Cell Respiration
Aprenda jogando
Responda a estas perguntas para ganhar energia, depois pesque e explore. Sem precisar de conta.
Notas de aula
ATP: The Universal Energy Currency
- ATP (adenosine triphosphate) is a small, soluble molecule that provides a short-term store of chemical energy for cells.
- It is described as a universal energy currency: universal because all organisms use it, and a currency because it can be used for many different reactions and reused countless times.
- ATP is a phosphorylated nucleotide made of a ribose sugar, an adenine base and three phosphate groups.
- Hydrolysis of ATP produces ADP and a phosphate ion (π), releasing energy that can be used for processes such as DNA synthesis.
- ATP is re-synthesised when ADP combines with an inorganic phosphate; this is an energy-requiring condensation reaction that releases water as a waste product.
- ATP is relatively stable at cellular pH and only breaks down when the enzyme ATPase is present, so energy is not wasted.
- ATP is not stored in large amounts; molecules such as glucose and fatty acids act as short-term energy stores, while glycogen, starch and triglycerides are long-term stores.
- The hydrolysis of ATP is quick and easy, allowing cells to respond to a sudden increase in energy demand.
Uses of energy from respiration

Cell Respiration: An Overview
- Cell respiration is the controlled release of energy from organic compounds to produce ATP.
- It is a catabolic process that happens in every cell and involves a series of enzyme-controlled reactions called a pathway.
- Glucose is the main respiratory fuel; it can enter glycolysis directly, making it easier to oxidise than lipids and proteins.
- Lipids and proteins can also be used as respiratory substrates but must undergo several changes before entering the respiratory pathway.
- Energy is released in small, controlled steps rather than one uncontrolled step, which would cause cell damage and tissue death.
- The energy released is used for anabolic processes, muscle contraction, active transport, moving molecules around the cell, and generating heat to maintain body temperature in warm-blooded animals.
- Aerobic respiration requires oxygen, completely oxidises the substrate and yields about 36 ATP per glucose molecule.
- Anaerobic respiration occurs without oxygen, partially oxidises glucose and yields only about 2 ATP per glucose molecule.
Balanced equation for aerobic respiration

Anaerobic Respiration
- Anaerobic respiration takes place in the cytoplasm and does not involve the mitochondria.
- It occurs when oxygen supply cannot keep up with demand (e.g. vigorous exercise) or when oxygen cannot reach the organism (e.g. waterlogged soil).
- Glucose is only partially oxidised, so only the first stage of respiration continues, producing a net yield of about 2 ATP per glucose molecule.
- Plants and yeasts produce ethanol and carbon dioxide during anaerobic respiration.
- Animals produce lactate during anaerobic respiration.
- In animals, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase, with reduced NAD transferring its hydrogens to pyruvate.
- Lactate can be oxidised back to pyruvate (requiring extra oxygen, an oxygen debt) or converted into glycogen for storage in the liver.
- In yeast, pyruvate is decarboxylated to ethanal, producing CO2, and ethanal is then reduced to ethanol by alcohol dehydrogenase.
Anaerobic respiration in muscle

Oxidation and Reduction in Respiration
- Oxidation is the loss of electrons, loss of hydrogen, gain of oxygen, and is exergonic (releases energy).
- Reduction is the gain of electrons, gain of hydrogen, loss of oxygen, and is endergonic (absorbs energy).
- Use OILRIG to remember: Oxidation Is Loss, Reduction Is Gain.
- NAD (nicotinamide adenine dinucleotide) is the primary electron carrier in respiration; FAD (flavin adenine dinucleotide) is another.
- Both NAD and FAD are coenzymes that act as oxidising agents, gaining electrons and hydrogen ions to become reduced NAD (NADH) and reduced FAD (FADH2).
- NAD+ + 2e- + 2H+ → NADH + H+; FAD + 2e- + 2H+ → FADH2.
- When reduced carriers lose electrons, they return to their original form: NADH → NAD+ + 2e- + 2H+.
Glycolysis
- Glycolysis is the first stage of respiration and takes place in the cytoplasm.
- It traps glucose in the cell by phosphorylating it and then splits the glucose molecule in two.
- Phosphorylation: glucose (6C) is activated by phosphorylation from two ATP to form fructose-1,6-bisphosphate (6C), making it less stable and more reactive.
- Lysis: fructose-1,6-bisphosphate (6C) splits into two molecules of triose phosphate (3C).
- Oxidation: hydrogen is removed from triose phosphate by dehydrogenase and transferred to NAD, forming two reduced NAD; triose phosphate is oxidised to glycerate-3-phosphate.
- ATP formation: phosphates are transferred from intermediate substrates to form four ATP through substrate-linked phosphorylation.
- The net gain is 2 ATP (4 produced, 2 used) and 2 reduced NAD per glucose molecule.
- The end product is two pyruvate (3C) molecules; each step is catalysed by a different enzyme.
The Link Reaction
- The link reaction takes place in the matrix of the mitochondria and links glycolysis to the Krebs cycle.
- It is an oxidative decarboxylation reaction: carbon dioxide is removed to produce a 2C molecule.
- The 2C molecule is then oxidised (loss of hydrogen and 2 high-energy electrons), reducing NAD to NADH.
- The acetyl compound combines with coenzyme A to form acetyl CoA.
- The products are acetyl CoA, carbon dioxide and reduced NAD.
- Equation: pyruvate + NAD+ + CoA → acetyl CoA + carbon dioxide + reduced NAD.
- Acetyl CoA is supplied to the Krebs cycle where aerobic respiration continues.
The Krebs Cycle
- The Krebs cycle (citric acid cycle) takes place in the matrix of the mitochondria and consists of a series of enzyme-controlled reactions.
- Acetyl CoA (2C) enters the cycle and combines with oxaloacetate (4C) to form citrate (6C); coenzyme A is released to be reused.
- Citrate (6C) is converted back to oxaloacetate (4C) through a series of redox reactions.
- Decarboxylation of citrate releases two CO2 as waste gas.
- Oxidation (dehydrogenation) of citrate releases hydrogen atoms that reduce coenzymes: 3 NAD+ and 1 FAD → 3NADH + H+ and 1 FADH2.
- Substrate-level phosphorylation transfers a phosphate from an intermediate to ADP, forming one ATP.
- As the link reaction produces two acetyl CoA per glucose, the Krebs cycle occurs twice per glucose molecule.
- Per glucose molecule, the Krebs cycle produces 4 CO2, 2 ATP, 6 NADH + H+ and 2 FADH2.
Oxidative Phosphorylation
- The electron transport chain is made of a series of redox reactions via membrane proteins (electron carriers) embedded in the inner mitochondrial membrane.
- Electron carriers are positioned close together so electrons can pass from carrier to carrier.
- The cristae are impermeable to protons, so electron carriers pump protons across the membrane to establish a proton (electrochemical) gradient.
- Energy is transferred when a pair of electrons is passed to the first carrier, converting reduced NAD back to NAD+.
- As electrons are transported along the carriers, energy is released in a controlled manner and used to form ATP by adding π to ADP.
- 3 ATP molecules are produced for every molecule of reduced NAD, contributing to a total yield of 32 ATP per glucose molecule during aerobic respiration.
- Chemiosmosis: protons accumulate in the intermembrane space and return to the matrix through ATP synthase, which catalyses phosphorylation of ADP to generate ATP.
- Oxygen acts as the final electron acceptor, is reduced by electrons, and combines with protons to form water.
Respiratory Substrates: Lipids and Carbohydrates
- Lipids are an excellent source of energy, transferring more than twice the energy per gram as carbohydrates when oxidised.
- This is because lipids have fewer oxygen atoms per molecule than carbohydrates, making their hydrogen and carbon atoms more oxidisable.
- Lipids are good energy storage molecules because they are insoluble and do not decrease the water potential of a cell.
- Lipids can be used as a source of metabolic water for desert animals, as their oxidation produces more water than carbohydrates.
- Glycolysis and anaerobic respiration can only occur if carbohydrate is the substrate.
- To enter respiratory pathways, lipids must first be broken down into fatty acids, which are further broken down into 2C acetyl groups.
- These acetyl groups combine with coenzyme A to form acetyl CoA, which can enter the Krebs cycle.
Investigating Respiration: Skills
- The rate of cell respiration varies with metabolic activity, organism size, oxygen supply, substrate supply, temperature and pH.
- Respirometers measure the rate of oxygen consumption by live organisms such as seeds or invertebrates.
- A respirometer contains a sealed container with live organisms, an alkaline solution (e.g. potassium hydroxide) to absorb CO2, and a capillary tube with a manometer fluid.
- As organisms respire, oxygen is absorbed and CO2 is absorbed by the alkali, reducing air pressure and causing the manometer fluid to move towards the organisms.
- Respirometers must be kept in temperature-controlled conditions (e.g. a thermostatically controlled water bath) because temperature fluctuations affect air pressure.
- Repeat readings should be carried out to identify and eliminate anomalies and to give a reliable mean.
- The volume of oxygen consumed can be calculated using the formula πr²h, where r is the radius of the capillary tube lumen and h is the distance moved by the manometer fluid.
- Oxygen sensors and CO2 monitors can measure gas concentrations in real-time without exposing subjects to hazards such as strong alkalis.
Slides
Sign up free to view the lesson slides
Step through every slide for this topic — plus flashcards and revision notes — with a free account.
Questões de prática
Prévia grátis — 8 de 62 perguntas. Cadastre-se para ver todas.
1.Which molecule is the universal energy currency of the cell?
Easy- AATP
- BGlucose
- CNADH
- DADP
2.Which of the following is NOT a component of ATP?
Easy- ADeoxyribose sugar
- BRibose sugar
- CAdenine base
- DThree phosphate groups
3.When ATP is hydrolysed, which products are formed?
Medium- AADP and a phosphate ion
- BAMP and two phosphate ions
- CAdenosine and three phosphate ions
- DADP and water
4.Which statement about the synthesis of ATP from ADP and π is correct?
Medium- AIt is a condensation reaction that releases water
- BIt is a hydrolysis reaction that uses water
- CIt is an exergonic reaction that releases energy
- DIt occurs only in the mitochondria
5.ATP is stored in large quantities in cells for long-term energy storage.
EasyTrue or false?
6.Which of the following are properties of ATP that make it suitable as an energy source for biological processes? (select all that apply)
Medium- AIt releases a small but sufficient quantity of energy
- BIt is relatively stable at cellular pH
- CIt can be recycled
- DIt is insoluble in water
- EIt is a large molecule that cannot move easily
7.Match each stage of aerobic respiration with its correct location in a eukaryotic cell.
Medium- Glycolysis
- Link reaction
- Krebs cycle
- Oxidative phosphorylation
- Cytoplasm
- Mitochondrial matrix
- Inner mitochondrial membrane
8.Place the stages of aerobic respiration in the correct sequence.
Medium- Glycolysis
- Link reaction
- Krebs cycle
- Oxidative phosphorylation
Unlock all 62 questions & more
Crie uma conta grátis para ver todas as perguntas, os slides, flashcards e resumos de revisão deste tópico.