Nutrition in human / Metabolism and enzymes / Movement of substances across the cell membrane / Nutrients / The variety of living

Your cell membrane is a vital boundary, separating the inside of your cell from its surroundings. It's not a solid wall but a flexible, fluid structure.

The main component is the phospholipid bilayer. Imagine two layers of tiny molecules called phospholipids. Each phospholipid has a "head" that loves water (hydrophilic) and two "tails" that hate water (hydrophobic). They arrange themselves with their water-hating tails facing inwards, away from the watery environments inside and outside the cell, and their water-loving heads facing outwards. This creates a stable barrier.

You know that everything around you, including yourself, is made of matter. In biology, we focus on specific types of matter called biological molecules, often called macromolecules because they're really big. These big molecules are polymers, meaning they're chains made of smaller, repeating units called monomers. Think of a pearl necklace: the whole necklace is the polymer, and each individual pearl is a monomer.

• Carbohydrates:
  • Function: Primary source of energy, especially for your brain and muscles.
  • Sources: Grains (bread, rice, pasta), fruits, vegetables, legumes, dairy.
  • Types: Simple (sugars) and complex (starches, fiber).
• Proteins:
  • Function: Essential for building and repairing tissues, making enzymes and hormones, and supporting immune function.
  • Sources: Meat, fish, eggs, dairy, legumes, nuts, seeds, soy products.
• Fats (Lipids):
  • Function: Concentrated energy source, help absorb fat-soluble vitamins, protect organs, and maintain cell membranes.
  • Sources: Oils, butter, nuts, seeds, avocados, fatty fish.
  • Types: Saturated, unsaturated (mono- and poly-), trans fats (best to limit).

When the substrate binds to the active site, it forms an enzyme-substrate complex. This binding often causes a slight change in the enzyme's shape (called induced fit), which further optimizes the fit and helps the enzyme perform its catalytic action. Once the reaction is complete, the enzyme releases the products, and its active site is ready to bind to another substrate molecule.

• Mouth: Chewing (mechanical) and saliva (chemical – amylase for starch). Forms a bolus.
• Oesophagus: Muscular tube that uses peristalsis (wave-like contractions) to push the bolus to the stomach.
• Stomach: Churns food (mechanical) and releases gastric juice (chemical – HCl and pepsin for protein). Forms chyme.
• Small Intestine: The main site for chemical digestion and absorption. Has three parts: duodenum, jejunum, and ileum. Receives bile from the liver/gallbladder (for fat emulsification) and pancreatic enzymes from the pancreas (for carbs, proteins, fats). Its inner lining has villi and microvilli, increasing surface area for absorption.
• Large Intestine: Primarily absorbs water and electrolytes. Forms faeces.
• Anus: Where faeces are expelled.