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Figure 1 shows a cell from the lining of the ileum specialised for absorption of products of digestion - AQA - A-Level Biology - Question 1 - 2020 - Paper 1
Question 1
Figure 1 shows a cell from the lining of the ileum specialised for absorption of products of digestion. SGLT1 is a carrier protein found in the cell-surface membrane... show full transcript
Worked Solution & Example Answer:Figure 1 shows a cell from the lining of the ileum specialised for absorption of products of digestion - AQA - A-Level Biology - Question 1 - 2020 - Paper 1
Step 1
The action of the carrier protein X in Figure 1 is linked to a membrane-bound ATP hydrolase enzyme.
Answer
The ATP hydrolase enzyme catalyzes the breakdown of ATP (adenosine triphosphate) into ADP (adenosine diphosphate) and inorganic phosphate (Pi). This reaction releases energy, which is utilized to transport ions, including sodium ions (Na+), against their concentration gradient. By maintaining a concentration gradient, this process facilitates the co-transport of glucose into the cell, as glucose is often transported alongside sodium ions.
Step 2
The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum.
Answer
Sodium ions (Na+) move out of the cell through the action of the Na+/K+ ATPase pump, which requires energy from ATP. This creates a low concentration of Na+ inside the cell compared to the intestinal lumen. As a result, Na+ can enter the cell from the lumen via co-transport mechanisms with glucose. This coupled movement increases the absorption of glucose into the cell lining of the ileum.
Step 3
Describe and explain two features you would expect to find in a cell specialised for absorption.
Answer
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Folded membrane or microvilli: These structures greatly increase the surface area available for absorption, allowing for more efficient uptake of nutrients.
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Large number of co-transporters: The presence of numerous co-transport proteins ensures a fast rate of nutrient uptake, especially for essential molecules like glucose and amino acids.
Step 4
Draw phospholipids on Figure 2 to show how the carrier protein, SGLT1, would fit into the cell-surface membrane.
Answer
The phospholipids should be oriented with their hydrophilic heads facing outward towards the aqueous environments on either side (inside the cell and the intestinal lumen) and the hydrophobic tails facing inward, forming a bilayer. SGLT1 would be represented as a protein embedded within this bilayer, with its structure extending through the membrane to facilitate the transport of glucose and Na+.
Step 5
Describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end.
Answer
Amino acids join together through peptide bonds, which form between the carboxyl group (COOH) of one amino acid and the amino group (NH2) of another. During this process, a molecule of water is released in a dehydration synthesis reaction. This results in a linear chain of amino acids where one end retains the free amino group (NH2) and the other end retains the free carboxyl group (COOH), maintaining the structural integrity of the polypeptide.