2.2. Carbohydrates and lipids
A subsection of Biology, 9700, through 2. Biological molecules
Listing 10 of 368 questions
In most plants, sucrose is the main sugar that is transported from sources to sinks. In the source, sucrose is transferred from a mesophyll cell to a phloem sieve tube through a companion cell. Describe and explain how the transfer of sucrose into a phloem sieve tube from a companion cell can lead to the transport of the sugar to a sink. shows part of one pathway that is used in plant cells to synthesise sucrose. The enzyme sucrose synthase catalyses the transfer of glucose from UDPG (uridine diphosphate glucose) to fructose. UDPG + fructose UDP + sucrose sucrose synthase The structure of UDPG is shown in . uracil ribose glucose Name the type of bond formed when sucrose is synthesised. Explain why UDP can be described as a phosphorylated nucleotide. Sucrose synthase acts by using an induced fit mechanism rather than a lock and key mechanism. With reference to sucrose synthase and the synthesis of sucrose, outline the difference between the induced fit mechanism and lock and key mechanism of enzyme action. UDPG is used in some algae (photosynthetic protoctists) to synthesise a storage compound known as floridean starch. The molecular structure of floridean starch has been described as an intermediate between amylopectin and glycogen, with little or no amylose. Describe the molecular structure of floridean starch by completing the passage. Floridean starch is a polysaccharide composed of monomers. The monomers are joined by and linkages, to give a branching structure that is less highly branched than .
9700_s20_qp_22
THEORY
2020
Paper 2, Variant 2
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Polysaccharides, such as glycogen, are composed of thousands of monomers. Oligosaccharides are carbohydrates that contain three to ten monomers in their chain. Nystose is one example of an oligosaccharide. The structure of nystose is shown in . H H H H H HOH2C CH2OH HO OH O O O C H H H H H CH2OH HO OH O O H H C H H H CH2OH HO OH O C H H H HO HO CH2OH H H OH O State three differences between the structures of nystose and glycogen, other than the number of monomers in the molecules. Cells use oligosaccharides to synthesise glycoproteins, which are transported to cell surface membranes. Describe the roles of the rough endoplasmic reticulum and the Golgi body in synthesising glycoproteins. State one role of glycoproteins in the cell surface membrane.
9700_s23_qp_23
THEORY
2023
Paper 2, Variant 3
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Questions Discovered
368