Adaptations for Nutrition



Nutrition is 'the process by which organisms obtain nutrients to provide energy to maintain life functions and matter to create and maintain structure'

Organisms are divided into;

  • Autotrophs (producers) - feed on complex organic compounds from simple inorganic molecules using a source of energy
  • Heterotrophs - organisms that feed on complex organic molecules

There are two types of autotrophs;

  • Photoautotrophs - Convert carbon dioxide and water into glucose using light energy from the sun during photosynthesis. Includes green plants, fungi and some bacteris
  • Chemoautotrophs - Use energy derived from special methods of respiration to build up organic molecules. Includes some bacteria
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Heterotrophs (Consumers)

  • Rely on complex organic molecules produced by autotrophs
  • These tend to be large, insoluble moleucules such as polysaccharides e.g. starch and proteins which cannot pass into the plasma membrane
  • Includes animals, some fungi, some protocists and bacteria

There are three types of heterotrophs;

Holozoic feeders

  • Take food into the body and break it down by digestion
  • They have a specialised digestive system 
  • Includes most animals

Types of holozoic feeders;

  • Herbivores - Feed on plants
  • Carnivores - Feed on other animals
  • Detrivores - Feed on dead and decaying material
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Saphrophytic Feeders

  • Digest food outside their body
  • Feed on dead and decaying matter
  • Have simple digestive systems
  • Includes some fungi and bacteria

Saphrophytic digestion

  • Enzymes are secreted onto dead organic matter outside the body
  • The enzymes hydrolyse the bonds in the organic matter to make smaller soluble molecules
  • These smaller molecules are absorbed across the cell membrane by diffusion
  • Microscopic saphrophytes are called decomposers 
  • Their activities are important in the decomposition of leaf litter and the recycling of valuble nutrients like nitrogen
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  • Live on or in another organism (host) and obtain nourishment at the expense of the host
  • No digestive system
  • Highly specialised and can adapt to ways of life 
  • E.g. tapeworm


Adaptations for nutrition

  • Organic molecules must be broken down by digestion and absorbed into the body tissues from the digestive system before they can be used in the body cells
  • Digestion and absorption take place in the gut
  • In simple organisms, feeding on only one type of food, the gut is undifferentiated
  • In advanced organisms, with a varied diet, the gut is divided into various parts along it's length and each part is specialised according to it's function
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Humans, like most animals use holozoic nutrition which comsists of these stages;

  • Ingestion - Taking food into the body through the mouth
  • Peristalsis - Muscular contraction of the gut wall to move food along the gut. Cellulose fibres provide bulk and stimulates peristalsis
  • Digestion - Breaking down large, insoluble food molecules into smaller, soluble ones. This is done by both mechanical and chemical digestion
  • Absorption - The passage of food through the gut wall into the blood
  • Egestion - The elimination of food that cannot be digested
  • The human digestive system consists of the alimentary canal (gut) which is a muscular tube with glands extending from the mouth to the anus
  • In adults it is up to 10m long and divided into distinct parts that have been adapted to carry out different functions
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The function of different tissues in the gut wall

Epithelium - Single layer of cells lining the gut

Mucosa - Contains glands that produce digestive secretions;

  • Digestive enzymes
  • Acid/alkaline fluid to provide the optimum pH for digestive enzymes
  • Mucus (secreted by the goblet cells) lubricates food and protects the gut wall from enzyme action

Sub-mucosa - 

  • Contains blood and lymph vessels to take away absorbed molecules
  • Contains nerves to control the muscles involved in peristalsis
  • In the duedenum, it contains glands that secrete an alkaline mucus
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Involuntary smooth muscle layer

  • Consists of two layers of muscle running in different direction;
  • Inner circular muscle 
  • Outer longitudinal muscle
  • Antagonistic muscles that cause peristalsis to move food along the gut


  • Tough connective tissue that forms a protective coat around the gut

Regions of the gut

Mouth - Chemical and mechanical digestion

  • Mechanical digestion of food begins in the mouth when food is chewed using the teeth 
  • This makes the food easier to swallow and increases it's surface area for enzyme action
  • Chemical digestion also occurs in the mouth
  • As it is chewed, food is mixed with saliva from the salivary glands
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  • Saliva is a watery secretion, containing mucus and amylase together with some mineral ions which help to keep the pH of the mouth slightly alkaline (pH 6.5 - 7.5)
  • Amylase breaks down the starch into maltose
  • After chewing, the food is rolled into a ball called a bolus by the tongue and swallowed. The mucus lubricates it's passage down the oesophagus

The oesophagus

  • Once inside the oesophagus, the bolus of food is pushed down to the stomach by a wave of muscular contraction known as peristalsis
  • Behind the bolus, the muscles lining the walls of the gut work as antagonistic pairs
  • The circular muscles contract and the longitudinal muscles relax, pushing food along in front of the wave of contraction
  • Peristalsis continues throughout the length of the gut
  • Goblet cells in the mucosa secrete musus for lubrication
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The stomach

  • Stores food for up to 4 hours

Main function;

  • Mechanical digestion - there are three layers of muscle that churn the food into liquid chyme 
  • Chemical digestion of protiens begins

Gastric juices secrete;

  • Endopeptidase enzymes that hydrolyse proteins to polypeptides
  • Hydrochloric acid provides the acidic conditions for the enzyme to work (pH2) and kills bacteria in food 
  • Mucus protects the stomach lining from digestive enzymes and acid. It lubricates food to assist movement in the stomach
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The small intestine

Divided into;

  • The duodenum - the first 20cm - main site of chemical digestion
  • The ileum -  up to 6m long and forms the majority of the small intestine - main site of absorption
  • Relaxation of the muscle at the base of the stomach allows small amounts of the partially digested food into the duodenum a little at a time
  • The duodenum recieves secretions of bile from the liver and pancreatic juice from the pancreas
  • The walls of the duodenum also contain brunner's glands that secrete;
  • Alkaline juices - keeps the contents of the small intestine at the correct pH for enzyme action
  • Mucus - lubricates and protects
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  • Sectreted by the liver
  • Stored in the gall bladder
  • Enters the duodenum via the bile duct
  • Contains bile salts which emulsify lipids into smaller droplets increasing the surface area for pancreatic lipase action
  • They also neutralise HCl from the stomach, making the pH of the small intestine slightly alkaline

Pancreatic juice

  • Secreted by the exocrine glands in the pancreas
  • Enters the duedenum via the pancreatic duct

Contains the digestive enzymes;

  • Amylase - Hydrolyse remaining starch into maltose
  • Lipase - Hydrolyse lipids into fatty acids and glycerol
  • Endopeptidases - Hydrolyses proteins to polypeptides
  • Maltase - Hydrolyses maltose into glucose
  • Exopeptidases - Hydrolyse polyptides into amino acids
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  • Main function is absorption
  • Some digestion also takes place;
  • Maltase - Hydrolyses maltose into glucose
  • Endopeptidases - Hydrolyse proteins to polypeptides
  • Exopeptidases - Hydrolyse short chains of polypeptides to amino acids

The large intestine: Colon

  • Absorption of remaining H2O, together with vitamins secreted by microorganisms living in the colon in order to produce solidified faeces
  • The faeces also contains undigested cellulose, bacteria and sloughed cells
  • Undigested cellulose is needed to provide bulk and stimulate peristalsis in the colon
  • This enables movement of faeces into the rectum and to the anus, from where it is egested in a process called defication
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Absorption takes place by simple diffusion, facilitated diffusion, active transport and osmosis. It takes place in the ilium which has several adaptations to increase the rate of diffusion

Large surface area

  • It is very long
  • It is highly folded
  • The mucosa forms finger like projections called villi
  • On the ends of the villi, the individual epithelial cells also have finger like projections called microvilli

Short diffusion pathway

  • The epithelium is one cell thick
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A steep diffusion gradient

  • Blood capillaried. These remove glucose and amino acids, keeping their concentration low
  • Lacteals - (part of lymph system) which remove fatty acids, glycerol  and monoglycerides, keeping their concentration low

Adaptations to different diets


  • Tough plant material must be thoroughly ground up before it is swallowed
  • Grazers e.g. cows and sheep, have small, flat top incisors on the lower jaw only and cuts against a horny pad on upper jaw
  • The canine teeth are not present or indistinguishable
  • The diastema (a gap) between front teeth and premolars. Allows tongue to move freshly cut grass to the large grinding surfaces of the molars
  • The horizontal jaw movement in a circular grinding action
  • Molars interlock in an W M arrangement
  • Teeth are continuously growing due to open, unrestricted roots
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  • Teeth for catching/killing prey, cutting or crushing bones and for tearing meat
  • The sharp incisors grip and tear flesh from bone
  • Large canine teeth , curved and pointed for siezing/killing prey firmly and help in crushing bone
  • Powerful, well developed jaw  movement allowing the jaw to open widely for capturing and killing prey
  • Carnassials (specialised cheek teeth) with a scissor like shearing action for cutting flesh
  • The premolars and molars are for cutting and crushing


  • In total, adult humans have 32 teeth
  • 8 incisors - for biting and cutting
  • 4 canines for biting and cutting
  • 10 premolars and molars for chewing, crushing and grinding
  • Our teeth are not particularly specialsed because we are omnivores
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Herbivore digestion

  • Herbivorous mammals have a diet that consists of cellulose
  • However, mammals do not produce the enzyme cellulase to break down cellulose
  • So, these animals have evolved a symbiotic relationship with bacteria which do produce cellulose. This relationship is called mutualism

Mutualism is 'a close association between organisms of two different species, where both organisms benefit from the relationship'

Herbivores can be divided into ruminants and non-ruminants;

Ruminants (fore-gut digesters)

  • Ruminants are animals with a specialised stomach (rumen) in which mutualistic bacteria live
  • Ruminants have a stomach made up of 4 chanbers
  • Three of the chambers are formed by the oesophagus and one is the true stomach
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The region of the gut occupied by the bacteria is kept seperate from the main digestive region so that;

  • Food can be kept there long enough for the bacteria to digest the cellulose
  • The bacteria are isolated from the cow's own digestive juices so they are in the optimum pH for their activities and not killed by the extremes of pH

Mouth - Grass chopped by teeth, mixed with saliva to form cud and then swallowed

Rumen - Cud is mixed with cellulose digesting bacteria to produce glucose. This is fermented to form organic acids that are absorbed into the blood to provide energy for the cow. Waste products - carbon dioxide and methane are passed out

Reticulum - The cud arrives here before being regurgitated into the mouth and chewed again

Omasum - The cud passes here to reabsorb water

Abomasum - This functionns like a normal stomach and protein is digested

Small intestine - Protein and lipid digestion followed by absorption

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Non Ruminants (Hind-gut digesters)

  • The caecum is enlarged to provide a region for mutualistic micro-organisms to break down cellulose
  • As the digestion of cellulose to glucose occurs after the ilium, the glucose can't be absorbed
  • Hind-gut digesters get round this by re-eating the faeces and absorbing the glucose 'second time around'

Ruminant v Non ruminant

  • Ruminants are more efficient than non-ruminants as the rumen contains a greater variety of mutualistic bacteria than the caecum
  • Ruminants are therefore able to achieve a more complete breakdown of celluose and consequently absorb more glucose

Carnivores have a short gut because protein is easy to digest, Herbivores have a long gut because cellulose is diffucult to digest

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