Heterotrophic nutrition

Heterotrophic organisms are organisms that feed on complex ready-made organic food.  They use it as a source of: –

(i)   energy for their vital activities,

(ii)  building materials, that is specific atoms and molecules for cell maintenance and repair and growth,

(iii) vitamins (co-enzymes) that cannot be synthesized in the organism but which are vital specific cellular processes.

The main forms of heterotrophic nutrition include

•  holozoic,
• saprotrophic (or saprophytic) e.g. mold, mushroom
• mutualistic
• parasitic, although some overlap between groups may occur.

Holozoic nutrition

It  is a type of heterotrophic  nutrition involves the following processes

•  Ingestion:   is taking in of complex organic food(solid or liquid).
• Digestion is the breakdown of large complex insoluble organic molecules into small, simple soluble diffusible molecules.  This is achieved by the mechanical breakdown and enzymatic hydrolysis.  Digestion may be either extra or intracellular.
• Absorption: is the uptake of the soluble molecules from the digestion region, across a membrane, and into the body tissue proper.  The food may pass into the bloodstream to be transported to appropriate regions within the body of the organism.
• Assimilation is the utilization of the absorbed molecules by the body to provide either energy or materials to be incorporated into the body.
•  Egestion is the elimination from the body of undigested waste food materials.

Animals which feed one plants are called herbivores, those that feed on other animals carnivores, and those that eat a mixed diet of animal and vegetable matter are termed omnivores. 

If they take in food in form of small particles the animals are microphagous feeders, for example, earthworms, whereas if the food is ingested in liquid form they are, classed as fluid feeders, such as aphids and mosquitoes.  Animals that take in food in the form of large pieces are termed as macrophagous.

Herbivores

Challenges of herbivores while feeding on the plant

(i) Indigestible cellulose

This is overcome in two ways

(a) Ruminant e.g. cow and goat; developed a porch at the anterior end of the stomach called a rumen that provides space for bacterial fermentation of ingested leaves.

(b) Other herbivores such as horse, zebra, rabbit, have well-developed cecum inside which bacteria digest cellulose. Unfortunately, the cecum is ta the end of the intestine that absorption of digested materials may not be efficient. Actually, rabbits have to feed on their feces to obtain enough nutrients from their food.

(ii) Low nutrient value in plant materials: this is overcome by eating a large volume of food.

(iii) Cellulose in plant cell walls is tough and difficult to digest, herbivores like cows and elephants have strong premolar and/or molar with ridges and large surface areas to grind food. These molar and premolar grow throughout life.

Herbivorous molluscs such as the snail possess a rasping organ, the radula.  The radula is like a serrated conveyor belt, which by rubbing backwards and forwards against the hardened roof of the mouth can tear plant food

Herbivorous insects like the locust have a pair of mandibles with a gagged edge for cutting through leaves of grass and other plants

Carnivores

The challenge is not so much digesting the food as obtaining it. 

Adaptation of carnivores to obtaining food

1. high-speed locomotion,
2. sharp claws, and dagger-like canine teeth in the great cats,
3. sucker – bearing tentacles in octopus and squids
4. tentacles armed with stinging cells in sea anemones and jellyfishes.
5. teeth for tearing flesh for instance dog has strong canine and carnassial teeth for tearing meat.

Insectivorous plants.

These insectivorous plants live in nitrogen-deficient soils.  All have green leaves and obtain their carbohydrate by photosynthesis; they obtain nitrogen from trapping and digestion of small animals, particularly insects

Table 1.:  Summary of the functions of the different parts of the human digestive system.

Specialized PART                             FUNCTION

Buccal cavity                                        Ingestion, mastication.

Pharynx                                                Swallowing.

Oesophagus                                          Links pharynx to the stomach

Stomach                                               Food storage and digestion of                                                                         proteins.

Duodenum                                           Digestion and absorption.

Liver (bile)                                           Emulsification of fats.

Pancreas (pancreatic juice)                  Digestion of starch, proteins                                                                         and fats.

Item                                                    Completion of digestion &                                                                       absorption of food

Colon                                                   Absorption of water.

Rectum                                                 Formation and storage of                                                                         faeces.

Anus                                                     Egestion

In the buccal cavity.

The food is broken up into smaller pieces by the chewing action of the teeth (mastication) and moistened by saliva from the salivary glands.

In the stomach.

The stomach is a dilated part of the gut where the food remains for two or more hours

Once in the stomach, the food gastric gland  in gastric wall secrete gastric juice

Gastric juice contains

• Hydrochloric acid  that kills germs and provides pH for the action of pepsin
• Enzyme pepsin which breaks down proteins into short polypeptide chains. Pepsin is secreted as an inactive precursor pepsinogen to prevent the gastric gland from being destroyed by its own enzyme (auto-digestion).
• Enzyme renin coagulates casein, the soluble protein of milk, forming insoluble card which is then attacked by pepsin.  

These enzymes are secreted by the chief (or peptic) cells in the walls of the gastric glands.

  Parts of gastric gland

• Mucous cells secrete mucus that protects stomach wall for pepsin,
• Parietal cells (oxyntic cells) are the epithelial cells that secrete hydrochloric acid (HCl) and intrinsic factor.
• The gastric chief cell (also known as a zymogenic cell or peptic cell) is a cell in the stomach that releases pepsinogen and chymosin. Pepsinogen is activated into the digestive enzyme pepsin when it comes in contact with acid produced by gastric parietal cells.

Duodenum

Receives secretion from pancreas and gall bladder

(a) Gall bladder produces bile that emulsifies fats

(a) Pancreas produces pancreatic juice that contains

• Pancreatic amylase:  breaks down starch to disaccharide maltose.
• Pancreatic lipase:  breaks down triglycerides in the emulsified fat into mono-glyceride and fatty acids.
• Protease: (Protein – splitting enzymes) which include trypsin, chymotrypsin, carboxy-peptidase, and elastase).

The four proteases are secreted as inactive precursors;  trypsinogen,  chymotrypsinogen,  pro-carboxypeptidase, and pro-elastase to prevents auto-digestion.  Trypsinogen is converted into trypsin by the action of the enzymes enterokinase, secreted by the wall of the small intestine.  The trypsin then activates the other three proteases.  This pancreatic protease breaks down proteins and polypeptides into tripeptides and dipeptides.

Pancreatic juice also contains nucleases that break down nucleic acids into nucleotides and a variety of peptidases that release some free amino acids from polypeptide chains.

• Bicarbonate neutralizes acidic chime from the stomach

Intestinal enzymes

Various enzymes, associated with the epithelial lining of small intestine complete the digestion of carbohydrates by breaking down disaccharides into monosaccharides.

These enzymes include: –

• Maltase: hydrolyzes maltose to glucose, thus completing the digestion of starch.
• Sucrase hydrolyzes sucrose (Sugar cane) to glucose & fructose.
• Lactase hydrolyzes lactose (Milk sugar) to glucose & galactose.

The end products of carbohydrate digestion are all mono-saccharides.  The final stage of carbohydrate digestion is intra-cellular, as disaccharides are absorbed by the plasma membrane of the epithelial cell before broken down into monosaccharides.

The epithelial cells also absorb tripeptides and dipeptides which are then broken down into amino acids by various peptidases.  Thus, the final stages of protein digestion are also intra-cellular.

Nucleotidases:  are also present in the epithelial cells of the small intestine.  They split nucleosides into constituent submits.

Brunner’s gland

Found in the walls of duodenum, secrete alkaline mucus which:

• protect the duodenum from the acidic content of chyme (which is introduced into the duodenum from the stomach)
• provide an alkaline condition for the intestinal enzymes to be active, thus enabling absorption to take place;
• lubricates the intestinal walls.

At the bottom of Brunner’s gland are cells called Crypts of lieberkutin that secrete an enzyme for final digestion.

Adaptation of small intestine for absorption of food

1. Long to allow food enough time for absorption
2. Villi and microvilli increase surface area for absorption
3. Well supplied by blood to carry away absorbed food so as to maintain diffusion gradient
4. The villi have a thin membrane to reduce diffusion gradient
5. Villi have a high concentration of mitochondria to provide energy for active transport.
6. circular fold increase surface area for absorption
7. Villi contain lacteal for the absorption of fats

Control of digestion

The regulation of food intake is complex. It involves gut hormones and the enteric nervous system as well as the brain.

The cells in the lining of the stomach and small intestine produce and release hormones that control the functions of the digestive system. These hormones stimulate the production of digestive juices and regulate appetite.

These hormones include

• Ghrelin is produced in the stomach, and its function is to tell the brain that the body has to be fed. It increases appetite.

• Gastrin is produced in the stomach when it is stretched. It
  • Stimulates the release of gastric juice rich in pepsin and hydrochloric acid.
  • Stimulates of smooth muscle contraction in the stomach, small intestine, and large intestine, which increases gastric and intestinal motility.
  • Relaxation of the pyloric sphincter, which promotes gastric emptying into the small intestine.
  • Acidic pH levels reduce the levels of gastrin.

• Secretin is produced in the duodenum and
  • stimulates the pancreas to produce alkaline secretions (rich in bicarbonate ions)
  • slows the emptying of the stomach
  • stimulates the liver to produce bile.
• Cholecystokinin (CCK) is produced in the duodenum.
  • It reduces appetite,
  • slows down the emptying of the stomach.
  •  stimulates the release of bile from the gall bladder.

• Peptide YY (PYY) is produced in the last part of the small intestine known as the ileum as well as parts of the large intestine. It plays a role in slowing down the passage of food along the gut, which increases the efficiency of digestion and nutrient absorption after a meal and inhibits appetite.

• Glucagon-like peptide 1 (GLP-1) is produced in the small intestine and colon and has multiple actions including inhibition of gastric emptying and appetite as well as the stimulation of insulin release.
• Motilin — stimulates the production of pepsin and speeds up peristalsis.

Nerve Regulators

Two types of nerves help control the action of the digestive system: extrinsic and intrinsic nerves.

Extrinsic, or outside, nerves connect the digestive organs to the brain and spinal cord. These nerves release chemicals that cause the muscle layer of the GI tract to either contract or relax, depending on whether food needs digesting.

The intrinsic, or inside, nerves within the GI tract are triggered when food stretches the walls of the hollow organs. The nerves release many different substances that speed up or delay the movement of food and the production of digestive juices.

For questions and answers; downloads

Sponsored by The Science Foundation College + 256 753 892709

Compiled by Dr. Bbosa Science