DigestionWe need to get all this goodness to our cells.

I’m helping to prepare this food

to cross a membrane.

Part 1: Digestion in Humans

Part 2: Digestion in other organisms

Who needs to eat?

•Autotrophs make their own food by converting sunlight into chemical energy. Therefore they do not need to eat food. Which type of organisms are autotrophs, and how do they make their own food?

•Some bacteria that live at the bottom of the ocean use the gases that rise up from deep-sea vents in order to make their own food. They are known as chemotrophs because they make organic chemicals which they use as food from inorganic chemicals.

•Heterotrophs cannot make their own food from either the sun or inorganic chemicals, they must eat other living things in order to gain the nutrients they need to survive.

•Heterotrophs eat autotrophs and other heterotrophs. The autotrophs are always at the start of the food chain and are responsible for bringing energy into the food chain.

In humans, digestion can be broken down

into 4 main sections:

1. Ingestion: this is where food is taken into our bodies through our mouths. We call this ‘eating’.

2. Digestion: large organic molecules are broken down either chemically or mechanically into their smaller components.

3. Absorbtion: these nutrients must pass across the intestine wall, then into the surrounding tissue fluid and then into the circulatory system or lymphatic system. They are then circulated around the body to cells where they are utilized.

4. Egestion: the nutrients that are not absorbed are stored temporarily in the rectum before elimination via the anus.

Part 1

Salivary Glands

Tongue

Epiglottis

Oesophagus

Stomach

Bile Duct

Gall Bladder

Pancreas

Liver

Pyloric Sphincter

Pancreatic Duct

Cardiac Sphincter

Duodenum

IliumAscending Colon

Appendix

Transverse Colon

Descending ColonRectum

Anus

The Main Parts of the Alimentary Canal

Ingestion

I am going to ingest this cake as soon as I have blown out the candles!

Ingestion is simply ‘eating’ our food. We need to place food at the opening of the alimentary canal (the mouth) to

start the process of digestion. Bon Appetite!

Mmnn lots of goodness here for our

cells to use.

Ingestion starts here

Digestion

Mouth:

Site of both Mechanical

and Chemical Digestion

1. The Oral Cavity

Consists of:

1. Teeth

2. Jaw

3. Tongue

4. Saliva

DigestionMechanical Digestion in the Mouth

•Teeth provide a means to break food into smaller parts by either ripping or slicing and then grinding. This not only makes the food easier to swallow, but increases the surface area to volume ratio of the food making chemical breakdown more efficient.

•The jaw acts like a lever to help the teeth breakdown tougher material that has been ingested.

•The tongue can move the food around the mouth which assists digestion. When the food is suitably masticated the tongue moves the food – now called a ‘bolus’ - to the back of the throat for swallowing.

Digestion

IncisorsMolarsMolars

Canines

•Different teeth are used for different types of mechanical breakdown

•The incisors are used to get food into the mouth, the canines to tear food apart, and the molars to grind food down to smaller pieces.

•What are the main differences between the herbivore and carnivore jaw?

Different Teeth Have Different Jobs

DigestionChemical digestion in The Mouth

•Saliva released from the salivary glands contains the digestive enzyme amylase. Amylase breaks down the more complex carbohydrates (such as starch) into simple carbohydrates (such as sugar).

Amylase

Long chain carbohydrate

Many short chain carbohydrates

Digestion

Epiglottis closes over the trachea to prevent bolus entering the respiratory tract and causing choking

Oesophagus moves the bolus down into the stomach with muscular contractions known as peristalsis. Some chemical digestion will still be taking place.

2. Moving food to the stomach

Digestion

Stomach: Mostly Chemical & some Mechanical Digestion. Food stays in the stomach from 1-4 hours on average

The fluid in the stomach is known as chyme

Pyloric sphincter holds the chyme in the stomach before it passes into the small intestine.

3. Digestion in the Stomach

Cardiac sphincter helps prevent reflux

Digestion

•The stomach consists of many layers of muscle that can churn the contents around as digestion takes place. As you may have experienced before, the stomach can expand to accommodate a great deal of food!

Mechanical Digestion in the Stomach

Digestion

Chemical Digestion in the Stomach

•Gastric juices in the stomach contain mucus, hydrochloric acid (HCl), enzymes and water

•One of these enzymes is pepsin. Pepsin (a protease) first acts on proteins here in the stomach

•The HCl can also help breakdown chemical bonds

•Amylase is still breaking down carbohydrates

Amylase Pepsin

Large protein chain

Smaller protein chains

Digestion4. Digestion in the Small Intestines

Duodenum:

While the chyme is here;

•bile made in the liver and stored in the gall bladder is

added to the mixture. Bile

emulsifies fats

•bicarbonates made in the pancreas are

used to neutralize all the acids

Jejunum

Ilium

Liver

Pancreas

Gall Bladder

Duodenum + Jejunum + Ilium = Small Intestine

DigestionChemical Digestion in the Small Intestines

Pancreatic Amylase

Glucose ready for absorption into blood

Pepsin

Amino Acids ready for absorption into the blood

Bile Salts

Pancreatic Lipase

Glycerol

Fatty Acids

Fatty Acids & Glycerol ready for absorption into the lymph

Peptidase

Digestion

+ H2O

Proteins + H2O

Polypeptides

Polypeptides

Amino AcidsPeptidase

Pepsin

Complex carbohydrates + H2O Disaccharides

Amylase

Disaccharides + H2O MonosaccharidesDisaccharase

Summary of Digestive Enzymes

+ H2OFats Fatty acidsLipase

+ Glycerol

Note: bile and HCl help break down food but are not enzymes

Absorption

•Some absorption can take place in the stomach.

•There is also some absorption in the colon, it consists mostly of water uptake. Up to 40mls of water a day is absorbed from the colon.

•Most absorption takes place in the small intestines immediately after digestion is complete. The surface of the small intestines is folded to increase surface area and therefore allow more absorption. The folded surface also has small projections called villi that protrude into the lumen & further increasing the area over which absorption can take place.

•Water, vitamins & minerals can be absorbed without digestion.

Absorption

a. Longitudinal section of small intestine showing folds.

b. Villi with an appearance like fingers. They contain blood capillaries which carry food from the gut to the cells.

c. Cell of the small intestine

Folds Villi

Blood capillaries

Lacteal:

Microvilli

AbsorptionMicrograph of microvili on the inner lining of the small intestines

Microvili

Absorption

•The Lacteal carries the glycerol and the fatty acids to the lymphatic system.

•The lymph in the lymphatic system carries them towards the head where they enter the blood stream near the shoulder.

•The blood in the circulatory system carries the digested food to all the cells in our bodies.

Absorption

What happens to the food once it enters the cell?

How long would we last without food?

Egestion

RectumAnus

•Faeces are stored in the rectum until they are expelled by the anus.

•Faeces contain food remains that have not been absorbed, water, dead cells from the digestive tract and bacteria

Part 2: Digestion in other organisms

Digestive Systems in Mammalian Herbivores

It says here that herbivores need to take a large proportion of the carbohydrates in their diets from the cellulose in plant cell walls. As mammals lack the enzymes to digest cellulose they need to enter

into a symbiotic relationship with bacteria that live in their gut.

This book says that the bacteria break down the cellulose by fermentation which makes it

available for the animals to absorb.

This one says that therefore the digestive systems of herbivores

are modified by being enlarged in certain portions to allow for the

fermentation to take place.

Digestive Systems in Mammalian Herbivores

Mammalian Digestive System

No ‘fermentation sac’

Carnivore

Fermentation sac

Herbivore

Foregut Fermenter Hindgut Fermenter

Modified oesophagus

Ruminant

Modified stomach

Non-Ruminant

Caecum fermenter Colon fermenter

Wombats!

Digestive Systems in Mammalian Herbivores

So which are foregut and which are hindgut fermentors?

Digestive Systems in Mammalian Herbivores

Coprophagy – faecal ingestion

We rabbits are unusual in that we produce two

kinds of faeces

The small dark round pellets you see is the type we egest during the day. But at night we egest a soft green faeces that you don’t usually see because we ingest it.

That’s great when we get the midnight munchies!

The night pellets are made up mostly of bacteria that ferment the carbohydrates we eat during the day….that’s very good for us.

The bacteria also provide us with some vitamins.

We rats are coprophiliacs too!!

Digestion in other organismsAmeoba

Ameoba change their shape to engulf their food. They secrete enzymes to break the food down.

Digestion in other organismsParamecium

Paramecium use their cilia to move food towards the food vacuole.

Digestion in other organisms

The tapeworm hangs on to the inside of the host gut and absorbs its digested food.

The earthworm has a long digestive tract. It digests nutrients in the soil and absorbs them directly into blood stream.

Digestion in other organisms

In the sponge, cells trap food when the water passes over it.

Digestion in other organisms

Crop stores undigested food ready to be used for overnight feeding

Stomach

Gizzard grinds food into smaller pieces

Intestines

Anus

Digestion in Birds

And that is the end of the digestive story!