Post on 05-Jun-2020
ABSORPTION IN THE SMALL INTESTINE
ABSORPTION IN THE SMALL INTESTINE
Objectives
Know how the small intestine is adapted for
the absorption of nutrients
Understand how carbohydrates are absorbed
Understand the roles of diffusion, active
transport and co-transport in digestion in the
small intestine
STARTER
Small intestine quick facts...
- ‘small’ because more narrow than
large intestine....BUT much
longer...about 14 feet!
How is the small intestine adapted
to adsorb digested materials?
In pairs – write down as may ways
as you can remember – 2 minutes
ADAPTATIONS OF THE SMALL INTESTINE FOR
ABSORPTION
Long... Large SA for efficient absorption
ADAPTATIONS OF THE SMALL INTESTINE FOR
ABSORPTION
Folded...further increases SA
ADAPTATIONS OF THE SMALL INTESTINE FOR
ABSORPTION
Villi...finger-like projections which increase
SA
1) Large surface area
ADAPTATIONS OF THE SMALL INTESTINE FOR
ABSORPTION
Epithelial cells have micro-villi (make up
the ‘brush border’ )
ADAPTATIONS OF THE SMALL INTESTINE FOR
ABSORPTION
Thin epithelium
(inner layer)...one
cell thick
This creates a
short diffusion
pathway
2) Thin walls – reduce
diffusion distance
ADAPTATIONS OF THE SMALL INTESTINE FOR
ABSORPTION
3) The villi have a good blood supply
4) The villi can move
Which component of this diagram might indicate this?
5) The villi secrete enzymes
Which component of this diagram might indicate this?
-Intestinal glands
(crypts)
– Release e.g.
Sucrase,
maltase,
petidase
- Nerve fibre
- What
do the
goblet
cells do?
Histology of small intestine
ABSORPTION OCCURS VIA 3 PROCESSES
Diffusion
Facilitated diffusion
Active transport
DIFFUSION
The net movement of substances from an
area of high concentration to an area of low
concentration.
Passive process
FACILITATED DIFFUSION
Involves a carrier
protein :
Substance binds to
protein
Protein changes
shape
Molecule released
Is a passive process
Some glucose can be
absorbed in this way but
not main process
ACTIVE TRANSPORT
Substances are moved against a
concentration gradient
This requires a supply of energy
This energy comes from ATP
Absorption of glucose
High concentration of glucose both in the
lumen and in the capillaries to the villi
Some glucose moves by diffusion ....
Glucose rich blood circulated quickly creating
a gradient
However, not enough as concentration
gradient can be equalized ....
Therefore co-transport required...using the
sodium-potassium pump
CO-TRANSPORT
A special type of active transport
Monosaccharides move across the
epithelium of the small intestine into the
capillaries in this way
Disaccharides & polysaccharides are too
large
CO-TRANSPORT
http://www.youtube.com/watch?v=Ry7l0Oe3wJ
M&feature=related
Watch the video simulation
....write down at least 4 key points ........
CO-TRANSPORT
Glucose must be moved from the lumen of
the gut into the epithelial cells
But the concentration of glucose is usually
higher in the epithelial cells than in the gut
Therefore transport into the cell requires
energy....KEY POINT: note exactly which
part of the process required energy...
CO-TRANSPORT
Transport proteins have to move glucose across the
membrane due to its size..through co-transort:
1) Sodium ions are actively transported out of epithelial
cells via the sodium-potassium pump (NA+ ions moved
out in exchange for K+ ions) – creating a low
concentration of NA+ ions in epithelial cells
This requires energy from
ATP
2) There is now a higher concentration of sodium ions
outside the cell (in the lumen) than inside cells...
3) Sodium ions can travel by diffusion on a
different carrier protein. They couple with
glucose molecules that are carried with them.
Glucose molecules move against their
concentration gradient
Sodium ions move with a concentration gradient
4) Glucose then moves from epithelial cells to blood via
facilitated diffusion on another type of carrier protein
CO-TRANSPORT
The pump which moves glucose and Na⁺ across the epithelial cell membrane together
is called the Na⁺-glucose symporter
NA⁺-GLUCOSE SYMPORTER
Both Na⁺ and glucose can bind to the pump
but the binding of one makes the other more
effective.
2 glucose molecules and 2 Na⁺ ions must
bind to the protein before they can be
transported across the membrane.
NA⁺-GLUCOSE SYMPORTER
Glucose and Na⁺ could in theory move back
across the membrane the other way.
But there is a very low concentration of Na⁺ on the inside of the cell so it is rare that Na⁺ will move into the symporter from this side.
Therefore glucose does not follow and we
say the import is unidirectional ....therefore
symport
Sodium ions move out of epithelial cell
Sodium ions move into the epithelial cell
Glucose molecules move into the epithelial
cell
State which of the following movements is
passive or active
SOME TASKS
Complete question on handout (from Churchman & Peddar (2000)
p.173)
Complete question on next slide (from s-cool.co.uk / A-level /cells
& organelles)
PLENARY
Spend 2 minutes reading your notes
Then write down at least 5 things you’ve
learned
If you can, try quickly writing out the co-
transport process in 3 sentences (in rough)