Autotrophic organisms make their own food from the two simple organic raw materials, carbon dioxide and water.
Autotrophic organisms can be:
These are organisms that use light as an energy source and undergo photosynthesis.
e.g. green plants
These are organisms that use energy from chemical reactions.
e.g. All prokaryotes
This is less efficient than photosynthesis and the organisms that do this are no longer dominant life forms.
Heterotrophic organisms cannot generate their own food, and so instead consume large complex organic molecules that have been produced by autotrophs. They are consumers.
They either consume autotrophs directly or organisms that have consumed autotrophs themselves.
For example: animals. fungi.
Types of Nutrition
There are three different types of nutrition you need to be aware of:
- Saprotrophic nutrition
- Parasitic nutrition
- Holozoic nutrition
Saprotrophic nutrition - organisms derive their energy and raw materials for growth from the extracellular digestion of dead or decaying matter.
e.g. fungi and some bacteria.
Saprotrophs/ saprobionts have no specialised digestive system: they secrete digestive enzymes, such as amylases, cellulases, proteases and lipases, on to the dead or decaying matter outside the body for extracellular digestion. They then absorb the soluble products of digestion across their membrane either by diffusion or active transport.
Parasitic and Holozoic nutrition
Parasitic nutrition - obtaining nutrients from another organism, the host.
Holozoic nutrition - This is the feeding method of most animals, and involves ingestion, digestion, absorption and egestion. It occurs internally in a specialised digestive system.
Nutrition in Unicellular Organisms
Amoeba use holozoic nutrition, as digestion occurs intracellularly. They obtain their nutrition by diffusion, active transport or facilitated diffusion across their cell membrane. They take in larger molecules/ microbes by endocytosis, into food vacuoles. Lysosomes then fuse with these food vacuoles and its contents are digested. The soluble products of digestion are then absorbed into the cytoplasm, whilst indigestible remains are egested by exocytosis.
Nutrition in Multicellular Organisms
Single body opening, e.g. Hydra
Hydra are comprised of two layers of cells, separated by a jelly layer containing a network of nervous fibres. They are cylindrical in shape and have tentacles around their mouth. These tentacles discharge and paralyse prey before pulling it through the mouth and into the hollow body cavity. The epithelial cells then secrete lipases and proteases for extracellular digestion. The soluble products of digestion are absorbed by the cells whilst the remains are egested back out the mouth.
Tube gut opening, e.g. Earthworm
Most animals have a tube gut, with two openings.
The Human Digestive System
Ingestion - This is the taking in of food into the body via the mouth.
Digestion - The breaking down of large, insoluble molecules into smaller, soluble molecules.
Absorption - The passage of molecules/ions from the lumen of the gut into the blood.
Egestion - The elimination of waste products.
Functions of the Digestive System
- Mouth - Ingestion and the digestion of starch.
- Oesophagus - passage of food to the stomach by a series of coordinated contractions, called peristalsis.
- Stomach - Mechanical digestion of proteins.
- Duodenum - The digestion of carbohydrates, fats and lipids.
- Ileum - The digestion of carbohydrates, fats and lipids and the absorption of these products.
- Large intestine - Absorption of water
- Rectum - storage of faeces
- Anus - Egestion
General structure of the gut wall
In order from the outside inwards:
- Longitudinal muscle and circular muscle
- muscle of mucosa
Serosa - tough connective tissue that protects the gut wall and reduces friction with other abdominal organs.
The circular and longitudinal muscles perform waves of coordinated contractions (peristalsis). Behind the ball of food, the circular muscles contract whilst the longitudinal muscles relax, pushing the food along.
Submucosa - this contains lymph and blood capillaries to remove the absorbed products of digestion, and has nerves to coordinate peristalsis.
Mucosa - the epithelium secretes mucus to lubricate the passage of food and to protect the mucosa. In some areas, it also secretes digestive juices, and in others, absorbs the products of digestion.
Regional Specialisation of the mammalian gut
1. Buccal cavity/mouth
The buccal cavity and oesophagus
The mouth is the site of mechanical digestion. The food is mixed with saliva by the tongue and chewed with the teeth.
This increases the food's surface area, allowing the enzymes to have more access.
- Amylase, which breaks down starch into maltose.
- HCO3- and CO32- ions, which make saliva slightly alkaline providing an optimum pH for the enzymes to function.
- Mucus, which lubricates the passage of food down the oesophagus.
The oesophagus carries food down from the mouth to the stomach using a series of coordinated contractions, called peristalsis.
The walls of the stomach have folds, called rugae, and an extra layer of muscle, increasing muscle contraction and therefore enhancing the mechanical digestion of proteins.
The walls of the stomach have gastric pits, which secrete gastric juices. These contain:
- Peptidases, secreted by chief cells for the digestion of protein.
- HCl, secreted by oxyntic cells, providing an acidic pH for the enzymes to function effectively and destroyed bacteria.
- Mucus, which is secreted by goblet cells, lubricates the passage of food and lines the stomach wall, protecting it from the digestive enzymes.
The Duodenum and its relationship with the liver a
The Duodenum receives secretions from both the liver and pancreas.
The liver secretes bile, which is stored in the gall bladder and enters the duodenum via the bile duct. It contains bile salts that emulsify fats by reducing their surface tension and breaking larger globules into smaller globules. This increases their surface area making lipid digestion more efficient. Bile is also alkaline, and so neutralises any acid that enters from the stomach, and provides an optimum pH for the enzymes to function effectively.
The pancreas secretes pancreatic juices that enter the duodenum via the pancreatic duct. This contains several enzymes including:
- Peptidases, to hydrolyse proteins to peptides.
- Trypsinogen, an inactive enzyme that is converted into trypsin by enterokinase.
- Lipases, for the digestion of lipids.
- Amylase, for the digestion of carbohydrates.
It also contains sodium hydrogen carbonate, which raises the pH slightly and neutralises any acid from the stomach, and provides a suitable pH for the enzymes to function.
The duodenum also has goblet cells which secrete mucus to lubricate the movement of food, and acid is neutralised by alkaline secretions from the cells at the base of the Crypts of Lieberkuhn, called Brunner's glands.
Adaptations of the small intestine
1. The walls of the small intestine are folded, forming villi. These increase the surface area for greater absorption of the products of digestion.
2. The aforementioned villi have, in turn, microvilli, increasing the surface area further.
3. The villi have a dense supply of blood capillaries to maintain a steep concentration gradient, increasing the efficiency of absorption.
4. The epithelium is one cell thick, shortening the diffusion pathway for the digested products.
5. Each villus has a lacteal for the absorption of fatty acids, glycerol and lipid-soluble vitamins.
The large intestine consists of the colon, the caecum, the rectum and the appendix. It is the main site of the absorption of water and contains mutualistic microbes that secrete vitamin K, which is important for blood clotting and folic acid.