Fig. 1.1 Fig. 1.1 shows a single villus from the lining of the small intestine. The inner wall of the small intestine is folded and is covered with millions...

This question tests absorption in the small intestine and how the villus is adapted for it, plus what happens to absorbed food (assimilation).

(a) The soluble products of digestion pass into the blood by absorption [1]. This is the movement of the small, soluble molecules from the gut cavity through the wall of the intestine into the blood.

(b) Fatty acids and glycerol (the products of fat digestion) are absorbed into the lacteal [1], the central lymph vessel of the villus, not into the blood capillaries.

(c) Any three of the following adaptations, for one mark each (max 3):

• The wall is thin / only one cell thick, giving a short diffusion distance for food molecules to cross [1].
• The villus (together with the millions of villi and their microvilli) gives a very large surface area for absorption [1].
• There is a rich network of blood capillaries that carries absorbed food away quickly, keeping a steep concentration (diffusion) gradient [1].
• There is a lacteal to absorb the products of fat digestion [1].

Each feature works by either shortening the diffusion path, enlarging the exchange surface, or maintaining the gradient that drives diffusion and active uptake.

(d) Carbohydrate is digested to glucose / simple sugars [1]; protein is digested to amino acids [1]. These small soluble molecules are the forms that can be absorbed across the villus wall.

(e) A very large number of villi provides a very large total surface area [1], so more food can be absorbed at the same time / absorption is faster [1]. Surface area is directly related to the rate of diffusion, so more surface means more molecules cross per unit time.

(f) Assimilation is the movement of absorbed food molecules into the cells [1] where they are built into the cell / used by the body (for example amino acids made into new proteins, glucose used in respiration) [1].

(g) After absorption the amino acids are carried in the blood to the liver [1]; they are used to build new proteins needed by the body (enzymes, membranes, etc.) [1]; and any excess is broken down / deaminated in the liver, forming urea that is excreted [1] (max 3). The key point is that amino acids cannot be stored, so surplus is dealt with in the liver.