Plasma Membranes

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a dynamic, flexible structure composed of various molecules that move within the membrane. This model explains how the membrane maintains its integrity while allowing movement and communication.

• Fluid: Membrane components (phospholipids, proteins) can move laterally within the bilayer.

• Mosaic: The membrane is a patchwork of phospholipids, proteins, cholesterol, and carbohydrates.

• Functions: Protection, selective transport, cell communication, recognition, and attachment.

Structure of the Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which forms a selective barrier between the cell and its environment.

• Phospholipid: Molecule with a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.

• Bilayer arrangement: Hydrophilic heads face outward (toward water), hydrophobic tails face inward.

• Barrier function: Controls passage of substances based on their properties.

Components of the Plasma Membrane

The plasma membrane contains several key components, each with distinct functions:

Membrane Fluidity

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Fatty acid tails: Unsaturated tails (with bends) increase fluidity; saturated tails decrease fluidity.

• Cholesterol: Prevents excessive fluidity at high temperatures and rigidity at low temperatures.

Functions of Membrane Proteins

• Transport: Channel and carrier proteins move substances across the membrane.

• Communication: Receptor proteins bind signaling molecules.

• Recognition: Proteins (often with carbohydrates) help cells identify each other.

• Attachment: Proteins anchor cells to neighbors or the extracellular matrix.

• Enzymatic activity: Some proteins catalyze reactions at the membrane.

Selective Permeability

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

• Easy passage: Small, nonpolar molecules (e.g., O2, CO2).

• Difficult passage: Ions (Na+, K+), large polar molecules (glucose), other polar substances.

• Transport proteins: Required for many substances that cannot cross the bilayer directly.

Passive Transport

Passive transport moves substances down their concentration gradient without energy input from the cell.

• Simple diffusion: Direct movement of molecules across the membrane (no protein required).

• Facilitated diffusion: Movement via transport proteins (no energy required).

• Osmosis: Net movement of water across a membrane in response to solute concentration differences.

Tonicity: Hypertonic, Hypotonic, Isotonic

• Plant cells: Hypotonic makes cell turgid; hypertonic causes plasmolysis.

Active Transport

Active transport moves substances against their concentration or electrochemical gradient, requiring energy (usually ATP).

• Sodium-potassium pump: Uses ATP to move 3 Na+ out and 2 K+ in per cycle.

• Coupled transport: Movement of one substance down its gradient drives another against its gradient (does not always use ATP directly).

Bulk Transport: Endocytosis and Exocytosis

Bulk transport moves large particles or volumes via vesicles.

• Endocytosis: Cell takes in substances by forming vesicles from the membrane.

• Types: Phagocytosis ("cell eating"), pinocytosis ("cell drinking"), receptor-mediated endocytosis (selective uptake).

• Exocytosis: Vesicles fuse with membrane to release contents outside (e.g., hormones, neurotransmitters).

Energy and Metabolism

Kinetic Energy vs. Potential Energy

Energy is the capacity to do work. It exists as:

• Kinetic energy: Energy of motion (e.g., moving molecules).

• Potential energy: Stored energy (e.g., chemical bonds, water behind a dam).

Oxidation and Reduction (Redox Reactions)

Redox reactions involve electron transfer:

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain.

• Example: Glucose oxidation in cellular respiration produces ATP.

Thermodynamics and the First Law

Thermodynamics studies energy transformations. The First Law of Thermodynamics states:

• Energy cannot be created or destroyed, only transformed.

• Cells convert chemical energy from food into ATP and other forms.

Entropy and the Second Law

Entropy measures energy dispersal and disorder. The Second Law of Thermodynamics states:

• Every energy transfer increases the total entropy of the universe.

• Energy transformations are not perfectly efficient; some energy is lost as heat.

• Organisms maintain order by using energy and releasing heat/waste.

Free Energy: Exergonic vs. Endergonic Reactions

Free energy (G) is the energy available to do work. The change in free energy is .

ATP Hydrolysis and Cellular Work

ATP (adenosine triphosphate) is the cell's energy currency. Hydrolysis of ATP releases energy for cellular processes:

• ATP consists of adenine, ribose, and three phosphate groups.

• Hydrolysis reaction:

• Three types of cellular work: chemical (building molecules), transport (moving substances), mechanical (movement).

• ATP is regenerated from ADP and Pi using energy from metabolism.

Catalysts and Activation Energy

Catalysts speed up reactions by lowering activation energy, without being consumed or altering or equilibrium.

• Activation energy: Energy barrier reactants must overcome.

• Enzymes: Biological catalysts (mostly proteins, some RNA called ribozymes).

Enzyme Structure and Substrate Binding

Enzymes bind substrates at their active site, forming an enzyme-substrate complex. The induced-fit model describes how the active site adjusts to fit the substrate.

• Enzymes orient reactants, strain bonds, create favorable environments, and stabilize transition states.

• Specificity is determined by active site structure and chemistry.

Types of Enzymes

• Protein enzymes: Folded proteins with specific active sites.

• Ribozymes: RNA molecules with catalytic activity.

• Cofactors: Nonprotein helpers (inorganic ions or organic coenzymes) required by some enzymes.

Effects of Temperature and pH on Enzymes

• Temperature: Low temperatures slow reactions; optimal range increases rate; high temperatures cause denaturation.

• pH: Each enzyme has an optimal pH; extreme pH alters charges and structure, causing denaturation.

• Example: Stomach enzymes function at low pH; intestinal enzymes at higher pH.

Metabolic Pathways

Metabolism is the sum of all chemical reactions in an organism. Metabolic pathways are sequences of enzyme-catalyzed reactions:

• Product of one reaction is substrate for the next:

• Catabolism: Breaks down molecules, releases energy.

• Anabolism: Builds molecules, requires energy.

• Pathways allow control, resource conservation, and coordination.

Allosteric Proteins and Regulation

Allosteric regulation occurs when a molecule binds to a protein at a site other than the active site, altering its activity.

• Allosteric regulators can activate or inhibit proteins.

• Feedback inhibition: End product of a pathway inhibits an early enzyme, preventing waste and maintaining balance.

Key Terms

• Plasma membrane, fluid mosaic model, phospholipid, hydrophilic, hydrophobic, bilayer, cholesterol, membrane protein, selective permeability, concentration gradient, simple diffusion, facilitated diffusion, osmosis, hypertonic, hypotonic, isotonic, passive transport, active transport, ATP, sodium-potassium pump, coupled transport, endocytosis, exocytosis, phagocytosis, pinocytosis, receptor-mediated endocytosis.

• Kinetic energy, potential energy, oxidation, reduction, redox reaction, thermodynamics, First Law of Thermodynamics, entropy, Second Law of Thermodynamics, free energy, , exergonic reaction, endergonic reaction, ATP hydrolysis, catalyst, activation energy, enzyme, substrate, active site, induced fit, ribozyme, cofactor, coenzyme, denaturation, metabolism, metabolic pathway, catabolism, anabolism, allosteric regulation, feedback inhibition.

Exam Preparation Checklist

• Explain the fluid mosaic model and identify plasma membrane components.

• Predict effects of hypertonic, hypotonic, and isotonic solutions on cells.

• Compare passive, active, and bulk transport.

• Explain sodium-potassium pump and ATP usage.

• Distinguish kinetic vs. potential energy.

• Explain oxidation, reduction, and redox reactions.

• State and explain First and Second Laws of Thermodynamics.

• Identify exergonic and endergonic reactions using .

• Explain ATP hydrolysis and cellular work.

• Describe enzyme function and effects of temperature/pH.

• Explain metabolic pathways and allosteric regulation.