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1 DIGESTION & ABSORPTION
Structure of the Digestive System


Function of the digestive system = break down the diverse mixture of large carbon
compounds in food, to yield ions and smaller compounds that can be absorbed



What digestion requires:
 SURFACTANTS to break up lipid droplets & enzymes to catalyse reactions
 GLANDULAR CELLS in lining of stomach & intestines to produce some of the enzymes
 ACCESSORY GLANDS that have ducts leading to the digestive system to secrete
surfactants & other enzymes

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ESOPHAGUS – movement of food by peristalsis from the mouth to stomach
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SMALL INTESTINE – final stages of digestion of lipids, carbohydrates, proteins and nucleic
acids, neutralizing stomach acid, + absorption of nutrients
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LIVER – secretion of surfactants in bile to break up liquid droplets
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LARGE INTESTINE – re-absorption of H20, further digestion especially of carbohydrates by
symbiotic bacteria, + formation & storage of faeces

Structure of the Wall of the Small Intestine
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4 layers:
 Serosa – outer coat
 Muscle layers – longitudinal muscle, and inside it, circular muscle
 Sub-mucosa – a tissue layer containing blood & lymph vessels
 Mucosa – lining of the small intestine + epithelium that absorbs nutrients on the inner
surface

Peristalsis
= Waves of contraction of circular & longitudinal muscle layers of the small intestine which mixes the
food with enzymes & moves it along the gut

- How peristalsis takes place
-

Waves of peristalsis pass along intestine
Contraction of circular muscles behind the food constricts the gut to prevent it from being
pushed back towards mouth
Contraction of longitudinal muscles where food is located moves it along gut
Contractions are controlled unconsciously by the enteric nervous system
Peristalsis occurs in 1 direction (away from mouth) & food moves quickly down oesophagus in
1 continuous peristaltic wave – in the small intestine, food is only moved a few cm at a time
allowing time for digestion

- Nature of the circular & longitudinal muscle in the wall of the gut
-

Smooth (not striated)
Consists of short cells (not elongated fibres)
Often exerts continuous moderate force with short periods of more vigorous contraction (not
remaining relaxed unless stimulated to contract)

Pancreatic Juice


2 types of gland tissue in the pancreas:
 ISLETS OF LANGERHANS secrete hormones insulin & glucagon into blood
 REMAINDER OF PANCREAS synthesizes & secretes digestive enzymes into the gut

- How the secretion of digestive enzymes is controlled
1) Hormones synthesised & secreted by stomach
2) Enteric nervous system

- How digestive enzymes are synthesised & secreted
1) Synthesised in pancreatic gland cells on ribosomes on the RER
2) Then processed in Golgi Apparatus & secreted by exocytosis
3) Ducts within pancreas merge into larger ducts, finally forming 1 pancreatic duct (about 1L of
pancreatic juice secreted per day into lumen of SI)
 Pancreatic juice contains:
 AMYLASE – starch
 LIPASES – triglycerides, phospholipids
 PROTEASES – proteins & peptides

Digestion in the Small Intestine


Hydrolysis reactions carried out by enzymes secreted by pancreas in SI lumen:
1) Starch – maltose by AMYLASE
2) Triglycerides – fatty acids & glycerol or fatty acids & monoglycerides by LIPASE
3) Phospholipids – fatty acids, glycerol & phosphate by PHOSPHOLIPASE
4) Proteins & polypeptides – shorter peptides by PROTEASE

-

However, this isn’t complete process of digestion into molecules small enough to be absorbed –
SO WHAT HAPPENS?
 Wall of SI produces other enzymes
 Some enzymes produced by gland cells in intestinal wall may be secreted into
intestinal juice BUT most remain immobilised in plasma membrane of
epithelium cells lining the SI



Hydrolysis reactions carried out by enzymes immobilised in epithelium cells lining SI
1) DNA & RNA – nucleotides by NUCLEASES
2) Maltose – glucose by MALTASE
3) Lactose – glucose & galactose by LACTASE
4) Sucrose – glucose & fructose by SUCRASE
5) Peptides – dipeptide by EXOPEPTIDASES (type of protease) – process? Removing single
amino acids from carboxyl or amino terminal of chain until only a dipeptide is left
6) Dipeptides – amino acids by DIPEPTIDASES


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Why isn’t cellulose digested?
Humans cannot synthesise necessary enzymes

Villi & the Surface Area for Digestion


ABSORPTION = process of taking substances into cells & the blood – in humans nutrients are
absorbed by the EPITHELIUM (single layer of cells forming the inner lining of the mucosa)
 Rate of absorption depends of SA of epithelium
 This is LARGE as the SI is about 7m long & 25-30mm wide + villi



VILLI = small finger-like projections of the mucosa on the inside of the intestine wall
 Villus = between 0
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5mm long & can be 40 per square mm of SI wall so increase the
SA by a factor of about 10

Absorption by Villi


Substances absorbed that are products of digestion of macromolecules in food:
1) Glucose, fructose, galactose & other MONOSACCHARIDES
2) Any of 20 AMINO ACIDS used to make proteins
3) FATTY ACIDS, monoglycerides & glycerol
4) BASES from digestion of nucleotides



Substances absorbed that are required by the body but not needing digestion:
1) MINERAL IONS e
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Ca, K & Na
2) VITAMINS e
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ascorbic acid (vitamin C)



Unwanted substances that are absorbed & how they are removed:
1) HARMFUL ones = removed from blood & detoxified by liver
2) HARMLESS e
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colourings = passed out in urine
3) BACTERIA = removed from blood by phagocytic cells in liver

Methods of Absorption
- Simple diffusion
E
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hydrophobic nutrients such as FATTY ACIDS & MONOGLYCERIDES


How fatty acids & monoglycerides are absorbed into the capillaries:
1) Start off as TRIGLYCERIDES – first are digested = fatty acids & monoglycerides
2) Absorbed into villus epithelium cells by simple diffusion as they can pass between
phospholipids in plasma membrane
3) Once inside epithelium cells, fatty acids are combined with monoglycerides = triglycerides,
so they cannot diffuse back into lumen

- Facilitated diffusion
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hydrophilic nutrients such as FRUCTOSE & FATTY ACIDS


How fructose is absorbed into the capillaries:
1) First broken down to GLUCOSE by digestion
2) Glucose cant pass through plasma membrane by simple diffusion as its polar & thus
hydrophilic
3) SODIUM-GLUCOSE TRANSPORTER PROTEINS in microvilli transfer a Na+ ion & glucose
molecule together from intestinal lumen to cytoplasm of epithelium cells
 = Facilitated diffusion & thus PASSIVE but depends on CG of Na+ ions created by
active transport
4) GLUCOSE CHANNELS allow glucose to move by FD from cytoplasm to interstitial spaces
inside villus & into capillaries

- Active transport
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mineral ions such as SODIUM, CALCIUM & IRON


How mineral ions are absorbed into the capillaries:
1) SODIUM-POTASSIUM PUMPS in inwards-facing part of plasma membrane pump Na+ ions
by AT from cytoplasm to interstitial spaces (between cells) inside villus
2) K+ ions are moved in the opposite direction
3) Low concentration of Na+ ions inside villus epithelium cells created

- Endocytosis and exocytosis
These are the processes in which larger substances (too big for facilitated diffusion or active
transport) are taken in (endocytosis) and out (exocytosis) of the cell via the plasma membrane
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g
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g
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1 seconds = electrical signal conveyed to ventricles
6) Time delay allows time for atria to pump blood into ventricles
7) Signal is spread throughout walls of ventricles = contract & pump blood to arteries

The Cardiac Cycle

E

A (0
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1 seconds)
- Atria contract = small pressure increase, which pumps blood into ventricles through open
atrioventricular valves
- Semilunar valves = closed & blood pressure in arteries gradually drops to minimum as blood
continues to flow along them but no more is pumped in
B (0
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15 seconds)
- Ventricles contract + rapid pressure build up = causes atrioventricular valves to close
- Semilunar valves remain closed
C (0
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4 seconds)
- Pressure in ventricles rises above pressure in arteries = semilunar valves open & blood is
pumped from ventricles into arteries = maximising blood pressure
- Pressure slowly rises in atria as blood drains into them from veins as they fill
D (0
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45 seconds)
- Contraction of ventricular muscles decreases & pressure inside ventricles drops below
pressure in arteries = semilunar valves close
- Atrioventricular valves remain closed
E (0
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8 seconds)
- Pressure in ventricles drops below pressure in atria = atrioventricular valves open
- Blood from veins drains into atria & from there into ventricles = slow increase in pressure

Changing the Heart Rate




INCREASING THE HEART RATE – Sinoatrial node responds to signals from 1 of the 2 nerves
originating in a region in the MEDULLA of the brain called the CARDIOVASCULAR CENTRE
 = Causes the ‘pacemaker’ to increase frequency of heartbeats – in young people can
increase up to 3X resting rate
DECREASING THE HEART RATE – SN responds to signals from the other nerve originating
from the same place


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HOW THE CARDIOVASCULAR CENTRE SENDS THESE SIGNALS:
Receives inputs from RECEPTORS that monitor blood pressure & its pH & O2 concentration
(pH of blood reflects CO2 concentration)
 Low BP, O2 concentration & pH = heart rate needs to SPEED UP to increase flow rate
of blood to tissues, deliver more O2 & remove more CO2
 High BP, O2 concentration & pH = heart rate needs to SLOW DOWN

Epinephrine (also called adrenalin)
-

Sinoatrial node responds to epinephrine in the blood by increasing the heart rate
Produced by adrenal glands
Secretion is controlled by the brain & rises when vigorous physical activity may be
necessary (threat/opportunity)



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