Photosynthesis: how plants make glucose
Green plants take carbon dioxide from the air and water from the soil. Using light energy caught by chlorophyll, they build glucose and release oxygen:
6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
Count the atoms: 6 C, 12 H and 18 O on each side, so the equation is balanced. Light energy is stored in the bonds of glucose. This is the start of almost every food chain.
Structure of biomolecules
Living things use four main families of large molecules. Most are polymers: long chains made of many small repeating units.
- Carbohydrates: made of sugar units (like glucose). Starch and cellulose are long chains of glucose.
- Proteins: chains of amino acids. There are 20 kinds; their order decides how the chain folds and what the protein does.
- Fats (lipids): one glycerol joined to three fatty acid tails. They store energy and are not true polymers.
- Nucleic acids (DNA and RNA): chains of nucleotides. They carry the instructions of life.
Units join by a reaction that removes a water molecule (condensation). Breaking them needs water added back (hydrolysis).
Breakdown of nutrients
Digestion cuts big molecules into small ones so the body can absorb them: starch to glucose, proteins to amino acids, fats to glycerol and fatty acids.
Inside cells, glucose is burned slowly with oxygen to release energy for work (respiration):
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
This is the reverse of photosynthesis. Enzymes are proteins that speed up each step. Each enzyme fits one kind of molecule, like a key in a lock, and is not used up.
Protein synthesis in steps
The order of nucleotides in a gene in DNA holds the recipe for a protein. Cells make it in steps:
- Transcription: a short copy of the gene, mRNA, is made in the nucleus.
- The mRNA moves to a ribosome in the cytoplasm.
- Translation: the ribosome reads the mRNA three letters at a time. Each triplet picks one amino acid.
- Amino acids are joined in that order into a chain, which then folds into the working protein.
A change in the DNA order can change the amino acids and so the protein.
Key formulas and definitions
- 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2
- C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
- Units join by condensation (water leaves); chains break by hydrolysis (water enters)
- DNA → mRNA (transcription) → protein (translation)
Worked examples
1. How many oxygen molecules are made when 3 glucose molecules are made by photosynthesis?
One glucose comes with 6 O2. So 3 glucose come with 3 × 6 = 18 oxygen molecules.
2. A chain of 10 amino acids is made. How many water molecules are removed when the units join?
Each join removes 1 water. 10 units need 9 joins, so 9 water molecules are removed.
3. Which enzyme-type job happens when bread tastes sweet after chewing?
Salivary amylase cuts starch into small sugars (maltose and glucose). The small sugars taste sweet.
4. A piece of mRNA has 12 letters. How many amino acids does it code for?
The ribosome reads 3 letters per amino acid. 12 ÷ 3 = 4 amino acids.
Common mistakes
- Thinking plants breathe only in light. They respire all day; photosynthesis happens only in light.
- Mixing up the two equations. Photosynthesis makes glucose and oxygen; respiration uses them.
- Saying enzymes are used up. They are not; they work again and again.
- Thinking DNA leaves the nucleus to make protein. A copy, mRNA, does the travelling.