Analysing what living things are made of

Asking what a tissue is made of

Take a piece of fresh liver, or a leaf, and ask a plain question: what is it made of? Chemists answer it in two ways. One way finds the elements. The other finds the compounds.

Finding the elements

Weigh a fresh sample. Dry it in an oven, and the water goes off. The mass that is left is the dry weight. Now burn the dry sample. Everything made of carbon compounds turns into gases, such as carbon dioxide, and goes off. What remains is the ash.

• The gases came from the carbon compounds, the organic matter.
• The ash holds the inorganic elements, such as calcium and magnesium.

So a tissue holds carbon, hydrogen, oxygen and nitrogen in large amounts. It holds small amounts of elements such as calcium, magnesium, phosphorus and sulphur.

You should find that $50\ \text{g}$ of fresh sample with $80\ \%$ water has $10\ \text{g}$ of dry matter. The ash is a small part of that. Raise the water share and the dry matter shrinks.

Finding the compounds

To find the compounds, grind the tissue with trichloroacetic acid in a mortar and pestle. This makes a thick slurry. Strain the slurry through a cheesecloth or cotton. Two parts result.

• The filtrate is the acid-soluble pool. It holds thousands of small molecules: amino acids, sugars, nucleotides and ions. These micromolecules have masses from about 18 up to about 800 daltons, written Da.
• The retentate, left on the cloth, is the acid-insoluble fraction. It holds the large molecules.

The acid-soluble pool is roughly the chemistry of the cytoplasm. The acid-insoluble fraction holds the large molecules of the cytoplasm and of the organelles. Together the two hold the whole chemical make-up of the tissue. All these compounds are called biomolecules.

The four kinds of large molecule

The acid-insoluble fraction holds only four kinds of organic compound: proteins, nucleic acids, polysaccharides and lipids. Proteins, nucleic acids and polysaccharides have masses of ten thousand daltons and more. They are the biomacromolecules.

Lipids are a surprise. Their masses are below 800 Da, so their molecules are small. Why do they come down with the large ones? Grinding breaks the cell membranes into pieces. The pieces curl into vesicles, which do not dissolve in water. They settle with the acid-insoluble fraction, and the lipids of the membrane go with them. So lipids are not true macromolecules.

How much of a cell is what?

Cells are mostly water. The average make-up of a cell is below.

Water is the most abundant compound. Proteins are the most abundant organic compounds.

Primary and secondary metabolites

A cell makes thousands of compounds. They fall into two groups.

• Primary metabolites have known jobs in normal life. Amino acids and sugars are examples.
• Secondary metabolites are products whose roles in the plant's own life are not clear. Examples are alkaloids, flavonoids, rubber, essential oils, antibiotics, pigments, scents, gums and spices.

Many secondary metabolites matter to people. Rubber, spices, scents and pigments come from them. The drugs vinblastin and curcumin do too. Some also protect a plant from animals that eat it.

Try it in Python

The program decides which pool each molecule goes to when a tissue is ground in acid.

Practise with new numbers

Common mistakes

• Putting all small molecules in the acid-soluble pool. Lipids are small, but their membrane vesicles come down with the large molecules.
• Calling lipids macromolecules. Their molecules are small; they only settle with the large ones.
• Mixing up dry weight and ash. Dry weight is what is left after the water goes. Ash is what is left after the carbon compounds burn.
• Saying secondary metabolites are waste. Many are useful drugs, spices and pigments.

Remember these

• Dry the sample for water. Burn it for ash, which holds the inorganic elements.
• Grind in trichloroacetic acid: the filtrate is the acid-soluble pool, the retentate the acid-insoluble fraction.
• The four large kinds are proteins, nucleic acids, polysaccharides and lipids. Lipids are not true macromolecules.
• Cells are 70 to 90 per cent water; proteins are the most abundant organic compounds.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the liquid that passes through the cloth called, and what does it hold?

2. Lipids are small, yet they appear in the acid-insoluble fraction. Why?

3. What does the ash of a burned tissue hold?

4. Which makes up the larger share of a typical cell, proteins or carbohydrates?

5. Is rubber a primary or a secondary metabolite?