Cell Membranes and Transport

Flashcards on cell membrane structure, function, transport, and related concepts.

Cell Membrane Functions

Organize and regulate cellular processes through compartmentalization and selective permeability; facilitate cell communication via vesicle transport and receptors.

Membrane Role in Communication

Actively regulate transport, signaling, and interaction between cells, ensuring efficient communication at the right time and place.

Plasma Membrane as Boundary

Acts as a selective barrier, controlling the passage of substances into and out of the cell.

Membrane Compartmentalization

Creates specialized compartments inside cells, enabling specific functions within organelles like the Golgi apparatus, ER, and mitochondria.

Membrane Proteins in Transport

Allow regulated transport of key molecules, such as the sodium-potassium pump which helps maintain ion balance.

Membrane Proteins in Signal Detection

Function as receptors, detecting external signals (e.g., hormones) and triggering intracellular changes.

Membrane Proteins in Cell-to-Cell Interactions

Connect neighboring cells, permitting direct molecule exchange important in cardiac and plant cells.

Passive Transport

The movement of molecules across membranes without the need for energy input, following the concentration gradient.

Active Transport

The movement of molecules across membranes requiring energy, typically ATP, against their concentration gradient.

Exocytosis

A type of bulk transport where large molecules are secreted from the cell by vesicle fusion with the plasma membrane.

Endocytosis

A type of bulk transport where large molecules are taken into the cell as the plasma membrane pinches inward, forming a vesicle.

Fluid Mosaic Model

Cellular membranes are composed of lipids (phospholipids, cholesterol), proteins (integral, peripheral), and carbohydrates (glycolipids, glycoproteins) working together dynamically.

Asymmetry in Membranes

Membranes have distinct E face (extracellular) and P face (cytoplasmic) compositions contributing to function.

Membrane Synthesis and Sidedness

Asymmetry is established during synthesis in the ER and Golgi and maintained as vesicles transport components to the plasma membrane.

Phospholipid Structure

Amphipathic molecules with a hydrophilic head (phosphate, glycerol, variable group) and hydrophobic tails (non-polar fatty acids).

Phospholipid Diversity

Allows membranes to function according to their specific cellular role, with variations in phospholipid composition across organisms and within cells.

Membrane Fluidity

The phospholipid bilayer is dynamic, allowing movement that supports membrane flexibility and function.

Lateral Diffusion

Common movement of phospholipids side to side within the same layer of the membrane.

Transverse Diffusion

Rare movement where phospholipids switch layers in the membrane, energetically unfavorable.

Cell Fusion Experiment

Demonstrates membrane protein movement by fusing mouse and human cells and observing the intermixing of labeled proteins over time.

FRAP (Fluorescence Recovery After Photobleaching)

Experiment used to test membrane fluidity by tracking the movement of fluorescently labeled molecules into a bleached area.

Role of Unsaturated Fatty Acids in Membrane Fluidity

Increase membrane fluidity due to cis double bonds creating kinks that prevent tight packing of phospholipids.

Role of Saturated Fatty Acids in Membrane Fluidity

Decrease membrane fluidity, as their tails pack closely together, making the membrane more rigid.

Cholesterol as Fluidity Buffer

Maintains membrane stability by reducing fluidity at high temperatures and preventing solidification at low temperatures.

Effect of Fatty Acid Tail Length on Membrane Fluidity

Longer tails increase viscosity (less fluid), while shorter tails increase fluidity.

Transition Temperature

The temperature at which a membrane shifts from a solid to a more fluid state.

Homeoviscous Adaptation

The process where cells modify lipid composition to maintain consistent membrane viscosity despite environmental changes.

Functions of Membrane Proteins

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton & ECM.

Role of Membrane Proteins in HIV Infection

HIV requires the CD4 receptor and CCR5 co-receptor on host cells for viral entry; absence of CCR5 confers resistance.

Integral Membrane Proteins

Embedded in the membrane, difficult to remove; includes monotopic and transmembrane proteins.

Peripheral Membrane Proteins

Not embedded; attach via non-covalent bonds to integral proteins or phospholipid heads; hydrophilic.

Lipid-Anchored Proteins

Covalently attached to a lipid molecule, which anchors it in the membrane; includes GPI-anchored and fatty acid-anchored proteins.

Selective Permeability

The ability of membranes to regulate the passage of substances based on phospholipid composition and structure.

High Membrane Permeability

Associated with short, unsaturated tails in phospholipids, preventing tight packing and increasing fluidity.

Low Membrane Permeability

Associated with long, saturated tails in phospholipids, packing tightly together and reducing fluidity.

Fluidity vs. Permeability

How freely phospholipids move versus how easily molecules cross the membrane; related but distinct.

Relationship Between Cholesterol and Membrane Permeability

Reduces permeability at high temperatures by stabilizing the membrane, and prevents solidification at low temperatures.

Small, Nonpolar Molecules Permeability

High permeability, diffuse freely across the membrane (e.g., O₂, CO₂).

Small, Uncharged Polar Molecules Permeability

Moderate permeability, can pass but not as easily as nonpolar molecules (e.g., H₂O, glycerol).

Large, Uncharged Polar Molecules Permeability

Very low permeability, highly restricted (e.g., glucose, sucrose).

Ions Permeability

Blocked, need transport proteins to cross the membrane (e.g., Na⁺, Cl⁻, K⁺).

Large, Nonpolar Molecules Permeability

High permeability, can diffuse across membranes (e.g., steroid hormones).

Amphipathic Nature of Transport Proteins

Hydrophilic regions near aqueous environments and hydrophobic regions interacting with the bilayer core.

Aquaporins

Transmembrane channel proteins that allow rapid water movement across the membrane.

Characteristics of Simple Diffusion

Requires no energy, moves molecules from high to low concentration, and does not need transport proteins.

Osmosis

The diffusion of water across a selectively permeable membrane from areas of lower solute concentration to higher solute concentration.

Hypotonic Solution

Lower solute concentration than the cell's interior; water moves into the cell.

Isotonic Solution

Equal solute concentration inside and outside the cell; no net water movement.

Hypertonic Solution

Higher solute concentration than the cell's interior; water moves out of the cell.

Plasmolysis

Occurs when a plant cell is in a hypertonic solution, causing the plasma membrane to pull away from the cell wall.

Turgidity

Occurs when a plant cell is placed in a hypotonic solution, creating internal pressure against the cell wall.

Facilitated Diffusion

Passive transport aided by proteins, moving molecules down their concentration gradient without energy.

Channel Proteins

Create a hydrophilic corridor for specific solutes to pass through the membrane.

Carrier Proteins

Undergo conformational shape changes to transport molecules across the membrane.

Uniport

Carrier protein transports a single solute in one direction.

Symport

Coupled transporter where both solutes move in the same direction.

Antiport

Coupled transporter where solutes move in opposite directions.

Active Transport Requirement

Requires energy input, often ATP, to move solutes against their concentration gradients.

Sodium-Potassium Pump

Active transport mechanism that moves sodium out of the cell and potassium into the cell against their concentration gradients.

Membrane Potential

Voltage difference across a membrane determined by charge distribution.

Electrochemical Gradient

Combination of electrical and chemical forces influencing ion movement across the membrane.

Cotransport

A type of secondary active transport that couples the movement of one solute down its gradient with another moving against its gradient.

Exocytosis and Endocytosis

Bulk transport mechanisms that moves large molecules in and out of the cell.

Phagocytosis

A cell engulfs large particles by extending pseudopodia and forming a food vacuole.

Pinocytosis

A cell takes in extracellular fluid and its dissolved solutes through vesicle formation.

Receptor-Mediated Endocytosis

A cell selectively takes in specific molecules using receptors on the plasma membrane.

Hypercholesterolemia

Results from defective receptor-mediated endocytosis of LDL, leading to high cholesterol levels.