Different types of nutrition

• There are two main categories of nutrition, autotrophic and heterotrophic.

Autotrophic

• These create their own nutrition from simple organic raw materials.

Photoautotrophic

• These use sunlight as an energy source - perform photosynthesis.

• These include green plants, some protista and some bacteria.

• Described as holophytic.

Chemoautotrophic

• Use energy from chemical reactions - perform chemosynthesis.

• All prokaryotes.

• Less efficient than photosynthesis.

Heterotrophic

• Consume other organisms for nutrition. Eat autotrophs or organisms that have consumed autotrophs.

Saprotrophic

• Used by all fungi and some bacteria.

• Known as saprotrophs or saprobionts.

• They feed on decaying or dead matter.

• They have no specialised digestive system, so instead secrete enzymes such amylases onto food material outside the body for extracellular digestion.

  • They then absorb the soluble products of digestion across cell membranes via diffusion and active transport.

• Decomposers are microscopic saprotrophs, and are important in decaying leaf litter and recycling nutrients, such as nitrogen.

Parasitic

• Obtaining nutrition from a living organism, the host.

• The host will suffer harm from this, and may even die.

• There are two types:

  • Endoparasitic - Live in the bodies of the host, such as tapeworms.

  • Ectoparasitic - Live on the surface of the host, such as head lice.

Holozoic

• Used by most animals.

• Food is ingested, digested, and indigestible remains are egested.

• Food is processed inside the body, absorbed into bodily tissues and used by cells.

• There are different types of animals who use this nutrition:

  • Herbivores - Eat only plant material.

  • Carnivores - Eat other animals only.

  • Omnivores - Eat plants and animals.

  • Detritivores - Feed on dead and decaying matter.

Unicellular organisms

• An example is the animal-like protista, amoeba, which uses holozoic nutrition.

• They obtain nutrients using diffusion or active transport, due to their high SA:V ratio.

• Larger molecules and microbes are taken in via endocytosis.

  • They are converted to food vacuoles, and fuse with lysosomes and digested by the lysosomal enzymes.

  • The products of this are then absorbed into the cytoplasm and indigestible remains and egested by exocytosis.

Multicellular organisms

• An example is the hydra, which is related to sea anemones and is diploblastic - has two layers of cells, and ectoderm and an endoderm, separated by a jelly layer containing a network of nerve fibres.

  • It is cylindrical, usually have six tentacles surrounding its mouth, the only bodily opening.

  • It lives in freshwater, and attaches to leaves or twigs by a basal disc.

• It extends its tentacles and catches small organisms, releasing stinging cells to any who touch its tentacles.

  • The prey is then moved through the mouth into the hollow body cavity.

  • Some endodermal cells secrete lipase, protease, but not amylase. This means the prey is digested extracellularly, and products are absorbed into the cells.

  • Other cells engulf food particles, which are digested in food vacuoles.

  • The indigestible remains are egested from the mouth.

• British species of hydra have photosynthesising protista, which have been proved via radioactive tracing to pass sugars to the hydra.

Human digestive system

• Digestion is necessary to break down molecules into their monomers, and to make them soluble.

Gut functions

• Digestion and absorption occurs in the gut, a long, hollow, muscular tube which moves its contents in one direction only, using peristalsis, the contraction of gut muscles.

  • It moves food along the organs in the digestive system, each with a certain purpose.

• The gut itself has four main functions:

  • Ingestion - Taking in of food via the buccal cavity.

  • Digestion - Breaking down of large insoluble molecules to soluble molecules for absorption. There are two types:

    • Mechanical - Mainly occurs in the mouth via teeth crushing food, but also muscle contractions of the gut wall. This increases SA for enzymes to act.

    • Chemical - Occurs throughout internal process, with usage of enzymes, bile and stomach acid. Breaks down food.

  • Absorption - Molecules and ions entering the blood via the gut wall.

  • Egestion - Disposal of non-bodily created waste, including indigestible food such as cellulose.

Gut structure

• The gut wall is comprised of four layers surrounding the inner lumen:

  • Serosa - The outermost layer. Made of tough connective tissue, and protects the gut wall. The serosa reduces friction of the gut against other abdominal organs.

  • Muscles - There are two types, the inner circular muscles and outer longitudinal muscles. These work together to perform peristalsis via coordinated contractions, with circular muscles contracting behind the ball of food and longitudinal muscles relaxing to push it along.

  • Submucosa - Connective tissue that contains nerve and lymph vessels, coordinating peristalsis and removing absorbed products of digestion.

  • Mucosa - Innermost layer, lines the gut wall. It secretes mucus for lubrication and protection, and sometimes secretes digestive juices and absorbs digested food.

Digestion of molecules

• Different enzymes are needed for each type of macromolecule, in some cases multiple are needed.

Carbohydrates

• Amylase hydrolyses starch and glycogen to disaccharides.

  • Maltase breaks down the disaccharide maltose down into glucose.

  • Lactase breaks down the disaccharide lactose down into glucose.

  • Sucrase breaks down the disaccharide sucrose into glucose.

• The general name for these enzymes is carbohydrase.

Proteins

• They are digested from polypeptides into dipeptides and then amino acids.

• There are two types of these enzymes:

  • Endopeptidases - Hydrolyse bonds within the protein molecule.

  • Exopeptidases - Hydrolyse terminal or penultimate bonds at the ends of shorter polypeptides.

• The general name for these enzymes is proteases of peptidases.

Fats

• Fats are broken down into fatty acids and monoglycerides.

• There is one enzyme involved, lipase.

Mouth

• Also known as the buccal cavity.

• Chewing is used to perform mechanical digestion, and saliva is used to perform chemical digestion.

• Saliva contains:

  • Amylase, for starch and glycogen digestion to maltose.

  • HCO3- (hydrogen carbonate) and CO3 2- (carbon trioxide).

  • pH varies between 6.2-7.4, but salivary amylase has an optimum of 6.7-7.0.

  • Mucus to lubricate passage down the oesophagus.

• This increases food SA for enzymes.

Oesophagus

• It carries food to the stomach.

• It has the same layers as the gut wall.

Stomach

• Food is kept in the stomach by contractions of the two sphincters (rings of muscles) and is structured with an extra layer of oblique muscles between the longitudinal and circular.

  • It also has large folds in the mucosa, known as rugae.

• It has a volume of 2 dm3 and food can stay there for several hours.

  • Walls contract rhythmically to mix food with gastric juice, which is secreted by glands in depression in the mucosa, known as gastric pits.

• This gastric juice contains:

  • Peptidases, secreted by zymogen/chief cells at the base of the gastric pit.

    • Pepsinogen is secreted, an inactive enzyme which is activated by H+ ions to pepsin, which is an endopeptidase.

  • Hydrochloric acid is secreted by oxyntic cells.

    • This lowers the pH of stomach contents to around pH2, which is the optimum enzyme pH and kills most of the bacteria in the food.

  • Mucus, secreted by goblet cells at the top of the gastric pit.

    • It forms a lining to protect the stomach wall from enzymes and lubricates the food.

Small intestine

• It has two regions:

  • Duodenum, the first region that receives secretions from liver and pancreas. Takes up the first 25cm.

  • Ileum, where absorption occurs, have folds, villi and microvilli. It is several metres long.

• The liver secretes bile into the duodenum, which is stored in the gall bladder, than passes through the bile duct:

  • It has no enzymes.

  • It contains bile salts which are amphipathic (molecules have hydrophilic and hydrophobic parts).

  • It emulsifies lipids in the food, lowering their surface tension and breaking up large globules to increase SA. This makes lipase digestion more efficient.

  • It is alkaline, neutralising the stomach acid and providing optimum pH for the enzymes in the small intestine.

• The pancreas secretes pancreatic juice from islet cells, which are exocrine glands (glands that secrete substances onto an epithelial surface) in the pancreas. It contains:

  • Trypsinogen - An inactive enzyme that is converted to the endopeptidase trypsin by the duodenal enzyme, enterokinase.

  • Endopeptidases, amylase and lipase.

  • Sodium hydrogen carbonate (NaHCO3)- Raises pH to slightly alkaline to neutralise stomach acid and provide optimum pH for enzymes.

• When food enters, it is lubricated by mucus and neutralised by the alkaline secretions from cells at the base of the crypts of Lieberkuhn, called Brunner’s glands.

• Epithelial cells lining the ileum have villi, with microscopic projections known as microvilli. These synthesise digestive enzymes:

  • Peptidases - Secreted by villus cells, and digestion continues in gut lumen.

    • Dipeptidases are in cell surface membranes.

  • Carbohydrases are secreted, and digestion continues in gut lumen.

    • In the cell surface membrane, they are digested from disaccharides to monosaccharides.

    • Some disaccharides are absorbed and then digested intracellularly.

Absorption

• Occurs via active transport and facilitated/ simple diffusion.

• Ileum is specialised to increase SA via it’s folds, villi and microvilli. It absorbs each molecule differently.

  • Amino acids are absorbed using active transport, and then use facilitated diffusion to enter the capillaries. As they are water-soluble, they dissolve in the plasma.

  • Glucose co-transports with sodium ions. They then move into the blood separately, sodium using active transport and glucose using facilitated diffusion, where they both dissolve in the plasma.

    • However, as diffusion can be slow, some glucose is absorbed using active transport.

  • Fatty acids and monoglycerides diffuse into epithelial cells and lacteals, which are lymph (a colourless fluid containing white blood cells) capillaries in the villi.

    • This is part of the lymphatic system, which transports fat-soluble molecules into the subclavian (just below the clavicles) vein near the heart.

  • Minerals enter the blood via simple/facilitated diffusion and active transport, where they dissolve in the plasma.

  • Vitamins B and C are absorbed into the blood and dissolve in the plasma, while vitamins A, D and E are absorbed into lacteals where they dissolve in fats.

  • Water is diffused into epithelial cells and capillaries via osmosis.

Large intestine

• This organ is around 1.5 metres long, and has many different parts:

  • Caecum - Where undigested food, mucus, bacteria and dead cells enter.

  • Appendix - Connects to the large intestine, with no set purpose.

  • Colon - Food then enters this part. This has less villi then the ileum, and which has a major role in water and mineral absorption.

    • Vitamin K and folic acid and secreted by mutualistic organisms in the colon.

  • Rectum - Food is semi-solid once it reaches this section. It is egested here as faeces - a process known as defecation.

Holozoic adaptations

Amphibians and reptiles

• Reptiles and amphibians swallow food whole, while mammals hold food in their mouth for chemical digestion.

  • This is because mammals are the only vetrebrates with separate nasal and mouth cavities, separated by a palate.

  • This allows them to chew and breathe simultaeneously.

Carnivores

Organs

• Carnivores have shorter small intestines, as protein makes up most of their diet and is easily digested.

  • Their large intestines are straight with a smooth lining.

Dentition

• Incisors are located at the front of the mouth, and are sharp but smaller than canines.

  • They grip and tear muscle from bone.

• Canines are located around incisors, and are large, curved and pointed.

  • They tear muscle from bone, and kill, pierce and seize prey.

• Premolars and molars are located towards the back of the mouth.

  • They have cusps, sharp points which cut and crush.

• Carnassials are specialised cheek teeth - with two on each side, which slide past each other. They are large, sharp and are located towards the back of the mouth.

  • They shear muscle off the bone.

• Jaw muscles move vertically, and need to open their mouths wide to fit prey.

  • They are well developed and powerful, in order to crush bone and grab prey firmly. They leave protrusions on the skull for these muscles to insert into bone.

Herbivores

Organs

• Herbivores have a large small intestine, as plant material is harder to digest and requires more time for absorbtion.

  • Pouched large intestine, to allow for larger volume of faeces produced in plant digestion. Much of this is cellulose.

Dentition

• For grazing animals, incisors are on the lower jaw and front of the mouth only and are indistinguishable from canines. They are thin.

  • They slice through grass.

• The dental pad is above the incisors and canines, and the front of the mouth and the upper jaw. It is leathery.

  • The tongue pulls grass tight across this, allowing incisors and canines to slice through grass.

• Molars and premolars and flat, and located at the back of the mouth. The top and bottom row interlock.

  • They grind food circularly.

  • Over time, grinding ridges are worn down and expose sharp edged enamel ridges, which improves grinding efficiency.

  • Teeth have open, unrestricted roots, allowing for teeth to grow throughout the animal’s life. This replaces material worn down by chewing.

• Herbivores jaws move side-to-side (horizontally).

  • They do not need strong jaws, as their food is not moving or extremely solid.

  • They have a relatively smooth skull, absent of muscle protrusions.

Ruminants

• This is a group of herbivores that use a rumen, a chamber which has mutualistic microbes to aid in food digestion.

• It is mainly necessary for cellulose, as cellulase is not made by animals so they cannot digest the bonds.

  • Therefore, the mutualistic enzymes inside the gut are used to produce the enzymes needed. These microbes live in a 150dm3 chamber, and include:

    • Bacteria.

    • Fungi.

    • Protista.

• It takes place as follows:

  • In the mouth, grass is cut by teeth and mixed with saliva to form the cud, which is then swallowed by the oesophagus to the rumen.

    • These teeth are designed to ground up lignin, cellulose and silica.

  • The rumen mixes the food with microbes, which secrete enzymes that break down cellulose to glucose.

    • This is then fermented to organic acids which are absorbed into the blood, providing energy to the cow.

    • It releases the waste products methane and carbon dioxide.

  • The reticulum is where the fermented grass is passed to, where it is reformed to the cud and regurgitated to the mouth for extra chewing.

    • The cud can be regurgitated and swallowed multiple times.

  • The omasum is next, where water and organic acids made from the fermented glucose are absorbed.

  • The abomasum is known as the ‘true stomach‘, and is where pepsin is used to digest proteins at pH2.

  • The small intestine then absorbs the products of digestion.

  • The large intestine absorbs water, like in humans.

Omnivores/humans

Organs

• Have an intermediate sized gut, and pouched large intestine.

Dentition

• Have incisors, canines, molars and premolars, which are unspecialised due to a mixed diet.

Parasites

• There are many different types of parasites, which are specialised to different hosts.

  • Plants and animals are parasitised by bacteria, fungi, nematodes (slender worms) and insects.

  • Animals are parasitised by protista, tapeworms and mites.

  • Bacteria are parasitised by viruses (bacteriophages).

• Many organisms are parasitised for a part of their lives.

Pork tapeworm

• This is an example of a gut endoparasite. These must survive harsh conditions in the gut, including:

  • Peristalsis.

  • Digestive juices and mucus.

  • The organism’s immune system.

  • pH changes from gut to duodenum.

• Pork tapeworms are structured to withstand these environmental factors:

  • They are ribbon shaped to avoid blocking the host’s food entirely.

  • Can be up to 10m long.

  • The anterior end is the scolex, made of muscles carrying suckers and a double row of curved hooks to attach strongly to the duodenum wall.

  • It is made of a linear series of sections called proglottids.

  • It has a thick body coating known as the cuticle, to protect from the host’s immune system and enzymes.

  • It makes its own enzyme-inhibitors known as anti-enzymes.

  • It has a large SA:V ratio for absorption of pre-digested food.

  • It is a hermaphrodite, with each proglottid having male and female reproductive parts, each of which can have up to 40,000 eggs, which pass out via the faeces.

  • Eggs have resistant shells, and can survive until ingested by a pig.

• The pork tapeworm has two sections to it’s life, with two hosts:

  • It’s secondary host is pigs, which are infected by eating food contaminated by human faeces.

    • Here larval forms develop. Once eggs are eaten by pigs, they hatch and move through the intestinal wall and remain dormant in the pig’s muscles until consumed by a human.

  • It’s primary host is humans, which are infected by eating undercooked pork which contains live larval forms. This is treated with drugs.

    • The tapeworm causes cause discomfort, but long-term infections cause taeniasis, which causes abdominal pains and weakness.

    • However, if humans are infected by eating eggs directly, the dormant embryos can form cysts in organs, including eyes and brain, and causes significant damage to surrounding tissues. This is harder to treat.

Pediculus

• These are examples of ectoparasites.

• There are many species, which are specialised to a specific host and sometimes even a specific region of that host’s body.

• They are wingless and poorly adapted to movement, and are therefore transferred by direct contact.

• Humans are infected by close relatives head and body lice, and distant relative pubic lice.

  • They feed on blood.

• There are three stages in their life cycle:

  • Adult - An adult lays eggs.

  • Eggs - Take 1-2 weeks to hatch, and leave empty egg cases known as nits.

  • Nymphs - Identical to an adult, except smaller. Becomes adult after 10 days.