1. Cells require a constant supply of energy to maintain their internal organization and carry out reactions through metabolism. Energy flows through ecosystems in a unidirectional manner as it is transformed and some is lost as heat. 2. ATP is the energy currency of cells that is regenerated through exergonic reactions and used to drive endergonic reactions. Metabolic pathways involve series of enzyme-catalyzed reactions that convert food into ATP. 3. Photosynthesis and cellular respiration are key redox reactions that allow the flow of energy from the sun through living things by cycling molecules between chloroplasts and mitochondria.

1. UNIT 5 – METABOLISM:
ENERGY AND ENZYMES

2. ENERGY
! Energy – The ability to do work.
! Living organisms need to acquire energy for survival
! Cells require energy to:
! Maintain organization
! Carry out reactions
! Develop, Grow and Reproduce

3. FORMS OF ENERGY
! Kinetic Energy – energy of motion
! Potential Energy – stored energy
! Source of Potential Energy
! FOOD – called Chemical Energy
! Organisms convert chemical energy into a form of
kinetic energy called Mechanical Energy.

4. ENERGY FLOW
! Is unidirectional in ecosystems (no cycling)
! Law of thermodynamics explains this:
! 1st Law: Energy cannot be created or destroyed,
but can be transformed.
! 2nd Law: Energy transformation results in a loss
of usable energy.
! Loss of usable energy is typically in the form of
heat.

5. 6-5
FLOW OF ENERGY

6. CELLS AND ENTROPY
! The term entropy is used to indicate the
relative state of disorganization.
! Cells need a constant supply of energy to
maintain their internal organization.
! Complex molecules tend to break apart into
their building blocks.
! Ex: Glucose " Carbon Dioxide + Water
! Greater Organization = less stable
! The result is a loss of potential energy and an
increase in entropy.

7. 6-7
CELLS AND ENTROPY

8. METABOLIC REACTIONS AND
ENERGY TRANSFORMATIONS
! Metabolism is the sum of all the chemical
reactions that occur in a cell.
! Reactants are substances that participate in a
reaction; products are substances that form as a
result of a reaction.
! A reaction will occur spontaneously if it increases
entropy.
! Biologists use the term “free energy” instead of
entropy for cells.
! Free energy, ∆G, is the amount of energy to do
work after a reaction has occurred.

9. 6-9
! ∆G (change in free energy) is calculated by
subtracting the free energy of reactants from that of
products.
! A negative ∆G means the products have less free
energy than the reactants, and the reaction will occur
spontaneously.
! Exergonic reactions have a negative ∆G and energy
is released. (EXIT)
! Endergonic reactions have a positive ∆G and occur
only if there is an input of energy. (ENTER)
! Energy released from exergonic reactions is used to
drive endergonic reactions inside cells.
! ATP is the energy carrier between exergonic and
endergonic reactions.

10. ATP: ENERGY FOR CELLS
! ATP (adenosine triphosphate) is the energy
currency of cells.
! ATP is constantly regenerated from ADP
(adenosine diphosphate) after energy is
expended by the cell.
! Use of ATP by the cell has advantages:
! It can be used in many types of reactions.
! When ATP --> ADP + P, energy released is
sufficient for cellular needs and little energy is
wasted.
! ATP is coupled to endergonic reactions in such a
way that it minimizes energy loss.

11. THE ATP CYCLE

12. 6-12
COUPLED REACTIONS
! In coupled reactions, energy released by an
exergonic reaction drives an endergonic reaction.

13. COUPLED REACTIONS

14. FUNCTION OF ATP
! Cells make use of ATP for:
1. Chemical work – ATP supplies energy
to synthesize macromolecules, and
therefore the organism
2. Transport work – ATP supplies energy
needed to pump substances across the
plasma membrane
3. Mechanical work – ATP supplies
energy for cellular movements

15. METABOLIC PATHWAYS AND
ENZYMES
! Cellular reactions are usually part of a
metabolic pathway, a series of linked
reactions, illustrated as follows:
E1 E2 E3 E4 E5 E6
A " B " C " D " E " F " G
! A-F are reactants or substrates
! B-G are the products in the various reactions
! E1-E6 are enzymes.

16. ! An enzyme is a protein molecule that
functions as an organic catalyst to speed a
chemical reaction.
! An enzyme brings together specific molecules
and causes them to react.
! The reactants in an enzymatic reaction are
called the substrates for that enzyme.

17. ENERGY OF ACTIVATION
! The energy that must be added to cause
molecules to react with one another is
called the energy of activation (Eact).
! The addition of an enzyme does not
change the free energy of the reaction,
rather an enzyme lowers the energy of
activation.

18. 6-18
ENERGY OF ACTIVATION (EA)

19. ENZYME-SUBSTRATE COMPLEXES
! Every reaction in a cell requires a specific
enzyme.
! Enzymes are named for their substrates:
SUBSTRATE ENZYME
Lipid Lipase
Urea Urease
Maltose Maltase
Ribonucleic acid Ribonuclease

20. ! Only one small part of an enzyme, called the
active site, complexes with the substrate(s).
! The active site may undergo a slight change in
shape, called induced fit, in order to
accommodate the substrate(s).
! The enzyme and substrate form an enzyme-
substrate complex during the reaction.
! The enzyme is not changed by the reaction,
and it is free to act again.

21. ENZYMATIC REACTION

23. INDUCED FIT MODEL

24. FACTORS AFFECTING ENZYMATIC
SPEED
! Enzymatic reactions proceed with great speed
provided there is enough substrate to fill active
sites most of the time.
! Enzyme activity increases as substrate
concentration increases because there are more
collisions between substrate molecules and the
enzyme.

25. TEMPERATURE AND PH
! As the temperature rises, enzyme activity
increases because more collisions occur
between enzyme and substrate.
! If the temperature is too high, enzyme
activity levels out and then declines
rapidly because the enzyme is
denatured.
! Each enzyme has an optimal pH at which
the rate of reaction is highest.

26. RATE OF AN ENZYMATIC REACTION
AS A FUNCTION OF TEMPERATURE
AND PH

27. ! A cell regulates which enzymes are present or
active at any one time.
! Genes must be turned on or off to regulate the
quantity of enzyme present.
! Another way to control enzyme activity is to
activate or deactivate the enzyme.
! Phosphorylation is one way to activate an
enzyme.

28. ENZYME INHIBITION
! Enzyme inhibition occurs when an active
enzyme is prevented from combining with its
substrate.
! When the product of a metabolic pathway is in
abundance, it binds competitively with the
enzyme’s active site, a simple form of
feedback inhibition.
! Other metabolic pathways are regulated by the
end product binding to an allosteric site on
the enzyme.

29. 6-29
FEEDBACK INHIBITION

32. 6-32
ENZYME COFACTORS
! Presence of enzyme cofactors may be
necessary for some enzymes to carry out their
functions.
! Inorganic metal ions, such as copper, zinc, or
iron function as cofactors for certain enzymes.
! Organic molecules, termed coenzymes, must
be present for other enzymes to function.
! Some coenzymes are vitamins.

33. 6-33
OXIDATION-REDUCTION AND THE
FLOW OF ENERGY
! Oxidation is the loss of electrons and
reduction is the gain of electrons.
! Because oxidation and reduction occur
simultaneously in a reaction, such a
reaction is called a redox reaction.
! Oxidation also refers to the loss of
hydrogen atoms, and reduction refers to
the gain of hydrogen atoms in covalent
reactions in cells.

34. ! These types of oxidation-reduction, or redox,
reactions are exemplified by the overall
reactions of photosynthesis and cellular
respiration.
! The two pathways of photosynthesis and
cellular respiration permit the flow of energy
from the sun though all living things.

35. 6-35
PHOTOSYNTHESIS
! The overall reaction for photosynthesis can be
written:
6CO2 + 6H2O + energy ---> C6H12O6 + 6O2
! During photosynthesis, hydrogen atoms are
transferred from water to carbon dioxide, and
glucose is formed.
! Water has been oxidized; carbon dioxide has
been reduced.
! Energy to form glucose comes from the sun.

39. 6-39
CELLULAR RESPIRATION
! The overall equation for cellular respiration is
opposite that of photosynthesis:
C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy
! In this reaction, glucose is oxidized and oxygen
is reduced to become water.
! The complete oxidation of a mol of glucose
releases 686 kcal of energy that is used to
synthesize ATP.

40. GLYCOLYSIS
Citric Acid
Cycle

41. ELECTRON TRANSPORT CHAIN

43. 6-43
ORGANELLES AND THE FLOW OF
ENERGY
! During photosynthesis, chloroplasts
capture solar energy and use it to convert
water and carbon dioxide into
carbohydrates that provide food for other
living things.
! Cellular respiration, the breakdown of
glucose into carbon dioxide and water,
occurs in mitochondria.
! It is the cycling of molecules between
chloroplasts and mitochondria that allows
a flow of energy from the sun through all
living things.

44. 6-44
RELATIONSHIP OF CHLOROPLASTS
TO MITOCHONDRIA

45. 6-45
CHAPTER SUMMARY
! Two laws of thermodynamics state that energy
cannot be created or destroyed, and energy
transformations result in a loss of energy, usually
as heat.
! As a result of these laws, we know the entropy of
the universe is ever increasing, and that it takes
energy to maintain the organization of living
things.

46. 6-46
! Metabolism refers to all the chemical reactions in the
cell.
! Only reactions with a negative free energy occur
spontaneously.
! Endergonic reactions are thus coupled with exergonic
reactions.
! Energy is stored in cells in ATP molecules.
! Metabolic pathways are a series of enzyme-catalyzed
reactions.

47. 6-47
! Each reaction requires a specific enzyme.
! Substrate concentration, temperature, pH, and
enzyme concentration affect the rates of reactions.
! Most metabolic pathways are regulated by feedback
inhibition.
! Photosynthesis and cellular respiration involve
oxidation-reduction reactions and account for the flow
of energy through all living things.