Bio - Digestion

Structure of the digestive system

Production of an annotated diagram of the digestive system.

The part of the human body used for digestion

can be described in simple terms as a tube

through which food passes from the mouth to

the anus. The role of the digestive system is to

break down the diverse mixture of large carbon

compounds in food, to yield ions and smaller

compounds that can be absorbed. For proteins,

lipids and polysaccharides digestion involves

several stages that occur in different parts of

the gut.

Digestion requires surfactants to break up lipid

droplets and enzymes to catalyse reactions.

Glandular cells in the lining of the stomach

and intestines produce some of the enzymes.

Surfactants and other enzymes are secreted

by accessory glands that have ducts leading

to the digestive system. Controlled, selective

absorption of the nutrients released by digestion

takes place in the small intestine and colon, but

some small molecules, notably alcohol, diffuse

through the stomach lining before reaching the

small intestine.

Mouth - Voluntary control of eating and

swallowing. Mechanical digestion

of food by chewing and mixing with

saliva, which contains lubricants and

enzymes that start starch digestion

Esophagus - Movement of food by peristalsis

from the mouth to the stomach

Stomach Churning and mixing with secreted

water and acid which kills foreign

bacteria and other pathogens in

food, plus initial stages of protein

digestion

Small intestine - Final stages of digestion of lipids,

carbohydrates, proteins and nucleic

acids, neutralizing stomach acid,

plus absorption of nutrients

Pancreas - Secretion of lipase, amylase and

protease

Liver - Secretion of surfactants in bile to

break up lipid droplets

Gall bladder - Storage and regulated release of bile

Large intestine - Re-absorption of water,

further digestion especially of

carbohydrates by symbiotic

bacteria, plus formation and storage

of feces

The wall of the small intestine is made of layers

of living tissues, which are usually quite easy

to distinguish in sections of the wall. From the

outside of the wall going inwards there are

four layers:

● serosa – an outer coat

● muscle layers – longitudinal muscle and inside

it circular muscle

● sub-mucosa – a tissue layer containing blood

and lymph vessels

● mucosa – the lining of the small intestine,

with the epithelium that absorbs nutrients on

its inner surface.

pancreatic juice

The pancreas secretes enzymes into the lumen of the

small intestine.

The pancreas contains two types of gland tissue. Small groups of cells secrete

the hormones insulin and glucagon into the blood. The remainder of the

pancreas synthesizes and secretes digestive enzymes into the gut in response

to eating a meal. This is mediated by hormones synthesized and secreted

by the stomach and also by the enteric nervous system. The structure of

the tissue is shown in gure 4. Small groups of gland cells cluster round the

ends of tubes called ducts, into which the enzymes are secreted.

The digestive enzymes are synthesized in pancreatic gland cells on ribosomes

on the rough endoplasmic reticulum. They are then processed in the Golgi

apparatus and secreted by exocytosis. Ducts within the pancreas merge into

larger ducts, nally forming one pancreatic duct, through which about a litre

of pancreatic juice is secreted per day into the lumen of the small intestine.

Pancreatic juice contains enzymes that digest all the three main types of

macromolecule found in food:

● amylase to digest starch

● lipases to digest triglycerides, phospholipids

● proteases to digest proteins and peptides.

Digestion in the small intestine

Enzymes digest most macromolecules in food into

monomers in the small intestine.

The enzymes secreted by the pancreas into the lumen of the

small intestine carry out these hydrolysis reactions:

● starch is digested to maltose by amylase

● triglycerides are digested to fatty acids and glycerol or fatty

acids and monoglycerides by lipase

● phospholipids are digested to fatty acids, glycerol and

phosphate by phospholipase

● proteins and polypeptides are digested to shorter peptides by

protease.

This does not complete the process of digestion into molecules small

enough to be absorbed. The wall of the small intestine produces

a variety of other enzymes, which digest more substances. Some

enzymes produced by gland cells in the intestine wall may be secreted

in intestinal juice but most remain immobilized in the plasma

membrane of epithelium cells lining the intestine. They are active

there and continue to be active when the epithelium cells are abraded

off the lining and mixed with the semi-digested food.

● Nucleases digest DNA and RNA into nucleotides.

● Maltase digests maltose into glucose.

● Lactase digests lactose into glucose and galactose.

● Sucrase digests sucrose into glucose and fructose.

● Exopeptidases are proteases that digest peptides by removing single

amino acids either from the carboxy or amino terminal of the chain

until only a dipeptide is left.

● Dipeptidases digest dipeptides into amino acids.

Because of the great length of the small intestine, food takes hours to

pass through, allowing time for digestion of most macromolecules to

be completed. Some substances remain largely undigested, because

humans cannot synthesize the necessary enzymes. Cellulose for example

is not digested and passes on to the large intestine as one of the main

components of dietary fibre.

Villii and the surface area for digestion

Villi increase the surface area of epithelium over which

absorption is carried out.

The process of taking substances into cells and the blood is called

absorption. In the human digestive system nutrients are absorbed

principally in the small intestine. The rate of absorption depends on

the surface area of the epithelium that carries out the process. The

small intestine in adults is approximately seven metres long and

25–30 millimetres wide and there are folds on its inner surface, giving

a large surface area. This area is increased by the presence of villi.

Villi are small finger-like projections of the mucosa on the inside of the

intestine wall. A villus is between 0.5 and 1.5 mm long and there can

be as many as 40 of them per square millimetre of small intestine wall.

They increase the surface area by a factor of about 10.

Absorbtion by villi

Villi absorb monomers formed by digestion as well as

mineral ions and vitamins.

The epithelium that covers the villi must form a barrier to harmful substances, while at the same time being permeable enough to allow

useful nutrients to pass through.

Villus cells absorb these products of digestion of macromolecules in food:

● glucose, fructose, galactose and other monosaccharides

● any of the twenty amino acids used to make proteins

● fatty acids, monoglycerides and glycerol

● bases from digestion of nucleotides.

They also absorb substances required by the body and present in foods

but not needing digestion:

● mineral ions such as calcium, potassium and sodium

● vitamins such as ascorbic acid (vitamin C).

Some harmful substances pass through the epithelium and are

subsequently removed from the blood and detoxied by the liver. Some

harmless but unwanted substances are also absorbed, including many

of those that give food its colour and avour. These pass out in urine.

Small numbers of bacteria pass through the epithelium but are quickly

removed from the blood by phagocytic cells in the liver.

methds f absrtin

Dierent methods of membrane transport are required to

absorb dierent nutrients.

To be absorbed into the body, nutrients must pass from the lumen of

the small intestine to the capillaries or lacteals in the villi. The nutrients

must rst be absorbed into epithelium cells through the exposed

part of the plasma membrane that has its surface area enlarged with

microvilli. The nutrients must then pass out of this cell through the

plasma membrane where it faces inwards towards the lacteal and blood

capillaries of the villus.

Many different mechanisms move nutrients into and out of the villus

epithelium cells: simple diffusion, facilitated diffusion, active transport

and exocytosis. These methods can be illustrated using two different

examples of absorption: triglycerides and glucose.

● Triglycerides must be digested before they can be absorbed. The

products of digestion are fatty acids and monoglycerides, which can

be absorbed into villus epithelium cells by simple diffusion as they

can pass between phospholipids in the plasma membrane.

● Fatty acids are also absorbed by facilitated diffusion as there are fatty

acid transporters, which are proteins in the membrane of the microvilli.

● Once inside the epithelium cells, fatty acids are combined with

monoglycerides to produce triglycerides, which cannot diffuse back

out into the lumen.

Triglycerides coalesce with cholesterol to form droplets with a

diameter of about 0.2 μm, which become coated in phospholipids

and protein.

● These lipoprotein particles are released by exocytosis through the

plasma membrane on the inner side of the villus epithelium cells.

They then either enter the lacteal and are carried away in the lymph,

or enter the blood capillaries in the villi.

● Glucose cannot pass through the plasma membrane by simple

diffusion because it is polar and therefore hydrophilic.

● Sodium–potassium pumps in the inwards-facing part of the plasma

membrane pump sodium ions by active transport from the cytoplasm

to the interstitial spaces inside the villus and potassium ions in the

opposite direction. This creates a low concentration of sodium ions

inside villus epithelium cells.

● Sodium–glucose co-transporter proteins in the microvilli transfer

a sodium ion and a glucose molecule together from the intestinal

lumen to the cytoplasm of the epithelium cells. This type of

facilitated diffusion is passive but it depends on the concentration

gradient of sodium ions created by active transport.

● Glucose channels allow the glucose to move by facilitated diffusion

from the cytoplasm to the interstitial spaces inside the villus and on

into blood capillaries in the villus.

Starch diestin in the sa intestine

Processes occurring in the small intestine that result in the digestion of starch and

transport of the products of digestion to the liver.

Starch digestion illustrates some important

processes including catalysis, enzyme specicity

and membrane permeability. Starch is a

macromolecule, composed of many α-glucose

monomers linked together in plants by

condensation reactions. It is a major constituent

of plant-based foods such as bread, potatoes and

pasta. Starch molecules cannot pass through

membranes so must be digested in the small

intestine to allow absorption.

All of the reactions involved in the digestion of

starch are exothermic, but without a catalyst they

happen at very slow rates. There are two types of

molecule in starch:

● amylose has unbranched chains of α-glucose

linked by 1,4 bonds;

● amylopectin has chains of α-glucose linked

by 1,4 bonds, with some 1,6 bonds that make

the molecule branched.

The enzyme that begins the digestion of both

forms of starch is amylase. Saliva contains

amylase but most starch digestion occurs in the

small intestine, catalysed by pancreatic amylase.

Any 1,4 bond in starch molecules can be broken

by this enzyme, as long as there is a chain of at

least four glucose monomers. Amylose is therefore digested into a mixture of two- and three-glucose

fragments called maltose and maltotriose.

Because of the specificity of its active site, amylase

cannot break 1,6 bonds in amylopectin. Fragments

of the amylopectin molecule containing a

1,6 bond that amylase cannot digest are called

dextrins. Digestion of starch is completed by

three enzymes in the membranes of microvilli

on villus epithelium cells. Maltase, glucosidase

and dextrinase digest maltose, maltotriose and

dextrins into glucose.

Glucose is absorbed into villus epithelium cells

by co-transport with sodium ions. It then moves

by facilitated diffusion into the fluid in interstitial

spaces inside the villus. The dense network of

capillaries close to the epithelium ensures that

glucose only has to travel a short distance to

enter the blood system. Capillary walls consist of

a single layer of thin cells, with pores between

adjacent cells, but these capillaries have larger

pores than usual, aiding the entry of glucose.

Blood carrying glucose and other products of

digestion flows though villus capillaries to venules

in the sub-mucosa of the wall of the small

intestine. The blood in these venules is carried

via the hepatic portal vein to the liver, where

excess glucose can be absorbed by liver cells and

converted to glycogen for storage. Glycogen is

similar in structure to amylopectin, but with

more 1,6 bonds and therefore more extensive

branching.