Cells and organisation, Year 10: specialised cells and microscopes

mitochondria

HintThey are the site of aerobic respiration.

The whySwimming 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

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

The whySome 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)

HintThink what your biceps does when you bend your arm.

The whyWhen 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

HintThink of drinking straws joined in a line.

The whyWith 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

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

The whyPhloem 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

HintThe word is related to "different".

The whyAn 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

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

The whyIn 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

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

The whyMost 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

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

The whyOnce 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

HintImagine a body in which every single cell was exactly alike.

The whyA 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)

HintA sharp photograph has more of it than a blurred one.

The whyResolution 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

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

The whyAn 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

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

The whyWith 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

HintZooming in on a blurry phone photograph gives you a bigger blur.

The whyMagnifying 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

HintIt uses a beam of tiny charged particles in place of light.

The whyRibosomes 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

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

The whyMagnification = 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

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

The whyImage 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

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

The whyMultiply 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

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

The whyReal 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

HintOne length might be given in millimetres and the other in micrometres.

The whyDividing 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.