Download Energy - Doctor Jade Main

Energy
Energy is the capacity to perform work. The First Law of
Thermodynamics tells us that energy cannot be destroyed nor created but
can be converted from one form into another. Energy flows into the
ecosystem as sunlight. This kinetic energy is transformed into chemical
energy, in the form of food and fuels by photosynthesis. Animals consume
the food products to provide ATP or energy for cells to perform work. Cells
release energy by cellular respiration. In this process oxygen is used to
convert the chemical energy stored in fuel molecules (glucose) to chemical
energy (ATP) the cell can use to carry on its processes.
ATP is the molecule that provides energy for cellular work. ATP consists
of adenine, a nitrogenous base, ribose, a five carbon sugar and three PO4
groups. Phosphate bonds are unstable in this molecule and can be easily
broken by hydrolysis. Each PO4 released from ATP yields 7Kcal of energy.
When one phosphate group is removed ATPADP + pi + 7Kcal of energy.
When another phosphate is removedAMP + pi + 7Kcal of energy. PO4
groups don’t just fly off; they are transferred to other molecules. This
transfer is called phosphorylation.
Photosynthesis
Photosynthesis is the process by which green plants and some organisms
use light energy to convert CO2 + water glucose. Glucose is the basic
energy source for virtually all organisms. Plants use much of this glucose as
energy to build leaves, flowers, fruits, and seeds. They are referred to as
autotrophs or self feeders. Most plants produce more glucose than they can
use and store it as starch and other carbohydrates. Humans and other animals
depend on glucose as an energy source. They cannot produce it. All living
things that do not carry out photosynthesis are consumers or heterotrophs
meaning other feeders. Energy is obtained by eating plants or animals that
have eaten plants.
An important byproduct of photosynthesis is O2. Humans and other
animals breathe in oxygen and use it in cellular respiration. Humans are also
dependent on ancient products of photosynthesis, the fossil fuels of natural
gas, coal, and petroleum for modern industrial energy. Virtually all life on
Earth, directly or indirectly, depends on photosynthesis as a source of food,
energy, and O2.
Photosynthesis requires, CO2 and water. CO2 enters plants via pores
called stomata in the leaves. Water is absorbed by roots from the soil and
sent to the leaves via a series of veins. Membranes in the chloroplasts
(chlorophyll containing organelles) provide the sites for the reactions of
photosynthesis.
Photosynthesis is a redox process. The equation is: 6CO2 + 12H2O
C6H12O6 + 6O2 +6H2O in the presence of light. In a redox reaction there
must be both an oxidation and a reduction. In photosynthetic reactions
water is oxidized, that is it loses electrons and hydrogen ions while carbon
dioxide is reduced to sugar as it gains electrons and hydrogens. These
reactions proceed in an uphill fashion. As water is oxidized and CO2 is
reduced electrons gain energy by being boosted up an energy hill. Light
energy causes electrons in chlorophyll and other light-trapping pigments to
boost electrons up and out of their orbit. Hydrogens along with electrons are
transferred to CO2sugar and require that H2O is split into H and O2. O2
escapes to the air. Light drives electrons from H2O to NADP+ which is
oxidized NADPH which is reduced.
There are 2 stages to photosynthesis. Light-dependent reactions, in
which chloroplasts trap light energy and convert it to chemical energy,
contained in nicotinamide adenine dinucleotide phosphate (NADPH) and
ATP. Both of which are used in the second stage of photosynthesis, the
light-independent reactions or the Calvin cycle.
Light Energy for Photosynthesis
The energy of sunlight is called radiation or electromagnetic energy. This
type of energy travels as waves. The distance between 2 waves is the
wavelength. Light contains many colors, each with a defined range of
wavelengths measured in nanometers. This range of wavelengths is called
the electromagnetic spectrum. A part of this spectrum can be seen by
humans and is called visible light.
Light absorbing molecules or pigments are built into plant thylakoid
membranes. These pigments absorb some wavelengths of light and reflect
others. Plants appear green because the main photosynthetic pigment they
contain-chlorophyll, does not absorb green light. It is reflected back.
As light is absorbedenergy is absorbed. Chlorophyll A absorbs blueviolet and red light and reflects green. It participates in light reactions.
Chlorophyll B absorbs blue and orange light and reflects yellow-green
wavelengths and does not directly participate in light reactions. It serves to
broaden the range of light a plant can use by sending its absorbed energy to
chlorophyll A. Carotenoids are yellow-orange pigments that absorb bluegreen wavelengths and reflect yellow-orange. Some pass their absorbed
energy to chlorophyll A and others have a protective function.
When a pigment absorbs a photon of light one can say the pigment’s
electrons have gained energy. The electrons are excited which is an unstable
state. Electrons do not stay in this state. They rapidly fall back to their
original orbits. Pigments release absorbed energy from excited electrons to
neighboring molecules as electrons fall back to their ground state after being
excited by a photon of light.
Chlorophyll and other pigments are found clustered next to one another
in a photosystem. Energy passes rapidly from one chlorophyll pigment
molecule to another. Energy jumps from pigment to pigment until it arrives
at the reaction center of the photosystem. The reaction center contains a
chlorophyll a molecule sitting next to a primary electron acceptor. The
electron acceptor traps a light excited electron from the reaction center
chlorophyll and passes it to an electron transport chain which uses trapped
energy to make ATP & NADPH.
Light-Independent Reactions-Calvin Cycle
The energy of ATP and the electrons and hydrogen ions of NADPH made
in the light dependent reactions are used in the next stage of photosynthesisthe Calvin cycle. The Calvin cycle takes place in the stroma of the
chloroplast. Each step is controlled by a different enzyme. It is a cycle of
reactions that makes sugar molecules from CO2 and energy-containing
products of the light reaction.
Steps of the Calvin Cycle
The starting material for the light independent reactions is ribulose
bisphosphate (RuBP). The first step is carbon fixation. In this step rubisco
(an enzyme) attaches CO2 to RuBP. Next a reduction reaction takes place.
NADPH reduces 3-phosphoglyceric acid (3-PGA) to glyceraldehyde 3phosphate (G3P) with the assistance of ATP. To do this the cycle must use
carbons from three CO2 molecules. In order to complete the cycle it must
regenerate the beginning component-RuBP. For every three molecules of
CO2 fixed, one G3P molecule leaves the cycle as the product of the cycle
and the remaining five G3P molecules are rearranged using ATP to make
three RuBP molecules. The regenerated RuBP is used to start the Calvin
cycle again. The process occurs repeatedly in each chloroplast as long as
CO2, ATP, and NADPH are available. The thousands of glucose molecules
produced in this reaction are processed by plants to produce energy in the
process known as aerobic respiration, used as structural materials, or stored.
Cellular Respiration
The products of photosynthesis are used in cellular respiration where
oxygen is consumed (an aerobic process) as glucose is broken into CO2 and
H2O. Respiration means breathing but that is not it’s meaning in this context
although the processes are related. Cellular respiration refers to an
exchange of gases. In cellular respiration O2 is taken from the environment
and CO2 is released.
The purpose of cellular respiration is to provide ATP for cellular work.
The process is an oxidation. It requires oxidization of food molecules, like
glucose, to CO2 and water. The overall equation is 6C6H12O2 + 6O2   
6CO2 + 6H2O + ATP. Energy is released trapped in the form of ATP to be
used for all energy-consuming activities of cells.
During this process electrons are transferred from sugar molecules to O2
making H2O. You do not see any electrons transferred in the equation above.
But you can see changes in H ions. Glucose molecules lose hydrogen atoms
as it is converted to CO2 while O2 gains hydrogen atoms to form water.
These hydrogen movements represent electron transfers because each
hydrogen atom consists of one electron and one proton. Electrons move
along with hydrogens from glucose to O2. Energy is released in the process.
Oxidation-Reduction Reactions/Role of Coenzymes
Chemical reactions involved in the transfer of hydrogens are oxidationreduction reactions or Redox reactions because for one thing to be oxidized
another must be reduced. Oxidation refers to the combing of O2 with other
elements. O2 combines with CCO2 +H20 + energy in the form of heat and
light. Oxidation also occurs when H+ atoms are removed from compounds.
Oxidized things lose electrons. They move to substances that more strongly
attract them. The loss of electron is called oxidation. Glucose is oxidized. It
loses electrons to O2. One cannot have an oxidation reaction without also
having a reduction reaction. O2 is reduced. It gains electrons from glucose.
When electrons change partners energy is released. When an electron is
lost or a substance is oxidized, it looses energy. When an electron is gained a
compound is reduced and it gains energy. As food fuels are oxidized, they
loose energy. That energy is transferred to other moleculesATP. These
reactions are catalyzed by enzymes which require coenzymes. Enzymes
cannot accept H atoms therefore coenzymes act as reversible hydrogen or
electron acceptors and are reduced each time a substrate is oxidized. NAD+
or nicotinamide adenine dinucleotide and FAD or flavin adenine
dinucleotide are the major coenzymes used in cellular respiration.
Oxidation of food energy in the body must be done slowly. There is a step
by step removal of pairs of H+ s or pairs of electrons from substrate
molecules.
Phases of Cellular Respiration
Cellular respiration consists of three stages: glycolysis, Kreb’s cycle and
electron transport chain. These three are related and occur in the order
shown above. Glycolysis begins respiration by breaking down glucose into
two molecules of pyruvic acid; a process that takes place in the cytosol of a
cell and may or may not use oxygen. The Kreb’s Cycle or citric acid cycle is
the complete oxidation of pyruvic acid to CO2 and water and occurs in
mitochondria. The electron transport chain or oxidative phosphorylation
also takes place in mitochondria. NADH and FADH2 made in the citric acid
cycle shuttle their electrons to the electron transport chain where ATP is
produced by oxidative phosphorylation.
Anaerobic Respiration-Glycolysis
The first step of cellular respiration is glycolysis when the six carbon
glucose is split into two 3-carbon molecules of pyruvic acid. Glycolysis is
sometimes termed anaerobic respiration because it does not need oxygen.
The process occurs in all living organisms.
Stages of Glycolysis
The entire process of glycolysis can be broken into several stages each
with several reactions. Stage 1 is a preparatory phase. It consumes energy. In
this stage ATP is used to energize a molecule of glucose which is then split
into two small sugars that are primed to release energy. This is sometimes
termed the sugar activation or the energy investment phase. It uses 2 ATPs
and provides activation energy needed to prime later states.
Glucose diffuses into cells and is phosphorlyated + ATP catalyzed by
hexokinase Glu6PO4 + ADP. In this reaction a PO4 group is added.
Glucose 6 Phosphate is isomerizedfructose 6 Phosphate. Fru6P + ATP 
fructose-1,6-disphosphate (Fru 1,6diP). Fru 1,6 diP + Aldolase (enzyme)2,
3-carbon products glyceraldehyde-3-PO4-.
Glyceraldehyde-3 phosphate enters the second stage of the process which
yields an energy payoff. Glyceraldehyde-3-P dehydrogenase catalyzes the
NAD+ dependent oxidation of glyceraldehyde 3P 1,3 diphosphoglycerate
+ 2 NADH. In this reaction an H+ is removed and picked up by NAD+
NADH + H+. The NADH will continue on to the electron transport chain.
In the third and last stage of glycolysis ATP and pyruvate are produced.
1,3 diphosphoglycerate + ADP3 phosphoglycerate + 2 ATPs. 3
phosphoglycerate2 phosphoglycerate. 2 phosphoglycerate splits off water
 phosphoenolpyruvate. Phosophoenolpyruvate + ADP2 pyruvate +
2ATPs. The final products of glycolysis: 2 pyruvic acids-C3H4O3 + 4 ATP +
2 NADH + H+ + 2H2O. In total 4 ATPs are made. Remember the net ATP
produced is two because for each molecule of glucose two ATPs are
consumed.
Pyruvate is an important branch point in glucose metabolism. Its fate
depends on oxygen availability. Glycolysis requires NAD+ which must be
regenerated from NADH to maintain the pathway. During aerobic glycolysis
when oxygen is present electrons of cytoplasmic NADH are transferred to
mitochondrial carriers of the oxidative phosphorylation pathway generating
a continuous pool of cytoplasmic NAD+ and glycolysis continues. When
there is not enough oxygen, NAD+ is regenerated by converting pyruvate to
lactic acid via lactate dehydrogenase. When O2 is available lactic
acidpyruvic acid oxidationenters aerobic pathways-Krebs cycle &
electron transport chain.
Anaerobic metabolism is limited by the buildup of lactic acid which
begins in minutes. A build up of lactic acid disrupts the acid base balance in
the body. It degrades athletic performance by impairing muscle cell
contraction and produces physical discomfort. Glycolysis is used only for
short bursts of high level activity lasting several minutes at most. This
process cannot supply ATP for longer, endurance activities.
Citric Acid Cycle
A specific mechanism transports pyruvate into mitochondria where
aerobic processes occur. Inside the mitochondrion, a multi-enzyme complexpyruvate dehydrogenase converts pyruvateacetyl CoA, a two carbon
metabolite. This is a major branch point in metabolism. Acetyl CoA can be
converted into fatty acids, amino acids or made into energy.
The Krebs cycle was named for its discoverer Hans Krebs. It is often called
the citric acid or the tricarboxylic acid cycle. The process requires oxygen
and therefore is termed aerobic and occurs in mitochondria. The cycle
begins and ends with oxaloacetate. The two carbons of acetyl CoA’s
condense with the four carbons of oxaloacetate forming a six carbon
molecule called citrate. This is the first substrate of the citric acid cycle. The
cycle continues around several successive steps in which atoms of citric acid
are rearranged producing different intermediates called keto acids.
In summary there are two decarboxylations & four oxidations per turn of
the cycle. Two CO2, 3 NADH, 1 FADH2, and 1 GTP are produced for each
acetyl CoA that enters the cycle. NADH & FADH2 are used to carry
electrons to the electron transport chain, where more energy can be
harvested from them via further oxidization reactions.
Oxidative Phosphorylation/Electron Transport Chain
The final stage of cellular respiration is oxidative phosphorylation which
involves the electron transport chain, chemiosmosis and takes place on the
inner mitochondrial membrane. Oxygen is required. Unlike anaerobic ATP
production oxidative phosphorylation produces a tremendous amount of
energy. It is the primary method of energy production. The electron
transport chain transfers pairs of electrons from entering substrates to
oxygen. Oxidative phosphorylation captures free energy released during
electron transport and couples it to the rephosphorylation of ADP to make
ATP.
Electron Transport Chain
The electron transport chain consists of a system of electron carriers
embedded or built into the inner membrane of the mitochondria. Many
compounds making up the electron transport chain belong to a special group
of chemicals called cytochromes. Electrons captured in NADH and FADH2
are transported or passed from one compound to the next in the electron
transfer chain. As electrons are transferred energy is harvested and stored by
forming ATP.
Carriers pass electrons from one compound to the next via a series of
redox reactions. First the membrane accepts electrons and donates them to
the next acceptor with each transfer resulting in the release of a great deal of
energy. Pumping of electrons (H+) back and forth across the mitochondrial
membrane is key to this process. Hydrogen ions are pumped from where
they are less concentrated to where they are more concentrated. This
hydrogen ion gradient stores potential energy.
ATP synthases built into the inner mitochondrial membrane make ATP
as hydrogen ions are driven back across the membrane by the energy of the
concentration gradient. Since the membrane is not permeable to hydrogen
ions they can only cross via a channel in the membrane. This channel is
through the ATP synthase molecule. This process is called chemioosmosis.
The synthase attaches phosphate groups to ADPs producing ATP. The last
step in the electron transport chain occurs when used up electrons, along
with spare hydrogen ions combine with O2, the final acceptor, to form water.
The transfer of electrons from NADH + H+ to O2 releases a large amount of
energy. It is an exergonic reaction. Energy must be released in small steps
so not to loose all energy as heat.
Energy Yield
All of the steps of glucose oxidation are interrelated. Glycolysis makes
pyruvic acid which feeds into the citric acid cycle which makes reducing
equivalents (NADH & FADH2) that enter the electron transport chain.
Electrons captured in NADH and FADH2 are transported or passed from
one compound to the next in the electron transport chain and energy is
harvested and stored by forming ATP. For each molecule of NADH
approximately 3 molecules of ATP formed, and for each molecule of
FADH2, about 2 molecules of ATP are formed.
Glycolysis produces 4 ATPs, 2 of which are used in the reaction for a net of
2 ATP. It makes 2 NADH which in the electron transport chain yield 6
ATPs.
The citric acid cycle produces 2 GTP (ATP equivalents) and 8
NADH24 ATPs and 2 FADH24 ATPs resulting in 38 ATPs. The
complete oxidation of 1 mole of glucose to CO2 and H2O yields a total of 38
ATP molecules.
Fermentation
Another way certain cells harvest energy from food is fermentation.
During fermentation pyruvic acid made by glycolysis is turned into a waste
product and two ATP molecules. Formation of the waste product supplies
NAD+ needed to carry out glycolysis. Two of the most common types of
fermentation are lactic acid fermentation and alcohol fermentation. They are
named for the waste products they produce that is either lactic acid or
ethanol and CO2.