1.2. Explain the biochemical properties of carbohydrates, proteins and lipids

This guide will help you answer 1.2. Explain the biochemical properties of carbohydrates, proteins and lipids.

The Biochemical Properties of Carbohydrates, Proteins, and Lipids

Understanding the biochemical properties of carbohydrates, proteins, and lipids allows us to see how these molecules behave in the body. Their properties are largely based on their structure and interactions with other molecules, which influences their functions in biological processes.

Carbohydrates

Carbohydrates are energy-rich molecules with many functions in the body. Their biochemical properties come from their structure and how they interact with enzymes, water, and other chemicals.

Water Solubility

• Simple carbohydrates (like glucose and sucrose) are highly soluble in water.
• Their hydroxyl (OH) groups form hydrogen bonds with water, allowing them to dissolve easily.
• Solubility is important because it enables carbohydrates to be transported in the blood and used quickly for energy.

Reducing and Non-Reducing Sugars

• Monosaccharides and some disaccharides (like glucose and maltose) are known as reducing sugars. They have a free aldehyde or ketone group that can donate electrons.
• Reducing sugars participate in chemical reactions, such as the Benedict’s test for detecting the presence of sugar.
• Non-reducing sugars, like sucrose, do not have a free aldehyde or ketone group and do not give a positive result in these tests unless chemically broken down.

Energy Release through Respiration

• Carbohydrates store energy in their chemical bonds. When they are broken down during cellular respiration, they release energy in the form of ATP (adenosine triphosphate).
• Glucose is the main carbohydrate used for this process. The equation for aerobic respiration is: [ C_6H_{12}O_6 + 6O_2 → 6CO_2 + 6H_2O + ATP ]

Polysaccharides and Storage

• Polysaccharides like starch and glycogen are insoluble in water due to their large size. This makes them excellent storage molecules, as they do not affect osmotic pressure in cells.
• Starch is a primary energy store in plants, while glycogen serves as an energy reserve in animals.

Structural Properties

• Some carbohydrates, like cellulose, have special biochemical properties for structural roles. Cellulose is made of long chains of glucose linked by beta-glycosidic bonds, making it rigid and strong.
• In humans, cellulose cannot be digested but contributes to dietary fibre, aiding bowel movements.

Sweetness

• Simple carbohydrates, especially monosaccharides like fructose, are sweet due to their molecular structure. This sweetness is recognised by receptors on taste buds.

Proteins

Proteins are incredibly versatile molecules. Their biochemical properties depend on the specific sequence of amino acids and their 3D structure, which is essential for their function.

Solubility

• Some proteins are soluble in water, like enzymes and haemoglobin, which is dependent on their amino acid composition. The hydrophilic (water-attracting) side chains on their surface make them dissolve better in water.
• Other proteins, such as keratin (in nails and hair), are insoluble due to the arrangement of their hydrophobic (water-repelling) amino acids.

Buffering Capacity

• Proteins act as buffers in the body by regulating pH. They can accept or donate hydrogen ions, maintaining the balance between acidity and alkalinity.
• This action involves the amino (-NH_2) and carboxyl (-COOH) groups of amino acids.

Enzymatic Activity

• Many proteins function as enzymes, which speed up biochemical reactions. Their activity relies on their 3D shape, which forms an active site. This site binds to a specific substrate (the molecule acted upon by the enzyme).
• Enzymes have specific conditions at which they work best, including an optimum pH and temperature.

Denaturation

• Proteins can lose their structure and function if exposed to extreme heat or changes in pH. This process is called denaturation.
• For example, cooking an egg denatures its proteins, causing the clear part (albumin) to solidify and turn white.

Hydrogen Bonding and Strength

• Proteins can form hydrogen bonds within their structure or with other molecules, contributing to their stability.
• Structural proteins like collagen derive their strength and flexibility from these bonds, making them vital for skin, bones, and tendons.

Roles in Transport and Storage

• Haemoglobin, a globular protein, carries oxygen in the blood. Its ability to bind oxygen is due to its structure and the presence of iron at the centre of the haem group.
• Ferritin, another protein, stores iron in cells for later use.

Lipids

Lipids have a diverse range of biochemical properties based on their structure. These properties allow them to store energy, form biological membranes, and serve as signalling molecules in the body.

Hydrophobicity

• Lipids are hydrophobic, meaning they repel water. This property comes from long hydrocarbon chains that do not interact well with water.
• Their hydrophobic nature allows lipids to form barriers in the body, such as cell membranes, which regulate substances entering or exiting cells.

Energy Density

• Lipids are energy-dense molecules. They provide about 9 kcal of energy per gram, which is more than double what carbohydrates and proteins provide (4 kcal per gram).
• Their structure, which includes long hydrocarbon chains, stores large amounts of energy in chemical bonds.

Insolubility and Transport

• Lipids do not dissolve in water. In the body, they are transported by lipoproteins, which are complexes of lipids and proteins.
• For example, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) transport cholesterol through the bloodstream.

Amphipathic Nature of Phospholipids

• Phospholipids have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions, making them amphipathic.
• This property allows them to form the lipid bilayer of cell membranes, with hydrophilic heads facing outward and hydrophobic tails pointing inward.

Melting Point and Saturation

• The saturation of fatty acids affects their melting point. Saturated fats, with no double bonds, are solid at room temperature (e.g., butter). Unsaturated fats, with one or more double bonds, are liquid at room temperature (e.g., olive oil).
• Double bonds in unsaturated fats introduce “kinks” in the fatty acid chains, preventing tight packing.

Signalling Molecules

• Lipids like steroids and eicosanoids act as chemical messengers in the body. For example:
• Cholesterol is a precursor for steroid hormones like oestrogen and testosterone.
• Prostaglandins, derived from fatty acids, regulate inflammation and other processes.

Thermal Insulation

• Adipose tissue, which stores lipids, provides insulation by trapping heat.
• In cold environments, lipid stores help maintain body temperature.

Comparison of Biochemical Properties

Biochemical Property	Carbohydrates	Proteins	Lipids
Solubility	Highly soluble (simple carbohydrates)	Variable (depends on structure)	Insoluble in water (except phospholipids)
Energy Yield	4 kcal/g	4 kcal/g	9 kcal/g
Structural Role	Provides rigidity (e.g., cellulose in plants)	Structural strength (e.g., collagen, keratin)	Forms membranes (phospholipids)
Chemical Reactivity	Reducing sugars interact with other molecules	Acts as enzymes to catalyse reactions	Acts as signalling molecules (e.g., steroids)
Storage Form	Glycogen in animals, starch in plants	No primary storage form	Stored in adipocytes (fat cells)

Final Thoughts

• Carbohydrates: Their solubility allows easy transport for energy. The large, insoluble polysaccharides make them ideal for storage.
• Proteins: The diversity in their structures makes them adaptable for various roles, from enzymes to structural components.
• Lipids: Their hydrophobic nature makes them highly efficient for forming cell membranes, storing energy, and acting as hormones.

Understanding these properties helps health and social care professionals evaluate dietary needs and metabolic conditions effectively.