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Flashcards in 2.1 The Digestive System Deck (64):

The human digestive system is made up of:

A long muscular tube and it's associated glands.


Glands produce:

Enzymes, that break down large insoluble molecules into small soluble ones ready for absorption.


Parts of the digestive system:

Salivary glands
Large intestine
Small intestine


Role of the Oesophagus in digestion:

Carries food from the mouth to the stomach, made up of thick muscular walls.


What part does the stomach play in digestion.

It is a muscular sac with an inner layer which produces enzymes. It stores and digests food, particularly proteins. It has glands which produce enzymes which digest protein and also glands producing mucus (so the stomach is not digested by its own enzymes).


What does the small intestine do in digestion?

It is a long muscular tube, food is further digested by enzymes produced in the wall glands. The inner walls are folded into villi and then there are microvilli on the epithelial cells of each villus (more surface area for absorption).


What does the large intestine do in digestion?

Absorbs water
(Most of which comes from secretions of the many digestive glands).
Becomes dry and thick = faeces


What role does the rectum play in digestion?

The faeces are stored here before being removed via the anus by egestion.


What is the role of the salivary glands in digestion?

The salivary glands pass their secretions via a duct into the mouth, they contain amylase which breaks down starch into maltose.


What is the role of the pancreas in digestion?

It produces pancreatic juice which contains proteases, carbohydrases (amylase) and lipases.


Digestion takes place in two stages...

1. Physical breakdown
2. Chemical digestion


What happens in physical digestion?

Broken down by teeth - provides large surface area for chemical digestion. Food is churned by the muscles in the stomach wall which also breaks it up.


What is absorption?

Taking in soluble molecules into the body.


What is assimilation?

Incorporating absorbed molecules into the body tissues.


What happens in chemical digestion?

Breaks down large insoluble molecules into smaller soluble ones.
Hydrolysis - splitting up molecules by adding water to the chemical bonds which hold them together. (These enzymes are hydrolases).


Name the three most important types of digestive enzymes.

Carbohydrases - break down carbohydrates to monosaccharides.
Lipases - break down lipids into fatty acids and glycerol.
Proteases - break down protein to amino acids.


What happens after large food molecules have been hydrolysed?

They are absorbed by various means from the small intestine into the blood. They are carried to different parts of the body and often built up again into large molecules ( which are not necessarily the same type the molecules from which they were derived.) They are assimilated.


Polysaccharides are...

Polymers (made up of monomers) (so are polypeptides) (not lipids)


Carbohydrates contain the elements...

Carbon, hydrogen and oxygen


Proteins contain the elements...

Carbon, hydrogen, oxygen and nitrogen


A single carbohydrate monomer is known as...

A monosaccharide


A pair of monosaccharides is...

A disaccharide


Many monosaccharides is...

A polysaccharide


Monosaccharides are sweet-tasting...

Soluble substances which have the general formula (CH2O)n where n is any number from 3 to 7


Examples of some monosaccharides are:



What is the test for reducing sugar?

2cm^3 of food sample (if not in liquid form grind it with water).

Add an equal volume of Benedict's reagent (an alkaline solution of copper sulfate).

Heat in a gently boiling water bath for 5 minutes.

If reducing sugar is present, a red precipitate will form (copper I oxide) (colour change from blue to brick red).

(The colour can vary depending on how much reducing sugar is in the sample; it is semi-quantitative). (Can go from green to red).


What are some examples of disaccharides and which monosaccharides are they made from?

Sucrose - fructose and glucose
Lactose - galactose and glucose
Maltose- glucose and glucose


What is the name of the bond produced by a condensation reaction?

A glycosidic bond


What is the reaction which splits up two monosaccharides called?

Hydrolysis (splitting with by adding water)


What is the test for non-reducing sugars?

Carry out Benedict's test. If no colour change proceeds reducing sugar is not present.

Add another 2cm^3 of food to 2cm^3 of dilute hydrochloride acid and place the test tube in a gently boiling water bath for 5 minutes. The dilute acid hydrolyses any disaccharide present into its monosaccharides.

Add some sodium hydrogencarbonate solution to neutralise the hydrochloride acid (Benedict's reagent will not work in acidic conditions). Test with pH paper to check that the solution is alkaline

Re do the Benedict's test. If a non-reducing sugar was present in the original sample a orange-brown colour change will be seen. This is due to the reducing sugars produced in the acid hydrolysis.


Fill the gaps...
All... Monosaccharides are ................. Some disaccharides are ................... (.............)

Fill the gaps...
All monosaccharides are reducing sugars. Some disaccharides are reducing sugars (Maltose).


Polysaccharides are...

Very large - Insoluble - storage
Polymers - monosaccharides joined by glycosidic bonds


Starch is found in plants in...

Starch grains in chloroplasts


What is the test for starch?

2cm^3 of the sample into a test tube
Add two drops of iodine solution and shake or stir
If starch is present there will be a colour change from orange to blue-black


Enzymes are specific and therefore...

It usually takes more than one enzyme to completely break down a large molecule.


The enzymes are produced in different parts of the digestive system because...

They function at different pHs


Talk through the various stages of the digestion of starch up to food going down the Oesophagus.

Food is chewed to provide a larger surface area.

Saliva mixed with food thoroughly.

Salivary amylase is secreted by the salivary glands and hydrolyses starch into maltose.(it contains mineral salts that maintain the neutral pH - the optimum pH for amylase).


Talk through the various stages of the digestion of starch from food going down the Oesophagus up to enzyme activity in the small intestine.

The food is swallowed and enters the stomach where the conditions are acidic. This denatures the amylase and prevents further hydrolysis of starch.

After a time, food is passed into the small intestine where it mixes with the secretions from the pancreas - pancreatic juice (containing pancreatic amylase - any remaining starch is hydrolysed to maltose.)

Alkaline salts are produced by the pancreas and the intestinal wall to maintain neutral pH so that amylase can function.

Muscles in the intestinal wall push the food along the intestine. It's epithelial lining produces the enzyme maltase. The maltase hydrolyses the maltose into alpha-glucose.


Aside from maltose which other two disaccharides are common components of the diet and must be broken down.

Sucrose- contained within cells and must be physically broken down by the teeth in order to release it. The epithelial lining produces sucrase which hydrolyses sucrose into glucose and fructose.

Lactose is found in milk - the epithelial lining in the small intestine produces lactase which hydrolyses lactose into glucose and galactose.


Why are some people lactose intolerant?

Milk is the only food of young babies so they produce lots of lactase, but in adulthood milk is a smaller part of the diet and so the amount of lactase produced naturally decreases but some people produce no lactase at all. They are lactose intolerant.


What happens when undigested lactose reaches the small intestine?

Microorganisms break it down giving rise to a large volume of gas. It may result in bloating, nausea, diarrhoea and cramps.


What is the nutritional problem with being lactose intolerant?

The main difficulty is taking in sufficient calcium in the absence of milk. This can be resolved by taking in foods rich in calcium or adding the enzyme lactase to milk before drinking it. Babies need to be fed special non-milk that is rich in calcium and vitamin D.


How large are proteins?

Very large


The types of carbohydrates and lipids are relatively few and they are very similar. How does this differ from proteins?

There are numerous proteins which differ from species to species.


Are the shape of proteins all the same?

No, every different type of protein is a different shape.


What are the basic monomer units for polypeptides (protein polymers) called?

Amino acids

20 occur naturally
100 have been found


What makes up every amino acid?

An amino group (-NH2)
A carboxyl group (-COOH) (acidic group)
Hydrogen atom (-H)
R group (a variety of different chemical groups; each amino acid has a different R group).


Amino acid monomers combine to form a DIPEPTIDE. What is this process called?



How are condensation reactions formed?

By combining an (-OH) from the carboxyl group of one amino acid with an (-H) from the amino group of another amino acid.


What bond joins amino acids?

A peptide bond


A peptide bond can be broken by which process?



What is the process in which a series of condensation reactions take place and a polymer is formed?



What forms the primary structure of a polypeptide?

The sequence of amino acids.


How is there almost an unlimited number of primary protein structures?

Each polypeptide has hundreds of the 20 naturally occurring amino acids joined in any sequence and so there are a limitless numbers of combinations.


Which structure determines a polypeptide's ultimate shape?

It's primary structure. A change in the primary sequence can lead to a change in the shape of the protein and may stop it carrying out its function.


How many polypeptide chains are there in a protein?

A single protein may consist of a single polypeptide chain but usually it has a number of chains.


How do hydrogen bonds form in proteins?

The linked amino acids both possess a-NH and -C=O on either side of every peptide bond.

The H on the -NH has a positive charge and the O on the -C=O has a negative charge. The two groups form WEAK hydrogen bonds.

The polypeptide is twisted into a 3-D a-helix. This is the secondary structure.


How is the tertiary structure maintained?

By a number of different bonds...
Disulfide bonds - fairly strong (not easily broken)

Ionic bonds - which are formed between any carboxyl and amino group not involved in peptide bonds. They are weaker than disulfide bonds and are easily broken in changes of pH.

Hydrogen bonds - numerous but weak


What is the tertiary structure?

The secondary structure of a protein can be twisted and folded even more to give the tertiary structure.


What makes proteins so specific?

Their 3D shape


What is a quaternary structure?

It is the combination of a number of different polypeptide chains and associated non-protein (prosthetic) groups in a large complex protein molecule. E.g iron containing heam group


What is the test for protein?

Place 2 cm^3 of the sample in a test tube
Add an equal volume of Biuret reagent
If protein is present the sample will go from blue to purple.


What are the two basic protein types?

Fibrous proteins - collagen - have structural functions
Globular proteins - enzymes/heamaglobin - carry out metabolic functions.


Fibrous proteins run parallel to each other. The chains are linked by cross-bridges and so form very stable molecules. What is the molecular structure of the fibrous protein collagen?

Primary - unbranched polypeptide chains
Secondary - very tightly wound
Tertiary - twisted into a second helix
Quaternary - made of of three such polypeptide chains sound together in the same way as individual fibres are wound in a rope.