WHAT IS HOLOZOIC NUTRITION? STAGES AND CHARACTERISTICS - ANATOMY AND PHYSIOLOGY - 2025

The holozoica nutrition is a method involving nutrition intake liquid or solid organic material, digestion, absorption and assimilation for use as an energy source in the body. Holozoic nutrition includes taking complex substances and converting them into simpler forms. For example, proteins can be divided into amino acids.

This method suggests phagocytosis where the cell membrane completely surrounds a food particle. Most free living animals, including humans, exhibit this type of nutrition.

In this mode of nutrition, the food can be a small bacterium, a plant or an animal. Holozoic nutrition is the process used by most animals. In this process, food that is ingested as a solid particle is digested and absorbed.

Holozoic nutrition can be further classified in terms of food source: herbivores, like cows, obtain food from plants; carnivores, like wolves, obtain nutrients from other animals; omnivores, like man, use both plants and animals for food.

Stages of the holozoic nutrition process

There are five stages in the holozoic nutrition process that most higher invertebrates and vertebrates make use of.

1. Ingestion

Ingestion is the act of consuming any substance, be it liquids, food, drugs, poisons, pathogenic bacteria, or even indigestible nutritional elements.

Simply put, ingestion simply refers to the act of introducing any substance into the digestive system.

The food is introduced as large or small particles. This can be by specialized organs such as the mouth in higher animals or by the general surface of the body with the help of structures such as pseudopods in lower organisms (such as amoebae). Ingestion of pseudopods is called phagocytosis.

2. Digestion

Digestion is defined as the process by which complex food molecules are broken down into simpler molecules so that they can be absorbed by the body. Digestion can be mechanical or chemical.

In mechanical digestion, food is physically broken down into smaller particles through processes such as chewing.

Chemical digestion, meanwhile, makes use of certain chemicals called enzymes. They are proteins that help in the simplification of the nutritional matter.

The required enzymes are secreted by the body itself depending on the type of food to be digested.

Enzymes break covalent bonds in food molecules and release energy. This reaction is chemically called hydrolysis and is the breakdown of a bond by the addition of the water molecule. The enzymes that catalyze these reactions are therefore called hydrolases.

Digestion converts food into soluble form. This is done in order to absorb food into cells. Foods like glucose and vitamin C, which are already small and water soluble, do not need to undergo digestion. They can enter cells directly.

Digestion can take place outside of cells (extracellular) or within cells (intracellular). In unicellular organisms, digestion is intracellular with the enzymes present in the vesicles.

In more advanced multicellular forms, digestive enzymes are secreted out into the surrounding environment. The digested products are absorbed back into the cell.

In higher invertebrates and vertebrates, digestion takes place in a separate specialized channel called the alimentary canal.

In lower organisms such as Hydra, ingestion and excretion take place through the same opening. Features such as ingestion and excretion with different openings and each portion of the channel with specific enzymes targeting specific types of food increase the efficiency of the digestive system.

3. Absorption

This involves the absorption of food in the soluble form from the region of digestion into the tissues or into the bloodstream that transports it to the different tissues. This occurs through cell membranes. Absorption can be passive or active.

Passive absorption is through diffusion or osmosis without using energy. It takes place in both directions. For example, water is absorbed by osmosis. Active absorption requires energy and can be inhibited by poisons such as cyanide. It only takes place in one direction.

The small intestine is 5 to 6 meters in length, and most chemical digestion occurs within the first meter. Once the food has been digested into smaller molecules, absorption can take place.

Millions of tiny finger-like structures, called villi, project inward from the lining of the small intestine.

These structures greatly increase the contact surface that the products of digestion have with the small intestine, allowing their rapid absorption into the bloodstream. Once absorbed, they are transported to the liver through the hepatic portal vein.

4. Assimilation

The digested food is assimilated by the cellular cytoplasm by diffusion. Food vacuoles are constantly moving in the cytoplasm to deliver digested food to each part of the body through cells.

Assimilation involves the utilization of nutrients that have been acquired from food for various functions of the body.

5. Excretion

Upon reaching the end of the small intestine, all digested food products, along with minerals and vitamins that are useful to the body, should have been removed from the watery contents, that is, they should have been assimilated to benefit the body.

What is left consists of the indigestible components of food such as cellulose from consuming plant-based foods. These materials are then passed into the large intestine.

In the large intestine the following functions are carried out:

• Recover water and electrolytes (sodium, chloride) from indigestible food material.
• Form and store stool.
• Fermenting some of the food material indigestible by bacteria.
• Maintain a bacterial population.

As the undigested material accumulates in the rectum, it stimulates a response that leads to the evacuation of waste through the anus.

References

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