为什么An 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

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

为什么Move 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

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

为什么There 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

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

为什么Divide 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

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

为什么Ribosomes 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

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

为什么Cellulose 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

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

为什么Chloroplasts 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

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

为什么At 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

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

为什么Onion 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

提示Share the distance out among the cells first, then change the unit.

为什么Each 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.