Definition -

The digestive tract - basically a tube with a mouth (for ingestion) at one end and an anus (for egestion) at the other.

- food processing is organised as a one-way conveyor belt system.

- food can be taken in while previously ingested food is being digested.

- parts of the tube may have specialised structure and functions.

Near the front end most animals have structures - teeth, bill (birds, turtles), gizzard (earthworms, insects, birds) that physically break down the food.

The intestine may have a specialised stomach that stores food and secretes digestive enzymes.

Overall, food is broken down step by step and the nutrients released are absorbed further down the gut

Mouth, pharynx, oesophagus, stomach, small intestine, large intestine, anus.

What are its functions? (Table 10.7; Fig. 10.14, 10.15)

- Food intake

- mechanical and chemical breakdown

- storage and transport

- adsorption of nutrients

- formation & evacuation of faeces

 Food is taken into the mouth, chewed (physical / mechanical digestion), mixed with ?-amylase (chemical digestion) and mucous (to moisten & lubricate) in saliva, rolled into a bolus and pushed to the back of the throat.

The muscular walls of the alimentary canal contract in waves (peristalsis) starting from the end closest to the mouth, pushing the food along.

Produced by glands lining the stomach and small intestine, and by pancreas and salivary glands.

(See section 10.6 - fate of absorbed food materials)

When does food enter the body? Food does not enter the body until sometime after a meal when absorption of small molecules occurs.

In the stomach, fat-soluble substances, e.g. alcohol & some drugs are absorbed.

Most absorption occurs in the small intestine, a tube about 3 m long, highly adapted for absorption.

Glucose and aa’s are taken up by active transport from the lumen

Di-and tri-peptides are absorbed and digested intracellularly and the aa’s passed on to the bloodstream.

Long chain fatty acids diffuse into cells lining the intestine, converted to fats and these are transported by lacteals to the bloodstream.

Water is absorbed indirectly as a result of movement of Na⁺. how? Na⁺ is transported out of the intestinal cells into the villi. Cl? follows passively, this raises the osmotic potential of the lumen relative to the cells and water moved from the lumen by osmosis

Na, other ions, vit K and H₂O are main substances absorbed by the large intestine

What remains after absorption is faeces:

Terrestrial plants have tough cell walls (why?) so breaking the cell walls and digesting cellulose are problems to be overcome.

Some animals have symbiotic micro-organisms in their g-i tract that produce cellulase and allow digestion of cellulose into substances their hosts can absorb. E.g. Termites & herbivorous mammals (compare these with locusts and grasshoppers).

Aquatic plants are supported by surrounding water, they have little tough supporting tissues and are easily digested.

Filter feeding is non-selective, everything which is filtered is ingested, what can be used is digested & the remainder is egested.

Includes animals that have mechanisms for

(a) Ingestion of inactive masses e.g. earthworms and detritus feeders

(b) Scraping, boring, biting, chewing e.g. sea-urchins, snails, insects and vertebrates

(c) Capture and swallowing of prey e.g. fish, snakes, birds, bats

There is an extensive range of modifications for acquiring food (radula, teeth, beaks, fangs)

parasite - an organism that for part or all of its life derives its food from a living organism of another species (the host).

Some parasites (e.g. tapeworms) live in nutrient-rich media and no longer have a digestive system as their food is predigested by the host and the host’s enzymes. They absorb their nutrients directly across their body surface. (Humans can become infected by eating meat containing the tapeworm larvae.)

Saprotroph - a bacterium, fungus or plant that feeds on dead or decaying organic matter. Most fungi and bacteria are saprobes - involved in the decay of organic matter.

Important for returning complex organic matter to CO₂ and H₂O and recycling of nutrients.

Without these activities, essential nutrients would remain locked up in dead animal bodies, faeces, branches, logs etc and be unavailable for further use by new generations of organisms.

Generally these organisms live in/on their food, secrete enzymes to break down the food and absorb the simpler products of the digestion.

An important aspect of this is the specialisation for degradation of cellulose and lignin, the major part of terrestrial plant’s dry weight, unlike bacteria many fungi do this.