CELLULAR RESPIRATION:Harvesting Chemical Energy

Chapter 9

CATABOLIC PATHWAYS• Complex molecules that are high in

potential energy are broken down into smaller waste products that have less energy

• Some of this released energy can later do work, but most is given off as heat

• Two major catabolic pathways–Aerobic Respiration–Anaerobic respiration (fermentation)

AEROBIC RESPIRATIONC6H1206 + 602 6CO2 + 6H20 + energy (ATP and

heat)

• Aerobic (uses oxygen) respiration• Exergonic• ΔG = -686 kcal/mole of glucose• Big picture – chop up glucose and

make ATP• Transfer energy in glucose to ATP• Oxidation of glucose by oxygen

ATP (ADENOSINE TRIPHOSPHATE)• The last phosphate of ATP can be

removed by enzymes and added to another molecule.

• This turns ATP into ADP (adenosine diphosphate).

• Molecules that receive a phosphate group have been phosphorylated.

• This makes the molecule change shape, which allows the molecule to do work.

• After the work is done, the phosphate group is released.

Figure 9.2 A review of how ATP drives cellular work

REDOX REACTIONS

• Oxidation - loss of electrons• Reduction - gain of electrons• In respiration, transferring

electrons releases energy to make ATP

• An e- loses potential energy when it moves from a less electronegative atom toward a more electronegative atom.

• In respiration, hydrogen’s electrons are transferred to oxygen (the fall of electrons), which liberates energy.

NAD+

• Hydrogen atoms are removed gradually from glucose.

• They are transferred to oxygen by a coenzyme called NAD+ (nicotinamide adenine dinucleotide).

• Dehydrogenase enzymes remove a pair of hydrogen atoms (2 e- and 2 protons) from sugar.–Remember, protons (H+) are hydrogen

cations or an H atom without its electron

• The enzyme delivers 1 proton and 2 e- to its coenzyme NAD+

making NADH.• The remaining proton (H+)is

released into surrounding solution.

• The e- lose very little energy in this transfer.

Figure 9.4 NAD+ as an electron shuttle

The three metabolic stages of respiration:

1. Glycolysis2. The Kreb’s cycle3. The electron transport chain

and oxidative phosphorylation

Figure 9.6 An overview of cellular respiration (Layer 3)

GLYCOLYSIS: “splitting of sugar”• Occurs in cytoplasm• Series of 10 steps, each with its own

enzyme• No oxygen needed (anaerobic)• Needs 2 ATP to start process• Makes 4 ATP by substrate-level

phopsphorylation (when an enzyme removes a phosphate from a substrate to make ATP)

• Transfers electrons and H+ to NAD+ to make 2 NADH (to go to ETC)

Figure 9.7 Substrate-level phosphorylation

• By the end, one glucose molecule will been broken in half to form two 3-carbon molecules of pyruvate.

• Only if oxygen is present, puruvate moves into the Kreb’s cycle (Citric Acid Cycle) to continue aerobic respiration.

Figure 9.8 The energy input and output of glycolysis

Figure 9.9 A closer look at glycolysis: energy investment phase

Figure 9.9 A closer look at glycolysis: energy payoff phase

Pyruvate converts to acetyl CoA • Pyruvate enters mitochondria• Pyruvate loses CO2, and the

resulting 2-carbon compound is oxidized making acetate.

• The e- and H+ are transferred to NAD+ to make NADH (to go to ETC)

• Coenzyme A (a vitamin B derivative) attaches to acetate making acetyl CoA

Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle

• Acetyl CoA combines with a 4-carbon molecule

• This molecule is oxidized over a series of steps that are cyclic

• e- and H+ are transferred to NAD+ and FAD+ to make 3 NADH and 1 FADH2 (flavin adenine dinucleotide).

• 2 molecules of CO2 are given off

THE KREB’S CYCLE

• 1 ATP is made by substrate-level phosphorylation

• Only 2 carbons can go through the cycle at one time so the cycle must “turn” twice to oxidize both pyruvates.

• CO2 diffuses out of cell, into blood, and is exhaled.

• NADH and FADH2 take their electrons to the electron transport chain (ETC)

Figure 9.12 A summary of the Krebs cycle

Figure 9.11 A closer look at the Krebs cycle

ELECTRON TRANSPORT CHAIN• Made up of a chain of molecules

embedded in the inner membrane of mitochondria

• Mostly proteins with prosthetic groups that can easily donate and accept e- (redox) – many are cytochromes with heme groups (Fe)

• NADH transfers e- to first molecule and FADH2 transfers e- to a lower molecule.

• e- move down the chain via redox reactions

• They move down the ETC because oxygen is electronegative and pulls the e- along

• Oxygen captures the e- at bottom and along with 2 H+ (from solution) forming water

• The energy released by falling e- causes H+ to be pumped out into intermembrane space.

• H+ move back into mitochondria by diffusion (a proton-motive force) only through a protein called ATP synthase (oxidative phosphorylation)

• These protons change ATP synthase’s shape so that it acts as an active site for Pi and ADP to make ATP.

• Each NADH eventually yields ~3 ATP.• Each FADH2 eventually yields ~2 ATP.

Figure 9.13 Free-energy change during electron transport

Figure 9.14 ATP synthase, a molecular mill

Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis

SUMMARY OF AEROBIC RESPIRATION

• Approximately 38 ATP’s made from one glucose

• About 60% of energy from glucose is “lost” as heat

• This heat helps to keep our warm body temperature

Figure 9.16 Review: how each molecule of glucose yields many ATP molecules during cellular respiration

ANAEROBIC RESPIRATION (FERMENTATION)• NO oxygen = anaerobic = no Kreb’s

–Alcohol fermentation (yeast)• Pyruvate is converted to ethanol

–Lactic acid fermentation (humans)• Pyruvate is converted to lactic acid

• 2 ATP and 2 NAD+ are made• Makes NAD+ so glycolysis can continue

– otherwise NADH has no where to go (without oxygen at bottom of ETC) and is not converted back to NAD+.

Figure 9.17a Fermentation

Figure 9.x2 Fermentation

Figure 9.17b Fermentation

Figure 9.18 Pyruvate as a key juncture in catabolism

• Facultative anaerobes – organisms that make ATP through fermentation if no oxygen and through respiration if oxygen is present (ex. yeast and some bacteria)

Evolutionary Significance of Glycolysis

• No oxygen required (early earth had no oxygen in atmosphere)

• No mitochondria required (prokaryotes do not have)

• Most common metabolic pathway

Versatility of Respiration

• Proteins and lipids enter at different locations than glucose

• Intermediates of respiration can be used to make other necessities (like amino acids)

• Intermediates and products of respiration inhibit enzymes to slow respiration down.

Figure 9.19 The catabolism of various food molecules

Figure 9.20 The control of cellular respiration 

PHOTOSYNTHESIS

Chapter 10

BASIC VOCABULARYAutotrophs – producers; make their own “food”Heterotrophs – consumers; cannot make own food

LEAF STRUCTUREStomata (stoma) – microscopic pores that allow water, carbon dioxide and oxygen to move into/out of leafChloroplasts – organelle that performs photosynthesis

Found mainly in mesophyll – the tissue of the interior leafContain chlorophyll (green pigment)Stroma – dense fluid in chloroplastThylakoid membrane – inner membrane of chloroplastGrana (granum) – stacks of thylakoid membrane

Figure 10.2 Focusing in on the location of photosynthesis in a plant

PHOTOSYNTHESIS SUMMARY

6CO2 + 6H20 + light energy C6H12O6 + 6O2

Converting light energy into chemical energy (using sunlight to make sugar)Oxygen comes from water, not CO2

Two parts: Light Reactions The Calvin Cycle (Dark Reactions or Light Independent)

Figure 10.3 Tracking atoms through photosynthesis

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle

LIGHTPhotons – discrete packets of light energyChlorophyll a – (blue-green)only pigment that is directly used in light reactionsChlorophyll b – (yellow-green) accessory pigmentCarotenoids - (yellow-orange)

Figure 10.6 Why leaves are green: interaction of light with chloroplasts

Figure 10.8 Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for photosynthesis in an alga

PHOTOEXCITAIONWhen photons hit chlorophyll and other pigments, electrons are excited to an orbital of higher energyIn solution when the excited electrons fall, they give off energy (a photon) and fluoresce

Figure 10.9 Location and structure of chlorophyll molecules in plants

LIGHT REACTIONSPhotosystems:

Made of proteins and other molecules surrounding chlorophyll aContain a primary electron acceptor Photosystem I – P700Photosytem II – P680

Figure 10.11 How a photosystem harvests light

Require light to occurTwo pathways:

Noncyclic (predominant route)CyclicNoncyclic animationAnother animation

NONCYCLIC ELECTRON FLOW

Photosystem II absorbs lightTwo electrons excited and captured by primary electron acceptor“Hole” in photosystem II is filled by 2 electrons that come from the splitting of waterH2O 2H+ + ½ O2 + 2e-

Oxygen is releasedExcited electrons pass from primary electron acceptor down an electron transport chain to photosystem I (filling its “hole”)ATP is made by photophosphorylation as electrons fall down ETC

Photons excite 2 electrons from Photosystem I and are captured by its primary electron acceptorElectrons then move down another ETC to ferredoxin (Fd)Fd gives electrons to NADP+ (nicotinamide dinucleotide phosphate) making NADPHThe enzyme that helps this transfer of e- is called NADP+ reductase

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH

Figure 10.13 A mechanical analogy for the light reactions

Figure 10.14 Cyclic electron flow

CYCLIC ELECTRON FLOW

Only Photosystem I is usedFd passes electrons back to Photosystem I via ETC

Some ATP madeNo NADPH madeNo oxygen released

Used when cell needs more ATP than NADPH

ETC

Food (chemical energy) to ATP (chemical energy) ATP synthasePumps H+ into intermembrane space

Light energy to ATP (chemical energy) ATP synthasePumps H+ into thylakoid space

MITOCHONDRIA

CHLOROPLAST

Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts

Figure 10.17 The Calvin Cycle

CALVIN CYCLEAlso called Dark Reactions because light is not needed; however products from light reactions are needed.Carbon Fixation – initial incorporation of carbon into organic moleculesCO2 attaches to a 5-carbon sugar called ribulose bisphosphate (RuBP)The enzyme that catalyzes this is called rubiscoCalvin cycle animation

Immediately splits into two 3-carbon molecules called 3-phosphoglycerate3-phosphoglycerate is phosphorylated by ATP (from light reactions) making 1,3-bisphosphoglycerate1,3-bisphosphoglycerate is reduced by taking electrons from NADPH making glyceraldehyde 3-phosphate (G3P)One G3P molecule leaves cycle to be used by plantThe remaining G3P’s are converted into RUBP in several steps and by getting phosphorylated by ATP

Recall, G3P is the sugar formed by splitting glucose in glycolysisG3P can be made into glucose, sucrose, cellulose etc. by plant

C3 PLANTS – have a problem

Examples : rice, wheat, and soy beansProblem - produce less food when stomata are closed during hot days because low CO2 starves Calvin Cycle and rubisco can accept O2 instead of CO2

High oxygen levels = O2 passed to RUBP (not CO2) and Calvin cycle stopsWhen this oxygen made product splits, it makes a molecule that is broken down by releasing CO2

This process is called photorespiration.

Occurs during daylight (photo)Uses O2 and makes CO2 (respiration)

NO ATP made (unlike respiration) and NO food made

Early earth had low O2 when first plants appeared so this would not have mattered as muchPhotorespiration drains away as much as 50% of carbon fixed by Calvin Cycle in many plants.

C4 PLANTS – have a solutionExamples: sugarcane, corn and grassesLeaves contain bundle-sheath cells and mesophyll cellsBundle sheath surrounds veins of leaf (location of Calvin cycle)Mesophyll – between bundle and surface

In mesophyll cells: CO2 fixed to phosphoenolpyruvate (PEP)PEP carboxylase is the enzyme that does thisPEP carboxylase has higher affinity for CO2 than rubisco so less danger of O2 interferingThe fixed CO2 is then taken to Calvin cycle (in bundle-sheath) as part of a 4-carbon molecule (malate)Malate gives CO2 to Calvin cycle

Figure 10.18 C4 leaf anatomy and the C4 pathway

CAM PLANTS – have another solution(crassulacean acid metabolism)

Examples: succulent plants (pineapples and cacti etc.)Open stomata at night and close during dayAt night CO2 is fixed into organic acids in mesophyll and then taken to Calvin cycle (also in mesophyll) during day.

Figure 10.19 C4 and CAM photosynthesis compared

PHOTOSYNTHESIS FACTS

50% of organic material made is used by plant in respirationOrganic molecules often leave leaves as sucroseLarge amounts of cellulose are made (for cell walls)“And no process is more important than photosynthesis to the welfare of life on Earth.” (Campbell and Reece, 2005)

Figure 10.20 A review of photosynthesis

Carbon Cycle

Carbon CycleHuman Impact on C CycleClimate change = global warmingIncreased levels of CO2 (and some other gases) increase Greenhouse effect (traps heat)Increased Greenhouse Effect equals warming earthHow does deforestation and burning fossil fuels impact the carbon cycle?