Carbohydrates and Lipids – Key Concepts

These flashcards review key points on starch structure, maltose formation/breakdown, lipid types, structures of triglycerides, phospholipids and steroids, cholesterol functions, and general roles of lipids in biology.

What is the significance of the coiled structure of starch?

It makes the molecule compact and therefore suitable for energy storage.

How is starch converted into a substrate for energy production?

Starch is hydrolysed by α-amylase into α-glucose, which then enters cellular respiration.

Which monosaccharide units form maltose?

Two α-glucose molecules.

Explain how maltose is formed from two α-glucose molecules.

A condensation reaction removes one water molecule and creates an α-1,4 glycosidic bond between C1 of one glucose and C4 of the other.

How is maltose broken down into its monosaccharides?

Maltase catalyses hydrolysis; water is added to break the α-1,4 glycosidic bond, yielding two α-glucose molecules.

Which enzyme catalyses both the formation and hydrolysis of maltose?

Maltase (the reaction is reversible).

What elements are present in lipids, and how does their oxygen proportion compare with carbohydrates?

Lipids contain carbon, hydrogen and oxygen, but have a lower proportion of oxygen than carbohydrates.

Why do lipids store more energy per gram than carbohydrates?

They possess a higher ratio of energy-rich C–H bonds, giving over twice the energy per gram compared with carbohydrates.

Name the three major classes of lipids and give one example of each.

Triglycerides (e.g., fat/oil), phospholipids (e.g., lecithin), and steroids (e.g., cholesterol).

Describe the structure of glycerol.

A three-carbon alcohol; each carbon bears a hydroxyl (-OH) group.

Which functional groups react to form an ester bond in triglycerides?

The hydroxyl group of glycerol and the carboxyl group of a fatty acid.

How many water molecules are removed during formation of one triglyceride?

Three water molecules—one for each ester bond formed.

What is the key structural difference between saturated and unsaturated fatty acids?

Saturated fatty acids have no C=C double bonds; unsaturated fatty acids have one or more double bonds.

How does saturation level influence the physical state of triglycerides at room temperature?

Unsaturated fats (with double bonds) are usually liquid (oils), whereas saturated fats are solid.

What effect do saturated vs. unsaturated fats have on LDL cholesterol levels?

Saturated fats tend to increase LDL levels; unsaturated fats tend to decrease them.

Give typical dietary sources of saturated and unsaturated triglycerides.

Saturated fats: animal fats such as butter and lard; unsaturated fats: plant oils such as olive, sunflower and corn oil.

What components make up a phospholipid molecule?

One glycerol, two fatty acid tails, and a phosphate-containing hydrophilic head group.

Why are phospholipids essential to cell membranes?

They form lipid bilayers that provide membrane fluidity and permit passage of lipid-soluble and small non-polar substances.

Describe the carbon-ring backbone common to all steroids.

Four fused rings: three six-carbon rings and one five-carbon ring (total of 17 carbons).

Which part of a cholesterol molecule is hydrophilic?

The hydroxyl (-OH) group attached to the ring structure.

Name three important substances for which cholesterol is a precursor.

Bile salts, vitamin D, and steroid hormones such as progesterone and testosterone.

State two roles of cholesterol in cell membranes.

It helps maintain membrane fluidity and provides structural stability.

How do stored fats act as thermal insulation in animals?

Adipose tissue under the skin reduces heat loss; in marine mammals this forms blubber.

What is the role of lipids in nerve transmission?

They form the myelin sheath around axons, insulating them and speeding impulse conduction.

How can desert animals obtain water from lipids?

Oxidation of stored lipids produces metabolic water.

What type of linkage joins fatty acids to glycerol in a triglyceride?

An ester linkage.

Why are lipids insoluble in water?

Their long non-polar hydrocarbon chains make them hydrophobic.

Which parts of a phospholipid are hydrophilic and hydrophobic?

The phosphate head is hydrophilic; the fatty acid tails are hydrophobic.