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