Unit 2.4

• Heterotrophs cannot synthesise their own complex organic molecules, so the must digest the necessary chemicals produced by other organisms.
• Autographs produce their own complex organic products from an energy source.
• Chemoautotrophs use chemical energy to synthesise complex organic chemicals from inorganic chemicals. Photoautotrophs (plants) are the same but use light energy instead of chemical energy. All photoautotrophic organisms photosynthesise
• Saprotrophs will undergo extra cellular digestion of dead or decaying matter. Enzymes are secreted by the organisms to digest the organic chemicals. The products are then absorbed by the saprotroph.
• Parasites live on or in a host organism, from which they obtain nourishment. This usually has no benefit or is harmful to the host organism.
• Symbiosis is when organisms live on or in other organisms but both benefit from this relationship.
• Holozoic nutrition involves the ingestion and then digestion of a food source. The organism does not have to remain attacked to its food source in order to digest it.
• Endoparasites live in the host organism, exoparasites live on the host organism.
• A primary host is where the adult forms of parasites develop
• Secondary hosts are where larvae or intermediate forms of the parasites are found.
• Vectors are secondary hosts which actively and directly transfer parasite from one primary host to another.
• A tapeworm is a type of endoparasite. It has a range of adaptions.
• The head of a tapeworm is called the scolex. It has hooks and suckered which it uses to attach to the gut wall. The scolex is also embedded in the gut wall. These adaptions prevent the tapeworm from being dislodged by peristalsis.
• Tapeworms contain a long chain of proglottids. They are hermoaphrodites, meaning they contain both female and male gametes. They can self fertilise and do not need to sexually reproduce.
• Tapeworm proglottids are covered by a thick cuticle which resists digestive enzymes. They secrete enzyme inhibitors (preventing their digestion) and neutralise secretions of the gut.
• Tapeworms are long and flat. This increases the surface area for absorption and decreases the diffusion distance for the digested particles.
• A digestive system is not needed as tapeworms are surrounded by predigested food, which diffuse through its external surface.
• Tapeworms undergo anaerobic respiration as there is no oxygen in the gut lumen (where the tapeworm resides).
• The serosa is the layer of connective tissue carrying blood and lymphatic vessels as well as nerves.
• The longitudinal muscles are the outer muscles, and the circular muscles are th inner muscles. They are both responsible for peristalsis.
• The submucosa contains glands and carries the main arterioles amd venules
• The mucosa is covered by a layer of epithelial cells, some produces mucus whereas others absorb nutrients.
• Muscularis mucosa is a thin muscle which helps move the inner wall of the gut.
• The duodenum and ileum are both adapted for efficient digestion and absorbtion. They have folds in their walls to increase the surface area. These are further folded into villi. Villi are covered in microvili on epithelial cells, to increase the surface area even more.
• The ileum is the mainly the site of absorption, so it contains longer villi. The duodenum is mainly the site of digestion, so shorter villi are found there.
• Villi are adapted to increase absorption efficiency. They are thin, reducing the diffusion distance. They have laterals, which absorb the products of fat digestion, and so maintain concentration gradients. They contain mircovilli to increase their surface area. They have a rich capillary network to absorb and remove the products of digestion, maintaining concentration gradients. Epithelial cells contain lots of mitochondria which produce lots of ATP for active transport.
• Incisors are involved in biting and tearing flesh (in carnivores)
• Canines are for gripping prey (in carnivores)
• Molars and premolars are adapted to gring plant tissue iflr shear flesh off the bone depending on the organisms diet.
• A carnivores jaw moves up and down as animal tissue is easier to digest. Most carnivores don't chew their food before swallowing, however they still need strong jaw muscles to hold their prey or even crack bones.
• Herbivores move their jaws side to side as well as up and down. This is due to their plant based diet, as cellulose is hard to digest. Plant tissue requires herbivores to grind their food in order to break down the cellulose
• Herbivores have no incisors on their upper jaw. The presence of a diastema (a gap) between the modified canines and the 1st premolar means they can use their tongue to separate their buccal cavity in two. This means they can chew and bite their food at the same time.
• Herbivores have flat amd ridged molars to grind plant tissue and release nutrients.
• Carnivores have a relatively short digestive system as meat is easily digestible. Their stomachs are quite large to increase the digestion of proteins.
• The oesophagus of a ruminant herbivore is split into 3 chambers. One leads to the rumen, where cellulose is digested into fatty acids, proteins and vitamins by bacteria. Protein digestion begins in the abomasum, the true stomach, and occurs after the bacterial digestion in the rumen. Food is often regurgitated numerous times to be chewed more in order to further breakdown tough fibres and release more nutrients. The omasum is the other chamber.
• Non ruminant herbivores have bacteria present in the caecum which can digest cellulose fibres to produce protein. Digestion occurs after the ileum (the main site of absorption) so the products of bacterial digestion are not absorbed. Therefore, some animals may eat their faeces to absorb extra nutrients (eg rabbits).
• Insectavores, organisms with an insect based diet, have a short gut as the products of chitin and insect tissue digestion are easily absorbed.
• Multicellular organisms have a higher level of organisation so a wider range of foods are able to be digested.
• Digestive enzymes carry out extracellular digestion within the organism. Products of digestion are absorbed whereas undigested food (waste) is egested.
• A hydra, a simple multicellular organisms related to a Jellyfish, carries out both intracellular and extracellular digestion. Large scale digestion occurs in the gastrovascular cavity. It breaks down food which will be absorbed into the cells lining the cavity, where intracellular digestion completes the breakdown of food. Mechanical and chemical digestion both occur on the gut cavity, as the Hydra's body will contract to 'churn' the food and digestive enzymes are secreted to digest the food.