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Notes from Dr. Jones BSC 1010C lecture
Chapter 9
Cellular Respiration and Fermentation
Brief review of chap. 8
•Metabolism
–All chemical reactions in an organism
–These reactions are ordered into metabolic pathways or sequences of steps
–Each step sequence is controlled by enzymes
•Enzymes convert specific molecules into simpler compounds or products
–Catabolic pathways: releases the energy stored in the complex molecules by breaking
them down into simpler compounds
–Anabolic pathways (aka biosynthetic pathways): combines simpler molecules into
complex molecules and uses energy from catabolic pathways to do this
•Energy: the ability to do work or to cause change
•Various forms of energy:
–Thermal (heat), light, mechanical, chemical, solar, atomic, sound, kinetic, potential,
gravitational potential, chemical potential
–Kinetic energy: energy of motion and can be transferred to another object
–Potential energy: stored energy with the potential to become kinetic energy or do work
–Chemical potential energy: potential energy stored in chemical bonds and released
during chemical reactions
•Energy can be converted from one form to another
–Plants convert light energy to the chemical energy in sugar; cells release this chemical
energy to use as fuel for their own use
•First law of thermodynamics
–Energy cannot be created or destroyed. The amount of energy in the universe and on the
planet is always constant.
–Principle of conservation of energy: energy can be transferred or transformed into one
form or another.
•Second law of thermodynamics
–Every energy transfer or transformation results in increasing disorder within the
universe and some of that disorder is in the form of heat.
•Cellular respiration: the process through which the body breaks down the stored
chemical energy in food into a form that is usable by cells
–Because all living organism have cells, cellular respiration occurs in all living
organisms.
–Chemical potential energy
–A complex series of chemical reactions where glucose and other fuels are broken down
to release or store energy using oxygen.
•Energy is released from molecules in a controlled manner rather than all at one time as
occurs in an autumn leaf fire.
•All of the energy in a gas tank does not come out at one time; it is released little by little.
•In the same way, cells take the energy from food and package that energy into
manageable bursts that provide just the right amount of energy for the organism’s many
activities. Cellular respiration occurs in steps or stages.
•The point of cellular respiration is to harvest electrons from organic compounds such as
glucose and use that energy to make a molecule called ATP. ATP in turn is used to
provide energy for most of the immediate work that the cell does. ATP can be thought of
as being like a small package of energy that has just the right amount of energy that can
be used in a controlled manner.
•Adenosine tri-phosphate, ATP is a nucleotide with three phosphate groups instead of
one phosphate group. It is reduced to ADP by removal of the last phosphate group.
•Nucleotides are important not only as a component of DNA but also as an important
energy carrying compound.
•An important activity of cellular respiration is to harvest chemical energy from food and
store it in the chemical bonds of ATP.
•When ATP releases energy, the energy release only involves breaking the last of three
phosphate bonds in the molecule. This results in the production of a small controlled
amount of energy that is just the right amount for most of the energy using processed of
the cell.
•Energy release since it involves breaking just the one phosphate bond means that ATP
can easily be put together by taking ADP and adding phosphate to it. Thus the products
of energy release from ATP can be recycled by the cell to make new ATP. If this sort of
cycling between ADP+Phosphate and ATP were not possible our bodies would require
huge amounts of ATP.
•Energy release is almost spontaneous. It does not take much energy for the cells to get at
the energy in that third phosphate bond in the ATP molecule. Other molecules have nore
energy, but the cell has to "spend" a lot more energy than its worth to get at the energy.
•The relationship between ADP, ATP and work done by the cell and cellular respiration
can be viewed as a cycle with the following steps:
–1. Cellular respiration makes ATP by joining phosphate groups to ADP to make the
ATP
–2. The cell uses the ATP to do work. As part of this process the chemical bond between
the 3rd phosphate group in the ATP is broken and energy is transfered to other molecules
of the cell and lost as heat.
–3. ADP and phosphate produced by the breakdown are now recycled into cellular
respiration. See step 1.
Catabolic pathways
–Produce energy by oxidizing organic fuels
•Two basic types of cellular respiration:
–Aerobic: requires oxygen
•Most prevalent and efficient catabolic pathway
•Uses oxygen to breakdown glucose or other energy rich organic compounds such as
other sugars, fats, and proteints to yield carbon dioxide and water and to release energy as
ATP and heat.
–Anaerobic: does not require oxygen (ex. Fermentation)
•Partial degradation of sugars or other organic fuels that occur without the use of oxygen
•Catabolism does not directly perform cellular work such as moving flagella, or change
monomers into polymers. It provides the energy that is then used by the cell.
•For simplicity, we will use glucose to explain the steps of cellular respiration as glucose
is the fuel that cells most often use.
•How do catabolic pathways yield energy?
Through the transfer of electrons during the
chemical reactions.
–Remember from Chapter 2, when electrons are moved from one energy level to another
energy level or from one atom to another, energy is released.
•Redox reactions: the partial or complete transfer of ome or more electrons from one
reactant to another
•oxidation
and reduction reactions occurring together
•Oxidation:
the loss of electrons from one substance; this substance becomes oxidized
and is called the electron donor or reducing agent
•Reduction:
the gaining or addition of electrons to another substance
; this
substance is the electron acceptor or oxidizing agent. It becomes reduced because the
electrons have a negative charge and decrease its positive charge.
•Oxygen strongly attracts electrons and is one of the most powerful oxidizing agents.
•As electrons shift toward a more electronegative atom, the potential energy in them
changes form. Chemical energy is released in a redox reaction that relocates electrons
closer to oxygen.
•When the electron shifts closer to an oxygen atom or a more electronegative atom, it
loses or releases chemical energy that can be used for work, or in the case of respiration,
by the cell.
•The released energy is used for ATP synthesis.
•Organic molecules, such as carbohydrates and fats, with an abundance of hydrogen are
rich in “hilltop” electrons which release their potential energy when they relocate closer
to oxygen.
•Primary steps
of cellular respiration
each glucose molecule is broken down into two molecules of the compound
pyruvate; takes place in the cytosol
–Glycolysis:
–Pyruvate
oxidation: pyruvate enters the mitochondrion and is oxidized to acetyl CoA.
–Citric
acid cycle: the breakdown of glucose to CO2 is completed heres. Acetyl CoA is
further oxidized to CO2. NADH and a coenzyme called FADH2 transfer electrons
derived from glucose to electron transport chains, which are built into the inner
mitochondrial membrane. Here they are combined with H+ and oxygen to form water
–In
prokaryotes, these chains are located in the plasma membrane.
–Oxidative
phosphorylation: The electron transport chain accepts electrons from the
breakdown products of the first two stages and passes them to one molecule after another.
At the end of the chain, the electrons are combined with molecular oxygen and hydrogen
ions where they form water. The energy released at each step of the chain is stored in a
form the mitochondrion can use to make ATP from ADP. Electron transport chains
convert the chemical energy to a form used for ATP synthesis during chemiosmosis. The
energy released in the previous chain of redox reactions is used to synthesize ATP by this
process.
Glycolysis
•Glycolysis takes place in the cytosol where the following happens:
–2 molecules of ATP are consumed as glucose is split into 2 three-carbon sugars.
–This results in the production of NADH and ATP
•The ATP Is generated from substrate-level phosphorylation which occurs when an
enzyme transfers a phosphate group from a substrate molecule to ADP. This is different
from oxidative phosphorylation in which an inorganic phosphate is added to ADP
instead.
•The NADH is generated from the reduction of NAD+ which is an electron carrier,
nucleotide, and a coenzyme. It is a derivative of the vitamin niacin. It cycles easily
between its oxidized form, NAD+, and its reduced form, NADH.
–For each molecule of glucose, glycolysis produces 2 ATP and 2 NADH and two
pyruvates.
•Enyzmes
catalyze each step in glycolysis.
transfer phosphate groups
–Dehydrogenase oxidizes the three-carbon sugars and reduces NAD+
•This enzyme captures a pair of hydrogen atoms from the glucose and delivers the 2
electrons and 1 of the protons to its coenzyme, NAD+. It releases the other proton into
the surrounding solution.
–Kinases
–Other
enzymes break the six-carbon sugar and rearranges atoms in substrate molecules
•Glycolysis is the sugar splitting phase of catabolism.
•The smaller sugars are oxidized and their remaining atoms are rearranged to form 2
molecules of a compound called pyruvate.
•All of the carbon originally present in glucose is accounted for in the 2 pyruvates.
•No carbon is released as CO2 during this phase.
•Glycolysis occurs whether oxygen is present or not.
•If O2 is present, the chemical energy stored in pyruvate and NADH can be extracted
during the next three stages
•Less than 25% of the chemical energy in glucose is released during glycolysis. Most of
the energy remains in the 2 pyruvate molecules.
Pyruvate Oxidation
•The pyruvates are transported into the mitochondria
•In a series of steps that take place within a multienzyme complex, the following
happens:
– a carboxyl group is removed from the three-carbon pyruvate and is released as CO2.
–The two-carbon group of the pyruvate that still remains is oxidized to form acetate
–NAD+ is reduced to NADH which is a storage form of energy
–Coenzyme A, a compound derived from a B vitamin, is attached by its sulfur atom to
the acetate and forms acetyl CoA
•In a nutshell, CO2 is released,NAD+ is reduced to NADH, and acetate is formed and
attached to the coenzymeA which creates acetylCoA.
•The molecule is now moved into the citric acid cycle for further oxidation
•Described in another way
–Pyruvate is a charged molecule so it is brought into the mitochondrion through active
transport by a transport protein.
–Several enzymes (the pyruvate dehydrogenase complex) catalyze the conversion of
pyruvate to acetylCoA.
–This results in
•the release of CO2
• the reduction of NAD+ resulting in the storage of energy in NADH
•the formation of acetate through oxidation of the remains of the pyruvate,
•the attachment of the acetate to a coenzymeA compound located in the mitochondrion to
form acetylCoA.
•This part of the process produces:
– CO2 which diffuses out of the cell
– NADH which stores energy in the form of electrons
–Acetyl CoA whose acetyl group will enter the citric acid cycle and the sulfur of the CoA
will attach to a Hydrogen and be reoved from the cycle.
Citric Acid cycle
•The energy yielding oxidation of organic molecules is completed here.
•Also called the Kreb’s cycle.
•This pathway is a metabolic furnace that oxidizes organic fuel derived from pyruvate.
•Occurs in mitochondrion
•GTP (guanosine triphosphate) is produced during step 5. It is a molecule that is similar
to ATP in tis structured and cellular function. It may be used to make an ATP molecule
or to directly power work in the cell.
•Step 5 is the only place in the citric acid cycle where ATP is generated.
•The citric acid cycle summarized:
–Pyruvate is broken down to 3 CO2 molecules, one of which was released during the
conversion of pyruvate to acetylCoA
–One ATP is generated but most of the chemical energy is transferred to NAD+ and an
electron carrier and coenzyme called FAD (flavin adenine dinucleotide) which is derived
from riboflavin, a B vitamin during the redox reactions. The NAD+ is reduced to NADH
and two hydrogens are transferred to FAD, forming FADH2.
–NADH and FADH2 transport their stored cargo in the form of high-energy electrons
into the electron transport chain.
•Each turn of the citric acid cycle produces:
–CO2
–Three NADH
–One FADH2
–1 ATP (GTP in most animal cells)
•It takes two turns of the citric acid cycle to oxidize a single glucose molecule.
•Compounds formed during the citric acid cycle are oxaloacetate, citrate, isocitrate,
alpha-ketoglutarate, succinyl CoA, succinate, fumarate, and malate.
•Citric acid cycle described in more detail
–Acetyl CoA enter the cycle and adds its cetyl group to oxaloacetate which produces
citrate. The CoA is released.
–Citrate is converted to isocitrate and one molecule of water is removed while another is
added.
–Isocitrate is oxidized which results in the reduction of NAD+ to NADH and in the loss
of a CO2 molecule.
–Another CO2 molecule is lost and the compound remaining is oxidized which produces
a reduction reaction in which NAD+ is reduced to NADH. The oxidized compound
molecule is then attached to a CoA in an unstable bond.
–The CoA is displaced by a phosphate group which is transferred to GDP which causes
GTP to be formed, a molecule like ATP.
–Two hydrogens are transferred to FAD, forming FADH2 which travels to the electron
transport chain
–A water molecule is added which rearranges bonds in the substrate
–The substrate is then oxidized, reducing NAD+ to NADH and regenerating oxaloacetate
–The cycle begins again for a second time to fully oxidize one glucose molecule.
Electron Transport chain
•A collection of molecules embedded in the infoldings or inner membrane of the
mitochondrial membrane or in the plasma membrane of prokaryotes
•The foldings increase the surface area thus providing space for thousand of copies of the
chain in each mitochondrion (like the looped lines at Disney)
•Most components of the chain are proteins existing in multiprotein complexes which the
other components are non proteins bound to the protein in prosthetic groups.
•The prosthetic groups are essential for the catalytic functions of certain enzymes.
•During electron transport along the chain, electron carriers alternate between reduced
and oxidized states as they accept and donate or release electrons.
• When accepting an electron from its neighbor, the chain component is reduced.
•When it donates the electron to its neighbor, it becomes oxidized again.
•Electron transport process
–Electrons from the citric acid cycle are transported by NADH to the first molecule of
the electron transport chain, a flavoprotein. Here it is transferred to the flavoprotein.
–The flavoprotein (FMN) passes the electron in a redox reaction to an iron-sulfur protein.
–The Iron sulfur protein passes the electrons to a compound called ubiquinone (CoQ or
coenzyme Q) which is the only member of the electron transport chian that is not a
protein.
•Ubiquinone is mobile within the membrane and does not reside in any one complex.
–Most of the remaining electron carriers between CoQ and oxygen are proteins called
cytochromes. Their prosthetic group, called a heme group, has an iron atom that accepts
and donates electrons.
•There are several types of cytochromes, each with a different protein and heme group.
–The last cytochrome of the chain, cyt a3, passes the electrons to oxygen which is very
electronegative thus attractive to the electrons. Each oxygen atom also picks up a pair of
hydrogen ions from the aqueous solution around it, forming water.
•What about the FADH2 from the citric cycle?
–It adds its electrons to the transport chain at a lower energy level than that of NADH.
–As a result, even thos it donates an equivalen number of electrons for oxygen reduction,
the electron transport chain provides 1/3 less energy for ATP synthesis when the electron
donor is FADH2.
•ATP is not made directly from the electron transport chain.
It proves the stair stepping
process down from food to oxygen and helps bread a large free energy drop into a series
of smaller steps that release energy in manageable amounts.
Chemiososis
•Chemiosmosis:
the process in which energy stored in the form of a hydrogen ion
gradient across a membrane is used to drive cellular work such as the synthesis of ATP
•A
process in which ATP synthase, a protein complex in the inner mitochondrial
membrane, uses the energy of a proton (H+) gradien to make ATP.
•The
flow of H+ protons down a gradient in the ATP complex causes parts of the
complex to rotate and activate catalytic sites where ADP and inorganic phosphate join to
make ATP.
•Approximately 30-32
ATPs may be produced per glucose molecule oxidized
•The
proton gradient is a result of the movement of protons down the electron chain at the
same time electrons are being moved. The protons are deposited at three sites.
•Exergonic
redox reactions produce the proton-motive force energy of the proton gradient
in animal mitochondria while chloroplasts in plants use light energy to create the proton
motive force. Prokaryotes use proton gradients generated across the plasma membrane to
transport molecules.
Anaerobic respiration and Fermentation
•Anaerobic respiration does not use oxygen as the final electron acceptor at the end of the
electron transport chain
•Some bacteria use sulfate ions to accept electrons instead
•Fermentation
generates ATP without using oxygen or an electron transport chain.
Oxygen is not the final electron acceptor.
•Glucose oxidation produces 2 ATP
•NADH is recycled to NAD+ by the transfer of electrons to pyruvate
•In lactic acid fermentation, pyruvate is reduced directly by NADH to lactate, recycling
NAD+.
•Muscle cells make ATP by lactic acid fermentation when energy demand is high and 02
supply is low.
Comparing different types of cellular respiration
•Fermentation and aerobic and anaerobic all use glycolysis with NAD+ as the oxidizing
agent to convert glucose and other organic fuels to pyruvate.
•The two
respiration processes use oxygen or another electronegative molecule as the
final electron acceptor after electrons are passed down an electron transport chain.
•Fermentation
oxidizes NADH back to NAD+ and to do this, it uses pyruvate or
acetaldehyde as the final electron acceptor. It does not use oxygen or an electron
transport chain.
–As an alternative to respiratory oxidation of organic nutrients, fermentation is actually
an extension of glycolysis that allows the continuous generation of ATP through
substrate-level phosphorylation of glycolysis. This recycling keeps the cell’s pool of
NAD+ from being totally reduced to NADH which would cause the cell to shut down for
lack of an oxidizing agent.
–Under aerobic conditions, NAD+ is recycled from NADH by the transfer of electrons to
the electron transport chain. Under anaerobic conditions, the alternative is to transfer
electrons from NADH to pyruvate, the end product of glycolysis.
•Obligate anaerobes: carry out only fermentation or anaerobic respiration and they cannot
survive in the presence of oxygen.
•Facultative anaerobes can make enough ATP to survive using either fermentation or
respiration.
–Our muscle cells behave in this manner.
•Read “evolutionary significance of glycolysis” on page 179
•Also read and be familiar with all of Section/Concept 9.6
Photosynthesis
•In photosynthesis, light energy from the sun is converted into chemical energy stored in
organic molecules.
•Autotrophs: make their own organic molecules form inorganic raw materials; they are
producers of the biosphere. Plants, alage, and some other organisms are photoautotrophs.
•Heterotrophs are consumers: they eat plants or animals or decompose organic litter but
almost all ultimately depend on photoautotrophs for food and oxygen.
•Chloroplasts
–Located mainly in the mesophyll tissue of the leaf
–CO2 enters and O2 exits the leaf through stomata
–Veins carry water from the roots to leaves and distribute sugar to non-photosynthetic
tissue
–Chloroplasts consist of a double membrane surrounding a dense fluid called the stroma.
–Thylakoids are an elaborate membrane system which enclose the thylakoid space.
Thylakoid sacs may be stacked to form grama.
–Chlorophyll, the green pigment that absorbs the light energy that drives photosynthesis
is embeddedin the thylakoid membrane.
•The two stages of photosynthesis
–1) Solar energy is converted into chemical energy in the light reactions
•Light energy absorbed by chlorophyll drives the transfer of electrons and hydrogen ions
from water to the electron acceptor NADP+, which is reduced to NADPH and
temporarily stores electrons.
•Oxygen
is released when water is split and ATP is formed during the light reactions
using chemiosmosis in a process called photophosphorylation.
–2) Calvin cycle
•CO2 is incorporated into existing organic compounds by carbon fixation and these
compounds are then reduced to form carbohydrates.
•NADPH and ATP from the light reactions supply the reducing power and chemical
energy needed for the Calvin cycle.
•The chloroplast uses light energy to make sugar by coordinating the two stages of
photosynthesis.
•How does photosynthesis happen?
–Photosynthetic pigments serves as light receptors or absorbers.
–Different pigments serve different functions and absorb light of different wave lengths
or excess sunlight (plant antioxidants).
–When a pigment molecule absorbs energy from a light photon (a discrete particle of
light that has a fixed amount of energy), one of the molecule’s electrons is elevated to an
orbital where it has more potential energy.
–It is unable to stay in this excited state and eventually drops back to its original orbital,
releasing energy as heat in the process.
–Embedded in thylakoid membranes are numerous photosystems that each have several
light-harvesting complexes and one reaction center complex.
•The reaction center complex has 2 special chlorophyll a molecules and a primary
electron acceptor.
–When a photon is absorbed by a pigment molecule in one of the light harvesting
complexes, the energy is passed from pigment to pigment until it reaches the reaction
center.
–A redox reaction occurs and an excited electron of a reaction center chlorophyll a is
captured by the primary electron acceptor before it can return to the ground state.
–Two types of photosystems in the thylakoid membrane
•Photosystem I : chlorophyll a molecule called P700
•Photosystem II: chlorophyll a molecule called P680.
•In a process called linear electron flow, electrons pass from water to NADP+ through
the two photosystems.
•A pigment molecule abosrobs a photon of light and the energy is relayed through other
pigment molecules of the light-harvesting complex to the P680 pair of chlorophyll a
moleculess in the PS II reaction center complex.
•An excited electron of P680 is trapped by the primary electron acceptor.
•P680 is a strong oxidizing agent and its electron hole is filled when an enzyme removes
electrons from water, splitting it into 2 electrons, two H+, and an oxygen atom that
immediately combines with another oxygen atom to form O2.
•The primary electron acceptor passes the photoexcited electron to an electron transport
chain made up of plastoquinone (Pq), a cytochrome complex and plastocyanin (Pc).
•The energy released as electrons fall through the electron transport chain is used to
pump protons into the thylakoid space, contributing to the proton gradient used for the
synthesis of ATP.
•At the bottom of the electron transport chain, the electron passes to P700+ in photo
system I where it replaces the photoexcited electron that was captured by its primary
electron acceptor when PSI absorbed a photon.
•This primary electron acceptor passes the electron down a second electron transport
chain through ferredoxin (Fd), from which the enzyme NADP+ reductase transfers
electrons to reduce NADP+ to NADPH.
•Calvin cycle
–Uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
–It takes 3 turns of the Calvin cycle to fix 3 molecules of CO2 and produce on molecule
of the three-carbon sugar (G3P). It does not make glucose.
–Can occur at night but typically occurs during the day to take advantage of sunlight.
–Anabolic process unlike citric cycle: it builds carbohydrates from smaller molecules and
consumes energy.
–Carbon enters in the form of CO2 and leaves in the form of sugar.
–It spends ATP as an energy source and consumes NADPH as reducing power for adding
high-energy electrons to make the sugar.
–Three phases in the cycle
•Carbon fixation
•Reduction
•Regeneration of CO2 acceptor.
•Carbon fixation: CO2 is added to a 5-carbon sugar, ribulose biphophate (RuBP) in a
reation catalyzed by the enzyme RuBP carboxylase (rubisco). The resutling unstable 6carbon intermediate splits into two molecules of 3-phosphoglycerate.
•Reduction: each molecule of 3-phosphoglycerate is then phophorylated by ATP to form
1,3-biphosphoglycerate. Two electrons from NADPH reduce this compound to G3P. The
cycle must turn 3 times to create one molecule of G3P.
•Regeneration of CO2 acceptor (RuBP): the rearrangement of five molecules of G3P into
three molecules of RuBP requires three more ATP.
•Cyclic electron flow
–Electrons excited from P700 in PSI are passed from FD to the cytochrome complex and
back to P700. Several groups of photosynthetic bacteria have only photosystem I and
photosynthesis may have evolved in a form similar to cyclic electron flow.
–This may be a photoprotective mechanism in eukaryotic photosynthesizers
•Comparison of chemiosmosis in chloroplasts and mitochondria
–Very similar
–Electron transport chains in a membrane pump protons across the membrane as
electrons are passed down the chain in a series of redox reactions.
–In respiration, organic molecules provide the electrons and chemical energy is
transferred to ATP
–In chloroplasts, water provides the electrons and light energy is transformed to the
chemical energy of ATP.
–Also in chloroplasts, the electron transport chain pumps protons from the stroma into
the thylakoid space. As H+ diffuses back through ATP synthase, ATP is formed on the
stroma side where it is available to the Calvin cycle.
•Different methods of carbon fixation.
•C3 plants
–They close their stomata on hot dry days to limit water loss. CO2 concentration in the
leaf air spaces falls and slows the Calvin cycle. Os more O2 than CO2 accumulates,
rubisco adds O2 to RuBP in place of CO2.
–The product splits and a two-carbon compound exits the chloroplast and is broken down
to release CO2. This is called photorespiration.
•C4 plants
–CO2 is first added to the three carbon compound PEP with the aid of an enzyme that has
a high affinity for CO2.
–The resulting 4 carbon compound formed in the mesophyll cells of the leaf is
transported to bundle sheath cells tightly packed around the veins of the leaf.
–The compound is then broken down to release CO2 which rubisco then fixes in the
Calvin cycle.
–Increasing atmospheric CO2 concentration and the resulting global climate change may
affect the distribution of C3 and C4 plants and change the structure of various plant
communities.
•CAM plants
–Many desert succulent plants close their stomata during the day to help prevent water
loss.
–At night,they open their stomata and take up CO2 incorporating it into a variety of
organic acids in a mode of carbon fixation called crassulacean acid metabolism (CAM).
During daylight, CAM plants break these compounds down and release CO2 allowing the
Calvin cycle to proceed.
–Unlike the C4 pathway, the CAM pathway does not structurally separate carbon fixation
from the Calvin cycle; instead the two processes are separated in time.
–Read and be familiar with all of Section/Concept 10.4
•Why is photosynthesis important
–Earth’s oxygen
–CO2 uptake
–50% of the organic material produced by photosynthesis is used as fuel in cellular
respiration in the mitochondria of plant cells; the rest is used as carbon skeletons for the
synthesis of organic molecules (proteins, lipids, and a great deal of cellulose), stored as
starch or lost through photorespiration.
–Each year about 160 billion metric tons of carbohydrate are produced by
photosynthesis.