IB DP Biology HL C1.2 Cell respiration Flashcards

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[h] IB DP Biology HL C1.2 Cell respiration Flashcards

[q] C1.2.3—How is energy released and stored by ATP?

[a] Energy is released when ATP is hydrolyzed to ADP (adenosine diphosphate);

and a phosphate; known as Pi;

Conversely, synthesizing ATP from ADP and phosphate requires energy input;

[q] C1.2.4—How is ATP produced in cells using energy released from carbon compounds? What is the process?

[a] Cell respiration is a biochemical process that produces ATP;

by releasing energy from carbon compounds like glucose and fatty acids;

cell respiration is distinct from gas exchange as it involves a series of reactions that convert biochemical energy from nutrients into ATP;

not just breathing in and out;

[q] C1.2.5—What are the differences between anaerobic and aerobic cell respiration in humans?

[a] Anaerobic respiration occurs without oxygen;

producing less ATP and yielding lactic acid as a waste produce;

primarily in the cytoplasm;

Aerobic respiration requires oxygen;

occurs in mitochondria;

and produces more ATP along with carbon dioxide and water as waste;

Both processes use glucose, but only aerobic respiration requires mitochondria;

[q] C1.2.6—What variables affecting the rate of cell respiration?

[a] The rate of cell respiration can be influenced by various factors such as;

temperature, availability of substrates, and oxygen concentration;

Experimental measurements and calculations from data can determine the respiration rate,

through measuring oxygen use or carbon dioxide production;

[q] HL ONLY – C1.2.7—What is the role of NAD in cell respiration,

particularly in relation to hydrogen and oxidation? 

[a] NAD (Nicotinamide adenine dinucleotide) acts as a hydrogen carrier; in cell respiration;

Oxidation occurs when hydrogen, along with its electron, is removed from a substrate (dehydrogenation);

thereby oxidizing the substrate;

NAD is reduced when it gains hydrogen;

[q] HL ONLY – C1.2.8— What is the process used to convert glucose to pyruvate in and what are the products?

[a] In glycolysis;

glucose is converted to pyruvate through stepwise reactions involving phosphorylation; which is adding phosphate;

lysis; breaking the molecule apart;

oxidation; removing hydrogen and oxygen;

and ATP formation;

Each step is catalyzed by a different enzyme;

the net yield is a small amount of ATP and reduced NAD;

[q] Describe glycolysis  – IB Question

[a] Occurs in the cytoplasm;

Two ATP used to phosphorylate glucose (phosphorylation);

the glucose is broken down (lysis) into two three-carbon pyruvate molecules;

through removing hydrogen (which has electrons) and adding them to NAD+ (NAD);

this is oxidation of glucose;

with a net yield of 2 pyruvate and 2 ATP;

as four ADP are phosphorylated into four ATP*;

and two reduced NAD (NADH) are produced;

[q] HL ONLY – C1.2.9—How is pyruvate converted to lactate in anaerobic respiration, and why is this significant?

[a] In anaerobic respiration;

pyruvate is converted to lactate to regenerate NAD;

allowing glycolysis to continue;

This process yields a net of two ATP molecules per glucose molecule;

and is essential for energy production when oxygen is not available;

[q] HL ONLY – C1.2.10—What is the role of anaerobic respiration in yeast, and how is it utilized in brewing and baking?

[a] Anaerobic respiration in yeast is similar to humans except for pyruvate is not used to regenerate NAD;

pyruvate is further broken down;

producing carbon dioxide and alcohol;

The final products of this process are utilized in brewing;

and carbon dioxide makes bread rise in baking;

[q] HL ONLY – C1.2.11—What is the link reaction?

Where does it happen?

What is produced as a result?

[a] pyruvate enters the mitochondria;

the link reaction occurs in the matrix of the mitochondria;

enzymes remove carbon from pyruvate; this is decarboxylation;

pyruvate is converted to acetyl CoA; by joining to CoA enzyme;

by oxidative decarboxylation; (as pyruvate is oxidised as it loses electrons);

Reduced NAD (NADH) and CO2 formed;

fatty acids also can be converted to acetyl CoA;

acetyl CoA then enters the Krebs cycle;

[q] HL ONLY – C1.2.12—What happens in the Krebs cycle?

Where does it happen?

What is formed as a result?

[a] Krebs cycle occurs in matrix of mitochondria;

acetyl CoA enters the Krebs cycle:

acetyl group (2C) joins a 4C sugar; oxaloacetate;

to form a 6C sugar;  citrate;

the 6C sugar is then turned into a 5C sugar; by oxidative decarboxylation;

four oxidations happen and 2 decarboxylation;

oxidation is via dehydrogenation, removing hydrogen from citrate;

which are added to NAD and FAD;

which are reduced by the addition of hydrogen;  to Reduced NAD and Reduced FAD;

decarboxylation is removing carbon dioxide from the sugars; producing CO2;

the 5C compound is then turned back into the 4C compound oxaloacetate; by oxidative decarboxylation;

[q] What is the net yield from Krebs?

[a] 2 x CO2 are produced per molecule of pyruvate;

3 x reduced NAD and one FADH2 per molecule of pyruvate;

one ATP is produced by substrate-level phosphorylation per molecule of pyruvate;

NADH and FADH2 provide electrons to the electron transport chain;

[q] HL ONLY – C1.2.13—How is energy transferred to the electron transport chain in the mitochondrion?

What is made there?

[a] Energy is transferred when reduced NAD;

passes a pair of electrons;

to the first carrier in the electron transport chain;

the electrons carry energy;

the NAD is converted back to NAD;

Reduced NAD is comes from glycolysis, the link reaction, and the Krebs cycle;

[q] HL ONLY – C1.2.14—What is energy from electrons used to do in the electron transport chain? 

[a] the electron transport chain receives energy from oxidation reactions from Krebs cycle and glycolysis;

in the form of electrons from Reduced NAD and Reduced FAD;

energy is released as electrons pass from carrier to carrier (in the chain);

the release of energy (from electron flow) is coupled to proton pumping;

protons are pumped into inter-membrane space;

this creates a proton gradient;

[q] HL ONLY – C1.2.15—What is chemiosmosis and how is it lnked to ATP production?

[a] chemiosmosis is the coupling of the energy from the proton gradient; 

with the production of ATP-by-ATP synthase;

the protons diffuse down their concentration gradient (across the membrane);

the protons pass through ATP synthase;

the protons return to the matrix;

the flow of protons provides energy for generating ATP;

[q] HL ONLY – C1.2.16—What is oxygen’s role in aerobic respiration?

Why do we need it for respiration?

[a] In aerobic respiration, oxygen acts as the terminal electron acceptor in the electron transport chain;

the electrons are transferred to oxygen at end of electron transport chain;

It accepts electrons and protons from the matrix of the mitochondrion to produce metabolic water;

allowing the continuous flow of electrons along the chain;

which would otherwise stop due to a build up;

[q] Aerobic cell respiration

[a] respiration requiring oxygen, involving the oxidation of glucose to carbon dioxide and water.

[q] Adenosine triphosphate (ATP)

[a] a nucleotide that releases energy when its phosphate bonds are hydrolyzed.

[q] Carbon dioxide (CO2)

[a] molecule resulting from oxidation of organic carbon compounds, and formed in the tissues and eliminated by the lungs.

[q] Cell respiration

[a] the controlled release of energy from organic compounds to produce ATP.

[q] Ethanol

[a] alcohol formed by microbial fermentation of carbohydrates.

[q] Fermentation

[a] anaerobic breakdown of glucose with the end-products of ethanol and carbon dioxide or lactic acid

[q] Glucose

[a] monosaccharide that is an end product of carbohydrate metabolism, and is the chief source of energy for living organisms.

[q] Metabolic pathways

[a] a series of enzymatic reactions that converts one biological material to another.

[q] Yeast

[a] a unicellular fungus that lives in liquid or moist habitats.

[q] Glycolysis

[a] Glucose is split into pyruvate during a biochemical pathway

[q] Krebs Cycle

[a] process of aerobic respiration that fully harvests the energy of glucose; also known as the citric acid cycle

[q] Electron Transport Chain

[a] A sequence of electron carrier molecules (membrane proteins) that shuttle electrons during the redox reactions that release energy used to make ATP.

[q] ADP

[a] (Adenosine Diphosphate) The compound that remains when a phosphate group is removed from ATP, releasing energy

[q] ATP ADP cycle

[a] ATP stores energy, phosphate breaks, releases energy and ATP turns into ADP.

(To form ATP the reverse of this is performed)

[q] Mitochondria

[a] An organelle in eukaryotic cells that serves as the site of cellular respiration;

uses oxygen to break down organic molecules and synthesize ATP

[q] Oxidation

[a] The loss of electrons from a substance involved in a redox reaction.

[q] Reduction

[a] The addition of electrons to a substance involved in a redox reaction.

[q] Cristae

[a] Series of inner membranes in mitochondria where cell respiration occurs

[q] Chemiosmosis

[a] a process for making ATP using the energy stored in an electrochemical gradient of hydrogen ions.

[q] Aerobic Respiration

[a] Catabolic pathway in which oxygen is consumed as a reactant along with the organic fuel

[q] Anaerobic Respiration

[a] The process by which cells convert glucose into energy in the absence of oxygen.

[q] Phosphorylation

[a] The transfer of a phosphate group, usually from ATP to a molecule.

[q] Pyruvate

[a] Three-carbon compound that forms as an end product of glycolysis.

[q] Acetyl CoA

[a] The entry compound for the citric acid cycle in cellular respiration

Two carbon molecules

[q] Outline the purpose of cellular respiration

[a] The purpose of cellular respiration is to help organisms produce energy in the form of ATP of energy using molecules that are taken in.

It is the controlled release of energy from organic compounds in cells to form ATP.

[q] Compare and contrast aerobic and anaerobic respiration– Aerobic properties

[a] 1. Process takes place in the mitochondria

2. Takes place in the presence of oxygen

3. Uses the Krebs cycle. Electron transport chain (in mitochondrial membrane)

4. Produces a large amount of ATP in animals. Carbon dioxide and water are products

[q] Compare and contrast aerobic and anaerobic respiration– Anaerobic properties

[a] 1. Process takes place in the cytoplasm

2. Without the presence of oxygen

3. Products in animals: lactate and bacteria

Only a small amount of ATP is produced

4. Products in plants: ethanol and carbon dioxide in yeast and plants.

5. Yields only a small amount of ATP

[q] Outline aerobic respiration

[a] A. Aerobic respiration is the process that produces energy in the presence of oxygen.

1. This process takes place in the mitochondria where glucose is converted into pyruvate through glycolysis.

2. The steps of aerobic respiration are glycolysis, the link reaction, Krebs cycle, the electron transport chain, and chemiosmosis.

3. Glycolysis: a six carbon sugar is broken down into two pyruvate molecules.

Two ATP molecules are used per glucose but four are produced so the net yield is two ATP

4. The two pyruvate molecules move into the matrix of the mitochondrion for the link reaction.

Pyruvate is converted to acetyl CoA (acetyl coenzyme A).

The conversion is known as oxidative decarboxylation because hydrogen is removed (oxidation) and carbon dioxide is removed (decarboxylation)

5. The acetyl-CoA moves to the Krebs cycle

6. In the Krebs cycle, NADH molecules are created and moved to the Electron transport chain to be exchanged for ATP molecules

7. In chemiosmosis

[q] Aerobic respiration equation

[a] Glucose + oxygen yields carbon dioxide, water, and energy

[q] Anaerobic respiration equation (animals)

[a]

[q] Anaerobic respiration equation (plants)

[a]

[q] Outline anaerobic respiration

[a] A. Anaerobic respiration is the process that produces energy in the absence of oxygen.

1. It takes place in the cytoplasm during glycolysis when glucose is broken down into pyruvate and energy (2 ATP).

2. Then fermentation turns pyruvate into ethanol or lactate (lactic acid).

3. In animals, anaerobic respiration, pyruvate is converted into lactic acid and occurs during intense muscular activity.

4. In plants, pyruvate is converted into ethanol and carbon dioxide

5. Few ATPs are created

[q] oxidation and reduction are…

[a] Oxidation and reduction are chemical reactions that involve the exchange of electrons.

[q] oxidation

[a] 1. Addition of oxygen atoms to a substance

2. Removal of hydrogen atoms from a substance

3. Loss of electrons from a substance

[q] reduction

[a] 1. Removal of oxygen atoms from a substance

2. addition of hydrogen atoms to a substance

3. Addition of electrons to a substance

[q] Four main steps of Glycolysis

[a] phosphorylation, Lysis, Oxidation, ATP formation.

[q] Outline Glycolysis

[a] A. Glycolysis has four main steps: phosphorylation, Lysis, Oxidation, ATP formation.

1. Glucose is the substrate of the metabolic pathway of glycolysis. 2 ATP molecules are used to begin glycolysis

2. Glycolysis has four main steps: phosphorylation, Lysis, Oxidation, ATP formation.

3. In Phosphorylation: A six carbon hexose molecule (glucose), through phosphorylation, uses two phosphates to become a molecule of hexose biphosphate.

The two phosphate molecules come from 2 ATP molecules used at the beginning of the process.

4. In Lysis: the phosphorylated glucose splits into two triose phosphate molecules

5. The two phosphorylated triose phosphate molecules are oxidized.

The oxidation is of the CARBONS (removal of hydrogen)– from products of lysis to 2 pyruvate is oxidation

6. NAD+ —> NADH + H^+ is a reduction reaction that happens during the overarching oxidation reaction

7. glycolysis results in 2 pyruvate molecules that move to link reaction

[q] Link reaction

[a] A. Pyruvate from glycolysis enters the mitochondrial matrix

1. Enzymes remove CO2 and hydrogen from the pyruvate

2. hydrogen is accepted by NAD. The removal of hydrogen is oxidation.

The removal of CO2 is decarboxylation

3. The whole conversion is therefore, oxidative decarboxylation.

The end product od decarboxylation of pyruvate is an acetyl group, which attaches to coenzyme A to form acetyl coenzyme A.

[q] Krebs Cycle

[a] In the first reaction of the cycle, an acetyl group is transferred from acetyl-CoA to a four-carbon compound (oxaloacetate) to form a six-carbon compound (citrate).

1. The six carbon compound undergoes decarboxylation where CO2 is removed.

It also undergoes oxidation (NAD is reduced) where a hydrogen is removed to from a five carbon compound.

2. The five carbon compound is then decarboxylated and oxidized (NAD reduced) to a four-carbon compound

3. These oxidation reactions release energy, much of which is stored by the carriers when they accept hydrogen.

This energy is later released by the electron transport chain and used to make ATP.

4. ^With each reduction of NAD to NADH, they yield 3 ATP molecules per reduction.

FAD is reduced to FADH2 and yields 2 ATP molecules.

These carriers are exchanged for ATP in the electron transport chain.

5. The four carbon then undergoes substrate-level phosphorylation this reaction produces ATP directly.

[q] Electron Transport Chain (ETC)

[a] A series of electron carriers (NAD) located on the inner membrane of the mitochondrion including the cristae.

Reduced NAD comes and drops off its protons (H+) and electrons (e-)

Oxygens attract the electrons.

The oxygen i the final electron acceptor. (it then joins with other protons and becomes water)

The final goal of ETC is to increase concentration of protons in the intermembrane space.

[q] Role of oxygen in ETC

[a] It is the final electron acceptor in the electron transport chain.

Without oxygen, the electrons would not move (they would be blocked) and ATP production would not occur.

[q] Explain the role of H+ ions in the electron transport chain

[a] When energy is released as electrons pass from carrier to carrier, helps protons to move across proton carriers.

The energy from electrons moving helps protons get pumped across into the intermembrane space.

They create a concentration gradient by making a high H+ concentration on the side that the protons move to.

The final goal is to increase the concentration of protons in the intermembrane space.

[q] What is Chemiosmosis

[a] The generation of ATP using the energy released by the movement of hydrogen ions across a membrane is called chemiosmosis (The coupling of ATP synthesis to electron transport)

[q] Process of Chemiosmosis

[a] After the ETC process, the high concentration of H protons in the intermembrane space will diffuse through an enzyme called ATP synthase to the matrix.

This enzyme ATP synthase makes ATP with ADP and inorganic phosphate.

It does this by using the energy from flowing protons (down the concnetration gradient)

ADP is phosphorylated to ATP using energy from oxidation (NADH + H+ is oxidized to NAD)
^ this stage is called oxidative phosphorylation

[q] Substrate Level phosphorylation

[a] In substrate-level phosphorylation, the PO43- from a phosphorylated substrate is transferred to ADP to form ATP.

This is what happens during glycolysis in CONTRAST to oxidative phosphorylation involving the electron transport chain

[q] Oxidation Phosphorylation

[a] Oxidative Phosphorylation is the metabolic pathway where ADP is phosphorylated to ATP using energy from oxidation (NADH + H+ is oxidized to NAD)

ETC is the site of oxidative phosphorylation and chemiosmosis is a process that happens during OXPHOS.

When hydrogen protons are in high concentration outside the matrix (in the space between the inner and outer mitochondrial membranes), they return through diffusion into the matrix through ATP synthase.

As they go through ATP synthase, they release energy.

This energy is used by the ATP synthase to convert ADP into ATP

[q] Mitochondria structure

[a] A.  The (mitochondrial) Matrix
1. Contains enzymes for the Krebs cycle and the link reaction

2. Contains 70S ribosomes

3. Contains Mitochondrial DNA which has their own genetic material and the facility to produce their own ribonucleic acids and proteins

B. The Cristae
1. Tubular projections of inner membrane that increase the surface area (SA) available for oxidative phosphorylation

2. Folds of the inner mitochondrial membrane

C. Intermembrane Space

1. Protons are pumped into this space by the ETC

2. Protons accumulate as a result of electron transport chain (ETC).

In order to make ATP the protons diffuse through ATP synthase

3. Small volume space into which protons are pumped into\due to its small volume, a high concentration gradient can be reached very quickly which is vital to chemiosmosis

D. Inner mitochondrial membrane
1. Contains ETC and STP synthase, which carry out oxidative phosphorylation

E. Outer mitochondrial membrane
1. Separates contents of mitochondria from rest of cell

2. Creates environment for cellular respiration

[q] C1.2.1—ATP as the molecule that distributes energy within cells 

Include the full name of ATP (adenosine triphosphate) and that it is a nucleotide.

Students should appreciate the properties of ATP that make it suitable for use as the energy currency within cells.

[a] 

ATP is used by cells to provide energy for metabolic reactions.

This energy is stored within the molecule’s chemical bonds, specifically the high-energy bonds connecting the phosphate groups together.

When one of these phosphate bonds are broken, sufficient energy is released which can then be used as input for a metabolic reaction.

[q] C1.2.2—Life processes within cells that ATP supplies with energy 

Include active transport across membranes, synthesis of macromolecules (anabolism), movement of the whole cell or cell components such as chromosomes. 

[a] ATP is used by protein carriers to carry out active transport (i.e. sodium-potassium pump), synthesize molecules (i.e. photosynthesis), and move the organism (i.e. muscle contraction), among others.

[q] C1.2.3—Energy transfers during interconversions between ATP and ADP

Students should know that energy is released by hydrolysis of ATP (adenosine triphosphate) to ADP (adenosine diphosphate) and phosphate, but energy is required to synthesize ATP from ADP and phosphate.

Students are not required to know the quantity of energy in kilojoules, but students should appreciate that it is sufficient for many tasks in the cell.

[a] ATP is quite instable due to its high energy, so if it is not quickly used it will break down into ADP + Pi (aka lose a phosphate group) and the energy is lost as heat.

This is also called ATP hydrolysis, since it involves a water (‘hydro’) molecule.

When ATP is needed for a metabolic reaction, hydrolysis occurs, and the released phosphate group does not simply float away but is transferred onto another molecule in a process called phosphorylation (attaching a phosphate to a molecule).

[q] C1.2.4—Cell respiration as a system for producing ATP within the cell using energy released from carbon compounds

Students should appreciate that glucose and fatty acids are the principal substrates for cell respiration but that a wide range of carbon/organic compounds can be used. Students should be able to distinguish between the processes of cell respiration and gas exchange.

[a] Cell respiration is a system of metabolic pathways that produces ATP within the cell using energy released from carbon compounds like glucose, fatty acids, and proteins.

Gas exchange is a physical process by which gases move in and out of an organism or cell (i.e. oxygen moves into the blood during inhalation and carbon dioxide moves out).

[q] C1.2.5—Differences between anaerobic and aerobic cell respiration in humans 

Include which respiratory substrates can be used, whether oxygen is required, relative yields of ATP, types of waste product and where the reactions occur in a cell.

Students should be able to write simple word equations for both types of respiration, with glucose as the substrate.

Students should appreciate that mitochondria are required for aerobic, but not anaerobic, respiration.

[a] Cell respiration can be carried out aerobically (with oxygen), or anaerobically (without oxygen).
Aerobic respiration equation: glucose + oxygen → carbon dioxide + water + ATP
Anaerobic respiration equation: glucose → lactate + ATP

[q] C1.2.6—Variables affecting the rate of cell respiration

Application of skills: Students should make measurements allowing for the determination of the rate of cell respiration.

Students should also be able to calculate the rate of cellular respiration from raw data that they have generated experimentally or from secondary data. 

[a] When measuring variables affecting the rate of cell respiration, consider the following:
• Ethical implications of using different types of living organisms (i.e. animals, plants, etc.)
• A method to measure the dependent variable (i.e. gas probes, gas syringe, etc.)
• Controlled variables (temperature, pH, etc.)

[q] C1.2.7—Role of NAD as a carrier of hydrogen and oxidation by removal of hydrogen during cell respiration

Students should understand that oxidation is a process of electron loss, so when hydrogen with an electron is removed from a substrate (dehydrogenation) the substrate has been oxidized.

They should appreciate that redox reactions involve both oxidation and reduction, and that NAD is reduced when it accepts hydrogen.

[a]

Redox reactions are those involving reduction and oxidation.
NAD (Nicotinamide adenine dinucleotide) is a molecule used as a hydrogen and electron carrier during cell respiration.

It does this by gaining a hydrogen ion (proton) or electron during one step of respiration and then donating it in another step, thus carrying it from one molecule to another.
NAD in its oxidized form exists with a positive charge, NAD+, but when it is reduced it exists as NADH.

[q] C1.2.8—Conversion of glucose to pyruvate by stepwise reactions in glycolysis with a net yield of ATP and reduced NAD

Include phosphorylation, lysis, oxidation and ATP formation.

Students are not required to know the names of the intermediates, but students should know that each step in the pathway is catalyzed by a different enzyme.

[a] Glycolysis (‘lysis’ = breakdown, ‘glyco’ = glucose, hence glucose breakdown) is the process of breaking down a glucose monomer into two pyruvate molecules in the cytoplasm via the following steps:
1. A glucose monomer is phosphorylated twice (using 2 ATP molecules) to prevent the molecule from leaving the cell, causing instability, which leads to lysis (splitting) into two 3C compounds
2. Each of the two 3C compounds is phosphorylated again without energy from ATP molecules, resulting in 2 phosphate groups attached to each 3C compound
3. Both 3C compounds now lose their 2 phosphate groups (4 in total), which join with 4 ADP to form 4 ATP molecules in total (net ATP yield is 2).

During this process, 2 NAD+ molecules are reduced to NADH
4. 2 pyruvate molecules are formed at the end Each of these steps is catalyzed by a different enzyme.

Instead of forming pyruvate from glucose right away, this stepwise mechanism reduces the overall activation energy

[q] C1.2.9—Conversion of pyruvate to lactate as a means of regenerating NAD in anaerobic cell respiration

Regeneration of NAD allows glycolysis to continue, with a net yield of two ATP molecules per molecule of glucose.

[a] After glycolysis in humans and in the absence of oxygen, pyruvate is reduced to lactate and NADH is oxidized back into NAD+ in order to regenerate NAD+ supplies in anerobic respiration to allow glycolysis to continue.

[q] C1.2.10—Anaerobic cell respiration in yeast and its use in brewing and baking 

Students should understand that the pathways of anaerobic respiration are the same in humans and yeasts apart from the regeneration of NAD using pyruvate and therefore the final products.

[a] Yeast undergoes the same glycolysis pathway as humans, however instead of converting pyruvate into lactate, fungi carry out ethanol / alcohol fermentation to produce CO2 and ethanol while also oxidizing NADH to NAD+ per pyruvate molecule.
Brewing involves allowing yeast to anaerobically respire in grape juice until a specific ethanol concentration or until its levels become too toxic for the yeast.
Baking involves allowing yeast to anaerobically respire in dough to make it more fluffy and less sticky due to the carbon dioxide bubbles that form.

[q] C1.2.11—Oxidation and decarboxylation of pyruvate as a link reaction in aerobic cell respiration

Students should understand that lipids and carbohydrates are metabolized to form acetyl groups (2C), which are transferred by coenzyme A to the Krebs cycle.

[a] The link reaction occurs in the mitochondrial matrix with the presence of oxygen and involves 3 main steps:
1. Each pyruvate molecule is decarboxylated into a 2C compound and the removed carboxyl group is turned into CO2
2. The 2C compound is oxidized into acetyl while simultaneously reducing a NAD+ molecule
3. Acetyl is joined to the CoA enzyme which carries it to the next stage of respiration
The result of the link reaction per glucose monomer is 2 CO2, 2 NADH, and 2 acetyl CoA molecules.

[q] C1.2.12—Oxidation and decarboxylation of acetyl groups in the Krebs cycle with a yield of ATP
and reduced NAD 

Students are required to name only the intermediates citrate (6C) and oxaloacetate (4C).
Students should appreciate that citrate is produced by transfer of an acetyl group to oxaloacetate and that oxaloacetate is regenerated by the reactions of the Krebs cycle, including four oxidationsand two decarboxylations.

They should also appreciate that the oxidations are dehydrogenation reactions. 

[a] The Krebs cycle occurs in the mitochondrial matrix and includes the following steps:
1. The acetyl CoA joins its acetyl to oxaloacetate (a 4C compound), forming a 6C compound (citrate)
2. Citrate is decarboxylated twice to a 4C compound, simultaneously reducing 2 NAD+ molecules
3. The 4C compound is then oxidized into oxaloacetate by reducing 1 NAD+ to NADH and 1 FAD to FADH2 (another electron carrier), as well as forming 1 ATP molecule from ADP
At the end of the Krebs cycle, all the carbon atoms from the single glucose monomer have been converted into CO2, and what remains of this monomer is its electrons, which are carried by NADH and FADH2.

[q] C1.2.13—Transfer of energy by reduced NAD to the electron transport chain in the mitochondrion 

Energy is transferred when a pair of electrons is passed to the first carrier in the chain, converting reduced NAD back to NAD. Students should understand that reduced NAD comes from glycolysis, the link reaction and the Krebs cycle.

[a] Reduced NAD+ (NADH) from glycolysis, the link reaction, and the Krebs cycle – in addition to reduced FAD (FADH2) – move to the inner mitochondrial membrane and donate their electrons to a group of proteins (cytochromes).

NADH and FADH2 are both oxidized back into NAD+ and FAD in order to be reduced again in glycolysis and the Krebs cycle.

In turn, the cytochromes become reduced and pass these electrons from one carrier to another, harnessing the energy lost by each transfer.

The Electron Transport Chain (ETC) is a group of proteins bound to the inner mitochondrial membrane, which electrons pass through in a series of redox reactions and release energy

[q] C1.2.14—Generation of a proton gradient by flow of electrons along the electron transport chain 

Students are not required to know the names of protein complexes. 

[a] The energy released by the ETC is used to pump protons (H+ ions) by cytochromes from the matrix into the intermembrane space of the mitochondria, creating a steep proton gradient between the two spaces.

[q] C1.2.15—Chemiosmosis and the synthesis of ATP in the mitochondrion

Students should understand how ATP synthase couples release of energy from the proton gradient with phosphorylation of ADP.

[a] Due to the steep proton gradient generated from the ETC, protons are prompted to move down this gradient from the intermembrane space to the matrix (chemiosmosis), which due to their polarity can only be achieved via a protein channel, ATP synthase (a transmembrane channel and enzyme).
ATP synthase is composed of two main parts:
• F0 – the part embedded in the inner mitochondrial membrane
• F1 – the part projecting into the mitochondrial matrix
Around 3-4 of the accumulated protons in the intermembranous space enter the one of two half channels in the F0 portion and exists through the other one.

Since these channels are not aligned, the F0 complex has to rotate in order for protons to pass through.

The energy released by protons moving down their gradient is harnessed by the F0 portion, causing the F0 and stalk (which both make up the rotor of ATP synthase) to rotate in a clockwise direction until the proton exists.
The rotations that occur in the F0 and stalk portions induce conformational changes (rotations) in the F1 complex, which contains an active site for catalyzing the phosphorylation of ADP to ATP.

[q] C1.2.16—Role of oxygen as terminal electron acceptor in aerobic cell respiration 

Oxygen accepts electrons from the electron transport chain and protons from the matrix of the mitochondrion, producing metabolic water and allowing continued flow of electrons along the chain. 

[a] After the electrons pass through several electron carriers in the ETC, an oxygen molecule accepts these electrons (thus becoming reduced).

This attracts nearby protons (hydrogen ions) from the surrounding medium, and water is formed.

Not only does this create a steeper proton gradient by further decreasing the proton concentration in the matrix, but it also ensures a continuous flow of electrons along the chain.
Without oxygen, the ETC, chemiosmosis, and ATP synthesis would be incapable of occurring as there is no terminal electron acceptor.

The phosphorylation of ADP is dependent on the presence of oxygen, thus this process is called oxidative phosphorylation.

[q] C.1.2.17—Differences between lipids and carbohydrates as respiratory substrates 

Include the higher yield of energy per gram of lipids, due to less oxygen and more oxidizable hydrogen and carbon.

Also include glycolysis and anaerobic respiration occurring only if carbohydrate is the substrate, with 2C acetyl groups from the breakdown of fatty acids entering the pathway via acetyl-CoA (acetyl coenzyme A).

[a] Both carbohydrates and lipids can be used as respiratory substrates. Lipids have a higher ATP/energy yield per gram as they have less oxygens and more oxidizable hydrogen and carbon.

This makes them able to reduce more NAD+ molecules than carbohydrates, providing more electrons to the ETC thus producing more ATP.
Glucose and lipid metabolism are interconnected. Triglycerides are broken down into glycerol and fatty acids upon digestion.

Glycerol can be phosphorylated into glycerol-3-phosphate, which is an intermediate in glycolysis and thus continues down that pathway.

Fatty acids are catabolized in the mitochondrial matrix and converted to acetyl groups, which join to form acetyl CoA and proceed into the Krebs cycle.

Therefore, glycolysis and anaerobic respiration can only be initiated by glucose, with other organic nutrients entering (but not initiating) the pathway

[x] Exit text

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