Membranes, Cell Structure, and Cell-Cell Interactions: Study Guide for Biology 161 Test 2

Membranes

Fluid Mosaic Model

The fluid mosaic model describes the structure of biological membranes as a dynamic combination of lipids, proteins, and carbohydrates. The membrane is flexible, with proteins embedded or attached to a bilayer of phospholipids.

• Phospholipid bilayer: Provides the basic structural framework.

• Proteins: Integral and peripheral proteins serve functions such as transport, signaling, and structural support.

• Carbohydrates: Often attached to proteins or lipids, important for cell recognition.

Example: The plasma membrane of animal cells contains cholesterol, which modulates fluidity.

Permeability and Van der Waals Forces

Membrane permeability is influenced by the interactions between phospholipid tails and the presence of cholesterol.

• Length and saturation of tails: Longer and more saturated tails increase van der Waals forces, making the membrane less fluid and less permeable.

• Cholesterol: Reduces permeability by filling gaps between phospholipids.

Example: Membranes with unsaturated fatty acids are more fluid and permeable than those with saturated fatty acids.

Tonicity, Osmosis, and Selective Permeability

Tonicity refers to the relative concentration of solutes outside versus inside the cell, affecting water movement by osmosis.

• Isotonic: Equal solute concentration; no net water movement.

• Hypertonic: Higher solute outside; water moves out of cell.

• Hypotonic: Lower solute outside; water moves into cell.

• Selective permeability: Membranes allow some substances to cross more easily than others.

Example: Aquaporins facilitate water movement across membranes.

Passive and Active Transport

Transport across membranes can be passive (no energy required) or active (requires energy).

• Passive transport: Includes simple diffusion, facilitated diffusion via channels or carriers.

• Active transport: Uses energy (usually ATP) to move substances against their concentration gradient.

• Channels: Often gated; open or close in response to signals. Usually do not undergo conformational change.

• Transporters: Undergo conformational change to move molecules across the membrane.

• Pumps: Example: Na+/K+ pump moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.

Electrochemical gradients: Combined effect of concentration and electrical charge differences across the membrane.

Example: The Na+/K+ pump maintains membrane potential.

Equation:

$\text{Na}^+/\text{K}^+ \text{ pump ratio: } 3\ \text{Na}^+ \text{ out} : 2\ \text{K}^+ \text{ in}$

Cell Structures

Cell Size

Cell size is limited by surface area-to-volume ratio, affecting transport efficiency and metabolic activity.

• Small cells: Higher surface area relative to volume, more efficient exchange.

• Large cells: May require specialized structures for transport.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on internal structure.

• Prokaryotes: No nucleus, no membrane-bound organelles, smaller size.

• Eukaryotes: Nucleus, membrane-bound organelles, larger size.

• Similarities: Both have plasma membrane, cytoplasm, ribosomes, and genetic material.

Example: Bacteria are prokaryotes; plants and animals are eukaryotes.

Structure and Function of Eukaryotic Organelles

• Nucleus: Stores genetic material, site of transcription.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosome: Digests macromolecules.

• Peroxisome: Breaks down fatty acids and detoxifies.

• Mitochondria: Site of cellular respiration.

• Chloroplast: Site of photosynthesis (plants).

Example: Peroxisomes break down long fatty acid chains.

Endomembrane System

The endomembrane system is a network of membranes involved in protein and lipid synthesis, modification, and transport.

• Components: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles.

• Purpose: Compartmentalizes cell functions, facilitates transport and modification.

• Mechanisms: Vesicle budding and fusion move substances between compartments.

Example: Proteins synthesized in the rough ER are sent to the Golgi for modification.

Cytoskeleton Components and Functions

The cytoskeleton provides structural support and facilitates movement.

• Microfilaments (actin): Cell shape, movement.

• Intermediate filaments: Structural stability.

• Microtubules: Vesicle transport, flagellar motion.

Example: Microtubules move vesicles and are involved in flagellar motion.

Extracellular Structures and Functions

• Cell wall: Provides rigidity (plants, fungi, bacteria).

• Extracellular matrix (ECM): Provides structural support and cell signaling (animals).

Exocytosis, Endocytosis, Phagocytosis

Cells transport large molecules via vesicle-mediated processes.

• Exocytosis: Vesicles fuse with membrane to release contents outside.

• Endocytosis: Membrane engulfs material to bring it inside.

• Phagocytosis: Cell engulfs large particles.

• Receptor-mediated: Endocytosis is triggered by specific receptors binding to target molecules.

Cell-Cell Interactions

Extracellular Matrix and Cell Wall

The ECM in animal cells and cell wall in plant cells provide structural support and mediate cell interactions.

• ECM: Composed of proteins (collagen, elastin) and polysaccharides.

• Cell wall: Composed of cellulose (plants), peptidoglycan (bacteria).

Cell-Cell Attachments

Cells are connected by specialized junctions.

• Tight junctions: Seal cells together, prevent leakage.

• Gap junctions: Allow communication via small molecules; present in most tissues except skeletal muscle.

• Desmosomes: Provide mechanical strength.

• Plasmodesmata: Plant cell equivalent of gap junctions.

Selective Adhesion and Cadherins

Animal cells adhere selectively via cadherins, which are important for tissue formation and embryonic development.

• Cadherins: Calcium-dependent adhesion proteins.

• Role: Recognition, tissue cohesiveness, development.

Cell Signaling

Cell signaling involves the reception, processing, response, and deactivation of signals.

• Signal reception: Binding of signaling molecule to receptor.

• Signal processing/transduction: Conversion of signal to cellular response.

• Signal response: Activation of cellular processes.

• Signal deactivation: Termination of signal.

• Signal amplification: One signal can trigger many responses.

• Phosphorylation cascades: Series of protein phosphorylations amplify signal.

• Second messengers: Small molecules (e.g., cAMP) relay signals inside cell.

• Enzyme-linked receptors: Trigger phosphorylation cascades.

• G-protein-coupled receptors: Activate G-proteins, which relay signals.

Example: Binding of a hormone to a G-protein-coupled receptor activates a second messenger cascade.

Higher Order Thinking Questions (Expanded)

Selective Permeability and Membrane Fluidity

• Selective permeability: Due to lipid composition, protein channels, and transporters.

• Factors affecting fluidity: Fatty acid saturation, cholesterol, temperature.

Transport Mechanisms

• Simple diffusion: Movement of small, nonpolar molecules.

• Facilitated diffusion: Movement via channels or carriers.

• Active transport: Movement against gradient, requires energy.

• Osmosis: Water movement.

• Aquaporins: Water channels.

• Ion channels: Allow ions to pass.

• GLUT-1: Glucose transporter.

• Na+/K+ pump: Maintains gradients.

Cellular Organelles and Functions

• Each organelle: Specific function contributing to cell's overall activity.

Endomembrane System

• Successive compartments: Synthesis (ER), modification (Golgi), sorting, transport (vesicles).

Cell-Cell Interactions

• ECM/cell wall: Influence cell behavior and attachment.

• Attachments: Tight junctions, gap junctions, desmosomes, plasmodesmata.

Cell Signaling Steps

• Reception: Signal binds receptor.

• Processing/transduction: Signal relayed/amplified.

• Response: Cellular change.

• Deactivation: Signal terminated.

Example Questions and Applications

Sample Multiple Choice

• Which organelle breaks down long fatty acid chains? Answer: Peroxisome

• Signal transduction involving a receptor: Answer: The receptor changes shape due to the binding of the signaling molecule.

Membrane Transport Example

Given concentrations:

• Glucose: Higher outside (10mM) than inside (1mM); net movement into cell via GLUT-1.

• Na+: Higher inside (100mM) than outside (10mM); net movement out via Na+ channel, but pump moves Na+ out and K+ in.

• Ca2+: Higher outside (1000mM) than inside (100mM); net movement into cell via Ca2+ channel.

• CO2: Higher inside (400ppm) than outside (300ppm); net movement out by diffusion.

Table: Comparison of Transport Mechanisms

Mechanism	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	GLUT-1, aquaporins
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Additional info: Table expanded for clarity and completeness.