Digestive System

Parts of Dijestive system

• Mouth
• Teeth
• Tongue
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Rectum
• Anus

There are four steps in the digestion process: 

Ingestion: Ingestion is the process of taking in food through the mouth. In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing the food into bolus). While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well.

The mechanical and chemical breakdown of food: Mechanical digestion involves physically breaking down food substances into smaller particles to more efficiently undergo chemical digestion. The role of chemical digestion is to further degrade the molecular structure of the ingested compounds by digestive enzymes into a form that is absorbable into the bloodstream.

Nutrient absorption: Absorption is a complex process, in which nutrients from digested food are harvested. Absorption can occur through five mechanisms: 

(1) Active transport– Active transport is the process of moving molecules across a cellular membrane through the use of cellular energy. … Active transport is used by cells to accumulate needed molecules such as glucose and amino acids. Active transport powered by adenosine triphosphate (ATP) is known as primary active transport.

Example: uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.

Here are some examples of active transport in animals and humans:

• Sodium-potassium pump (exchange of sodium and potassium ions across cell walls)
• Amino acids moving along the human intestinal tract.
• Calcium ions moving from cardiac muscle cells.
• Glucose moving in or out of a cell.
• A macrophage ingesting a bacterial cell.

(2) Passive diffusion– Passive diffusion is the process by which molecules diffuse from a region of higher concentration to a region of lower concentration. It is the most important mechanism for passage of drugs through membranes. Lipid soluble drugs penetrate lipid membranes with ease.

Example: For example, oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood.

Simple diffusion is the passive movement of solute from a high concentration to a lower concentration until the concentration of the solute is uniform throughout and reaches equilibrium.

(3)Facilitated diffusion:Facilitated diffusion is the transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration with the help of a transport molecule. Since substances move along the direction of their concentration gradient, chemical energy is not directly required.

Example:A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar.

(4) Co-transport (or secondary active transport):In secondary active transport, also known as coupled transport or cotransport, energy is used to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP.

Example:An example of secondary active transport is the movement of glucose in the proximal convoluted tubule.

(5) Endocytosis:Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material.

Endocytosis is the process of actively transporting molecules into the cell by engulfing it with its membrane. Endocytosis and exocytosis are used by all cells to transport molecules that cannot pass through the membrane passively. Exocytosis provides the opposite function and pushes molecules out of the cell.

Example: The three main types of exocytosis are phagocytosis, pinocytosis and receptor-mediated endocytosis. Pinocytosis is non-specific. Phagocytosis targets large structures (e.g., bacteria, food particles…) and is not particularly specific. … Exocytotic pathways also deliver membrane proteins made in cells to the cell surface.

Elimination of indigestible food:The food molecules that cannot be digested or absorbed need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces, is defecation or elimination.