GCSE · Biology · AQA · Spec 8461
Metabolism
Most people think metabolism means burning food. Biologists mean something far bigger — sort eight of its reactions and count how many actually break anything down.
Sort the reactions
Build up, or break down?
Every reaction here is part of metabolism. Place each one: does it build new molecules, or break molecules down?
Still to sort
Builds new molecules (0)
Smaller molecules are joined or combined to make a new molecule. This is synthesis.
Where the line is: Ask what the reaction ends with. If it makes a new molecule out of the ones it started with, it builds — even when the new molecule is small, like an amino acid.
Breaks molecules down (0)
A molecule is taken apart into smaller ones.
Where the line is: Ask what happens to the starting molecule. If it is taken apart into smaller products, it breaks down — even when one of those products, like urea, has a name of its own.
Eight of the reactions that metabolism includes.
Where the energy for building comes from
Reason it through
Why does a cell need respiration in order to build new molecules?
First link · your turn
Respiration breaks glucose down. As it does, what does it do for the cell?
Is glucose enough?
Glucose is the building block of starch, glycogen and cellulose. Decide whether that makes it enough for everything.
A plant cell has plenty of glucose but no nitrate ions at all. Can it make new amino acids — and so build new proteins?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Every reaction in a cell or the body — the ones that build new molecules as well as the ones that break molecules down.
What you need to know
- Metabolism is the sum of all the reactions in a cell or the body.
- Sugars, amino acids, fatty acids and glycerol are used to synthesise carbohydrates, proteins and lipids, and are what those molecules are broken back down into.
- Metabolism includes the conversion of glucose to starch, glycogen and cellulose.
- Metabolism includes forming a lipid molecule from one molecule of glycerol and three molecules of fatty acids.
- Metabolism includes using glucose and nitrate ions to form amino acids, which in turn are used to synthesise proteins.
- Metabolism includes respiration, and the breakdown of excess proteins to form urea for excretion.
- The energy transferred by respiration is used for the continual enzyme-controlled reactions of metabolism that synthesise new molecules.
The big picture
Metabolism is the sum of all the reactions in a cell or the body. It includes reactions that build new molecules — glucose into starch, glycogen and cellulose; one glycerol and three fatty acids into a lipid; glucose and nitrate ions into amino acids, and amino acids into proteins — and reactions that break molecules down, such as respiration and the breakdown of excess proteins to form urea. The building reactions are enzyme-controlled and run continually on energy transferred by respiration.
Key points
Worked example
Problem
A leaf cell has a good supply of glucose and of nitrate ions. Describe what its metabolism can do with the glucose, say whether each reaction builds up or breaks down, and say which reaction supplies the energy for the others.
⚠ Watch out
Writing that a lipid is made from 'glycerol and a fatty acid', or from fatty acids alone. A lipid molecule needs both building blocks, in a fixed ratio: one molecule of glycerol and three molecules of fatty acids.
Memory hook
On the list, building beats breaking down six to two — and the one reaction that burns glucose, respiration, pays for the building.
Check yourself
Close the page. For each of starch, glycogen, cellulose, proteins and lipids, name what it is built from — then name the one reaction on the list that supplies the energy to build them.
Flashcards
(11)What is metabolism?
Which three carbohydrates are made from glucose?
What is one lipid molecule formed from?
Which two substances are used to form amino acids?
Why can't glucose alone make an amino acid?
What are proteins synthesised from?
What happens to excess proteins in the body?
Is respiration part of metabolism?
What is the energy transferred by respiration used for?
What controls the reactions of metabolism?
What are carbohydrates, proteins and lipids broken down into?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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