Chapter 5

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113 Terms

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**active transport**
method of transporting material that requires energy
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**amphiphilic**
molecule possessing a polar or charged area and a nonpolar or uncharged area capable of interacting with both hydrophilic and hydrophobic environments
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**antiporter**
transporter that carries two ions or small molecules in different directions
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**aquaporin**
channel protein that allows water through the membrane at a very high rate
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**carrier protein**
membrane protein that moves a substance across the plasma membrane by changing its own shape
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**caveolin**
protein that coats the plasma membrane's cytoplasmic side and participates in the liquid uptake process by potocytosis
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**channel protein**
membrane protein that allows a substance to pass through its hollow core across the plasma membrane
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**clathrin**
protein that coats the plasma membrane's inward-facing surface and assists in forming specialized structures, like coated pits, for phagocytosis
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**concentration gradient**
area of high concentration adjacent to an area of low concentration
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**diffusion**
passive transport process of low-molecular weight material according to its concentration gradient
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**electrochemical gradient**
a combined electrical and chemical force that produces a gradient
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**electrogenic pump**
pump that creates a charge imbalance
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**endocytosis**
type of active transport that moves substances, including fluids and particles, into a cell
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**exocytosis**
process of passing bulk material out of a cell
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**facilitated transport**
process by which material moves down a concentration gradient (from high to low concentration) using integral membrane proteins
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**fluid mosaic model**
describes the plasma membrane's structure as a mosaic of components including phospholipids, cholesterol, proteins, glycoproteins, and glycolipids (sugar chains attached to proteins or lipids, respectively), resulting in a fluid character (fluidity)
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**glycolipid**
combination of carbohydrates and lipids
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**glycoprotein**
combination of carbohydrates and proteins
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**hydrophilic**
molecule with the ability to bond with water; “water-loving”
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**hydrophobic**
molecule that does not have the ability to bond with water; “water-hating”
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**hypertonic**
situation in which extracellular fluid has a higher osmolarity than the fluid inside the cell, resulting in water moving out of the cell
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**hypotonic**
situation in which extracellular fluid has a lower osmolarity than the fluid inside the cell, resulting in water moving into the cell
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**integral protein**
protein integrated into the membrane structure that interacts extensively with the membrane lipids' hydrocarbon chains and often spans the membrane
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**isotonic**
situation in which the extracellular fluid has the same osmolarity as the fluid inside the cell, resulting in no net water movement into or out of the cell
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**osmolarity**
total amount of solutes dissolved in a specific amount of solution
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**osmosis**
transport of water through a semipermeable membrane according to the water's concentration gradient across the membrane that results from the presence of solute that cannot pass through the membrane
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**passive transport**
method of transporting material through a membrane that does not require energy
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**peripheral protein**
protein at the plasma membrane's surface either on its exterior or interior side
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**pinocytosis**
a variation of endocytosis that imports macromolecules that the cell needs from the extracellular fluid
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**plasmolysis**
detaching the cell membrane from the cell wall and constricting the cell membrane when a plant cell is in a hypertonic solution
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**potocytosis**
variation of pinocytosis that uses a different coating protein (caveolin) on the plasma membrane's cytoplasmic side
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**primary active transport**
active transport that moves ions or small molecules across a membrane and may create a difference in charge across that membrane
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**pump**
active transport mechanism that works against electrochemical gradients
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**receptor-mediated endocytosis**
variation of endocytosis that involves using specific binding proteins in the plasma membrane for specific molecules or particles, and clathrin-coated pits that become clathrin-coated vesicles
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**secondary active transport**
movement of material that results from primary active transport to the electrochemical gradient
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**selectively permeable**
membrane characteristic that allows some substances through (also known as semipermeable)
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**solute**
substance dissolved in a liquid to form a solution
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**symporter**
transporter that carries two different ions or small molecules, both in the same direction
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**tonicity**
amount of solute in a solution
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**transport protein**
membrane protein that facilitates a substance's passage across a membrane by binding it
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**transporter**
specific carrier proteins or pumps that facilitate movement
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**uniporter**
transporter that carries one specific ion or molecule
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Which plasma membrane component can be either found on its surface or embedded in the membrane structure?
protein
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Which characteristic of a phospholipid contributes to the fluidity of the membrane?
double bonds in the fatty acid tail
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What is the primary function of carbohydrates attached to the exterior of cell membranes?
identification of the cell
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Plasma membranes range from
5 to 10 nm in thickness
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A plasma membrane's principal components are
lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and proteins
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A phospholipid is a molecule consisting of
glycerol, two fatty acids, and a phosphate-linked head group
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Carbohydrates are present only on the
plasma membrane's exterior surface and are attached to proteins
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The membrane's main fabric comprises
amphiphilic, phospholipid molecules
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The phospholipids' hydrophilic regions form
hydrogen bonds with water and other polar molecules on both the cell's exterior and interior
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the membrane surfaces that face the cell's interior and exterior are
hydrophilic.
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the cell membrane's interior is
hydrophobic and will not interact with water
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phospholipids form an excellent two-layer cell membrane that
separates fluid within the cell from the fluid outside the cell
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A phospholipid molecule consists of a
three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon
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vital to the plasma membrane's structure because, in water
phospholipids arrange themselves with their hydrophobic tails facing each other and their hydrophilic heads facing out
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lipid bilayer
a double layered phospholipid barrier that separates the water and other materials on one side from the water and other materials on the other side
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Single-pass integral membrane proteins usually have a
hydrophobic transmembrane segment that consists of 20–25 amino acids
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These carbohydrate chains may consist of
2–60 monosaccharide units and can be either straight or branched
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carbohydrates form
specialized sites on the cell surface that allow cells to recognize each other
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This recognition function is very important to cells, as it allows the
immune system to differentiate between body cells (“self”) and foreign cells or tissues (“non-self”)
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The glycocalyx is highly
hydrophilic and attracts large amounts of water to the cell's surface
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The membrane is not like a balloon, however, that can expand and contract; rather, it is
fairly rigid and can burst if penetrated or if a cell takes in too much water
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Plasma membranes are asymmetric: the
membrane's interior is not identical to its exterior
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Non-polar and lipid-soluble material with a low molecular weight can
easily slip through the membrane's hydrophobic lipid core
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Ions such as
sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes
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concentration gradients are a form of
potential energy, which dissipates as the gradient is eliminated
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Molecules move constantly in a
random manner, at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature.
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A substance moves into any space available to it until it
evenly distributes itself throughout
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lack of a concentration gradient in which the
substance has no net movement dynamic equilibrium
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A concentration gradient exists that would allow these materials to
diffuse into the cell without expending cellular energy
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Facilitated transport proteins shield these materials from the
membrane's repulsive force, allowing them to diffuse into the cell
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Passage through the channel allows polar compounds to avoid the
plasma membrane's nonpolar central layer that would otherwise slow or prevent their entry into the cell
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Channel proteins are either
open at all times or they are “gated,” which controls the channel's opening
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When all of the proteins are bound to their ligands, they are
saturated and the rate of transport is at its maximum
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glucose transport proteins, or GLUTs, are involved in
transporting glucose and other hexose sugars through plasma membranes within the body
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Channel proteins facilitate diffusion at a rate of
tens of millions of molecules per second; whereas, carrier proteins work at a rate of a thousand to a million molecules per second
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Osmosis is the movement of
free water molecules through a semipermeable membrane according to the water's concentration gradient across the membrane
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If the solution's volume on both sides of the membrane is the same, but the solute's concentrations are different, then there are
different amounts of water, the solvent, on either side of the membrane
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A solution with low osmolarity has a
greater number of water molecules relative to the number of solute particles
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A solution with high osmolarity has
fewer water molecules with respect to solute particles
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the solute cannot move across the membrane, and thus
the only component in the system that can move the water, moves along its own concentration gradient
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Scientists use three terms
hypotonic, isotonic, and hypertonic to relate the cell's osmolarity to the extracellular fluid's osmolarity that contains the cells
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In a hypotonic environment
water enters a cell, and the cell swells
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In an isotonic condition
the relative solute and solvent concentrations are equal on both membrane sides
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In a hypertonic solution
water leaves a cell and the cell shrinks
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If either the hypo- or hyper- condition goes to excess, the cell’s functions become
compromised, and the cell may be destroyed
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If the cell swells, and the spaces between the lipids and proteins become too large, the cell will
break apart
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when excessive water amounts leave a red blood cell, the cell
shrinks, or crenates
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Various living things have ways of
controlling the effects of osmosis a mechanism we call osmoregulation
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The interior of living cells is electrically negative with respect to the extracellular fluid in which they
are bathed, and at the same time, cells have higher concentrations of potassium (K+) and lower concentrations of sodium (Na+) than the extracellular fluid
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To move substances against a concentration or electrochemical gradient, the cell must use
energy.
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An important membrane adaptation for active transport is the
presence of specific carrier proteins or pumps to facilitate movement
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These three types of carrier proteins are also in
facilitated diffusion, but they do not require ATP to work in that process
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Some examples of pumps for active transport are
Na+-K+ ATPase, which carries sodium and potassium ions, and H+-K+ ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins
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Two other carrier proteins are
Ca2+ ATPase and H+ ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.
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One of the most important pumps in animal cells is the
sodium-potassium pump (Na+-K+ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na+ and K+) in living cells
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The process consists of the following six steps

1. With the enzyme oriented towards the cell's interior, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it.
3. As a result, the carrier changes shape and reorients itself towards the membrane's exterior. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the cell's interior.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions moves into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.
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Secondary active transport uses the
kinetic energy of the sodium ions to bring other compounds, against their concentration gradient into the cell
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As sodium ion concentrations build outside of the plasma membrane because of the primary active transport process, this creates an
electrochemical gradient.