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Fluid Mosaic Model

The non-polar regions of phospholipids and membrane proteins interact to form an insoluble barrier, which serves as a lipid "lake" in which a variety of proteins "float"


The capacity of phospholipids to associate with one another and maintain a bilayer organization helps biological membranes fuse during vesicle formation, phagocytosis, and related processes

Lipid Composition (Membrane Fluidity)

Cholesterol and long-chain, saturated fatty acids pack tightly together, resulting in less fluid membranes

Temperature (Membrane Fluidity)

Membrane fluidity declines under cold conditions because molecules move more slowly at lower temperatures. Some organism simply change the lipid composition of their membranes when their environment getes cold, replacing saturated with unsaturated fatty acids and using fatty acids with shorter chains


A membrane that encloses portions of some neutrons (nerve cells) and acts as an electrical insulator - has only 1 protein for every 70 lipids

Peripheral Membrane Proteins

Lack exposed hydrophobic groups and are not embedded in the bilayer.
Polar or charged regions that interact with exposed parts of integral proteins, or with polar heads of phospholipid molecules

Integral Membrane Proteins

At least partly embedded in the phospholipid bilayer. They have both bydrophilic and hydrophobic regions.


The polar ends of proteins can interact with the polar ends of lipids, and the non-polar regions of both molecules can interact hydrophobically

Transmembrane Protein

An integral membrane protein that extends all the way through the phospholipid bilayer and protrudes on both sides.
Transmembrane proteins are always oriented the same way - domains with specific functions inside or outside the cell are always found on the correct side of the membrane


Consists of a carbohydrate covalently bonded to a lipid. The carbohydrate may serve as a recognition signal fro interactions between cells


Consists of a carbohydrate covalently bonded to a protein. These oligosaccharides often function as signaling sites, as do the carbohydrates attached to glycolipids.


Membranes are constantly forming, transforming from one type to another, fusing with one another, and breaking down

Selective Permeability

Biological membranes allow some substances, but not others, to pass through them

Passive Transport

Process that does not require direct input of metabolic energy to drive them

Active Transport

Process that does require the input of metabolic energy to drive them


The energy for passive transport of a substance is found in the difference between its concentration on one side of the membrane and its concentration on the other


The process of random movement toward a state of equilibrium


Not surprising that as plants and animals evolved and became larger and multicellular, those with circulatory systems to distribute vital molecules such as O2 had a distinct advantage over organisms relying on simple diffusion

Simple Diffusion

Some small molecules can pass through the phospholipid bilayer of the membrane


A molecule that is hydrophobic and soluble in lipids can enter the membrane readily and pass through it


Electrically charged or polar molecules, such as amino acids, sugars, ions and water, do not pass readily through a membrane.
These molecules are not very soluble in the hydrophobic interior of the bilayer, and also charged molecules will form hydrogen bonds with water and ions in the aqueous environment


Water molecules pass through specialized channels in membranes (diffusion process)


In a particular solution, the higher the total solute concentration, the lower the concentration of water molecules


Solution has a higher solute concentration than the other solution


Solutions have equal solute concentrations


Solution has a lower solute concentration than the other solution


The concentration of solutes in the environment determines the direction of osmosis in all animal cells


The integrity of blood cells is absolutely dependent on the maintenance of a constant solute concentration in the surrounding blood plasma - the plasma must be isotonic to the blood

Turgor Pressure

Cells with sturdy walls take up a limited amount of water, and in so doing they build up internal pressure against a cell wall, which prevents further water from entering

Channel Proteins

Integral membrane proteins that form channels across the membrane through which certain substances can pass

Carrier Proteins

Binding proteins that speed up their diffusion through the bilayer

Facilitated Diffusion

Substances diffuse according to their concentration gradients, but their diffusion is made easier by channel or carrier proteins

Ion Channels

Channel protein
Movement of ions across membranes is important in many biological processes, including respiration.
Show basic structure of hydrophilic pore that allows a particular ion to move through

Gated Channel

Most ion channels are "gated": opened or closed to ion passage
Opens when a stimulus causes change in the 3D shape of the channel


Binding of a chemical signal
Causes a stimulus

Ligand-Gated Channel

Channels controlled by a ligand (chemical stimulus)

Voltage-Gated Channel

Stimulated to open or close by a change in voltage (electrical charge difference) across the membrane


Allow large amounts of water to move along the concentration gradient


Another kind of facilitated diffusion involves the actual binding of the transported substance to a membrane protein called a carrier protein (speeds up diffusion through membrane)


Binding of glucose to a specific 3D site on one side of the transport protein to change its shape and release glucose on the other side of the membrane


Since glucose is usually broken down as soon as it enters the cell, there is almost always a strong concentration gradient favoring glucose entry.
The transporter allows glucose molecules to cross the membrane and enter the cell much faster than they would by simple diffusion through the bilayer


Because there are only a limited number of carrier protein molecules per unit of membrane area, the rate of diffusion reaches a maximum when all the carrier molecules are fully loaded with solute molecules.


Often the energy source is the nucleotide adenosine triphosphate (ATP)
In eukaryotes, ATP is produced in the mitochondria and plastids, and it has chemical energy stored in its terminal phosphate bond


Simple and facilitated diffusion (passive transport) follow concentration gradients and can occur in either direction across the membrane.
Active transport is directional, and moves a substance either into or out of a cell against its concentration gradient

Primary Active Transport

Involves the direct hydrolysis of ATP, which provides the energy required for transport

Secondary Active Transport

Does not use ATP directly. Energy is supplied by a ion concentration gradient or an electrical gradient, established by primary active transport


In primary active transport, energy released by hydrolysis of ATP drives the movement of specific ions against their concentration gradients

Sodium-Potassium Pump

Integral membrane glycoprotein that is found in all animal cells
Breaks down ATP into ADP and Pi and uses the released energy to bring two K+ ions into the cell, and export three Na+ ions


In secondary active transport, the movement of a substance is accomplished by energy "regained" by letting ions move across the membrane with their gradient


Group of processes that bring small molecules, macromolecules, large particles and even small cells into the cell
3 types: phagocytosis, pinocytosis, and receptor-mediated endocytosis


"cellular eating"
Part of the plasma membrane engulfs a large particle or even an entire cell
Phagosome -- food vacuole, forms usually fuses with a lysosome where its contents are digested


"cellular drinking"
The vesicles are smaller and bring fluids and dissolved substances, including proteins into the cell

Receptor-Mediated Endocytosis

Molecules at the cell surface are recognized and trigger the uptake of specific materials


Proteins that bind to specific molecules (ligands) and set off specific cellular responses
Receptors are integral membrane proteins located at particular regions on the extracellular surface of the plasma membrane

Coated Pits

Membrane regions that form slight depressions in the plasma membrane, coated by a protein called clathrin


Receptor-mediated endocytosis is used to take cholesterol up into the cell
Most cholesterol is packaged into LDL (low-density lipoproteins) and circulated via the bloodstream
-->LDL's bind to the receptors and taken into the cell via endocytosis


Process by which materials are packaged in vesicles are secreted by the cell
Contents of vesicles are released into the environment, and the vesicle is incorporated back into the membrane


Secreted proteins are transported out of the cell via exocytosis. The proteins are folded and modified in the ER, transported in vesicles to the Golgi and are modified and packaged in vesicles for secretion again

Signal Transduction Pathway

Sequence of molecular events and chemical reactions that lead to a cell's response to a signal.
Ability of cells to sense and respond to their environment is key to the maintenance of cellular and organismal homeostasis


Signals that affect the same cells that release them


Signals diffuse to and affect nearby cells


Signals to distant cells travel through the circulatory system


Chemical signals do not always come from within the multicellular organism -- some come from the external environment


The target cell must be able to sense the signal and respond to it
In a multicellular animal, the cells may receive chemical signals that are circulated in the blood, but most body cells are not capable of responding to the signals

Allosteric Regulation

Alteration in the 3D shape of a protein as a result of the binding of another molecule at a site other than the protein's active site


A signal transduction pathway may end in a response that is short term, such as the activation of an enzyme, or long term such as an alternation in gene expression


Binding of the ligand causes the receptor to change its 3D shape, and that conformational change initiates a cellular response

Cytoplasmic Receptors

Small and nonpolar ligands can be t

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