Cellular Respiration

Respiration

the process that converts energy from food mlcs to ATP

4 major steps of cellular respiration

Glycolysis, Link rxn, Krebs cycle, OxPhos

Major goal of first 3 steps of cellular respiration

energize electron carriers so they can be used in OxPhos

What two steps can OxPhos be divided into?

Electron transport chain (ETC), Chemiosmosis

Electron carriers

• Energy is carried through the rxns in electrons of mlcs called electron carriers.

• Electron carriers in cellular respiration are NADH and FADH2

Reduced vs Oxidized 

• Reduced: e- is added to mlc/gains e-, has more energy 

• Oxidized: e- is taken away from mlc/loses e-, has less energy 

Are NADH and FADH2 oxidized or reduced mlcs?

Reduced mlcs.

• oxidized form of NADH is NAD+

• oxidized form of FADH2 is FAD

• (NAD+) + (H+) + 2e- → NADH

• (FAD) + (H+) + 2e- → FADH2

NAD+ and NADH, FAD and FADH2

NAD+ and FAD are the starting mlcs and are empty so it can receive e-’s. Once they receive e-’s, they become NADH and FADH2, which are e- carriers 

Reducing agent

A substance that is oxidized so it makes another substance reduced

Oxidizing agent

A substance that is reduced so it makes another substance oxidized

Glycolysis

• glucose + 2ATP + 2NAD+ → 2 pyruvate + 4ATP + 2NADH

• breaking down of one glucose mlc into 2 pyruvate mlc 

• 2 net ATP (2 ATP is used to break down glucose, 4 ATP is produced, 4-2=2 total ATP)

• pyruvic acid=pyruvate but with added H 

Anaerobic vs aerobic

• Anaerobic= no oxygen

• Aerobic= yes oxygen

• Anaerobic respiration creates only 2 ATP (from glycolysis)

• Aerobic respiration creates 38 ATP 

Fermentation

process that occurs under anaerobic conditions as a means to regenerate NAD+

Lactic Acid Fermentation

• pyruvate + NADH → lactic acid + NAD+

• happens after glycolysis if oxygen is NOT present

• more NAD+ produced allows for glycolysis to occur again

• glycolysis occurs with no oxygen → lactic acid fermentation occurs bcs there is no oxygen → fermentation creates NAD+ so glycolysis occurs again → goes back to lactic acid fermentation. creates 2 ATP in glycolysis and a build up of lactic acid which is bad for the body, which explains why we need O2

• humans undergo lactic acid fermentation 

• occurs in cytoplasm

Alcoholic Fermentation

• process where microorganisms (like yeast) convert sugars into ethanol and carbon dioxide in an anaerobic environment

• essential for production of alcoholic beverages and bread making 

• occurs in cytoplasm

• does NOT occur in humans

• pyruvate + NADH → alcohol(ethanol) + CO2 + NAD+

Link Reaction

• links glycolysis to the Krebs cycle

• occurs twice for each mlc of glucose (glucose is broken down into 2 pyruvate, both pyruvates go through link rxn)

• occurs in the mitochondrial matrix: pyruvate is decarboxylated and converted to high-energy Acetyl-CoA

• pyruvate decarboxylation = pyruvate is a 3-carbon mlc. during decarboxylation, a carbon atom is removed, and a two-carbon acetyl group is left behind. this attaches to coenzyme A to form Acetyl-CoA 

• 2 pyruvate + 2 CoA (coenzyme A) + 2 NAD+ → 2 Acetyl-CoA + 2 NADH + 2 CO2

• Goal of converting NAD+ → NADH is so it can be transferred to OxPhos through ETC and generate large amounts of ATP 

Decarboxylate

process of removing carboxyl (COOH) group from mlc and releasing it as CO2

Krebs Cycle (Citric Acid Cycle)

• Goal is to energize electron carrier mlcs for OxPhos 

• Acetyl-CoA combines with one oxaloacetate to form citric acid → critic acid is broken down one carbon at a time (CO2 is produced and released, high energy electrons are stripped and captured by NAD+ and FAD, converting them to NADH and FADH2) to form original oxaloacetate mlc → cycle continues with the original oxaloacetate mlc 

• For each mlc of glucose, we get 6 mlcs of NADH, 2 mlcs of FADH2, 2 CO2, and 2 mlcs of ATP 

• Occurs in inner matrix 

What do we get in the first 3 steps of CR for one mlc of glucose?

10 NADH, 2 FADH2, 4 ATP, and 6 CO2

Oxidative Phosphorylation (OxPhos)

Split into 2 main processes:

1) Electron Transport Chain (ETC)

2) Chemiosmosis 

OxPhos- ETC

• TWO main goals. 1) Return e- carriers back to their “empty” state (as NAD+ and FAD) to go through more CR. 2) use energy from the e-’s to actively transport H+ into intermembrane space, which creates a proton gradient

• ETC occurs in inner mitochondrial membrane 

• e- carriers pass electrons along the chain, which powers transport of H+ ions out of mitochondrial matrix into intermembrane space 

• creates an electrical gradient, charged and polar H+ wants to move back into matrix but cannot diffuse because of the nonpolar fatty acid tails.

• O2 is final e- acceptor, combines w/ H+ to form H2O/water (which is why CR is an aerobic process)

Why is oxygen being the final e- acceptor important for CR?

Oxygen’s high electronegativity pulls e- through the chain towards it, clearing the way for more e- to pass along and continue the cycle. If oxygen was not present, ETC would stop and large numbers of ATP won’t be created. 

OxPhos- Chemiosmosis

• ATP synthase- an enzyme that allows H+ to flow back into matrix

• Proton gradient is established by ETC because it pumps protons (H+) from matrix into intermembrane space, creating a low concn in matrix and high concn in intermembrane space 

• H+ cannot flow back into matrix by itself (because of fatty acid tails), so it flows back through the ATP synthase

• This movement causes a part of ATP synthase to spin, which adds ADP to an inorganic phosphate to form ATP 

• produces 34 ATP 

Difference between proton gradient vs. electrical gradient

Proton gradient: refers to difference in concn of protons across mitochondrial membrane. forms as H+ (proton) is pumped from matrix into intermembrane space, making concn of proton in there greater than in the matrix

Electrical gradient: refers to difference in electrical charge across a membrane. forms as H+ (proton) is pumped from matrix into intermembrane space, making the charge of matrix more negative while intermembrane space becomes more positive 

Where does glycolysis occur?

Cytoplasm/cytosol 

Where does link rxn occur?

Mitochondrial matrix for eukaryotes, cytoplasm for prokaryotes because they lack mitochondria

Where does Krebs cycle occur?

Mitochondrial matrix for eukaryotes, cytoplasm for prokaryotes because they lack mitochondria

Where does OxPhos- ETC occur?

Inner mitochondrial membrane for eukaryotes, plasma membrane for prokaryotes

Where does OxPhos- Chemiosmosis occur?

Inner mitochondrial membrane for eukaryotes, plasma membrane for prokaryotes

What are the parts of a mitochondria?

1) Outer membrane 

2) Inner membrane (folded into cristae which increases surface level for more chemical rxns) 

3) Intermembrane space between the two membranes 

4) Matrix- fluid filled space inside inner-membrane