Download File - Ms. Richards IB Biology HL

Topic 8
Metabolism, cell respiration,
and photosynthesis
8.2 Essential idea: Energy is converted to a usable form in cell
respiration
8.2 Cell respiration
Understandings
• Cell respiration involves the oxidation and reduction of electron carriers
• Phosphorylation of molecules makes them less stable
• In glycolysis, glucose is converted to pyruvate in the cytoplasm
• Glycolysis gives a small net gain of ATP without the use of oxygen
• In aerobic respiration pyruvate is decarboxylated and oxidized and converted into acetyl compound and attached to
coenzyme A to form acetyl coenzyme A in the link reaction
• In the Kreb cycle, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers, liberating carbon
dioxide
• Energy is released by oxidation reactions is carried to the cristae of the mitochondria by reduction of NAD and FAD
• Transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is couple to
proton pumping
• Oxygen is the final electron acceptor
• In chemiosmosis protons diffuse through ATP synthase to generate ATP
• Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation of water
• The structure of the mitochondrion is adapted to the function it performs
8.2 Cell respiration
Applications and Skills:
• Application: Electron tomography used to produce images of active
mitochondria
• Skill: Analysis of diagrams of the pathways of aerobic respiration to
deduce where decarboxylation and oxidation reactions occur
• Skill: Annotation of a diagram of a mitochondrion to indicate the
adaptations to its function
Cellular Respiration: Harvesting Chemical
Energy
• This is the part of biology that deals with recycling the ATP used in
cellular work. All living organisms use ATP for:
1. Transport work- by phosphorylating membrane proteins
2. Mechanical work- phosphorylating motor proteins
3. Chemical work- energizing other molecules (unstable
phosphorylated intermediates)
ATP and Cellular Work
Cellular Respiration
• ATP is obtained or recycled in all organisms, including plants, in the
catabolic processes of fermentation (without oxygen= anaerobic) and
cellular respiration
• Most efficient catabolic pathway is cellular respiration, in which
oxygen is consumed as a reactant along with the organic fuel
• Most of cellular respiration takes place in the mitochondria
Overall Process
• Organic compounds + Oxygen  Carbon dioxide + Water + Energy
• For convenience we usually start with glucose, but can use lipids,
proteins and other carbohydrates
• C6H12O6 + 6 O2  6 CO2 + 6H2O + Energy
• Glucose is oxidized and oxygen is reduced
Oxidation-Reduction
• Always coupled
• Chemical reactions which involve a partial or complete transfer of
electrons from one reactant to another
• Oxidation: partial or complete loss of e- from a substance; e- donor is
the reducing agent
• Reduction: partial or complete addition of e- to another substance; eacceptor is an oxidizing agent
Methane Combustion: Oxidation and
Reduction
Methane Combustion
1. Covalent electrons of methane are equally shared because carbon
and hydrogen have similar electronegativities
2. As methane reacts with oxygen to form carbon dioxide, electrons
shift away from carbon and hydrogen to the more electronegative
oxygen
3. Since electrons lose potential energy when they shift closer to the
more electronegative atoms, redox reactions that move electrons
closer to oxygen release energy
• Look back at the equation for cellular respiration and note:
1. Valence e- of carbon and hydrogen lose potential energy as they
shift toward electronegative oxygen
2. Released energy is used by cells to produce ATP
3. Carbs – fats: excellent energy stores, because they are rich in C to H
bonds
• Electrons “fall” from organic molecules to oxygen during cellular
respiration.
• The “fall” of electrons is stepwise, via NAD+ and an electron
transport chain
Oxidation vs. Reduction
Oxidation
Addition of oxygen atoms
Removal of H atoms
Loss of e- from a substance
Reduction
Removal of oxygen atoms
Addition of H atoms
Addition of e- to a substance
NAD+
• Co-enzyme which acts as an e- acceptor for the hydrogens stripped
from glucose
• Nicotinamide adenine dinucleotide- found in all cells, assists enzymes
in e- transfer during redox
• Functions as an oxidizing agent by trapping e- from food
Dehydrogenases
• Remove a pair of hydrogen atoms (2 e- and 2 protons) from substrate
• Deliver the two electrons and one proton to NAD+
• Release the remaining proton into the surrounding solution.
• High energy e- transferred from substrate to NAD+ and then passed
down the electron transport chain to oxygen, powering ATP synthesis
in a process called oxidative phosphorylation
NAD: Electron Shuttle
Oxidizing agent when it is reduced
Electron Transport Chain
• Convert some of the chemical energy extracted from food to a form
that can be used to make ATP
• Composed of electron-carrier molecules built into the inner
mitochondrial membrane
• Accept energy-rich e- from reduced coenzymes (NADH and FADH2)
and pass down chain to oxygen. Water is formed
• Energy is released in a controlled stepwise fashion. E- transfer is
exergonic
Electron Transport: Controlled release of
energy
Overview of Cell Respiration
Glycolysis
• Glyco-lysis  splitting of glucose
• Catalyzed by enzymes in the cytoplasm
• Glucose is partially oxidized and a small amount of ATP is produced
• Accomplished without the use of oxygen
• Is part of both aerobic and anaerobic respiration
Glycolysis Overview
Glycolysis
• Energy investment phase: 2 phosphate groups from ATP are added
to a molecule of glucose to form a hexose biphosphate
Glycolysis
• Stage 2 Lysis: The hexose biphosphate is split to form two molecules
of triose phosphate.
Glycolysis
• Stage 3 Oxidation: 2 molecules of NAD+ are reduced to 2 NADH and
2H+ so the triose phosphate is oxidized
• The energy is used to add another phosphate group to each triose
• NADH can enter the electron transport chain in the mitochondria and
be used to produce more ATP in the process called oxidative
phosphorylation
Glycolysis
• Stage 4 ATP Formation: Two phosphate groups are removed from the
two trioses and passed to ADP to form ATP
• So 4 ATPs are generated for a net gain of 2 ATPs
• ATP is produced by a process called substrate-level phosphorylation
because an enzyme transfers a phosphate group from a substrate
(organic molecule generated by the sequential breakdown of glucose)
to ADP
Substrate Level Phosphorylation
Glycolysis
End of Glycolysis
• A 6-Carbon compound has been turned into 2 3-Carbon compounds
called pyruvate (or oxopropanoate)
• Glucose has been oxidized
• Glycolysis also yields 2 water molecules for each glucose
Hexose biphosphate
Lysis
2 triose phosphate molecules
Oxidation
ATP Formation
Link Reaction
• Pyruvate oxidation and the formation of acetyl
• Acetyl is a 2-Carbon compound aka ethanol
• Occurs after pyruvate molecules are translocated from the cytosol
into the mitochondrion by carrier proteins in the mitochondrial
membrane
• Involves the removal of a molecule of carbon dioxide
• Oxidizes 2 carbon fragment to acetate while reducing NAD+ to NADH
2 per glucose
Link Reaction
• Attaches coenzyme A to the acetyl group, forming acetyl CoA
• This bond is unstable, making acetyl (ethanol) very reactive
• It is acetyl CoA that enters the Krebs Cycle
The Link Reaction: oxidative decarboxylation
The Kreb Cycle
• Explicated by German-British scientist Hans Krebs
• Also called citric acid cycle or tricarboxylic acid cycle
• Oxidation of the remaining acetyl fragments to CO2
• Energy from this exergonic process is used to reduce coenzymes
NAD+ and FAD (both electron carrier molecules) and to
phosphorylate ATP (once again through substrate level
phosphorylation)
Entry into the Krebs cycle (Citric Acid Cycle):
2 C + 4C = 6C
Kreb Cycle
• A 2-carbon acetyl group bonds to the 4-carbon oxaloacetate to form
the 6-carbon citrate
• The 6-C compound loses CO2 and becomes a 5-C compound
• 5-C compound is oxidized and NAD+ is reduced
• Another CO2 is removed
• 4 C compound is oxidized and NAD+ is reduced
6C – 1CO2= 5 C
5C - 1CO2= 4 C
Kreb Cycle
• A molecule of ATP is formed
• Co-enzyme FAD is reduced to FADH2
• Another NAD+ is reduced and oxaloacetate is regenerated to begin
this cycle again
Hydrogen-carrying coenzymes: NAD and FAD
The starting material is regenerated and we
go around again!
Kreb Cycle Summary
Krebs cycle results per glucose
• 2 molecules of pyruvate are oxidized
• 2 ATPs by substrate level phosphorylation
• 6 NADH and 2 FADH2
• Starting material is regenerated
• Electron transport chain couples electron flow down the chain to ATP
synthesis
Electron Transport Chain
• Most molecules of ATP are produced during oxidative
phosphorylation
• The reduced coenzymes NADH and FADH2 link glycolysis, the link
reaction, and the Krebs cycle to oxidative phosphorylation by passing
their e- down the electron transport chain to oxygen
• This exergonic transfer of e- is coupled to ATP synthesis
Electron Transport Chain
Cristae
• Movement of electrons
through progressively more
electronegative molecules
down to oxygen
Electron Transport Chain
• Made of electron carrier molecules embedded in the inner
mitochondrial membrane
• Each successive carrier in chain has a higher electronegativity than
the carrier before it, so that the e- are pulled down hill to oxygen
• Most carriers are proteins bound to nonprotein cofactors which
alternate between reduced and oxidized states as they accept and
donate electrons
• As molecular oxygen is reduced, it also picks up 2 protons from the
medium. For every 2 NADHs, one O2 is reduced to 2 water molecules
• FADH2 also donates e- but at a lower energy level
• Chain does not make ATP directly
• It generates a proton gradient across inner mitochondrial membrane
which stores potential energy that can be used to phosphorylate ADP
Chemiosmosis
Oxidative Phosphorylation
• The electron transport chain (oxidative because of oxygen’s pull on e-)
• Combines with the flow of H+ through ATP synthase called
chemiosmosis (phosphorylation because ATP is made with the
energy)
• To give the process of Oxidative Phosphorylation
The Energy-Coupling Mechanism
• The exergonic e- flow from the oxidation of food is used to pump H+
across the inner mitochondrial membrane from the mitochondrial
matrix to the intermediate space
• The energy from the flow of electrons is used to set up a gradient of
pH (H+)
• The pH of the intermembrane space is 1 to 2 pH units lower that the
matrix but same as cytosol
ATP Synthase
• H+ can leak back across the inner membrane at specific sites since
the membrane’s phospholipid bilayer is impermeable to H+ and
prevents diffusion back
• This specific site is a protein complex embedded in the mitochondrial
membrane – ATP Synthase
• ATP Synthase is the enzyme that makes ATP
ATP Synthase
• ATP Synthase uses the potential energy stored in a proton gradient to
make ATP by allowing H+ to diffuse down the gradient back across
the membrane
• Protons diffuse through ATP synthase complex which causes
phosphorylation of ADP
• H+ gradient is called proton-motive force to emphasize gradientpotential energy
ATP Synthase: Converts H+ gradient into ADP
Electrochemical gradient
• Has a concentration gradient of protons
• Has a voltage across the membrane because of a higher
concentration of positively charged protons on one side
• Tends to drive protons across the membrane back into the matrix
Chemiosmosis
Cell Respiration Review
Oxidation of organic nutrients in the absence
of oxygen
• Anaerobic fermentation
• Glycolysis oxidizes glucose to pyruvate
• The oxidizing agent is NAD+, not oxygen
• Net production of ATP = 2
• In fermentation, pyruvate is reduced and NAD+ is regenerated with
no additional ATP produced
Alcohol Fermentation
• Alcohol fermentation- pyruvate is converted to ethanol in two steps
1. Loses CO2  acetaldehyde
2. NADH is oxidized to NAD+ and acetaldehyde is reduced to ethanol
• Common in bacteria and yeast in anaerobic conditions
Fermentation: Regenerating NAD+
Lactic Acid Fermentation
• NADH is oxidized to NAD+ and pyruvate is reduced to lactate
• Commercially important products include cheese and yogurt
• Occurs when O2 is scarce in human muscle cells which switch from
aerobic respiration to lactic acid fermentation
• Lactate accumulates, carried to liver, converted back to pyruvate
when O2 available
Lactic Acid Fermentation
Branching off point for pyruvate
Oxidation of other organic compounds
• Glycolysis and Krebs cycle connect to many other metabolic pathways
• Complex molecules such as fats, proteins, disaccharides and
polysaccharides must be hydrolyzed to simpler molecules or
monomers that can enter the intermediate reaction of glycolysis or
Krebs if they are to be oxidized to make ATP
Fats
• Rich in hydrogens and high energy e• Fats are digested into glycerol and fatty acids. Glycerol can be
converted to an intermediate of glycolysis
• Most of the energy in fats is in fatty acids which can be converted
into acetyl Co-A by beta oxidation
• Can then enter the Krebs cycle
Proteins and fats in cell respiration: Central
role of Acetyl CoA
Carbohydrates and Proteins
• Notice that complex carbohydrates would be hydrolyzed to glucose
• Proteins are hydrolyzed to amino acids. Excess amino acids are
enzymatically converted to intermediates of glycolysis and Krebs
cycle. Common intermediates are pyruvate, acetyl Co-A
• Conversion process deaminates amino acids and the resulting
nitrogenous wastes are excreted
Feedback and Control of cellular respiration
• 1st on-off switch occurs in glycolysis. Enzyme phosphofructokinase is
inhibited by ATP and stimulated by ADP. ATP is allosteric inhibitor of
this enzyme
• It is also sensitive to the concentration of citrate.
Helps to synchronize the rates of glycolysis and
Krebs cycle
A 8.2.1 Electron tomography used to produce
images of active mitochondria
• Used to obtain a three dimensional model of active mitochondria
• Tomography is technique that images sections through the body using
X-rays or ultrasound
• This has shown that cristae are connected with the intermembrane
space between the inner and outer membranes by narrow openings
• When the mitochondria is active the volume and shape of cristae
change
S 8.2.1 Analysis of diagrams of the pathways of aerobic
respiration to deduce where decarboxylation and oxidation
reactions occur
S 8.2.1 Analysis of diagrams of the pathways of aerobic
respiration to deduce where decarboxylation and oxidation
reactions occur
S 8.2.1 Analysis of diagrams of the pathways of aerobic
respiration to deduce where decarboxylation and oxidation
reactions occur
Where does decarboxylation occur?
S 8.2.1 Analysis of diagrams of the pathways of aerobic
respiration to deduce where decarboxylation and oxidation
reactions occur
S 8.2.1 Analysis of diagrams of the pathways of aerobic
respiration to deduce where decarboxylation and oxidation
reactions occur
S 8.2.2 Annotation of a diagram of a mitochondrion to
indicate the adaptations to its function
• Structure and function in organisms is closely related
• It is a result of natural selection and is called adaptation