8 An Introduction to Metabolism

Question 1

What is a metabolic pathway?

Answer 1

A sequence of reactions, catalysed by enzymes, which converts one substance to another through various intermediaries.

Question 2

What are the types of metabolic pathway and what do they refer to?

Answer 2

Catabolic pathways: breakdown

Anabolic pathways: build up

Question 3

Generally speaking, which type of metabolic pathway release energy?

Answer 3

Catabolic i.e. respiration.

Question 4

What is bioenergetics?

Answer 4

The study of how energy is used within a cell, within an organism and within an ecosystem.

Question 5

Which disciple deals with the study of the flow of energy through living entities?

Answer 5

Bioenergetics.

Question 6

What is the first law of thermodynamics?

Answer 6

Energy is neither created nor destroyed, only transferred between states.

Question 7

What is the implication of the first law of thermodynamics?

Answer 7

No new energy can ‘appear’ so it must be taken from sources i.e. food

Question 8

What is the second law of thermodynamics?

Answer 8

No energy transfer or transformation is completely efficient and thus the entropy of a system must increase.

Question 9

Define entropy.

Answer 9

The randomness of energy in the universe i.e higher entropy = more spread out.

Question 10

What is the implication of the second law of thermodynamics?

Answer 10

An organism can’t just reuse its energy-it must replace the energy it has lost.

Question 11

What is an isolated system? What is its opposite?

Answer 11

A system in which no energy enters or leaves. The opposite is an open system in which energy enters and/or leaves/

Question 12

What is an example of an open and an isolated system?

Answer 12

Open: organism
Isolated: earth (pretty much)

Question 13

For a reaction to occur without outside help what must it do?

Answer 13

Increase the entropy of the universe i.e. release energy.

Question 14

What is the name for a process i.e. reaction that can occur without the input of energy?

Answer 14

Spontaneous.

Question 15

True or False, spontaneous reactions are fast?

Answer 15

Not necessarily some are (explosions), other aren’t (rusting)

Question 16

Why does catabolism generally release energy?

Answer 16

Anabolism decreases the entropy of the universe so must take energy. Therefore the opposite, catabolism, must release energy.

Question 17

At what temperature is absolute zero?

Answer 17

-273.15ºC

Question 18

How can the energy change of a reaction be quantified?

Answer 18

By calculating the change in ‘Gibbs free energy’ (ΔG)

Question 19

How is the change in free energy calculated?

Answer 19

ΔG = ΔH - TΔS

where:
ΔG is change in free energy
ΔH is change in enthalpy (total energy)
ΔS is change in the system's entropy
and T is temperature as measured in Kelvin

Question 20

What does a change in free energy mean?

Answer 20

If the number is negative this means the system has less free energy. Therefore this free energy must have been emitted.

Therefore a negative ΔG means energy has been released.

Question 21

What is a reaction which the system loses free energy termed?

Answer 21

Exergonic

Question 22

What is a reaction which the system absorbs free energy termed?

Answer 22

Endergonic

Question 23

What type of reaction is spontaneous?

Answer 23

Exergonic

Question 24

If a system reached equilibrium, what can it no longer do?

Answer 24

Work

Question 25

What is ATP composed of?

Answer 25

3 phosphate groups, a ribose sugar and an adenine

Question 26

How does broadly ATP power cellular work?

Answer 26

By energy coupling: the use of an exergonic reaction to drive an endergonic reaction.

Question 27

What is energy coupling?

Answer 27

The use of an exergonic reaction (hydrolysis of ATP) to drive an endergonic reaction.

Question 28

Specifically, how does ATP drive cellular work?

Answer 28

It transfers one of its phosphate groups (phosphorylation) This forms a high energy _phosphorylated intermediate_ which can then undergo a exergonic reaction to the final product

Question 29

What is the symbol for activation energy?

Answer 29

Ea (big E, subscript a)

Question 30

What is activation energy?

Answer 30

Energy that must be added before an exergonic reaction can take place. i.e. heat to ignite a fire.

Question 31

How can activation energy be lowered?

Answer 31

With enzymes.

Question 32

How do enzymes increase the rate of a reaction?

Answer 32

By lowering the activation energy by guiding the reactants into their correct orientation.

Question 33

What is an enzyme-substrate complex?

Answer 33

The enzyme and its bound substrate.

Question 34

What is an enzyme bound to its substrate called?

Answer 34

An enzyme substrate complex.

Question 35

How does the enzyme bond with its substrate?

Answer 35

As the substrate enters the active site it forms weak bonds with the enzyme which change its shape and 'clamp' it shut. This is known as 'induced fit'

Question 36

How does an enzyme speed up a reaction?

Answer 36

1) If 2 or more reactants guides them together 2) As the enzyme clamps during 'induced fit' it may pull apart 3) The active site might provide a beneficial environment - for example if it is made of acidic amino acids it could provide low pH

Question 37

Why do spontaneous reactions take time?

Answer 37

It is random which reactants actually have the activation energy (and orientation) to react.

Question 38

What conditions affect the rate of enzymatic catalysis?

Answer 38

Concentration of enzyme and substrate, temperate, and pH.

Question 39

What is the name for helpers needed for an enzyme to function?

Answer 39

Cofactors or more specifically coenzymes if they are organic (not proteins ie. vitamins)

Question 40

What role do vitamins often play?

Answer 40

Coenzymes (organic cofactors) of enzymatic reactions.

Question 41

How can enzymes be inhibited?

Answer 41

Competitive inhibition and Noncompetitive inhibition.

Question 42

What is competitive inhibition?

Answer 42

When a substance, but not the normal substrate, binds to the active site of the enzyme and thus blocks it.

Question 43

What is non-competitive inhibition?

Answer 43

When a substance binds to the enzyme outside the active site but in a way that changes the shape of the enzyme so that the active site is inaccessible.

Question 44

Is non-competitive/competitive inhibition permanent?

Answer 44

Sometimes, depending whether the bonds to the active site/enzyme are strong and permanent or not.

Question 45

In what ways can enzyme activity be regulated?

Answer 45

(Non)competitive inhibition, allosteric regulation and feedback inhibition.

Question 46

What is allosteric regulation.

Answer 46

Reversible regulation of enzyme activity based on molecules that bind outside the active site of an enzyme.

Question 47

How does allosteric regulation occur?

Answer 47

When the protein etc. binds it changes the shape of the enzyme and thus blocks its active site or makes it accessible.

Question 48

What forms does allosteric regulation divide into?

Answer 48

Allosteric activation and allosteric inhibition. A special form of allosteric activation is cooperativity.

Question 49

What is cooperativity?

Answer 49

A form of allosteric activation that occurs in proteins that can bind to more than one molecule of the same substrate at the same time. When one substrate binds to one of the enzymes active sites it changes an protein's shape so that the other active sites are more accessible, causing the protein to become saturated. As one substrate molecule leaves the reverse happens so they all might be dumped at the same time. Note: this often occurs in transport proteins

Question 50

Where is cooperativity seen and why?

Answer 50

The binding of an oxygen molecule to one binding site increases the affinity for oxygen of the remaining binding sites. Thus, where oxygen is at high levels, such as in the lungs or gills, hemoglobin’s affinity for oxygen increases as more binding sites are filled. In oxygen-deprived tissues, the re- lease of each oxygen molecule decreases the oxygen affinity of the other binding sites, resulting in the release of oxygen where it is needed.

Question 51

What is feedback inhibition?

Answer 51

A feature seen in many metabolic pathways where the end product of the pathways acts as an allosteric inhibitor to the enzyme that starts the pathway. This means that when the end product is plentiful no resources are wasted to continue making it. When the end product is used production starts again.