Energy & Respiration

Topic 7.1

The Need for Energy…

Living Organisms need a constant supply of energy to drive the metabolic reactions that take place inside their cells.

This includes:• Movement (muscular contractions)• Maintaining constant Body Temperature• Making new compounds (biosynthesis - as

forming new chemical bonds requires energy)• Getting things in and out of cells (active

transport)

Summary of Energy Uses:All organisms get their energy from Respiration, this involves the breakdown of glucose (which comes from our food) and it usually involved oxygen from the air.

Can you remember the equation for Respiration?

The chemical bond energy in your food could be release by burning (combustion) but this reaction is uncontrolled and would be fatal inside our cells!

Respiration involves the gradual release of energy in controlled small steps which release small amounts of energy to make a molecule called ATP…

Mechanical (Muscles)

Molecular transport (Across

Membranes)

Heat (Temp control)

Biosynthesis (making

molecules in the cell)

Types of respirationDuring aerobic respiration, a respiratory substrate, e.g.

glucose, is split in the presence of oxygen to release carbon dioxide and water. A large number of ATP molecules are

produced, releasing the energy from the glucose.

C6H12O6 + 6 O2 6 CO2 + 6 H2O + 36 ATP

In anaerobic respiration, glucose is converted (in the absence of oxygen) to either lactate or ethanol. A small

number of ATP molecules are produced.

C6H12O6 2 C3H6O3 + 2 ATPlactate

C6H12O6 2 C2H5OH + 2 CO2 + 2 ATP

ethanol

Where does respiration occur?

Mitochondria contain highly folded inner membranes that hold key respiratory proteins (including the enzyme that

makes ATP) over a large surface area.

Respiration occurs in all living cells. In eukaryotes the early stages of respiration occur in the cytoplasm. The later stages

of respiration are restricted to the mitochondria.

Mitochondria have their own DNA and ribosomes, so can manufacture their

own respiratory enzymes.

Mitochondria provide an isolated environment to maintain optimum

conditions for respiration.

The structure of the mitochondria

Adaptations of mitochondria

An overview of respiration

Stages in Aerobic RespirationThere are four main stages in the breakdown of glucose during aerobic respiration:

• glycolysis (in the cytosol – cytoplasm of cell)

• the link reaction (in mitochondrial matrix)

• Krebs Cycle (in matrix)

• electron transport chain (on cristae, inner membrane of mitochondria)

You need to know about all 4 of these stages but not in as much detail as your text suggests!

Glycolysis – the 1st stageThis is the splitting (–lysis) of glucose. It takes place in a number

of enzyme-controlled reactions and oxygen is not required. The process yields little energy but takes little time.

1. Glucose molecule is phosphorylated to make hexose phosphate* (6C) [caused by the addition of a 2 phosphates from 2 ATP molecules]

2. The 6-carbon hexose phosphate is split into two molecules of triose phosphate** (3C sugars)

3. Each 3-carbon sugar is converted into pyruvate [takes places in a series or reactions where Hydrogen is removed reducing a molecule of NAD+ -> NADH and producing 2 ATP molecules!]

Overall Glycolysis produces 4 molecules of ATP (but 2 are used), 2 molecules of pyruvate and 2 molecules of NADH…see flowchart on handout

*sometimes referred to as fructose-6-phosphate

**sometimes referred to as glyceraldehyde 3-phosphate

The first stage of respiration: glycolysis

The Link Reaction – 2nd stage

If oxygen is present this next stage of aerobic respiration can take place:

• Occurs in the matrix of mitochondria (Pyruvate from the end of the glycolysis stage enters through the double membrane from the cytoplasm)

• Pyruvate (3C) combines with coenzyme A to form acetyl coenzyme A [Acetyl-CoA] (2C molecule)

• In the process a molecule of CO2 and Hydrogen (2H) are removed

• Acetyl-CoA then enters the next stage of Respiration… The Krebs Cycle … where it combines with a 4C compound Oxaloacetate to form Citrate (6C)

Pyruvate

[3C]

Acetyl-CoA

[2C]

2 H

CO2 [1C]

Coenzyme A

The fate of pyruvate

The Krebs Cycle

This series of reaction was discovered by Sir Hans Kreb in 1937, it is also known as the Citric Acid Cycle.

• The Krebs cycle occurs in matrix of mitochondria • Remember: Acetyl coenzyme A (2C) combines with

Oxaloacetate (4C) to form Citrate (6C)• A series of other reactions take place in which Citrate (6C) is

both decarboxylated [CO2 removed] and dehydrogenated [Hydrogens removed]

• Carbon dioxide is removed as a waste product & Hydrogen atoms are picked up by H acceptors (like NAD).

The most important part of the Krebs Cycle is the release of hydrogen ions to be used in the Electron Transport System for generation of ATP

Krebs cycle

Counting carbons

Keeping track of the productsFor each molecule of glucose, glycolysis produces:

2 ×

2 ×

2 ×

For each molecule of glucose, the link reaction produces:

2 ×

2 ×

2 ×

Keeping track of the products

For each molecule of glucose, Krebs cycle generates:

The NADH and FADH2 contain the potential energy originally locked in glucose. This energy is now transferred to ATP by

oxidative phosphorylation in the electron transport chain.

4 × produced by decarboxylation

2 × produced by substrate-level phosphorylation

2 × produced by redox reactions

6 × produced by redox reactions

The Electron Transport Chain – 4th & final stage

The ETC allows the energy from Hydrogen atoms (removed from compounds in the previous stages) to be used to make large amounts of ATP. Oxygen is required and the reactions take place on the inner membrane (cristae) of the mitochondria.

• NADH & FADH2 deliver H+ (protons) and e-(electrons) to cristae. • e-’s "transported" along cristae through electron acceptors, providing energy to pump H+ from matrix into the intermembrane space.• Concentration of H+ is now higher in intermembrane space. H+ pass through ATP synthases in the cristae back out to matrix.• Last step involves H+ & e- added to oxygen. This frees NAD+ to return to glycolysis & Krebs Cycle to pick up more H+ & e-.• The overall process of forming ATP in this way is called oxidative phosphorylation.

28 ATP are made in total. •The movement of H+ ions across a membrane to generate ATP is called chemiosmosis.

The electron transport chain

Outline of the Electron TC

[Reduced Carrier 1]

NADH + H+

[Oxidised Carrier 1] NAD+

[Oxidised Carrier 2]

FAD

[Reduced Carrier 2] FADH2

[Reduced Carrier 3]

[Oxidised Carrier 3]

[Oxidised Carrier 4]

[Reduced Carrier 4]

Water

Oxygen

2H2e-2e-

2e-

ATPATPATP

2H+

Electrons being transported have come from these two H atoms

Understanding the ETC

How much ATP is produced?Process ATP in ATP produced Net ATP out

glycolysis 2 4 2

link reaction 0 0 0

Krebs cycle 0 2 (per glucose) 2 (per glucose)

Via the electron transport chain and chemiosmosis, each NADH can yield 2.5 ATP and each FADH2 1.5 ATP.

From one molecule of glucose, glycolysis yields 2 NADH, the link reaction yields 2 NADH and the Krebs cycle yields 6

NADH and 2 FADH2.

10 × 2.5 = 25 ATP from NADH 2 × 1.5 = 3 ATP from FADH2

total = 2 + 2 + 25 + 3

= 32 ATP overall

Anaerobic RespirationAnaerobic respiration takes place in the absence of oxygen,

and occurs via Glycolysis. Some organisms carry put aerobic respiration if O2 is available but are able to respire anaerobically in its absence.

Most anaerobes fall into this category, some bacteria however can thrive in the absence of O2 – such as C. welccii which causes gangrene!

The two main types of Anaerobic Respiration are:• Alcoholic Fermentation –

carried out by yeast and plants• Lactic Acid (Lactate) Fermentation –

takes place in animals

Lactic Acid Fermentation

The fate of pyruvate

Comparison of aerobic and anaerobic respiration

Aerobic respiration

Anaerobic Respiration

in animals in plants and yeast

Oxygen required? yes no noGlycolysis occurs yes yes yesATP yield 32ATP 2ATP 2ATPGlucose completely

broke down?yes no no

End products Carbon dioxide and water

Lactic acid Ethanol and carbon dioxide

Respiratory rateThe respiratory rate is the rate at which an organism

converts glucose to CO2 and water. It can be calculated by measuring an organism’s rate

of oxygen consumption.

Studies on simple animals often use a respirometer.

Respirometers measure the change in gas volume in a closed system. Any change is due to the respiratory activity of the study organisms. Potassium hydroxide or soda lime is used to absorb the carbon dioxide produced, meaning any

changes in volume are due to oxygen consumption.

The respirometer

Respirometer experiments