Cells and organisation, Year 10: specialised cells and microscopes

mitochondria

提示They are the site of aerobic respiration.

为什么Swimming to the egg needs a constant supply of energy for the tail. Mitochondria release energy by aerobic respiration, so having many of them, right next to the tail, suits the cell to its job.

It can carry electrical impulses over long distances in the body

提示Think what has to get all the way to a toe when you decide to wiggle it.

为什么Some nerve cells run all the way from the spinal cord to a toe. Being long means one cell can carry a signal the whole distance without passing it on. Branches at each end let it connect with many other cells.

Contract (get shorter)

提示Think what your biceps does when you bend your arm.

为什么When the fibres slide over each other the whole cell shortens, and when thousands of cells do this together the muscle pulls on a bone. Muscle cells also contain many mitochondria, because contracting needs a lot of energy.

They form long continuous tubes that water can flow through

提示Think of drinking straws joined in a line.

为什么With no cytoplasm and no end walls, nothing blocks the flow, so water and dissolved mineral ions move easily from the roots up to the leaves. The side walls are strengthened so the tubes do not collapse.

To let dissolved sugars pass from one cell to the next

提示Phloem is the plant's food-carrying tissue, and the food has to get along it somehow.

为什么Phloem cells are joined end to end in tubes. Unlike xylem cells they keep their end walls, but the pores let cell sap, carrying dissolved sugars, move along the tube from cell to cell.

Differentiation

提示The word is related to "different".

为什么An organism starts as unspecialised cells. As it develops, cells change: each gains the sub-cellular structures it needs for one function and becomes, for example, a nerve cell or a root hair cell.

early

提示Think about when in life an embryo forms its heart, nerves and muscles.

为什么In animals, most cells become specialised while the organism is still an embryo. After that, most of them cannot change into another type. This is very different from plants.

Many plant cells can differentiate throughout the plant's life

提示A cutting taken from an old stem can still grow brand new roots.

为什么Most animal cells lose the ability to differentiate early on. Many plant cells keep it, which is why a plant goes on producing new roots, leaves and flowers for as long as it lives, and why cuttings work.

Repairing tissues and replacing cells

提示Think about a grazed knee, or the skin flakes you lose every day.

为什么Once an animal has finished growing, it no longer needs division for growth. Cells still wear out, die or get damaged, so division carries on to replace them and to mend injuries.

It produces the different specialised cells needed to do different jobs

提示Imagine a body in which every single cell was exactly alike.

为什么A large organism needs cells that carry oxygen, cells that carry signals, cells that contract and many more. Differentiation is how one fertilised egg cell gives rise to all of these. Without it there could be no tissues or organs.

Resolution (resolving power)

提示A sharp photograph has more of it than a blurred one.

为什么Resolution decides how much detail an image holds. Two microscopes can magnify by the same amount, but the one with better resolution shows fine structures separately while the other shows a blur.

Both are much higher

提示It was the newer of the two inventions, and it changed what biologists could see.

为什么An electron microscope uses a beam of electrons instead of light. It can separate points less than a nanometre apart, where a light microscope is limited to about 200 nanometres, and it can magnify far more before the image blurs.

It showed sub-cellular structures in much finer detail, including many never seen before

提示Think about what lies inside the cytoplasm that a school microscope shows only as dots, or not at all.

为什么With a light microscope, a mitochondrion is a faint speck and a ribosome is invisible. The electron microscope showed the inside of mitochondria and chloroplasts and revealed ribosomes, so biologists could work out what each structure does.

Enlarging adds no detail: the light microscope's resolution is too low

提示Zooming in on a blurry phone photograph gives you a bigger blur.

为什么Magnifying an image only spreads out the detail that is already there. If two points were not separated by the microscope in the first place, no amount of enlargement will separate them.

electron

提示It uses a beam of tiny charged particles in place of light.

为什么Ribosomes are far smaller than anything a light microscope can resolve. Electron microscopes were developed in the 1930s, about three centuries after the first light microscopes, and in the following decades they revealed ribosomes and the detail inside other structures.

800

提示Both lengths are already in the same unit, so compare them directly.

为什么Magnification = size of image ÷ size of real object, so 40 ÷ 0.05. Magnification has no unit, because it is one length divided by another; it is written as ×800.

3 mm

提示Find the length of the image in the original unit first, then convert.

为什么Image size = magnification × real size, so 1500 × 2 µm = 3000 µm. There are 1000 micrometres in a millimetre, so divide by 1000.

Real size = image size ÷ magnification

提示Of the three quantities, the picture is the largest, so it is never the one you divide by.

为什么Multiply both sides by real size, then divide both sides by magnification. The specification states the equation in this section; the saved pages do not say that it is given in the exam, so learn it and its two rearrangements.

1 × 10⁻⁷ m

提示Work out the real width in the unit you were given, then change that unit, then tidy the number.

为什么Real size = image size ÷ magnification = 5 mm ÷ 50 000 = 0.0001 mm. A millimetre is 10⁻³ m, so 0.0001 mm is 10⁻⁴ × 10⁻³ m. That is the same as 100 nanometres.

units

提示One length might be given in millimetres and the other in micrometres.

为什么Dividing 40 mm by 50 µm gives a meaningless 0.8. Convert first: 40 mm is 40 000 µm, and 40 000 ÷ 50 = 800. The safest habit is to change the larger measurement into the smaller unit before dividing.