Biological Molecules - Gcse

Biological Molecules

Carbohydrates

These are molecules made up of carbon, hydrogen and oxygen only. It s in the name carbo(carbon)hydr(hydrogen)ate(a suffix which means containing oxygen).

You ll encounter two forms of carbohydrate sugars, called monosaccharides and disaccharides, and complex carbohydrates, called polysaccharides.

A monosaccharide is made up of a single sugar molecule. An example of this is glucose or fructose.

A disaccharide is made up of two sugar molecules stuck together. An example of this is sucrose or lactose.

A polysaccharide is numerous sugar molecules stuck together. An example of this is starch.

The monosaccharide you ll encounter the most is glucose and is normally shown as a hexagon:

Using this system of representation, a monosaccharide would be:

A disaccharide would be:

A polysaccharide would be:

Carbohydrates are mostly used in animals for energy! They are used in respiration to generate energy to allow processes in the cell to occur. The equation for (aerobic) respiration is:

glucose + oxygen -> carbon dioxide + water + ENERGY

The enzyme amylase breaks down starch (a polysaccharide) into maltose (a disaccharide) and then maltase breaks maltose down into glucose (a monosaccharide)

Tip you can recognise enzymes because their names end with -ase.

Monosaccharides are normally immediately used for energy, whereas polysaccharides are normally used to store energy, e.g. starch in plants and glycogen in humans.

To detect the presence of carbohydrates, we use Benedict s solution or iodine:

Benedict s solution is heated with the sample, and if it turns from blue to orange or red then we can say that mono or disaccharides are present.

Iodine is added to the sample and if it turns from orange to purple/black then starch is present.

We do these tests separately - you don t do Benedict s then iodine to the same sample.

Proteins

The atoms proteins are made up of are carbon, hydrogen and oxygen, as well as nitrogen and sometimes sulfur.

Proteins are made up of a long chain of amino acids stuck together. There are 20 amino acids which are used to make proteins, and proteins can be chains of thousands of amino acids, and so we can make a huge variety of proteins by sticking these amino acids together in different orders.

Proteins are used to do most of the jobs inside cells, particularly as enzymes.

Enzymes

An enzyme is a type of protein which speeds up a particular reaction. Something which speeds up a reaction is called a catalyst and so enzymes are often referred to as biological catalysts.

Enzymes can either break molecules down into smaller molecules, which is what happens in digestion:

o Lipases break lipids down into glycerol and fatty acids

o Proteases break proteins down into amino acids

o Carbohydrases break carbohydrates down into smaller sugars

When enzymes perform their job, they re not broken down or destroyed they can perform the reaction again and again and again.

The molecule an enzyme works on is called its substrate and the molecule(s) it produces is called the product.

The part of the enzyme which actually performs the reaction is called the active site. This has a very specific shape so specific that normally only one particular molecule can fit into it, e.g. it might allow glucose in, but not fructose or lactose. This means enzymes will only work on a single molecule they don t just break things down at random.

Enzymes can be damaged or prevented from working this is called being denatured. They re not alive, so you can t say they re killed. Enzymes can be denatured by temperature or pH. They become denatured because the conditions cause their shape to change, and so their active site changes shape, and so they can t interact with the substrates which they normally would.

Enzymes have an optimum temperature/pH this is the condition at which they work their best. As we go further and further from their optimum conditions, they are more and more likely to become denatured. Most enzymes in humans have an optimum temperature of ~37 C and an optimum pH of ~7, but enzymes in different parts of the body will have different preferences.

Proteins are not only used as enzymes they can be used to give cells structure, to let cells give each other information, and even to help cells move around.

Lipids

Lipids are also known as fats. They are made up of two components a molecule of glycerol and three fatty acid molecules attached to the glycerol:

Lipids have several roles in the body:

To provide insulation and protection by being stored under the skin.

To store excess energy.

To make up the membranes of cells

Lipids are insoluble in water this means they have to be emulsified by bile from the gallbladder so that they can dissolve and enter the body.

DNA

DNA is the molecule inside every cell which carries the instructions to make the cell how it is! You can think of it like a recipe book it contains the recipe for every molecule to let your cells exist.

DNA is an extremely long molecule, made by chaining lots of individual units together. Two long strands of DNA coil around each other to form a double helix structure:

The two strands are held together by bonds being created between them. The bonds are created by the interactions of DNA bases. There are four bases, referred to as A, T, C and G. A bonds to T and C bonds to G these bases act like the rungs of a ladder, holding the two strands of DNA together.

DNA is held in the nucleus in eukaryotic cells, and in prokaryotic cells it sits in the cytoplasm as a large circular molecule. In eukaryotes,

DNA is stored in numerous individual strands. These are called chromosomes. Each chromosome is just a long DNA molecule, coiled up to be stored. In human cells, there are 46 chromosomes, 23 from each parent.

Eukaryotic DNA

Prokaryotic DNA

Stored in the nucleus

Stored in the cytoplasm

Numerous chromosomes, which are strands of DNA

A single circular piece of DNA (and optional plasmids, which are also DNA)

Double helix where A bonds to T and C bonds to G

Double helix where A bonds to T and C bonds to G

Earlier we said that DNA is a set of instructions - these instructions are called lt;b>genes in DNA, and a gene is defined as a piece of DNA which gives the information to make a particular protein. Remember earlier when we said that proteins do most of the jobs in a cell DNA contains the instructions to make the proteins, and so controls what happens in the cell by deciding which proteins are made.

There are numerous genes on each chromosome you have ~21,000 genes and 46 chromosomes so each chromosome must contain lots of genes.

The genome of a cell is all of its DNA together, so your genome contains 46 chromosomes and ~3 billion individual DNA bases (i.e As, Ts, Cs and Gs).

We should also mention what the DNA backbone is made of in the middle, we have the bases. The long portion which holds the bases (in the diagram above, the two blue ribbons) is called the sugar-phosphate backbone. It is made of molecules of a sugar, specifically a pentose sugar, and a phosphate molecule. These units bond to each other to form the DNA backbone.

nucleotide. We talk about DNA as being made up of nucleotides bonded together. When 2 DNA strands bond to one another, the strands actually bond together upside down one strand is flipped upside down. This is called being antiparallel and is shown in the diagram above Strand 2 is upside down.

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