EOY IGCSE BIOLOGY – SECTION 3: HUMAN NUTRITION, TRANSPORT, DISEASES AND IMMUNITY

Components of a Balanced Diet

• A balanced diet is defined as one that contains carbohydrates, proteins, fats, vitamins, minerals, fibre, and water, all present in the correct amounts.

• Carbohydrates: These molecules provide the energy required for various biological processes.

• Proteins: These are essential for the growth and repair of body tissues.

• Fats: Used to store energy and provide insulation for the body.

• Iron: A mineral necessary for the production of haemoglobin in red blood cells.

• Calcium: A mineral required for the development and maintenance of strong bones and teeth.

The Process and Types of Digestion

• Digestion is the biological process of breaking down large insoluble food molecules into small soluble molecules that are capable of being absorbed by the body.

• Physical Digestion:

  • Involves the mechanical breakdown of food into smaller pieces.

  • Does not change the chemical composition of the food.

  • Occurs through chewing in the mouth and churning within the stomach.

• Chemical Digestion:

  • Uses enzymes to break down food molecules through chemical reactions.

  • Amylase: An enzyme that breaks down starch into maltose.

  • Protease: An enzyme that breaks down proteins into their constituent amino acids.

  • Lipase: An enzyme that breaks down fats into fatty acids and glycerol.

Nutrient Absorption and the Small Intestine

• The majority of absorption takes place in the small intestine.

• Villi: These are finger-like projections located on the inner lining of the small intestine that serve to increase the surface area available for absorption.

• Structural features of a villus:

  • Thin walls: Allow for a short diffusion path.

  • Rich blood supply: Ensures that absorbed nutrients are transported away quickly, maintaining a steep concentration gradient.

The Heart and the Double Circulatory System

• Humans possess a double circulatory system, meaning blood passes through the heart twice during a single complete circulation of the body.

• The heart is divided into four distinct chambers:

  • Right atrium

  • Right ventricle

  • Left atrium

  • Left ventricle

• The left ventricle is characterized by having the thickest muscular wall of all chambers because it is responsible for pumping blood to the entire body, necessitating higher pressure.

Classification and Function of Blood Vessels

• Arteries: These vessels carry blood away from the heart. They operate under high pressure and possess thick, elastic walls to accommodate this pressure.

• Veins: These vessels carry blood back toward the heart. They operate under low pressure and contain valves, which are structures designed to prevent the backflow of blood.

• Capillaries: Highly narrow vessels with walls that are only one cell thick. This thinness allows for the efficient exchange of substances between the blood and surrounding tissues.

Composition and Roles of Blood

• Red Blood Cells: Function to carry oxygen throughout the body using the protein haemoglobin.

• White Blood Cells: Responsible for defending the body against disease. They achieve this by destroying pathogens and producing antibodies.

• Platelets: These cell fragments are responsible for the process of blood clotting.

• Plasma: The liquid component of blood that transports various substances, including:

  • Nutrients

  • Hormones

  • Carbon dioxide ($CO_2$)

  • Waste products

Diseases, Pathogens, and Natural Defences

• Pathogens: Microorganisms that cause disease. The primary categories include:

  • Bacteria

  • Viruses

  • Fungi

  • Protoctists

• Natural Body Defences: The body has several innate barriers to infection, including the skin, mucus, stomach acid, and white blood cells.

• Antibodies: Specific proteins produced by white blood cells that are designed to target and destroy pathogens.

Active and Passive Immunity

• Active Immunity:

  • Occurs when the body’s own immune system produces antibodies following an infection or a vaccination.

  • This form of immunity typically provides long-lasting protection.

• Passive Immunity:

  • Occurs when the body receives antibodies from an external source rather than producing them internally.

  • Examples include antibodies received via breast milk or an antiserum.

  • The protection provided is immediate but only temporary.

Specific Infectious Diseases: Cholera and HIV

• Cholera:

  • A disease caused by bacteria.

  • Spread specifically through contaminated food or water supplies.

  • Symptoms include severe diarrhoea and subsequent dehydration.

  • Prevention is achieved through good sanitation practices and ensuring clean water supplies.

• HIV (Human Immunodeficiency Virus):

  • A virus that targets the immune system by destroying specific types of white blood cells.

  • Transmission Methods:

    • Unprotected sexual intercourse.

    • Contact with infected blood.

    • Sharing of needles.

    • Transmission from mother to baby.

  • Non-transmission Modes: HIV cannot be spread through hugging, sharing food, or other forms of casual contact.