The Human Digestive System - GCSE AQA Biology

Specification: 4.2.2.1

Biological molecules

Produced by cells

Nutrients

substances needed for growth, repair and metabolism

Biological molecules/Nutrients

Carbohydrates, proteins and lipids

What are Carbohydrates made from? (Simple and Complex)

Carbon, hydrogen and oxygen
Small: simple sugars - glucose and fructose - monomers
Complex: Glycogen and starch - polymers

What are Carbohydrates broken down into 

There are chemical bonds between monomers -- can be broken down into simple carbs: Starch ----> Glucose molecules

• Carried out by enzymes in mouth and small intestine 

Glucose - Starch, glycogen and cellulose 

Proteins

• Amino acids made up of glucose and nitrate ions and are monomers and protein is polymer

• Made up of Carbon, hydrogen, oxygen and nitrogen 

• Enzymes in stomach and small intestine break down proteins to amino acids

Lipids (Fats and Oils)

• Fats - solid at room temperature 

• Oils - lipids that are liquid at room temperature

• Contain single glycerol molecule attached to 3 fatty acid molecules (often different lengths)

• Length and structure of fatty acid molecule determines whether lipid is fat or oil 

• Made up of carbon, hydrogen and oxygen atoms - are not polymers as they aren’t formed from monomers 

• Enzymes in small intestine break lipids down inside the body

What are enzymes?

Proteins that speed up rate of chemical reactions
Biological catalysts that increase the speed of a reaction without being changed or used up in the process
Made by living organisms - large proteins are made of amino acids

How do enzymes work?

Substrate ----catalyst---> products - Hydrogen peroxide ---catalyst---> water + oxygen 

Reactant (substrates) broken down into small pieces (Product) 

Enzymes contain active site - complementary to substrates - otherwise reaction won’t be catalysed  - Allows enzymes to be specific to certain reactions

Lock and Key model and Induced fit model

Lock and Key model - Original model - assumes that substrate has to fit perfectly into active site

Induced fit model - More realistic - enzymes actually changes shape slightly as it binds to the substrate (complementary)

Factors affecting rate of enzyme action

Temperature and pH

Factors affecting rate of enzyme action: Temperature 

As temperature increases, so does rate of reaction as all particles have more kinetic energy therefore they are more collisions and more energy to react 

After 37C (Optimum temperature for most enzymes), rate drops rapidly as high temperature starts to break apart the bonds holding the enzymes together - active site denatures so enzymes won’t bind to substrate and catalyse the reaction 

Once the enzyme’s active site denatures, the damage is permanent 

All enzymes have optimum temperature

Factors affecting rate of enzyme action: pH

Measure of acidity - if it gets too high or too low, lower rate of reaction as some of the bonds holding the enzymes together will start to break down and active site will start to change shape, slowing down rate of reaction until enzyme’s active site completely denatures 

Optimal pH - depends on where enzymes work - stomach enzymes optimum pH is 2 as it is an acidic environment 

How are Carbohydrates broken down?

Most carbohydrates are starch - amylase used to break down starch into maltose molecules 

Maltose molecules broken down by maltase into glucose molecules which are small so can be absorbed into blood stream

How is Protein broken down?

Proteins broken down by protease into amino acids (many type of amino acids) - such as trypsin and pepsin

How are Lipids broken down?

Lipids broken down by lipase enzymes into glycerol and fatty acids 

Bile helps in breakdown of lipids however is not an enzymes - emulsifies lipids

Emulsifies - Takes big droplets of the lipids and breaks them down into smaller droplets which increases the surface area for lipase enzyme to break down lipids properly

What does bile do?

Bile helps in breakdown of lipids however is not an enzymes - emulsifies lipids

Emulsifies - Takes big droplets of the lipids and breaks them down into smaller droplets which increases the surface area for lipase enzyme to break down lipids properly

Where is is amylase, lipase and protease made?

All made by pancreas and small intestine
Amylase made in salivary glands
Protease made in stomach

What is Digestion?

Process by which large food molecules that are eaten are broken down into much smaller molecules

What is Absorption?

Process of absorbing small molecules (vitamins, minerals, water absorbed into blood stream to be sent round the body)

Role of Mouth in Digestion

Contains salivary glands and teeth ‘

Salivary glands are watery so make food easier to swallow

Salivary glands release saliva which contains amylase which digests starch 

Teeth - physically breaks down food via chewing, increasing SA of food and making it easier for enzymes to break down the food - easier to swallow

Process of moving food down the digestive system (through oesophagus as well)

Peristalsis 

Role of Stomach in Digestion

Contracts muscular walls 

Produces pepsin  

Produces HCl acid which kills bacteria and provides suitable pH for pepsin

Role of Pancreas in Digestion

Pancreas releases pancreatic juices into small intestine - liquid mix of enzymes - amylase, protease and lipase 

Role of Gall Bladder in Digestion

Bile is alkaline

Neutralises acid from the stomach making the pH more ideal for digestive enzymes in the small intestine

Emulsifies lipids

Made in the liver and stored in gall bladder

Role of Small Intestine in Digestion

Has mixture of food, pancreatic juices and bile 

Where most of the digestion takes place

Where nutrients are absorbed from the intestines into the blood stream 

Releases digestive enzymes

Role of Villi in the Intestines

Inside of intestines are covered in villi

Have huge surface area for diffusion 

Single layer of surface cells - short distance for diffusion 

Really good blood supply to maintain concentration gradient for absorption of nutrients (blood vessels on the inside)

All villi have microvilli which increase the surface area and rate of diffusion/absorption

Rest of Digestion from Small Intestine

When all nutrients are broken down and all useful materials are absorbed in the small intestine - the leftover is still watery 

Leftover passes into large intestine which absorbs most of the excess water and leaves behind the faeces which is stored in the rectum but is removed through the anus when ready