Metabolism (AQA GCSE Biology Combined Science): Revision Notes
Metabolism
What is metabolism?
Metabolism refers to all the chemical reactions that take place inside cells and throughout your body. These reactions help your body break down food for energy and build new substances it needs to function properly.
Think of metabolism as your body's chemical factory - it's constantly breaking things down and building things up!
The factory analogy is particularly helpful: just like a factory has different production lines for different products, your body has different metabolic pathways for processing different types of molecules.
Carbohydrates
Carbohydrates are one of the main types of molecules your body uses for energy and storage.
Glucose and sugar molecules
Glucose is a simple sugar that your body loves to use for quick energy. When sugar molecules join together, they can form larger, more complex carbohydrates:
- Glycogen - stored in muscles and liver for energy when you need it
- Starch - how plants store energy in their cells
- Cellulose - strengthens plant cell walls (like the tough bits in vegetables)
Breaking down carbohydrates
Your body uses an enzyme called amylase to break down complex carbohydrates like starch back into simple sugars. Most animals can't digest cellulose, but they can break down glycogen and starch to get glucose for energy.
Enzymes are like molecular scissors - they're highly specific and can only cut certain types of chemical bonds. This is why amylase only works on carbohydrates, not on fats or proteins.
Lipids
Lipids are fats and oils that your body uses for long-term energy storage and protection.
What are lipids?
- At room temperature, fats are solid and oils are liquid
- A lipid molecule forms when one glycerol molecule joins with three fatty acid molecules
Uses of lipids
Your body uses lipids for two main jobs:
- Storage - keeping energy for later use in both plants and animals
- Insulation - protecting animals from losing too much body heat
Lipids are incredibly efficient for energy storage because they contain more than twice as much energy per gramme compared to carbohydrates or proteins. This is why your body prefers to store long-term energy as fat rather than carbohydrates.
Breaking down lipids
The enzyme lipase breaks down lipids back into glycerol and fatty acids so your body can use them for energy.
Proteins
Proteins are complex molecules that do many important jobs in your body.
What are proteins?
Proteins include enzymes (which speed up reactions) and structural substances like collagen (in your skin) and keratin (in your hair and nails). Different amino acids join together in various combinations to make different proteins.
Proteins are incredibly diverse - your body can make thousands of different proteins from just 20 different amino acids. The sequence and combination of these amino acids determines what job each protein will do.
Breaking down proteins
The enzyme protease breaks down proteins into individual amino acids. Your body can then use these amino acids to build new proteins it needs.
The liver's role in metabolism
Your liver is like your body's processing centre. It has three key jobs in metabolism:
- Converting lactic acid - changes lactic acid (produced during exercise) back into glucose
- Storing excess glucose - turns extra glucose into glycogen for storage
- Processing amino acids - converts excess amino acids into ammonia (called deamination), then changes this ammonia into urea for removal from the body
The liver is absolutely essential for metabolism - it processes over 500 different chemical reactions! Without a functioning liver, your body couldn't properly process nutrients or remove toxic waste products.
Remember!
Key Points to Remember:
- Metabolism includes all chemical reactions in cells and the body
- Three main molecule types: carbohydrates (for energy), lipids (for storage and insulation), and proteins (for structure and enzymes)
- Key enzymes: amylase breaks down carbohydrates, lipase breaks down lipids, protease breaks down proteins
- The liver processes many substances and converts waste products for safe removal
- Synthesis builds up complex molecules while digestion breaks them down into simpler parts