Biology - Chapter 12: Respiration

2 types of cellular respiration and what each produces

- Aerobic respiration: requires oxygen; produces CO2, water and much ATP

- Anaerobic respiration: takes place in absence of oxygen; produces lactate in animals, ethanol and CO2 in plants + little ATP in both

How does lactate move out of muscles?

By facilitated diffusion

Name and briefly describe the 4 stages of aerobic respiration

1. Glycolysis (glucose molecule split into 2 pyruvate molecules)

2. Link reaction (pyruvate molecules undergo series of reactions, forming acetyl-CoA)

3. Krebs cycle (acetyl-CoA undergoes cycle of oxidation-reduction reactions, producing some ATP and a lot of reduced NAD + FAD)

4. Oxidative phosphorylation (electrons associated with reduced NAD and FAD are used to make ATP + water)

Where does glycolysis occur?

Cytoplasm

Where does the link reaction occur?

Matrix of the mitochondria

Where does the Krebs cycle occur?

Matrix of the mitochondria

Where does oxidative phosphorylation occur?

The membrane of the crista in the mitochondria

Describe the process of glycolysis (4)

1. Glucose (6 carbons) is phosphorylated using ATP (after ATP is hydrolysed - this makes glucose more reactive so it can undergo following reactions)
2. Phosphorylated glucose splits into 2 triose phosphate molecules (3 carbons)
3. Each triose phosphate molecule is oxidised (hydrogen removed) and a molecule of NAD is reduced, forming pyruvate (3 carbons), NADH and 2 ATP - this produces 4 molecules of ATP in total
4. Overall, there is a net gain of 2 ATP molecules (4 produced, 2 used)

Does glycolysis need oxygen to take place?

No

Glycolysis

The first part of cellular respiration in which glucose is broken down anaerobically in the cytoplasm to two molecules of pyruvate

Describe what happens during the link reaction (4)

1. Pyruvate is actively transported from cytoplasm into matrix of mitochondria
2. Pyruvate (3 carbons) is decarboxylated and oxidised (carbon and hydrogen removed), producing acetate (2 carbons) and CO2
3. NAD is also reduced, producing NADH
4. Acetate then combines with Co enzyme A, forming acetyl-CoA (2 carbons)

How many times does the link reaction occur for every glucose molecule?

Twice as it occurs once for each pyruvate molecule produced

Describe what occurs during the Krebs cycle (7)

1. Acetyl-CoA (2 carbons) combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate)
2. 6-carbon molecule is oxidised and decarboxylated (losing hydrogen and carbon), producing a 5-carbon molecule and CO2
3. The hydrogen is used to reduce NAD, forming NADH
4. 5-carbon molecule is oxidised and decarboxylated too, producing a 4-carbon molecule and CO2.
5. The hydrogen reduces NAD and FAD, forming NADH and FADH.
6. Substrate-level phosphorylation occurs and ATP is produced in a condensation reaction between ADP and Pi
7. 4-carbon molecule combines with acetyl-CoA and the cycle repeats.

What does glycolysis produce? (3)

1. 2x ATP (4 produced, 2 used)
2. 2x NADH
3. 2x Pyruvate

What does the link reaction produce? (3)

1. 2x CO2
2. 2x NADH
3. 2x Acteyl-CoA

(x1 for each pyruvate molecule, x2 overall as glucose makes 2x pyruvate)

What does the Krebs cycle produce? (4)

1. 6x NADH
2. 2x FADH
3. 2x ATP
4. 4x CO2

(half of above for each pyruvate molecule, x2 overall as the Krebs cycle turns twice per molecule of glucose)

What are coenzymes?

Molecules that some enzymes need in order to function

What does NAD do?

It works with dehydrogenase enzymes to catalyse the removal of hydrogen atoms from substrates and transfers them to other molecules involved in oxidative phosphorylation

Give 4 reasons why the Krebs cycle is important

1. It breaks down larger molecules (pyruvate) into smaller ones (CO2)
2. It produces hydrogen ions, which are carried by NAD to the electron transfer chain and provide energy for oxidative phosphorylation (resulting in the production of ATP)
3. It regenerates the 4-carbon molecule that combines with acetyl-CoA (acetyl-CoA would accumulate otherwise)
4. It produces intermediate compounds used by cells to produce other important substances (e.g. fatty acids, amino acids, chlorophyll)

Electron transfer chain

A series of electron carrier proteins that transfer electrons through a series of oxidation-reduction reactions

Describe the chemiosmotic theory of oxidative phosphorylation (7)

1. Hydrogen atoms produced during glycolysis and Krebs cycle combine with NAD and FAD, reducing them to form NADH and FADH
2. NADH and FADH donate the electrons of the H atoms they are carrying to the first molecule in the ETC
3. Electrons pass along ETC through a series of oxidation-reduction reactions, losing energy as they move along the chain
4. The energy they release is used to actively transport protons across the inner mitochondrial membrane and into the inner-mitochondrial space
5. Protons accumulate in inner-mitochondrial space before diffusing back into mitochondrial matrix through ATP synthase channels in the mitochondrial membrane
6. This causes ATP synthase to change shape so it catalyses the formation of ATP from ADP and Pi
7. At the end of the chain, the electrons combine with protons and oxygen to form water

Redox / oxidation-reduction reaction

A chemical reaction in which electrons are transferred from one substance to another substance

What is the final acceptor of electrons in the ETC?

Oxygen

Why are the electrons carried by NAD and FAD not released in a single step?

The greater the energy release in a single step, the more of it is released as heat and the less of it is available for more useful purposes

Apart from carbohydrates, what other respiratory substrates may be used?

Lipids and proteins

How are lipids used as a respiratory substrate? (3)

1. They are first hydrolysed to form glycerol and fatty acids
2. The glycerol is phosphorylated and converted to triose phosphate (then enters glycolysis pathway, link reaction, Krebs cycle)
3. The fatty acids are broken down to 2-carbon fragments, which are converted to acetyl-CoA (then enters Krebs cycle)
4. Oxidising lipids produces 2-carbon fragments and many H atoms - as H atoms are used to produce ATP, lipids release more than double the energy of carbohydrates

How are proteins used as a respiratory substrate? (2)

1. They are first hydrolysed to form amino acids
2. Their amino group is removed, then they enter respiratory pathway at different points depending on the number of carbons they contain

Krebs cycle

Series of aerobic biochemical reactions by which energy is obtained through the oxidation of acetyl-CoA

Can the Krebs cycle or the ETC continue in the absence of oxygen?

No - all the FAD and NAD will be reduced so there will be none available to take up the H+ produced during Krebs, so the enzymes will stop working

In the absence of oxygen, how does glycolysis continue?

The pyruvate molecules accept the hydrogen from NADH, oxidising it to form NAD so glycolysis can continue

How is energy released during anaerobic respiration?

Through the production of ATP in glycolysis

In anaerobic respiration, how is ethanol produced in plants/yeast? (2)

1. After glycolysis, a pyruvate molecule is decarboxylated and reduced
2. This produces ethanol, CO2 and NAD

In anaerobic respiration, how is lactate produced in animals? (2)

1. Each pyruvate molecule take up two hydrogen atoms from NADH
2. This produces lactate and NAD

What happens to the lactate produced in anaerobic respiration?

It is further oxidised to release energy or converted into glycogen when oxygen is available again

What does lactate cause if it accumulates?

Cramp and muscle fatigue

2 ways the energy from cellular respiration is derived from

1. Substrate-level phosphorylation: the direct transfer of phosphate from a respiratory intermediate to ADP to form ATP
2. Oxidative phosphorylation: the indirect linking of energy from phosphate to ADP, producing ATP - this involves energy from the H atoms carried by NADH and FADH

Why must glucose be broken down to pyruvate?

Glucose cannot cross the mitochondrial membrane, while pyruvate can

When measuring the volume of gases using a gas syringe during aerobic respiration, why does the level of the gas syringe stay the same?

The volume of carbon dioxide produced = the volume of oxygen taken in

What would happen if more acetyl CoA entered the Krebs cycle?

The Krebs cycle would produce more reduced coenzymes, so more reduced coenzymes would pass their electrons to the ETC - this causes more ATP to be produced

In plants, what is the effect of increasing temperature on the rate of CO2 release? (3)

- Enzymes act faster (greater KE, move faster and collide more frequently, form more enzyme-substrate complexes)
- Higher rate of respiration so higher rate of CO2 production
- Stomata open more often to get rid of CO2

When measuring the volume of gases using a gas syringe during aerobic respiration, if CO2 was absorbed by a chemical, why would the liquid in the tubing move to the left? (2)

1. O2 is taken up by [the respiring organism], while CO2 produced is absorbed by the chemical

2. So the pressure decreases