6. Absorption

Question 1

Define ‘absorption’.

Answer 1

the process of taking substances into cells and the blood

Question 2

What are epithelial cells?

Answer 2

single layer of cells forming the inner lining of the mucosa

Question 3

What is the role of the epithelium in the human digestive system?

Answer 3

absorbing nutrients

Question 4

What does the rate of absorption (in the epithelium) depend on?

Answer 4

the surface area of this epithelium

Question 5

Where does absorption of nutrients principally occur?

Answer 5

in the small intesting

Question 6

How long is the small intestine in an adult? How wide? How is its surface area increased?

Answer 6

• 7m long
• 25-30mm wide
• folds on its inner surface, giving a large surface area of epithelium

Question 7

How else, apart from folds on the inside of the small intestine, is the surface area of the epithelium increased?

Answer 7

by the presence of villi, which are small finger-like projections of the mucosa on the inside of the intestinal wall

Question 8

What are villi?

Answer 8

small finger-like projections of the mucosa on the inside of the intestinal wall

Question 9

How long is a villus? How many villi can there be per square millimetre of small intestinal wall?

Answer 9

• 0.5 - 1.5mm long

- 40 per square millimetre

Question 10

By how much do the villi increase the surface area of the small intestine?

Answer 10

by about a factor of 10

Question 11

What do the villi absorb?

Answer 11

• mineral ions and vitamins

- also monomers formed by digestion such as glucose

Question 12

Draw and label a diagram of a villus. (p73)

Answer 12

outside to in:

• layer of microvilli
• epithelium
• goblet cells (secrete mucus)
• blood capillary
• lacteal (a branch of the lymphatic system)

Question 13

State 4 different methods membrane transport used in the epithelium to absorb nutrients.

Answer 13

• simple diffusion
• facilitated diffusion
• active transport
• endocytosis

Question 14

How is ‘simple diffusion’ used by the epithelium cells to absorb different nutrients?

Answer 14

simple diffusion: nutrients pass down concentration gradient between phospholipids in the membrane

example: hydrophobic nutrients such as fatty acids and monoglycerides

Question 15

How is ‘facilitated diffusion’ used by the epithelium cells to absorb different nutrients?

Answer 15

facilitated diffusion: nutrients pass down the concentration gradient between phospholipids in the membrane

example: hydrophilic nutrients such as fructose

Question 16

How is ‘active transport’ used by the epithelium cells to absorb different nutrients?

Answer 16

active transport: nutrients are pumped through the membrane against the concentration gradient by specific pump proteins

example: mineral ions such as sodium, calcium and iron

Question 17

How is ‘endocytosis’ used by the epithelium cells to absorb different nutrients?

Answer 17

endocytosis: small droplets of fluid are passed through the membrane by means of vesicles
example: triglycerides and cholesterol in lipoprotein particles

Question 18

Give an example of a more complex method of transport used in epithelium cells to absorb nutrients, other than the 4 main types of membrane transport. Explain how this works.

Answer 18

• glucose is absorbed by sodium co-transporter proteins
• which move a molecule of glucose together with a sodium ion across the membrane together into the epithelium cells. Glucose can be moved against its concentration gradient because the sodium ion is moving down its concentration gradient. The sodium gradient is generated by active transport of sodium out of the epithelium cell by a pump protein.

Question 19

How can we model the absorption be the epithelium of the intestine?

Answer 19

with dialysis tubing

Question 20

Sketch a dialysis tubing model. (p73)

Answer 20

-

Question 21

What is expected to happen in the dialysis tubing model?

Answer 21

• cola drink contains mixture of substances which can be used to model digested and undigested foods in the intestine
• water outside bag tested at intervals to see if substances in cola have diffused through dialysis tubing

expected result: glucose and phosphoric acid, which have small-sized particles, diffuse through the tubing but caramel, which consists of larger polymers of sugar, does not

Question 22

Define a ‘scientific model’.

Answer 22

a theoretical representation of the real world

Question 23

What do biological models represent?

Answer 23

a structure or process non-mathematically

Question 24

What happens once a model has been proposed?

Answer 24

predictions are made, and the predictions are tested

Question 25

How are predictions of a biological model tested? How can a model be trusted more? What happens if predictions are not as close as they could have been?

Answer 25

• with experiments or observations of the real world
• if the predictions based on a model fit experimental data or observations
• the model is modified

Question 26

What is falsification? What happens when a model or theory is falsified?

Answer 26

• when evidence shows that a model or theory is incorrect

- the model or theory must then be discarded and replaced

Question 27

Give an example of a physical model.

Answer 27

modelling absorption with dialysis tubing