Cellular Respiration

parts of a nucleotide

• nitrogenous base

• pentose/five-carbon sugar

• one or more phosphate groups

properties of ATP which make it suitable for energy currency of the cell

• soluble in water, can move freely through the cytoplasm and other aqueous solutions in the cell

• stable at pH levels close to neutral as in cytoplasm

• cannot pass freely through phospholipid bilayer of membranes, allowing movement between membrane-bound organelles to be controlled

• third phosphate group of ATP can be easily removed and reattached by hydrolysis and condensation reactions

• hydrolyzing ATP to ADP and phosphate releases a relatively small amount of energy

ATP hydrolysis and condensation reactions

ATP + H2O <—> ADP + phosphate + energy

hydrolyzing ATP to ADP and phosphate releases a relatively small amount of energy

• enough energy for many processes within the cell

• not an excess which would be wasted by conversion to heat

life processes within cells that ATP supplies with eenergy

1. synthesizing macromolecules

2. active transport

3. movement

role of ATP in synthesizing macromolecules

anabolic reactions that link monomers together into large polymers would be endothermic and therefore unlikely to happen without coupling them to conversion of ATP to ADP (e.g. synthesis of DNA during replication)

role of ATP in active transport

• ATP energy used to cause reversible changes in conformation of pump protein, allowing particle to enter pump protein from one side of the membrane

• when the pump is in the other conformation, the particle can exit on the other side of the membrane

• one of the two shapes is more stable than the other

• ATP used to cause change from more stable to less stable conformation, change back to stable conformation happens without need for energy

role of ATP in movement

• movement of cell components

• change shape of cell, locomotion

source of energy required to convert ADP and phosphate back to ATP

• cell respiration - oxidation of carbohydrates, fats, or proteins

• photosynthesis - light energy converted to chemical energy

• chemosynthesis - oxidation of inorganic substances such as sulfides

efficiency of interconversion of ATP and ADP

not 100%, some energy transformed into heat

gas exchange

carbon dioxide and oxygen move across the plasma membrane independently by simple diffusion (not one-for-one swapping), interdependent process with cell respiration

aerobic respiration in humans and many other animals and plants

C6H12O6 + 6O2 -(ADP→ATP)→ 6CO2 + 6H2O

anaerobic respiration in humans, other animals, and some bacteria

glucose -(ADP→ATP)→ lactate

anaerobic respiration in yeast and other fungi

glucose -(ADP→ATP)→ ethanol + carbon dioxide

features unique to aerobic cell respiration

• oxygen used as electron acceptor in oxidation reactions

• carbohydrates such as glucose, lipids including fats and oils, and amino acids after deamination can be used

• CO2 and H2O are waste products

• ATP yield of more than 30 ATP molecules per glucose

• initial reactions are in the cytoplasm, more reactions in mitochondria

features unique to anaerobic cell respiration

• oxygen is not used - other substances act as oxygen acceptors in oxidation reactions

• only carbohydrates can be used

• CO2 plus either lactate or ethanol are the waste products, water is not produced

• ATP yield of only 2 ATP per glucose

• all reactions happen in the cytoplasm, mitochondria not required

lactate/lactic acid

waste product of anaerobic respiration in muscles, limited tolerance in human body, breakdown requires oxygen

oxygen debt

demand for oxygen that builds up during a period of anaerobic respiration

oxidation reaction

loss of electrons and hydrogen atoms, release energy ex. C6H12O6 → 6CO2

reduction reaction

gain of electrons and hydrogen atoms, gain energy ex. 6O2 → 6H2O

electron carriers

substances that can accept and lose electrons reversibly, often linking oxidations and reductions in cells

NAD reduction

NAD+ + 2H+ + 2e- → NADH + H+

role of FAD as a carrier of Hydrogen

FAD → FADH2

stages of cellular respiration

1. glycolysis

2. link reaction

3. Krebs/citric acid cycle

4. electron transport chain and chemiosmosis

glycolysis

conversion of glucose to pyruvate in the cytoplasm of cells by aa chain of reactions, each catalyzed by a different enzyme, creating a small yield of ATP without oxygen consumption

1. phosphorylation of glucose

2. lysis

3. oxidation

4. ATP formation

1. phosphorylation of glucose

addition of PO₄^3- group(s), makes molecule more unstable and therefore more likely to participate in subsequent reactions

• glucose -(ATP→ADP)→ glucose-6-phosphate (phosphorylation)

• glucose-6-phosphate → fructose-6-phosphate

• fructose-6-phosphate -(ATP→ADP)→ fructose-1,6-bisphosphate

2. lysis

fructose-1,6-bisphosphate → 2 triose phosphate

3. oxidation

triose phosphate + phosphate -(NAD→reduced NAD)→ bisphosphoglycerate

• energy released by oxidation of triose allows second phosphate group to become attached

4. ATP formation

bisphosphoglycerate -(2ADP→2ATP)→ pyruvate

• transfer of phosphate groups to ADP occurs twice because bisphosphoglycerate has two phosphates

• 4 ATPs produced per glucose

• glycose (6C) converted to 2 pyruvate (3C each)

• 2 NAD converted to 2 reduced NAD

• net yield of 2 ATPs

uses of pyruvate

anaerobic respiration (alcoholic fermentation, lactic acid fermentation) and aerobic respiration depending on oxygen availability

anaerobic respiration

• NADH + H+ donates its H+ and e- to pyruvate during ethanol or lactate production, respectively

• NAD+ becomes available again so that e- and H+ can continue to be transferred from glucose to NAD+ when oxidized

• thus pyruvate synthesis during glycolysis continues

alcoholic fermentation

C6H12O6 → 2 pyruvate → 2 Ethanol + 2CO2 (net 2 ATP)

• in yeast (eukaryotic cell) and some bacteria (prokaryotic cell)

• cytoplasm

yeast as an example of anaerobic cell respiration

facultative anaerobe single-celled fungus, can respire aerobically or anaerobically

• breaks down starch and sugars in dough by alcoholic fermentation to make CO2 and ethanol

• CO2 released in fermentation trapped in dough, causing bread to rise

• bread baked in oven to kill yeast and trap ethanol

lactic acid fermentation

C6H12O6 → 2 Lactate (net 2 ATP)

• some bacteria (prokaryotic cell) and some mammals

• cytoplasm

aerobic respiration

C6H12O6 → 6CO2 + 6H2O (net 32-34 ATP)

before the link reaction

pyruvate/pyruvic acid enters mitochondrial matrix by facilitated diffusion to be processed further

link reaction

1. decarboxylation of pyruvate (3C) to acetate (2C)

   1. CO2 leaves cell, diffuses into the bloodstream and is expired

2. binding of enzyme CoA to acetate

3. oxidation of acetate into acetyl-coenzyme A

Krebs/citric acid cycle

oxidation and decarboxylation of acetyl groups

• acetyl groups (2C) fed into cycle by transfer from coenzme A to oxaloacetate (4C) → citrate (6C)

• citrate converted back to oxaloacetate by enzyme-catalyzed reactions, two carbons lost through decarboxylation reactions, creating CO2 waste product

• 4 oxidation reactions release energy, mainly held in electrons removed from oxidation and transferred through reduction of NAD+ and FAD

• net products per acetyl-CoA molecule: 2CO2, 3NADH, 1 FADH2, 1 ATP

change in free energy graph in electron transport chain

in each drop, energy is transferred to energy-storing molecules NAD+ and FAD, which later become oxidized again in electron transport chain

• energy gained from oxidation reaction used to make ATP

transfer of energy to the electron transport chain

electron carriers NAD+ and FAD bring electrons and hydrogen ions to electron transport chain in cristae of the mitochondria

electron transport chain

• reduced electron carriers NADH + H+ and FADH2 from glycolysis and Krebs cycle move to inner mitochondrial membrane

• membrane proteins accept electrons from NADH and FADH2

• each carrier in chain has slightly higher electronegativity and therefore a stronger attraction for electrons than previous carrier

• electrons passed down an energy gradient until they reach the end of the chain

• electrons “fall” from higher levels to lower ones, energy released used to pump protons from matrix into intermembrane space against the concentration gradient

• proton gradient drives chemiosmosis

membrane proteins in electron transport chain

integral carrier and channel proteins embedded within phospholipid bilayer have a high tendency to become reduced by accepting electron

chemiosmosis

• transfer of H+ sets up concentration gradient across the membrane as H+ accumulates in the intermembrane space

• protons follow natural concentration gradient by moving through the ATP synthase into matrix

• oxidative phosphorylation

• oxygen as the terminal electron acceptor

oxidative phosphorylation in between chemiosmosis and ATP synthesis

movement of protons through ATP synthase down the concentration gradient releases energy used to phosphorylate ADP to ATP

ATP synthase

complex of integral proteins located in the mitochondrial inner membrane where it catalyzes the synthesis of ATP from ADP and phosphate, driven by a flow of protons

role of oxygen as terminal electron acceptor

in the reduction of the oxygen molecule, the O2 accepts electrons and forms a covalent bond with hydrogen to produce H2O

• in the matrix, H+ combines with ½ O2 + 2e- to form water

carbohydrates as respiratory substances

simple sugars (glucose or fructose) can be used straight away in glycolysis and anaerobic respiration

lipids as respiratory substances

lipids broken down into glycerol and fatty acids, fatty acids converted to acetyl groups to be used in Krebs cycle