Stomach: The stomach is a J-shaped organ that acts as a storage reservoir for food and initiates the digestion of proteins. Its muscular walls churn and mix food with gastric juices, which contain hydrochloric acid (HCl) and pepsin. HCl denatures proteins, unfolding their complex structures and making them more accessible to pepsin. Pepsin, a protease enzyme, begins the breakdown of proteins into smaller peptide chains. The stomach lining is protected from the acidic environment by a layer of mucus. The stomach's muscular walls allow for powerful contractions to mix and churn the food, increasing the surface area for chemical digestion.

Small Intestine: The small intestine is the primary site for the digestion and absorption of nutrients. It is divided into three sections: the duodenum, jejunum, and ileum. The duodenum receives chyme (partially digested food) from the stomach, along with digestive juices from the pancreas and liver. The pancreas secretes enzymes like amylase (for carbohydrates), lipase (for fats), and protease (for proteins) to further break down food. The liver produces bile, which emulsifies fats, increasing their surface area for lipase action. The jejunum and ileum are specialized for absorption. They have a large surface area due to circular folds, villi, and microvilli. The villi contain blood capillaries and lacteals (lymphatic vessels) which absorb digested nutrients. Nutrients are absorbed into the blood stream via the capillaries and into the lymphatic system via the lacteals.

Mouth: The mouth is the entry point for food into the alimentary canal. It's involved in mechanical digestion through chewing (mastication) and chemical digestion through the action of salivary amylase, which begins the breakdown of starch.

Oesophagus: The oesophagus is a muscular tube that connects the mouth to the stomach. It transports food via peristalsis, a series of rhythmic muscle contractions, to the stomach.

Stomach: The stomach is a J-shaped organ that stores food and mixes it with gastric juices (hydrochloric acid and pepsin). This initiates chemical digestion of proteins. The stomach also regulates the release of chyme (partially digested food) into the small intestine.

Small Intestine: The small intestine is the primary site of nutrient absorption. It's divided into the duodenum, jejunum, and ileum. Here, enzymes from the pancreas and bile from the liver/gallbladder further digest carbohydrates, proteins, and fats. Nutrients are absorbed into the bloodstream through the intestinal walls.

Large Intestine: The large intestine absorbs water and electrolytes from the undigested material, forming faeces. It also houses bacteria that produce some vitamins. The large intestine then stores the faeces until elimination.

Assimilation refers to the process by which absorbed nutrients are used by the body for growth, repair, and energy. It involves the uptake of digested food molecules into cells and their subsequent utilization in metabolic processes. This is intricately linked to absorption, which is the process by which nutrients pass from the lumen of the small intestine into the bloodstream or lymphatic system.

Absorption Mechanisms:

• Simple Diffusion: Small, lipid-soluble molecules like fatty acids and some vitamins can diffuse directly across the epithelial cells of the small intestine lining.
• Facilitated Diffusion: Some molecules require a carrier protein to cross the cell membrane. This process doesn't require energy. For example, fructose absorption.
• Active Transport: This requires energy (usually ATP) to move molecules against their concentration gradient. Glucose and amino acids are absorbed via active transport. This process often involves carrier proteins.
• Endocytosis: Large molecules, such as antibodies, can be absorbed by endocytosis, where the cell membrane engulfs the molecule.

Transport Mechanisms:

• Bloodstream: Monosaccharides, amino acids, water-soluble vitamins, and water are absorbed into the blood capillaries within the villi. These are then transported to the liver via the hepatic portal vein.
• Lymphatic System: Fatty acids and glycerol are absorbed into the lacteals, which are lymphatic vessels within the villi. These are then transported to the bloodstream.

Once in the bloodstream or lymphatic system, nutrients are transported to various tissues and cells throughout the body, where they are used for energy, growth, and repair. The liver plays a crucial role in processing and storing some of these nutrients.