B2.2.1 - The human digestive system

Define the human digestive system

an organ system where several organs work together to digest and absorb food

Ingestion

taking food into the body (occurs in the mouth)

Mechanical Digestion

food broken down from larger pieces into smaller pieces by the action of chewing in the mouth and churning in the stomach (therefore there is increased surface area for the enzymes to act on)

Chemical Digestion

breakdown of large, insoluble molecules into smaller soluble molecules using enzymes

Absorption

the small soluble molecules passing from the small intestine into the bloodstream (there is an increased rate of diffusion because of the large surface area, due to the villi and microvilli, and a short diffusion distance)

Assimilation

the cells making use of the molecules (e.g. glucose being used in respiration and amino acids being used for growth and repair)

Egestion

the removal of undigested material (faeces) from the body

Excretion

the removal of waste products from chemical reactions inside the body (e.g. removing urea and carbon dioxide)

Function of the mouth

• The beginning of digestion - ingestion occurs here

• Mechanical digestion occurs here: large food particles are chewed up by the teeth into smaller food particles, which provide a larger surface area for the enzymes to act on

• Chemical digestion occurs here: saliva contains amylase, which catalyses the breakdown of starch into glucose

Function of the oesophagus

The oesophagus connects the mouth to the stomach - it is a muscular tube that pushes chewed up food (bolus) into the stomach via a series of contractions called peristalsis

Function of the stomach

• Secretes hydrochloric acid (via the glandular tissue) which kills ingested bacteria and provides the optimal pH for protease (HCl has a pH of 1.5-2, and the optimal pH is 2)

• It also produces protease enzymes (mainly pepsin) which digest protein

• The muscular walls of the stomach churn the food, breaking it up into smaller pieces and increasing its surface area)

Function of the small intestine

• The small intestine produces protease, lipase, and amylase, thus continuing chemical digestion

• Soluble products of digestion are absorbed into the blood through the walls which are covered in projections called villi (which increase surface area thus increasing the rate of digestion)

Function of the large intestine

The large intestine (colon) connects the small intestine to the rectum, and absorbs the water from stool into the blood (stool is mainly made up of food debris and bacteria), and it also processes waste

Function of the rectum

Stores faeces

Function of the anus

Expels faeces

Function of the salivary glands

Produces and excretes saliva into the mouth (triggered by the smell of food) which contains amylase to digest starch into sugar

Function of the liver

• The liver processes absorbed food and detoxifies the blood

• It also makes bile, which is alkaline so it neutralises HCl from the stomach (to ensure that the enzymes in the small intestine are at their optimum pH)

• Bile also emulsifies fats, by breaking up large lipid drops into smaller lipid droplets (mechanical btw) which increases the surface area of lipids, thus increasing the rate at which lipase catalyses the breakdown of lipids into fatty acids and glycerol, which then diffuse across the lining of the small intestine into the bloodstream

Function of the gall bladder

Stores excess bile before being released via the bile duct into the small intestine

Function of the pancreas

• Produces digestive enzymes (carbohydrase, protease, lipase) and secretes them into the duodenum

• Also produces and secretes insulin (which metabolises sugar)

Function of the appendix

Maintains the gut microbiome

What elements make up carbohydrates

Carbon, hydrogen, oxygen

What are the monomers for carbohydrates

The monomer for carbohydrates are glucose (fructose is also a monomer, and sucrose and maltose are dimers (sucrose is one glucose and one fructose, while maltose is two glucoses), and all of these are simple sugars but sucrose isn't a reducing sugar)

What are the polymers for carbohydrates

The polymers for carbohydrates are starch (a long chain of glucose, used as an energy store in plants), glycogen (used as an energy store in animals), and cellulose (a support material used in plants, e.g. in the cell wall)

What is starch broken down into, and what is that product used for?

Starch is broken down into glucose, so that they can be used in cellular respiration to release energy, used in metabolic reactions in the cell

What foods contain carbohydrates

Foods that contain carbohydrates are rice, potatoes, bread, and pasta

What elements make up proteins

The elements that make up proteins are carbon, hydrogen, oxygen, and nitrogen

What are the monomers and polymers of proteins

The monomers are amino acids - there are around 20 amino acids and they are joined together by special bonds in long chains to form proteins (the polymers), and different arrangements of amino acids form various proteins (examples include keratin and collagen)

Describe the shape of proteins, and the reason for that shape

The protein is folded, coiled, and twisted into a specific shape so that smaller molecules can fit into it

State the functions of proteins in the body

• Acting as structural components of muscles and tendons

• Acting as hormones (e.g. insulin), which are chemical messengers

• Acting as antibodies (which are necessary for the immune system as they destroy pathogens)

• Acting as enzymes (biological catalysts)

• Growth and repair

• Enzyme (protein) synthesis

What are foods that contain proteins

Foods that contain protein include meats, fish, eggs, cheese, and pulses

What elements make up lipids

Oxygen, carbon, and hydrogen (note that lipids are insoluble in water)

What are the monomers of lipids

Fatty acids and glycerol

What are the polymers of lipids

Phospholids, triglycerides, and oils, but lipids themselves are not polymers

What are the functions of lipids in the body

• Acting as a (very efficient) store of energy

• Protecting organs (insulation)

• Used with other molecules for cell membranes

• Used with other molecules for hormones

• Used with other molecules for the nervous system

What are some foods that contain lipids

Cream, oils, cheese, and butter

Define enzymes

Enzymes are biological catalysts (they catalyse the breakdown of polymers)

Define a catalyst

Catalysts speed up chemical reactions without being used or changed themselves

What are enzymes made up of

Enzymes are proteins, which are made of amino acids (enzymes have around 100-1000 amino acids), and the amino acids are joined together in a long chain, which is folded into complex shapes so that smaller molecules can fit into it

What does carbohydrase do

Breaks down carbohydrates into simple sugars

What does amylase do

Amylase is a type of carbohydrase, and breaks down starch into glucose

What does protease do

Breaks down proteins into amino acids

What does lipase do

Breaks down lipids into fatty acids + glycerol

Explain the Lock and Key theory

• Enzymes have an active site - this is shaped specifically so that it exactly matches the shape of a substrate (the molecule that is being broken down/joined together) - this is called enzyme specificity

• The substrate (by chance) enters the active site, and binds to it, forming an enzyme-substrate complex

• The enzyme catalyses the breakdown of the substrate, and releases those products

• The enzyme is not changed/used up after the reaction

• Note: the enzyme is the lock; the substrate is the key

How do you prepare a food sample for RP4 (Food Tests)

Crush up the food using a mortar and pestle.

How do you test for starch

1. Add one spatula of the food sample onto a dimple tile

2. Add a few drops of iodine solution (orange-brown) to the dimple tile using a dropper

3. Record the name of the food sample and whether the colour changed (to blue-black)

How do you test for glucose (reducing sugars)

1. Add one spatula of the food sample into a test tube

2. Add 1cm³ of water and stir to mix

3. Add 10 drops of Benedict's solution (blue) and stir to mix

4. Put the test tube in a water bath for 10 minutes at 95 degrees

5. Record the name of the food sample and whether the colour changed (if the solution goes brick red glucose is present (but the colour can go green, yellow, and orange too, which mean a little, a medium amount, and quite a lot of sugar))

How do you test for proteins

1. Add 1 spatula of the food sample into a test tube

2. Add 1cm³ of water and stir to mix

3. Add 5-10 drop of Biuret reagent (blue)

4. Record the name of the food and if the colour changed (if protein is present the solution goes purple/lilac)

How do you test for lipids

1. Add 1 spatula of the food sample into a test tube

2. Add 2cm³ of ethanol to the test tube, put a bung firmly on the end of the test tube, and shake vigorously

3. Allow the contents to settle

4. Pour the liquid from the top of the mixture into a test tube half-filled with water

5. Record the name of the food and whether the water is cloudy or clear

What are safety precautions for RP4 (Food Tests)

• Wear safety goggles

• Wash hands immediately after coming into contact with solutions, because they are irritants

What is the optimum temperature for most enzymes in the human body

37°C, which is essentially the same as the normal body temperature

What happens if the temperature of the solution the enzyme is in is too low

There is less kinetic energy, so collisions between the substrate and the enzyme are less frequent and less successful, which means that less substrates will bond to the active site of an enzyme, so less enzyme-substrate complexes are formed, so the rate of the breakdown of the substrate will decrease

What happens if the temperature of the solution the enzyme is in is too high

The bonds that hold together the complex shape of the enzyme will start to break down, so the active site of the enzyme will change shape - the enzyme is denatured. This means that the substrate's shape is no longer complementary to the active site of the enzyme, so it will not bond to it, so less enzyme-substrate complexes are formed, so the rate of the breakdown will decrease

What is an optimum pH

The specific pH that an enzyme works best in

Is the optimum pH of all enzymes 7.0

• Not all enzymes have an optimum pH of 7.0 (but some enzymes like amylase do)

• An example is protease enzymes, like pepsin, which is produced in the stomach and has an optimum pH of 2.0, and because the stomach produces hydrochloric acid which results in a pH between 1.5 and 2.0, the protease enzymes are at their optimum pH

• Some lipases and proteases have very alkaline optimum pH (e.g. trypsin, with an optimum pH of 9) and these enzymes are used in laundry detergents

What happens if the enzyme is put in a solution that is not its optimum pH (it’s too acidic or alkaline)

• If the pH becomes too acidic or alkaline the rate of enzyme activity decreases, because that extreme acidity/alkalinity causes the active site of the enzyme to change shape because the bonds that hold the enzyme's structure together start to break - the enzyme is denatured

• This means that the substrates aren't able to bind to the active site of the enzyme anymore, because the shape of the substrate is no longer complementary to the shape of the enzyme's active site, so less enzyme-substrate complexes are formed, so the rate of the breakdown of the substrate decreases