NOTES FOR BIOLOGY 1001

SECTION 005

Spring 2005

DR. STEVEN POMARICO

CHAPTER 8

HOW CELLS RELEASE STORED ENERGY

Energy flows through ecosystems while the chemicals within an ecosystem are recycled

Energy flow: Light => organic molecules => ATP + heat

The first half:

                     Light => organic molecules

          Is done by photosynthesis.

The second half:

                     organic molecules => ATP + heat

          Is done by glycolysis and cellular (or aerobic) respiration

>>>>>Cellular respiration and fermentation are catabolic pathways

---Fermentation is a means of allowing the ATP production by glycolysis to continue by transferring electron to an organic final electron acceptor.

          -ATP production via glycolysis

          -organic electron donors and acceptors

---Cellular respiration is a means of allowing the ATP production started by glycolysis to continue by transferring electron to an inorganic final electron acceptor.

          -catabolism of organic molecules

          -ATP production

          -inorganic electron acceptor

>>>>>Cellular respiration is a cumulative function of glycolysis, the Krebs cycle, and electron transport.

Three metabolic stages of cellular respiration (See fig 8.3):

          1. Glycolysis

          2. Krebs cycle

          3. Electron transport chain and oxidative phosphorylation

An overview

---Glycolysis

          -Occurs with or without O₂

          -Occurs in the cytoplasm

          -Partially oxidizes glucose (C6) in two pyruvate (C3) molecules.

---Krebs Cycle

          -Occurs in the mitochondrial matrix

          -Completes the oxidation of glucose that glycolysis started, by breaking down a

                     pyruvate derivative (acetyl CoA) into CO₂

The products of glycolysis and the Krebs cycle:

                     -NADH and another reduced coenzyme, FADH

                     -a small amount of ATP generated by substrate-level phosphorylation

---substrate-level phosphorylation is the production of ATP by the enzymatic transfer od a phosphate from a substrate to an ADP

---Electron transport chain and oxidative phosphorylation

          -Located in the inner mitochondrial membrane

          -Accepts electrons from reduced coenzymes (NADH and FADH)

          -Use the energy from electron transfers to make ATP via oxidative

                     phosphorylation

          -Produces most (90%) of the ATP of cellular respiration.

---oxidative phosphorylation is the production of ATP by using the energy captured by the oxidation of organic molecules.

The reactions of glycolysis occur in two phases: (See fig. 8.4)

          1. Energy-requiring steps

                     -uses cellular ATP to phosphorylate glycolysis intermediates

                     -costs two ATP molecules per glucose

          2. Energy-releasing steps

                     -produces ATP

                     -yields 4 ATP molecules per glucose

                     -2 molecules of NAD⁺ to NADH per glucose

The steps in glycolysis (see page 137):

          YOU DO NOT NEED TO MEMORIZE THESE STEPS.

          Step 1: Phosphorylation of glucose

                     -makes glucose more reactive

                     -gives glucose a charge and traps it in the cytoplasm

          Step 2: Rearrangement

                     -shuffle some functional groups

          Step 3: Second phosphorylation

                     -the enzyme at this step is an allosteric enzyme that controls the                                      pathway

                     -makes the substrate (fructose-6-phosphate) more reactive

Steps 1-3 make up the energy-requiring steps

          Step 4: Splitting the 6-carbon sugar into two 3-carbon sugars.

                     -glycolysis is named for this sugar (glyco) split (lysis)

                     -for each glucose there are now 2 product molecules to proceed                           through the pathway

          Step 5: Rearrangement of the two 3-carbon sugars

                     -only one form proceeds through the       remainder of the pathway.

          Step 6: Two steps in one.

                     Step 6a: Rearrangement of the 3-carbon sugar

                                           -2 NADH molecules per glucose

                     Step 6b: Phosphorylation of the 3-carbon sugar

                                           -creates a high energy phosphate bond (like in ATP).

          Step 7: Substrate level phosphorylation

                     -the high energy phosphate of the sugar is transferred                                                                 to ADP to produce ATP.

                     -2 ATP molecules per glucose

          Step 8: Transfer of the phosphate within the sugar molecule.

          Step 9: Rearrangement of the 3-carbon sugar

                     -the rearrangement makes the phosphate bond a high-energy one.

          Step 10: Substrate level phosphorylation

                     -high-energy phosphate of the sugar is transferred

                                           to ADP to produce ATP.

                     -2 ATP molecules per glucose

                     -end up with 2 pyruvate molecules

>>>>>The Krebs cycle completes the energy-yielding oxidation of organic molecules.

The Krebs cycle also goes by two other names:

                                The Citric Acid Cycle

                                The TCA (tricarboxylic acid) Cycle

The reaction that connects glycolysis to the Krebs cycle is the preparatory step or bridge reaction which converts pyruvate to acetyl-CoA. (See fig. 8.6)

                     1. Removal of CO₂

                     2. Production of NADH from NAD⁺.

                                Two molecules of NADH per glucose molecule.

                     3. Attachment of coenzyme A (a.k.a. CoA) to form acetyl-CoA.

---The Krebs cycle (see fig. 8-5) 

          -Pathway discovered by Hans Krebs (thus the name Krebs cycle)

          -occurs in the mitochondrial matrix

The steps of the Krebs cycle (see page 139):

          Step 1: Two Carbons (from acetyl-CoA) enter the cycle.

                     -The 2 carbons entering the cycle combine with 4 carbons from                             oxalacetate to form a 6 carbon citrate molecule.

          Step 2: Rearrangement of Citrate to Isocitrate

          Step 3: Rearrangement of Isocitrate and loss of CO₂

                     Step 3a: Loss of CO₂

                     Step 3b: Production of NADH from NAD⁺

          Step 4: Three steps in one, catalyzed by a multi-enzyme complex

                                (Similar to the bridge reaction)

                     Step 4a: Loss of CO₂

                     Step 4b: Production of NADH from NAD⁺

                     Step 4c: Production of ATP from ADP

          Step 5: Rearrangement of carbons - Succinic acid to fumaric acid

                                -Production of FADH₂ from FAD

          Step 6: Rearrangement of carbons - fumaric acid to malic acid

          Step 7: Rearrangement of carbons - malic acid to oxaloacetic acid

                                -regeneration of one of the starting materials.

                                -production of NADH from NAD⁺

>>>>>The inner mitochondrial membrane couples electron transport to ATP synthesis.

The energy stored (as electrons) in NADH and FADH₂ is harvested by the passage of electrons through the electron transport system. The harvested energy is used to produce ATP

>>>>>Chemiosmosis: The energy-coupling mechanism. (See fig 8.7)

The electron transport chain does not make ATP directly. Instead it generates a proton gradient across the inner mitochondrial membrane.

---Chemiosmosis is the coupling of exergonic electron flow down an electron transport chain to endergonic ATP production by the creation of a proton gradient across a membrane. The proton gradient drives ATP synthesis as protons diffuse back across the membrane.

The inner mitochondrial membrane is the site of chemiosmotic ATP synthesis.

          -the proton gradient formed by the electron transport chain exists across              the inner mitochondrial membrane.

          -there are many copies of the protein complex that makes ATP (ATP                    synthase) in the inner mitochondrial membrane.

          -the enfoldings of the inner mitochondrial membrane (cristae) increase the                      surface area available for chemiosmosis.

                     SO WHERE IS ALL THE ATP??

                                (See page 141)

For every NADH that feeds into the electron transport chain 3 protons are moved from the mitochondrial matrix to the outside of the inner membrane.

For every FADH₂ that feeds into the electron transport chain 2 protons are moved from the mitochondrial matrix to the outside of the inner membrane.

For every proton that crosses back into the mitochondrial matrix one ATP is synthesized by ATP synthase.

>>>Fermentation

If there is no oxygen present then the pyruvic acid molecules from glycolysis go through fermentation

          -can be anaerobic (i.e., take place without O₂)

          -No ATP production

          -results in the partial degradation of sugars

          -regenerates NAD⁺

>Two types of fermentation (see fig 9.16)

The two most common products of pyruvate reduction are either ethanol or lactic acid.

          1. Alcoholic or Ethanol fermentation

                      pyruvate loses a CO₂ and ethanol is produced

.

                                Glucose => pyruvate => ethanol + CO₂

          2. Lactate fermentation

                     pyruvate is reduced to lactate.

                                Glucose => pyruvate => lactate

Many bacteria and yeast carry out ethanol fermentation under anaerobic conditions.

Under anaerobic conditions muscles carry out lactate fermentation instead of oxidative phosphorylation.

>>>>>Glycolysis and the Krebs cycle are at a metabolic crossroads

Cellular respiration can accept components from most of the major types of macromolecules found in food:

          -Carbohydrates

                     -most of the polysaccharide breakdown products can be converted to                                 either glucose or fructose.

          -Fats

                     -Glycerol can enter glycolysis at the start of the energy yielding step

                     -Fatty acids are broken down to 2-carbon acetyl groups         and enter at                                 the Krebs cycle.

          -Proteins

                     -Proteins are hydrolyzed to amino acids, the amino group is                       removed and enter at pyruvate or later (bridge reactions                              or Krebs cycle)

>>>>>Feedback mechanisms and control of cellular respiration

Cells can switch off the pathways they don’t need by feedback inhibition.

The ratio of ATP/ADP reflects the energy state of a cell.

          Energy high      =>   ATP/ADP high

          Energy low      =>   ATP/ADP low

The key, regulatory point is in glycolysis