what does controlled stepwise oxidation of sugar allow
- it allows the capture of energy in small portions that is captured by activated carriers like ATP and NADH
What are the 3 stages of catabolism
Stage 1: breakdown of food to subunits
Stage 2: breakdown of subunits to acetyl-CoA
Stage 3: oxidation of acetyl-CoA
Describe stage 1 of catabolism
- breakdown of food to subunits
polysaccarides –> simple sugars
proteins –> amino acids
fats –> fatty acids and glycerol - this can take place in either outside of the cell (like in the intestines) or in lysosomes
Describe stage 2 of catabolism
- breakdown subunits into acetyl-CoA:
- glucose is broken down in cytosol into two molecules of pyruvate through a process known as glycolysis
- pyruvate is transported into the mitochondrial matrix and converted into acetyl-CoA by the multi enzyme complex known as pyruvate dehydrogenase
- fats and some amino acids are converted into acetyl-CoA
- some ATP, CO2 and NADH is produced
- some amino acids are broken down into pyruvate
Describe stage 3 of catabolism
Oxidation of acetyl-CoA:
- acetyl-CoA enters the citric acid cycle (CAC) in the mitochondrial matrix
- NADH, GDP and FADH2 are produced
- electrons from NADH and FADH2 are passed down the electron transport chain driving the production of ATP at the inner mitochondrial membrane
Glycolysis
occurs in 2 phases:
1. preparatory phase: 2 ATP are required to activated the glucose molecule
2. pay off phase: energy investment pays off in form of 4 ATP and 2 NADH for each glucose
Glucose —> pyruvate + 2ATP + 2NADH
What 2 steps result in the formation of ATP in glycolysis
step 7 and 10
- these steps are known as substrate level phosphorylations because a phosphate is transferred from a substrate (glycolysis intermediate) onto ADP
- no oxygen is required
No oxygen is required for
glycolysis to occur. Does
glycolysis involve any oxidation
reactions?
Yes:
- glyceraldehyde 3-phosphate is oxidized and NAD+ is reduced to NADH
Coupled reactions in step 6 and 7 in glycolysis
Step 6: C-H bond oxidation (very favourable) is coupled with the unfavourable reduction of NAD+ and the formation of high energy phosphate bond
Step 7: hydrolysis of the high energy phosphate bond (very favourable) is coupled with the formation of ATP
When can transfers of phosphate occur?
transfers of phosphate can only occur when the change in free energy for hydrolysis is more negative for the donor than the acceptor
What is fermentation
- it is the breakdown of organic molecules in the absence of oxygen
- occurs in active muscle cells and yeast
- come organisms rely on glycolysis as a principle source of energy
- fermentation regenerated NAD+ needed for glycolysis
Fermentation in yeast is especially important with brewing and bread making. Why?
- because the byproducts of fermentation are CO2 and ethanol
- CO2 is important in helping bread rise and ethanol is the key component in most alcoholic beverages
Fat derived acetyl-CoA
- Fatty acids are broken down into acetyl-coA in a process known as B-oxidation
- they produce 1 NADH and 1 FADH2 each round
- the fatty acid is shortened by 2 carbons each round and a molecule of acetyl-CoA is released
The citric acid cycle
- known as tricarboxylic acid cycle or the Krebs cycle
- takes place in mitochondrial matrix
- does not use O2 directly, but still depends on it
- O2 is used by electron transport chain which regenerates NAD+
- produces 3 NADH, 1 GTP, 1 FADH2 and releases 2 molecules of CO2, and results in regeneration of oxaloacetate
- then electrons of NADH and FADH2 are passed down the electron transport chain
Why are intermediates formed during glycolysis and CAC important
- because they serve as precursors to the production of important molecules like amino acids, nucleotides and fats
The electron transport chain
- drives majority of synthesis of ATP in cells (around 30 ATP from 1 molecule of glucose)
- takes place using proteins embedded in inner mitochondrial matrix
- electrons from NADH and FADH2 are transferred through a series of acceptors and donors which pump H+ ions to the intermembrane space
- the ultimate accepter is O2 which produces H2O
- the import of H+ is coupled to the production of ATP
What is gluconeogenesis
- production of glucose from small molecules like pyruvate
- many steps are the reverse steps of glycolysis but not all
What is the difference between glycolysis and gluconeogenesis
- 3 steps in glycolysis are essentially irreversible and require a different set of enzymes
- for example phosphofructokinase is used during glycolysis, but fructose 1,6-biphosphatase is used during gluconeogenesis
- the balance between glycolysis and gluconeogenesis is made by positive and negative inhibition
- AMP and ADP activate phosphofructokinase while ATP inhibits it. Note that opposite affects are seen in fructose 1,6-bisphosphatase
Why do the opposite affects in glycolysis and gluconeogenesis make sense?
- because if ATP is abundant you don’t want to undergo glycolysis to produce more
Why is gluconeogenesis energetically expensive
- because it requires 4 ATP and 2 GTP for each molecule of glucose to be produced
Storing glucose
- glucose can be stored as glycogen mostly in cytoplasm of liver and muscle cells
what is glycogen
glycogen is a branched polymer that can be broken down into glucose when blood sugar is low or synthesized when it is plentiful
storing fats
- fats can be stored as triacylglycerols in cells known as adipocytes
- fatty acids are released from adipocytes to enter bloodstream when needed (like when fasting) and then broken down into acetyl-CoA
- storage of fats is important because it yields much more energy than sugars
energy storage in plants
- plants store fats and starches in chloroplasts which are branched polymers of glucose similar to glycogen
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