TOPIC 1: RESPIRATION

Definition: Aerobic respiration is the release of energy (ATP) from food in the presence of oxygen

Balanced chemical equation:

C6H12O6 + 6O2➝ 6CO2 + 6H2O + energy (ATP)

Energy carriers: Respiration is divided into two stages 1. Glycolysis and 2. Krebs cycle

Respiration is required to form a large amount of ATP which can then be used by the cell as a source of energy (catabolic reaction).

Stage 1: Glycolysis

Location: Cytosol (cytoplasm without any cell organelles) Anaerobic: No oxygen required

Energy: Small amount of energy (ATP) produced

• Glucose (C6) is broken down into two compounds containing three carbons, each known as pyruvic acid (pyruvate) (C3)

• As the bonds between the carbons are broken high energy electrons and hydrogen protons are released

• High energy electrons and a hydrogen proton are captured by NAD+ to form NADH, which transfers them to the electron transport system

• Two molecules of ADP are converted to two molecules of ATP, which carries energy to where it is needed in the cell

Stage 2: Krebs Cycle

Location: Mitochondria (matrix)

Aerobic: Oxygen required

Energy: Large amount of energy (ATP) produced

• Stage 2 begins when pyruvic acid (C3) enters the mitochondrion and is converted to two-carbon molecule (acetyl-CoA) (C2) with release of carbon dioxide.

• High energy electrons and hydrogen protons are released forming NADH

• CoA (C2) enters the Krebs cycle where it is broken down into carbon dioxide, high energy electrons and hydrogen protons which join NAD+ to form NADH

• At one point in the cycle ADP is converted to ATP

Electron transport system

• NADH enters the electron transport system, where it is broken down to form NAD+ and H+ and 2 high energy electrons

• The high energy electrons move through the electron transport system and lose their energy as they move

• The energy released is picked up by ADP to form ATP

• At the end of the system the two low energy electrons join with oxygen and two hydrogen protons to form water

2009 Question 12(a)(b) 36 marks

(a) ATP and NAD/NADP+ play important roles in cell activities.

(i) Name the substance X, formed by the loss of a phosphate group. ADP (3)

(ii) The ATP cycle is kept going By Y. What is Y? Energy (3) (iii) Suggest a role for NAD/NADP+ in cell activities.

Transfers hydrogen protons and high energy electrons (3) (b)

(i) What name is given to the first stage of respiration?

Glycolysis (3)

(ii) The first stage ends with the formation of pyruvate (pyruvic acid).

In aerobic conditions, what is produced from this pyruvate:

1. In muscle cells? Lactic acid (3)

2. In yeast cells? Ethanol and carbon dioxide (3)

(iii) If conditions are aerobic, pyruvate next passes to an organelle in which the second stage of respiration takes place. Name this organelle. Mitochondria (3)

(iv) In this organelle pyruvate is broken down to CO2 and a two-carbon compound. Name this two carbon compound. Acetyl co-enzyme A (CoA) (3)

(v) This two-carbon compound passes directly into a series of reactions in the second stage of respiration. Name this series of reactions and give one product, other than electrons, of these reactions. Krebs cycle and carbon dioxide/ATP/hydrogen protons (2x3)

(vi) The electrons released from the above reactions pass along a transport chain and in the process energy is released

1. To what use is this energy put? Energy combines with ADP to form ATP (3)

2. At the end of the transport chain what happens to the electrons? They join with hydrogen protons and oxygen to form water (3)