Architecture of biological membranes and transport mechanisms through membrane

Question 1

Selective permeability

Answer 1

Like all biological membranes, the plasma membrane exhibits selective permeability; that is, it allows some substances to cross more easily than others. The resulting ability of the cell to discriminate in its chemical exchanges with its environment is fundamental to life.

Question 2

Amphipathic

Answer 2

A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region. A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to the water.

Question 3

Fluid mosaic model

Answer 3

In the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. The proteins are not randomly distributed, groups of them are often associated in long-lasting, specialized patches, as are certain lipids.

Question 4

Integral proteins

Answer 4

Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic interior. The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids , usually coiled into alfa helixes. The hydrophilic parts are exposed to the aqueous solutions on either side of the membrane. Some proteins also have one or more hydrophilic channels that allow passage for hydrophilic substances, like water.

Question 5

Peripheral proteins

Answer 5

Peripheral proteins are not embedded in the lipid bilayer at all, they are loosely bound to the surface of the membrane, often to exposed parts of integral proteins.

Question 6

Membranous proteins and their functions

Answer 6

There are a number of different kinds of membranous proteins. For instance, they are used for transport across the membrane, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Question 7

Transport

Answer 7

A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane

Question 8

Enzymatic activity

Answer 8

A protein built into the membrane may be an enzyme with its active site (where the target molecule binds), exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

Question 9

Signal transduction

Answer 9

A membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein.

Question 10

Cell-cell recognition

Answer 10

Some glycoproteins, serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell-cell binding is usually short-lived compared to intercellular joining.

Question 11

Intercellular joining

Answer 11

Membrane proteins of adjacent cells may hook together in various kinds of junctions, like gap junctions or tight junctions. This type of binding is more long lasting than cell-cell recognition.

Question 12

Attachement to the cytoskeleton and extracellular matrix (ECM)

Answer 12

Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes location of certain membrane proteins. Proteins that bind to the ECM molecules can coordinate extracellular and intracellular changes.

Question 13

Glycolipids

Answer 13

Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. Some are covalently bonded to lipids, forming molecules called glycolipids. However, most are covalently bonded to proteins, which are thereby called glycoproteins.

Question 14

Glycoproteins

Answer 14

Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. Some are covalently bonded to lipids, forming molecules called glycolipids. However, most are covalently bonded to proteins, which are thereby called glycoproteins.

Question 15

Transport proteins

Answer 15

Specific ions and a variety of polar molecules can’t move through cell membranes on their own. However, these hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span through the membrane. Other transport proteins, called carrier proteins hold onto their passengers and change shape in a way that shuttles them across the membrane. Transport proteins are specific for the substance it translocates, allowing only a certain substance or a small group of related substances to cross the membrane.

Question 16

Aquaporins

Answer 16

Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. For example, the passage of water molecules through the plasma membrane of certain cells is greatly facilitated by channel proteins called aquaporins. Most aquaporin proteins consist of 4 identical subunits. The polypeptide making up each subunit forms a channel that allows single file passage of up to 3 billion water molecules per second, many more than would cross without it.

Question 17

Diffusion

Answer 17

Molecules have a type of energy called thermal energy, due to their constant motion. One result of this motion is diffusion, the movement of particles of any substance so that they tend to spread out into the available space. Each molecule moves randomly, yet diffusion of a population of molecules may be directional. In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient.

Question 18

Concentration gradient

Answer 18

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient.

Question 19

Passive transport

Answer 19

The diffusion of a substance across biological membrane is called passive transport, because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. Remember, however, that membranes are selectively permeable, and therefore have different effects on the rates of diffusion in various molecules.

Question 20

Osmosis

Answer 20

The diffusion of free water across a selectively permeable membrane, whether artificial or cellular, is called osmosis. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms.

Question 21

Tonicity

Answer 21

To explain the behavior of a cell in a solution, we must consider both solute concentration and membrane permeability. Both factors are taken into account in the concept of tonicity, the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrative solutes) relative to that inside the cell. If there is a higher concentration of nonpenetrative solutes in the surrounding solution, water will tend to leave the cell and vice versa.

Question 22

Isotonic

Answer 22

If a cell without a cell wall, such as an animal cell, is immersed in an environment that is isotonic to the cell (iso means same), there will be no net movement of water cross the plasma membrane. Water diffuses across the membrane, but at the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable, as shown in the middle.

Question 23

Hypertonic

Answer 23

Let’s transfer the cell to a solution that is hypertonic to the cell (hyper means “more”, in this case referring to nonpenetrating solutes). The cell will lose water, shrivel, and probably die.

Question 24

Hypotonic

Answer 24

If we place the cell in a solution that is hypotonic to the cell (hypo means less), water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled water balloon.

Question 25

Osmoregulation

Answer 25

In hypertonic or hypotonic environments, however, organisms that lack rigid cell walls must have other adaptions for osmoregulation, the control of solute concentrations and water balance.

Question 26

Plant cell

Answer 26

Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the wall pushing back on the cell.

Question 27

Animal cell

Answer 27

An animal cell fares best in an isotonic, environment unless it has special adaptions that offset the osmotic uptake or loss of water.

Question 28

Turgid

Answer 28

Like the animal cell, the plant cell swells as water enters by osmosis. However, the relatively inelastic cell wall will expand only so much before it exerts a back pressure in the cell, called turgor pressure, that opposes further water uptake. At this point, the cell is turgid (very firm), which is the healthy state for most plant cells.

Question 29

Flaccid

Answer 29

Plants that are not woody, such as most houseplants, depend on mechanical support on cells kept turgid by a surrounding hypotonic solution. If a plant’s cells and their surroundings are isotonic, there is no net tendency for water to enter, and the cells become flaccid.

Question 30

Plasmolysis

Answer 30

The cell wall is of no advantage if the cell is immersed in a hypertonic environment. In this case, a plant cell, like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places. This phenomenon is called plasmolysis, causes the plant to wilt and can lead to plant death. The walled cells of bacteria and fungi also plasmolyze in a hypertonic environment.

Question 31

Facilitated diffusion

Answer 31

Many polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon is called facilitated diffusion. Most transport proteins are very specific.

Question 32

Ion channels

Answer 32

Channel proteins that transport ions are called ion channels. Many ion channels function as gated channels, which open or close in response to a stimulus. For some gated channels, the stimulus is electrical. In a nerve cell, for example an ion channel opens in response to an electrical stimulus, allowing a stream of potassium ions to leave the cell. This restores the cells ability to fire again. Other gated channels opens or close when a specific substance other than the one to be transported binds to the channel. These gated channels are also important in the functioning of the nervous system.

Question 33

Gated channels

Answer 33

For some gated channels, the stimulus is electrical. In a nerve cell, for example an ion channel opens in response to an electrical stimulus, allowing a stream of potassium ions to leave the cell. This restores the cells ability to fire again. Other gated channels opens or close when a specific substance other than the one to be transported binds to the channel. These gated channels are also important in the functioning of the nervous system.

Question 34

Active transport

Answer 34

To pump a solute across a membrane against its gradient requires work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport. The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins. For example, pumping calcium ions into the ER. ATP supplies most of the energy for active transport.

Question 35

Sodium-potassium pump

Answer 35

One way ATP power can power active transport is by transferring its terminal phosphate group directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a solute bound to the protein across the membrane. One transport system that works this way is the sodium potassium pump, which exchanges Na+ for K+ across the plasma membrane of animal cells.

Question 36

Membrane potential

Answer 36

All cells have voltages across their plasma membranes. Voltage is electrical potential energy, a separation of opposite charges. The cytoplasmic side of the membrane is negative in charge relative to the extracellular side, because of an unequal distribution of anions and cations on the two sides. The voltage across a membrane is called membrane potential, ranges from about -50 to -200 millivolts (mV). The minus sign indicates that the inside of the cell is negative relative to the outside. The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the membrane. Because the inside of the cell is negative compared to the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell. Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient) and an electrical force (the effect of the membrane potential on the ion’s movement). This combination of forces acting on an ion is called the electrochemical gradient.

Question 37

Electrochemical gradient

Answer 37

All cells have voltages across their plasma membranes. Voltage is electrical potential energy, a separation of opposite charges. The cytoplasmic side of the membrane is negative in charge relative to the extracellular side, because of an unequal distribution of anions and cations on the two sides. The voltage across a membrane is called membrane potential, ranges from about -50 to -200 millivolts (mV). The minus sign indicates that the inside of the cell is negative relative to the outside. The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the membrane. Because the inside of the cell is negative compared to the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell. Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient) and an electrical force (the effect of the membrane potential on the ion’s movement). This combination of forces acting on an ion is called the electrochemical gradient.

Question 38

Electrogenic pump

Answer 38

A transport protein that generates a voltage across a membrane is called an electrogenic pump. The sodium-potassium pump appears to be the major electrogenic pump of the animal cells. The main electrogenic pump of plants, fungi and bacteria is a proton pump, which actively transfers protons out of the cell.

Question 39

Proton pump

Answer 39

The main electrogenic pump of plants, fungi and bacteria is a proton pump, which actively transfers protons out of the cell. The pumping of H+ transfers positive charge from the cytoplasm to the extracellular solution. By generating a voltage across membranes, electrogenic pumps help restore energy that can be tapped for cellular work. One important use of proton gradients in the cell is for ATP synthesis during cellular respiration. Another is a type of membrane traffic called cotransport.

Question 40

Cotransport

Answer 40

One important use of proton gradients in the cell is for ATP synthesis during cellular respiration. Another is a type of membrane traffic called cotransport. In a mechanism called cotransport, a transport protein (a cotransporter) can couple the “downhill” diffusion of the solute to the “uphill” transport of a second substance against its own concentration (or electrochemical) gradient.

Question 41

Exocytosis

Answer 41

The cell secretes certain biological molecules by the fusion of vesicles with the plasma membrane; this process is called exocytosis. A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. When the vesicle membrane and the plasma membrane come into contact, specific proteins rearrange the lipid molecules of the two bilayers so that the two membranes fuse. The contents of the vesicle then spill to the outside of the cell, and the vesicle membrane becomes part of the plasma membrane.

Question 42

Endocytosis

Answer 42

In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane. Although the proteins involved in the two processes are different, the events of endocytosis look like the reverse of exocytosis.

Question 43

Phagocytosis

Answer 43

In phagocytosis, a cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes.

Question 44

Pinocytosis

Answer 44

In pinocytosis, a cell continually “gulps” droplets of extracellular fluid into tiny vesicles, formed by infoldings of the plasma membrane. In this way, the cell obtains molecules dissolved in the droplets. Because any and all solutes are taken into the cell, pinocytosis as shown here is nonspecific for the substances it transports. In many cases, as above, the parts of the plasma membrane that form vesicles are lined on their cytoplasmic side by a fuzzy layer of coat protein; the “pits” and resulting vesicles are said to be coated.

Question 45

Receptor-mediated endocytosis.

Answer 45

It is a specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the plasma membrane are proteins with receptor sites exposed to the extracellular fluid. Specific solutes bind to the sites. The receptor proteins then cluster in coated pits, and each coated pit forms a vesicle containing the bound molecules. There are relatively more bound molecules inside the vesicle, but other molecules are also present. After the ingested material is liberated from the vesicle, the emptied receptors are recycled to the plasma membrane by the same vesicle.