2.4

• Modes of nutrition


  • Autotrophic
  • Heterotrophic
• Autotrophic organisms


  • Photoautotrophic
  • Chemoautotrophic
• Photoautotrophic


  Use light energy to perform photosynthesis. Examples include green plants, Protoctista and some bacteria
• Chemoautotrophic


  Use energy from chemical reactions
• Heterotrophic organisms


  • Saprotrophic
  • Parasitic
  • Holozoic
• Saprotrophic


  Feed on dead or decaying matter by secreting enzymes extracellularly and then absorbing the products, eg Rhizopus (bread mould)
• Parasitic


  Obtain nutrition from another living organism called the host over a long period of time, whilst causing it harm. Endoparasites live within the host's body, eg. tapeworm, Taenia. Ectoparasites live on the surface, e.g human head louse, Pediculus
• Holozoic


  Form of nutrition used by most animals where they ingest and then digest food absorbing nutrients. They possess a specialised digestive system. Examples include herbivores (plant material), carnivores (animal material), omnivores (plant and animal material) and detritivores (dead or decaying material)
• Protoctista such as amoeba


  Holozoic heterotrophs. They absorb nutrients directly through their cell membrane by diffusion, ingesting larger molecules by endocytosis and fluids by pinocytosis into food vacuoles. Lysosomes fuse with the vacuoles and release digestive enzymes. Nutrients are absorbed through the membrane of the food vacuole and waste is egested by exocytosis
• Organisms with a single body opening


  • Tentacles paralyse prey and move it into the hollow body cavity through the mouth. Protease and lipase enzymes digest the food extracellularly, and the products are absorbed before the indigestible remains are egested back out through the mouth
• More developed organisms


  • Possess a tube gut, ingesting at one end, egesting at the other, with the most advanced possessing specialised regions
• Functions of the gut


  • Ingestion
  • Digestion
  • Absorption
  • Egestion
• Ingestion


  Taking food into the body via the mouth bringing it into contact with the digestive surface
• Digestion


  Results in large biological molecules being hydrolysed to smaller molecules that can be absorbed across cell membranes. Digestion begins with mechanical digestion in the mouth involving the teeth, which breaks large food pieces into smaller pieces. Digestion is then completed by enzymes
• Absorption


  Passage of nutrient molecules through the wall of the gut into the blood
• Egestion


  Elimination of undigested material, e.g. cellulose fibre
• Layers of the gut wall


  • Serosa
  • Muscle
  • Submucosa
  • Mucosa
• Serosa


  Outermost layer consisting of tough connective tissue, which protects the gut and reduces friction from other abdominal organs
• Muscle

  Consists of two layers: circular and longitudinal smooth muscles which contract in a coordinated fashion to push food along by peristalsis
• Submucosa


  Connective tissue containing blood and lymph vessels to take away the absorbed products of digestion. Nerves are also present, which co-ordinate muscular contractions
• Mucosa


  Innermost layer lining the gut; it secretes mucus (lubrication and protection from enzymes). Depending upon the region, it secretes enzymes and absorbs digested food and nutrients
• Digestion of food groups


  • Carbohydrates
  • Proteins
  • Fats
• Carbohydrate digestion


  Amylase hydrolyses starch to maltose, and then maltase hydrolyses maltose to glucose. Sucrase hydrolyses sucrose to glucose and fructose. Lactase hydrolyses lactose to glucose and galactose
• Protein digestion


  Proteins are digested into polypeptides, dipeptides and eventually amino acids. The enzymes involved are peptidases, named according to where they break peptide bonds
• Fat digestion


  Fats are emulsified by bile and then hydrolysed to fatty acids and glycerol
• Regions of the mammalian gut


  • Mouth (buccal cavity)
  • Oesophagus
  • Stomach
  • Duodenum
  • Ileum
  • Large intestine
• Mouth (buccal cavity)


  Digestion begins here. Teeth mechanically digest food, the tongue contains the enzyme, amylase, and mucus, which lubricates the food. The saliva mixes this with saliva and rolls it into a bolus for swallowing. The saliva enzyme amylase initiates starch digestion
• Oesophagus


  Muscles contract to move the food towards the stomach via peristalsis
• Stomach


  Food is digested for about four hours by muscular action from the stomach walls and gastric juice, which contains hydrochloric acid (from oxyntic cells in gastric pits) and pepsin. Pepsin is an endopeptidase that is secreted in an inactive form pepsinogen, and activated by H+ ions: this prevents pre-digestion. The acidic pH of around 2 also kills bacteria. Mucus is produced by goblet cells in the gastric pits, which lubricates food and protects the lining
• Duodenum


  Receives secretions from the liver and pancreas. Bile, which contains bile salts, neutralises acidic food from the stomach and emulsifies fats. Pancreatic juice is slightly alkaline (due to the presence of sodium hydrogen carbonate) and contains endopeptidases and trypsinogen (which is inactive, and converted to the active form trypsin by enterokinase), amylase and lipase. Brunner's glands at the base of the crypts of Lieberkühn produce alkaline secretions that also neutralise acidic food from the stomach. The mucosa of the small intestine is heavily folded to form villi. In the duodenum, endopeptidases and exopeptidases are secreted by cells at the tips of the villi, and peptidases bound to epithelial cells complete the digestion to amino acids. Maltase, lactase and sucrase enzymes are also bound to the epithelial cells and complete the digestion of carbohydrates
• Ileum


  Responsible for the absorption of digested food. Villi and microvilli greatly increase the surface area of the ileum (over 600 times) for absorption by diffusion, facilitated diffusion, co-transport and active transport and for the action of membrane bound enzymes. Glucose enters into epithelial cells by co-transport and active transport and by facilitated diffusion into capillary of villus. Amino acids enter by active transport into epithelial cells and then by facilitated diffusion into the capillary of villus. Fatty acids and glycerol enter epithelial cells via diffusion where they recombine into triglycerides and enter lacteal of villus. Water is absorbed by osmosis into epithelial cells and the capillary of villus. Water-soluble vitamins (e.g. B and C) are absorbed directly into the blood, whilst fat-soluble vitamins (e.g. A, D and E) are absorbed into the lacteals by diffusion
• Large intestine


  Has small villi present and is responsible for absorption of water and the formation of faeces, which is stored in the rectum until it is egested. Mutualistic bacteria present in the colon are responsible for the production of vitamin K and folic acid. Glucose and amino acids are transported by the hepatic portal vein to the liver where they are processed. The lacteals drain into the lymphatic system which drains into the blood via the thoracic duct in the right subclavian vein
• Carnivores


  • Sharp incisors to tear flesh
  • Pointed canines to pierce flesh and kill prey
  • Specialised molars (carnassials) that shear flesh and bone
  • Powerful jaw muscles that move the lower jaw vertically up and down
  • Able to open jaws wide to accommodate large prey
  • Relatively short intestines
• Herbivores


  • Incisors and canine teeth that slice through vegetation
  • Some lack incisors in upper jaw, instead have a horny pad
  • Gap (diastema) allows food mixing during chewing
  • Interlocking molar teeth with sharp enamel ridges
  • Teeth worn down and grow continuously
  • Jaws able to move sideways to aid grinding
• Ruminants


  • Highly modified oesophagus with 3 chambers including rumen
  • Grass first chewed into 'cud' which enters rumen and mixes with cellulose-digesting bacteria
  • Bacteria anaerobically respire cellulose producing glucose, organic acids, CO2 and methane
  • Cud regurgitated, re-chewed, then swallowed again
  • Organic acids absorbed in omasum
  • Bacteria killed and digestion begins in abomasum (true stomach)
  • Water absorbed in large intestine
• Parasites


  Live in (endoparasites) or on (ectoparasites) a host organism, causing it harm
• Pork tapeworm (Taenia solium)


  • Has two hosts - human (primary) and pig (secondary)
  • Evolved adaptations like suckers and hooks to attach to gut wall
  • Thin body with large surface area to volume ratio to maximise absorption
  • Produces enzyme inhibitors to prevent digestion
  • Thick cuticle to protect from immune responses
  • Has male and female reproductive structures
  • Produces vast numbers of eggs to increase chances of finding new host
  • Eggs have resistant shells to survive until eaten by secondary host
  • No digestive system, absorbs host's digested food directly through cuticle
• Head louse (Pediculus)


  • Wingless insect that can only pass to new host via direct contact
  • Legs adapted as claws to grip onto hairs
  • Lays eggs glued to base of hairs
• Autotrophic


  Synthesis of complex organic chemicals from inorganic substances using an energy source