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gang green gang

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Using fig 3.2 and your knowledge about this structure, explain why fig 3.1 is not a good representation of absorption in the small intestine.

Fig 3.1 shows the walls of the small intestine to be smooth. It does not show the extensively folded inner walls of the small intestine with the numerous villi present on the walls of the small intestine which increases the surface area to volume ratio for faster rate of absorption. During absorption, the amino acids, glucose, fatty acids and glycerol diffuse into the structures of villi through the one-cell thick epithelium that reduced barrier to diffusion and allows  a faster rate of diffusion. It then diffuses into the capillaries and the lacteal. The capillaries and lacteal are not outside the small intestine as suggested by Fig 3.1, the blood capillaries and hymphatic capillaries which absorb glucose and amino acids and the lacteal which absorbs fatty acids and glycerol are within the structure of the villus shown in fig 3.2 on the folded walls of the small intestine.

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(c) Describe the fate of glucose after absorption.

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(c) Describe the fate of glucose after absorption.

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Excess glucose will be converted into glycogen by insulin and stored in the liver and muscles, while the remaining glucose leaves the liver via the hepatic vein to be transported to the rest of the body. Glucose will be used during respiration by mitochondria in cells in order to release energy for cellular activities.

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3b:
The extensively folded inner walls of the small intestine is not shown on figure 3.1 as it displays the walls of the small intestine to be smooth. The capillary and lacteal should also be within the small intestine, not on the outside of it.

3c:
Glucose will be absorbed directly into the blood capillaries found in the villi. The hepatic portal vein will then carry glucose in the blood to the liver.

3d:
1. Villi have thin walls and epithelium is only one-cell thick. This reduces diffusion distance for faster diffusion as it allows it to proceed faster.
2. It is richly supplied with blood capillaries and lymphatic capillaries. This carries away absorbed food nutrients quickly ( blood capillaries absorb glucose and amino acids while lacteal absorb fatty acids and glycerol) and maintains a concentration gradient for diffusion.

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(b) The inner walls of the small intestine in fig 3.1 are flat and without folds. This is not a good representation of absorption in small intestine as the small intestine will have a smaller surface area to volume ratio hence reduces the rate of absorption of food substances.
The inner walls of small intestine in fig 3.1 do not have the numerous minute finger-like projections called villi. This is not a good representation of absorption in the small intestine as villi increase the surface area to volume ratio, hence the absence of villi will then lower the rate of absorption.

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(c) After the absorption of glucose, glucose will be used to release energy for cell activities during respiration which will occur in the mitochondria.
Excess glucose is converted to glycogen by insulin and will be stored in the liver and muscles. Remaining glucose will leave the liver via hepatic vein to be transported to the rest of the body.

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1. Villi have thin walls and epithelium is only one-cell thick. This reduces barrier to diffusion, reducing diffusion distance for faster diffusion of food substances.
2. It is richly supplied with blood capillaries and lymphatic capillaries. This helps to carry away absorbed food substances quickly by diffusion and active transport. It maintains concentration gradient for diffusion of food substances.

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The inner walls of the small intestine are supposed to be extensively folded, to provide a larger surface area to volume ratio for faster rate of absorption of digested food substances. However, Figure 3.1 shows that the inner walls of the small intestine are smooth.

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3b) Figure 3.1 did not show the structure of the villus which enabled it to increase surface area to volume ratio for quicker absorption of digested food molecules such as fats,proteins and starch molecules. It is not good as it does not show that the lacteal is surrounded by capillaries as the lacteal and the capillary are not separated.

3c) Glucose is used by all cells as a source of energy. It is used during respiration to release energy for cellular activities.Excess glucose are returned to liver, excess glucose are then converted to glycogen by insulin and stored in liver and muscles.

3d) 1)Villus has walls which are one cell thick to reduce diffusion distance for faster diffusion of digested food substances.
2)Villus has a lymphatic capillary surrounded by blood capillaries. the lymphatic capillaries of villus transports fats while the blood capillaries transport simple sugars and amino acids away from the small intestine. The continual transport of digested food substances maintains concentration gradient for the absorption for the absorption of digested food substances.

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b) The small intestine bears numerous villi. The epithelium of the villi,in turn has numerous microvilli. Therefore, the representation for the inner surface of the small intestine should be extensively folded. Also, lacteal and capillary is found inside the small intestine. Hence, their representation should be inside the small intestine.

c)Glucose would be used by cells for respiration to release energy. Excess glucose would be converted into glycogen by insulin in the liver and is stored in the liver and muscles.

d) 1. The villi has a one-cell thick wall that reduces diffusion distance for faster diffusion of food substances.

2. The villi is richly supplied with blood capillaties and lymphatic capillaries which carries away food substance quickly and maintains concentration gradient for diffusion.

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3b) From fig.3.2, the lacteal is being coiled around by the blood capillaries. fig 3.1 has the lacteal and blood capillaries at the sides of the small intestine which is inaccurate because they are located in the walls of the small intestine to carry out the function of absorbing amino acids and glucose by the blood capillaries and fatty acid and glycerol by the lacteal to be transported to liver after they pass through the epithelial cells of the villi.

c)it is transported from the small  to the liver through the hepatic portal vein. In the liver, glucose is used to release energy for respiration in the mitochondria. Excess is converted to glycogen and stored in the liver and muscles.

d)Villi walls are one cell thick to reduce diffusion distance for faster diffusion. Richly supplied with blood capillaries that carries away glucose and amino acids and lacteal that carries away fatty acids and glycerol quickly and these structures to maintain the concentration gradient for diffusion.

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3c) glucose will be transported to the liver via the hepatic portal vein . Glucose will be transported to other cells of the body by the hepatic vein for release of energy by respiration . Excess glucose is converted to glycogen by insulin and stored in the liver or muscles

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Well written but too much description on what occurs during absorption. Focus on the *structures*

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good

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You correctly spotted the poor representation, but you did not specify what are the correct *structures* found in the small intestine. Have to specify e.g. small intestine have folded internal walls so as to....(function)....

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well done

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1. "food substances" is inaccurate. it must be "digested food substances/nutrients" or state the names of the nutrients. And faster diffusion to where??

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Lacking 1 more structure

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good

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Be precise, it is "blood capillary"

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Did not answer the question. Your answer describes the absorption and transport of glucose. Fate = how will glucose be *used*

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1. What thing to "proceed"?? What process is happening? Not clear, no marks!
2. acceptable.

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Can be clearer. First, it is "blood capillaries", next explain where lacteal & blood cap can be found. Then explain that they are not separated like in Fig 3.1 .

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Excess glucose is *transported*, not returned to liver

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1) "food substances" is inaccurate. it must be "digested food substances/nutrients" or state the names of the nutrients. And faster diffusion to where??

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good that you have spotted many errors. But you must contrast against what is shown in Fig. 3.1.
I would write "The inner walls of small intestine are extensively folded but the Fig. 3.1 shows the walls to straight and smooth." This is to show compare and contrast.

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acceptable

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1. is ok
2. "food substances" is not acceptable at your level. Be precise to specify which nutrients. Give exact names.
"carries away" is also not acceptable. Give exact name of the process: absorption and transport

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Be precise: the lacteal and blood capillaries are located *in the villi* which are on the walls of small intestine

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2 improvements. First, glucose is used to release energy *not only in the liver* but in all cells. So "in the liver" is not needed and wrong.
Second, state the hormone to cause excess glucose to be converted.

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1. for faster diffusion of what molecules? And diffuse into where? Not clear enough, no marks!
2. "Villi are richly supplied....."

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d)Epithelial cells of villus in the small intestine have microvilli to increase surface area to volume ratio for faster rate of absorption, diffusion, and digestion of food molecules like glucose.

One cell thick epithelial cell to reduce diffusion distance and barrier for faster diffusion and absorption of food molecules into the lacteal and blood capillaries.

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aric :(((