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 physical properties of phospholipids and the presence of cholesterol.

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

• Cholesterol: Reduces membrane fluidity at high temperatures and prevents solidification at low temperatures.

Additional info: Unsaturated tails (with double bonds) create kinks, increasing fluidity and permeability.

Tonicity, Osmosis, and Selective Permeability

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

• Osmosis: Diffusion of water across a selectively permeable membrane.

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

Example: Water moves from areas of low solute concentration to high solute concentration.

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, allowing selective passage of ions; gating can be voltage, ligand, or mechanically controlled.

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

• Pumps: Such as the Na+/K+ pump, actively transport ions.

Na+/K+ pump ratio: Moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.

$\text{3 Na}^+ \text{(out)} : \text{2 K}^+ \text{(in)}$

Electrochemical Gradients

An electrochemical gradient is the combined effect of concentration and electrical charge differences across a membrane.

• Drives movement of ions such as Na+, K+, and Ca2+.

• Important for nerve impulse transmission and muscle contraction.

Cell Structures

Cell Size

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

• Smaller cells have higher ratios, allowing efficient exchange.

• Larger cells may require specialized structures.

Prokaryotic vs. Eukaryotic Cells

Prokaryotes (bacteria, archaea) lack membrane-bound organelles, while eukaryotes (plants, animals, fungi, protists) have complex internal structures.

• Similarities: Both have plasma membranes, cytoplasm, ribosomes, and DNA.

• Differences: Eukaryotes have nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, etc.

Structure and Function of Eukaryotic Organelles

• Nucleus: Stores genetic material, site of transcription.

• Mitochondria: ATP production via cellular respiration.

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

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

• Lysosomes: Digestive enzymes for breakdown of macromolecules.

• Peroxisomes: Break down fatty acids and detoxify.

• Vacuoles: Storage and structural support in plants.

Endomembrane System

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

• Includes ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Proteins are synthesized in the ER, modified in the Golgi, and transported via vesicles.

Additional info: The endomembrane system is essential for secretion, endocytosis, and compartmentalization.

Cytoskeleton Components and Functions

• Microfilaments (actin): Cell shape, movement, muscle contraction.

• Intermediate filaments: Structural support.

• Microtubules: Vesicle transport, flagellar/ciliary motion, mitosis.

Extracellular Structures and Functions

• Cell wall: Provides rigidity in plants, fungi, and some prokaryotes.

• Extracellular matrix (ECM): Animal cells; provides structural support, cell signaling, and adhesion.

Exocytosis, Endocytosis, Phagocytosis

• Exocytosis: Release of substances from cells via vesicle fusion with the membrane.

• Endocytosis: Uptake of substances by membrane invagination.

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Receptor-mediated endocytosis: Specific uptake via receptors.

Cell-Cell Interactions

Extracellular Matrix and Cell Wall

The extracellular matrix (ECM) in animal cells and the cell wall in plant cells provide structural support and mediate cell interactions.

• ECM is composed of proteins (collagen, elastin) and polysaccharides.

• Cell wall is mainly cellulose in plants.

Cell-Cell Attachments

• Tight junctions: Seal cells together, prevent leakage.

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

• Desmosomes: Provide mechanical strength.

• Plasmodesmata: Plant cell equivalent of gap junctions.

Selective Adhesion and Cadherins

Selective adhesion is mediated by cadherins, which are proteins important for cell recognition, tissue cohesion, and embryonic development.

Cell Signaling

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

• Signal amplification: One signal can trigger many responses.

• Phosphorylation cascades: Series of protein activations via phosphorylation.

• Second messengers: Small molecules (e.g., cAMP, Ca2+) that relay signals inside the cell.

• Enzyme-linked receptors: Signal transduction via receptor-associated enzymes.

• G-protein-coupled receptors: Signal transduction via G-proteins.

Higher Order Thinking Questions

• Selective permeability: Membranes are selectively permeable due to lipid composition, protein channels, and transporters.

• Fluidity and permeability: Affected by phospholipid tail length/saturation, cholesterol, and temperature.

• Transport mechanisms: Simple diffusion (no proteins), facilitated diffusion (channels/carriers), active transport (pumps).

• Osmosis: Water movement; aquaporins facilitate rapid water transport.

• GLUT-1: Facilitated diffusion of glucose.

• Na+/K+ pump: Maintains ion gradients; essential for cell function.

• Cellular organelles: Each organelle has a specific function contributing to cell survival.

• Protein targeting: Signal sequences direct proteins to correct locations.

• Endomembrane system: Sequential modification and sorting of proteins/lipids.

• Cell surface and attachments: ECM/cell wall and junctions determine cell interactions.

• Cell signaling steps: Reception, transduction, response, deactivation.

Example Questions

• 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:

• Inside: 1 mM glucose, 100 mM Na+, 100 mM Ca2+, 400 ppm CO2

• Outside: 10 mM glucose, 10 mM Na+, 1000 mM Ca2+, 300 ppm CO2

Molecule	Net Movement Direction	Transport Mechanism
Glucose	Into cell	GLUT-1 carrier (facilitated diffusion)
Na+	Out of cell	Na+ channel (passive), Na+/K+ pump (active)
Ca2+	Out of cell	Ca2+ channel (passive), Ca2+ pump (active)
CO2	Out of cell	Simple diffusion

Additional info: Net movement is determined by concentration gradients and transport protein specificity.