Energy Transfers in and between Organisms - Respiration Flashcards
What do cells use as an immediate energy source?
Glucose produced by photosynthesis cannot be used directly by cells as a source of energy. Instead, cells use ATP as their immediate energy source.
What is cellular respiration?
The formation of ATP from the break down of glucose (a respiratory substrate) takes place during the process of cellular respiration. There are two different forms of cellular respiration depending on whether oxygen is involved or not: aerobic and anaerobic.
What is aerobic respiration?
Aerobic respiration requires oxygen and produces carbon dioxide, water and much ATP.
What is anaerobic respiration?
Anaerobic respiration takes place in the absence of oxygen and produces lactate (in animals) or ethanol and carbon dioxide (in plants and fungi) but only a little ATP in both cases.
What four stages can aerobic respiration be divided into?
Respiration is a multi-step process with each step controlled and catalysed by a specific intracellular enzyme.
- glycolysis
- link reaction
- Krebs cycle
- oxidative phosphorylation
What is glycolysis (summary)?
The splitting of the 6-carbon glucose molecule into two 3-carbon pyruvate molecules.
What are link reactions (summary)?
The 3-carbon pyruvate molecules enter into a series of reactions which lead to the formation of acetylcoenzyme A, a 2-carbon molecule.
What is the Krebs cycle (summary)?
The introduction of acetyl coenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large quantity of reduced NAD and FAD. The Krebs cycle goes round twice for each glucose molecule (once for each pyruvate -> acetyl CoA).
What is oxidative phosphorylation?
The use of the electrons, associated with reduced NAD and FAD, released from the Krebs cycle to synthesise ATP with water produced as a by-product.
When and where does glycolysis occur?
Glycolysis is the initial stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm of all living cells and is the process by which a hexose sugar is split into two molecules of the 3-carbon molecule, pyruvate (an acid).
What are the four stages that the smaller enzyme-controlled reactions in glycolysis can be grouped into?
- phosphorylation of glucose to glucose phosphate
- splitting of the phosphorylated glucose
- oxidation of triose phosphate
- the production of ATP
What happens in the phosphorylation of glucose to glucose phosphate?
Before it can be split into two, glucose must first be made more reactive by the addition of two phosphate molecules (phosphorylation). The phosphate molecules come from the hydrolysis of two ATP molecules to ADP. This provides the energy to activate glucose and lowers the activation energy for the enzyme-controlled reactions that follow.
What happens in the splitting of the phosphorylated glucose?
Each glucose molecule is split into two 3-carbon molecules known as triose phosphate.
What happens in the oxidation of triose phosphate?
Hydrogen is removed from each of the two triose phosphate molecules and transferred to a hydrogen-carrier molecule known as NAD to form reduced NAD.
What happens in the production of ATP?
Enzyme-controlled reactions convert each triose phosphate into another 3-carbon molecule called pyruvate. In the process, two molecules of ATP are regenerated from ADP.
Why must the gross yields be doubled?
For each molecule of glucose at the start of the process, two molecules of triose phosphate are produced. Therefore the gross yields must be doubled (four molecules of ATP and two molecules of reduced NAD).
What is the overall yield from one glucose molecule undergoing glycolysis?
- two molecules of ATP (four molecules of ATP are produced, but two were used up in the initial phosphorylation of glucose and so the net increase is two molecules)
- two molecules of reduced NAD (these have the potential to provide energy to produce more ATP)
- two molecules of pyruvate
Why does glycolysis provide indirect evidence for evolution?
Glycolysis is a universal feature of every living organism.
What organelles are needed for glycolysis?
The enzymes for the glycolytic pathway are found in the cytoplasm of cells and so glycolysis does not require any organelle or membrane for it to take place.
Can glycolysis take place without oxygen?
Glycolysis does not require oxygen and therefore it can take place whether or not it is present.
In the absence of oxygen, the pyruvate produced by glycolysis can be converted into either lactate or ethanol during anaerobic respiration with the help of NADH. Lactate or ethanol is then converted back to pyruvate in the liver. This is necessary in order to re-oxidise NAD so that glycolysis can continue.
Why do organisms need oxygen in respiration?
Anaerobic respiration yields only a small fraction of the potential energy stored in the pyruvate molecule. In order to release the remainder of this energy, most organisms use oxygen to break down pyruvate further.
What is the importance of the link reaction?
The pyruvate molecules produced during glycolysis possess potential energy that can only be released in a process called the Krebs cycle. Before they can enter the Krebs cycle, these pyruvate molecules must first be oxidised in a procedure known as the link reaction.
Where do the Krebs cycle and the link reaction take place in eukaryotic cells?
exclusively inside mitochondria
What is a link reaction (detailed)?
In the link reaction, the 2 pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the matrix of mitochondria.
The enzyme decarboxylase then removes a molecule of CO2 with a hydrogen also being lost, going on to reduce NAD. The acetate formed then combines with coenzyme A to form a molecule of acetyl coenzyme A. Per glucose molecule, 2 molecules of acetyl coenzyme A are formed and 0 ATP.
