Chapter 7: Membrane Function and Structure

just read this.

Models of membranes were developed long before membranes were first seen with electron microscopes in the 1950s. In 1915, membranes isolated from red blood cells were chemically analyzed and found to be composed of lipids and proteins. In 1925, E. Gorter and F. Grendel reasoned that cell membranes must be a phospholipid bilayer two molecules thick. The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water. Actual membranes adhere more strongly to water than do artificial membranes composed only of phospholipids. One suggestion was that proteins on the surface of the membrane increased adhesion. In 1935, H. Davson and J. Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins. Early images from electron microscopes seemed to support the Davson-Danielli model, and until the 1960s, it was widely accepted as the structure of the plasma membrane and internal membranes.

laterally, flip-flop, rapid, slowly

Most of the lipids and some proteins drift _____ in the plane of the membrane, but rarely ______ from one phospholipid layer to the other. The lateral movements of phospholipids are _____, about 2 microns per second (or a switching of positions 10⁷ times per second). Many larger membrane proteins may also drift within the phospholipid bilayer, although they move more _______ than the phospholipids. Some proteins move in a very directed manner, perhaps guided or driven by motor proteins attached to the cytoskeleton. Other proteins never move and are anchored to the cytoskeleton.

cytoskeleton, ECM (extracellular matrix)

On the cytoplasmic side of the membrane, some membrane proteins connect to the ___________ . On the exterior side of the membrane, some membrane proteins attach to the fibers of the ____.

hypotonic

The solution with the lower concentration of solutes is ____________ relative to the other solution. A cell in this environment will have more water enter in comparison to the water that leaves the membrane, and the cell will swell and lyse (burst).

tonicity

the ability of a solution to cause a cell to gain or lose water, which depends partly on the concentration of solutes that cannot cross the membrane. If there is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell, and vice versa.

ion, gated

A group of channel proteins called ___ channels, that may also function as ____ channels. These channels open or close depending on the presence or absence of a chemical or physical stimulus. If chemical, the stimulus is a substance other than the one to be transported. For example, stimulation of a receiving neuron by specific neurotransmitters opens gated channels to allow sodium ions into the cell. When the neurotransmitters are not present, the channels are closed.

turgid

A plant cell in a hypotonic solution will swell until the elastic cell wall opposes further uptake. At this point the cell is _____ (very firm), a healthy state for most plant cells, as it helps to contribute to the mechanical support of the plant.

cotransport

A single ATP-powered pump that transports one solute can indirectly drive the active transport of several other solutes in a mechanism called _______. As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration gradient.

freeze-fracture

A specialized preparation technique, ___________, splits a membrane along the middle of the phospholipid bilayer. When a preparation is viewed with an electron microscope, protein particles are interspersed in a smooth matrix, supporting the fluid mosaic model.

voltage

All cells maintain a __________ (ranging from −50 to −200 millivolts) across their plasma membranes, an electrical potential energy due to the separation of opposite charges. The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations and anions on opposite sides of the membrane. The membrane potential acts like a battery. The membrane potential favors the passive transport of cations into the cell and anions out of the cell.

lowers, unsaturated, saturated

As the temperature __________, the phospholipids settle into a closely packed arrangement and the membrane solidifies. The more a membrane has phospholipids with __________ hydrocarbon tails, the more fluid it is at lower temperatures compared to membranes with __________ lipids.

endocytosis

Essentially the reverse of exocytosis, in _____________, a vesicle is formed from the plasma membrane to take in molecules. Three types are: 1. phagocytosis (cell-eating) 2. pinocytosis (cell-drinking) 3. receptor-mediated endocytosis (specific phagocytosis)

flaccid

If a plant cell and its surroundings are isotonic, there is no movement of water into the cell. The cell becomes ______ (limp), and the plant may wilt.

fluid mosaic

In 1972, S. J. Singer and G. Nicolson presented a revised model that proposed that the membrane proteins are dispersed and individually inserted into the phospholipid bilayer. In this ______________ model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water, and the hydrophobic regions are in a nonaqueous environment within the membrane.

pinocytosis

In ____________, a cell creates a vesicle around a droplet of extracellular fluid. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplets. All included solutes are taken into the cell in this nonspecific process.

phagocytosis

In _________________, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole. The contents of the vacuole are digested when the vacuole fuses with a lysosome.

concentration gradient, diffusion

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its ____________. No work must be done to make this happen, because _________ is a spontaneous process, needing no input of energy.

glycolipids, glycoproteins

Membrane carbohydrates are usually short, branched chains with fewer than 15 sugar units. They may be covalently bonded to lipids, forming ________, or more commonly to proteins, forming _________. The proteins on the external side of the plasma membrane vary from species to species, from individual to individual, and even from cell type to cell type within the same individual. This variation distinguishes each cell type. The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

weak

Membrane molecules are held in place by relatively _____ hydrophobic interactions.

osmoregulation

Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for ____________, the control of water balance, to maintain their internal environment. For example, Paramecium, a protist, is hypertonic to the pond water in which it lives. In spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell. To solve this problem, Paramecium cells have a specialized organelle, the contractile vacuole, which functions as a bilge pump to force water out of the cell (so cool :D)

amphipathic

Phospholipids and most other membrane constituents (both lipids and proteins) are _____________ molecules. These molecules have both hydrophobic regions and hydrophilic regions.

proteins

Phospholipids form the main fabric of the membrane, but _______ determine most of the membrane's functions.

isotonic

Solutions with equal solute concentrations are ________. There will be no *net* movement; water flows in both directions across the plasma membrane but at the same rate in both directions. An animal cell fares best in this environment.

Active transport, carrier, ATP

Some transport proteins move solutes across membranes against their concentration gradient. This __________ requires the cell to expend metabolic energy. All transport proteins used for this are _______ proteins. ____ supplies the energy for most of these transports, by transferring its phosphate group to the protein, making the protein change shape (an example of this is the Na-K pump)

electrogenic, Na-K, proton

Special transport proteins, __________ pumps, generate a voltage across a membrane in order to store energy outside the cell that can later be used for cellular work. The ____ pump is the major pump of this type in animals. The ______ pump is the main pump in plants, fungi and bacteria.

facilitated diffusion

Speeds the transport of a solute by providing an efficient passage through the membrane, but doesn't alter the direction of transport. This passive movement of molecules down their concentration gradient via transport proteins is called ______________, and does not require work.

cotransport

The H+ gradient is maintained by a ATP-driven proton pump that concentrates H+ ions outside the cell, storing potential energy that can be used for active transport. This is an example of ____________.

asymmetrical

The __________ orientation of proteins, lipids and associated carbohydrates begins during the synthesis of membrane in the ER and Golgi apparatus. Membrane lipids and proteins are synthesized in the endoplasmic reticulum. Carbohydrates are added to proteins in the ER, and the resulting glycoproteins are further modified in the Golgi apparatus. Glycolipids are also produced in the Golgi apparatus. These are all transported to the membrane via vesicles, which then fuse with the membrane.

negative, cations

The cytoplasm is ____________ relative to the outside of the cell. Sodium is pumped out by the Na-K pump, and potassium is pumped in. Because of this, the cell favors the transport of ___________ into the cell.

passive transport

The diffusion of a substance across a biological membrane is called _______________ because it requires no energy from the cell to make it happen.

hypertonic

The solution with the higher concentration of solutes is ______________ relative to the other solution. A cell in this environment loses water to its surroundings, shrivel up and probably die.

Transport, enzymes, Signal transduction, recognition, joining, Attachment

The proteins of the plasma membrane have six major functions: 1. _____________ of specific solutes into or out of cells (sometimes by changing shape, or hydrolyzing ATP as an energy source to actively pump stuff across membranes) 2. Act as _______: active site exposed to substances in the adjacent solution. They sometimes act as a team with other enzymes by catalyzing one of a number of steps of a metabolic pathway. 3. ____________: relaying hormonal messages to the cell. (hormone fits into active site, causes shape change in protein, which then relays message into cell) 4. Cell-cell ____________: some glycoproteins serve as identification tags that are recognized by membrane proteins of other cells. 5. Intercellular _________ of adjacent cells with gap or tight junctions. 6. ____________ to the cytoskeleton and extracellular matrix: maintaining cell shape and stabilizing the location of certain membrane proteins. These proteins can then coordinate extracellular and intracellular changes.

passive

The two types of _______ transport proteins are channel and carrier proteins.

specific

Transport proteins are highly ____________ for the substance it moves across membrane.

electrochemical gradient

Two combined forces, collectively called the ____________, drive the *diffusion of ions* across a membrane. One is a *chemical force* based on an ion's concentration gradient. The other is an *electrical force* based on the effect of the membrane potential on the ion's movement.

inside,outside

When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the inside layer of the plasma membrane. In that way, molecules that originate on the ______ face of the ER end up on the _______ face of the plasma membrane.

peripheral

_________ proteins are not embedded in the lipid bilayer at all. Instead, they are loosely bound to the surface of the protein, often connected to integral proteins.

integral

__________ proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (as transmembrane proteins). The hydrophobic regions built into the membrane's core consist of stretches of nonpolar amino acids, often coiled into alpha helices. Where they are in contact with the aqueous environment, they have hydrophilic regions of amino acids.

cholestorol

____________ acts as a "temperature buffer" for the membranes of animal cells. At warm temperatures (such as 37°C), it restrains the movement of phospholipids and reduces fluidity. At cool temperatures, it maintains fluidity by preventing tight packing.

Cell-cell recognition

____________, the ability of a cell to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. This attribute is important in the sorting and organization of cells into tissues and organs during development. It is also the basis for rejection of foreign cells by the immune system. Cells recognize other cells by binding to surface molecules, often carbohydrates, on the plasma membrane.

hydrophobic, hydrophillic, ion

_____________ molecules, such as hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily. However, the hydrophobic core of the membrane impedes the direct passage of ions and polar (_________) molecules, which cross the membrane with difficulty. This includes small molecules, such as water, and larger molecules, such as glucose and other sugars. An ____, whether a charged atom or molecule, and its surrounding shell of water also has difficulty penetrating the hydrophobic core.

Receptor-mediated

______________ endocytosis allows greater specificity, transporting only certain substances. This process is triggered when extracellular substances, or *ligands*, bind to special receptors on the membrane surface. The receptor proteins are clustered in regions of the membrane called *coated pits*, which are lined on their cytoplasmic side by a layer of coat proteins. Binding of ligands to receptors triggers the formation of a vesicle by the coated pit, bringing the bound substances into the cell. This type of endocytosis enables a cell to acquire bulk quantities of specific materials that may be in low concentrations in the environment.

exocytosis

the process by which a substance is released from the cell through a vesicle that transports the substance to the cell surface and then fuses with the membrane to let the substance out

channel protein

transport protein that has a hydrophilic channel for certain molecules and atomic ions to pass through in the membrane instead of coming in contact with the phospholipid bilayer.

aquaporin

type of channel protein for water passage. Each can allow the entry of up to 3 billion water molecules per second.

carrier protein

type of channel protein that binds to molecules and changes its shape to shuttle materials across the membrane.

plasmolysis

when a plant cell is in a hypertonic environment, the cell will lose water to its surroundings, shrink, and its plasma membrane will pull away from the wall.