Module 5 Respiration

Question 1

Need for cellular respiration

Answer 1

• Respiration involves the transfer of chemical potential energy to useable energy in the form of ATP. Glucose + oxygen -> carbon dioxide + water.
• Energy is required for active transport in sodium/potassium ion pumps and exocytosis of pathogenic material from macrophages. Energy is also required for anabolic reactions such as the synthesis of DNA and proteins, as well as the movement of chromosomes along spindle fibres and muscular contraction, as well as metabolic reactions in the liver.

Question 2

Mitochondria

Answer 2

• The mitochondria are the site of aerobic respiration in eukaryotic cells, synthesizing ATP. They are 0.5 -1.0 micrometres and smaller than chloroplasts.
• The mitochondria have a double membrane. The outer membrane is smooth and permeable to many small molecules. The inner membrane is highly folded with cristae and the site of electron transport chains and ATP synthase and less permeable.
• The intermembrane space has a low pH, as it contains many protons and forms a proton gradient.
• The matrix is an aqueous solution containing enzymes, mitochondrial DNA and 70S ribosomes.
• The cristae result in a large surface area for many electron transport chains and ATP synthase enzymes.
• Many mitochondria are present in active cells.

Question 3

Glycolysis

Answer 3

• Glycolysis takes place in the cell cytoplasm, phosphorylating glucose to trap it in the cell splitting it into two molecules of pyruvate. The products are 2 molecules of pyruvate, a net gain of two ATP and two reduced NAD.
• Phosphorylation involves the reaction of glucose with 2 molecules of ATP to form one 6 carbon molecule of hexose bisphosphate.
• This hexose bisphosphate is split into 2 molecules of 3 carbon TP in lysis.
• The 2 molecules of TP are then oxidised by NAD, forming 2 molecules of 3 carbon pyruvate and 2 reduced NAD.
• 4 phosphate groups are removed and react with ADP to form 4 molecules of ATP, at substrate level phosphorylation.

Question 4

Link Reaction

Answer 4

• The link reaction takes involves the decarboxylation and dehydrogenation of pyruvate in the mitochondrial matrix.
• When oxygen is available pyruvate is actively transported by transport proteins and ATP into the matrix of the mitochondria.
• There, pyruvate is oxidised by enzymes to produce 2 carbon acetate, carbon dioxide and reduced NAD. The acetate then combines with coenzyme A to form acetyl CoA.
• Coenzyme A carries out a function, while not being used up in the reaction. It supplies the acetyl group to the Krebs cycle.
• The products of the link reaction are 1 carbon dioxide, 1 acetyl CoA, 1 reduced NAD per pyruvate.

Question 5

Krebs cycle

Answer 5

• The Krebs cycle is a series of enzyme-controlled reactions that take place in the matrix of the mitochondria.
• 2 carbon acetyl CoA reacts with 4 carbon oxaloacetate to form 6 carbon citrate. 6 carbon citrate is then reconverted back to oxaloacetate through a series of redox reactions.
• Citrate is converted to a 5-carbon intermediate through the reduction of 1 molecule of NAD and decarboxylation, producing 1 molecule of carbon dioxide.
• The 5-carbon intermediate is then converted to oxaloacetate through decarboxylation, producing 1 molecule of carbon dioxide. 2 molecules of NAD and 1 molecule of FAD are also reduced. Substrate level phosphorylation produces 1 molecule of ATP.
• The end products are 1 molecule of ATP, 3 molecules of reduced NAD and 1 molecule of reduced FAD.

Question 6

Coenzymes

Answer 6

• Coenzymes carry out a function without being used up in the reaction. Coenzyme A supplies acetyl groups to the Krebs cycle. NAD and FAD are hydrogen carriers and transfer hydrogen ions and electrons to the electron transport chain.

Question 7

Oxidative Phosphorylation

Answer 7

• Oxidative phosphorylation is the last stage of respiration and takes place on the inner mitochondrial membrane, resulting in the production of many ATP and water.
• Hydrogen atoms are donated by reduced NAD and FAD and split into hydrogen ions and electrons. The high energy electrons enter the electron transport chain and move down the carrier proteins, releasing energy.
• This energy is used to actively transport hydrogen ions across the impermeable inner membrane into the intermembrane space, creating a proton gradient.
• Protons move back down their concentration gradient into the matrix of the mitochondria by facilitated diffusion through ATP synthase. This release energy allowing for the synthesis of ATP.
• Oxygen acts as the final electron acceptor, combining with protons and electrons to form water, so the reaction can’t take place in the absence of water.
• The electron transport chain is a series of proteins imbedded in the inner membrane. Energy is needed to actively transport hydrogen ions as the membrane is impermeable to them.

Question 8

Anaerobic respiration

Answer 8

• When little to no oxygen is present, there is no final electron acceptor, and the Krebs cycle stops.
• Anaerobic pathways can oxidise reduced NAD produced in glycolysis, so it can be recycled to produce more ATP, by being used for hydrogen transport.
• Ethanol fermentation takes place in yeast and microorganisms. Pyruvate is converted to ethanal by decarboxylation, producing carbon dioxide. Ethanal is then reduced to ethanol, by alcohol dehydrogenase, oxidising reduced NAD to produce NAD. Ethanol is a waste product. ATP is produced in glycolysis. Ethanal is the hydrogen acceptor.
• Lactate fermentation takes place in mammals. Reduced NAD reduces pyruvate to lactate, forming NAD, by lactate dehydrogenase. Pyruvate is the hydrogen acceptor. Lactate can be further metabolised. It can be converted to glycogen and stored in the liver or oxidised back to pyruvate. Oxygen is required for this, which is why mammals breathe deeply after exercise.
• The energy yield of aerobic respiration is much greater because glucose is only partially oxidised in anaerobic respiration. The net gain of ATP is 2, from glycolysis.
• The rate of respiration in yeast anaerobically can be measured using a redox indicator, which will accept electrons instead of NAD.
• The rate of aerobic respiration is measured by oxygen uptake using a respirometer.

Question 9

Respiratory Substrates

Answer 9

• Glucose is the main respiratory substrate. Amino acids are only respired when all other substrates are used up, as they have essential functions elsewhere.
• Lipids have the highest energy value, then proteins, then carbohydrates.
• Hydrogen atoms are used in chemiosmosis. More hydrogen atoms result in a greater proton gradient.
• Lipids have long hydrocarbon chains, so a high hydrogen content.

Question 10

Respiratory Quotient

Answer 10

• The ratio of carbon molecules produced to oxygen molecules taken in. RQ = CO2 produced/O2 consumed.
• Carbohydrates, lipids and proteins all have different RQ values as they have different numbers of carbon-hydrogen bonds.
• Carbohydrates – 1.0. Proteins – 0.9. Lipids – 0.7.
• RQ can’t be calculated for anaerobic respiration in muscle cells, as no carbon dioxide is produced, and no oxygen is used. It tends towards infinity in yeast cells, as carbon dioxide is produced but no oxygen is used.

Question 11

Respirometers

Answer 11

• Respirometers can be used to measure the effects of temperature, substrate concentration and respiratory substrates on the rate of respiration.
• They measure the rate of oxygen uptake.
• Glass beads are used as control, soda lime absorbs carbon dioxide, a U-tube manometer measures the oxygen uptake. Pi x radius squared should be used in calculations.