MCBG Session 5 - Energy Reactions in Cells

Question 1

Define metabolism and explain its function.

Answer 1

Metabolism is the set of processes which derive energy and raw materials from food, using them for repair, growth and activity of tissues to sustain life.

Question 2

Give a brief overview of how metabolism works?

Answer 2

Food enters the GI tract and is broken down into nutrients. These nutrients are taken up into the blood and utilised by tissues to produce energy via oxidation reactions (waste products reabsorbed into blood and lost) or converted and stored if they are in excess.

Question 3

What are catabolic and anabolic pathways?

What are the key differences between the two?

Answer 3

Catabolic = breaking down larger molecules into smaller ones (intermediary metabolites)

• Releases large amounts of free energy
• Oxidative reaction - releases H+ ions with reducing power.

Anabolic = synthesise larger important molecules from intermediary metabolites

• Uses energy released from catabolism
• Reductive - uses H+ from catabolism

Question 4

How are catabolic and anabolic pathways linked to each other?

Answer 4

The energy and H+ ions (reducing power) released from catabolic pathways are used for anabolic reactions.

Question 5

What are the 4 main products of catabolic metabolism?§

Answer 5

1) Building block materials (sugars, AA’s, fatty acids)
2) Organic precursors (mainly Acetyl CoA)
3) Biosynthetic reducing agents (NADH, NADPH, FADH2)
4) Energy for cell functions (ATP)

Question 6

What should the average daily energy intake (food) be equal to in healthy adults?

Answer 6

Daily energy expenditure (work + heat).

Question 7

What is the standard unit of energy?
How many Kj’s in a Kcal?
Which food molecules have high energy values?

Answer 7

• Joule or Kilo Joule (Kj)
• 1 kcal (1000 cals) = 4.2Kj (4,200 J)
• Fats + Alcohol.

Question 8

What are the energy requirements in Kcal and Kj for a 70kg male and 58kg female for the following:

• BMR
• Activity
• Specific dynamic action of food

Answer 8

BMR = 1,700/1,400 Kcal - 7,000/5800Kj
Activity = 1,000-3,000 Kcal - 4,000-12,000 Kj
SPAF = 150Kcal/650Kj

Question 9

What happens if:

1) Energy intake>expenditure
2) Energy expenditure>intake

Answer 9

1) Growth - synthesis of new tissue and production of adipose tissue
2) Tissue is lost (weight lost)

Question 10

What kind of energy is used to drive functions in the body?

Why is man ‘isothermal’?

Answer 10

• Chemical bond energy

- Man cannot use heat energy for work

Question 11

Which is the difference between exergonic and endergonic reactions?

Answer 11

Exergonic:

• Can occur spontaneously
• Release energy - products are lower in energy than reactants
• Have -deltaG values (Gibbs free energy)

Endergonic:

• Can’t occur spontaneously
• Require energy input - products higher energy than reactance
• Have + deltaG values

Question 12

What kind of reactions release chemical bond energy?

Why are these actually known as redox reactions?

Answer 12

• Oxidation reactions (removal of electrons or hydrogen ions)
• As all oxidation reaction are accompanied by reduction reactions - therefore they are known as Redox.

Question 13

When protons and electrons are released by oxidation reactions, where are they transferred to?

Answer 13

Carrier molecules, that act as carriers of ‘reducing power’ for ATP production and biosynthesis. E.g.: NADH/NADPH and FAD2H. - converted to reduced form by adding 2 x H+ (which dissociate in solution - NADH + H+).

Question 14

What is the energy and reducing power release from oxidative (exergonic) reactions used for?

Answer 14

To drive the synthesis of new molecules (anabolism) or to produce work (e.g.: exercise).

Question 15

Describe the structure of ATP

What is the energy released from the breakdown of ATP –> ADP or ADP –> AMP

Answer 15

• Adenosine (adenine base + ribose) and 3 phosphate groups.

- -31kJ.mol (+31 for production of ATP from ADP)

Question 16

Can ATP ‘store’ energy?

Why is the flow of energy from ATP controlled?

Answer 16

• No, there are limited amounts of ATP - it is only a carrier that cycles, not a store. Only 250g of ATP at any one point in time in the body, but your body weight amount of ATP is cycled each day - THEREFORE NEED A CONTINUOUS SUPPLY.
• ATP is stable in the absence of specific catalysts.

Question 17

What pathways are activated when energy signals are high or low?
What are the high energy signals?
What are the low energy signals?
What is the role of adenylate kinase? (myokinase)

Answer 17

High energy = Anabolic pathways activated
Low energy = Catabolic pathways activated

High energy = ATP, NADH, NADPH, FAD2H
Low energy = ADP, AMP, NAD+, NADP+, FAD+

Adenylate kinase scavenges ADP when energy signal is low to reproduce ATP and AMP. AMP produced also acts as a low energy signal to further activate catabolic pathways.

Question 18

What is used as a reserve of high energy stores that can be used immediately when cell types such as skeletal muscle require it?

Answer 18

Phosphocreatine (creatine phosphate) - When energy levels are high, phosphate bond energy may be stored in phosphocreatine.

Question 19

How is phosphocreatine synthesised?

When is it synthesised?

Answer 19

• From creatine & ATP via creatine kinase enzyme
• When ATP concentrations are high, when they are low this reaction reverses to provide a short term boost of ATP to tissues.

Question 20

What is creatine kinase used as a marker for and why?

Answer 20

• Myocardial infarction
• CK is made from 2 subunits - one isoform (CK-MM = 70% & CK-MB = 30%) is specific to heart muscle. Elevated levels of this in blood indicated MI - as CK is released from damaged myocytes.

Question 21

What is creatinine a clinical marker for and why?

Answer 21

• Marker for urine dilution
• Breakdown product of creatine and phosphocreatine, excrete via kidneys. Can be used as a marker of urine dilution or to estimate true urinary loss of different substances, e.g.: hormones in pregnancy.