Nutrition in Humans
Nutrition
• Nutrition is the intake of food and the
processes that convert food substances into
living matter.
• Comprises of the following
o
o
o
o
FEEDING: the intake of food into the body
DIGESTION: the process whereby large food
molecules are broken down into soluble and
diffusible molecules that can be absorbed into the
body cells.
ABSORPTION: the process whereby digested
food materials are taken into the body cells.
ASSIMILATION: the process whereby some of
the absorbed food materials are converted into
new protoplasm or used to provide energy
Types of Nutrition in
Organisms
• Autotrophic: most autotrophic organism
make their own food by photosynthesis. They
use carbon dioxide, water, and the sun’s
energy to accomplish this.
• Heterotrophic: where the organism obtains
food from the environment. Heterotrophs can
be further categorized as being parasitic if
they obtain nutrition from a live host or
saprophytic if they obtain theirnutrition from
dead organisms.
 Herbivores: consume plant matter
 Carnivores: consume animal matter
 Omnivores: obtain their nutrition from both.
Physical & Chemical
Digestion
• Physical digestion:
Mechanical breakdown of food into small particles
Increases the surface area to volume ratio of the ingested
food so that digestive enzymes can act on the food more
efficiently.
o Occurs in the mouth, when you chew food using your teeth
and in the stomach, where churning of the stomach walls
breaks up the food particles and mixes them with digestive
enzymes.
o
o
• Chemical digestion:
Enzymatic hydrolysis (recall Chap 4) of large food molecules
into small soluble molecules which can be absorbed.
o Achieved by the digestive enzymes found in the mouth,
o
stomach, duodenum and ileum.
Human Alimentary Canal
• Alimentary canal is a hollow tube extending from the mouth to
the anus.
• It receives secretions from various accessory organs or
glands.
• The walls of the alimentary canal have four basic layers.
o Serosa: It is an outer most tough coating layer of the wall
of the alimentary canal.
o Muscular layer: Below serosa is a muscle bilayer. It
consists of muscles of two kinds:
 Circular muscles towards inner side
 Longitudinal muscles on outer side These are
involuntary muscles and they help to move the food
along the alimentary canal.
o Submucosa
Below muscular layer is a nutritive layer that contains blood
supply and plexus of nerve fibers.
o Mucosa
The inner most of the alimentary canal is lined with a wet
Human Alimentary Canal
Peristalsis
• Refers to the rhythmic, wave-like contractions of
the gut walls caused by the action of 2
antagonistic muscles.
o
o
circular and longitudinal muscle fibers.
The circular muscles, by contracting and relaxing
alternately, urge the food in a wave like motion through the
various regions of the alimentary canal.
• Functions
o Moves the food along the gut
o Mixes the food with digestive enzymes
Antagonistic Muscles
Means that when 1 muscle contracts, the other relaxes and
vice versa.
Circular muscles (inner wall)
Longitudinal muscles (outer wall)
Peristalsis
• Contraction of the circular muscles always constricts the
lumen (becomes narrower and longer)
• Contraction of the longitudinal muscles always dilates/
expands the lumen (becomes widen and shorter).
• When the circular muscles contract, the longitudinal
muscles relax.
• The diameter of the gut decreases: lumen constricts
(becomes narrower and longer)
• Food is squeezed forward from original position.
• When the circular muscles relax, the longitudinal muscles
contract.
• The diameter of the gut increases: lumen dilates/ expand
(becomes widen and shorter)
• Food enters the lumen created.
Peristalsis
Pharynx
• The part of the gut which
leads from the mouth to
the oesophagus and to the
trachea by way of the
larynx or voice-box.
• The larynx bears a slit-like
opening called the glottis
• The pharynx is a common
passage for food and air
• During swallowing, the
larynx moves up so that
the larynx is covered by
the epiglottis and no food
particles can enter the
trachea
The Mouth
• The mouth is where food is mechanically
digested. Here, the teeth physically break
down the food.
• The food is broken down so that it can be
easier to swallow and has a greater surface
are for enzymes to act on.
• Starting from the front of the mouth the four
types are:
incisors — These are chisel shaped teeth and are used
for cutting, slicing and biting.
o canines — These are long, pointed teeth that resemble
fangs. They grip, stab and tear food.
o premolars —These are teeth with projections on the
surface called cusps. They crush and chew food.
o molars — These are large teeth located in the back of
o
Digestion in the Mouth
Chemical Digestion
• The mouth contain the enzyme amylase (from saliva)
o secreted by 3 salivary pairs of salivary glands located
under the tongue, at the back of the jaws, and in the cheeks.
o Saliva also contains lysozyme which helps destroy microorganisms.
o The saliva helps to soften the food so that it is easy to
swallow.
• Amylase digests starch into maltose.
Physical Digestion
• Once food enters the mouth the teeth break it down into smaller
and smaller pieces.
o This has the dual function of making the food easier to
swallow and increasing the surface area of food on which
the saliva can act.
o The tongue, lips and cheeks assist the teeth in the process
by allowing the food to be "rolled" around the oral cavity.
o The mechanical action described above produces a
softened bolus of food which is now ready to be swallowed.
In the mouth
• After the food is chewed it is formed
into a ball or bolus and pushed back
to the pharynx. This is where the food
will leave the mouth and enter the
oesophagus.
• To stop food from entering the
trachea (or windpipe) there is a small
flap called the epiglottis. This flap
closes over the trachea to stop food
from entering.
Oesophagus
• The oesophagus connects the mouth
with the stomach.
• Food moves through the oesophagus by
involuntary wavelike muscular contraction
called peristalsis.
• Peristalsis continues through the
stomach, small intestine, and large
intestine.
• Finally, peristalsis forces food into the
rectum.
• Here it is ready to be discharged out of
the anus.
• No digestion occurs here!
Stomach
• There is a sphincter muscle at the end of the oesophagus
that opens and closes to allow food into the stomach and to
stop food from going back up into the oesophagus.
• The stomach lining is called the mucosa, which has many
folds in it forming millions of gastric glands.
• These glands secrete gastric juice consisting of:
o Mucous: produced by goblet or mucous cells of the
mucosa. It prevents the stomach from digesting itself.
o Pepsinogen: this is converted into pepsin by acid in the
stomach. Pepsin converts protein into smaller peptides.
Pepsin is said to be a protease - an enzyme that digests
protein.
o Prorenin which is activated into renin by HCl.
o Hydrochloric Acid (HCl): gives the stomach a pH of 1
to 2.
Functions of HCl
1.stops the action of salivary amylase by
denaturing it;
2.changes the inactive forms of the
enzymes, pepsinogen and prorennin, in
the gastric juice, to the active forms,
pepsin and rennin respectively
3.provides a slightly acidic medium
suitable for the action of the gastric
enzymes; and
4.kills certain potentially harmful
microorganisms in food.
Action of Pepsin and Rennin
• The proteases pepsin and rennin act on
proteins.
• Pepsin: convert proteins to polypeptides
• Rennin: convert caseinogen to casein
o Rennin clots or curdles milk proteins by
converting the soluble protein caseinogen into
insoluble casein.
o This is necessary because milk proteins would
pass through the the duodenum as easily as water
and would not be digested by pepsin.
o Insoluble casein remains long enough in the
stomach to be digested by pepsin.
Stomach
• The presence of food in the stomach
stimulates the gastric glands to secrete
gastric juice into the stomach cavity.
• Peristalsis in the stomach wall churns
and breaks up the food. Peristalsis also
mixes the food well with gastric juice.
• Food normally remains in the stomach for
about three to four hours.
• The partly digested food becomes
liquefied, forming chyme.
Small Intestines
• Chyme passes in small amounts into the duodenum when
the pyloric sphincter relaxes and opens.
• The lining of the small intestine consists of many folds
called villi.
o Each villus has about 600 microvilli.
o These foldings and microvilli increase the surface area of
the small intestine for increased digestion or absorption.
• Intestinal glands are located between the villi. These
glands produce numerous enzymes, which are collectively
called intestinal juices.
• Structure:
o About 6m long
o Consists of
 U-shaped duodenum
 Jejunum
 the much coiled ileum.
Digestion in the Small
Intestines
• 3 types of digestion occurs here
o Carbohydrate digestion
 continuing from the mouth
o Protein digestion
 continuing from the stomach
o Fat digestion
 first time digestion
• 3 other organs associated with digestion here:
o Liver – produces bile and store in gall bladder
o Gall bladder – releases the bile
o Pancreas – secretes pancreatic juice
Digestive Juices in the Small
Intestines
• Chyme enters the small intestine and
stimulates:
1.Intestinal glands to secrete intestinal juice.
2.Pancreas to secrete pancreatic juice. The
pancreatic juice passes through the pancreatic
duct into the duodenum.
3.Gall bladder to release bile. Bile does not
contain enzymes. Bile passes through the bile
duct into the duodenum.
Digestive Juices in the Small
Intestines
• All three fluids (intestinal juice, pancreatic juice &
bile) are alkaline.
• The alkalis:
o
o
Neutralise the acidic chyme
Provide a suitable alkaline medium for the action of the
pancreatic and intestinal enzymes.
• The digested food (simple sugars, amino acids,
fatty acids & glycerol) are absorbed by the villi of
the small intestine, especially of the jejunum &
the ileum.
• Water and mineral salts are absorbed from the
undigested food material.
A. Intestinal Juice
• Intestinal juice is secreted by intestinal
gland.
• Intestinal juice consists of 6 digestive
enzymes
o
o
o
o
o
o
Enterokinase: inactive trypsinogen to active
trypsin
Maltase: maltose to glucose + glucose
Lactase: lactose to glucose + galactose
Sucrase: sucrose to glucose + fructose
Erepsin: polypeptides to amino acids
Intestinal lipase: fats to fatty acids + glycerol
B. Pancreatic Juice
• Pancreas secretes pancreatic juice
• Pancreatic juice consists of 3
digestive enzymes:
o
o
o
Pancreatic amylase: starch to maltose
Pancreatic lipase: fats to fatty acids + glycerol
Trypsin: protein to polypeptides
C. Bile (from Liver)
• Liver produces bile and stores in the gall
bladder.
• Gall bladder releases the bile via the bile
duct.
o
o
o
Bile is not an enzyme
Bile is an alkaline greenish-yellow liquid
containing bile salts and bile pigment.
Function of bile:
 emulsifies fats: breaks up fat into minute fat
globules thereby increasing the surface area
of the fat molecules for lipase to act on.
Functions of Liver
• The production of bile
• Detoxifying the body, i.e. breaking down alcohol
and drugs
• Breaking down excess amino acids to form urea
• Converting glucose to glycogen for storage
• Converting excess carbohydrates to fats
• Storing vitamins
• Storing minerals such as Iron, Copper, and Zinc.
• Making plasma proteins such as fibrinogen which
is used in blood clotting
• Making cholesterol which is used to form many
hormones
• Producing heat for the blood and body
Fat Digestion
• In the small intestine, bile salts emulsify fats.
• lower the surface tension of the fats, that is, they reduce
the attractive forces between the fat molecules.
• This causes the fats to break into tiny fat droplets
suspended in water, forming an emulsion.
• Emulsification increases the surface area to volume
ratio of the fats, speeding up their digestion by lipase.
• Emulsified fats are digested by lipases (pancreatic and
intestinal lipases) to fatty acids and glycerol.
• Note that this is just a physical break-up, no chemical
digestion of fat molecules has occurred.
In the Jejunum and Ilium
• By the time the food enters the ileum it is fully digested.
• The job of the jejunum and ileum is to absorb the food.
• The lining of these sections of the small intestine contain
many villi.
• The food is then taken to the hepatic portal vein which
takes it to the liver. Here the food is stored and released
when needed by the body.
• Amino acids are not stored but, those not used by the
body, are broken down by the liver and form urea. This
process is called deamination.
• Wastes created at the liver go from the liver to the hepatic
vein and then to the kidneys. Here they are excreted.
• Fats (fatty acids and glycerols) are absorbed into the villis’
lacteals. The lacteals have lymph fluid in them. From the
lacteal the fats are transported by the lymph and carried to
the bloodstream
Absorption of nutrients into the
body
• Simple Sugars & Amino acids
o
o
Absorbed into blood capillaries of the villus
via diffusion or active transport
• Glycerol & fatty acids
Absorbed into the lacteal (lymphatic capillaries) of the
villus
o via diffusion
o
• Mineral salts
o
o
Absorbed into blood capillaries of the villus
via diffusion or active transport
Structure of Villi
• Between the bases of the villi are the minute
openings of the intestinal glands that secrete
intestinal juice.
• The intestinal wall and the villi are richly
supplied with blood vessels & lymphatic
vessels to carry away the absorbed
substances.
• In each villus is a lacteal or lymphatic
capillary by blood capillaries.
• The lymphatic capillaries of the villi transport
fats while the blood vessels transport sugars
and amino acids away from the intestine.
Villi
Adaption of villus for
absorption
• Minute/ small opening at the base of the
villi: helps in the production of intestinal juice
that contains enzymes.
• Each villi has its own blood capillary
system: aids in the absorption of amino
acids and simple sugars
• Each villi has its own lacteal system: fatty
acids and glycerol can be absorbed easily
• Finger-like projections: Allows for more
efficient rate of absorption
Adaption of small intestine for
absorption
1.reduces barrier to diffusion
o
Villi have thin walls
cell thick.
epithelium is only ONE
2.Increase surface area for absorption
The inner walls of the small intestine have
numerous transverse folds and furrows.
o The small intestine also has numerous minute
finger-like projections called villi projecting into
the intestinal cavity.
o These villi further increase the surface area; the
epithelial cells of the villi, in turn, bear numerous
microvilli
o
Adaption of small intestine for
absorption
3. provides sufficient time for absorption
• The length of the small intestine is around 6m
which is long enough to provide sufficient time
for absorption.
4. carries away absorbed food substances +
continual removal maintains a concentration
gradient for absorption
• The villi of the small intestine are richly supplied
with blood capillaries and lymphatic capillaries
(lacteal).
Large Intestines
• Structure:
o
Consists of the colon & rectum (muscular
tube)
• Processes:
No digestion occurs here.
The colon absorbs water and mineral salts
from the remaining food that has passed
through the alimentary canal.
o The waste then becomes a semi-solid called
faeces.
o Faeces are stored in the rectum and then
egested through the anus.
o
o