1. Which of the following is correct regarding the enzymes listed in the table?
Enzyme
Amylase
Lipase
Protease
A. Substrate polysaccharide
emulsified fat
dipeptide or polypeptide
B. Substrate emulsified fat
dipeptide or polypeptide polysaccharide
C. Product
amino acids
small polysaccharides
or monosaccharides
D. Product
small polysaccharides amino acids
or monosaccharides
fatty acids and glycerol
fatty acids and glycerol
2. Figure 1 below shows a cross-section through the small intestine and Figure 2 an
enlarged longitudinal section through a single villus.
Using these diagrams, outline three
ways in which the structure of the
small intestine is related to its function
of absorbing food.
(3 mks)
Figure 1
Figure 2
Enzyme
Amylase
1. A
A. Substrate polysaccharide
Lipase
emulsified fat
Protease
dipeptide or polypeptide
2. villus intestinal wall has many folds to increase surface area ( : volume ratio);
surface of villus close to blood vessels so materials can easily diffuse;
surface of villus close to lymph vessels so lipids can be easily absorbed;
greater surface area related to greater rate of diffusion;
villus wall consists of single layer of cells;
3 max
Do not accept microvilli – not visible in diagrams.
The intestinal wall of the villus has many folds to increase surface area, or the
SA:Vol. This increased surface area is related to an increased rate of
diffusion. Aiding in diffusion is the fact that the wall of the villus consists only
of a single layer of cells. The surface of the villus itself is located close to the
blood vessels, so that materials can easily diffuse into them. It is also located
close to the lymph vessels, so that lipids can easily be absorbed.
6.1 Digestion
6.1.1 Explain why digestion of
large food molecules is essential.
• Most food molecules are large polymers & insoluble
• Too big to pass through membrane or into blood
• Must be digested  smaller , soluble molecs absorbed to blood
• Series of chem reactions  smaller & smaller molecules
• Protein  amino acids
• Example: you eat egg  albumin (egg white) contains
amino acid serine  hydrolysis reactions break down prtn
into aa’s  serine absorbed @ small intestine  capillary,
bloodstream  pancreas cell  Transcription/Translation 
used to make a new protein hormone insulin molecule
• Lipids (triglycerides)  glycerol, fatty acids
• Carbs (poly/di/monosaccharides)  monosaccharides
• Nucleic acids (DNA/RNA)  nucleotides
• THEN, can be absorbed through cells of dig. system and blood
• Circulatory sys transports nutrients to body cells
• Body uses to make your “own” DNA, protein, etc.
6.1.2 Explain the need for enzymes in digestion.
• Enzymes = biol catalysts, increase rate of reaction
• Increase rate of digestion at body temperature!
• Activation energy reduced: normal rxn – higher act
energy –higher body temp----not possible in humans!
• Digestive enzymes specific for specific food types
• Secreted into lumen of gut
• Hydrolysis of insoluble food molec into soluble
products
• Break down macromolecules
• Starts @ mouth—salivary amylase
• Starch (amylose)  maltose  glucose
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.
• Pancreatic amylase:
• Source: pancreas cells
• Optimal pH 7.5 – 7.8
• Substrate: starch (amylose)
• End product: maltose (disaccharide)
• Action: hydrolysis of 1-4 glycosidic bonds
6.1.3 State the source, substrate, products and
optimum pH conditions for one amylase, one
protease and one lipase.
• Pepsin: (a protease)
• Source: stomach cells
• Optimal pH 2 - 3
• Substrate: polypeptide chains of amino acids
• End product: amino acids
• Action: hydrolysis of peptide bonds
6.1.3 State the source, substrate, products and
optimum pH conditions for one amylase, one
protease and one lipase.
• Pancreatic lipase:
• Source: pancreas cells
• Optimal pH 7
• Substrate: triglyceride lipid
• End product: glycerol & fatty acid chains
• Action: requires bile salts to emulsify lipids
(increases SA of lipid, for fat digestion, exposes
glycerol structure to enzyme
• Hydrolysis of ester bonds b/w glycerol and fatty
acids
6.1.4 Draw and label a diagram of the
digestive system.
• “Alimentary Canal”
• 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.
1. Stomach: @ end of esophagus (peristalsis, muscular)
Stores food, begins protein digestion.
(a) Lumen: mixes food w/gastric juice (pepsin, HCl, mucus)
(b) Gastric pits: mucus , enzymes and acid are secreted
(c) Mucus secreting cells. (protect surface of stomach from autodigestion)
(d) Parietal cells: produce HCl which kills microorganisms that enter
the digestive system (food & tracheal mucus). This also converts
inactive pepsinogen to active pepsin & helps degrade foods some.
(e) Chief cells: produces pepsinogen (protease enzyme)
 activated by HCl  pepsin, most active in acidic pH
6.1.5 Outline the function of the stomach,
small intestine and large intestine.
2. small Intestine: where digestion is completed & products
of digestion are absorbed into the blood stream
Duodenum:
bile from liver/gall bladder
trypsin (protease), lipase, amylase, bicarbonate from
pancreas
 Small molecules absorbed
1000s of villi (each w/cap bed and lacteal) increase Sfc Area
Most molecules absorbed @ cap bed
Lipids absorbed @ lacteal
To body cells via another capillary & used for energy or
building new macromolecules (assimilation)
(a) Villus: increase SA for absorption of products of digestion
(b) Microvilli: border of epithelial cell, increase the SA for absorption.
(c) Lacteals connect to lymphatic system for the transport of lipids
(d) In the wall of the small intestine are the blood vessels to transport absorbed products to
the general circulation, And muscles to maintain peristalsis
6.1.5 Outline the function of the
stomach, small intestine and large
intestine.
3. Large Intestine or colon: responsible for reabsorption of
water from gut.
Undigested food (unabsorbed) excreted
Water we drink/eat – much is reabsorbed, some passes
E. coli (mutualistic) synthesize Vitamin K for us!
(a) The lumen of the colon
(b) The mucus producing goblet cells
(b) Muscular walls to maintain peristalsis
6.1.6 Distinguish between
absorption and assimilation.
Insoluble food molecules are digested to soluble
products in the lumen of the gut.
Absorption:
• soluble products are 1st taken up by various
mechanisms into epithelial cells lining gut.
• These cells then load various absorbed
molecules into blood stream.
Assimilation:
• soluble products of digestion then transported to
various tissues by circulatory system.
• cells of tissues then absorb molecules for use
within tissues
6.1.7 Explain how the structure of the villus is
related to its role in absorption and transport of
the products of digestion.
• structure increases SA for absorption of digested food molecules
into circ/lymph systems for transport
• Digested nutrients from lumen of small int pass through single-cell
thickness of villus epithelium to get to cap bed or lacteal
• Undigested nutrients can’t pass through
• Blood/lymph from arteriole/lacteal enters villus, picks up nutrients,
leaves via venule/lacteal
(a) folds increase SA:VOL ratio by X 3
(b) Villi project into the lumen of the gut increasing the surface area by X 10
(c) Microvilli are outward folds of the plasma membrane increasing the surface area another
X10
6.1.7 Explain how the structure of the villus is
related to its role in absorption and transport of
the products of digestion.
• Blood supply in villus absorbs end products of
digestion from epithelial cells
• lacteals receive lipoproteins before transporting to
circulatory system.