Cell Membrane Structure and Function

• The cell membrane, also known as the plasma membrane, surrounds all cells as a thin, flexible boundary (~8nm thick)
• Under a light microscope, it appears as a single line, but is actually a bilayer.
• An electron microscope reveals the bilayer structure.
• It controls what enters and exits the cell
• The main function is to selectively control what enters and exits

Phospholipid Bilayer

• Phospholipids are composed of glycerol, two fatty acids, and a phosphate group.
• These form the structure and provide selective permeability
• Hydrophilic heads (phosphate group) are attracted to water and face outward.
• Hydrophobic tails (fatty acids) repel water and face inward.
• The heads interact with water, while the tails avoid water.
• This self-assembly occurs due to the properties of the heads' attraction to water and the tails' repulsion of water.
• There are no covalent bonds between phospholipids, resulting in fluidity and flexibility.
• Covalent bonds would create a strong, rigid structure.
• Examples of phospholipids in water are micelles (small spheres) or bilayers.

Fluid Mosaic Model (1972)

• Model named Fluid because the membrane is not rigid, and phospholipids and proteins move.
• Model named Mosaic because it is a mix of lipids, proteins, and carbohydrates.
• Proteins can float within the bilayer because they are not locked in place.

Factors Affecting Membrane Fluidity

• More unsaturated fatty acids result in more fluid membranes because double bonds prevent tight packing.
• More saturated fatty acids result in less fluid membranes because tails pack tightly.
• Cholesterol helps:
  • At high temperatures it prevents excess fluidity.
  • At low temperatures it lessens solidification.

Membrane Proteins & Their Functions

• Membrane proteins have varied structures that dictate their diverse functions.
• Integral (transmembrane) proteins: These span the entire bilayer and are involved in transport.
• Peripheral proteins: These are attached to one side and are used in recognition and signaling.
• Nonpolar amino acids in proteins interact with hydrophobic tails, keeping proteins embedded.
• Polar regions of proteins face water on both sides, allowing specific molecules to pass through channels.
• Aquaporins are water channels with hydrophobic exteriors and hydrophilic interiors for rapid water movement.

Transport Across the Cell Membrane

• Cells must transport materials in and out to survive.
• The cell membrane regulates what enters, exits, and how movement occurs.
• Some molecules pass easily (oxygen and carbon dioxide).
• Others require the help of proteins (glucose and ions).
• Some require direct energy to move (sodium and potassium).
• Metaphor: the cell membrane acts like a nightclub with bouncers (proteins) and VIP entrances (active transport) versus people freely walking in/out (passive transport).

Types of Transport

Simple Diffusion

• No energy is needed, molecules move from high to low concentration across the membrane (e.g., O₂, CO₂)

Facilitated Diffusion

• No energy is needed, large or charged molecules move through protein channels (e.g., glucose, ions).

Osmosis

• No energy is needed, water moves toward higher solute concentration for to aid balance the solution within cells

Active Transport

• Energy (ATP) is needed.
• Molecules move from low to high concentration (against the gradient) (e.g., Na+, K+ pump).

Endocytosis

• Energy (ATP) is needed.
• The cell engulfs large molecules by wrapping its membrane around them (e.g., white blood cells eating bacteria).

Exocytosis

• Energy (ATP) is needed.
• The cell releases substances by fusing a vesicle within the membrane (e.g., hormone secretion).

Passive Transport Details

• Molecules move from high to low concentration.
• The cell doesn't use energy, molecules use random motion for movement.
• Small, nonpolar molecules slip through the membrane (O2, CO2) without protein assistance.
• Large or charged molecules (like glucose, Na+, Cl-) need transport proteins (channels/carriers)
• An example is glucose transporters in the intestines letting sugar into the bloodstream without energy use.

Active Transport Details

• Moves molecules from low to high concentration and requires energy (ATP).
• Membrane proteins "push" molecules across using ATP (e.g., Sodium-Potassium Pump).
• Bulk Transport: small membrane bubbles (vesicles) transport materials.
  • Endocytosis: Cell uptakes large particles.
  • Exocytosis: Cell releases contents.

Osmosis

• Osmosis involves water diffusion across a selectively permeable membrane.
• Water moves toward higher solute concentration to balance solute levels.
• Water follows salt concentrations, moving to balance it out.
• Isotonic solutions: Equal solute concentrations inside and outside the cell result in no net water movement.
• Hypotonic solutions: Lower solute outside the cell -> water enters -> animal cells may swell/burst (lysis); plant cells become turgid.
• Hypertonic solutions: Higher solute outside the cell -> water leaves -> animal cells shrink (crenation), plant cells undergo plasmolysis.
• Plants wilt in dry soil as water leaves cells (plasmolysis); IV fluids must be isotonic to prevent cell damage.
• Osmosis keeps cells hydrated, active transport enables neuron signals; endocytosis and exocytosis are used in the immune and cell communication system.
• A freshwater fish in saltwater loses water from its cells (hypertonic solution), causing dehydration

Organelles

• Organelles are specialized structures inside cells that perform specific jobs.
• Each organelle is essential for the cell to work correctly.

Cell Factory

• The nucleus is the manager (controls everything)
• he ribosomes are the workers (make proteins)
• The golgi apparatus is the shipping center (packages proteins)
• The mitochondria is the power plant (makes energy)

Role of cell membrane components

• Phospholipids form the bilayer, providing membrane flexibility.
• Proteins transport materials, send signals, and help provide structure. Cholesterol keeps the membrane stable and fluid at different temperatures.
• Carbohydrates (glycoproteins/glycolipids) act as "ID tags" for cell recognition.

Cell membrane variation

• Nerve cell membrane has more ion channels for fast signals.
• Muscle cell membrane has more protein pumps for contraction.
• Intestinal cells have many transport proteins for nutrient absorption.
• Skin cells have extra cholesterol for strength and durability.

Key components

• *A neuron needs many ion channels to send electrical signals
• skin cells need extra cholesterol to prevent damage

Additional definitions and analogies

• Cell Membrane Definition: Selectively permeable barrier that regulates molecule movement in/out, maintaining cell environment.
• Fluid Mosaic Model: Lipid bilayer (water) with floating proteins (rafts).
• Lipid Bilayer Composition: Phospholipids with hydrophilic heads and hydrophobic tails self-assemble to form a barrier.
• Analogy for lipid bilayer: bread (hydrophilic heads), peanut butter (hydrophobic tails).
• Membrane Proteins are Gates, channels and receptors
  • Integral Proteins that go all the way through the membrane for transportation of molecules
  • Peripheral proteins that sit on the surface and help with signaling
  • Glycoproteins & Glycolipids Protein and lipids linked with sugar chains for surface recognition
• Cholesterol: Stabilizes membrane fluidity and structure and stabilizes the cells at different tempuratures
• Analogy Cholestrol: like butter in cookie dough

Explore the cell membrane, a flexible boundary around cells. Learn about its structure as a phospholipid bilayer with hydrophilic heads and hydrophobic tails. Understand how this arrangement controls what enters and exits the cell, providing selective permeability.