Biological Macromolecules and Enzymes

Four macromolecule groups have characteristic roles

Four large molecule groups are important in cells. Carbohydrates include sugars and often provide quick energy. Lipids store long-term energy and form cell membranes. Proteins are made of amino acids and perform many jobs. Nucleic acids, DNA and RNA, are made of nucleotides and carry genetic information.

Structure supports function. For example, phospholipids have a water-loving end and a water-fearing end, so they form membrane layers.

Proteins do many jobs because amino-acid chains can fold into many shapes. Enzymes, receptors, antibodies, and some structural fibers are proteins. Shape matters: a receptor binds only signals that fit its binding region.

Common trap: a molecule’s group does not force it to have only one job. Carbohydrates can be structural, proteins can transport or signal, and lipids can form membranes. Also, lipids are grouped by shared properties and roles; not every lipid is a repeating-chain polymer.

Dehydration builds and hydrolysis separates

Cells join small units to build larger molecules. Dehydration synthesis removes water as a bond forms. Hydrolysis adds water to break a bond.

Digestion often uses hydrolysis. For example, a large carbohydrate can be split into smaller sugars by adding water across its bonds.

Dehydration and hydrolysis are opposite patterns. Building a protein joins amino acids while releasing water. Digesting a protein adds water as bonds are broken. The same logic applies to many biological polymers.

Energy depends on the overall reaction. Breaking one bond alone is not the whole story: all bonds broken and formed matter. ATP hydrolysis can supply energy for a coupled cellular process; hydrolysis does not always mean energy release.

Enzymes lower activation energy

An enzyme is a biological catalyst. It speeds a reaction by lowering the activation energy, the energy needed to start. The enzyme is not used up.

A reactant called the substrate binds to the enzyme’s active site. The fit is specific. If the active site changes shape, the substrate may not bind well.

Worked example. With an enzyme, a reaction reaches its products sooner because the activation barrier is lower. The starting materials and final equilibrium do not change. Without the enzyme, the same reaction may still occur more slowly.

Substrate binding and product release

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Conditions change enzyme activity

Temperature and pH affect enzyme shape and motion. Warming usually speeds reactions up to an enzyme’s best range. Extreme heat can unfold the enzyme, a change called denaturation.

Very high or low pH can also change enzyme shape. More substrate increases rate only until all active sites are busy. Enzymes change reaction speed, not the final equilibrium.

As substrate rises, reaction rate rises until most enzyme active sites are busy. At that point, extra substrate has little effect unless more enzyme is added. This is enzyme saturation.

A temporary pH change may reduce binding without permanently destroying the enzyme. Extreme heat is more likely to cause lasting denaturation.

Interpret enzyme data comparatively. Find the condition with the highest measured rate, then compare one changed variable at a time. A lower rate at extreme pH may reflect altered ionization or shape; recovery after returning to the original pH argues against permanent denaturation. A plateau as substrate rises supports active-site saturation when enzyme amount is fixed.

Your quick summary

• Carbohydrates, lipids, proteins, and nucleic acids have different jobs.
• Dehydration joins units; hydrolysis splits them.
• Enzymes lower activation energy.
• An enzyme’s shape fits certain substrates.
• Extreme temperature or pH can reduce enzyme activity.