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					                                           10.6 – Digestion

Digestion in a Saprophytic Fungus

Like animals, fungi need to use ______________ compounds as a source of energy

They are one of the principle groups of decay organisms – many feed on organic compounds in the
dead bodies of plants and animals

Organisms which feed on dead organic matter are called ______________________

A main group of saprophytic fungi are the MOULD FUNGI (e.g. _____________ _______________)

The body of a mould fungus consists of extremely thin threads called ________________

The hyphae have a wall made out of a polymer – ___________ (similar to cellulose) and inside the
hyphae are all the organelles which can be found in animal cells but NO _________________!

Notice that the hyphae are _________ made out of separate cells. They are said to be a
multinucleate cell or are said to have a non-cellular (coenocytic) arrangement

The extensive branched network of hyphae forms a ___________________ – this spreads over the
food source. The mycelium may be vast – biggest one known = 6km2

The hyphae penetrate the food on which they are growing and feed by secreting ______________
from the tips of the hyphae which digest the material on which they live

Enzymes secreted include carbohydrases, lipases and proteases

These enzymes catalyse the _________________ of __________________, ____________ and
__________________ (in a similar way to human enzymes)

This produces smaller molecules (_______________) such as amino acids and glucose which are
absorbed into the hyphae by facilitated diffusion and active transport. The products are then
assimilated (_____________) by the organism

As they feed, the hyphae ____________ and _____________ through the decaying food material.

The thin and much branched nature of the hyphae ensures that the mould has a ___________
surface area to volume ratio for efficient digestion and absorption

This digestion is _______________________ – it occurs outside the body of the fungus

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 This is similar to the first stage in the digestion of proteins and carbohydrates in the human gut –
 enzymes secreted by gastric glands mix with food in the lumen of the gut

Structure and Feeding in Rhizopus:

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Use of starch agar plates to measure carbohydrate digestion

The activity of a digestive enzyme can be determined in 2 ways:

1. ___________________________________________________________________________

2. ___________________________________________________________________________

One convenient method to measure the activity of the starch digesting enzyme amylase is to use
________________     _________________          ______________

Starch agar is made by adding starch to liquefied __________                _____________. The molten starch
agar is then poured into a petri dish and allowed to set

Samples to be tested are placed in cavities cut into the agar or solid samples can be placed on the

After several hours, the surface of the agar plate is covered with _______________ solution

Areas that contain starch are stained _____________/__________ whereas where starch has been
digested they remain clear

The _______________ of the clear area can be used as a measure of the concentration of amylase
in the sample

The larger the clear area, the further the amylase must have diffused since more concentrated the
amylase the steeper the diffusion gradient

This technique, in which the quantity of a substance is found by comparing its activity with a standard
sample is called an _______________

Similar techniques can be used with other enzymes

      Diagram to show use of starch-agar plates (Course text page 78):

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Human Digestive System - Overview
All animals depend on organic substances which have been originally made by ________________

They are _______________________

The organic substances (food) eaten by heterotrophs has 2 main functions in the body – to act as a
source of ______________ and to provide the raw materials for cell _______________ and _______

In humans, much the greater proportion is used as a fuel – as much as half may be needed to
provide the energy to maintain __________ ___________________

Energy is also needed for muscular activity, building new cells, active transport and other metabolic

The amount of energy used for each type of process varies according to the age of the person, their
activity levels and environmental temperature

Different individuals therefore need different proportions of carbohydrate, protein and fat

A _______________ ___________ should contain all the nutrients the body needs – including
vitamins, minerals, fibre and water

Heterotrophic organisms need to digest food compounds like starch, protein and lipids. This is
because they are __________ and _____________ and cannot be absorbed directly into the blood

Digestion is breaking down of food into molecules which are small enough to be absorbed into the

All heterotrophs use CHEMICAL DIGESTION – which breaks down large food molecules into smaller

The reactions involved are ______________ reactions in which water is added when the bonds
between different parts of the molecule are broken

The reactions are catalysed by _______________

___________________________ catalyse the hydrolysis of POLYSACCHARIDES e.g. starch and
DISACCHARIDES e.g. maltose

___________________ catalyse the breakdown of PROTEINS

__________________ catalyse the hydrolysis of LIPIDS

Vitamins, minerals and water need no digestion – they are already small or lipid soluble and so can
pass directly into the bloodstream

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A Summary of Chemical Digestion:

    Digestion of Carbohydrates

    Digestion of Proteins

    Digestion of Lipids

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The Structure of the Human Gut
Total length = _________ metres. So it fits into the body, parts of it are highly coiled and folded

Along the whole length of the canal there are muscles in the walls which produce waves of
contraction and relaxation – ________________________ WAVES – this moves and mixes food

____________________ is secreted along the whole length of the tube by cells lining its walls

This helps food to pass through without damaging the lining and protects the living cells from coming
into contact with digestive juices

Humans, like most other animals, secrete enzymes into the ________________ – the central cavity
of the gut where digestion takes place

              Diagram of Human Digestive System:

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There are 3 basic layers in the gut wall:

1. Mucosa

2. Submucosa

3. Muscluaris externa (outer muscle layer)


This is the innermost layer
All the glands in the gut wall are formed from the mucosa


The submucosa contains vessels of the circulatory system whose main function is to transport food
materials away from the gut

The submucosa in some regions of the gut contains glands formed from the mucosa

Muscularis externa (outer muscle layer)

This is made up of 2 layers of muscle – the outer layer of muscles run lengthways (along the tube),
the inner sheet is arranged circularly (around the tube)

Alternate contractions of these muscles, peristalsis, forces food along the gut

Peristalsis is a slow, continuous process. It is like a wave along the gut


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Regions of the Gut - Introduction
As food travels along the gut it passes through several different regions

The structure of the wall in these different regions is __________ the same. Each region is adapted
to performing a particular function and different regions have different structures. These functions





The specific features of the gut wall found in each region are called ____________________
(see table 2 page 76 Course text )

    Table to show structure and function of the gut – Complete!

    Region of Gut         Functions              Adaptations of          Adaptations of         Adaptations of
                                                 muscle layer            submucosa              mucosa




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The Mouth
Mechanical breakdown of food

3 pairs of salivary glands (gland = organs which secrete substances) secrete _________ into the

This is a mixture of water, mucus and the enzyme __________________

Amylase catalyses the breakdown (hydrolysis) of ___________________

It breaks the  1-4 links between glucose molecules within the starch molecule BUT is not able to
break links of glucose molecules on the end of a chain

Digestion of amylase produces ____________ (a disaccharide) and small chains of 3 or 4 glucose
molecules but NOT individual glucose molecules

Saliva also contains an enzyme called ______________ – this destroys bacteria causing infections in
the mouth and throat

In this part of the alimentary canal, digestion is ___________________ – because it happens outside
the cells that make up the gut wall

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The Stomach – Structure and Function
Muscle layers in the muscularis externa (outer muscle layer) produce strong, rhythmical churning
movements when there is food in the stomach

This ______________ food with juices secreted in the stomach and continues the mechanical
breakdown of food

Mucosa of the stomach wall is specialised to produce large quantities of ______________
_____________ – this contains protease and lipase enzymes and hydrochloric acid

                   The Stomach:

To protect the walls from damage from acid here they are covered by a ______________containing
hydrogencarbonate ions which ______________________ the acid

The protease enzyme is called _______________ and it is secreted from large cells in the gastric
glands called CHIEF / PEPTIC CELLS

It is secreted in an inactive form called ___________________, to prevent it from digesting proteins
in the cells which produce it

Pepsinogen is activated by removing a strip of several amino acids from it – this exposes the
___________ ___________of the enzyme – rendering it active

This happens automatically when pepsinogen is exposed to the ________________ conditions in the

Pepsin catalyses the hydrolysis of peptide bonds within the protein molecule. It makes polypeptide
chains from proteins. It ____________ _________ break the bonds holding the ‘end’ amino acids
of the polypeptide chain.

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Proteases like pepsin are therefore called _____________________

Pepsin therefore breaks proteins into short chains of amino acids but ___________ individual amino
acid molecules

Pepsin molecules are only stable in acidic conditions . Optimum pH = 2/3. This is the pH of the
stomach because of the secretion of HCl

                             A Gastric Gland in the Stomach:

HCl also destroys potentially harmful microbes and is secreted by PARIETAL / OXYNTIC CELLS it
the gastric glands

_________________ in the gastric juice begins to hydrolyse triglycerides into fatty acids and glycerol
(only a small amount of this digestion occurs here though) In addition to this, the warmth and the
churning actions of the stomach turn solid fats into a fatty liquid

At this point, digestion is still _____________________

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The Small Intestine - Structure
Made up of the _____________ and _______________

Overall length = 5m

Duodenum = first 25cm

It is in the SI where most of the digestion and ___________________ occurs

The mucosa of the small intestine is greatly folded forming tiny projections called ______________

The muscles of the muscularis mucosa (muscles in the mucosa) contract and relax, so the villi sway
about, helping to bring their surfaces into contact with more of the contents of the small intestine

The cells making up the epithelium of the villi have a very folded cell surface membrane on the side
nearest to the lumen of the small intestine = _________________ or BRUSH BORDER

Villi and microvilli produce an enormous ____________ ______________ in the small intestine –
increasing the rate at which absorption can occur (see handout)

In ‘troughs’ between villi in duodenum, glands called CRYPTS OF LIEBERKUHN secrete mucus

They constantly produce new cells which move up the villi until they ‘drop off’

Deeper in the walls of the duodenum, in the submucosa, there are BRUNNERS GLANDS which
secrete a watery mucus containing hydrogencarbonate ions which neutralises acidic
______________ coming from the stomach

             Detail of villus structure

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Digestion in the Small Intestine
Brought about by enzymes from 2 sources:


The pancreas secretes _______________ ______________ which flows into the duodenum along
the pancreatic duct

Pancreatic juice contains ________________________ions (to neutralise acidic chyme) and

The enzymes it contains are amylase, the proteases _______________ and ________________
(both endopeptidases) and lipase

These enzymes continue to digest substances coming from the stomach

Digestion is _________________ by the enzymes produced by the cells on the surface of the villi
which is useful because it means products of digestion are next to the surface across which they can
be absorbed – it therefore increases the rate at which they are taken up into cells

Digestion of Carbohydrates

Amylase from the pancreas acts in the same way as amylase in saliva – catalysing the hydrolysis of
starch to maltose

Cells on the villi also produce carbohydrase enzymes:

            Maltase – catalyses hydrolysis of ______________ to

      Sucrase – hydrolyses sucrose to glucose and ____________

   Lactase – hydrolyses lactose to glucose and _____________
This final part of carbohydrate digestion is therefore ______________________ (since occurs on

    Carbohydrate Digestion in the Small Intestine:

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Digestion of Proteins

Trypsin and chemotrypsin and ______________________ secreted by the pancreas in
____________________ form

They need to have a piece removed before they can begin to digest proteins

An enzyme called ENTEROKINASE does this

Both of these proteases are endopeptidases and produce polypeptides from protein molecules

Most of the polypeptides produced are adsorbed onto the brush border of the cells on the villi

Here, exopeptidases  e.g. carboxypeptidase (attack end of polypeptide with free carboxyl group) /
aminopeptidase (attack end of polypeptide with free amino group)  secreted by these cells
breakdown polypeptides into dipeptides or individual amino acids

Any dipeptides are broken down by dipeptidases in the cytosol (Solution in the cell cytoplasm) This
part of digestion is therefore ___________________

 Protein Digestion in the Small Intestine

Digestion of Lipids

Only a little fat digestion occurs in the stomach, most occurs in the _______________

Lipase catalyses the hydrolysis of some triglycerides into ___________ _________ and glycerol

Other triglycerides are only partially broken down – leaving 1 fatty acid still attached to glycerol =

Lipids are hard to digest because they are not water soluble

Mechanical mixing in the stomach breaks large droplets of lipid into tiny droplets which become
suspended in a watery chyme. This sort of mixture is called an __________________

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Lipase can readily act on lipid in a lipid – water emulsion, because a ____________ ___________
_____________of lipid droplets is in contact with the watery part of the liquid in which the lipase
molecules are found

Once the chyme leaves the stomach, the tiny droplets would coalesce back into big droplets if it

These are present in ________________ and they increase the surface area available for lipase

Bile is produced constantly and is stored in the ____________ ________________

It is released down the bile duct into the duodenum when food enters the small intestine from the

Once lipase has hydrolysed the triglycerides to monoglycerides, fatty acids and glycerol, these,
together with the phospholipids and cholesterol in food are absorbed / diffuse thorough the
membrane of the microvilli as structures called ____________________

 Lipid Digestion in the Small Intestine

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Absorption in the Small Intestine
The area of the alimentary canal in which almost all absorption of nutrients occurs

A very large surface area from villi and microvilli increase the rate at which absorption can occur

Inside each villus is a blood capillary which can transfer absorbed nutrients to a branch of the hepatic
portal vein

There is also a blind ending lymph vessel called a LACTEAL – This is important in the absorption of

To get to blood or lacteal, nutrients must first cross the cell surface membrane on the outer brush
border surface of one of the cells on the surface of the villus and leave it across the csm on the side
furthest from the lumen

They then have to cross either the wall of the blood capillary or the wall of the lacteal – these walls
are adapted to allow various substances to pass in and out

Molecules can cross the csm of the villus cells by diffusion, facilitated diffusion and active transport.

                   Diagram to show epithelial cell, capillary and lacteal from a villus:


Is absorbed into the cells by _________________    active transport, involving the co-transport of
sodium ions. The passage of glucose is much more rapid when the concentration of sodium in the
lumen is relatively high

The presence of enzymes in the membrane also assists the uptake of glucose

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The glucose then moves out of the opposite side of the cell by simple and facilitated diffusion, into the
tissue fluid inside the villus then into blood vessels in the submucosa, then onto blood in the hepatic
portal vein which takes them to the liver

Glucose therefore ______ ___________ need to be digested and it is quickly absorbed by the
microvilli. It therefore has an _______________ effect on blood sugar levels

Starch needs to be digested before its monomers can be absorbed. The sugar is released slowly
from starchy foods and this is easier for diabetics to control

In wholemeal foods the outer surfaces of the cells need to be digested by protein and fat digesting
enzymes to release the starch grains before the starch can be digested – this slows down the rate of
sugar absorption even more

Amino Acids

These are absorbed into the villus by _____________ __       ___________ and pass out of the
opposite side by diffusion. They then follow the same pathway into circulation as monosaccharides

The presence of enzymes in the membrane also assists the uptake of amino acids

Fatty Acids and Glycerol

Are easily absorbed across the csm of the villus epithelial cells because they are ______________
soluble and so pass through by simple diffusion

Once inside the cell they are taken to the smooth ER where some are converted to triglycerides by
condensation reactions

Then they are moved to the golgi apparatus, where they are surrounded in a coat of protein,
phospholipid and cholesterol to form _____________________. These are moved out of the far side
of the cell into the tissue fluid in the villus by diffusion

                      A Chylomicron:

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They are too big to enter the blood capillary and so enter the _______________ (more porous)

The chylomicrons suspended in lymph inside the lacteals form a milky emulsion

The lacteals eventually drain into the lymph vessels then into the blood. From here the chylomicrons
pass to the ______________

Other Substances

Large amounts of water and inorganic ions are also absorbed in the small intestine – water moves by
_________________, inorganic ions by a combination of diffusion and active transport

Vitamins are also absorbed in the small intestine

Fat soluble vitamins e.g. A, D and E cross the csm by diffusion – either on their own or as part of a

Water soluble vitamins like Vitamin C and B are moved across csm by specific transporters, by
diffusion and active transport

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