Digestion
Starter
Quiz
Interactive quiz 1
Assessment objectives
6.1.1 Explain why digestion of large food molecules is essential.
6.1.2 Explain the need for enzymes in digestion.
3 The need for increasing the rate of digestion at body temperature should be
emphasized.
6.1.3 State the source, substrate, products and optimum pH conditions for one
amylase, one protease and one lipase.
Any human enzymes can be selected. Details of structure or mechanisms of action are
not required.
6.1.4 Draw and label a diagram of the digestive system.
The diagram should show the mouth, esophagus, stomach, small intestine, large
intestine, anus, liver, pancreas and gall bladder. The diagram should clearly show the
interconnections between these structures.
6.1.5 Outline the function of the stomach, small intestine and large intestine.
6.1.6 Distinguish between absorption and assimilation.
6.1.7 Explain how the structure of the villus is related to its role in absorption and
transport of the products of digestion.
Assessment objectives
6.1.1 Explain why digestion of large food molecules is essential.
Food needs to be broken down and reassembled.
Large food molecules need to be broken down into smaller ones.
6.1.2 Explain the need for enzymes in digestion.
Enzymes break down large food molecules into smaller ones.
Speed up the process of digestion by lowering the activation energy for the reaction.
Work at body temperature
The need for increasing the rate of digestion at body temperature should be emphasized.
Assessment objectives
6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one
protease and one lipase.
Amylase
Lipase
Protease
Enzyme
Source
Substrate
Product
Optimum pH
Any human enzymes can be selected. Details of structure or mechanisms of action are not
required.
Assessment objectives
6.1.4 Draw and label a diagram of the digestive system.
The diagram should show the mouth, esophagus, stomach, small intestine, large intestine,
anus, liver, pancreas and gall bladder. The diagram should clearly show the interconnections
between these structures.
6.1.5 Outline the function of the stomach, small intestine and large intestine
(Homework)
Stomach:
Secretes HCL which kills bacteria.
HCL provides optimum pH for pepsin.
Secretes pepsin for protein digestion.
Small intestine:
Intestinal wall secretes enzymes
Receives enzymes from the pancreas.
Has villi for absorption of food particles.
Large intestine:
Moves material that has not been digested along.
Absorbes water.
Produces faeces.
Assessment objectives
6.1.6 Distinguish between absorption and assimilation.
Absorption occurs when the food enters the body as the food molecules pass through a layer of
cells and into the bodies tissues. This occurs in the small intestine which has many villi that are
specialised for absorption.
Assimilation occurs when the food molecules becomes part of the bodies tissue. Therefore,
absorption is followed by assimilation.
6.1.7 Explain how the structure of the villus is related to its role in absorption and transport
of the products of digestion.
Summary:
Many villi increase the surface area for absorption.
Epithelium is only one cell layer thick and so food is quickly absorbed.
Microvilli on the villi increase the surface area for absorption further.
Protein channels and pumps are present in the microvilli for rapid
absorption.
The mitochondria in the epithelium provide ATP needed for active
transport.
Blood capillaries are very close to the epithelium so diffusion distance
is small.
The lacteal takes away fats after absorption.
IA practice
Investigation into the effect of villi on the rate of absorption
Task
•Record any possible limitations/weaknesses in the investigation as you do
it and try to suggest improvements. This will help you to complete the CE
part of the IA.
•DCP – record quantitative data in a table, including uncertainties and
making sure that all data is written to the same precision.
•DCP – Processing data. Show calculations for standard deviations and
statistical tests.
•DCP – Presenting data in a graph.
6.1.4 Draw and label a diagram of the
digestive system.
Humans are HETEROTROPHS
• Get their nutrients by breaking down
large molecules such as carbohydrates
and proteins into smaller ones.
SYNOPTIC
• Name three Biological molecules.
• Why are some Biological molecules known
as polymers?
• What is the monomer for Proteins?
• By what process do monomers join to form
larger molecules?
• What type of bond is formed between
mono-saccharides?
• What are the different types of glucose
molecules?
• Biological molecule made up of amino acid
monomers
• Biological molecule made up of glycerol
and 3 fatty acids
• Biological molecule made up of alphaglucose monomers. Composed of amylose
and amylopectin.
• Proper scientific term is oesophagus
• A muscular bag with an acidic
environment
• Ingestion- taking large pieces of food into
the body
• digestion- breaking down the food by
mechanical and chemical means
• absorption- taking up the soluble digestion
products into the body's cells
• assimilation- using the absorbed materials
• egestion- eliminating the undigested
material
• Egestion is NOT the same as
EXCRETION.
• Excretion is the removal of waste products
formed by biochemical reactions.
What is the function of the
digestive system?
• To break down large, insoluble molecules
into small, soluble ones so that they can
be absorbed into our bloodstream.
(This topic has strong, synoptic links with
BYA1 unit)
Large, insoluble
molecule
1. Protein
2. Starch (Complex
carbohydrate)
3. Lipid
Small, soluble
molecule
1. __________
2. __________
3. ___________ and
________
Digestion
• Physical – muscular
contractions of the
alimentary canal
• Chemical - enzymes
Practical:
1. What is the aim of your investigation? Write a null
hypothesis.
2. What is the Independent variable?
3. What is the dependent variable?
4. What variables need to be controlled and why?
5. What control experiment could have been done?
6. Write a conclusion based on the evidence collected.
• It comprises a long tube, the alimentary
canal or digestive tract (or simply gut)
which extends from the mouth to the anus,
together with a number of associated
glands. The digestive systems made up of
different tissues doing different jobs. The
lining wall of the alimentary canal appears
different in different parts of the gut,
reflecting their different roles, but always
has these four basic layers:
• The mucosa, which secretes digestive juices and absorbs
digested food. It is often folded to increase its surface area.
On the inside, next to the lumen (the space inside the gut) is a
thin layer of cells called the epithelium. Mucosa cells are
constantly worn away by friction with food moving through the
gut, so are constantly being replaced.
• The submucosa, which contains blood vessels, lymph vessels
and nerves to control the muscles. It may also contain
secretory glands.
• The muscle layer, which is made of smooth muscle, under
involuntary control.
• The serosa, which is a tough layer of connective tissue that
holds the gut together, and attaches it to the abdomen.
Chemistry of Digestion
• 1. Digestion of Carbohydrates
• By far the most abundant carbohydrate =
starch found in __________________ but
there may also be a lot of sugar (mainly
sucrose) and some glycogen (in meat).
• Salivary amylase starts the digestion of
starch. Very little digestion actually takes
place, since amylase is quickly denatured
in the stomach, but is does help to clean
the mouth and reduce bacterial infection.
• Pancreatic amylase digests all the
remaining starch in the duodenum.
Amylase digests starch molecules from
the ends of the chains in two-glucose
units, forming the disaccharide maltose.
Glycogen is also digested here.
• Disaccharidases in the membrane of the
ileum epithelial cells complete the
digestion of disaccharides to
monosaccharides. This includes maltose
from starch digestion as well as any
sucrose and lactose in the diet. There are
three important disaccharidase enzymes:
• Active transport requires energy in the
form of ATP, but it allows very rapid
absorption, even against a concentration
gradient.
• The carbohydrates that make up plant
fibres (cellulose, hemicellulose, lignin, etc)
cannot be digested, so pass through the
digestive system as fibre
Digestion of Proteins
• Pepsin (in gastric juice) digests proteins to
peptides, 6-12 amino acids long.
• Pepsin is an endopeptidase, which means it
hydrolyses peptide bonds in the middle of a
polypeptide chain. It is unusual in that it has an
optimum pH of about 2 and stops working at
neutral pH.
• Pancreatic endopeptidases continue to
digest proteins and peptides to short
peptides in the duodenum.
• Different endopeptidase enzymes cut at
different places on a peptide chain
because they have different target amino
acid
• Exopeptidases in the membrane of the
ileum epithelial cells complete the
digestion of the short peptides to individual
amino acids.
• Carboxypeptidases work from the Cterminal end,
• aminopeptidases work from the N-terminal
end, and dipeptidases cut dipeptides in
half.
• Protease enzymes are potentially
dangerous. SUGGEST why this is.
• Because they can break down other
enzymes (including themselves!) and
other proteins in cells.
• To prevent this they are synthesised in
the RER(BYA1) of their secretory cells
as inactive forms.
• The enzymes are only activated in the
lumen of the intestine when they are
required
• Pepsin is synthesised as inactive pepsinogen, and
activated by the acid in the stomach
• The pancreatic exopeptidases are activated by
specific enzymes in the duodenum
• The membrane-bound peptidase enzymes are
already in their active form – suggest why there is
no need for it to be first in an inactive form?
• do not have this problem since they are fixed, so
cannot come into contact with cell proteins.
• The lining of mucus between the
stomach wall and the food also protects
the cells from the protease enzymes
once they are activated.
Digestion of Triglycerides
• Fats are emulsified by bile
salts to form small oil
droplets called micelles,
which have a large surface
area.
• Pancreatic lipase enzymes
digest triglycerides to fatty
acids and glycerol in the
duodenum.
• Fatty acids and glycerol are
lipid soluble and diffuse
across the membrane (by
lipid diffusion) into the
epithelial cells of the villi in
the ileum.
• In the epithelial cells of the
ileum triglycerides are resynthesised (!) and combine
with proteins to form tiny
lipoprotein particles called
chylomicrons.
• The chylomicrons diffuse into
the lacteal - the lymph vessel
inside each villus.
• The chylomicrons are carried through
the lymphatic system to enter the
bloodstream at the vena cava, and are
then carried in the blood to all parts of
the body. They are stored as
triglycerides in adipose (fat) tissue.
• Fats are not properly broken down until
they used for respiration in liver or
muscle cells.