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)