AQA A-Level Biology Topic 1.2: Carbohydrates

AQA A-level Biology Topic 1.2: Carbohydrates

Definition of Carbohydrates

  • Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen.

  • They are primarily used as a source of energy and for structural purposes in cells.

Types of Carbohydrates

  1. Monosaccharides:

    • Simplest form of carbohydrates.

    • Examples: Glucose, Fructose, Galactose.

    • General formula: (CH2O)n, where n is usually 3-7.

  2. Disaccharides:

    • Formed by the condensation of two monosaccharides.

    • Examples:

      • Sucrose (Glucose + Fructose)

      • Lactose (Glucose + Galactose)

      • Maltose (Glucose + Glucose)

    • Linked by glycosidic bonds.

  3. Polysaccharides:

    • Long chains of monosaccharide units linked by glycosidic bonds.

    • Examples:

      • Starch: Storage form of glucose in plants (made of amylose and amylopectin).

      • Glycogen: Storage form of glucose in animals (highly branched structure).

      • Cellulose: Structural component of plant cell walls (beta-glucose units).

Properties of Carbohydrates

  • Solubility varies: Monosaccharides and disaccharides are generally soluble in water, while polysaccharides are usually insoluble.

  • Sweetness varies: Monosaccharides are sweet, while polysaccharides are tasteless.

  • Energy storage: Glycogen (in animals) and starch (in plants) serve as energy reserves.

Functions of Carbohydrates

  • Energy Source: Immediate energy source for cellular respiration.

  • Energy Storage: Stored in the form of starch in plants and glycogen in animals.

  • Structural Role: Cellulose provides rigidity to plant cell walls, while chitin is a structural component in fungal cell walls and exoskeletons of arthropods.

  • Cell Recognition and Signaling: Carbohydrates attached to proteins and lipids on cell surfaces play crucial roles in cell recognition and signaling processes.

Here are some example past paper questions and answers related to carbohydrates for AQA A-level Biology:

Question 1:

Explain the difference between a monosaccharide and a disaccharide. Provide two examples of each.Answer:

  • Monosaccharides are the simplest form of carbohydrates and consist of single sugar units. Examples include glucose and fructose.

  • Disaccharides are formed from two monosaccharide units joined by a glycosidic bond through condensation. Examples include sucrose (glucose + fructose) and lactose (glucose + galactose).

Question 2:

Describe the structure and function of starch and glycogen.Answer:

  • Starch is a polysaccharide made up of amylose and amylopectin. It serves as the primary energy storage molecule in plants. Amylose has a linear structure, while amylopectin is branched, making starch suitable for energy storage.

  • Glycogen is also a polysaccharide, highly branched and composed of glucose units. It serves as the primary energy storage molecule in animals and can be quickly mobilized when energy is required.

Question 3:

Explain the role of carbohydrates in cell recognition.Answer:

  • Carbohydrates attached to proteins and lipids on the cell surface form glycoproteins and glycolipids. These carbohydrate chains play crucial roles in cell recognition and signaling processes, allowing cells to identify and communicate with each other, which is essential for processes such as immune response and cell adhesion.