Lecture 5
Energy Conversion
in Eukaryotic cells
1. Introduction to Metabolism.
2. Forms of Energy.
3. Laws of thermodynamics.
4. Photosynthesis.
5. Cellular Respiration
Prepared by Mayssa Ghannoum
Overview

The living cell is a chemical factory in miniature, where
thousands of reactions occur within a microscopic space.

Small molecules are arranged into polymers, which may
be hydrolyzed later as the needs of the cell change.

In multicellular organisms, many cells export chemical
products that are used in other parts of the organism.
Introduction to Metabolism
 The
totality of an organism’s chemical reactions is called
metabolism.

Metabolism is an emergent property of life that arises from
interactions between molecules within the cell.
 A metabolic
pathway begins with a specific molecule, which
is then altered in a series of defined steps, resulting in a certain
product.
 Each
step of the pathway is catalyzed by a specific enzyme:
 The
metabolic pathway can be either catabolic or anabolic.
 Catabolic
pathway: is the process that release energy by
breaking down complex molecules to simpler ones.
Example: cellular respiration.
 Anabolic
pathway: consumes energy to build complicated
molecules from simpler ones.
Example: the synthesis of proteins from amino acids.
 Energy
 Energy
is fundamental to all metabolic processes.
released from reactions of catabolic pathways can be
stored and then used to drive reactions of anabolic pathways.
Forms of energy
 Energy
is the capacity to cause change.
 Energy
exists in various forms, and the work of life depends
on the ability of cells to transform energy from one form into
another.
 Kinetic
energy: it’s associated with the relative motion of
objects.
Thermal energy: it’s the heat, energy associated with the
random movement of atoms or molecules.
Potential energy: it is energy that matter possesses because of
its location or structure.
Chemical energy: the energy released in chemical reactions.
Laws of thermodynamics
 The
study of the energy transformations that occur in a
collection of matter is called thermodynamics.
 Organisms,
are open systems that absorb energy for
instance, light energy or chemical energy in the form of
organic molecules- and release heat and metabolic waste
products, such as CO2, to the surroundings.
 Two
laws of thermodynamics govern energy transformations
in organisms and all other collections of matter.
The First Law of Thermodynamics
 According
to the first law of thermodynamics, the energy
of the universe is constant.
 Energy
can be transferred and transformed, but it cannot
be created or destroyed.
 This
first law is also known as
the principle of conservation of energy.
 By
converting sunlight to chemical energy, a plant acts as
an energy transformer, not an energy producer.
The second law of
thermodynamics
The Second Law of Thermodynamics
 Every
energy transfer or transformation increases the
disorder (entropy) of the universe.
 Scientists use a quantity called entropy as a measure of
disorder, or randomness.
 We know that certain events occur spontaneously and
others do not.
 A process that cannot occur on its own is said to be
nonspontaneous.
 In fact, another way to state the second law is:
For a process to occur spontaneously, it must increase the
entropy of the universe.
Exergonic and Endergonic reactions in Metabolism
 Based
on their free-energy changes, chemical reactions can
be classified as:
Exergonic (energy outward)
Endergonic (energy inward)
 An
exergonic reaction proceeds with a net release of free
energy. It occurs spontaneously.
P.S: the word spontaneous does not imply that a reaction
will occur instantaneously or even rapidly.
 An
endergonic reaction absorbs free energy from its
surroundings. It stores free energy in molecules.
The structure and hydrolysis of ATP
 ATP (adenosine
triphosphate) is the molecule that releases
free energy when its phosphate bonds are hydrolysed. This
energy is used to drive endergonic reactions in cells.
 It
contains the sugar ribose, with the nitrogenous base
adenine and a chain of three phosphate groups bonded to it.
 The
bonds between the phosphate groups of ATP can b
broken by hydrolysis.
ATP + H2O
ADP + Pi + Energy
this reaction is exergonic and releases free energy.
ATP powers cellular work

A cell does three main kinds of work:
1. Chemical work
2. Transport work
3. Mechanical work

Energy coupling: is the use of an exergonic process to drive
an endergonic one.

ATP is responsible for mediating most energy coupling in
cells, and in most cases it acts as the immediate source of
energy that powers cellular work.
Enzymes speed up metabolic reactions
 A spontaneous
chemical reaction occurs without any
requirement for outside energy, but it may occur so slowly.
So addition of enzymes make it occur in seconds.
 An
enzyme is a macromolecule, protein that acts as a catalyst,
a chemical agent that speeds up a reaction without being
consumed by the reaction.
 The
activation energy barrier: in a chemical reaction, the
energy necessary to break the bonds of the reactants is the
activation energy, Eª
It is the enough energy, reactants must absorb to attain the
transition state.

Enzymes catalyze a reaction by lowering the
activation energy barrier, enabling the reactant
molecules to absorb enough energy to reach the
transition state even at moderate temperature.

Enzymes are very specific for the reactions they
catalyze, they determine which chemical process
will be going on in the cell at any particular time.
 The reactant an enzyme acts on is referred to as the
enzyme’s substrate.
 Only a restricted region of the enzyme molecule
binds to the substrate, it’s called the active site.
 The enzyme binds to its substrate forming an
enzyme-substrate complex
Catalysis in the Enzyme’s Active site
Effects of local Conditions on Enzyme Activity

The activity of an enzyme is affected by general
environmental factors such as:
Temperature and pH ( each enzyme has its optimal
temperature and pH)
2. Cofactors
3. Enzyme inhibitors
4. Allosteric regulation: Activation or inhibiton
5. Specific localization of Enzymes within the cell.
1.
Photosynthesis
Photosynthesis: conversion of sunlight energy to
chemical energy stored in sugar.
Autotrophs: self-feeders organisms, produce their
own organic molecules (producers).
Heterotrophs: organisms that live on compounds
produced by other organisms (consumers).
6CO2+6H2O-----light,chlorophyll----→ C6H12O6+6O2
Carbon dioxide+ water------→ sugar+ oxygen
Chloroplasts: the site of photosynthesis
 Chloroplast:
found in all plant cells, but leaves are
the major site where photosynthesis occur
 Chlorophyll: is the green pigment which gives the
leaves their color.
 The chlorophyll is in the membranes of thylakoids
(connected sacs in the chloroplast).
 Chloroplast are found mainly in the cells of
Mesophyll (the tissue in the interior of a leaf).
Figure 10.3, page
187; Zooming in
on the location of
photosynthesis in
a plant mesophyll
cell.
Stages of Photosynthesis
Two stages are involved in photosynthesis process:
1- Light reaction
2- Calvin cycle
1-Light reaction
 Light absorbed by chlorophyll drives a transfer of
electrons and hydrogen ions from water to an acceptor
called NADP+ (nicotinamide adenine dinucleotide
phosphate); which temporarily stores the energized
electrons and will act as electron carrier in cellular
respiration).
 The
•
•
•
•
light reactions (in the thylakoids):
Split H2O
Release O2
Reduce NADP+ to NADPH
Generate ATP from ADP by
phosphorylation adding phosphate group
(Pi) to ADP(adenosine diphosphate).
Figure 10.5; an overview of photosynthesis: cooperation of the light reactions and
the Calvin cycle.
2-Calvin Cycle
 A cycle
reaction is where the starting material is
regenerated after molecules enter and leave the cycle, it’s
considered anabolic cycle which build sugar from smaller
molecules and consumes energy.

At Calvin cycle carbon enters in the form of CO2 and
leaves in the form of sugar.
 Carbon
fixation: CO2 is incorporated from air and
attached into organic molecule found in the chloroplast.
 Reduction : carbon is reduced to carbohydrate by addition
of electron from NADPH, and requires energy from ATP
(which is produced from the light reaction).
Cellular Respiration
 Respiration:
is a process in which organisms breathe
oxygen and excrete CO2 and water.
 Aerobic
respiration: when O2 is consumed as a
reactant along with organic fuel.
 Anaerobic
respiration: No O2 is used, other
substances are reactants that are harvesting chemical
reaction.
 Cellular respiration:
includes both aerobic &
anaerobic processes where fuel breaks down by
Mitochondria generating ATP.
 Organic
compound+O2→CO2 +water + energy
The chemical elements essential to life are
recycled
Photosynthesis generates
O2 and organic molecules
used by Mitochondria as
fuel for Cellular
respiration.
 The waste products of
cellular respiration are CO2
and water which are the
raw materials for
Photosynthesis.

Figure 9.2; Energy flow and chemical recycling
in ecosystems. Page 162.
Stages of Cellular Respiration
1- Glycolysis
2- The Citric Acid
Cycle
3- Oxidative
Phosphorylation
(Electron
transport &
Chemiosmosis).
Figure 9.6; an overview of
cellular respiration. Page
166.
1-Glycolysis
 Glycolysis: means Sugar splitting
 Glucose (6Carbon sugar) splits into two 3C sugars which are
oxidized and rearranged to form two molecules of Pyruvate.
The net energy produced by glycolysis is 2ATP and 2 NADH.
2-The Citric Acid Cycle
 Or Krebs Cycle: where Pyruvate enters the Mitochondria via
active transport, and converts to compound called Acetyl coA ,
which is the step junction between Glycolysis and Citric Cycle.
 Acetyl coA is broken down to 2 CO2 molecules;
 The cycle generates 1 ATP per turn, but most chemical energy is
transferred to NAD+ and FAD (vit Riboflavin) producing
3NADH and FADH.
3-Oxidative Phosphorylation
 A- The electron transport chain: is a collection of
molecules embedded in the inner membrane of mitochondria
during chain electron carriers which alternate between reduced &
oxidized states by accepting & donating electrons. Electrons drop
in free energy as they go down the chain and are finally passed to
O2, forming H2O.
 B- Chemiosmosis: a process in which energy stored in the
form of hydrogen ion across a membrane is used to drive cellular
work, such as the synthesis of ATP using enzyme ATP Synthase.
(ADP+Pi-----ATP synthase---→ATP)
 The net ATP produced are about 32-34 ATP.
Figure 9.17 ATP yield per molecule of glucose at each stage of cellular
respiration: Since oxygen is required to complete the citric acid cycle and
oxidative phosphorylation, these two processes are known as aerobic respiration,
and generate about 32-34 ATP per glucose.
Fuels for cellular
respiration.
Carbohydrates, fats,
and proteins can all be
used as fuel for cellular
respiration.
Monomers of these
molecules enter
glycolysis or the citric
acid cycle at various
points.
Figure 9.20; the catabolism of
various molecules from food.
Fermentation and Anaerobic Respiration
 A process
where organic fuels are oxidized and generate
ATP without the use of oxygen.
 Anaerobic
respiration: has electron transport
chain, but the final electron acceptor is other than
oxygen → it is sulfide.
 Fermentation:
the electrons from NADH are passed to
pyruvate, regenerating the NAD+ required to oxidize
more glucose.
Two types: Alcohol fermentation( yeast is used in baking)
& Lactic acid fermentation (Human muscles)