Cells and organisation, Year 10: using glucose, and metabolism

cellulose

HintIts name begins with the same four letters as the structure it builds.

The whyMany glucose molecules are joined into long chains to make this tough material. It is one of the five uses of the glucose from photosynthesis that GCSE lists.

It is insoluble

HintThink about what glucose does in water that a storage substance must not do.

The whyBecause it does not dissolve, starch stays where it is put and does not draw water into the cell by osmosis. It can be converted back to glucose when the plant needs it, for example at night.

nitrate

HintProteins contain an element that glucose lacks, and farmers add it in fertiliser.

The whyGlucose contains only carbon, hydrogen and oxygen. Amino acids also need nitrogen, which the plant takes in through its roots as these ions. A plant short of them makes too little protein and grows poorly.

From glucose, as an energy store

HintTrace it back to the product of photosynthesis, and ask what a seed needs before it has leaves.

The whyPlants convert some of their glucose into fats and oils for storage. Seeds are often rich in them, because the young plant must live on its store until it can photosynthesise for itself.

respiration

HintAnimal cells carry out exactly the same process.

The whyPhotosynthesis makes glucose only in the green parts and only in the light. Every living plant cell, including those in the roots, uses glucose in this process all the time to release the energy it needs.

Metabolism

HintPeople talk about having a 'fast' or a 'slow' one.

The whyIt includes reactions that build large molecules from small ones and reactions that break large molecules down. The building reactions use energy transferred by respiration, and respiration is itself part of the total.

glycerol; fatty acids

HintThey are the same two products that lipase releases when it digests a fat.

The whyBuilding lipids is one of the synthesis reactions of metabolism. Digestion runs it in reverse: lipase breaks each lipid molecule back into these four smaller molecules.

starch; glycogen; cellulose

HintThe plant store turns iodine blue-black; the animal store is kept in liver and muscle; the third builds cell walls.

The whyAll three are long chains of glucose molecules, made in synthesis reactions that are part of metabolism. Plants make two of them; animals make the third.

urea

HintIt gives urine its name.

The whyThe body cannot store spare protein. It is broken down, and the nitrogen-containing waste is made into this substance, which the kidneys remove from the blood.

enzymes

HintThey are biological catalysts, each with an active site.

The whyEvery step in building or breaking down a molecule in a cell needs its own protein catalyst. This is why conditions such as temperature and pH, which affect these catalysts, affect the whole of metabolism.