Cellular Membranes, Organelles, and Transport Mechanisms

Endomembrane System and Protein Trafficking

Endoplasmic Reticulum (ER) and Golgi Apparatus

The endomembrane system is a network of membranes within eukaryotic cells that is involved in the synthesis, modification, and transport of proteins and lipids.

• Proteins and lipids destined for secretion are synthesized in the rough ER (endoplasmic reticulum).

• These molecules bud off from the ER in transport vesicles, which carry them to the Golgi apparatus.

• The Golgi apparatus accepts vesicles from the ER, chemically modifies ER products, and sorts and packages materials into new transport vesicles.

• Modifications in the Golgi help sort proteins based on chemical "tags" for their final destinations.

• Transport vesicles deliver their contents to specific parts of the cell or to the plasma membrane for secretion.

Lysosomes

Lysosomes are membrane-bound sacs containing hydrolytic enzymes that digest macromolecules. They play a key role in cellular digestion and recycling.

• Lysosomes are acidic (pH 4-5), which is optimal for their enzymes.

• Hydrolytic enzymes and lysosomal membranes are made by the rough ER and processed in the Golgi apparatus.

• Lysosomes perform "cleaning up" and "recycling" functions, breaking down damaged cellular structures, macromolecules, and protein aggregates.

• Specialized processes include phagocytosis (engulfing large particles) and autophagy (recycling the cell's own organelles).

• In animal bodies, phagocytosis is mainly carried out by immune cells and specialized retinal cells.

• Lysosomal dysfunction is linked to diseases such as Tay-Sachs and Alzheimer's; lysosomes become less effective with aging.

Cytoskeleton and Cellular Movement

Cytoskeleton Structure and Function

The cytoskeleton is a dynamic network of protein fibers that provides structural support and enables directed movement within eukaryotic cells.

• Functions include:

  • Structural support

  • Directed movement of organelles, vesicles, and the cell itself

• Three main types of cytoskeletal fibers:

  • Microtubules: Hollow tubes made of tubulin; involved in cell shape, chromosome movement, and motility (cilia and flagella).

  • Microfilaments (actin filaments): Thin strands involved in cell movement and muscle contraction.

  • Intermediate filaments: Provide mechanical strength to cells.

Motor Proteins and Organelle Movement

• Motor proteins (e.g., dyneins, kinesins, myosins) move vesicles and organelles along cytoskeletal fibers using ATP.

• Microtubules control the beating of cilia and flagella (motile appendages on cell surfaces).

• Cilia are usually numerous and short; flagella are few and long per cell.

• Both cilia and flagella are composed of microtubules and move via motor proteins.

• Microtubules also control chromosome movement during cell division (mitosis and meiosis).

• The cytoskeleton allows eukaryotic cells to grow larger by overcoming diffusion limitation through directed movement.

Mitochondria, Endosymbiont Theory, and Gradients

Mitochondria and ATP Production

Mitochondria are organelles that produce ATP from oxygen and food molecules, serving as the cell's "power stations." ATP is the universal energy currency of the cell.

• Mitochondria (and chloroplasts) share similarities with prokaryotes: circular chromosomes, ribosomes, binary fission, and double membranes.

• These similarities support the endosymbiont theory: mitochondria (and chloroplasts) evolved from prokaryotic cells engulfed by ancestral eukaryotes.

• Mitochondria are involved in creating chemical gradients and synthesizing macromolecules (DNA, RNA, proteins, carbohydrates).

Chemical Gradients and Life

• A gradient is a difference in particle concentration across space.

• Organisms use energy to create and maintain chemical gradients, which perform chemical and mechanical work.

• Death is the loss of gradients.

• Plasma membranes allow organisms to create and maintain gradients, which are essential for life.

Plasma Membranes and Selective Permeability

Structure and Function of Plasma Membranes

The plasma membrane is a selectively permeable barrier that controls the inflow and outflow of molecules, maintaining chemical gradients essential for cellular function.

• Composed mainly of a phospholipid bilayer with embedded proteins.

• Phospholipids are amphipathic molecules, containing hydrophobic (tails) and hydrophilic (heads) regions.

• Hydrophobic interactions hold the bilayer together and inhibit diffusion of polar molecules.

• Membranes exhibit selective permeability:

  • Nonpolar molecules (e.g., hydrocarbons, CO2, O2) pass through easily.

  • Polar molecules (e.g., sugars, water, ions) pass through slowly or not at all.

• Proteins determine most membrane functions and are responsible for creating and maintaining gradients.

• Membrane protein composition varies among cell types and organelles.

Types of Membrane Proteins

• Peripheral proteins: Bound to the membrane surface.

• Integral proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

• Integral proteins must be amphipathic to interact with both hydrophobic and hydrophilic regions.

• Channel proteins: Provide hydrophilic channels for specific molecules or ions.

• Carrier proteins: Bind to molecules and change shape to shuttle them across the membrane.

Membrane Transport Mechanisms

Passive and Active Transport

Transport across membranes can be passive (no energy required) or active (requires energy, usually from ATP).

• Passive transport: Movement down a concentration gradient; no energy used.

• Diffusion: Random movement of particles from high to low concentration.

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

• Free water moves from high to low free water concentration (equivalent to low to high solute concentration).

• Equilibrium is reached when concentrations are equal on both sides.

• Two types of passive transport:

  • Simple diffusion: Direct movement across the membrane (no proteins involved).

  • Facilitated diffusion: Movement via transport proteins (channel or carrier proteins); may be gated (open/close in response to stimuli).

• Active transport: Moves substances against their concentration gradient; always requires carrier proteins and energy (ATP).

• Carrier protein shape changes when a phosphate group from ATP is attached or removed, enabling transport against the gradient.

Equations and Key Terms

• Diffusion (generalized):

$J = -D \frac{dC}{dx}$ Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Osmosis (osmotic pressure):

$\Pi = iMRT$ Where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.

Cell Surface Molecules and Cell Recognition

Glycolipids and Glycoproteins

Cells recognize each other by binding to molecules on the cell surface, many of which are carbohydrates attached to lipids or proteins.

• Glycolipids: Carbohydrates bonded to lipids.

• Glycoproteins: Carbohydrates bonded to proteins.

• Diversity of surface carbohydrates enables cell identification and communication.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction Relative to Gradient	Protein Involved?	Example
Simple Diffusion	No	Down	No	O2, CO2
Facilitated Diffusion	No	Down	Yes (channel/carrier)	Glucose, ions
Active Transport	Yes (ATP)	Against	Yes (carrier)	Na+/K+ pump

Key Definitions

• Phagocytosis: Engulfing large particles or cells by the cell membrane.

• Autophagy: Recycling of the cell's own organelles and macromolecules.

• Selective permeability: Property of membranes that allows some substances to pass more easily than others.

• Amphipathic: Molecule with both hydrophobic and hydrophilic regions.

• Gradient: Difference in concentration of a substance across a space or membrane.