Digestion, absorption and metabolism
digestion enables nutrients to be absorbed,
absorption into the blood enables them to be utilized
metabolism refers to all the chemical changes which take place in the
body.
Digestion is simply the breakdown of large molecules into smaller
ones and it occurs in the gastrointestinal tract through the action of
enzymes and other substances like bile and hydrochloric acid.
proteins – amino acids
fats – fatty acids, monoglycerides
starch - glucose
Digestion is under nervous and hormonal control.
The gastrointestinal tract is a muscular tube
varying diameter
mixing the food with the digestive secretions (peristalsis)
producing mucus (lining the tract, lubricates the passage)
Mouth
• food is chewed
• producing saliva
dilutes the food
allows water-soluble constituents to dissolve
allows tasting,
it contains alpha-amylase
Swallowing is a complex action in which food is directed into the
oesophagus and prevented from entering the trachea.
Oesophagus is simply a muscular tube, which carries food to the stomach.
Stomach
temporary store of food,
producing chyme (the mixture of food and gastric secretion)
hydrochloric acid and pepsins secretion
Small intestine
three parts (duodenum, jejunum, ileum)
most of the digestion and absorption of food occurs.
the mucosa is folded into finger-like projections, called villi
the inner surface of every cell is folded in microvilli.
the surface area of absorption is more hundred m2
Liver
• produces bile
• excreting cholesterol and the bile pigments
Pancreas
• enzymes (digest fat, starch, protein, RNA and DNA).
• contains two types of tissue.
• the islets of Langerhans, produces the hormones of insulin and
glucagon
Proteins are digested to two or three amino acids,
Fats to fatty acids and monoglycerides,
Starches to two- and three-unit sugars.
Digestion of these molecules is completed by enzymes produced by the
cells of the small intestine itself (enterocytes).
PANCREATIC ENZYMES
Enzyme
Substrate
Digestive products
Trypsin
Chymotrypsin
Carboxypeptidases
Pancreatic lipase
Phospholipases
Protein
Protein
Protein
Fats
Lecithin and related
compounds
Starch
RNA
DNA
Connective tissue
Elastin
To smaller fragments
To smaller fragments
To smaller fragments
Fatty acids, monoglycerides
Free fatty acids
Alpha-amylase
Ribonuclease
Deoxynuclease
Collagenase
Elastase
Large intestine
• water absorption
• producing faeces
• bacterial activity
To smaller fragments
Absorption
Absorption is the passage of nutrients from the gut into the blood
stream.
It occurs mainly in the small intestine
active transport - passive diffusion.
Fat-soluble vitamins are absorbed along with fat, water-soluble
vitamins by diffusion.
Minerals
Some, like sodium, are fully absorbed and the body content is
controlled by variable excretion.
Others, like copper, iron, calcium and zinc are only partly absorbed.
Some dietary substances hinder the absorption of iron, calcium, zinc
and copper
phytates,
oxalates
none starch polysaccharides (NSP)
Metabolism
The term metabolism means the sum of all the chemical reactions
which occur in the body.
• anabolism. (synthesis of glycogen, fat and proteins)
• catabolism (breakdown of substances either in normal
turnover
or to provide energy)
Energy sources
• amino acids (from proteins),
• fatty acids (from fats)
• monosacharides (from starch and sugars)
These different molecules are broken down to a two-carbon molecule
called acetyl coenzyme A. Acetyl coenzyme A enters a cycle of
enzymatic reactions called the citric acid cycle (also called Kreb's
cycle). In this cycle acetyl coenzyme A is broken down to carbon dioxide
and water and high-energy molecules of adenosine triphosphate (ATP) are
formed.
ATP is the `unit' in which energy is produced by the body. Any
reaction which needs energy (like the synthesis of protein, the contraction
of muscles or a nerve impulse) requires and uses ATP.
A simplified scheme indicating the release of food
energy
After a meal
• amino acids, fats and sugars are taken up by the cells
• insulin is released
• cells take in glucose and amino acids
• stimulation of glycogen, fat and protein synthesis
• it is the state of anabolism or synthesis.
Between meals
• the reverse occurs
• no longer a supply of energy-producing molecules
• stored nutrients are used for cellular energy supplies
• glucagon, growth hormone and adrenaline control metabolism during
fasting.
In pregnancy, lactation, growth and healing there is an increased need for
protein. These are anabolic states. A catabolic state is where there is
loss of body tissue, as in starvation, fever.
Total metabolic demand will depend
• on the size of the body,
• on its composition (muscle cells are more active metabolically
than fat cells)
• and on its state (for example growth, pregnancy, or recovery
from injury).