Brown Fat & Cellular Respiration

White vs. Brown Fat

• White fat stores calories, while brown fat burns energy.
• Brown fat is rich in mitochondria and burns fat through thermogenesis.

Cellular Respiration

• Chemical equation: glucose + 6 O2 → 6 CO2 + 6 H2O + ATP + heat
• Oxidation involves the loss of electrons or hydrogen.
• Reduction involves the gain of electrons or hydrogen.
• Compounds with hydrogen are typically reduced.
• Substrate-level phosphorylation involves transferring a phosphate group from a reactant to ADP, forming ATP.
• Exergonic process where energy is transferred from glucose bonds to form ATP.
• Oxygen is a reactant, breaking down sugars and other food molecules to generate ATP and heat.
• Brown fat generates heat only, without producing ATP in cellular respiration.

Gas Exchange

• Breathing involves the exchange of O2 and CO2.
• Oxygen moves from the lungs to the bloodstream, then to cells for ATP production.
• CO2 is produced as a byproduct.

Redox Reactions

• Oxidation is the loss of electrons or hydrogen.
• Reduction is the gain of electrons or hydrogen.
• Glucose is oxidized to CO2 by losing hydrogen atoms.
• Oxygen is reduced to H2O by gaining hydrogen atoms.
• NAD+ is crucial for oxidizing glucose, accepting electrons to become NADH.

Stages of Cellular Respiration

• Glycolysis
• Pyruvate oxidation and the citric acid cycle
• Oxidative phosphorylation

Glycolysis

• Occurs in the cytosol.
• Glucose (6-C) is split into two molecules of pyruvate (3-C).
• Two molecules of NAD+ are reduced.
• There is a net gain of two ATP molecules.
• ATP is formed through substrate-level phosphorylation, where an enzyme transfers a phosphate group from a substrate to ADP.

Pyruvate Oxidation and Citric Acid Cycle

• Occurs in the mitochondria.
• Pyruvate is oxidized to a two-carbon compound.
• Supplies the third stage with electrons and makes two ATPs.
• Pyruvate is transported from the cytosol to the mitochondrion.
• Coenzyme A combines with a two-carbon group to form acetyl coenzyme A (acetyl CoA).
• For each glucose molecule, there are two CO2, two NADH, and two acetyl CoA.
• Acetyl CoA enters the citric acid cycle and generates multiple NADH and FADH2 molecules.
• After two cycles: six NADH, two FADH2, two ATP, and four CO2 are produced.

Electron Transport Chain (ETC)

• Generates the majority of ATP.
• Occurs in the inner mitochondrial membrane.
• Cristae folds increase the surface area for ATP production.
• Involves electron transport and chemiosmosis.
• REQUIRES OXYGEN
• Utilizes the energy of the gradient to drive ATP synthesis.
• Electrons travel through the ETC to O2 (final electron acceptor), which picks up H+ to form water.
• Chemiosmosis involves H+ diffusing back across the inner membrane through ATP synthase complexes, driving ATP synthesis.
• Carrier molecules, such as NADH, deliver electrons to the ETC.
• Oxygen accepts two electrons, picks up two hydrogens, and is reduced to water.
• Each glucose molecule produces approximately 36 ATP.
• Four ATPs are produced in the first two steps, and 32 are made in the electron transport chain.

Fermentation

• Allows cells to produce ATP without oxygen.
• Lactic acid fermentation: muscle cells oxidize NADH and reduce pyruvate to lactate,also used to make cheese and yogurt.
• Alcoholic fermentation: yeast cells oxidize NADH back to NAD+ and convert pyruvate to CO2 and ethanol.
• Obligate anaerobes require anaerobic conditions and are poisoned by oxygen.
• Facultative anaerobes can produce ATP via fermentation or oxidative phosphorylation e.g., yeast and many bacteria.

Energy Measurement

• A kilocalorie is the amount of heat needed to raise the temperature of 1 liter of water by 1 degree Celsius.

Photosynthesis vs. Cellular Respiration

• Photosynthesis: sunlight is captured by chloroplasts, atoms of CO2 and H2O are rearranged, glucose and O2 are produced.
• Cellular respiration: glucose is broken down to CO2 and H2O, and the cell captures some of the released energy to make ATP.

Diffusion and Osmosis

• Diffusion: movement of molecules from high to low concentration areas.
• Osmosis: diffusion of water across a semi-permeable membrane.
• Water can move freely across the plasma membrane due to aquaporins.
• Water moves towards areas with higher solute concentration.
• Tonicity: the ability of a solution to cause a cell to gain or lose water.
• Hypotonic: low solute concentration.
• Isotonic: same solute concentration.
• Hypertonic: high solute concentration.

Cell Behavior in Different Solutions

• Water moving out of an animal cell causes it to shrivel.
• Water moving into an animal cell causes it to lyse.
• Water moving out of a plant cell causes it to plasmolyze.
• Water moving into a plant cell makes it turgid (normal).
• Turgidity gives fresh vegetables their crispness.
• In hypertonic solutions, plant cells undergo plasmolysis where the plasma membrane pulls away from the cell wall.
• Cell walls prevent plant cells from bursting.
• Animal cells must regulate water balance (osmoregulation) to prevent swelling.

Enzymes

• Speed up reactions by lowering the activation energy.
• Substrates: reactants that enzymes act upon.
• Substrates bind to the active site of the enzyme.
• Substrates are converted to products in chemical reactions.
• Optimal temperature: 35-40°C.
• Optimal pH: around 7.
• Cofactors are non-protein helpers (e.g., zinc, iron, copper).
• Coenzymes are organic cofactors, often vitamins.
• Enzyme inhibition regulates enzyme function.
• Competitive inhibition: inhibitors bind to the active site, blocking substrate binding.
• Non-competitive inhibition: inhibitors bind elsewhere, changing the enzyme structure and affecting the active site.
• Inhibitor binding involves weak bonds and is reversible.
• Feedback inhibition: the product acts as an inhibitor to an enzyme in the pathway.

Passive Transport

• Requires no energy.
• Oxygen and water transport.
• Water moves to where there is less water across a membrane (osmosis).
• Hypertonic: solution concentration higher inside the animal cell.
• Hypotonic: solution concentration lower inside the animal cell
• Channel proteins allow ion passage without energy expenditure.
• Channels are hydrophilic.
• Aquaporins channel water.
• The Sodium-potassium pump is a channel protein that need lots of energy and is used in muscle and brain cells.
• Collects three positive sodium ions and pushes them into the positively charged environment.
• ATP is adenosine triphosphate.
• When the reaction is done and the sodium ions are let go potassium ions are let in.

Vesicular Transport (Cytosis)

• Vesicles transport materials.
• Exocytosis: vesicles transport materials outside the cell.
• Endocytosis: vesicles transport materials inside the cell.
• Phagocytosis: "devouring" cell action; engulfs a solid particle.
• Pinocytosis: "drinking" action; surrounds dissolved substances.
• Receptor-mediated endocytosis: molecules bind to receptors, triggering vesicle formation.

Membrane Proteins

• Attachment proteins: provide cell structure, support, and strength.
• Transport proteins: allow specific ions to enter or exit the cell.
• Receptor proteins: bind to signaling molecules and relay the message.
• Junction proteins: form intercellular junctions that attach adjacent cells.

Additional Cellular Components

• Luciferase is an enzyme that converts luciferin into a molecule that emits light, requiring oxygen and ATP.
• The fluid mosaic model describes membrane structure.
• Glyco refers to an attached carbohydrate.

Membrane Protein Functions

• Attachment points to the extracellular matrix and cytoskeleton.
• Catalyzing enzymatic reactions.
• Receptors for chemical messengers.
• Glycoprotein receptors for cell recognition.
• Cell-to-cell junctions.
• Transport proteins (channels in passive transport and pumps in active transport).

Diffusion

• Particles spread evenly in available space from high to low concentration.
• It is driven by the concentration gradient and aims for dynamic equilibrium (no concentration change on either side of the membrane).
• Small, nonpolar molecules diffuse freely across the membrane.
• Passive transport does not require energy (e.g., oxygen and carbon dioxide exchange).

Osmosis

• Diffusion of water across a membrane.
• Membranes are permeable to water, but not solutes.
• Tonicity: ability of a solution to cause a cell to gain or lose water.
• Movement of water is always towards high solute concentration.
• Hypertonic: water exits the cell (more solute outside).
• Hypotonic: water enters the cell (more solute inside).
• Hydrophobic substances easily diffuse across a membrane, in contrast, polar or charged substances do not and require transport proteins in facilitated diffusion.
• Channel-mediated: proteins form a channel for ion/molecule passage down the concentration gradient that requires no energy input.
• Water diffuses via aquaporin channels.

Active Transport

• Requires energy.
• Moves solutes against their concentration gradient.
• E.g., the sodium-potassium pump moves three sodium ions out for every two potassium ions in.

Bulk Transport

• Exocytosis exports large molecules (proteins, polysaccharides).
• Endocytosis imports large molecules.
• Phagocytosis: engulfment of a particle by wrapping the cell membrane around forming a vacuole.
• Receptor-mediated endocytosis: membrane-bound receptors for specific solutes trigger inward pinching to form a vesicle.
• Pinocytosis: taking in extracellular fluid with no receptor proteins involved.

Energy

• Cells use energy when performing work.
• Kinetic energy: energy of motion.
• Potential energy: energy matter possesses due to location or structure.
• Thermal energy: random movement of atoms/molecules, transferred as heat.
• Chemical energy: potential energy available for release in a chemical reaction.

Thermodynamics

• Study of energy transformations.
• First law: energy in the universe is constant (cannot be created or destroyed).
• Second law: energy transformations are inefficient, releasing energy as heat which increases entropy - measure of disorder or randomness.
• Exergonic reactions release energy stored in covalent bonds where cellular respiration stores energy in ATP.

Other Reaction types

• Endergonic reactions require energy input and yield products rich in potential energy.
• Energy coupling uses energy released from exergonic reactions to drive endergonic reactions. Photosynthesis is an endergonic process that uses energy from sunlight and converts the energy into the chemical energy of sugar.

Metabolism

• The total of an organism's chemical reactions.
• Metabolic pathway: series of chemical reactions that either builds or breaks down a complex molecule.
• Hydrolysis of ATP releases energy by transferring its third phosphate to another molecule, which is called phosphorylation.
• ATP drives chemical, mechanical, and transport work.
• Energy released from breaking down glucose during cellular respiration converts ADP to ATP.

Enzymes - Specifics

• Substrate: specific reactant an enzyme acts on.
• Active site: where the substrate binds to the enzyme.
• Inhibitors: chemicals that interfere with enzyme activity.
• Competitive inhibitors: block substrates from entering the active site.
• Non-competitive inhibitors: bind elsewhere and change the active site shape.
• Feedback inhibition: the product acts as an inhibitor to an enzyme in the pathway.
• Enzyme inhibitors can reduce sensations of pain and inflammation.