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Cellular Respiration and Fermentation & Photosynthesis - Coggle Diagram
Cellular Respiration and Fermentation & Photosynthesis
Catabolic pathways yield energy by oxidizing organic fuels
Energy enters ecosystems as light and exits as heat
The chemical elements essential to life are recycled
Photosynthesis uses CO2 and H2O to make organic molecules and O2
Cellular respiration uses O2 and organic molecules to make ATP; CO2 and H2O are produced as waste
Catabolic pathways release stored energy by breaking down complex molecules
Catabolic Pathways and Production of ATP
The breakdown of organic molecules is exergonic
Fermentation is a partial degradation of sugars that occurs without oxygen
Aerobic respiration consumes organic molecules and oxygen and yields ATP
Anaerobic respiration is similar to aerobic respiration but consumes compounds other than oxygen
Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration
Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)
Catabolic pathways do not directly power work in the cell; they are linked to work by ATP
Cells must constantly regenerate their supply of ATP from ADP and phosphate
Redox Reactions: Oxidation and Reduction
The transfer of electrons during chemical reactions releases energy stored in organic molecules
This energy is ultimately used to synthesize ATP
The Principle of Redox
Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions
In redox reactions, the loss of electrons from a substance is called oxidation
The addition of electrons to a substance is called reduction (the amount of positive charge is reduced)
Oxidation and reduction always go hand in hand
The electron donor is called the reducing agent, it reduces the electron acceptor
The electron acceptor is called the oxidizing agent, it oxidizes the electron donor
Electron transfer from food molecules to other molecules plays a major role in these pathways
These processes are central to cellular respiration
Oxidation of Organic Fuel Molecules During Cellular Respiration
During cellular respiration, fuel molecules (such as glucose) are oxidized, and O2 is reduced
Organic molecules with an abundance of hydrogen are excellent sources of high-energy electrons
Cellular respiration is a redox process; energy is released as hydrogen and electrons are transferred to O atoms
The oxidation of glucose transfers electrons from a higher energy state (in glucose) to a lower energy state with O atoms
This releases energy that is be used to synthesize ATP
Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
In cellular respiration, glucose and other organic molecules are oxidized in a series of steps
Each electron travels with a proton—thus, as a hydrogen atom
Hydrogen atoms are usually first passed to electron carriers, rather than directly to O2
Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate
The 2 electrons and 1 proton is transferred to NAD+ forming NADH
The other proton is released as a hydrogen ion (H+) into the surrounding solution
The Stages of Cellular Respiration:
Harvesting energy from glucose by cellular respiration has three stages
Glycolysis breaks down glucose into two molecules of pyruvate
Pyruvate oxidation and the citric acid cycle complete the breakdown of glucose to CO2
During oxidative phosphorylation the electron transfer chain and chemiosmosis facilitate synthesis of most of the cell’s ATP
The process that generates almost 90% of the ATP is called oxidative phosphorylation because it is powered by redox reactions
Some ATP is also formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
Substrate-level phosphorylation occurs when an enzyme transfers a phosphate group directly from a substrate to ADP
CONCEPT 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis occurs in the cytoplasm and has two major phases
In the energy investment phase, 2 ATP are used to split glucose into 2 three-carbon sugar molecules
In the energy payoff phase, 4 ATP are synthesized, 2 NAD+ are reduced to NADH, the small sugars are oxidized to form 2 pyruvate and 2 H2O
A net of 2 ATP are produced by substrate-level phosphorylation during glycolysis
CONCEPT 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules
Most of the energy in glucose remains stored in the pyruvate molecules produced by glycolysis
In eukaryotic cells, if O2 is present, pyruvate enters a mitochondrion to complete glucose oxidation
This occurs in the cytosol for aerobic prokaryotes
Oxidation of Pyruvate to Acetyl CoA
Pyruvate is converted to acetyl coenzyme A (acetyl CoA) before entering the citric acid cycle
Pyruvate dehydrogenase catalyzes three reactions
Oxidation of pyruvate’s carboxyl group, releasing the first CO2 of cellular respiration
Reduction of NAD+ to NADH
Combination of the remaining two-carbon fragment with coenzyme A to form acetyl CoA
The Citric Acid Cycle
The citric acid cycle, also called the Krebs cycle, oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn
Another 2 CO2 are produced as a waste product
Because 2 pyruvate are produced per glucose, the cycle runs twice per glucose molecule consumed
The citric acid cycle has eight steps, each catalyzed by a specific enzyme
First the acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate
The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle
The NADH and FADH2 produced by the cycle carry electrons to the electron transport chain
CONCEPT 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
Molecules of NADH and FADH2 produced during glycolysis and the citric acid cycle account for most of the energy extracted from glucose
NADH and FADH2 donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
The Pathway of Electron Transport
Molecules of the electron transport chain are embedded in the inner mitochondrial membrane in eukaryotic cells
The membrane is folded into cristae to increase surface area for electron transport chains
In prokaryotes, the electron transport chain is embedded in the plasma membrane
Electrons drop in free energy as they are transferred down the chain, finally passing to O2 to form H2O
The electron transport chain breaks the large free-energy drop from glucose to O2 into smaller steps, releasing energy in manageable amounts
No ATP is produced directly by the chain
An Accounting of ATP Production by Cellular Respiration
During cellular respiration, most energy flows in this sequence:
glucose → NADH → electron transport chain → proton-motive force → ATP
About 34% of the energy in a glucose molecule is transferred to ATP, making about 32 ATP
The rest of the energy is lost as heat
CONCEPT 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen
Most cellular respiration depends on electronegative oxygen to pull electrons down the transport chain
Without oxygen, the electron transport chain will cease to operate
In that case, glycolysis couples with anaerobic respiration or fermentation to produce ATP
Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen
For example, some organisms use a sulfate ion (SO42-) as a final electron acceptor
In this case, H2S (hydrogen sulfide) is made as a by-product instead of H2O
Under aerobic conditions, NAD+ is regenerated from NADH by transferring electrons to the electron transport chain
Fermentation must use alternate mechanisms to regenerate NAD+
Fermentation is an extension of glycolysis that oxidizes NADH by transferring electrons to pyruvate or its derivatives
Two common types are alcohol fermentation and lactic acid fermentation
In alcohol fermentation, pyruvate is converted to ethanol in two steps
The first step releases CO2 from pyruvate
The second step produces NAD+ and ethanol
Alcohol fermentation by yeast is used in brewing, winemaking, and baking
In lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate and NAD+
There is no release of CO2 in lactic acid fermentation
Lactic acid fermentation by fungi and bacteria is used to make cheese and yogurt
CONCEPT 10.1 Photosynthesis feeds the biosphere
Photosynthesis is the process that converts solar energy into chemical energy within chloroplasts
Photosynthesis nourishes almost the entire living world directly or indirectly
The Process That Feeds the Biosphere
Autotrophs are “self-feeders” that sustain themselves without eating anything derived from other organisms
Autotrophs are the producers of the biosphere; they produce organic molecules from CO2 and other inorganic molecules
Almost all plants are photoautotrophs, that is, they use the energy of sunlight to make organic molecules
Photosynthesis also occurs in algae, certain other protists, and some prokaryotes
Heterotrophs obtain organic material from other organisms; they are the consumers of the biosphere
Some consume other living things; others, called decomposers, eat dead organic material or feces
Fossil fuels were formed from the remains of organisms that died hundreds of millions of years ago, representing ancient stores of the sun’s energy
Almost all heterotrophs depend on photoautotrophs, either directly or indirectly, for food and O2
CONCEPT 10.2: Photosynthesis converts light energy to the chemical energy of food
Plants and other photosynthetic organisms contain organelles called chloroplasts
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
The structural organization of these organelles allows for the chemical reactions of photosynthesis
Chloroplasts: The Sites of Photosynthesis in Plants
Most photosynthesis in plants occurs in the leaves
Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
CO2 enters and O2 exits the leaf through microscopic pores called stomata
Veins transport water from the roots and export sugar to nonphotosynthetic parts of the plant
A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma
Thylakoids are connected sacs in the chloroplast that compose a third membrane system
Thylakoids may be stacked in columns called grana
Chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes
Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis is a complex series of reactions that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
The overall chemical change during photosynthesis is the reverse of cellular respiration
The Splitting of Water
Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing O2 as a by-product
All photosynthetic organisms require a hydrogen source, but the source varies among organisms
For example, sulfur bacteria use H2S instead of water, forming yellow globules of sulfur as a waste product
Photosynthesis as a Redox Process
Photosynthesis reverses the direction of electron flow compared to respiration
Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
Photosynthesis is an endergonic process; the energy boost is provided by light
Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis is a complex series of reactions that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
The overall chemical change during photosynthesis is the reverse of cellular respiration
CONCEPT 10.3: The light reactions convert solar energy to the chemical energy of ATP and NADPH
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the chemical energy of ATP and NADPH
The Nature of Sunlight
Light is electromagnetic energy, also called electromagnetic radiation
Electromagnetic energy travels in rhythmic waves
Wavelength is a measure of the distance between crests of electromagnetic waves
It can range from less than a nanometer (gamma rays) to more than a kilometer (radio waves)
The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
Visible light (wavelengths 380 nm to 740 nm) drives photosynthesis and produces the colors seen by the human eye
Light also behaves as though it consists of discrete particles, called photons
Each photon has a fixed quantity of energy which is inversely related to the wavelength of light; shorter wavelengths have more energy per photon of light
Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths, and the wavelengths that are absorbed disappear
Wavelengths that are not absorbed are reflected or transmitted
For example, most leaves appear green because chlorophyll absorbs violet-blue and red light while reflecting and transmitting green light
A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes
A photosystem consists of a reaction-center complex surrounded by light-harvesting complexes
The reaction-center complex is an association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Chloroplasts and mitochondria both generate ATP by chemiosmosis
Electron transport chains pump protons (H+) across a membrane as electrons are passed through carriers with progressively higher electron affinity
ATP synthase couples the diffusion of H+ down their gradient to the phosphorylation of ADP to ATP
CONCEPT 10.4: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
The Calvin cycle is anabolic; it builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
3 turns of the Calvin Cycle = 1 G3P 2 G3Ps = 1 glucose 2 turns of the Kreb Cycle = breakdown of 1 glucose
