Metabolism: Topic 8.2 Cell Respiration Flashcards
What is ATP?
Adenosine triphosphate (ATP) is a high energy molecule that functions as an immediate power source for cells One molecule of ATP contains three covalently bonded phosphate groups – which store potential energy in their bonds
What is the purpose of phosphorylation in ATP function?
Phosphorylation (the addition of a phosphate group to a molecule) makes molecules less stable and hence ATP is a readily reactive molecule that contains high energy bonds (Phosphate group was added to ADP to make ATP)
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?
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
What is the benefit of breaking down organic molecules via a number of linked processes?
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)
Outline where does energy released in the oxidation of organic molecules go?
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
What are hydrogen carriers?
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)
Function of hydrogen carriers
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
Why lipids and proteins are not the preferred source of energy for cell respiration?
The main organic compound used in cell respiration is carbohydrates (glucose) – although lipids and proteins can be used
Lipids are not preferentially used as they are harder to transport and digest (although will yield more energy per gram)
Proteins are not preferentially used as they release potentially toxic nitrogenous compounds when broken down
Define glycolysis
The word glycolysis means ‘sugar splitting’. 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)
What is the first stage of glycolysis?
Phosphorylation
A hexose sugar (typically glucose) is phosphorylated by two molecules of ATP (to form fructose-1,6-biphosphate)
This phosphorylation makes the molecule less stable and more reactive, and also prevents diffusion out of the cell
What is the second stage of glycolysis
The less stable 6-carbon phosphorylated fructose is split into two 3-carbon sugars called glyceraldehyde-3-phosphate (G3P). This splitting process is known as lysis.
What is the third stage 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)
What is the fourth stage of glycolysis?
As NADH is being formed, released energy is used to add an inorganic phosphate to the remaining 3-carbon compound. This results in a compound (each of the two glyceraldehyde-3-phosphate compounds) with two phosphate groups. Enzymes then remove the phosphate groups so that they can be added to adenosine diphosphate (ADP) to produce 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)
What two alternate processes can occur after glycolysis? (faith of pyruvate)
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
Describe the anaerobic respiration path of pyruvate (recommended to write it down on paper)
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
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
What is link reaction?
The first stage of aerobic respiration is the link reaction, which transports pyruvate into the mitochondria
The link reaction is named thus because it links the products of glycolysis with the aerobic processes of the mitochondria
What happens in link reaction?
Pyruvate is transported from the cytosol into the mitochondrial matrix by carrier proteins on the mitochondrial membrane
The pyruvate loses a carbon atom (decarboxylation), which forms a carbon dioxide molecule
The 2C compound then forms an acetyl group when it loses hydrogen atoms via oxidation (NAD+ is reduced to NADH + H+)
The acetyl compound then combines with coenzyme A to form acetyl coenzyme A (acetyl CoA)
How many times does the link reaction occur and thus what is the result?
As glycolysis splits glucose into two pyruvate molecules, the link reaction occurs twice per molecule of glucose
Per glucose molecule, the link reaction produces acetyl CoA (×2), NADH + H+ (×2) and CO2 (×2)
What is a coenzyme?
A coenzyme is a molecule that aids an enzyme in its action. Coenzymes usually act as electron donors or acceptors.
What is the Kreb’s Cycle?
The second stage of aerobic respiration is the Krebs cycle, which occurs within the matrix of the mitochondria
The Krebs cycle is also commonly referred to as the citric acid cycle or the tricarboxylic acid (TCA) cycle
What’s the first step of the Krebs Cycle?
In the Krebs cycle, acetyl CoA transfers its acetyl group to a 4C compound (oxaloacetate) to make a 6C compound (citrate)
Coenzyme A is released and can return to the link reaction to form another molecule of acetyl CoA
What’s the second step of the Krebs Cycle?
Citrate (a 6-carbon compound) is decarboxylated/oxidised to form a 5-carbon compound. In this process, the carbon is released from the cell (after combining with oxygen) as carbon dioxide. While the 6-carbon compound is oxidized, NAD+ is reduced to form NADH.
What’s the third step of the Krebs Cycle?
The 5-carbon compound is oxidized and decarboxylated to form a 4-carbon compound. Again, the removed carbon combines with oxygen and is released as carbon dioxide. Another NAD+ is reduced to form NADH
