Mesophyll cells in plant leaves synthesise sucrose from the products of photosynthesis. The enzyme sucrose synthase catalyses the final reaction in the synthesis of sucrose.

Name the two monosaccharides that are used to make sucrose.

Two answers required.

5(b)Plant Structure TransportHard5 marks

Scientists studying a species of flowering plant investigated the movement of sucrose from mesophyll cells through phloem parenchyma cells and companion cells into sieve tubes.

Figure 5.1 shows the movement of sucrose from a mesophyll cell to a phloem sieve tube element in the vein of a leaf.

Figure 5.1, a labelled diagram (not to scale) of a mesophyll cell connected by plasmodesmata to a row of three phloem parenchyma cells, the central one of which connects on to a companion cell and then to a phloem sieve tube element, with dashed arrows showing the passive movement of sucrose and solid arrows showing the cotransport of sucrose between cells, from the mesophyll cell down to the sieve tube

Explain how sucrose moves from the mesophyll cell to the phloem sieve tube element, as shown in Figure 5.1.

Answer

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5(c)(i)Cell MembranesMedium3 marks

The scientists discovered a protein in the cell surface membrane of phloem parenchyma cells that transports sucrose out of the cells.

Figure 5.2 shows the structure of the membrane protein, known as SWEET, in the cell surface membrane.

Figure 5.2, a ribbon diagram of the SWEET membrane protein sitting within the phospholipid bilayer of the cell surface membrane, showing several coiled alpha-helices spanning the membrane and short loops of the polypeptide extending above and below the bilayer

Figure 5.3 shows the membrane protein viewed from the surface of the cell.

Figure 5.3, the SWEET membrane protein viewed from the surface of the cell, showing seven circles (cross-sections of the alpha-helices) arranged in a ring around a central hydrophilic pore, surrounded by the phospholipid head groups of the membrane

Describe the structure and arrangement of the SWEET protein as shown in Figure 5.2.

Answer

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5(c)(ii)Diffusion Osmosis Membrane TransportMedium2 marks

Explain why SWEET proteins are needed for the movement of sucrose out of the phloem parenchyma cell shown in Figure 5.1.

Answer

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