IB BIOLOGY UNIT 6: HUMAN PHYSIOLOGY
6.1.1 HUMAN DIGESTIVE SYSTEM
● There are two major groups of organs which comprise the human digestive system:
● The alimentary canal consists of organs through which food actually passes — oesophagus,
stomach, small & large intestine.
● The accessory organs that are connected to the alimentary canal through ducts aid in
digestion but do not actually transfer food — salivary glands, pancreas, liver, gall bladder.
● Oesophagus (yemek borusu) is a hollow tube connecting the oral cavity to the stomach and is
separated from the trachea by the epiglottis. Here, food is mixed with saliva and then is moved in a
bolus form via the action of peristalsis.
● Stomach is a temporary storage tank where food is mixed and protein digestion begins. It is lined by
gastric pits that release digestive juices, which create an acidic environment with around a pH of 2.
● Small intestine is a long, highly folded tube where usable food substances — nutrients — are
absorbed. It consists of three sections – the duodenum, jejunum and ileum.
● Large intestine is the final section of the alimentary canal, where water and dissolved minerals are
absorbed. It consists of the ascending colon, as well as the rectum.
● Salivary glands release saliva to moisten food and contain enzymes such as amylase to initiate
starch breakdown. Salivary glands include the parotid gland, submandibular gland and sublingual
gland.
● Pancreas produces a broad spectrum of enzymes that are released into the small intestine via
the duodenum. It also secretes certain hormones, such as insulin and glucagon, which regulate
blood sugar concentrations.
,● Liver takes the raw materials absorbed by the small intestine and uses them to make key chemicals.
Its role includes detoxification, storage, metabolism, bile production and haemoglobin
breakdown.
● Gall bladder stores the bile produced by the liver as bile salts are used to emulsify fats. Bile stored
in the gall bladder is released into the small intestine via the common bile duct.
● The abdominal cavity is the space containing all the digestive structures except for mouth and
oesophagus.
6.1.2 STAGES OF DIGESTION
● The main purpose of the digestive system is to break large molecules down into smaller subunits due
to the fact that:
● Large molecules are typically chemically inert and need to be broken down and
reassembled into usable products.
● Large molecules are typically insoluble and cannot be absorbed into cells, whereas smaller
subunits are soluble.
● Hence, all digestion reactions are hydrolysis reactions.
● The process of digestion occurs across a number of stages, including:
● Ingestion – food is taken into the body via the act of eating
● Digestion – food is broken down both physically (mechanical digestion) and chemically
(chemical digestion)
● Absorption & Transportation – digested food products are absorbed into the bloodstream and
transported to cells
● Assimilation – digested food products are converted into the fluid and solid parts of a cell or
tissue
● Elimination – undigested food residues are egested from the body as semi-solid faeces
6.1.3 MECHANICAL DIGESTION
● Food can be digested by a combination of two methods – mechanical digestion and chemical digestion.
,● In mechanical digestion, food is physically broken down into smaller fragments via the acts of
chewing in mouth, mixing in stomach and segmentation in small intestine.
1. Food is initially broken down in the mouth by the grinding action of teeth which is chewing.
2. The tongue pushes the food towards the back of the throat, where it travels down the
esophagus to the stomach as a bolus.
● The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the
bolus from entering the nasal cavity.
● Peristalsis is the principal mechanism of movement in the oesophagus, although it
also occurs in both the stomach and gut. In peristalsis, longitudinal smooth muscles
rhythmically contract and relax and help the food move unidirectionally along the
alimentary canal in a caudal direction from mouth to anus.
3. Food is digested within the stomach with the help of the muscles on the stomach lining which
physically squeeze and mix the food with strong digestive juices for several hours and is turned
into a creamy paste called chyme.
● Peristalsis is also used here by the stomach to mix food with digestive secretions,
including proteases. This movement is called churning.
● The peristaltic movement is relatively fast within the oesaphagus and slows
dramatically in the intestines.
4. Eventually the chyme enters the small intestine from the duodenum where absorption will occur.
● Segmentation involves the contraction and relaxation of non-adjacent segments
of circular smooth muscle in the intestines. Segmentation contractions move chyme
in both directions (both from stomach to small intestine and from small intestine to
stomach), allowing for a greater mixing of food with digestive juices. While
segmentation helps to physically digest food particles, its bidirectional
propulsion of chyme can slow overall movement.
6.1.4 CHEMICAL DIGESTION
● In chemical digestion, food is broken down by the action of chemical agents such as acids, bile
and enzymes.
STOMACH ACIDS
● The stomach contains gastric glands which release digestive acids to create a low pH
environment.
● The acidic environment functions to denature proteins and other macromolecules, aiding in
their overall digestion.
● The stomach epithelium contains a mucous membrane which prevents the acids from damaging
the gastric lining.
● The pancreas releases alkaline compounds such as bicarbonate ions, which neutralise the acids as
they enter the intestine.
BILE
, ● Bile is a watery fluid that contains bile salts and pigments and is made by the liver and released from
the gall bladder to the intestines.
● Bile contains bile salts which interact with fat globules and divide them into smaller droplets in a
process called emulsification.
● The emulsification of fats increases the total surface area available for enzyme activity of lipase and
hence, facilitates the digestion of lipids.
6.1.4.1 ENZYMES
● Enzymes are biological catalysts which speed up the rate of a chemical reaction by lowering activation
energy.
● Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient
speeds for survival requirements.
● Warm-blooded organisms such as humans have an advantage over cold-blooded organisms for
efficient digestion and many other metabolic processes, because of their constantly warm
internal environment.
● Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur
independently in distinct locations.
● Digestive enzymes are secreted predominantly by the pancreas, although other organs such as
salivary glands and stomach also contribute.
● The type of enzyme secreted and location of secretion depends on the specific macromolecule
required for hydrolysis.
CARBOHYDRATES
● Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands.
● Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the
small intestine.
● Enzymes for disaccharide hydrolysis such as maltase are often immobilised on the epithelial lining
of the small intestine, near channel proteins.
● Humans do not possess the enzyme capable of digesting cellulose — cellulase — and hence it passes
through the body undigested.
PROTEINS
● Protein digestion begins in the stomach with the release of proteases that function optimally in an
acidic pH.
● Smaller polypeptide chains enter the small intestine where they are broken down by
endopeptidases released by the pancreas.
● These endopeptidases work optimally in neutral environments as the pancreas neutralises the
acids in the intestine.
LIPIDS
6.1.1 HUMAN DIGESTIVE SYSTEM
● There are two major groups of organs which comprise the human digestive system:
● The alimentary canal consists of organs through which food actually passes — oesophagus,
stomach, small & large intestine.
● The accessory organs that are connected to the alimentary canal through ducts aid in
digestion but do not actually transfer food — salivary glands, pancreas, liver, gall bladder.
● Oesophagus (yemek borusu) is a hollow tube connecting the oral cavity to the stomach and is
separated from the trachea by the epiglottis. Here, food is mixed with saliva and then is moved in a
bolus form via the action of peristalsis.
● Stomach is a temporary storage tank where food is mixed and protein digestion begins. It is lined by
gastric pits that release digestive juices, which create an acidic environment with around a pH of 2.
● Small intestine is a long, highly folded tube where usable food substances — nutrients — are
absorbed. It consists of three sections – the duodenum, jejunum and ileum.
● Large intestine is the final section of the alimentary canal, where water and dissolved minerals are
absorbed. It consists of the ascending colon, as well as the rectum.
● Salivary glands release saliva to moisten food and contain enzymes such as amylase to initiate
starch breakdown. Salivary glands include the parotid gland, submandibular gland and sublingual
gland.
● Pancreas produces a broad spectrum of enzymes that are released into the small intestine via
the duodenum. It also secretes certain hormones, such as insulin and glucagon, which regulate
blood sugar concentrations.
,● Liver takes the raw materials absorbed by the small intestine and uses them to make key chemicals.
Its role includes detoxification, storage, metabolism, bile production and haemoglobin
breakdown.
● Gall bladder stores the bile produced by the liver as bile salts are used to emulsify fats. Bile stored
in the gall bladder is released into the small intestine via the common bile duct.
● The abdominal cavity is the space containing all the digestive structures except for mouth and
oesophagus.
6.1.2 STAGES OF DIGESTION
● The main purpose of the digestive system is to break large molecules down into smaller subunits due
to the fact that:
● Large molecules are typically chemically inert and need to be broken down and
reassembled into usable products.
● Large molecules are typically insoluble and cannot be absorbed into cells, whereas smaller
subunits are soluble.
● Hence, all digestion reactions are hydrolysis reactions.
● The process of digestion occurs across a number of stages, including:
● Ingestion – food is taken into the body via the act of eating
● Digestion – food is broken down both physically (mechanical digestion) and chemically
(chemical digestion)
● Absorption & Transportation – digested food products are absorbed into the bloodstream and
transported to cells
● Assimilation – digested food products are converted into the fluid and solid parts of a cell or
tissue
● Elimination – undigested food residues are egested from the body as semi-solid faeces
6.1.3 MECHANICAL DIGESTION
● Food can be digested by a combination of two methods – mechanical digestion and chemical digestion.
,● In mechanical digestion, food is physically broken down into smaller fragments via the acts of
chewing in mouth, mixing in stomach and segmentation in small intestine.
1. Food is initially broken down in the mouth by the grinding action of teeth which is chewing.
2. The tongue pushes the food towards the back of the throat, where it travels down the
esophagus to the stomach as a bolus.
● The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the
bolus from entering the nasal cavity.
● Peristalsis is the principal mechanism of movement in the oesophagus, although it
also occurs in both the stomach and gut. In peristalsis, longitudinal smooth muscles
rhythmically contract and relax and help the food move unidirectionally along the
alimentary canal in a caudal direction from mouth to anus.
3. Food is digested within the stomach with the help of the muscles on the stomach lining which
physically squeeze and mix the food with strong digestive juices for several hours and is turned
into a creamy paste called chyme.
● Peristalsis is also used here by the stomach to mix food with digestive secretions,
including proteases. This movement is called churning.
● The peristaltic movement is relatively fast within the oesaphagus and slows
dramatically in the intestines.
4. Eventually the chyme enters the small intestine from the duodenum where absorption will occur.
● Segmentation involves the contraction and relaxation of non-adjacent segments
of circular smooth muscle in the intestines. Segmentation contractions move chyme
in both directions (both from stomach to small intestine and from small intestine to
stomach), allowing for a greater mixing of food with digestive juices. While
segmentation helps to physically digest food particles, its bidirectional
propulsion of chyme can slow overall movement.
6.1.4 CHEMICAL DIGESTION
● In chemical digestion, food is broken down by the action of chemical agents such as acids, bile
and enzymes.
STOMACH ACIDS
● The stomach contains gastric glands which release digestive acids to create a low pH
environment.
● The acidic environment functions to denature proteins and other macromolecules, aiding in
their overall digestion.
● The stomach epithelium contains a mucous membrane which prevents the acids from damaging
the gastric lining.
● The pancreas releases alkaline compounds such as bicarbonate ions, which neutralise the acids as
they enter the intestine.
BILE
, ● Bile is a watery fluid that contains bile salts and pigments and is made by the liver and released from
the gall bladder to the intestines.
● Bile contains bile salts which interact with fat globules and divide them into smaller droplets in a
process called emulsification.
● The emulsification of fats increases the total surface area available for enzyme activity of lipase and
hence, facilitates the digestion of lipids.
6.1.4.1 ENZYMES
● Enzymes are biological catalysts which speed up the rate of a chemical reaction by lowering activation
energy.
● Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient
speeds for survival requirements.
● Warm-blooded organisms such as humans have an advantage over cold-blooded organisms for
efficient digestion and many other metabolic processes, because of their constantly warm
internal environment.
● Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur
independently in distinct locations.
● Digestive enzymes are secreted predominantly by the pancreas, although other organs such as
salivary glands and stomach also contribute.
● The type of enzyme secreted and location of secretion depends on the specific macromolecule
required for hydrolysis.
CARBOHYDRATES
● Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands.
● Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the
small intestine.
● Enzymes for disaccharide hydrolysis such as maltase are often immobilised on the epithelial lining
of the small intestine, near channel proteins.
● Humans do not possess the enzyme capable of digesting cellulose — cellulase — and hence it passes
through the body undigested.
PROTEINS
● Protein digestion begins in the stomach with the release of proteases that function optimally in an
acidic pH.
● Smaller polypeptide chains enter the small intestine where they are broken down by
endopeptidases released by the pancreas.
● These endopeptidases work optimally in neutral environments as the pancreas neutralises the
acids in the intestine.
LIPIDS
