Tagged: starch
Starch Digestion: Grade 9 Understanding for IGCSE Biology 2.29
You must remember that “Digestion” has a specific meaning in Biology. It is the term used for the process that involves the chemical breakdown of large, insoluble food molecules into smaller, simpler molecules that can be absorbed into the blood. Many of the molecules in food are polymers – that is macromolecules made from long chains of repeating subunits. Examples of dietary macromolecules include proteins, polysaccharides and fats. These molecules are too large to be able to pass into the blood in the villi of the small intestine and so the body has evolved to chemically break them down into their constituent monomers or building blocks. Digestion is the process in the alimentary canal that achieves this.
Digestion reactions are also known as hydrolysis reactions because a molecule of water is required in the reaction to break the covalent bond holding the monomers together. These reactions are all catalysed (sped up) by specific molecules called digestive enzymes.
Why do different food types need different digestive enzymes to speed up their breakdown in the digestive system?
(If you are unsure, you need to revise the way enzymes work to catalyse reactions by a “lock and key” theory?)
Digestion of Carbohydrates
Many simple carbohydrates (e.g. glucose) do not need digesting. This is because they are already small enough to be absorbed into the blood directly in the ileum (small intestine). But larger disaccharide sugars (e.g. maltose and sucrose) do need to be broken down, as do all polysaccharides (e.g starch).
The family of enzymes that break down carbohydrates are called carbohydrases.
Starch is a large polysaccharide made up of many hundreds of glucose residues linked together. It is way too big to be able to cross the epithelial lining of the small intestine and so needs to be digested. This happens in a two-stage process. Firstly there is an enzyme amylase that can catalyse the following reaction:
starch + water ——-> maltose
Amylase is made in the salivary glands and so works in the mouth. But the main region for the digestion of starch is in the duodenum. This is because amylase is also made in the pancreas.
Maltose is a disaccharide molecule made of two glucose residues joined together. Maltose itself requires digesting to its constituent glucose molecules in order to be absorbed. So the second stage in the digestion of starch involves a second enzyme, maltase that is found embedded into the epithelial lining of the ileum. Maltase catalyses the breakdown of a molecule of maltose into two molecules of glucose which can be absorbed into the blood.
maltose + water ——> glucose
Starch Digestion: Grade 9 Understanding for IGCSE Biology 2.29
You must remember that “Digestion” has a specific meaning in Biology. It is the term used for the process that involves the chemical breakdown of large, insoluble food molecules into smaller, simpler molecules that can be absorbed into the blood. Many of the molecules in food are polymers – that is macromolecules made from long chains of repeating subunits. Examples of dietary macromolecules include proteins, polysaccharides and fats. These molecules are too large to be able to pass into the blood in the villi of the small intestine and so the body has evolved to chemically break them down into their constituent monomers or building blocks. Digestion is the process in the alimentary canal that achieves this.
Digestion reactions are also known as hydrolysis reactions because a molecule of water is required in the reaction to break the covalent bond holding the monomers together. These reactions are all catalysed (sped up) by specific molecules called digestive enzymes.
Why do different food types need different digestive enzymes to speed up their breakdown in the digestive system?
(If you are unsure, you need to revise the way enzymes work to catalyse reactions by a “lock and key” theory?)
Digestion of Carbohydrates
Many simple carbohydrates (e.g. glucose) do not need digesting. This is because they are already small enough to be absorbed into the blood directly in the ileum (small intestine). But larger disaccharide sugars (e.g. maltose and sucrose) do need to be broken down, as do all polysaccharides (e.g starch).
The family of enzymes that break down carbohydrates are called carbohydrases.
Starch is a large polysaccharide made up of many hundreds of glucose residues linked together. It is way too big to be able to cross the epithelial lining of the small intestine and so needs to be digested. This happens in a two-stage process. Firstly there is an enzyme amylase that can catalyse the following reaction:
starch + water ——-> maltose
Amylase is made in the salivary glands and so works in the mouth. But the main region for the digestion of starch is in the duodenum. This is because amylase is also made in the pancreas.
Maltose is a disaccharide molecule made of two glucose residues joined together. Maltose itself requires digesting to its constituent glucose molecules in order to be absorbed. So the second stage in the digestion of starch involves a second enzyme, maltase that is found embedded into the epithelial lining of the ileum. Maltase catalyses the breakdown of a molecule of maltose into two molecules of glucose which can be absorbed into the blood.
maltose + water ——> glucose
Biological Molecules: Grade 9 Understanding for IGCSE Biology 2.7 2.8
You will have studied the Biological Molecules section in some detail I would imagine, perhaps in more detail than is absolutely required for the specification. This post is meant to help you focus your understanding onto those points that are most likely to be tested in iGCSE questions. Here goes…
You do need to understand some chemistry for this topic to make sense. In particular you need to understand what is meant by the following terms:
- atom
- molecule
- element
- compound
My personal definitions would be as follows:
Atom: the smallest particle that retains the chemical properties of the element – a structure made up of protons, neutrons and electrons
Molecule: a particle made of two or more atoms chemically bonded together – may contain just one type of atom or several
Element: a substance in which all the atoms are the same
Compound: a substance containing more than one type of element
Back to safer ground…..
Living organisms are made from a fairly small group of molecules. The commonest molecule in every organism is water and in humans water makes up about 70% of the mass. But if you were to remove all water, leaving behind just the dry mass, the most common molecules could be grouped into proteins, lipids, carbohydrates and nucleic acids (e.g DNA)
Carbohydrates contain just three elements – carbon, hydrogen and oxygen
Lipids (fats and oils) contain three elements – carbon, hydrogen and oxygen
Proteins contain four or five elements – carbon, hydrogen, oxygen, nitrogen and sometimes sulphur
Big idea: many of the molecules that living things are made from are examples of polymers. A polymer is a large molecule made up of a long chain of repeating subunits (called monomers)
Carbohydrates are grouped into three main types:
Simple sugars like glucose or fructose – these are called monosaccharides.
Some sugars like sucrose are made of two simple sugars joined together – these are called disaccharides
Some carbohydrates are macromolecules (polymers) made of many hundreds of sugar residues joined together – these are called polysaccharides.
You can see from the diagram above that there are three important polysaccharides in living organisms. All three are polymers of the sugar glucose but the arrangement of the glucose residues is different. Cellulose is the main constituent of plant cell walls. Starch is a storage polysaccharide found in plants and Glycogen is a similar storage molecule found in liver and muscle tissue in animals.
Glucose is detected using a Benedict’s Test. Heat the solution with Benedict’s,reagent to 90 degrees for 5 minutes. A positive test for glucose is a brick red colour.
Starch is tested for using iodine solution (in potassium iodide) Iodine solution turns blue-black in the presence of starch.
Proteins are also polymers but this time the individual monomer is not a sugar but a molecule called an amino acid.
This protein is then folded up into a complex 3D shape using a whole load of weak bonds that can easily be broken at high temperatures. This is why enzymes, made of protein, denature at high temperatures.
There are 20 different amino acids that could be incorporated into a protein so there are an almost limitless variety of different proteins that can be made.
Lipids are a group of water-repelling molecules that again contain C,H and O atoms. They used to be separated into fats and oils depending in whether they are a solid (fat) or liquid (oil) at room temperature. Many lipids are a type of molecule called a triglyceride and this is made of a single molecule of glycerol attached to three fatty acid tails.
PMG Biology 