1.2 Cell Respiration

Adenosine triphosphate (ATP)

A small and soluble molecule that provides a short-term store of chemical energy that cells can use to do work

Structure of ATP

ATP is a phosphorylated nucleotide. It is made up of ribose sugar, an adenine base, and 3 phosphate groups

Features of ATP

Releases a small but sufficient quantity of energy, contains chemical energy in its structure, can be recycled, solubility and size allows it to move easily in cells and living organisms by facilitated diffusion

Interconversions between ATP and ADP

ATP is a very reactive molecule and is readily converted to ADP and phosphate when releasing its energy. ADP and phosphate can then be re-converted to ATP during respiration

Organic compounds

Molecules which contain carbon-carbon or carbon-hydrogen bonds, such as glucose or fatty acids

What are the principal substrates for cell respiration?

Glucose and fatty acids

Breakdown of ATP

ATP is broken down using a hydrolysis reaction where water splits ATP into ADP and Pi. The chemical energy stored within bonds is released

Synthesis of ATP

ATP is formed from ADP and Pi through a condensation reaction, where water is released as a waste product. This process absorbs energy

Cell respiration

The controlled release of energy from breaking down organic compounds to produce ATP

Principal substrates for cell respiration

Glucose and fatty acids, other organic compounds can also be used but they must undergo several changes before they can enter the respiratory pathway

Why is glucose a principal substrate?

Glucose can enter glycolysis directly which makes it easier to oxidise compared to lipids and proteins

Will proteins be used in cell respiration?

Since proteins are primarily structural molecules, they will only be used as a respiratory fuel in conditions where both glucose and lipids are not available

Where does the energy for ATP synthesis come from?

The breakdown of organic molecules

Uses of ATP

Fuelling anabolic processes, muscle contraction, active transport, moving molecules around the cell, generating heat to maintain body temperature in warm blooded animals

Cell respiration vs gas exchange

Cell respiration: The release of ATP from organic molecules

Gas exchange: The exchange of oxygen and carbon dioxide in the alveoli and the bloodstream

2 forms of cell respiration

Aerobic and anaerobic

Aerobic respiration

The process where glucose in the presence of oxygen is completely oxidised and broken down in cells to produce carbon dioxide (waste product), water (by-product), and large amounts of ATP. Takes place in the mitochondria

Anaerobic respiration

The process where glucose in the absence of oxygen is incompletely oxidised and broken down in cells to produce ethanol, carbon dioxide, and a small amount of ATP. Takes place in the cytoplasm. For animals, lactate is produced instead of ethanol and carbon dioxide

Situations where anaerobic respiration is used

• When oxygen supplies run out in respiring cells

• When a short supply of ATP is needed

• In environments which are oxygen deficient (eg. waterlogged soil)

What variables affect the rate of cell respiration?

How metabolically active the cell is, the size of the organism, oxygen availability, glucose availability, temperature, pH

Respirometer

A device used to measure the rate of respiration of an organism by measuring its oxygen uptake

Common features of respirometers

Sealed container containing live organisms and air, alkaline solution to absorb carbon dioxide, capillary tube connected to the container and set against a graduated scale (a manometer)

How does a respirometer work?

The organisms respire aerobically and absorb oxygen from the air. The carbon dioxide they release is absorbed by the alkali. This reduces the air pressure inside the sealed chamber, causing the manometer fluid to move towards the organisms due to the pressure drop inside the container

Oxidation

Loss of electrons, loss of hydrogen, gain of oxygen, releases energy to surroundings (exergonic)

Reduction

Gain of electrons, gain of hydrogen, loss of oxygen, absorbs energy from surroundings (endergonic)

Reducing agents

Molecules that have a strong tendency to lose/donate their electrons

Oxidising agents

Molecules that that have a strong tendency to gain electrons

NAD⁺

The primary electron carrier involved in respiration, it serves as an oxidising agent and is reduced to NADH when it gains one or more hydrogen ions from hydrogen. NAD⁺ transports the electrons it gains to other reactions in respiration

Glycolysis

The first stage of respiration, takes place in the cytoplasm, results in the production of 2 pyruvate (3C) molecules, net gain 2 ATP, and 2 reduced NAD (NADH)

Steps of glycolysis

1. Phosphorylation: 1 ATP molecule is used to phosphorylate glucose, converting it into glucose-6-phosphate. It undergoes further phosphorylation using another ATP to produce fructose-1,6-bisphosphate.

2. Lysis: Fructose-1,6-bisphosphate (6C) splits into two molecules of triose phosphate (3C).

3. Oxidation: Triose phosphate (3C) is oxidised to glycerate-3-phosphate, and NAD⁺ is reduced to NADH and H⁺. Glycerate-3-phosphate is converted into pyruvate. 2 molecules of ATP are produced at the same time.

Equation for NAD⁺ reduction

NAD⁺ + 2e^- + 2H⁺ → NADH + H⁺

Converting pyruvate to lactate

Reduced NAD (NADH) transfers its hydrogens to pyruvate to form lactate

Yeast and microorganisms

Convert pyruvate to ethanol

Mammalian muscle cells

Convert pyruvate to lactate

Metabolisation of lactate

After lactate is produced it is either oxidised back to pyruvate which is channelled into the Krebs cycle or it can be converted into glycogen for storage in the liver

Alcoholic fermentation

Pyruvate is decarboxylated to ethanal, producing carbon dioxide. Reduced NAD (NADH) then transfers its hydrogens to ethanal to form ethanol

The link reaction

Takes place in the mitochondrial matrix. It links glycolysis and the Krebs cycle

Steps of the link reaction

1. Each pyruvate molecule is decarboxylated (carbon dioxide is removed) to produce a 2C compound

2. The 2C compound is oxidised into acetyl while simultaneously reducing 1 NAD⁺ molecule

3. Acetyl is joined to the CoA enzyme to form acetyl coenzyme A

4. Acetyl coenzyme A is supplied to the Krebs cycle where aerobic respiration continues

Krebs cycle

Takes place in the mitochondrial matrix

Steps of the Krebs cycle

1. Acetyl coenzyme A enters the circular pathway from the link reaction

2. Oxaloacetate (4C) accepts the 2C acetyl fragment from acetyl coenzyme A to form a 6C compound called citrate. Coenzyme A is released in this reaction to be reused in the link reaction

3. Citrate (6C) is decarboxylated twice to a 4C compound, simultaneously reducing 2 NAD⁺ molecules

4. The 4C compound is then oxidised into oxaloacetate by reducing 1 NAD+ to NADH and 1 FAD to FADH₂, as well as forming 1 ATP molecule from ADP

Electron transport chain

Made up of a series of redox reactions that occur via electron carriers embedded into the inner mitochondrial membrane. The electrons received from NADH and FAD are transported along the electron carriers, releasing energy

Generation of proton gradient

The energy released from the transfer of electrons down the ETC is used to pump protons (H⁺ ions) from the matrix into the intermembrane space, creating a proton gradient

Oxygen

Acts as the final electron acceptor, ensuring a continuous flow of electrons along the chain. It is reduced by electrons, and when combined with protons from the mitochondrial matrix, water is formed. Without oxygen, the ETC, chemiosmosis, and ATP synthesis would be incapable of occurring as there would be no terminal electron acceptor

ATP synthesis

Protons that have built up in the intermembrane space can only pass through the phospholipid bilayer by facilitated diffusion through ATP synthase. As protons flow through ATP synthase, it catalyses phosphorylation of ADP, generating ATP

Lipids vs carbohydrates

They have a higher yield of energy per gram. Glycolysis and anaerobic respiration can only occur if a carbohydrate is the substrate