Pp 69 – 73 &

Define cell respiration Cell respiration is the controlled release of energy from organic compounds in cells to form ATP Glucose is the major substrate for respiration Adenosine triphosphates (ATP) is the molecule which directly fuels the majority of biological reactions.

Why cell respiration? Cells require a constant source of energy to perform various tasks e.g. Movement Transport Division

Types of Respiration: (i) Anaerobic Respiration (ii) Aerobic Respiration Occurs in the absence of Oxygen Occurs in the cells’ cytoplasm Yields small amount of ATP (2 molecules) per molecule of glucose Involves fermentation of pyruvate to lactate in muscles/CO2 & ethanol in plant & yeast Occurs in presence of Oxygen Occurs in the cells’ mitochondria Yields large amount of ATP (38 molecules) per molecule of glucose Does not involve fermentation

Types of Respiration Occurs in the absence of Oxygen (ii) Aerobic Respiration Occurs in presence of Oxygen Occurs in the cells’ cytoplasmOccurs in the cells’ mitochondria Yields small amount of ATP (2 molecules) per molecule of glucose Yields large amount of ATP (38 molecules) per molecule of glucose Does not involve fermentation Involves fermentation of pyruvate to lactate in muscles/CO2 & ethanol in plant & yeast (i) Anaerobic Respiration

Comparison between Aerobic & Anaerobic Respiration

Adenosine triphosphate (ATP): ATP the molecule which directly fuels the majority of biological reactions About ATP molecules are hydrolysed to ADP and Pi daily ADP is reduced back to ATP using the free energy from the oxidation of organic molecules

ATP Cycle

in cell respiration, glucose in the cytoplasm is broken down by glycolysis into pyruvate, with a small yield of ATP

Glycolysis and Cell Respiration: Glycolysis occurs in the cytoplasm of the cell 1 glucose molecules is broken down into 2 pyruvate molecules There is a net production of 2 ATP molecules Glycolysis does not require oxygen Fate of pyruvate depends on presence or absence of oxygen

Anaerobic Cell Respiration:

Summary Equation The summary equation for cellular respiration is: C 6 H 12 O 6 + O 2  CO 2 + H 2 O + Glucose + oxygen  carbon dioxide + water + ATP ATP

Key players in this process Glucose: source of fuel NAD + : electron carrier Enzymes: mediate entire process Mitochondria: site of aerobic respiration ATP: principal end product Protons/Electrons: sources of potential energy Oxygen: final electron acceptor

Redox reactions Reduction: reducing overall positive charge by gaining electrons Oxidation: loss of electrons

NAD+: an electron carrier In order for electrons to be passed from one compound to another, an electron carrier is needed NAD+ is reduced to NADH when picking up electrons It is oxidized back to NAD+ when losing them

Where do the electrons come from? Remember all those hydrogen atoms that make up glucose? Hydrogens are a part of fats, too. Hydrogen = 1e-, so here, H = e-

Respiration is a controlled release of energy It’s a highly exergonic, but well- controlled process Mediated by enzymes, electron carriers Otherwise, it would be like an explosion Not compatible with life!

Phosphorylation Addition of a phosphate group to a molecule; in this case, to ADP, forming ATP Substrate level phosphorylation vs. oxidative phosphorylation

Substrate-level phosphorylation An enzyme transfers a phosphate group from a substrate to ADP Ineffective in generating large amounts of ATP

Oxidative phosphorylation Refers to phosphorylation that occurs due to redox reactions transferring electrons from food to oxygen Happens on electron transport chains

Mitochondrion Structure Outer membrane Inner membrane Cristae (folds) Matrix (liquid) DNA (mtDNA) Intermembrane space

Three stages of respiration Stage 1: Glycolysis (energy investment) Some ATP is made, some is used Stage 2: Krebs Cycle (oxidation of pyruvate) Generation of CO 2 Stage 3: Oxidative Phosphorylation Generation of most ATP

Three stages of respiration

Stage 1: Glycolysis Where Cell’s cytoplasm Why To break glucose down into pyruvate, which feeds into the Krebs Cycle To regenerate NAD, an electron carrier

Glycolysis: How Glucose is phosphorylated. -1 ATP A series of enzymes produces intermediate products. An intermediate is phosphorylated. -1 ATP This diphosphate compound is unstable and breaks into 2 PGAL. The PGAL molecules generate ATP through substrate-level phosphorylation. +4 ATP NAD is reduced to NADH, 1 each per PGAL +2 NADH + H + Summary: 2 pyruvate produced 2 NADH + H + produced Net 2 ATP produced

Stage 2: Krebs cycle Where: Matrix of mitochondria, but only if O 2 present Why: To oxidize pyruvate to CO 2 To build up a H+ ion gradient used in electron transport

Krebs Cycle: How Pyruvate is decarboxylated. -1 CO 2 Resulting acetic acid (2C) is oxidized by NAD. +1 NADH + H + Acetyl group (2C) has Coenzyme A added 1 Acetyl co-A: link reaction Acetyl co-A (2C) is added to a 4C base molecule, forming a 6C intermediate NAD oxidizes these intermediates CO 2 is given off as a byproduct +2 NADH + H + -1 CO 2 To regenerate the original 4C base, ATP is generated and FAD oxidizes an intermediate +1 ATP +1 FADH 2 NAD oxidizes a 4C to form original 4C molecule: +1 NADH + H +

Krebs cycle summary Per pyruvate that enters: 1 ATP made 3 CO 2 given off 4 NADH produced 1 FADH 2 produced Think: how many pyruvates entered the cycle? How many times must this cycle happen to break down ONE glucose?

Stage 3: Oxidative phosphorylation Where: Inner membrane of mitochondria (on cristae) Why: To produce ATP from H + ion gradient generated during Krebs cycle Requires oxygen!

Oxidative Phosphorylation: How matrix Intermembrane space H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+ ions accumulate in the matrix as a result of NADH picking them up during the Krebs cycle. cytochromes

Oxidative Phosphorylation: How matrix Intermembrane space H+H+ The H + ions diffuse out of the matrix through protein channels into the intermembrane space where they split into H protons and electrons. H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ e- e- e- e- e- e- cytochromes

Oxidative Phosphorylation: How matrix Intermembrane space H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ The accumulated H + ions then move through a pump called ATP synthase to produce ATP. What powers the pump is the electrochemical gradient produced. cytochromes e- ADP ATP

Oxidative Phosphorylation: How matrix Intermembrane space H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ While the H+ protons move through ATP synthase, electrons carried by NADH are passed along electron transport chains composed of cytochromes. cytochromes e- ADP ATP

Oxidative Phosphorylation: How matrix H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Now the H+ atoms are in the matrix. The hydrogen atoms and electrons combine with oxygen, the final electron acceptor of oxidative phosphorylation, to form water. cytochromes e- ADP 34 ATP O H+H+ H+H+

Summary: Oxidative Phosphorylation 34 ATP made H2O generated NADH oxidized back to NAD Very efficient process! Produces a lot of energy.

Remember, the first living organisms lived in an anaerobic environment…

Without oxygen… NADH cannot be oxidized back to NAD+ In order for aerobic respiration to occur, NADH must be oxidized and some intermediate compound must be reduced

Fermentation Includes glycolysis Also side reactions that allow for NADH to be oxidized back to NAD+ by shuttling electrons to intermediate products such as ethanol and lactate

Alcoholic Fermentation Glycolysis happens Pyruvate is then converted to acetaldehyde, CO 2 is released Acetaldehyde is reduced by NADH to ethanol No additional ATP is made Occurs in yeasts, some bacteria

Lactic acid fermentation Glycolysis happens Pyruvate is reduced by NADH and forms lactate (lactic acid) No CO 2 is released No additional ATP is formed Done by bacteria, muscle cells

Other molecules can be used in respiration Proteins: must be deaminated, then converted to pyruvate Fats: undergo beta-oxidation Cells prefer carbs