Theme C: C1 Molecules - C1.2 Cell Respiration

Question 1

How do cells metabolise their organic nutrients?

Answer 1

cells break donw or metabolise their organic nutrients by slow oxidation. a molecule, such as glucose, is acted on by a series of enzymes.

the function of these enzymes is to catalyse a series of reactions in which the covalent bonds are broken (oxisidsed) one a time and new products are formed that have lower energy.

the goal of releasing enegry in a controlled way (through a series of enzyme-catalysed steps) is to store the released energy in the form of ATP moleucules. if a cell does not have glucose available, other organic molecules may substituted, such as fatty acids or amino acids.

Question 1

How do organic molecules store energy?

Answer 1

Organic moleucles contain energy stored in their molecular structures. each covalent bond in a molecule of glucose, an amino acid, or a fatty acid represents stored chemical energy.

Question 2

chemical structure of ATP

why it’s clssified as a nucleotide? contribution to its function?

Answer 2

ATP is nucelotide because it contains the 5-carbon suar ribose, the nitrogenous base adenine, and three phosphate groups.

ATP has a specific chemical structure that allows it to function as the energy currency of the cell:
* the last two phostphate groups of ATP are attatched to the main molecule by high-energy bonds.
* since the phosphate groups are negatively charged, they repel one another, resulting in an unstable covalent bond between the two, referred to as a high-energy bond.
* these unstable bonds have low activation energy and are easily browken by hydrolysis.
* this hydrolysis reaction is exergonic (energy releasing), the released energy is then free to perform cellular work

Question 3

cellular work carried out using the energy released form the high-energy bonds of ATP includes:

Answer 3

• active transport across cell membranes (discussed in Chapter B2.1)
• synthesis of macromolecules by anabolism (discussed in Chapter C1.1)
• movement of the whole cell by cilia or flagellum action
• movement within the cell of cell components, such as chromosome movement in mitosis or meiosis.

Generally, ATP is needed for all cell activities.

Question 4

the ATP cycle

Answer 4

The ATP cycle is a cyclic process where ATP is formed from ADP and inorganic phosphate. This reaction requires an input of energy and is endergonic, storing energy from oxidation of nutrients in the high-energy bond between the second and third phosphate groups.

When ATP is hydrolyzed, the third phosphate group is released, and the reaction becomes exergonic, releasing energy stored in high energy bond for cellular work. This also forms ADP and a free phosphate, which can be reused to regenerate ATP.

Question 5

cell (cellular) respiration

Answer 5

markscheme definition: cell respiration is the controlled release of energy from organic compounds to produce ATP. it envolves the oxidation and reduction of electron carriers

The process by which most organisms on Earth synthesize ATP for cellular functions. It involves the release of energy from carbon compounds, especially glucose (CH,20) and fatty acids. Carbohydrates (other than glucose), proteins and many other carbon-containing compounds can also be used in respiration.

The catabolic reaction involves the removal of electrons from glucose (oxidation) and the acceptance of thos eelectrons by oxygen (reduction).
C6H12O6 + 6O2 –> 6CO2 + 6H2O + energy

Question 6

how is cell respiration made more efficient?

Answer 6

glucose is a high-erngy moleucle comapred to caron diozide and water. therefore, as this reaction proceeds, energy is released. the pathways of cellular repsirtation allow the slow release of enegry from the glucose molecules so that ATP can be produced more efficiently.

Question 7

what is the initial stage of cell respiration (in all cells)?

Answer 7

glycolysis. glucose eners a cell through the cell membrane and is found in the cytoplasm. enzymes then catalyse reactions to ultimately cleave the 6-carbon glucose molecule into two 3-carbon molecules. each of these 3-carbon molecules is called pyruvate. Some, but not all, of the covalent bonds in the glucose are broken during the series of reactions.

some of the energy released from the breaking of these bonds is used to form a small number of ATP molecules. 2 ATP molecules are needed to begin glycolysis, and a total of four ATP molecules is formed. hence, a net gain of 2 ATPs.

Question 8

why is glycolysis the metabolic pathway that’s common to most organisms on Earth?

Answer 8

oxygen is not needed for glycolysis to proceed. some organisms derive all their ATP aithout the use of oxygen. these organisms are said to carry anaerobic cell respiration.

Question 9

fermentation

Answer 9

the brekadown of organic molecules for anaerobic ATP production. there are two types: alcoholic fermentation and lactic acid fermentation.

Question 10

how does lactic acid fermentation take place in humans?

Answer 10

if oxygen is not present after glycolysis, then in humans lactic acid fermentation commences. if your exercise rate exceeds your body’s capacity to supply adequate oxygen, at least some of teh glucose entering cell respiration will follow the anaerobic pathway called lactic acid fermentation.

the lack of adequate oxygen converts each pyruvate produced by glycolysis into lactic acid molecules (lactate). like pryuvate, lactic acid molecules are 3-carbon molecules. lactic acid fermentation allow glycolysis to continue because there’s no build-up of pryuvate. however, only two ATP molecules are generated from anaerobic respiration.

You may have experienced the muscle burn that occurs as a result of lactic acid accumulation during intense exercise. The burn goes away when adequate supplies of oxygen are provided to the muscle so that aerobic cell respiration can occur. The lactic acid is carried to the liver via the bloodstream, where it is converted to glucose, then glycogen.

Question 11

similarities and differences between anaerobic and aerobic cellular respiration

Answer 11

An: requires glucose but not oxygen.
Aer: requires both.

An: entirely cytoplasm
Aer: begins in the cytoplasm, the continues in the mitochondria.

An: the product of glycolysis is two molecules of pryuvate made from glucose.
Aer: same

An: if oxygen is unavailable, pyruvate is converted into lactic acid (in humans) through fermentation in the cytoplasm.
Aer: If oxygen is available, pyruvate is transported into the mitochondria for further breakdown in the Krebs cycle.

An: no mitochondria are needed.
Aer: pryuvate is converted into a 2-carbon compound in the matrix of mitochondria.

An: net gain of 2 ATPs.
Aer: the 2-carbon compound enters the Krebs cycle, also in the mitochondrial matriz.

An: N/A
Aer: carbon dioxide is produced as a waste product of the krebs cycle. 30-34 ATPs are produced in the cristae of the mitochondria.

Question 12

what happens after glycolysis?

Answer 12

glycolysis produces 2 molecules of pryuvate in the cytoplasm of the cell, which then enter the mitochondrion. once inside, the pruvate molecules are turned into a 2-carbon compound that enters the next stage of respiration called the Kreb cycle. the preperatory reaction is known as the link reaction and takes place in the matrix of the mitochondria. the Krebs cycle also takes place in the matrix, and is a series of reactions that begins and ends with the same molecule. a net gain of two ATPs occurs in the Krebs cycle.

Question 13

what is the final stage of aerobic respiration?

Answer 13

electron transport chain, whcih occurs in the cristae of the mitochondrion. most ATP molecules produced from the breakdown of glucose are made in the elcron transport chain: 30-34 ATPs are prodyced in this stage.

Question 14

key points about anaerobic and aerobic respiration

Answer 14

• both types of cellular respiration initially take place in the cytoplasm
• in both cases glucose (a 6-carbon molecule) is broken down into two molecules of pyruvate (a 3-carbon molecule)
• the production of ATP is very low in anaerobic cellular respiration compared to aerobic cellular respiration
• anaerobic cell respiration occurs outside the mitochondria (in the cytoplasm)
  and does not require oxygen
• aerobic cell respiration starts in the cytoplasm but finishes within the mitochondria and requires oxygen
• the final products of anaerobic respiration in humans are lactic acid and ATP
• the final products of aerobic respiration are carbon dioxide, water and ATP.

Question 15

what factor affect the rate of cell respiration (considering the fact that it involves a series of chemical reactions)?

Answer 15

• Temperature: the optimum temperature for the rate of cell respiration is 20-30°C. Significantly higher and lower temperatures greatly decrease the rate.
• Carbon dioxide concentration: an increase in carbon dioxide concentration adversely affects the rate of cell respiration.
• Oxygen concentration: lower concentrations of oxygen lower the rate of cell respiration. The absence of oxygen results in anaerobic respiration.
• Glucose concentration: low levels of glucose in the cell will decrease the rate of cell respiration.
• Type of cell: some types of cells require more energy than others. Those that require more energy have higher cell respiration rates.

Question 16

how do you determine the factors that affect cell respiration?

Answer 16

Factors that affect cell respiration can be determined experimentally by calculating the rate of cell respiration using raw or secondary data. Respirometers are often used to calculate the rate of cell respiration.

Question 17

& its graph

respirometers

Answer 17

devices used to measure an organism’s rate of respiration by measuring the oxygen rate of exchange. they are sealed units in which any carbon dioxide produced is absorbed by an alkali such as soda lime or potassium hydroxide. Absorbing the carbon diozide allows an accurate measurment of oxygen exchange. these devices may work at a cellular level or at a whole organism level. produces a graph where:
* y axis is oxygen consumption /ml
* x axis is time /min

Question 18

what is oxidation and reduction?

Answer 18

oxidation results in the loss of electrons and reduction results from the gain of electrons. these reaction always occyr together in what are called redox reactions. both of these take place in respiration.

Question 19

& what happens with hydrogen

nicotinamide adenine dinucleotide (NAD)

Answer 19

it is a coenzyme utilised by the enzymes of cell respiration to carry out oxidation and reduction. NAD is also known as a hydorgen carrier. when hydrogen is added to NAD, the moecule is said to be reduced. when hyrodgen is removed the molecule is said to be oxidised or dehydrogenated.

It is important to remember that hydrogen atoms consist of a proton and an electron. Therefore, when NAD receives hydrogen it is actually receiving one proton and one
electron.

Question 20

differences between oxidation and reduction

Answer 20

oxidation: loss of e, gain of oxygen, loss of hydrogen, results in many C-O bonds, results in a compound with lower potential energy.

reduction: gain of e, loss of oxygen, gain of hydrogen, results in many C-H bonds, results in a compound with higher potential energy.

Question 21

how can the energy released in cell respiration be tracked?

Answer 21

• energy release is tracked by movement of hydrogen atoms (and their electrons)
• glucose is oxidised (loses H), becoming CO2
• oxygen is reduced (gains H), forming H2O
• the movement of electrons in the conversion of glucose to CO2 is an energy-releasing process.
• This energy is captured by NAD⁺, forming NADH (reduced NAD).
• NADH carries energy-rich electrons to later stages of respiration to help make ATP.

Question 22

glycolysis

Answer 22

the word means “sugar splitting” and this pathway is thought to be one of the first biochemical pathways to evolve. it yses no oxygen and occurs in the cytoplasm of the cell. no organelles are required.

the lysis (splitting) of the sugar proceeds efficiently in both aerobic and anaerobic environments, and glycolysis occurs in both prokaryotic and eukaryotic cells. a hexose sugar, usually glucose, is split in the process. the splittign involves many steps and each is controlled by a diff enzyme.

Question 23

three steps of glycolysis

Answer 23

In summary, we see phosphorlylation, lysis, oxidation and reduction, and ATP formation. The final products are two pruvate molecules, four ATP molecules, and two molecules of NADH.

1) Two molecules of ATP are used to begin glycolysis. In the first reaction, the phosphates from the ATP molecules are added to (6-carbon) glucose to form fructose-1,6-bisphosphate, a process called phosphorylation. This step is important because it creates a less stable molecule, quite high energy.

2) The less stable 6-carbon phosphorylated fructose is split (undergoes lysis) into two 3-carbon sugars called triose phosphate (TP).

3) Each TP molecule undergoes oxidation to form a reduced molecule of NAD (NADH). As NADH is formed, released energy is used to add an inorganic phosphate to the remaining 3-carbon compound, resulting in a compound with two phosphate groups. Enzymes then remove the phosphate groups so they can be added to ADP to produce ATP. The result is the formation of four molecules of ATP, two molecules of reduced NAD (NADH) and two molecules of pyruvate. Pyruvate is the ionized (electrically charged) form of pyruvic acid.

Question 24

what happens during anaerobic respiration in humans and why is it limited?

Answer 24

* When oxygen is not present, anaerobic cell respiration in humans allows muscles to work vigorously for a short period of time. * However, the lactic acid that builds up in the muscle during this fermentation process creates a sensation of burning if intense exercise continues and can be toxic in higher concentrations. * Anaerobic respiration provides energy when it is needed but is hard to sustain for long periods of time.

Question 25

How is NAD⁺ regenerated in anaerobic respiration and why is this important?

Answer 25

* Anaerobic cell respiration also allows the regeneration of NAD⁺, which is important because without NAD⁺ being available to accept hydrogen, glycolysis would have to stop. * The regeneration of NAD⁺ occurs when reduced NAD (NADH) donates its hydrogen and electrons to the pyruvate molecules being formed. * In humans, this reduction of pyruvate results in the formation of lactate. * In anaerobic respiration, the only ATP being produced is that formed by glycolysis, which has a net yield of 2 ATP per glucose molecule. * Glycolysis must therefore continue if life functions are to continue.

Question 26

Yeast

Answer 26

a common, single-celled fungus that uses alcholic fermentation for ATP generation when oxygen is not present. Yeast cells that in glucose form their environment and generate a net gain of two ATPs through glycolysis. the organic products of glycolysis are always 2 pryuvate molecules. yeast then converts both 3-carbon pryuvate molecules to molecules of 2-carbon ethanol. the "lost" carbon atoms is given off in a carbon dioxide molecule. both the ethanol and carbon dioxide that are produced are waste products and are released into the environment. e.g. Bakers' yeast is added to bread products because the generation of carbon dioxide helps the dough to rise. Yeast is also commonly used in the production of alcoholic drinks.

Question 27

simplified events of alcohol fermentation

Answer 27

Ethanol and carbon dioxide are produced from ethanol fermentation. NAD is also regenerated in this process, when the reduced NAD (NADH), from glycolysis donates its electrons and hydrogens to two acetaldehyde molecules to form the two molecules of ethanol. e.g. bioethanol is produced by living organisms as a renewable energy resource, which is one approach being applied around the world to reduce fossil fuel usage.

Question 28

the link reaction

Answer 28

once glycolysis has ocurred, and if there is oxygen present, pryuvate enters the **matrix** of the mitochondria via active transport. inside the matrix: 1) pryuvate is **decarboxylated** to form the 2-carbon acetyl group. the removed carbon is released as carbon dioxide, a waste gas. 2) the acteyl group is then oxidised and reduced NAD (NADH) is formed. 3) Finally, the acetyl group combines with **coenzyme A** (CoA) to form **acetyl-CoA**. Acetyl-CoA can then **enter** the **Krebs cycle**. this **link reaction** is controlled by a system of enzymes. acetyl groups can be produced from most carbs and fats, not just glucose acts as a respiratory substrate. once acetyl groups are formed, they can be transferred by CoA into the Krebs cycle.

Question 29

the Krebs cycle

Answer 29

occurs in the matrix of mitochondria. this cycle includes two decarboxylation reactions and 4 points at which the carbon compound is oxidised for each acetyl group brought into the cycle by CoA. the Krebs cycle will run **twice** for each glucose molecule entering aerobic cell respiration. This is because a glucose molecule forms two pyruvate molecules. Each pyruvate produces one acetyl-CoA that enters the cycle. Here is an overview of the **products** produced from the breakdown of **1**glucose molecule in the Krebs cycle: * 2 molecules of ATP * 6 molecules of reduced NAD (NADH) * 2 molecules of reduced FAD (FADH2) * 4 molecules of carbon dioxide.

Question 30

# consider 2 carbon compounds plus a coenzyme what compounds are formed in the Krebs cycle?

Answer 30

**citrate is a 6-carbon compound** produced by the combination of the acetyl group from the link reaction with the **4-carbon oxaloacetate** from the cycle itself. there is an additional coenzyme involved in the Krebs cycle. the additional coenzymes if flavin adenine dinucleotide (FAD), and it acts as a hydrogen/electron acceptor just as NAD does.

Question 31

# also consider the spitting of hydrogen atoms in the matrix where does reduced NAD (NADH) come from?

Answer 31

the reduced NAD (NADH) from glycolysis, link reaction, and Krebs cycle carries electrons and a proton from a pair of hydrogen atoms, with the second proton released into the surrounding solution. hydrogen atoms are split in the mitochondrial matrix into a proton (H+) and electron (e-). the electrons have a high energy level and pass into the **elctron transport chain**, which occurs on the **inner mitochondrial membrane** and on the **membranes of the cristae**. embedded in these membranes are **electron carrier molecules** that are easily reduced and oxidised. NADH is converted back into NAD, which can then be used again in earlier parts of the process.

Question 32

role of reduced NAD molecules in the start of the electron transport chain?

Answer 32

NADH molecuels transfer high-energy electrons to the first carrier in the electron transport chain. these electrons are then passed from one carrier to another, with small amounts of energy released at each exchange.

Question 33

carriers in the electron transport chain | better demonstrated on a diagram. this FC ignore energy released.

Answer 33

**Carriers** FMN: This protein carrier has a flavin-containing group. Fe-S: This protein contains an iron-sulfur complex. Cyt: These are cytochromes (iron-containing proteins). Q: This is coenzyme Q, also called ubiquinone; it is not a protein.

Question 34

graph of electron transport chain

Answer 34

the y axis represents tha amount of energy available, while the x axis represents the occurrence of redox reactions. note that: * as high-energy electrons are bassed from carrier to carrier there is a decrease in available energy. * FADH2 brings its electrons into the chain at a slightly lower energy level than NADH. * as electrons move down the chain, the energy released in small increments is used to transfer protons across the inner membranes from the matrix into the space between the membranes.

Question 35

what does the electron transport chain produced? ## Footnote includes 2 key terms

Answer 35

the electron transport chain doe snot directly produce any molecules of ATP. however, it does produce a **proton gradient** which provides the energy for teh formation of ATP from ADP and inorganic phosphate (Pi). the formation of ATP driven by the proton gradient is called **chemiosmosis**.

Question 36

how do the electron transport chain and chemiosmosis work together to produce ATP?

Answer 36

* While electrons move down the electron transport chain, energy is released in small amounts and used to pump protons (hydrogen ions) out of the matrix and into the intermembrane space. * This creates a **proton gradient**, with a high concentration of protons in the intermembrane space compared to the mitochondrial matrix. * Protons move down the concentration gradient by diffusion through channels in an enzyme called ATP synthase. * As the protons move passively from the intermembrane space to the matrix, ATP synthase harnesses this energy, allowing the phosphorylation of ADP to form ATP. Remember that there are protons in the matrix as a result of NAD transporting hydrogen atoms to the electron transport chain. These hydrogen atoms are split, and the electrons enter the electron transport chain. The protons (hydrogen ions) are free in the matrix. H→H+ + 1e *Because of the hydrophobic region of the membrane, the protons (hydrogen ions) can only pass through the ATP synthase channels. Some poisons that affect metabolism act by establishing alternative pathways through the membrane, thus preventing ATP production.*

Question 37

role of oxygen in the electron transport chain

Answer 37

Oxygen is the final electron acceptor. When the electrons combine with the oxygen, so do two protons (hydrogen ions) from the matrix. The result is water. Because of the way it is formed, this water is known as **water of metabolism**. This acceptance of electrons by oxygen is what allows the continued flow of electrons along the electron transport chain.

Question 38

# specifically triglycerides lipids as a energy storer

Answer 38

lipids are carbon compounds that are insoluble in water. they include fats, waxes, oils, hormones and certain components of membranes. as important function of lipids is energy storage. lipids have less oxygen and more oxdisiable C-H bonds in their structure than carbs. **triglycerides** are simple lipids composed of glycerol and three molecules of fatty acids. triglycerides are the storage forms of lipid and can be used for energy production. long-chain fatty acids usually have an even number of carbon atoms with many C-H bonds.

Question 39

# specifically triglycerides lipids as a respiratory substrate

Answer 39

e.g: fatty acids --> oxidised in the mitochondrial matrix to remove individual 2-carhon acetyl groups --> each acetyl group combines with CoA to enter the Krebs cycle. when emasuring the energy output forma. 6-carbon faty acid and a 6-carbon carb such as glucose, we find that the fatty acid yields 20% more ATP than the carb. it is important to note that the full pathway of glycolysis and anaerobic repsiration can only occur if a carb is the substrate. when acetyl-CoA enter the pathway, aerobic cell respiration is occurring and the Krebs cycle is activated.

Question 40

where can different carbon compound enter the cell respiration pathway ## Footnote useful to refer to diagram in book

Answer 40

sugars and glycerol (from fats) can enter during glycolysis, when glucose is converted into pryuvate. sugars can also enter at the end of glycolysis/ start of link reaction when pruvate has been made. fatty acids (from fats) and amino acids can enter at the end of link reaction when there is acetyl-CoA present. amino acids can enter the Krebs cycle. ATP is then produced at the end of the kreb's cycle at a stage called oxidative phosphorylation.