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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 membran... 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
1. The action of the carrier protein X in Figure 1 is linked to a membrane-bound ATP hydrolase enzyme.
Answer
ATP hydrolase is responsible for breaking down ATP (adenosine triphosphate) into ADP (adenosine diphosphate) and inorganic phosphate (Pi). This reaction releases energy, which facilitates the active transport of ions against their concentration gradient, allowing Na+ ions to be moved into the cell.
Step 2
2. The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum.
Answer
The movement of Na+ ions out of the cell helps to maintain a concentration gradient that is essential for the co-transport of glucose. As Na+ ions flow back into the cell via SGLT1, they carry glucose molecules with them into the cell through a process called co-transport. This coupling of Na+ with glucose allows glucose to be absorbed even against its concentration gradient.
Step 3
3. Describe and explain two features you would expect to find in a cell specialised for absorption.
Answer
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Folded membrane/microvilli – This increases the surface area for absorption, allowing more efficient uptake of nutrients.
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Large number of mitochondria – These provide ATP for active transport processes required for absorbing substances, enhancing the cell's ability to transport nutrients.
Step 4
4. Draw phospholipids on Figure 2 to show how the carrier protein, SGLT1, would fit into the cell-surface membrane.
Answer
Phospholipids should be drawn in a bilayer, with the hydrophilic heads facing outward towards the aqueous environment and the hydrophobic tails facing inward. SGLT1 should be depicted embedded within this bilayer, ensuring the hydrophobic regions of the protein align with the phospholipid tails.
Step 5
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 are joined by peptide bonds, which form between the amino group (NH2) of one amino acid and the carboxyl group (COOH) of another. This reaction releases a molecule of water (condensation reaction). The sequence of amino acids will always have a free amino group (NH2) at one end (N-terminus) and a free carboxyl group (COOH) at the other end (C-terminus), maintaining this structure in the polypeptide chain.