Heterotrophic Nutrition

Syllabus 4.2.1 Heterotrophic nutrition

Definition of heterotrophic nutrition.

Description of the structure of the human alimentary canal in relation to digestion and absorption; histology of the ileum wall, to include the mucosa, submucosa, muscle layers and serosa.

The sources and effects of secretions concerned with the digestion of carbohydrates, lipids and proteins.

The nervous and hormonal control of enzyme release and gut activity (hormonal control to be exemplified by gastrin, CCK and secretin).

4.2.2 Adaptations in herbivorous and carnivorous mammals

Adaptation of ruminant mammals to their mode of nutrition:

i) dentition;

ii) the 4-chambered stomach of the alimentary tract, including mutualistic interactions;

iii) comparison of ruminants with hind-gut fermenters such as the rabbit.

Adaptations of carnivorous mammals to their mode of nutrition as shown by their dentition.

4.2.3 Saprophytic nutrition

Definition of saprophytic nutrition,

using Rhizopus as an example.

A) HETEROTROPHS B) DENTITION

C) CELLULOSE DIGESTION IN RUMINANTS

D) THE ALIMENTARY CANAL IN HUMANS

E) THE CONTROL OF DIGESTIVE SECRETIONS

Topic Overview

Heterotrophs are:

organisms that feed on complex, ready-

made organic food

all animals

fungi

majority of bacteria

on the synthetic activities of autotrophs

survival of heterotrophs depends:

either directly

or indirectly

Forms of heterotrophic nutrition :

1. holozoic

2. saprotrophic

3. parasitic

1. Holozoic nutrition

food taken in, is digested into smaller

soluble molecules which can be

absorbed and assimilated

Blood Food in intestine

enzyme

digestion

absorption

Processes involved in holozoic nutrition:-

1. Ingestion

2. Digestion

3. Absorption into the bloodstream

5. Egestion

4. Assimilation

Detritivores

e.g.earthworms and crabs, actively feed on dead organic material

Predators are animals that feed on living organisms:

herbivores prey on plants

carnivores prey on animals

omnivores prey on both

Filter feeders

e.g. clams and blue whales, prey on small organisms by filtering them from the aquatic environment

2. Saprotrophic nutrition organisms feeding on dead or decaying

organic matter

saprobes (also called saprotrophs or decomposers) are mostly protists and fungi

secrete enzymes on food

digested food is digested externally

Very soft!!

NOTE:

Saprotrophs used to be known as

saprophytes, but “-phyte” refers to a

plant and neither fungi nor bacteria are

plants, so saprotrophs or saprobes are

the terms now used to describe these

organisms.

[STILL: syllabus says „saprophytes‟]

3. Nutrients are

absorbed from all

over the hyphae.

1. Extracellular

enzymes from

growing tips.

2. Digestion takes place

outside the body.

The saprotrophic nutrition of Mucor and Rhizopus

Many of the simple substances formed are

not used by the saprotrophs themselves:

are absorbed by

plants

but

protease

the thin, much-branched

mycelium ensures a large

surface area for absorption

starch is broken down to glucose by:

carbohydrase

protein is broken down into amino

acids by:

Surplus food is stored in the hyphal cytoplasm

Glucose is

converted to:

glycogen & fat

Amino acids are

stored as:

protein granules

3. Parasitism is a close association between two living

organisms of different species which is

beneficial to one (the parasite) and harmful to

the other (the host)

the parasite obtains:

1. food &

2. generally shelter

a successful parasite is able to live with the

host without causing it any great harm

Parasites may be:

live on the outer

surface of a host e.g.

leech

ectoparasites cling to

host by:

ECTOPARASITES

Suckers e.g.

leech

Claws e.g.

lice

ENDOPARASITES

live within a host

Fasciola

Tapeworm

Ectoparasites in plants have haustoria for attachment:

Haustorium in the parasitic plant Cuscuta [dodder]

obligate parasites:

live parasitically at all times

Parasites may be:

facultative parasites:

e.g. fungi : feed parasitically at first

but having eventually killed their host:

continue to feed saprotrophically on

the dead body

A) HETEROTROPHS

B) DENTITION C) THE ALIMENTARY CANAL IN HUMANS

D) THE CONTROL OF DIGESTIVE SECRETIONS

E) CELLULOSE DIGESTION IN RUMINANTS

Topic Overview

Incisors:

situated at the front of the buccal cavity

have flat, sharp edges

used for cutting and biting food

Canines are pointed teeth

Highly developed

in carnivores Poorly developed

in humans

Canines are designed for:

piercing

killing prey

hold prey

tearing flesh

Premolars:

posses:

two cusps (projections on

the surface of a tooth)

used for:

crushing & grinding food,

in humans they may also

be used to tear food

root

cusp

Molars: each tooth has 4 or 5 cusps

used to crush & grind food

premolars are not present in the

milk teeth

Humans have two sets of teeth:

Deciduous

or milk teeth

[20 teeth]

Permanent

teeth [32 teeth]

2 1 2 32

2 1 2 3i c pm m

Total: 32

Upper jaw: 16 Lower jaw: 16

Dental Formula shows the arrangement of teeth:

Dental Formula for human permanent dentition

Dental Formula for a cat (permanent teeth):

Dental formula Upper jaw: 16

Lower jaw: 14 Canine [C]

Incisor [I]

Molar [M] Premolars [P]

3 1 3 12 30

3 1 2 1

i c pm m

Dentition in a carnivore (cat)

teeth are adapted to:

catch & break down animal food

Dentition in a carnivore (cat)

temporalis muscle [provides a powerful bite]

masseter muscle [needed to masticate]

1st molar (carnassial)

molar

canine

incisor

premolars

incisor

canine

3rd premolar (carnassial)

premolars

Incisors:

are closely fitting, small and chisel-

shaped

Canines:

enlarged, curved and fang-like

used to tear away flesh near the bone surface

Molars and premolars [Cheek teeth]:

carnassial teeth are the:

last upper premolar and

the first lower molar

Carnassials are:

enlarged with prominent ridges running parallel with the line of the jaw

shear (act like scissors) flesh from prey

Carnassial teeth:

p4 and m1

The other premolars and molars:

are flattened

possess sharp edges used for:

cutting flesh

cracking bones

Jaw point:

operates as a closely

fitting hinge

permits only up- and-down movement

Temporal / Temporalis muscle:

on contracting closes the

lower jaw [mandible]

provides a powerful bite

Function:

1. Food is crushed between molars

2. In carnivores, canines are clenched into the prey

Temporal muscle:

is attached to a prominent

bone extension from the

lower jaw which projects upwards towards

the ears

this arrangement provides efficient

leverage on the food:

as it is sheared by the teeth when the

mouth is snapped shut while killing

the prey

Masseter muscle: needed to masticate (chew)

pulls the base of the

lower jaw upwards

and reduces the

strain on the jaw joint

masseter

masseter

temporalis

temporalis

Temporalis & masseter muscles in carnivores & herbivores compared

temporalis

masseter

Carnivores Herbivores

Temporalis Large Small

Masseter Small Very large

Jaw movements Up & down jaw Sideways & back and forth

Jaw joint in herbivores: very loose and allows:

forward, backward and sideways movements

possible because: masseter is large in

herbivores to masticate the tough grass

during chewing the lower joint moves from

side to side

End-Of –Year June 2014

Write short notes to explain the following:

Despite the fact that carnivorous animals are meant

to feed on meat, they are occasionally observed to

chew on grass. (5 marks)

Carnivores do not digest the grass they consume,

however they use this as a source of roughage. The

relatively large undigested pieces of plant matter

prevent clogging of the intestines which can lead to

blockage of the intestines and over-absorption of

water – a condition known as constipation.

Also to get water.

Dentition in a herbivore (sheep)

Sheep crop grass

[bite off the ends]

Fig. 4 Jaws, dentition and musculature of the sheep.

Masseter muscle

[Large – needed for grinding]

Temporalis muscle

[Small – no powerful biting is

needed]

In a sheep:

upper incisors & canines are absent

a horny pad takes their place

chisel-shaped lower incisors & canines bite against the pad when the sheep is cropping grass

Herbivores can be divided into:

horse

Ruminants [horny pad

of gum]

Non-ruminants – hindgut fermenters [have teeth at the

front]

sheep

Dental Formula for a sheep

0 0 3 32

3 1 3 3

i c pm m

Total number of teeth on:

Lower jaw: Incisors: 6 Canines: 2 Premolars: 6 Molars: 6

Upper jaw: Incisors: 0 Canines: 0 Premolars: 6 Molars: 6

32

Function of:

Cheek teeth: grinding

Front teeth: cropping

Diastema:

is a gap between the:

front & cheek teeth

grass being chewed is kept apart from

that freshly gathered

provides space for the tongue to

manipulate cropped grass

Diastema:

Premolars and molars

possess broad grinding surfaces

Open roots in herbivores:

teeth grow throughout life

Closed roots & Open roots

Closed roots in

carnivores:

teeth stop growing

Write short notes to explain the following: The teeth in some mammals have open roots, meaning that the tooth keeps constantly growing. (5) [JC MAY 2014] Herbivores with gnawing habits continuously use teeth for grinding. This wears teeth and makes them unsuitable for mechanical digestion. Open roots ensure that teeth keep growing: so as to replace the layer of teeth removed due to friction and so that the teeth remain sharp even if they wear with use.

Ridges:

are composed of

hard enamel

Troughs:

composed of dentine

Overgrow

n teeth in

a rabbit.

The area of the premolars & molars is increased by the surfaces of the:

upper teeth being

folded into a W-shape

those of the lower teeth being folded into an M-shape

W-shaped ridges fit closely into the grooves of the M-shaped lower teeth during girding

Why do herbivores spend a great deal of time feeding?

Grass is low in energy and hard to digest

Question: MAY, 2012 This question is about herbivores.

The diagram below represents the lower

jaw of a herbivorous mammal.

Feature Function

1. Diastema A space that holds grass while

animal crops

2. Flat premolars and

molars

Offer a large surface area for

grinding grass

a) With reference to the diagram above, list TWO

features which indicate that the jaw is that of a

herbivore. Include the function of such features. (4)

b) Name ONE structural difference that

would be present in the jaw of a typical

carnivore but is not observed in that of a

herbivore. Give ONE reason for this

difference. (2)

Long and pointed canines.

Canines are needed by carnivores to kill

the prey, hold it in position and tear flesh.

Herbivores feed on grass and such

canines are useless.

A) HETEROTROPHS

B) DENTITION

C) CELLULOSE DIGESTION IN RUMINANTS

D)THE ALIMENTARY CANAL IN HUMANS

E) THE CONTROL OF DIGESTIVE SECRETIONS

Topic Overview

Herbivores have a problem:

They eat grass:

principal component is cellulose

BUT cannot produce cellulases

Mutualistic bacteria in gut produce

cellulase.

ruminants

Herbivores may be:

non-ruminants/hind-gut fermenters

e.g. cattle, sheep,

goats

e.g. rabbit, horse Monogastric herbivores: one-stomach chamber Four stomach chambers

A ruminant is:

any hooved animal that digests its food in two steps:

1. by eating the raw material

2. regurgitating a semi-digested form known as cud, then eating the cud

Ruminants have a specialised digestive

system capable of digesting cellulose:

stomach with 4 distinct compartments

3. Omasum

4. Abomasum 1. Rumen 2. Reticulum

Functions of stomach chambers:

2. Reticulum

3. Omasum

4. Abomasum

breakdown

of cellulose

true stomach: secretes HCl & proteases

1. Rumen

absorbs water & salts from food

Rumen & Reticulum:

are packed with

anaerobic microbes

A cow's rumen has a capacity of up to 95 litres.

Mutualistic microbes

act as fermentation

vats for cellulose

digestion

Fermentation is the enzymatic transformation of organic substances

[acids]

End-products of fermentation in rumen are:

ethanoic acid

propanoic acid

butyric acid

CO2

methane CH4

Are absorbed by ruminant & used as a source of energy

Waste gases

Why are microbes important to ruminants?

1. produce cellulase which digest

cellulose

2. microbes are themselves digested –

supply proteins to host

3. release organic acids by fermentation

– a source of energy to host

RUMEN has a very thick, muscular wall

food is mixed with saliva

SALIVA:

acts as a buffer to acids produced by microbes

The rumen provides an excellent environment for microbial growth:

1. Is anaerobic

2. Provides a stable temperature and osmotic conditions

3. Food & water are present

ABOMASUM very similar to the stomach of non-ruminants

where the majority of chemical breakdown of

food material occurs

mixes in digestive enzymes (pepsin)

killed by the acid & digested by proteases

passed into intestines for further digestion & absorption

What happens to the microbes

in the abomasum?

RUMINANTS: cud-chewers

Ruminant animals:

grasp mouthfuls of food and

swallow it before it is chewed.

wrap their tongue around a mouthful of

grass, clamp down their teeth, and pull to

break the grass at its weakest point, and

swallow.

Ruminants “chew their cud” (regurgitate)

their food material and then grind it with

their molars at a time when the animal is

resting.

–This is done until the food particles

are small enough to pass through the

reticulum into the rumen.

Rumination – chewing the cud Depending on the amount of fibre in their food, cattle

may spend between 3 – 6 hours per day chewing their cud

Where are microbes located in mammalian herbivores other than ruminants?

caecum

Hindgut fermenter

Compare gut of:

Ruminant Herbivore Four-chambered stomach with

large rumen, long small &

large intestine

Hindgut fermenter

Simple stomach,

Large caecum

caecum

caecum

Carnivore Short intestine &

colon, small caecum

Compare gut of:

Ruminant Herbivore Four-chambered stomach

with large rumen,

long small & large intestine

caecum caecum

REASON:

To get nutrients that

otherwise would be

lost

Coprophagy:

a behaviour of certain mammalian herbivores where

they ingest faeces

Caecum

Small intestine

Rabbits produce two types of faeces:

soft, edible, mucous covered packets of protein and vitamins

true faeces are drier and contain

undigested fibre

Question: MAY, 2012 Apart from their dentition, some herbivores have

additional adaptations that make them particularly

suited for a herbivorous mode of life. Give ONE

example of a hind-gut fermenter and briefly explain

how hind-gut fermenters have unique mechanisms

to maximise nutrient absorption from their food. (4)

Rabbit

A large caecum for absorption.

Animal practices coprophagy – pellets egested from

the caecum are eaten. As these pass through the

digestive system a second time, more nutrient

absorption occurs.

MAY 2009 Use your knowledge of biology to describe the selective

advantage of each of the following adaptations:

Gut fermentation in ruminants. (5)

Ruminants fed on grass containing a high content of cellulose

but cannot produce an enzyme to digest it.

Rumen contains mutualistic bacteria that can digest cellulose.

Microbes break the cellulose into sugars which they use for

anaerobic fermentation. Producing fatty acids and methane.

Methane is eliminated by belching and the fatty acids are

absorbed by the rumen – serve as a source of energy for the

ruminant.

Ruminants regurgitate the partially digested food and re-chew

it (chewing the cud) to increase the surface area where

enzymes can act.

Bacteria themselves are digested: serve as a source of protein

A) HETEROTROPHS

B) DENTITION

C) THE ALIMENTARY CANAL IN HUMANS

D) THE CONTROL OF DIGESTIVE SECRETIONS

E) CELLULOSE DIGESTION IN RUMINANTS

Topic Overview

The basic structure of the digestive tract consists of 4 layers:-

[liver, pancreas]

1. Mucosa [innermost layer of gut]

2. Submucosa [contains connective tissue]

3. Muscularis externa [muscle layer]

4. Serosa [outermost layer; composed of loose connective tissue]

Variations in the gut wall

Oesophagus Stomach

Large intestine Small

intestine

The mucosa is composed of:

1. glandular epithelium:

Secretes mucus + digestive enzymes

2. Lamina propria: contains

connective tissue

blood vessels

lymph vessels

3. Muscularis mucosa: smooth muscle to move the

mucosa

connective tissue

blood vessels

lymph vessels

sub-mucosa

contains blood vessels & nerves

epithelial cells rest on a

basement membrane

lamina propria lies beneath

the epithelial cells & contains:

epithelium

muscle or bone LS through oesophagus

Three functions of mucus:

i) lubricates food

ii) facilitates passage along digestive

tract

iii) prevents digestion of the gut wall by

its own enzymes

Goblet cell in human small

intestine

Submucosa: a layer of connective tissue containing:

nerves

blood vessels

lymph vessels

collagen

elastic fibres

The submucosa may contain some mucus-secreting glands

that deposit their contents via ducts e.g. Brunner‟s glands in the duodenum

[secrete mucus + alkaline fluids]

Two plexuses [nerve nets] in gut

Mucosa Epithelium

Lamina propria

Muscularis mucosa

Submucosa Meissner‟s plexus

Circular muscle

Longitudinal muscle

Auerbach‟s plexus

Serosa

Muscularis

externa

Both plexuses consist of nerves from the autonomic NS

Auerbach‟s

plexus

Meissner‟s

plexus

Location & Role of plexuses

Epithelium

Lamina propria

Muscularis mucosa

Submucosa Meissner‟s plexus

Auerbach‟s plexus

Serosa

Between the circular

muscle &

submucosa

Controls secretion of

glands in the

mucosal epithelium

Mucosa

Muscularis

externa

Longitudinal muscle

Circular muscle

Submucosa

circular muscle

longitudinal muscle Between: circular &

longitudinal muscle

layers

Controls peristalsis

Auerbach’s plexus: impulses in:

Sympathetic nerves

cause:

gut muscles to

relax

sphincters to

close

Parasympathetic

nerves stimulate :

gut wall to

contract

sphincters to

open

Muscularis externa: composed of an:

inner circular muscle

outer longitudinal muscle (smooth type)

action of these muscles propel food along the gut

at a number of points along the gut the circular muscle thickens into structures called sphincters

Sphincters contract or relax:

to move the food from one part of the

alimentary canal to another

1 Pyloric sphincter

contracted

Pyloric sphincter

relaxed 2

Sphincters are found at the junctions of:-

oesophagus & stomach

(cardiac sphincter)

stomach & duodenum

(pyloric sphincter)

ileum & caecum

the anus

Serosa: outermost coat of gut

composed of loose connective tissue

serosa

Peritoneum: is the serous membrane that

forms the lining of the abdominal cavity

forms the mesenteries

mesentery

peritoneum

mesentery

Peritoneum cells are moist & help to reduce friction :

when the gut wall slides over other portions of itself or other organs

Hernia: peritoneum is torn

Mesenteries:

stomach & intestines from

the dorsal body wall

consist of double layers of peritoneum containing:

nerves

blood vessels

lymph vessels that pass to and from the gut

mesentery

suspend &

support

Buccal cavity

region enclosing the jaws and

tongue

lined with stratified squamous

epithelium

Basal layer undergoes mitosis

The tongue:

mixes & moistens food

possesses taste buds sensitive to:

sweet

salty

sour

bitter

Taste bud

Saliva is produced by:

three pairs of salivary glands

Saliva contains: 1. salivary amylase :

begins digestion of starch to dextrins (shorter polysaccharides) and then to maltose

Salivary amylase

Saliva contains:

2. lysozyme:

kills bacteria by catalysing the

breakdown of their cell walls

3. various mineral salts, including

chloride ions:

Cl- speed up activity of enzymes

4. mucus:

moistens and lubricates the food

The semi-solid, partially digested food particles are:

stuck by mucin [a glycoprotein that is the chief component of mucus]

moulded into a bolus

food is swallowed

by a reflex action

Oesophagus

a narrow muscular tube lined by stratified squamous epithelium containing mucus glands

about 25 cm long in humans

quickly conveys food and fluids by peristalsis from:

pharynx

to the stomach

Pharynx is where the: mouth cavity &

the nasal passages join

Muscle contractions in the gut occur for two reasons:

1. move food forward

[peristaltic contractions]

2. stir & mix food

[segmental movements]

Peristaltic contractions: are progressive waves of contraction

that move progressively down the gut

1

2

Bolus Digestive tract

Wave of

contraction

Wave of

relaxation

Bolus

moves

2

Peristaltic contractions:

Step 1

Contraction of circular muscles behind mass

Step 2

Contraction of longitudinal muscles ahead of food mass

Step 3

Contraction in circular muscle layer forces food mass forward

What initiates peristaltic contractions?

Stretching of the smooth muscle of the gut

Swallowing a bolus of food stretches the upper end of the oesophagus:

and this stretching initiates a wave of contraction that slowly pushes the contents of the gut toward the anus

STOMACH

in humans - is situated:

below the diaphragm

on the left side of the

abdominal cavity

is a muscular bag

can stretch to hold

nearly 5 dm3 of food

2 smooth muscle layers in gut BUT 3 in stomach:-

Circular muscle (middle)

Longitudinal

muscle (outer)

Oblique muscle (inner)

The thick mucosa has:

1. numerous

gastric pits

2. mucus-secreting

epithelial cells

mucosa of the cardiac region of the stomach contains only mucus glands

Six functions of the stomach

1. Stores food temporarily after meals, releasing food slowly into the rest of the gut

food can be retained in the stomach up to 4-5 hours

Six functions of the stomach

2. The stomach continues mechanical digestion by the churning action.

Food digested

by churning

Food digested by enzymes

3. The mucus made by the stomach:

provides a barrier between the stomach mucosa & gastric juice

An ulcer results if the stomach wall is exposed to:

HCl

pepsin

prevents the stomach self-digesting

lesions started by the bacterial infection are made worse by: HCl pepsin

Helicobacter pylori is an infectious bacterium considered the basis for most

ulcers

4. The main part of the stomach, is dotted with numerous gastric pits.

There are two specialised types of cell:

Six functions of the stomach

Gastric pits lead into long tubular gastric glands.

Gastric glands are lined with cells which secrete the gastric juice (collective name for stomach

secretions)

zymogen or chief cells - secrete pepsinogen &

prorennin

oxyntic or parietal cells - secrete HCl

pH = 1 - 2.5

1

2

7 Functions of hydrochloric acid:

1. creates optimum pH for enzyme activity

2. kills many bacteria, thus acts as a defence mechanism

3. loosens fibrous and cellular components of tissue

Low pH

Active site

Masking sequence

A masking sequence is cleaved from the

pepsinogen molecule……

….…. transforming pepsinogen into the

active enzyme pepsin.

4. promotes conversion of pepsinogen to

pepsin

5. converts prorennin to rennin

6. renders Ca and Fe salts suitable for absorption in the intestine

7. begins hydrolysis of sucrose to glucose and fructose

7 Functions of hydrochloric acid:

5. Produces pepsin and rennin:-

Pepsin:

Six functions of the stomach

2. converts more molecules of pepsinogen to pepsin: process is autocatalysis

1. hydrolyses protein into smaller polypeptides

Rennin:

coagulates caseinogen (soluble protein

in milk) into the insoluble calcium salt

of casein in the presence of calcium

ions

Casein is then digested by pepsin

Changes into insoluble calcium salt: casein

rennin

Soluble protein in milk caseinogen

calcium

6. Stomach contains endocrine cells

which secrete the hormone gastrin.

Gastrin stimulates

gastric glands to

secrete gastric

juice rich in HCl.

Chyme is a semi-liquid mass consisting of food and gastric juice :

gradually the stomach squirts the chyme:

into the duodenum through the relaxed,

ring-shaped pyloric sphincter

A few substances can be absorbed from the chyme across the

stomach wall:

E.g.

alcohol

aspirin

caffeine

The intestines are named for their diameter, not length :

Small intestine:

Length: up to 7 m

Diameter: 2.5 cm

Large intestine:

Length: 1.5 m

Diameter: 7.6 cm

SMALL INTESTINE

4.5 m long in an adult - consists of the:

1. Duodenum (25 cm long)

the site of most digestion

pancreatic & bile ducts open into it

2. jejunum

3. ileum

(2-4 m long)

Pyloric sphincter

90% of the nutrient

absorption

The submucosa & mucosa together are folded

Numerous villi:

finger-like projections.

Microvilli

(brush border)

Structural features which

increase the surface area of

the small intestine

How is the structure of the small intestine

related to its function?

TS ileum showing

villus

villi on

mucosa

Walls of villi are richly supplied with:

blood capillaries

lymph vessels

smooth muscle

Villi contract and

relax to come in

close contact with

the food

Goblet cells

are special epithelial cells : secrete

mucus throughout the small intestine

Location: base of the villi

where the epithelium

folds inwards

Function:

make new epithelial

cells

secrete intestinal juice

(succus entericus)

Crypts of Lieberkühn are narrow tubes:

Paneth cells

at the base of the crypts

secrete lysozyme (antibacterial enzyme)

Submucosal Brunner’s glands secrete:

1. mucus

2. alkaline fluid in first part of

the duodenum (pH = 7 - 8)

Digestion by enzymes in the small intestine

in the small intestine:

1. the digestion of:

2. the absorption of nutrients begins

carbohydrates & proteins continues

fats begins

Intestinal juice (succus entericus): contains a number of enzymes secreted by the epithelial lining

of the small intestine

1. amylase - converts the amylose of starch to

maltose

2. lactase - converts lactose to glucose and galactose

3. sucrase converts sucrose to glucose and fructose

4. aminopeptidases and dipeptidases that convert

peptides and dipeptides to amino acids

5. enterokinase - converts trypsinogen to trypsin

Location of enzymes in small intestine

All other digestive enzymes are:

1. bound to the cell surface membranes of the microvilli

2. or located within the epithelial cells

Pancreatic enzymes: NOT membrane-bound

Digestion of proteins

whole

proteins

endopeptidases exopeptidases

dipeptidases

dipeptides shorter

polypeptides

amino acids

(a) Carboxypeptidase

cuts at C terminus

(b) Aminopeptidase:

cuts at N terminus

Endopeptidases

break proteins

Exopeptidases slit

off dipeptides or

single amino acids

Digestion of proteins involves endopeptidases & exopeptidases.

ENDOPEPTIDASES:

catalyze the

hydrolysis of peptide

bonds in the interior

of a polypeptide chain

EXOPEPTIDASES: catalyze the

hydrolysis of single amino acids from the end of a polypeptide

chain

Explain how the action of

endopeptidases increases the rate of

action of exopeptidases. (1)

Creates more ends / increases surface

area where exopeptidases work.

In addition to its own secretions, the small intestine receives:

2. alkaline

pancreatic juice

1. bile

Pancreas & Liver:

provide important

secretions

are two accessory organs that are not

part of the digestive system

The pancreas :

Exocrine tissue composed of acini

(groups of cells which produce enzymes)

is a large gland just beneath the stomach

To

pancreatic

duct Hormones to

bloodstream

The pancreas releases enzymes as zymogens:

[inactive enzyme precursors]

The pancreas :

pours enzymes via the

pancreatic duct

Pancreatic amylase

Trypsinogen

Lipase Pancreatic enzymes include Chymotrypsinogen

Pancreatic enzymes include- 1. amylase - converts amylose to maltose

2. lipase - converts fats to fatty acids and glycerol

3. trypsinogen

is activated by enterokinase

is converted to trypsin which changes:

i) proteins to smaller polypeptides

ii) more trypsinogen into trypsin (an example of autocatalysis)

4. chymotrypsinogen - when converted to chymotrypsin by trypsin, digests proteins to amino acids

Enterokinase is produced by: wall of duodenum

Enterokinase

[membrane-

bound enzyme]

The liver synthesises bile

Bile is a dark green to yellowish brown

fluid

Flow of bile:

To gallbladder through a side

branch of the hepatic duct

From liver to hepatic duct

Bile is stored in the

gallbladder until needed to

assist in fat digestion

Bile from gallbladder flows

down the common bile duct to

the duodenum

What causes the gallbladder to contract

to release bile?

The hormone cholecystokinin (CCK) causes the gallbladder to contract

Gallstones may block bile duct

Gallstones are formed

from a hardened precipitate

of cholesterol

Contraction of the gallbladder will cause pain

Bile contains:

water

bile salts

bile pigments

[bilirubin, biliverdin]

pH paper turns green in bile

Bile pigments:

do not participate in digestion

are waste products from the

destruction of old red blood cells

are finally eliminated

with the faeces

i.e. work like detergents, dispersing

large fat droplets into smaller ones

Bile salts emulsify fats:

Bile salts are made from cholesterol.

cholesterol is:

1. synthesised in the liver

2. taken in with the diet

Micelles are small fat particles that result from the action of

bile salts.

The importance of emulsification:

Large fat globules form smaller droplets, so increasing their total surface area, thus

lipase acts quicker

LIPASE

Fatty acids & monoglycerides enter epithelial cells by simple diffusion.

Fatty acid absorption Fatty acids & monoglycerides associated with

micelles in lumen of intestine.

Fatty acids & monoglycerides resulting from fat

digestion leave micelles & enter cell by

diffusion.

Fatty acids are used to synthesise triglycerides

in the smooth endoplasmic reticulum.

Fatty globules are combined with proteins to

form chylomicrons (within Golgi apparatus).

Lymph in the lacteal transports chylomicrons

away from intestine.

Vesicles containing chylomicrons migrate to

the basal membrane where by exocytosis

they move out of the epithelial cell & enter a

lacteal.

1

2

3

4

5

Chylomicron composition:

90% triglyceride

5% cholesterol

4% phospholipid

1% protein

What happens to the bile salts in the small intestine?

Bile salts are actively

reabsorbed and returned

to the liver via the blood

stream

Bile salts returned to liver

Absorption of food in the small intestine

monosaccharides

dipeptides

amino acids

Microvilli of epithelial cells on villi absorb:

absorbed by:

active transport & diffusion

Where does the absorbed food travel to

once absorbed into the blood stream?

To liver via the hepatic portal vein

Monosaccharide (glucose) transport

Glucose is absorbed by

symport with Na+ into

intestinal epithelial cells.

Symport is driven by a

sodium gradient

established by a Na+–K+

pump.

Glucose moves out of the

intestinal epithelial cells

by facilitated diffusion.

Glucose enters the capillaries of the

intestinal villi and is carried through the

hepatic portal vein to the liver.

Villus

Glucose 1

1

2

3

4

4

ADP ATP Na+ Na+

Na+ K+

Intestinal

epithelial cell

To liver

Capillary Lacteal

1

2

3

1

2

3

4

Villus

Amino acid transport

3

Amino acids are absorbed by symport into intestinal epithelial cells.

Symport is driven by a

sodium gradient established

by a Na+–K+ pump.

Amino acids move out

of intestinal epithelial

cells.

Amino acids enter the capillaries

of the intestinal villi and are

carried through the hepatic portal

vein to the liver.

Capillary Lacteal

Amino

acid

Na+

1

2

Villus

Intestinal

epithelial cell

K+

To liver

Na+ Na+

ADP ATP

4

2

1

1 3 4

2

Villus

Peyer’s patches in ileum

Peyer’s patch [function?]

Peyer’s patches: have a role in the immune response

aggregations of lymphoid tissue that are

usually found in the ileum in humans

SEP 2006 Paper 3 Draw an annotated map of the structure shown in Figure 1. (10 marks)

Scale: x 1 Scale: x 1.2

THE LARGE INTESTINE

THE LARGE INTESTINE

1.Caecum

Anus

2. Appendix

3. Colon

4. Rectum

The large intestine absorbs:

2. any remaining inorganic nutrients

8900 ml 1. bulk

of water

Epithelial cells secrete mucus to lubricate the solidifying food - faeces

Faeces consist of:-

dead bacteria

cellulose & other plant fibres

dead mucosal cells

mucus

cholesterol

bile pigment derivatives

water

Two anal sphincters: An internal one of smooth

muscle, controlled by the

autonomic nervous system

An outer one of striated

muscle, controlled by the

voluntary nervous system

1

2

1 2

Many mutualistic bacteria in the large intestine synthesise:-

Produce absorbable vitamins, especially vitamin K.

Produce some amino acids.

Digest complex carbohydrates to produce fatty acids which are used as a source of energy by the epithelial cells of the colon.

SEP, 2013 Relate the following to their biological function:

non-pathogenic bacteria in the large intestine of humans; (2)

Essay Title: [SEP, 2011]

Describe the structure of the alimentary canal of the human body in relation to its function.

A) HETEROTROPHS

B) DENTITION

C) THE ALIMENTARY CANAL IN HUMANS

D) THE CONTROL OF DIGESTIVE SECRETIONS

E) CELLULOSE DIGESTION IN RUMINANTS

Topic Overview

Production of digestive enzymes & HCl occurs only when food is

present - ADVANTAGE:

To save energy & materials in the body

The overall control of digestive activity is

coordinated and regulated by the:

1. Nervous system

2. Endocrine system

The control of the secretion of:

1.Saliva 2. Gastric juice

3. Pancreatic juice & bile

Salivary secretion is controlled by two reflex actions:-

1. an unconditioned cranial reflex occurs

when food is present in the buccal

cavity

Food contacts taste buds: impulses fired

Impulses travel to brain

Salivary glands secrete saliva

2. conditioned reflexes of seeing, smelling or thinking of food

Sight

Smell Thinking

of food

The control of the secretion of:

1. Saliva

2.Gastric juice 3. Pancreatic juice & bile

Three phases occur in the secretion of gastric juice:-

1. nervous / cephalic phase lasts 1h

occurs before food reaches stomach

2. gastric phase takes place in the stomach

involves both nervous & hormonal control

3. intestinal phase takes place in the small intestine secretion of gastric juice is inhibited and the

release of chyme from the stomach is slowed

1. nervous phase The presence of food in mouth + its

swallowing trigger reflex nerve impulses

which pass along the vagus nerve from

the brain to the stomach.

Sight

Smell

Taste

Thought of food

Gastric juice

can trigger the same reflex Gastric glands of the stomach are

stimulated to secrete gastric juice

2. gastric phase

Stretch receptors send nerve impulses to

Meissner‟s plexus in the submucosa, which in turn

sends impulses to the gastric glands, stimulating

the flow of gastric juice.

Food in stomach stimulates stretch

receptors in the wall of the stomach.

Stretching of the stomach + presence of

food also stimulate special endocrine

cells in the mucosa to secrete the

hormone gastrin.

Gastrin stimulates

gastric glands to

secrete gastric juice

rich in HCl for 4 h.

1

2

3

4

3. intestinal phase:

Acidified chyme in contact with the

walls of duodenum, triggers both

nervous & hormonal responses

duodenal mucosa secretes secretin & cholecystokinin (CCK)

Receptors in the small intestine are stimulated by the

presence of food, but the reflexes, which pass through the

brain:

1. inhibit secretion of gastric juice

2. slow the release of chyme from the stomach

secretin: inhibits secretion of gastric juice CCK: inhibits stomach emptying

The control of the secretion of:

1. Saliva

2. Gastric juice

3.Pancreatic juice

& bile

The secretion of bile & pancreatic juice is stimulated by:

produced in: duodenum

stimulus: acidified chyme from stomach

during the nervous & gastric phases of gastric

digestion, the vagus nerve also stimulates the :

liver to secrete bile

pancreas to secrete enzymes

1. HORMONES: secretin & CCK:

2. NERVOUS REFLEXES

Effects of CCK & secretin

Hormone Target organ Response

CCK pancreas Increased secretion of pancreatic juice rich in enzymes

gall bladder Contraction of gall bladder to release bile

Secretin pancreas

Increased flow of HCO3-

in pancreatic juice

liver Synthesis of bile rich in HCO3-

stomach Inhibits secretion of gastric juice

Question: MAY, 2011

Hormone that stimulates the production of

gastric acid by the stomach.

gastrin

Hormone that stimulates delivery into the small

intestine of digestive enzymes from the pancreas

and bile from the gallbladder.

cholecystokinin

Hormone that controls secretions into the

duodenum.

secretin

Site of initial carbohydrate digestion.

mouth

Complete the table by entering the most appropriate terms in the spaces below: (4)

REVISE:

Question: MAY, 2004

The broad dietary preferences of vertebrates may be inferred by observing their dentition.

a. Why should different dietary preferences necessitate specialist dentition? (2)

Teeth must be adapted to the type of food eaten. E.g. carnivores need sharp teeth for cutting while herbivores need flat surfaces for grinding.

Question: MAY, 2004

a. List two differences between the dentition of a generalised herbivore and that of a generalised carnivore. (2)

Herbivore : Small or no canines; diastema present; blunt incisors; flat premolars and molars.

Carnivore : Long, pointed canines; diastema absent; sharp incisors; premolars and molars with ridges.

Question: MAY, 2004

c. Name ONE adaptation, characteristic of some herbivores, that is not concerned with specialist dentition. (1)

Four chambered stomach; well-developed masseter muscle to allow grinding actions.

Question: MAY, 2011

Name, and briefly describe, THREE adaptations of ruminant mammals to their mode of nutrition. (6)

1) Rumen is full of bacteria that produce cellulase to digest cellulose.

2) Food is regurgitated and animal chews the cud to increase surface area for enzyme action.

3) The upper jaw is a hard tough pad and the grass is cut between the teeth and the pad.

Essay titles

1. Compare and contrast the mode of

nutrition of humans with that of

ruminant mammals. [SEP, 2005]

2. Give an overview of the adaptations of

ruminant and carnivorous mammals to

their respective modes of nutrition.

[SEP, 2008]

THE END