Cell Membrane Structure and Transport Professor Horton

Plasma membrane

The boundary that separates a living cell from its surroundings.

Fluid mosaic model

Describes the membrane as a fluid mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

Phospholipids

The most abundant lipid in the plasma membrane, forming the main fabric of the membrane.

Amphipathic molecules

Molecules (like phospholipids) that contain both hydrophilic ("water-loving") and hydrophobic ("water-fearing") regions.

Hydrophobic tails

The nonpolar regions of phospholipids that are sheltered inside the membrane.

Integral proteins

Proteins that penetrate the hydrophobic core of the lipid bilayer.

Transmembrane proteins

Integral proteins that span the entire membrane.

Peripheral proteins

Proteins bound only to the surface of the membrane.

Cholesterol

A steroid that acts as a fluidity buffer in animal cells; at warm temperatures, it restrains phospholipid movement, but at cool temperatures, it maintains fluidity by preventing tight packing.

Glycolipids

Membrane carbohydrates covalently bonded to lipids.

Glycoproteins

Membrane carbohydrates covalently bonded to proteins.

Membrane fluidity

A dynamic state, usually compared to salad oil, where most lipids and some proteins can move sideways within the membrane.

Selective permeability

The characteristic of the plasma membrane that regulates the exchange and passage of materials, controlling the cell’s molecular traffic.

Substances that cross the lipid bilayer easily

Hydrophobic (nonpolar) molecules, such as hydrocarbons.

Passive transport

The diffusion of a substance across a membrane that requires no energy expenditure by the cell.

Concentration gradient

The region along which the density of a chemical substance increases or decreases; passive transport moves substances down this gradient.

Diffusion

The tendency for molecules to spread out evenly into the available space until dynamic equilibrium is reached.

Osmosis

The diffusion of water across a selectively permeable membrane.

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water.

Isotonic solution

Solute concentration is equal inside and outside the cell, resulting in no net water movement.

Hypertonic solution

Solute concentration is greater outside the cell; the cell loses water (shrivels or undergoes plasmolysis).

Hypotonic solution

Solute concentration is less outside the cell; the cell gains water (lyses in animal cells or becomes turgid in plant cells).

Facilitated diffusion

Passive movement of molecules across the plasma membrane sped up by transport proteins.

Aquaporins

Channel proteins that greatly facilitate the diffusion of water molecules.

Channel proteins

Transport proteins that provide hydrophilic corridors for specific molecules or ions.

Carrier proteins

Transport proteins that bind to a molecule, undergo a subtle change in shape, and shuttle the solute-binding site across the membrane.

Active transport

Transport that moves solutes against their concentration gradients, requiring energy (typically ATP hydrolysis).

Sodium-Potassium Pump

The major electrogenic pump of animal cells; it uses ATP to pump $\text{Na}^+$ out and $\text{K}^+$ into the cell, maintaining concentration gradients.

Electrochemical gradient

The combined forces—chemical force (concentration gradient) and electrical force (membrane potential)—that drive the diffusion of ions across a membrane.

Electrogenic pump

A transport protein that generates voltage across a membrane, such as the sodium-potassium pump in animals or the proton pump in plants/fungi/bacteria.

Cotransport

Occurs when the active transport of one solute indirectly drives the transport of other substances against their own concentration gradient.

Bulk transport

The transport of large molecules (like polysaccharides and proteins) across the plasma membrane via vesicles, which requires energy.

Exocytosis

Transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell; used by secretory cells to export products.

Endocytosis

Process where the cell takes in macromolecules by forming vesicles from the plasma membrane.

Phagocytosis

A type of endocytosis known as "cellular eating," where the cell engulfs a particle into a vacuole for digestion.

Pinocytosis

A type of endocytosis known as "cellular drinking," where extracellular fluid with dissolved molecules is "gulped" into tiny vesicles.

Receptor-mediated endocytosis

A specific type of endocytosis triggered when solutes bind to receptor proteins, which then trigger vesicle formation (e.g., uptake of LDLs/cholesterol).

Enzymatic Activity Proteins

enzymatic activity proteins that facilitate biochemical reactions at the membrane surface. They often act as catalysts, lowering activation energy for reactions such as digestion or signaling.

Transport Proteins

Proteins that assist in the movement of ions and molecules across the cell membrane play a crucial role in maintaining homeostasis. These include channel proteins, carrier proteins, and pumps. Some require ATP (active transport), while some don’t require ATP (passive transport).

Cell-cell recognition (glycoproteins/glycolipids)

The process by which cells recognize and communicate with each other through specific interactions facilitated by glycoproteins and glycolipids on their surfaces. This recognition is essential for immune response, tissue formation, and cellular signaling.

Membrane Proteins

Membrane proteins function in transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Glycoprotiens/Glycolipids 

• Glycoproteins- membrane carbohydrates that have branched chains that are fewer than 15 sugar units, linked to proteins, play a role in cell recognition and communication.

• glycolipids- lipids with attached carbohydrate chains that also play a role in cell recognition and communication.

Signal Transduction Proteins

involved in transmitting signals from the cell surface to its interior, playing a crucial role in various cellular processes like responses to hormones and neurotransmitters.

Intercellular Joining

The connection between adjacent cells that allows for communication and coordination of their functions, facilitated by various proteins and adhesion molecules.

Membrane Proteins attached to Cytoskeletons

are integral or peripheral proteins that interact with the cell's cytoskeleton to provide structural support and facilitate cellular communication and signaling.

Passive transport

is the movement of molecules across the cell membrane without the use of energy, relying on concentration gradients from high to low concentrations to allow substances to move in and out of the cell.

Active Transport

is the process of moving molecules against their concentration gradient from low concentration to high concentration, requiring energy, usually in the form of ATP, to enable the transport of substances into or out of the cell.

membrane protein and lipid synthesis

Membrane proteins and lipids are synthesized in the ER and modified in the ER and Golgi apparatus. The inside and outside faces of membranes differ in molecular composition.

hydrophobic (nonpolar) molecules (hydrocarbons, CO2, O2)

These molecules do not interact favorably with water and can easily diffuse through the lipid bilayer of the cell membrane.

Hydrophilic (polar) molecules (sugars, water, and ions)

do not easily pass through the lipid bilayer of the cell membrane and often require specific transport proteins for entry.

Plasmolysis

is the process in which plant cells lose water in a hypertonic solution, causing the cell membrane to pull away from the cell wall.

Ion Channels

are protein structures that allow specific ions to pass through the cell membrane, often facilitating passive transport.

Sodium-Potassium Pump

is a membrane protein that actively transports sodium out of and potassium into the cell against their concentration gradients, utilizing ATP.

Channel transport protein

facilitates the movement of specific ions or molecules across the cell membrane through a channel, often functioning in passive transport.

Carrier transport protein (polar molecules) (active transport)

is a membrane protein that binds to specific molecules and changes shape to transport them across the cell membrane, often functioning in both passive and active transport.

Dialysis 

is a process used to separate small solutes from larger molecules in a solution, often occurring across a semipermeable membrane.

isotonic in plant cells

refers to a state where the concentration of solutes is equal inside and outside the plant cell, resulting in no net movement of water and maintaining cell turgor.

hypertonic in plant cells

describes a condition where the concentration of solutes outside the plant cell is greater than inside, leading to water moving out of the cell and causing plasmolysis.

hypotonic in plant cells

describes a condition where the concentration of solutes outside the plant cell is lower than inside, causing water to move into the cell, leading to cell swelling and potentially turgidity.

osmoregulation

the process by which cells maintain fluid balance and solute concentration, ensuring optimal cell function. Paramecium has the contractile vacuole.

ECM

(Extracellular Matrix) consists of a network of proteins and carbohydrates outside the cell that provides structural support, facilitates communication, and regulates various cellular functions.

pronton pump

a type of protein that transports protons (H+) across cellular membranes against the concentration gradient, playing a key role in maintaining pH and membrane potential.

Membrane potential

The difference in electric charge across a cell membrane, inside more negative than the outside, is crucial for nerve impulse transmission and muscle contraction.