Photosynthesis

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Photosynthesis

• Mrs. Forbes
• Biology 1

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8-1 Energy and Life

• Energy is defined as the ability to do work.
• Nearly every activity in society depends on one
  kind of energy or another.
• When a car runs out of fuel, it runs out of
  chemical energy.
• Without electrical energy, lights, appliances,
  and computers would not work.

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• Living things depend on energy, too
• Sometimes, the need for energy is easy to see.
• It is obvious that energy is needed to run a
  marathon.
• However, there are time when that need is less
  obvious.

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• For example, after you eat a Whopper, your body
  is busy using energy to break down the food into
  compounds your body can use.
• Clearly, without the ability to obtain and use
  energy, life would cease to exist.

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Autotrophs and Heterotrophs

• Where does the energy that living things need
  come from?
• Ultimately, all energy comes from the sun.
• Plants and some other organisms are able to use
  the light energy from the sun to produce food.
• Plants are called autotrophs autoself troph
  food

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• An autotroph is an organism that makes food for
  itself.
• Other organisms, such as animals, cannot use the
  suns energy directly.
• A heterotroph is an organism that needs to eat
  other organisms for food. hetero other
  troph food
• A leopard is a heterotroph it obtains energy by
  eating other organisms.

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Heterotrophs

• Organisms that cannot use the suns energy
  directly are heterotrophs. They must consume
  their food. Examples Impalas (herbivores) eat
  grasses , leopards (carnivores) eat impalas,
  fungi (decomposers or saprophytes) eat dead
  stuff.
• To live, all organisms, including plants, must
  release the energy in sugars and other compounds
  to accomplish their life processes (within cells
  active transport, movement of cell organelles
  along microtubules, ion pumps, etc. all need ATP
  to work).

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Chemical Energy and ATP

• Energy comes in many forms, including light,
  heat, electrical, and chemical.
• The activities of the cell are powered by
  chemical fuels.

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Chemical Energy and ATP

• All activities in your cells are powered by
  chemical fuel, and one of the main chemical fuels
  your body uses to store energy is ATP (Adenosine
  Triphosphate).
• ATP consists of a nitrogen containing compound
  called adenine, a 5-carbon sugar called ribose,
  and 3 phosphate groups.

Animation 1
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ADP

• Adenosine Diphosphate (ADP) has a structure that
  is similar to ATP but with one important
  difference ADP lacks a phosphate group
• This difference is the key to the way in which
  cells store energy.

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ADP vs. ATP

• Energy is released from ATP by removing a P
  (phosphate) group, forming ADP P.
• When a cell has energy available, it can store
  small amounts of energy by adding a phosphate
  group to ADP and store this energy as ATP.

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Battery comparison

• ATP Fully charged battery
• ADP Partially charged battery
• Again, to release energy from ATP, a cell must
  remove a P group from ATP. To store energy, a
  cell must add a P group to ADP. So the energy
  stored in ATP is released when ATP is converted
  into ADP and a phosphate group.

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• The cell is responsible for adding and
  subtracting a phosphate group or it is
  responsible for storing and releasing energy as
  needed.
• ATP is the basic energy source for all types of
  cells.
• One way cells use ATP is active transport.

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ATP and Glucose

• Most cells only have a small amount of ATP,
  enough to last for only a few seconds of
  activity.
• WHY IS THIS?
• Well
• Even though ATP is very efficient at transferring
  energy

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• ATP is only stored in small amounts, because it
  isnt very good for storing large amounts of
  energy over a long period of time.
• There are other molecules that are better for
  storing energy for example the sugar glucose.
• 1 Glucose stores over 90x the amount of chemical
  energy stored in 1 molecule of ATP!

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• This is why cells can regenerate ATP from ADP as
  needed by using the energy in glucose (and other
  carbs).

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Photosynthesis An Overview

• The study of energy capture and use begins with
  photosynthesis.
• In the process of photosynthesis, plants use the
  energy of sunlight to convert water and carbon
  dioxide into oxygen and high-energy
  carbohydrates.
• The experiments of many scientists have
  contributed to the modern understanding of the
  process of photosynthesis.

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Investigating Photosynthesis

• Research into photosynthesis began centuries ago
  with a simple question when a tiny seedling
  grows into a small tree with a mass of several
  tons, where does the trees increase in mass come
  from?
• In the 1600s, the Belgian physician Jan van
  Helmont devised an experiment to find out if
  plants grew by taking material out of the soil.

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• He determined the mass of a small pot of soil and
  a seedling, and then planted the seedling.
• He watered and observed the growth in the
  seedling for five years.
• He noticed at the end of five years that the
  plant had increased in mass to approximately
  75kg..the soil, however, was almost unchanged.

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• Van Helmont concluded that the mass of the plant
  must have come from the water because that is all
  he had added to the pot.
• Van Helmonts experiment accounts for the hydrate
  (water) portion of photosynthesis.
• What van Helmont did not realize was the carbon
  dioxide in the air made a major contribution to
  the mass of his tree
• The carbon in carbon dioxide is used to make
  sugars and other carbohydrates used in
  photosynthesis.

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Now, that was only half of the story

• More than 100 years after van Helmots
  experiment, the English minister Joseph Priestly
  performed an experiment that would give another
  insight into photosynthesis.
• Priestly took a candle, placed a glass jar over
  it, and watched as the flame gradually died out.

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• Something in the air was necessary to keep the
  flame burning, when the substance was used up,
  the flame went out.
• That substance was oxygen.
• Priestly then found if he placed a live sprig of
  mint under the jar and allowed a few days to
  pass, the candle could be re-lighted and would
  remain lighted for a while.

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• Hummmthe mint plant must have produced the
  substance required for burning.
• A Dutch scientist by the name of Jan Ingenhousz
  took Priestlys experiment one step further.
• Ingenhousz showed that the effect observed by
  Priestly only occurred when the plant was exposed
  to light.
• The result of Priestlys and Ingenhouszs
  experiments showed that light is necessary for
  plants to produce oxygen.

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The Photosynthesis Equation

• Because photosynthesis usually produces 6-carbon
  sugars (C H O ) as its final products, the
  overall equation for photosynthesis can be shown
  as follows

6
12
6
6CO 6H O ---gt C H O 6O
2 2 6 12
6 2
Carbon Water ---gt Sugar
Oxygen
dioxide
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• Photosynthesis uses the energy of sunlight to
  convert water and carbon dioxide into oxygen and
  high energy sugars.

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Light and Pigments

• Although the equation tells you that water and
  carbon dioxide are required for photosynthesis,
  it does not tell you how plants use these low
  energy raw materials to produce high energy
  sugars.
• To answer this question, you have to know how
  plants capture the energy of sunlight.

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• In addition to water and carbon dioxide,
  photosynthesis requires light and chlorophyll.
• Chlorophyll is the green pigment inside the
  chloroplasts.
• Energy from the sun travels to the Earth in the
  form of light.
• Sunlight is white light, which is a mixture of
  different wavelengths of light.
• Many of these wavelengths are visible to your
  eyes and make up what is known as the visible
  spectrum.

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• Your eyes see the different wavelengths of
  visible spectrum as different colors.
• Plants gather the suns energy with light
  absorbing molecules called pigments.
• The plants principal pigment is chlorophyll.

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• There are two main types of chlorophyll
  chlorophyll a and chlorophyll b.
• Because light is a form of energy, any compound
  that absorbs light also absorbs the energy from
  the light.
• This light will run the process of
  photosynthesis.

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The Reactions of Photosynthesis

• We now know that ultimately all energy comes from
  the sun.
• Through the experiments of van Helmont, Priestly,
  and Ingenhousz we learned that there are certain
  requirements for photosynthesis.

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• The requirements for photosynthesis are LIGHT,
  CHLOROPHYLL, RAW MATERIALS (CO and H 0), and
  ENZYMES.
• The rate of photosynthesis depends on
• Availability of raw materials
• The intensity of sunlight
• The temperature

2 2
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• In photosynthetic organisms, photosynthesis takes
  place inside the chloroplasts.
• Lets take a look inside a chloroplast

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• Chloroplasts contain stacks of thylakoids.
• Thylakoids contain chlorophyll and other pigments
  to capture energy of the sunlight.
• Photosynthesis has two parts light dependant
  reactions and the Calvin Cycle.

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• What materials come into the chloroplast that are
  used in the light-dependant reactions?
• What materials move out of the chloroplast from
  the light-dependant reactions?
• What materials come into the chloroplast that are
  used in the Calvin cycle?

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• What materials move from the light-dependant rxn
  to the Calvin Cycle?
• What materials move from the Calvin Cycle to the
  light-dependant rxn?

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What is NADPH and how is it created?

• When sunlight shines on a leaf, it excites
  electrons in the chlorophyll.
• These high energy electrons need to move, but
  they require a special carrier.

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• A carrier molecule is a compound that can accept
  a pair of high energy electrons and transfer them
  to another molecule.
• One of these carrier molecules is NADP.
  (Nicotinamide Adenine Dinucleotide Phosphate)
• NADP accepts and holds 2 high energy e- along
  with a hydrogen ion.

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• NADP is now converted into NADPH
• NADPH can now transfer these e- to be used in the
  cell.

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Light-dependant RXNS

• The light-dependant rxns require light.
• The light-dependant rxns use energy from light to
  produce ATP and NADPH.

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What actually happens in the light-dependant rxns?

• The light reactions take place in the thylakoid
  membrane.
• Photosynthesis begins when pigments in
  photosystem II absorb light.

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Why do we begin with photosystem II and not
photosystem I?

• For the simple reason that photosystem II was
  discovered after photosystem I.

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• Energy from the light is absorbed by electrons
  and increases their energy level.
• These high energy electrons are passed along the
  electron transport chain (ETC) to photosystem I.

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As the sun continues to shine, does the
chlorophyll run out of electrons?

• NO! The thylakoid has a system that is constantly
  replacing the lost electrons. These new electrons
  come from water molecules. Enzymes break the
  water molecule into 2 e-, 2 H, and 1 oxygen atom.

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• As the electrons move through the ETC they create
  energy.
• This energy is used to pump H ions from the
  stroma into the inner thylakoid.
• Once the electrons reach photosystem I, they have
  lost their energy.
• Light energy is absorbed by photosystem I and is
  used to re-energize the electrons.

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• NADP picks up these high energy electrons and H
  ion at the edge of the thylakoid membrane and
  become NADPH.
• As a result of H ions being released during
  water-splitting and ETC, the inside of the
  thylakoid membrane becomes positively charged and
  the outside (stroma) becomes negatively charged.

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• This unequal distribution of charge provide the
  energy to make ATP.
• H ions want to move into the stroma to equal the
  distribution of charge.
• H ions are too large to pass through the
  membrane directly.
• The thylakoid membrane contains proteins called
  ATP synthase that allows H ions to pass.

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• As a H ion passes through the ATP synthase, it
  rotates.
• As ATP synthase rotates it binds ADP and P
  together to form ATP.

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The Calvin Cycle

• ATP and NADPH contain an abundance of chemical
  energy, but they can not store this energy for
  more than a few seconds.
• The purpose of the Calvin Cycle is to use the
  energy that ATP and NADPH contain to build
  high-energy sugars that can be stored for a long
  time.

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• The Calvin Cycle is named after Melvin Calvin,
  the scientist who discovered this cycle.
• The Calvin Cycle begins with 6 carbon molecules
  from the atmosphere.
• These 6 carbon molecules combine with 6 five
  carbon molecules, the result is 12 three carbon
  molecules.

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• The 12 three carbon molecules are converted into
  high energy forms by ATP and NADPH.
• Two of these three carbon molecules are used to
  make a six carbon molecule.
• The ten remaining three carbon molecules are
  converted back into 5 carbon molecules and are
  used in the next cycle.

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Take a look at figure 8-11 on page 238

• The Calvin cycle uses 6 molecules of carbon
  dioxide to produce 1 six carbon sugar molecule.
• As the Calvin Cycle turns, it uses ATP, NADPH,
  and carbon from the atmosphere to make high
  energy sugar molecules.

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Chemical Pathways

• How much energy is actually present in food?
• Actually, quite a lot!
• The sugar glucose when burned, in the presence of
  oxygen, releases heat energy.
• Cells dont burn glucose, instead they
  gradually release the energy from the glucose and
  other food compounds.

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• This process begins with a pathway called
  GLYCOLYSIS.
• Glycolysis can happen in the presence of oxygen
  or not in the presence of oxygen.
• If there is oxygen present, the process is called
  AEROBIC RESPIRATION.
• If there is no oxygen present, the process is
  called ANAEROBIC RESPIRATION.
• Glycolysis itself releases little energy.

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Overview of Cellular Respiration

• In the presence of oxygen, glycolysis is followed
  by the Krebs Cycle and the ETC.
• These three processes make up what is called
  CELLULAR RESPIRATION.

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• The equation for cellular respiration is
• 6O C H O -gt 6CO 6H O energy
• Cellular respiration is the process that releases
  energy by breaking down molecules in the presence
  of oxygen.

2 6 12 6 2
2
Oxygen Glucose -gt carbon dioxide water
energy
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Take a look at Figure 9-2, pg. 252

• Where does the glucose used in respiration come
  from?
• Cells obtain glucose mainly by breaking down
  carbohydrates such as starch.
• How do you know that this series of reactions
  occurs in the presence of oxygen?
• Because if there were no oxygen present then
  anaerobic respiration would occur.

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• What does glycolysis supply to the Krebs Cycle
  and to the electron transport chain?
• It supplies pyruvic acid to the Krebs Cycle and
  high energy e- carried by NADH to the ETC.
• What stages of cellular respiration occur in
  mitochondria?
• Krebs Cycle and the ETC.

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Glycolysis

• First step in cellular respiration
• Glycolysis takes one molecule of glucose (six
  carbon sugar) and breaks it in half (two three
  carbon molecules) of pyruvic acid.
• As the process begins, two molecules of ATP are
  used.
• However, the process makes 4 ATP molecules.

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• The net total of ATP made during glycolysis is 2
  ATP.
• Also, during glycolysis, 4 high energy electrons
  are removed and passed to an electron carrier
  called NAD.
• Just like NADP in photosynthesis, NAD can only
  accept 2 electrons per molecule.
• Therefore, we need two NAD molecules to hold the
  four high-energy electrons.

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• Once NAD accepts the electrons it become NADH.
• Two NADH are made during glycolysis.
• Take a look at figure 9-4, pg 255

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Fermentation

• When oxygen is not present, glycolysis is
  followed by a different pathway.
• This pathway is called FERMENTATION.
• Fermentation releases energy from food molecules
  in the absence of oxygen.

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• During fermentation, 2 NADH are converted back
  into NAD by passing 2 electrons back to pyruvic
  acid.
• There are 2 types of fermentation alcohol and
  lactic acid fermentaion.
• Alcohol fermentation uses yeast and
  microorganisms to create ethyl alcohol from
  pyruvic acid.
• The products of alcohol fermentation are carbon
  dioxide and alcohol.

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• Lactic Acid fermentation produces lactic acid.
• Lactic acid is produced in your muscles during
  rapid exercise.
• An accumulation of lactic acid is what makes your
  muscles sore.
• Take a look at figure 9-8, pg. 263

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The Krebs Cycle and ETC

• Only a small amount of energy is created during
  glycolysis.
• 90 of the energy is still available in the
  pyruvic acid molecules.
• The pathways of cellular respiration are aerobic.
• Oxygen, one of the most powerful electron
  acceptors, is needed for the final steps of
  cellular respiration.

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Krebs Cycle

• The Krebs Cycle is named after Hans Kreb, who
  demonstrated it existence.
• During the Krebs Cycle, pyruvic acid is broken
  down into CO2 in a series of energy-extracting
  reactions.
• Because citric acid is the first compound formed,
  this cycle is also called the citric acid cycle.
• The Krebs Cycle takes place in the mitochondria.

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Take a look at figure 9-5, pg. 257

• 1 carbon from pyruvic acid becomes part of a
  molecule of CO2
• The 2 carbons that are left join with a compound
  called coenzyme A and become acetyl-CoA.
• The 2 carbons in acetyl-CoA join a 4 carbon
  molecule producing a 6 carbon molecule called
  citric acid.

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• As the cycle turns, citric acid is broken down
  into a 5-carbon molecule and then a 4-carbon
  molecule.
• As citric acid is broken down, 2 molecules of CO2
  are made and electrons join electron carrier
  molecules.

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• For each turn of the cycle, 5 pairs of
  high-energy electrons are captured by 5 carrier
  molecules 4 NADH and 1 FADH2
• For every molecule of pyruvic acid that enters
  the Krebs cycle, one molecule of ATP is made.

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What happens to each of these Krebs Cycle
products?

• The CO2 released is the sources of all the CO2 in
  your breath.
• The ATP is used for cellular activities.
• The electron carrier molecules transport
  electrons to the ETC where large amounts of ATP
  are made.

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ETC

• The electron transport chain uses the high-energy
  electrons from the Krebs Cycle to convert ADP to
  ATP.
• The ETC is located in the inner membrane of the
  mitochondria and is made up of a series of
  carrier proteins.

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• The high-energy electrons are passed from one
  carrier protein to the next.
• At the end of the ETC is an enzyme that combines
  the electrons with hydrogen ions and oxygen to
  form water.
• Take a look at figure 9-6, pg. 259

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• As the electrons move down the ETC, hydrogen ions
  are transported into the inter-membrane space.
• This build up of hydrogen ions causes one side of
  the membrane to be positively charged and the
  other side negatively charged.
• Just like the ETC in photosynthesis, a special
  enzyme called ATP synthase is found at the end of
  the ETC.

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• The job of ATP synthase is to transport hydrogen
  ions back across the membrane.
• As ATP synthase does this, ADP is converted into
  ATP.
• Each pair of electrons that pass through the ETC
  provides enough energy to convert 3 ADP into 3
  ATP.

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ATP Totals
Anaerobic
Aerobic
Glycolysis 2 Fermentation 0 Net Total
2 ATP Total 4 ATP
Glycolysis 2 Krebs 2 ETC
32 Net Total 36 ATP Total 38 ATP
___-2 36 ATP