Cell Energy Extraction & Obesity

Cell Energy Extraction

• Cell energy extraction breaks down food to create energy for the cell.

Obesity and Adipose Tissue

• Obesity involves an excess of white adipose tissue.
• White adipose tissue stores energy as fat, while brown adipose tissue burns fat to generate heat.
• Uncoupling protein 1 (UCP1) is in the inner mitochondrial membrane of brown adipose tissue.
• UCP1 disrupts the H+ gradient in mitochondria, and allows energy to be released as heat instead of ATP.
• Strategies to combat obesity include increasing brown fat activity or quantity.

Energy Extraction Stages

• Stage 1: Food molecules are broken down in the mouth and gut.
• Stage 2: Molecules are further broken down in the cytosol.
• Stage 3: Final breakdown occurs in the mitochondria.
• Glucose is the main chemical fuel for cells, converted from other carbohydrates, lipids, and proteins.
• Energy is stored in the chemical bonds of these molecules.

Glucose Oxidation

• Direct burning of sugar generates heat in nonliving systems
• Enzymes catalyze the breakdown of sugars in cells.
• Released energy is stored in activated carriers like ATP and NADH.

Thermodynamics in Cells

• The first law of thermodynamics states that energy is neither created nor destroyed
• Energy is the ability to cause change or do work.
• Kinetic energy is associated with relative motion.
• Potential energy is energy possessed by matter due to location or arrangement.
• Chemical energy is energy available in molecules for release in chemical reactions
• Thermal energy is kinetic from random atom and molecule motion, and cells transform energy from one form to another.
• The second law of thermodynamics states that disorder increases, so systems become more disordered as energy is lost as heat.
• Cells generate order, in apparent conflict with the second law, but they increase disorder in their surroundings.

Chemical Reactions in Cells

• Exergonic reactions release energy.
• Exergonic reactions are energetically favorable.
• Exergonic reactions have products with a lower free energy level than reactants.
• Endergonic reactions require energy input
• Endergonic reactions are energetically unfavorable
• Endergonic reactions have products with a higher free energy level than reactants, and an example of this is glucose synthesis.
• Cells couple exergonic and endergonic reactions if they share intermediates.
• Coupling energetically unfavorable reactions with favorable ones allows life.

Activated Carriers

• Activated carriers store energy released by exergonic reactions, like ATP, NADH, and NADPH.
• Adenosine triphosphate consists of 1 adenosine (nitrogenous base), 1 ribose (sugar), and 3 phosphate groups.
• ATP Synthesis is an energetically unfavorable phosphorylation reaction where to ADP (adenosine 5'-diphosphate) a phosphate group is added

Enzymes as Catalysts

• Even favorable reactions need activation energy to begin.
• Enzymes act as catalysts, and lower the activation energy.

Redox Reactions

• Gradual oxidation extracts energy from molecules.
• Transferring energy from one molecule to another usually involves electron transfer.
• Oxidation-reduction (redox) reactions involve one substance transferring electrons to another.
• Reduction is gaining electrons.
• Oxidation is losing electrons.
• Oxidizing agents gain electrons.
• Reducing agents lose electrons.
• Electron transfers can be complete or partial.

Catabolic Processes

• Glycolysis starts glucose catabolism and is anaerobic, converting glucose to pyruvate in the cytoplasm.
• Cellular respiration uses oxygen and pyruvate becomes carbon dioxide, including pyruvate oxidation, the citric acid cycle, oxidative phosphorylation.
• Fermentation is anaerobic, converting pyruvate into lactic acid or ethanol.

Glycolysis Details

• Glycolysis involves 10 enzyme-catalyzed reactions in three phases.
• It requires ATP during an energy-consuming phase
• Glucose is cleaved as part of a cleavage phase
• ATP and NADH are produced during an energy-releasing phase.
• The net result is 2 pyruvate + 2 ATP + 2 NADH.
• Glycolysis couples oxidation and energy storage using glycolytic enzymes to activated carriers.

Cellular Respiration

• Cellular respiration follows glycolysis if oxygen is present
• Cellular respiration consists of pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.
• The process happens in the mitochondria

Pyruvate Oxidation

• Pyruvate is oxidized to an acetate molecule and CO2 in the mitochondrial matrix.
• Acetate is bound to coenzyme A to form acetyl CoA.
• Pyruvate oxidation generates 1 NADH.

Citric Acid Cycle

• The citric acid cycle consists of 8 reactions and finishes glucose oxidation.
• Acetyl CoA begins this cycle
• The acetyl group (2 carbon molecule) is oxidized to 2CO2.
• The net result of the citric acid cycle (one turn) is 2 CO2 + 3 NADH + 1 GTP + 1 FADH2.
• The citric acid cycle operates twice for each glucose from glycolysis.
• Oxidation of one glucose molecule results in carbon dioxide, NADH, FADH2, and ATP.

Oxidative Phosphorylation

• Oxidative phosphorylation is membrane-based, happening in two steps: electron transport and chemiosmosis.
• During electron transport, electrons from NADH and FADH2 pass through the respiratory chain.
• Chemiosmosis has protons diffuse back to the mitochondrial matrix.
• Activated carriers pass high-energy electrons to the electron-transport chain.
• As electrons pass between carriers, protons are transferred from the matrix to the intermembrane space.
• Redox potential increases along the electron-transport chain.
• ATP synthase uses stored electrochemical gradient energy to produce ATP.
• Chemiosmosis drives most ATP synthesis in cells.

UCP1

• UCP1 is localized in the inner mitochondrial and permeabilizes it to protons in brown adipose tissue.

Fermentation

• In the absence of oxygen, fermentation allows ATP production by NADH converting back to NAD+.
• This is lactic acid fermentation in muscle cells, and is alcoholic fermentation in yeast cells

Respiration vs Fermentation

• Cellular respiration yields more energy than fermentation (32 ATP versus 2 ATP)