Biology AQA GCSE Year 9 - The Human Digestive System

What is the digestive system?

An organ system in which several organs work together to digest and absorb food.

What are enzymes?

. Proteins

. Act as biological catalysts (speed up reactions inside living organisms)

What types of reactions do enzymes catalyse?

. Extracellular reactions e.g digestion

. Intracellular reactions a.k.a metabolic reactions

What is an anabolic reaction?

Making larger molecules from smaller molecules

What is a catabolic reaction?

Breaking larger molecules into smaller, useful molecules

What is the active site?

The part of the enzyme that determines the function of the enzyme.

What molecules are enzymes made up of?

amino acids

How do enzymes work?

. The protein molecule folds up to give it a very unique 3D shape

. Each enzyme catalyses a specific reaction

. This is because the shape of the active site is complementary to that of a specific substrate

. The substrate will fit into the active site of the enzyme

. When together, they form the enzyme-substrate complex

. When the reaction is done, we are left with the enzyme and the products

What is the 'lock and key' hypothesis?

. In the same way that a key fits into a lock, so a substrate is thought to fit into an enzyme's active site

. The enzyme is the lock, and the substrate/reactant is the key (the lock and key hypothesis)

What are the key words you should always use in enzyme questions?

. Enzyme

. Substrate

. Active site

. Complementary

. Shape

. Enzyme-substrate complex

. Product

. Specific

Why do we have digestion?

. Most food molecules are too big to get through the small intestine wall into the blood

. Large insoluble molecules are broken down by digestive enzymes into small soluble molecules that can be absorbed into the bloodstream

What breaks down carbohydrates?

Carbohydrase (amylase is a carbohydrase which breaks down starch)

What is the product of starch being broken down by amylase?

glucose

Where is amylase produced?

. Salivary glands

. Pancreas

. Small intestine

Where does amylase activity take place?

. mouth

. small intestine

What breaks down proteins?

protease

What are the products of proteins and protease?

amino acids

Where is protease produced?

. stomach

. pancreas

. small intestine

Where does protease activity take place?

. stomach

. small intestine

What is the difference between a fat and an oil?

A fat is solid while an oil is liquid at room temperature.

What breaks down lipids?

lipase

What are the products of lipids and lipase?

. fatty acids

. glycerol

Where is lipase produced?

. pancreas

. small intestine

Where does lipase activity take place?

small intestine

Where is hydrochloric acid produced?

stomach

Where does hydrochloric acid act?

stomach

What is the function of hydrochloric acid?

. enzymes in the stomach work most effectively in acidic conditions

. kills bacteria in food

Where is bile produced?

liver

Where is bile stored?

gall bladder

Where does bile act?

small intestine

What is the function of bile?

. alkaline to neutralise the hydrochloric acid that was added to food in the stomach. This provides alkaline conditions in which enzymes in the small intestine work most effectively

. Emulsifies lipids to form small droplets which increase the surface area of the lipid so the lipase can work more effectively

. This increases the rate of fat breakdown by lipase

Are food tests qualitative or quantitative?

qualitative

What happens after food has been digested?

It passes through the wall of the small intestine and is absorbed into the blood stream

What does the small intestine have to increase its surface area?

villi

How are villi adapted for efficient exchange?

. Have very good blood supply that maintains a concentration gradient

. Having many villi and microvilli increases the surface area of the exchange surface

. Thin walls which create a short path for diffusion

What happens to undigested food?

. The muscular walls of the small intestine squeeze the undigested food into the large intestine

. Water is absorbed into the blood from the large intestine

. The material left forms faeces which are stored in the rectum before leaving the body via the anus

What is the relationship between time taken and rate of reaction?

. 1/time(s) = rate (s to the power of -1)

. They are opposites - the higher the rate of reaction, the lower the time taken

What factors can affect the rate of an enzyme-controlled reaction?

. temperature

. pH

. All enzymes work best at 1 particular temp and pH - this is called the optimum

. Different enzymes have different optimum temp and pH values

What do you always have to do in questions describing a graph?

Quote values WITH UNITS and describe trends

Describe the effect of temperature on an enzyme-controlled reaction

. At low temps, enzyme-controlled reactions go slowly because the molecules have low kinetic energy. So fewer collisions between the enzyme's active site and substrate, so slower reactions

. Rise in temp increases kinetic energy of enzyme and substrate molecules. More collisions occur more often. Rate of reaction increases

. The optimum temp is the temp at which the rate of reaction reaches its maximum

. The enzyme's active site starts to change shape (denature) and the substrate fits less easily into it. The rate of reaction decreases

. Rate of reaction reaches zero as the enzyme has been fully denatured. Substrate can no longer bind to the active site of the enzyme as their shapes are no longer complementary

Describe the effect of pH on an enzyme-controlled reaction

. At pH (quote optimum), which is the optimum pH, as the active site is complementary to the shape of the substrate the rate of reaction is at its fastest

. As the pH changes away from the optimum, extreme pH values denature the enzyme. The active site changes shape and is no longer complementary to the shape of the substrate so can't bind to it. This causes the rate of reaction to decrease

What is the structure of carbohydrates?

. made up of units of sugar

. Simple sugars contain 1 unit of sugar (glucose) or 2 (sucrose)

. Complex carbohydrates (starch and cellulose) are made up of long chains of sugar molecules bonded together

What happens to carbohydrates after they're digested?

. Used in respiration to provide cells with energy

. They're reassembled into the storage form of carbohydrate in animals - glycogen

. Cellulose can't be digested so it is egested from the gut

What is the structure of proteins?

. Long chains of small molecules called amino acids

. 20 different types and different combinations create different proteins

What happens to protein after it's digested?

. Proteins are big molecules that are too large to pass through the gut wall. So they must first be broken down into amino acids

. Once inside the body, the amino acids are reassembled into the proteins the individual requires (protein synthesis) and fold to produce a specific 3D shape

. So the protein can be an enzyme, hormone, or form another structure like collagen

. Excess amino acids are broken down in the liver

. Protein is used in the body for growth and repair

. also acts as structural components of tissues and as hormones

What is the structure of lipids?

. Esters of fatty acids and glycerol

. 3 fatty acid molecules joined to 1 glycerol

. Solid at room temp = fat

. Liquid at room temp = oil

What happens after lipids are digested?

. they are reassembled in the body's cells into the lipids the cell needs, e.g for the cell membranes

They help us with:

. Insulation

. Protection

. Help to make certain hormones e.g oestrogen and testosterone

Describe how to test for sugar

1) Add Benedict's solution to food in a boiling tube

2) Heat in a water bath for 10 mins

3) If sugar is present, Benedict's solution will go from blue to brick red

Safety: Wear safety goggles, Benedict's solution is an irritant, avoid contact with skin and eyes

Describe how to test for starch

1) Add a few drops of iodine solution to a small sample of food in a spotting tile

2) If starch is present, the iodine solution will go from brown to blue/black

Safety: Wear safety goggles, iodine solution is an irritant, avoid contact with skin and eyes

Describe how to test for protein

1) Add a 1cm³ of Biuret reagent to a small amount of food in a test tube

2) If protein is present, the Biuret reagent will go from blue to purple

Safety: Wear safety goggles, Biuret reagent is an irritant, avoid contact with skin and eyes

Describe how to test for lipids

1) Add a few drops of ethanol to a small amount of food in a test tube

2) Shake the test tube and leave for 1 min

3) Pour the solution into a test tube of water

4) If lipid is present, a cloudy white layer will appear

Safety: Wear safety goggles, Ethanol is highly flammable, avoid contact with skin and eyes