Cell Membranes, Transport, Energy, and Enzyme Regulation: General Biology Study Guide

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Cell Membrane Structure and Function

Components of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds cells, composed of a phospholipid bilayer with embedded proteins and other molecules.

Phospholipids: Form the basic structure of the membrane, creating a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.
Peripheral Proteins: Attached to the surface of the membrane; involved in signaling and maintaining cell shape.
Integral Proteins: Span the membrane; function as channels, transporters, or receptors.
Glycolipids: Lipids with carbohydrate chains; contribute to cell recognition and communication.
Glycoproteins: Proteins with carbohydrate chains; play roles in cell-cell interactions and immune response.
Cholesterol: Stabilizes membrane fluidity and integrity, especially at varying temperatures.

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a dynamic structure with proteins and other molecules floating in or on the fluid lipid bilayer.

Fluidity: Lipids and proteins can move laterally within the layer, allowing flexibility.
Mosaic: The membrane is a patchwork of different proteins embedded in the lipid bilayer.
Self-healing: The membrane can repair minor tears due to its fluid nature.

Effects of Temperature on Membrane

Temperature changes can affect membrane fluidity and function.

Too Cold: Membrane becomes rigid, reducing permeability and function.
Too Hot: Membrane becomes too fluid, possibly leading to loss of integrity and leakage.

Membrane Transport

Types of Membrane Transport

Cells transport substances across membranes using various mechanisms, classified as passive or active transport.

Selectively Permeable: Membranes allow certain molecules to pass while blocking others.
Passive Transport: Movement of substances down their concentration gradient without energy input.
Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).
Diffusion: Movement of molecules from high to low concentration.
Osmosis: Diffusion of water across a selectively permeable membrane.
Hypotonic: Solution with lower solute concentration than the cell; water enters the cell.

Simple Diffusion

Simple diffusion is the passive movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

Example: Oxygen entering a cell from the bloodstream.

Osmosis and Turgor Pressure

Osmosis is the movement of water across membranes, while turgor pressure is the pressure exerted by water inside plant cells against the cell wall.

Osmosis: Water moves from areas of low solute concentration to high solute concentration.
Turgor Pressure: Maintains plant cell rigidity and structure.

Facilitated Diffusion

Facilitated diffusion uses membrane proteins to help polar or large molecules cross the membrane without energy input.

Carrier Proteins: Bind and transport specific molecules across the membrane.
Channel Proteins: Form pores for ions or water to pass through.
Saturation: Carrier proteins can become saturated when all binding sites are occupied, limiting transport rate.

Active Transport and Co-Transport

Active transport requires energy to move substances against their gradient. Co-transport involves the simultaneous transport of two substances.

Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump).
Co-Transport: Coupled transport of two substances, such as glucose and Na+ in intestinal cells.

Endocytosis and Exocytosis

Cells use vesicles to transport large molecules or particles.

Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor binding.
Exocytosis: Release of substances from the cell via vesicle fusion with the membrane.

Energy and Metabolism

Forms of Energy

Cells use various forms of energy to drive biological processes.

Potential Energy: Stored energy due to position or structure.
Kinetic Energy: Energy of motion.
Chemical Energy: Energy stored in chemical bonds.

Metabolic Pathways

Metabolism includes all chemical reactions in a cell, organized into pathways.

Catabolism: Breakdown of molecules to release energy.
Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics.

First Law: Energy cannot be created or destroyed, only transformed.
Second Law: Every energy transfer increases the entropy (disorder) of the universe.
Entropy: Measure of disorder; related to the second law as systems tend toward increased entropy.

Enzymes and Regulation

Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

Catalyst: Substance that increases reaction rate without being consumed.
Enzyme: Protein catalyst in biological systems.
Ribozyme: RNA molecule with catalytic activity.
Substrate: Reactant acted upon by an enzyme.
Active Site: Region on enzyme where substrate binds.
Induced Fit: Enzyme changes shape to better fit the substrate.
Enzyme-Substrate Complex: Temporary association during reaction.
Cofactor/Coenzyme: Non-protein helpers required for enzyme activity.
Rate of Reaction: Speed at which reactants are converted to products.

Enzyme Reaction Graphs

Graphs of free energy vs. time show how enzymes lower activation energy.

Activation Energy (): Energy required to start a reaction.
Enzyme lowers : Makes reactions proceed faster.

Enzyme Kinetics

Enzyme activity depends on substrate concentration and can be graphed as reaction rate vs. substrate concentration.

Saturation: At high substrate concentrations, all enzyme active sites are occupied, and rate plateaus.

Enzyme Inhibition and Regulation

Enzymes can be regulated by inhibitors and feedback mechanisms.

Irreversible Inhibition: Inhibitor binds permanently, inactivating enzyme.
Reversible Inhibition: Inhibitor binds non-covalently; can be competitive (binds active site) or noncompetitive (binds elsewhere).
Allosteric Regulation: Regulation by binding at a site other than the active site.
Feedback Inhibition: End product of pathway inhibits an earlier step, maintaining homeostasis.

Metabolic Pathways and Regulation

Metabolic pathways are sequences of enzyme-catalyzed reactions. Regulation occurs via enzyme activity, feedback inhibition, and gene expression.

ATP and Cellular Energy

ATP: The Energy Currency

ATP (adenosine triphosphate) stores and transfers energy for cellular processes.

Structure: Adenine base, ribose sugar, three phosphate groups.
ATP Hydrolysis: Releases energy by breaking a phosphate bond:
AMP: Adenosine monophosphate, with one phosphate group.

OIL RIG and Redox Reactions

Redox reactions involve the transfer of electrons.

OIL RIG: "Oxidation Is Loss, Reduction Is Gain" (of electrons).
Reduction: Gain of electrons.
Oxidation: Loss of electrons.

Phosphorylation and ATP Production

ATP is produced by three types of phosphorylation:

Type	Description	Organisms
Substrate-level phosphorylation	Direct transfer of phosphate to ADP from a substrate	All cells
Oxidative phosphorylation	ATP synthesis using energy from electron transport chain	Eukaryotes, some prokaryotes
Photophosphorylation	ATP synthesis using light energy (photosynthesis)	Plants, algae, cyanobacteria

Chemiosmosis and Proton Motive Force

Chemiosmosis is the process by which ATP is generated using a proton gradient across a membrane.

Proton Motive Force: The electrochemical gradient of protons that drives ATP synthesis.
Difference: Chemiosmosis refers to the process; proton motive force is the driving force.

NAD and FAD

NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are electron carriers in cellular respiration.

NAD: Exists as NAD+ (oxidized) and NADH (reduced).
FAD: Exists as FAD (oxidized) and FADH2 (reduced); found in the citric acid cycle.

Additional info:

Graphs and diagrams referenced in the questions typically show enzyme reaction rates, activation energy, and substrate saturation curves. These are standard in biology textbooks.
All definitions and explanations are expanded for clarity and completeness.