8.2 Flashcards
One molecule of ATP contains…
three covalently bonded phosphate groups – which store potential energy in their bonds
Phosphorylation makes molecules…
less stable and hence ATP is a readily reactive molecule that contains high energy bonds
When ATP is hydrolysed (to form ADP + Pi), the energy stored in the terminal phosphate bond is…
released for use by the cell
ATP has two key functions within the cell:
- It functions as the energy currency of the cell by releasing energy when hydrolysed to ADP (powers cell metabolism)
- It may transfer the released phosphate group to other organic molecules, rendering them less stable and more reactive
ATP is synthesised from ADP using energy derived from one of two sources:
- Solar energy – photosynthesis converts light energy into chemical energy that is stored as ATP
- Oxidative processes – cell respiration breaks down organic molecules to release chemical energy that is stored as ATP
Cell respiration is the controlled release of energy from organic compounds to produce ATP
- Anaerobic respiration involves the incomplete breakdown of organic molecules for a small yield of ATP (no oxygen required)
- Aerobic respiration involves the complete breakdown of organic molecules for a larger yield of ATP (oxygen is required)
The breakdown of organic molecules occurs via a number of linked processes that involve a number of discrete steps:
- By staggering the breakdown, the energy requirements are reduced (activation energy can be divided across several steps)
- The released energy is not lost – it is transferred to activated carrier molecules via redox reactions (oxidation / reduction)
When organic molecules are broken down by cell respiration, the chemical energy is transferred by means of…
redox reactions
- Redox reactions involved the reduction of one chemical species and the oxidation of another (redox = reduction / oxidation)
Most redox reactions typically involve the transfer of electrons, hydrogen or oxygen
- Reduction is the gain of electrons / hydrogen or the loss of oxygen
- Oxidation is the loss of electrons / hydrogen or the gain of oxygen
Redox Mnemonics
LEO goes GER – Loss of Electrons is Oxidation ; Gain of Electrons is Reduction
Cell respiration breaks down organic molecules and transfers hydrogen atoms and electrons to carrier molecules
- As the organic molecule is losing hydrogen atoms and electrons, this is an oxidation reaction
- Energy stored in the organic molecule is transferred with the protons and electrons to the carrier molecules
The carrier molecules are called hydrogen carriers or electron carriers, as they gain electrons and protons (H+ ions)
- The most common hydrogen carrier is NAD+ which is reduced to form NADH (NAD+ + 2H+ + 2e– → NADH + H+)
- A less common hydrogen carrier is FAD which is reduced to form FADH2 (FAD + 2H+ + 2e– → FADH2)
The hydrogen carriers function like taxis, transporting the electrons (and hydrogen ions) to the cristae of the mitochondria
- The cristae is the site of the electron transport chain, which uses the energy transferred by the carriers to synthesize ATP
- This process requires oxygen to function, and hence only aerobic respiration can generate ATP from hydrogen carriers
- This is why aerobic respiration unlocks more of the energy stored in the organic molecules and produces more ATP
The first step in the controlled breakdown of carbohydrates is glycolysis, which occurs in the cytosol of the cell
In glycolysis, a hexose sugar (6C) is broken down into two molecules of pyruvate (3C)
first step of glycolysis: Phosphorylation
- A hexose sugar (typically glucose) is phosphorylated by two molecules of ATP (to form a hexose bisphosphate)
- This phosphorylation makes the molecule less stable and more reactive, and also prevents diffusion out of the cell
second step of glycolysis: Lysis
- The hexose biphosphate (6C sugar) is split into two triose phosphates (3C sugars)
third step of glycolysis: Oxidation
- Hydrogen atoms are removed from each of the 3C sugars (via oxidation) to reduce NAD+ to NADH (+ H+)
- Two molecules of NADH are produced in total (one from each 3C sugar)
fourth step of glycolysis: ATP formation
- Some of the energy released from the sugar intermediates is used to directly synthesise ATP
- This direct synthesis of ATP is called substrate level phosphorylation
- In total, 4 molecules of ATP are generated during glycolysis by substrate level phosphorylation (2 ATP per 3C sugar)
At the end of glycolysis, the following reactions have occurred:
- Glucose (6C) has been broken down into two molecules of pyruvate (3C)
- Two hydrogen carriers have been reduced via oxidation (2 × NADH + H+)
- A net total of two ATP molecules have been produced (4 molecules were generated, but 2 were used)
glycose is an…
Glycolysis occurs in the cytosol and does not require oxygen (it is an anaerobic process)
Depending on the availability of oxygen, the pyruvate may be subjected to one of two alternative processes:
- Aerobic respiration occurs in the presence of oxygen and results in the further production of ATP (~ 34 molecules)
- Anaerobic respiration (fermentation) occurs in the absence of oxygen and no further ATP is produced
Aerobic Respiration
- If oxygen is present, the pyruvate is transported to the mitochondria for further breakdown (complete oxidation)
- This further oxidation generates large numbers of reduced hydrogen carriers (NADH + H+ and FADH2)
- In the presence of oxygen, the reduced hydrogen carriers can release their stored energy to synthesise more ATP
- Aerobic respiration involves three additional processes – the link reaction, krebs cycle and the electron transport chain
Anaerobic Respiration (Fermentation)
- If oxygen is not present, pyruvate is not broken down further and no more ATP is produced (incomplete oxidation)
- The pyruvate remains in the cytosol and is converted into lactic acid (animals) or ethanol and CO2 (plants and yeast)
- This conversion is reversible and is necessary to ensure that glycolysis can continue to produce small quantities of ATP
Anaerobic Respiration
- Glycolysis involves oxidation reactions that cause hydrogen carriers (NAD+) to be reduced (becomes NADH + H+)
- Typically, the reduced hydrogen carriers are oxidised via aerobic respiration to restore available stocks of NAD+
- In the absence of oxygen, glycolysis will quickly deplete available stocks of NAD+, preventing further glycolysis
- Fermentation of pyruvate involves a reduction reaction that oxidises NADH (releasing NAD+ to restore available stocks)
- Hence, anaerobic respiration allows small amounts of ATP to be produced (via glycolysis) in the absence of oxygen