Nutrition and Digestion: A Comprehensive Look

Posted on Feb 13, 2025 in Biology

Nutrition and Substance Exchange

Nutrition involves the exchange of matter and energy between organisms and their surrounding environment. To perform this exchange, different tracts and systems work together to obtain nutrients and oxygen, which are distributed to each cell of the body. They also expel waste substances out of the body.

Substance Exchange in Unicellular Organisms

Unicellular organisms exchange substances directly with their surrounding environment through a plasma membrane.

Substance Exchange in Multicellular Organisms

Most cells in multicellular organisms don’t come into contact with the outside world and cannot exchange substances with it directly. The nutrition function is performed through the joint action of different systems that act as intermediaries between the outside world and cells.

Key Systems Involved in Nutrition

  1. The Digestive System: Transforms the food we eat into simpler substances (nutrients), which are absorbed and passed into the bloodstream.
  2. The Respiratory System: Takes O2 from the air and transfers it to the blood. It also takes CO2 from the blood and expels it out of the body.
  3. The Circulatory System: Uses blood to carry nutrients and O2 to the cells. It also transports waste from the cells to the excretory organs, where they are expelled.
  4. The Lymphatic System: Works with the circulatory system to transport substances.
  5. The Excretory System: Extracts waste substances produced by cellular activity from the blood before expelling them from the body.

The Digestive Tract

  • The Mouth: Cavity where food is chewed and ground by the teeth.
  • The Pharynx: (digestive and respiratory tracts). The epiglottis acts as a lid and closes the respiratory tract to prevent food from blocking it.
  • The Oesophagus: Food travels down to the stomach as a result of contractions in its walls (peristaltic movements).
  • The Stomach: It has strong muscles in its walls and glands that secrete gastric juices. It is connected to the small intestine through an opening called the pylorus and to the oesophagus through the cardia.
  • The Small Intestine: Long tube with glands that secrete intestinal juices and folds called intestinal villi (nutrients pass into the bloodstream through these capillaries).
  • The Large Intestine: Composed of three parts: the caecum, colon, and rectum (which leads to the anus where faeces are expelled).

Accessory Glands

Accessory glands are located outside the digestive tract, although the digestive juices they secrete are passed into it. These juices contain a great variety of digestive enzymes (types of proteins that accelerate the breakdown of complex food into nutrients).

  • The Salivary Glands: Three pairs of glands that secrete saliva into the mouth.
  • The Liver: The largest gland. It secretes bile, which is stored in the gallbladder. Bile helps us to digest lipids by acting as a detergent. The liver also performs other functions such as storing glucose, iron, and vitamins, and eliminating toxic substances (alcohol) from the blood.
  • The Pancreas: An arrowhead-shaped gland found under the stomach with two functions: digestive (pancreatic juices) and endocrine.

Digestion

The digestive process consists of various different stages: mechanical digestion, chemical digestion, absorption, and egestion or defecation.

Mechanical Digestion

Mechanical digestion involves processes designed to reduce the size of food particles and make them travel through the digestive system. It consists of three stages:

  • Mastication: Tearing, cutting, chewing, and grinding the food. It is carried out by the teeth and the movement of the lower jaw.
  • Insalivation: Mixing the ground-up food with saliva by moving the tongue. A bolus is formed as a result.
  • Deglutition: Using peristaltic movements to make the bolus travel through the pharynx and oesophagus to the stomach.