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Membrane Structure & Function

Terms in this set (35)

Plasma Membrane
*The boundary that separates the living cell from its surroundings.

*The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others.

*Transport proteins are often responsible for controlling passage across cellular membranes
Phospholipids
*The most abundant lipid in the plasma membrane.

*Phospholipids are amphipathic, containing hydrophobic and hydrophilic regions.
Fluid Mosaic Model
*In the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

*Proteins are not randomly distributed in the membrane
Membrane Fluidity
*As temps cool, membranes switch from fluid to solid state.

*Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids.

*The steroid cholesterol has different effects on the membrane fluidity of animal cells at different temperatures.
Evolution of Differences in Membrane Lipid Composition
*Variations in lipid composition of cell membranes of many species appear to be adaptations to environmental conditions.

*Ability to change the lipid compositions in response to temp changes evolved in organisms that live where temps vary.
Membrane Proteins and Their Functions
*Like a tile mosaic, a membrane is a collage of proteins, often clustered, embedded in the fluid matrix of the lipid bilayer.

*Phospholipids form the main fabric of the membrane.

*Proteins determine most of the membrane's functions
Peripheral Proteins
Bound to the membrane surface.
Integral Proteins
*Penetrate the hydrophobic core.

*Integral proteins that span the membrane are transmembrane proteins.
Cell-Surface Membrane Functions
*Transport.

*Enzymatic activity.

*Signal transduction.

*Cell-cell recognition.

*Intercellular joining.

*Attachment to the cytoskeleton and extracellular matrix.
The Role of Carbohydrates in Cell-Cell Recognition
*Cells recognize each other by binding to molecules, often carbohydrates, on the surface of the plasma membrane.

*Membrane carbohydrates may be covalently bonded to lipids, (glycolipids) or more commonly to proteins (glycoproteins).

*Carbohydrates on the extracellular side of the plasma membrane vary among species, individuals, and even cell types in an individual.
Synthesis and Sidedness of Membranes
*Membranes have distinct inside and outside faces.

*The asymmetrical distribution of proteins, lipids, and carbs in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus.
The Permeability of the Lipid Bilayer
*Hydrophobic molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly.

*Hydrophilic molecules including ions and polar molecules do not cross the membrane easily.

*Proteins in the membrane regulating transport.
Transport Proteins
*Allow passage of hydrophilic substances across the membrane.

*Some transport proteins (channel proteins) have a hydrophilic channel that certain molecules or ions can use as a tunnel.

*Channel proteins called aquaporins greatly facilitate the passage of water molecules.

*Transport proteins called carrier proteins bind to molecules and change shape to shuttle them across the membrane.

*A transport protein is specific for the substance it moves
Diffusion
*Diffusion is the tendency for molecules to spread out evenly into the available space.

*Although each molecule moves randomly, diffusion of a population of molecules may be directional.

*At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other.
Passive Transport and Gradients
*Substances diffuse down their concentration gradient.

*No work must be done to move substances down the concentration gradient.

*Diffusion of a substance across a biological membrane is passive transport because no energy is expended.
Effects of Osmosis on Water Balance
*Osmosis is the diffusion of water across a selectively permeable membrane.

*Water moves toward higher solute concentration until the solute concentration is equal on both sides.
Isotonic Solution
Solute concentration is the same; no net water movement across the plasma membrane.
Hypertonic Solution
Solute concentration is greater than that inside the cell; cell loses water.
Tonicity
*The ability of a surrounding solution to cause a cell to gain or lose water.

*Tonicity of a solution depends on its concentration of solutes:

Isotonic solution
Hypertonic solution
Hypotonic solution
Hypotonic Solution
Solute concentration is less than that inside the cell; cell gains water.
Osmoregulation
The control of solute concentrations and water balance.
Water Balance of Cells with Cell Walls
*Cell walls help maintain water balance.

*A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm).

*If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid.

*In a hypertonic environment, plant cells lose water.

*The membrane pulls away from the cell wall, causing the plant to wilt, a potentially lethal effect called plasmolysis
Facilitated Diffusion: Protein Aided Passive Transport
*In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane.

*Transport proteins include channel proteins & carrier proteins.

*Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane:

Aquaporins: facilitate the diffusion of water.
Ion Channels: facilitate the transport of
ions.
Gated Channels: open or close in response
to a stimulus.
Active Transport Uses Energy to move Solutes against their Gradients
*Facilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energy.

*Some transport proteins, however, can move solutes against their concentration gradients.
Active Transport
*Active transport requires energy (ATP) to move substances against their concentration gradients.

*All proteins involved in active transport are carrier proteins.

*Active transport allows cells to maintain concentration gradients that differ from their surroundings.
Membrane Potential
*Membrane potential is the voltage across a membrane.

*Voltage is created by differences in the distribution of positive and negative ions across a membrane.

*The cytoplasmic side of the membrane is negative in charge relative to the extracellular side.
Electrochemical Gradient
The electrochemical gradient drives the diffusion of ions across a membrane:

A chemical force (the ion's concentration
gradient).
An electrical force (the effect of the
membrane potential on the ion's
movement)
Electrogenic Pump
*A transport protein that generates voltage across a membrane.

*Electrogenic pumps help store energy that can be used for cellular work.
Proton Pump
*The main electrogenic pump of plants, fungi, & bacteria.

*Actively transports H+ out of the cell.
Cotransport
*Cotransport occurs when active transport of a solute indirectly drives transport of other substances.

*The diffusion of an actively transported solute down its concentration gradient is coupled with the transport of a second substance against its own concentration gradient.
Exocytosis
*In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell.

*Many secretory cells use exocytosis to export their products.
Endocytosis
*In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane.

*Endocytosis is a reversal of exocytosis.

*There are three types of endocytosis:

Phagocytosis ("cellular eating").
Pinocytosis ("cellular drinking").
Receptor-mediated endocytosis.
Phagocytosis
*In phagocytosis, a cell engulfs a particle in a vacuole.

*The vacuole fuses with a lysosome to digest the particle.
Pinocytosis
In pinocytosis, molecules dissolved in droplets are taken up when extracellular fluid is "gulped" into tiny vesicles.
Receptor-Mediated Endocytosis
*In receptor-mediated endocytosis, binding of specific solutes to receptors triggers vesicle formation.

*Receptor proteins, receptors, and other molecules from the extracellular fluid are transported in the vesicles.

*Emptied receptors are recycled to the plasma membrane.
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