Objectives & Announcementsfor Friday, September 16

• Objectives:– Continue Energy & Metabolism– Readings for today carry over through Mon. &

Wed.

• Announcements:– Exam I answer key will be posted on Web site

and outside 252 Davenport Hall (DH) by 3:00 PM today

– If you have questions about the actual grading of your exam, please direct them to Melissa Reedy in 208 Noyes

– If you have questions about the problems on the exam, please visit my office hours (do not email or post on LON-CAPA)

– If you took the conflict exam, you may pick up your copy of the exam in 252 Davenport Hall at your convenience

– Group tutoring begins next week; see Twitter for details

• So how does the energy from food get to ATP?

CELLULAR RESPIRATION

– Breakdown of glucose to CO2 and H2O

– Multiple reactions in 3 distinct pathways (phases)

• Glycolysis• Pyruvate Oxidation & The Krebs (or Citric Acid)

Cycle• Electron Transport Chain & Oxidative

Phosphorylation

– Let’s follow the path of energy and molecules from glucose to ATP…

• Glycolysis– First pathway in breakdown of glucose

• "glyco" (sugar) + "lysis" (splitting)• starts with a 6-carbon sugar (glucose), ends

with two 3-carbon molecules (pyruvate)

– Pathway is actually endergonic up to production of first 3-carbon molecules (uses cell's store of ATP)

– Occurs in the cytoplasm of all living cells

• Glycolysis

• 2 steps are endergonic

• 3 steps are exergonic

• E from first exergonic step harnessed by transfer of e- to NAD+ (E not used to directly synthesize ATP, but it’s “saved” and used later)

• E from other exergonic rxns is in form of P, which is used to phosphorylate ADP SLP

• Problems at end of Glycolysis:1.Still aren't at lowest energy state (most E

still in pyruvate)2.NAD+ is used up but not recycled, and is

still holding on to a lot of our energy (will be released when NADH gets oxidized)

• How is energy in pyruvate released?• How is NAD+ replaced?• How is energy in NADH transferred to

ATP?• If oxygen is present:

– Aerobic Respiration

• If oxygen is not present:– Anaerobic Respiration (Fermentation)

• Aerobic Respiration– Carbon source (2 molecules of pyruvate)

completely converted to carbon dioxide• only happens in presence of oxygen• pyruvate molecules are converted to acetyl-CoA,

which then enters the Krebs (or Citric Acid) Cycle• all C-H bonds converted to C-O bonds (6 CO2

released)• energy transferred to NAD+ and FAD (makes

more NADH & FADH2)

• another SLP reaction in Krebs cycle (GTP is ATP analog)

– Where does aerobic respiration take place?• Mitochondria of eukaryotic cells• Cytoplasm and plasma membrane of

prokaryotes

• The Mitochondria

Inner Membrane

Outer Membrane

Matrix

Cristae

• Organization of the Mitochondria:

– Krebs cycle enzymes– pyruvate gets brought into the matrix

Matrix

– Principle site of ATP generation– >70% protein! – Impenetrable to ions/small molecules

except at transporters

Inner Membrane

– Typical protein %– Porins

Outer Membrane

– Composition of ions and small molecules is same as cytoplasm

Intermembrane Space

• Pyruvate Oxidation & Krebs (Citric Acid) Cycle: • 6 CO2

• 8 NADH• 2 FADH2

• 2 GTP (2 ATP)

• Results of Krebs Cycle for every 2 pyruvate:1. 8 more NADH2. 2 FADH2

3. 2 GTP (which are equivalent in energy to ATP)

CO2, NADH

GTP

• Problems with results of Krebs Cycle:1. Still haven't replaced NAD+; in fact, more

NADH is made2. Now you have FADH2 that needs to be re-

oxidized3. Still haven't transferred energy carried by

cofactors to ATP

• Also, why is this dependent on oxygen?– Aerobic respiration requires oxygen, but

Krebs cycle itself does not– Because Krebs cycle is coupled to the

third pathway which does require oxygen –– ETC

• Electron Transport Chain– ETC is a series of "electron carriers" in

membranes:• in prokaryotes – cytoplasmic membrane• in eukaryotes – inner mitochondrial membrane