Cellular Respiration & Fermentation

Work for Cells

Require energy to do work, such as assembly and moving

Energy obtained through food (other organisms or self-produced)

Energy vs matter

Energy flows into an ecosystem as sunlight and leaves as heat

Elements recycled, chemical energy in organic molecules generate ATP

Catabolic Pathways

Release stored energy by breaking down complex molecules, powered by electron transfer

Anaerobic vs Aerobic Respiration

Both are degradations of sugars, fermentation is partial and occurs without O2 while aerobic consumes molecules + O2 to make ATP

Redox Reactions

Also known as oxidation-reduction reactions, transfer of electrons releases energy and makes ATP

Oxidation during CR

Glucose is oxidized and O2 is reduced, hydrogen often provides electrons and energy is released after the receptor takes them in

Energy harvest (electrons)

In CR, electrons are transferred to NAD+ coenzyme and oxidizing agent, stores energy as NADH until tapped to synthesize ATP

Electron Transport Chain

NADH passes electrons through the ETC, has a series of steps until accepted by O2 where energy yielded is to regenerate ATP

3 Stages of Cellular Respiration

Glycolysis - Breakdown of glucose to 2 molecules of Pyruvate

Pyruvate Oxidation and Citric Acid Cycle - Completes glucose breakdown

Oxidative Phosphorylation - Electron transport and chemiosmosis, ATP synthesis

3 Key Enzymes

PFK in glycolysis

Pyruvate Dehydrogenase - Pyruvate to acetyl CoA

Isocitrate Dehydrogenase - in Citric Acid Cycle

Oxidative Phosphorylation

Powered by redox reactions, compared to substrate-level phosphorylation in Krebs and Glycolysis that makes a small amount of ATP

Glycolysis

Doesn’t require O2 and in the cytoplasm, it splits glucose into 2 pyruvate molecules in an energy investment phase and an energy payoff phase

What would happen to pyruvate after?

If O2 is present, pyruvate will enter the mitochondria to fully oxidize glucose

Pyruvate Oxidation

Pyruvate converted into Acetyl COA through steps

Carried out by multi enzyme complex where Pyruvate oxidized and CO2 released, NAD+ reduced into NADH, 2 carbon fragment and Coenzyme A to make Acetyl COA

Citric Acid Cycle

Completes breakdown of Pyruvate to CO2 (x2), oxidizes fuel from Pyruvate to 1 ATP, 3 NADH, and 1 FADH2 each turn

What occurs during the Citric Acid Cycle?

Catalyzed by enzymes, Acetyl from Acetyl COA combines with oxaloacetate to make citrate

Next seven steps convert citrate back to oxaloacetate, producing NADH and FADH2 to donate electrons to ETC

Role of NADH and FADH2

Accounts for most energy extracted from food, carriers that donate electrons to power ATP synthesis

Electron Transport Pathway

In the mitochondria cristae, holds protein complexes where electrons go down the chain and are accepted by O2, forming H2O

ETC

Electrons transferred from NADH or FADh2 to ETC, they travel through proteins (including cytochromes) to O2

No ATP generated directly but drives ATP synthesis

Electron Transport Types

Mitochondrial membrane, thylakoid membrane, and cytoplasmic membrane

Prokaryotic ETC

Involves the cytoplasmic membrane, more simple and less branched

Endotherm

Organisms that regulate body temperature, maintains heat through metabolic rate (“burning food” through CR)

Decoupling Oxidative Phosphorylation

When the ETC is not connected with ATP synthesis, proton gradient flowing down a concentration gradient releases heat

Chemiosmosis

Energy as electrons passed through ETC pumps H+ from mitochondria matrix to inter-membrane space

H+ moves down its concentration gradient back across the membrane passing through ATP synthase to make ATP

How does ATP synthase make ATP?

H+ moves to binding sites on ATP Synthase, causing it to spin in a way that catalyzes ADP to ATP

A lot of energy is lost as heat

Proton Motive Force

Energy stored in H+ gradient couples redox reactions of ETC to ATP synthesis

Why is ATP a guestimate?

Phosphorylation and redox not directly coupled, ATP depends on electron acceptors (NAD or FAD), proton motive force can drive other work

Fermentation

Requires substrate-level phosphorylation instead of oxidative phosphorylation

Regenerates NAD+ to be reused for glycolysis (ATP production)

Alcoholic Fermentation

Pyruvate converted to Ethanol in two steps

Releases CO2 from Pyruvate then produces NAD+ and Ethanol

Lactic Acid Fermentation

Pyruvate reduced by NADH and forms NAD+ and Lactate (no CO2)

In bacteria to make cheese, yogurt, and can generate ATP in exercise