Metabolism Flashcards
What is metabolism in the cellular context
The sum of all enzyme-catalysed reactions in a cell, responsible for harvesting energy/electrons and using them to build and maintain the cell’s macromolecular structure
What are the four essential functions of metabolism in building a cell
- Harvest energy from the environment
- Harvest electrons from environmental sources
- Use energy + electrons to build monomers from C, N, P, S atoms
- Assemble macromolecules (e.g. lipids, proteins, polysaccharides) from those monomers
Why does protein biosynthesis require energy
Peptide bond formation is endergonic (ΔG > 0)
Coupling it to GTP hydrolysis during translation (1 GTP per amino acid) makes the reaction exergonic and favorable
Where is GTP used during translation
During tRNA loading into the ribosome
During translocation steps
Provides proofreading control to prevent errors
How is glucose activated for addition to starch
- Glucose-1-phosphate is formed from glucose
- Reacts with ATP → ADP-glucose (via ADP-glucose pyrophosphorylase)
- ADP-glucose is added to the starch chain by starch synthase
- Releases ADP → drives polymerization
Why does starch synthesis require 2 ATP equivalents per glucose unit
1 ATP to phosphorylate glucose → G1P, another ATP equivalent to form ADP-glucose
How is glucose polymerised into cellulose
- Glycogen → Glucose-1-phosphate
- Glucose-1-phosphate + UTP → UDP-glucose
- UDP-glucose is the activated form added by cellulose synthase
- Glucan chains form → aggregate into elementary fibrils → combine to form microfibrils
Give the key features of cellulose
It has β-1,4 glycosidic bonds
Forms rigid, crystalline microfibrils
Why are electrons required for biosynthesis
Many biosynthetic reactions are reduction reactions, adding electrons to molecules (e.g. reducing nitrate to ammonium, carbon to hydrocarbons)
Whats the major sources of electrons for biosynthesis
Autotrophs: H₂O → O₂ via photosynthesis; electrons passed to NADPH
Heterotrophs: Organic molecules (e.g. glucose) → NADPH via pentose phosphate pathway
How is glutamate synthesised in autotrophs
NO₃⁻ → NO₂⁻ → NH₄⁺ via nitrate reductase + nitrite reductase
NH₄⁺ + α-ketoglutarate + NADPH → Glutamate (via glutamate dehydrogenase)
How is glutamate linked to the TCA cycle
Glutamate is derived from α-ketoglutarate, a TCA intermediate
What is transamination and why is it important
It is the transfer of an amino group from glutamate to a keto acid to form other amino acids, allows you to form α-ketoglutarate
How is asparagine synthesised from aspartate
Aspartate + glutamine + ATP → Asparagine + glutamate + AMP + PPi
What are the components required for fatty acid synthesis
Acetyl-CoA – 2-C donor
Malonyl-CoA – made from acetyl-CoA + CO₂ (via acetyl-CoA carboxylase)
NADPH – reducing power
ATP – for carboxylation and elongation steps
Fatty Acid Synthase complex – multi-domain enzyme that catalyses chain elongation
Where does Acetyl-CoA come from
From glycolysis via pyruvate → acetyl-CoA, or from diverted TCA cycle intermediates
What is the role of Malonyl-CoA in fatty acid synthesis
Provides 3-carbon units for elongation, with one carbon released as CO₂ during chain building
What are the 3 phases of glycolysis
- Formation of doubly phosphorylated hexose (e.g. fructose-1,6-bisphosphate)
- Splitting into doubly phosphorylated triose sugars (e.g. G3P)
- Substrate-level phosphorylation – transfer of phosphate from intermediates to ADP → ATP
Why is early phosphorylation of glucose important
Prevents passive diffusion out of cell
Keeps glucose trapped inside
Prepares it for energy extraction
How do glycolysis avoid unstable chemistry
Uses stable intermediates
Avoids generation of highly reactive radicals or aldehydes
Intermediates are often phosphorylated, preventing unintended reactions
Despite thousands of reactions, what makes metabolism logically simple
Core principles are conserved
Most pathways are built on a small set of interconvertible intermediates
Reactions are often driven by phosphate transfer, redox reactions, or carbon rearrangement
How is glutamate synthesised from nitrate
- NO₃⁻ → NH₄⁺ via nitrate + nitrite reductase
- NH₄⁺ + α-ketoglutarate + NADPH → Glutamate
(alternative TCA route: α-ketoglutarate → glutamate)
