An Introduction toMetabolism
Organization of chemistry into metabolic pathways
Metabolism- emergent property of life that arises from orderly interactions between molecules
Metabolic pathways
Begins with a specific molecule
Then altered in a series of defined steps
Results in a certain product
Each step is catalyzed by a specific enzyme
(Starting molecule) A-enzyme 1/reaction 1->B-enzyme 2/reaction 2->C-enzyme 3/reaction 3->D (Product)
Manages the material and energy resources of the cell
Some metabolic pathways release energy by breaking down complex molecules to simpler compounds
Catabolic pathways
Breakdown pathways
Cellular respiration is a major pathway of catabolism
Sugar glucose and other organic fuels are broken down in the presence of oxygen to carbon dioxide and water
Pathways can have more than one starting molecule and/or product
Energy that was stored in the organic molecules becomes available to do the work of the cell
Anabolic pathways
Consume energy to build complicated molecules from simpler ones
Sometimes called biosynthetic pathways
Examples; synthesis of an amino acid from simpler molecules and the synthesis of a protein from amino acids
Forms of energy
Bioenergetics, study of how energy flows through living organisms
Energy, capacity to cause change
Some forms of energy can be used to do work
Moves matter against opposing forces, like gravity and friction
Ability to rearrange a collection of matter
Kinetic energy
Relative motion of objects
Moving objects can perform work by imparting motion to other matter
Thermal energy
Kinetic energy associated with the random movement of atoms or molecules
Thermal energy in transfer from one object to another is heat
Light can be harnessed to perform work
Potential energy
Energy that is not kinetic
Energy that matter possesses because of its location or structure
Molecules posses energy because of the arrangement of electrons in the bonds between their atoms
Chemical energy
Term used by biologists to refer to the potential energy available for release in a chemical reaction
Biochemical pathways
Enable cells to release chemical energy from food molecules and use the energy to power life processes.
Laws of Energy Transformation
Thermodynamics, study of energy transformations that occur in a collection of matter
First Law of Thermodynamics
Energy can be transferred and transformed, but it cannot be created or destroyed
Also known as the principle of conservation of energy
Second Law of Thermodynamics
Entropy, a measure of molecular disorder or randomness
Every energy transfer or transformation increases the entropy of the universe
Physical disintegration of a system’s organized structure is a good analogy for an increase in entropy
Helps understand why certain processes are energetically favorable and occur on their own
Spontaneous process
By itself leads to an increase in entropy that process can proceed without requiring an input of energy
Spontaneous reaction, keeps going
Spontaneous does not imply a process would occur quickly, it is energetically favorable
Free Energy and metabolism
Is the portion of a systems energy that can perform work when temperature and pressure are uniform throughout the system as in a living cell
Also known as Gibbs free energy
Chemical reaction equation; (delta)G=(delta)H-T(delta)S
(Delta)G represents free energy
(Delta)H represents the change in the systems enthalpy
(Delta)S the change in the systems entropy
(Delta)T the absolute temperature in Kelvin(K) units
For (delta)G to be negative, (delta)H must be negative
The system gives up enthalpy and H decreases
Or T(delta)S must be positive
The system gives up order and S increases
(Delta)H and T(delta)S are tallied, (delta)G is negative
Every spontaneous process decreases the systems free energy and processes that have a positive zero (delta)G are never spontaneous
(Delta)G=G(final state) - G(initial state)
(Delta)G can be negative only when
Unstable systems (higher G) tend to change in such a way that they become more stable (lower G)
Equilibrium, describes maximum stability
Includes chemical equilibrium
As a reaction moves to equilibrium, free energy of reactants and products decrease
Free energy increases when a reaction is away from equilibrium
A process is spontaneous and can perform work only when it is moving towards equilibrium
Exergnoic and endergonic reactions in metabolism
Exergonic reactions
Proceeds with a net release of free energy
Energy outward
Chemical mixture loses free energy (G decreases) (delta)G is negative for an exergonic reaction
Occur spontaneously
Endergonic reactions
Absorbs free energy from its surroundings
(G increases) (delta)G is positive
Non spontaneous
Magnitude of (delta)G is the quantity of energy required to drive the reaction
Uphill
ATP Hydrolysis
Cell does 3 main kinds of work
Chemical work
Transport work
Mechanical work
Pushing of endergonic reactions that do not occur spontaneously
Synthesis of polymers from monomers
Pumping of substances across membranes against the direction of spontaneous movement
Beating of cilia
Contraction of muscle cells
Movement of chromosomes during cellular reproduction
Energy coupling
Use of exergonic process to drive an endergonic one
ATP is responsible for mediating energy coupling in cells
Also acts as the immediate source of energy that powers cellular work
ATP (adenosine triphosphate)
Contains sugar ribose and nitrogenous base adenine and a chain of 3 phosphates
Is a nucleoside triphosphate used to make RNA
Bonds between the phosphate group can be broken by hydrolysis
When the terminal phosphate bond breaks by additional water molecules, a molecule of inorganic phosphate(HOPO3^2-) leaves ATP
It then becomes ADP, adenosine diphosphate
The reaction is exergonic
Releases 7.3 kcal of energy per mole of ATP hydrolyzed
ATP is high energy
Phosphorylated intermediate
Recipient molecule with the phosphate group covalently bonded to it
Is more reactive (less stable, with more free energy) than unphosphorylated molecules
Transport and mechanical work in cells are also powered by hydrolysis
ATP hydrolysis leads to a change in a proteins shape and ability to bind another molecule
Regeneration of ATP
Free energy is required to phosphorylation ADP comes from free exergonic breakdown reactions (catabolism)
Shuts inorganic phosphate and energy is called ATP cycle
Couples the cells energy yielding (exergonic) processes to the energy consuming (endergonic)
Enzymes speed up metabolic reactions by lowering energy barriers
Activation energy barrier
Enzyme, is a macromolecule that acts as a catalyst
Catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction
Activation energy
Free energy of activation
Initial investment of energy for starting a reaction, the energy required to contort the reactant molecules so the bonds can break
Abbreviation; E
Amount of energy needed to push the reactants to the top of an energy barrier, or uphill
Often supplied by heat in the form of thermal energy that reactant molecules absorb from its surroundings
Thermal energy accelerates the reactant molecules
Agitates the atoms within the molecules
Makes the breakage of bonds more likely
When molecules absorbed enough energy to break, reactants are unstable (transition state)
How enzymes speed up reactions
High temperature denatures proteins and kills cells
Heat speeds up all reactions
Catalysis, catalyst selectively speeds up a reaction without itself being consumed
Enzymes catalyzes a reaction by lowering the barrier E
Enables the reactant molecules to absorb enough energy to reach transition state even at moderate temperatures
An enzyme can not change (delta)G for a reaction; it cannot make an endergonic reaction exergonic
Enzymes can only hasten reactions that would eventually occur
This enables the cell to have dynamic metabolism, routing chemicals smoothly through metabolic pathways
Enzymes specificity and catalyst in the enzymes
Substrate specificity of enzymes
Substrate
Reactant an enzyme acts on is referred to as the enzymes substrate
Enzyme binds to its substrate when there are two or more reactants
Forms enzyme substrate complex
Enzyme and sun rates join, the catalytic action of the enzyme converts the substrate to the product of the reaction
Most enzyme names end in ase
Active site
Restricted region of the enzyme molecule actually binds to the substrate
Typically a pocket or groove on the surface of the enzyme where catalysis occurs
Usually formed by only a few of the enzymes amino acids, with the rest of the protein molecule providing a framework
Determines the shape
Specificity of an enzyme is attributed to a complementary fit between the shape of its active site and the shape of the substrate
An enzyme is not stiff structure locked into a given shape
Induced fit
Clasping handshake
Tightening of the binding after initial contact
Brings chemical groups of the activation site into positions that enhance their ability to catalyze the chemical reaction
Catalysis in the Enzymes Activation Site
When there are two or more reactants
The active site provides a template on which the substrates can come together in the proper orientation for a reactant to occur between them
As the active site of an enzyme clutches the bound substrates
The enzyme may stretch the substrate molecules toward their transition state forms
Stressing and bending critical chemical bonds to be broken during the reaction
The active site may also provide a microenvironment that is more conductive to a particular type of reaction
The solution would be without an enzyme
Amino acids in the active site directly participate in the chemical reaction
Sometimes this process involves brief covalent bonding between the substrate and the side chain of an amino acid of the enzyme
Effects of local conditions on enzyme activity
Effects of temperature and pH
Each enzyme works better under some conditions than under other conditions
Optimal conditions favor the most active shape for the enzyme
The rate of an enzymatic reaction increases with increasing temperature
Substrates collide with active sites more frequently when the molecules move rapidly
Thermal agitation of the enzyme molecule disrupts the hydrogen bonds, ionic bonds, and other weak interactions that stabilize the active shape of the enzyme
Protein molecule eventually denatures
Without denaturing the enzyme optimal temperature allows greater numbers of molecular collisions
Also allows fastest conversion of the reactants to product molecules
Human enzyme optimal temperature is 35-40C
Optimal pH values for most enzymes are in the range of pH 6-8
Cofactors
May be bound tightly to the enzyme as permanent residents or they might bind loosely and reversibly with the substrate
Metal atoms zinc, iron, and copper in ionic form are inorganic
Coenzyme
If the cofactor is an organic molecule
Enzyme inhibitors
Certain chemicals selectively inhibit the action of specific enzymes
At times the inhibitor attaches to the enzyme by covalent bonds, the inhibitor is then usually irreversible
Competitive inhibitors
Reduce the productivity of enzymes by blocking substrates from entering active sites
Can be overcome by increasing the concentration of substrate so the active sites become available
Noncompetitive inhibitors
Do not directly compete with the substrate to bind to the enzyme at the active site
They impede enzymatic reactions by binding to another part of the enzyme
This causes the enzyme molecule to change shape in a way that active site becomes less effective at catalyzing the conversion substrate to product
Regulation of enzyme activity
Allosteric regulation
Proteins function at one site is affected by the binding of a regulatory molecule to a separate site
May result in either inhibition or simulation of an enzymes activity
Most enzymes allosterically regulated are constructed from 2 or more subunits
Each is composed of a polypeptide chain with its own active site
The entire complex oscillated between 2 different shapes
1 catalyticaly active and the other inactive
Simplest allosteric regulation, and activating or inhibiting regulatory molecule binds to a regulatory site
Can also be called allosteric site
Often located where subunits join
Cooperativity
Substrate molecule binding to one active site in a multisubunit enzyme triggers a shape change in all subunits, increases catalytic activity at other active sites
Amplifies the response of enzymes to substrates
One substrate molecule primes an enzyme to act on additional substrate molecules more readily
Is also considered allosteric regulation
It’s binding affects catalysis in another active site
Feedback inhibition
Metabolic pathway is halted by inhibitory binding of its end products to an enzyme that acts early in the pathway
Cellular Respiration and Fermentation
Catabolic pathways and oxidizing organic fuels
Catabolic pathways and production of ATP
Organic compounds posses potential energy as a result of the arrangement of electrons in the bonds between their atoms
Some energy taken out of chemical storage can be used to do work, the rest dissipates as heat
Catabolic process
Fermentation
Partial degradation of sugars or other organic fuel that occurs without oxygen
Aerobic respiration
Oxygen is consumed as a reactant along with organic fuel that occurs without oxygen
Cells of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration
Anaerobic respiration
Prokaryotes use substances other than oxygen as reactants that harvests chemical energy without oxygen
Cellular respiration
Includes both aerobic and anaerobic processes
It’s often referred to as aerobic process
Formula: C6H12O6+6O2—>6CO2+6H2O+energy(ATP+heat)
Redox reactions
Transfer of one or more electrons from one reactant to another
Electron transfers are called oxidation reduction reactions
Oxidation
Loss of electrons from one substance
Reduction
Addition of electrons to another substance
Adding electrons
Adding negatively charged electrons to an atom reduces the amount of positive charge of that atom
the electron donor is the reducing agent
the electron acceptor is oxidizing agent
energy must added o pull an electron away from an atom
the more electronegative the atom the more energy is required
the electron transport chain
energy is released from a fuel all at once it cannot be harnessed efficiently for constructive work
glucose is not oxidized in a single step it is broken down
each glucose is catalyzed by an enzyme
electrons are stripped from the glucose
hydrogen atoms are not transferred directly to oxygen, they are passed to an electron carrier
this coenzyme is called nicotinamide adenine dinucleotide
NAD+ is oxidzied and the reduced form is NADH
NAD+ is an electron acceptor and functions as an oxidizing agent
consists of a number of molecules mosty proteins built into the inner membrane of the mitochondira of eukaryotic cels
stages of cellular respiration
Glycolysis
Pyruvate Oxidation and the Citric Acid Cycle
Oxidative Phosphorylation
occurs in the cytosol
begins the degradation process by breaking glucose into two molecules of a compound called pyruvate
pyruvate enters the mitochondrion and s oxidized to a compound called acetyl CoA
Acetyl CoA enters the citric acid cycle
the breakdown of glucose to carbon dioxide is completed
carbon dioxide produced by respiration represents fragments of oxidized organic molecules
energy released at each step of the chain is stored in a form the mitochondrion can make ATP from ADP
ATP synthesis is called oxidative phosphorylation
it is powered by the redox reactions of the electron transport chain
substrate level phosphorylation
smaller amount of ATP formed directly in few reactions of glycolysis and the citric acid cycle
occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP
Citric acid cycle completes energy yielding oxidation of organic molecules
oxidation of pyruvate to Acetyl CoA
entering the mitochondrion, active transport
pyruvate is first converted to a compound called acetyl coenzyme A, or Acetyl CoA
links glycolysis and the citric acid cycl
carried out by a multienzyme complex that catalyzes 3 reactions
pyruvates carboxyl group, somewhat oxidized and carries little chemical energy
2 carbon fragment is oxidized and the electrons transferred to NAD+, stores energy in NADH
coenzyme A (CoA) sulfur containing compound derived from a B vitamin attaches its sulfur atom to 2 carbon intermediate forming acetyl CoA
is high in potential energy and is exergonic
citric acid cycle
functions as a metabolic furnace that further oxidizes organic fuel derived from pyruvate
Acetyle CoA adds its 2 carbon acetyl group to oxaloacetate producing citrate
citrate is converted to its isomer, isocitrate by removing 1 water moleucle and adding another
isocitrate is oxidized reducing NAD+ to NADH resulting compound loses a CO2 molecule
another CO2 is lost and resulting compound is oxidized reducing NAD+ to NADH remaining molecule is attached to coenzyme A by an unstable bond
CoA is displaced by phosphate group which is transferred to GDP form GTP which can also be used to generate ATP
2 hydrogens are transferred to FAD forming FADH2 and oxidizing succinate
addition of a water molecule rearranges bonds in the substrate
substrate is oxidized reducng NAD+ to NADH and regenerating oxaloacetate
oxidative phosphorylation chemiosmosis electron transport to ATP synthesis
pathway of electron trasport
electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells
folding of inner membrane to form cristae increases its surface area
provides space for thousands of copies of each component of the electron transport chain in a mitochondrion
cytochromes
remaining electron carriers between ubiquinone and oxygen are proteins
chemiosmosis
energy coupling mechanism
ATP synthase
populating inner membrane of the mitochondrion or prokaryotic plasma membrane are many copies of a protein complex
enzyme that makes ATP from ADP and inorganic phosphate
chemiosmosis
energy stored in the form of a hydrogen ion gradient across a membrane is usd to drive cellular work such as synthesis of ATP
proton motive force
emphasizing the capacity of the gradient to perform work
chemiosmosis is an energy coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work
accounting of ATP production by cellular respiration
during respiration most energy flows in the sequence; glucose--> NADH---> electron transport chain---> proton motive force---> ATP
phosphorylation and the redox reactions are not directly coupled to each other
ATP yield varies slightly depending on the type of shuttle used to transport electrons from the cytosol into the mitchondrion
use of the proton motive force generated by the redox reactions of respiration to drive other kinds of work
fermentation and anaerobic respiration
types of fermentation
fermentation consists of glycolysis plus reactions that regenerate NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate
alcohol fermentation
pyruvate is converted to ethanol
releases carbon dioxide from the pyruvate
acetaldehyde is reduced by NADH to ethanol
converts to the 2 carbon compound acetaldehyde
regenerates the supply of NAD+ needed for the continuation of glycolysis
lactic acid fermentation
pyruvate is reduced directly by NADH to form lactate as an end product
regenerating NAD+ with no release of CO2
glycolysis and citric acid cycle connect to many other metabloic pathways
comparing fermentation with anaerobic and aerobic respiration
fermentation, anaerobic respiration, and aerobic respiration are 3 alternative cellular pathways for producing ATP
all 3 use glycolysis to oxidize glucose and other organic fuels to purivate
net production of 2 ATP by substrate level phosphorylation
obligate anaerobes
carry out only fermentation or anaerobic respiration
facultative anaerobes
organisms including yeasts and many bacteria can make enough ATP to survive using fermentation or respiration
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versatility of catabolism
organic molecules in food can be used by cellular respiration to make ATP
glycolysis can accept a wide range of carbohydrates for catabolism
the digestion of disaccharides including sucrose, provides glucose and other monosaccharides as fuel for respiration
proteins can also be used for fuel but must be digested to their constituent amino acids
catabolism can also harvest energy stored in fats obtained either from food or from fat cells in the body
after fats are digested to glycerol and fatty acids the glycerol is converted to glyceraldehyde 3 phosphate (intermediate of glycolysis)
beta oxidation breaks the fatty acid down to 2 carbon fragments
enter the citric acid cycle as acetyl CoA
NADH and FADH2 are also generated during beta oxidation
biosynthesis (anabolic pathways)
food provides the carbon skeletons that cells require to make their own molecules
organic monomers obtained from digestion can be used directly
compounds formed as intermediates of glycolysis and the citric acid cycle can be diverted into anabolic pathways as precursors from the cell can synthesize the molecules it requires
glucose can be made from pyruvate and fatty acids can be synthesized from acetyl CoA
anabolic or biosynthetic pathways do not generate ATP they consume it
glycolysis and the citric acid cycle function as metabolic interchanges that enable our cells to convert some kinds of molecules to others
metabolism is versatile and adaptable
regulation of cellular respiration
cell does not waster energy making more of a particular substance than it needs
feedback inhibition the end product of the anabolic pathway inhibits the enzyme that catalyzes an early step of the pathway
prevents the needless diversion of key metabolic intermediates from uses that are more urgent
they can enter the electron transport chain leading to ATP production
cell controls its catabolism
when the cell works hard and its ATP concentration begins to drop respiration speeds up
when there is plenty of ATP to meet demand respiration slows down
sparing organic molecules for other functions
phosphofructokinase is an allosteric enzyme with receptor sites for specific inhibitors and activators
inhibited by ATP and stimulated by AMP (adenosine monophosphate) which derives from ADP
ATP accumulates, inhibition of the enzyme slows down glycolysis
citrate accumulates glycosis slows down and the supply of pyruvate groups to the citric acid cycle decreases
if citrate consuption increases, more ATP demand or anabolic pathways are draining off intermediates
glycoolysis accelerates and meets the demand
metabloic balance is augmented by the control of enzymes that catalyze other key steps of glycolysis and the citric acid cycle
Photosynthesis
photosynthesis converts light energy to chemical energy
Chloroplasts
Plants and other photosynthetic organisms contain cellular organelles
Photosynthesis
Specialized molecular complexes in chloroplasts capture light energy tans converts it to chemical energy stored in sugar and other organic molecules
Autotrophs
Self feeders
Heterotrophs
Obtain organic material by the second major mode of nutrition
Found mainly in the cells of the mesophyll
Tissue in the interior of the leaf
Carbon dioxide enters the leaf and oxygen exits by way of microscopic pores called stomata
Chloroplast has two membranes surrounding a dense fluid called the stroma
Suspended within the stroma is a third membrane system made of sacs called thylakoids
Segregates the stroma from the thylakoid space inside the sacs
Chlorophyll 1 the green pigment that leaves their color resides in the thylakoid membranes of the chloroplasts
Two stages of photosynthesis
Light reactions
Calvin cycle
Convert solar energy to chemical energy
Named for Melvin Calvin and James Bashar and Andrew benson
Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor NADP+
Light reactions use solar energy to reduce NADP+ to NAPH by adding electrons with an H+
Light reactions also generate ATP using chemiosmosis to power the addition of a phosphate group to ADP called photo phosphorylation
The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast
Incorporation of carbon into organic compounds is known as carbon fixation
Reduces the fixed carbon to carbohydrates by the addition of electrons
Reducing power is provided by NADPH which acquired its cargo of electrons in the light reactions
Convert CO2 to carbohydrates also requires chemical energy in the form of ATP which is also generated by the light reactions
Light reactions convert solar energy to chemical energy of ATP and NADPH
The nature of sunlight
Light is a form of energy known as electromagnetic energy, travels in rhythmic waves
Electromagnetic waves are disturbances of electric and magnetic fields rather than disturbances of a material medium such as water
Distance between the crests of electromagnetic waves is called wavelength
Electromagnetic spectrum, narrow band from 380 no to 750 no in wavelength
Visible light can be detected as various colors by the human eye
Photons, fixed quantity of energy
Photosynthetic pigments; light receptors
Substances that absorb visible light are known as pigments
Spectrometer, ability of a pigment to absorb various wavelengths of light can be measured
Absorption, graph plotting a pigments light absorption versus wavelength
Absorption spectra of 3 types of pigments in chloroplasts
Chlorophyll a
Key light capturing pigment that participates directly in the light reactions
Chlorophyll b
Separate group of accessory pigments called carotenoids
Action spectrum
Profiles the relative effectiveness of different wavelengths of radiation in driving the process
Carotenoids
Hydrocarbons that are various shades of yellow and orange because they absorb violet and blue green light
Photosystem
Composed of a reaction center complex surrounded by several light harvesting complexes
Reaction center complex is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
Light harvesting complex consists of various pigment molecules bound to proteins
Primary electron acceptor, molecule capable of accepting electrons and becoming reduced
Thylakoid membrane is populated by 2 photosystems
Photosystem II (PS II)
Photosystem I (PS I)
P680 Pigment is best at absorbing light having a wavelength of 680 nm
P700 pigment absorbs light of wavelength of 700nm
Linear electron flow
Occurs during the light reactions of photosynthesis
Photon of light strikes one of the pigment molecules in a light harvesting complex of PS II, boosting one of its electrons to a higher energy level
Electron is transferred from the excited P680 to the primary electron acceptor
Enzyme catalyzes the splitting of a water molecule into 2 electrons, 2 hydrogen ions and an oxygen atom
Each photoexcited electron passes from the primary electron acceptor of PS II to PS I
Potential energy stored in the proton gradient is used to make ATP in a process called chemiosmosis
Light energy transferred light harvesting complex pigments to the PS I reaction center complex exciting an electron of the P700 pair of chlorophyll a molecules
Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor down a second electron transport chain through protein ferredoxin
Enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+
Cyclic electron flow, uses photosystem I but not photosystem II
Calvin cycles uses chemical energy of ATP and NADPH to reduce CO2 to sugar
Anabolic, building carbohydrates from smaller molecules and consuming energy
Cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high energy electrons to make sugar
Glyceraldehyde 3 phosphate (G3P)
Carbohydrate produced directly from the Calvin cycle is not glucose it is 3 carbon sugar
Phase 1: carbon fixation
Incorporates each CO2 molecule, one at a time by attaching it to a five carbon sugar named fibulae bisphosphate
Enzyme that catalyzes this first step is RuBP carboxylase oxygenate or rubisco
Phase 2 reduction
Each molecule of 3 phosphoglycerate receives an additional phosphate group from ATP becomes 1 3-bisphosphoglycerate
Pair of electrons is donated from NADPH reduces 1 3-bisphosphoglycerate also loses a phosphate group in the process becomes glyceraldehyde 3-phosphate
Phase 3 regeneration of the CO2 acceptor (RuBP)
Carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle into 3 molecules of RuBP
Alternative mechanisms of carbon fixation evolved in hot arid climates
Photorespiration
Occurs in the light and consumes O2 while producing CO2
C3 plants
First organic product of carbon fixation is a 3 carbon compound, 3-phosphoglycerate
C4 plants
Preface Calvin cycle with an alternate mode of carbon fixation that forms a 4 carbon compound as its product
2 types of photosynthetic cells
Bundle sheath cells
Mesophyll cells
Arranged into tightly packed sheaths around the veins of the leaf
Between the bundle sheath and the leaf surface are the more loosely arranged cells
Carried out by an enzyme present only in mesophyll cells called PEP carboxylase
Adds CO2 to phosphoenolpyruvate forming a 4 carbon product oxaloacetate
CO2 is fixed in the mesophyll cells the 4 carbon products are exported to bundle sheath cells through plasmodesmata
Within bundle sheath cells the 4 carbon compounds release CO2 which is refined into organic material by rubisco and the Calvin cycle
Crassulacean acid metabolism (CAM)
When the stomata is open, plants take up CO2 and incorporate it into a variety of organic acids
CAM plants store organic acids they make during the night in their vacuoles until morning when the stomata closes
Photosynthesis connects to metabolism pathways
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Energy transformations
Process of photosynthesis uses the energy of light to convert CO2 and H2O to organic molecules
Light reactions capture solar energy and use it to make ATP and transfer electrons from water to NADP+ forming NADPH
Calvin cycle uses the ATP and NADPH to produce sugar from carbon dioxide
Energy enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds
Photosynthetic products
Enzymes in the chloroplasts and cytosol convert G3P made in the Calvin cycle to many other organic compounds
50% of the organic material made by photosynthesis is consumed as fuel for cellular respiration in plant cell mitochondrion
In most plants carbohydrateis transported out of the leaves to the rest of the plant in the form of sucrose a disaccharide
After arriving at nonphotosynthetic cells, the sucrose provides raw material for cellular respiration and a multitude of anabolic pathways that synthesize proteins, lipids, and other products
Other photosynthesizers
Make more organic material each day than they need to use as respiratory fuel and precursors for biosynthesis
Stockpile extra sugar by synthesizing starch and storing some in the chloroplasts themselves and some in storage cells roots, tubers, seeds, and fruits
Photosynthesis is the process responsible for the presence of oxygen in our atmosphere
Photosynthesis makes an estimated 150 billion metric tons of carbohydrates per year