Unit 2 Cell Membranes and Homeostasis Flashcards Preview

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Flashcards in Unit 2 Cell Membranes and Homeostasis Deck (65):
1

selective permeability

plasma membranes allow some substances to cross more easily than others

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phospholipids

the most abundant lipids in most membranes
-amphipathic; have both a hydrophilic and hydrophobic region

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fluid mosaic model

membrane is a fluid structure with a "mosaic" of various proteins embedded or attached to a double (bilayer) of phospholipids

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The Fluidity Membranes

-membrane is held together mainly by weak hydrophobic interactions
-most lipids and some proteins can shift laterally (this occurs rapidly)
-some proteins seem to be moved and directed by cytoskeleton while others are held in place
-membranes must be within a certain range of fluidity to function properly

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The Fluidity of Membranes and Temperature

membranes solidify at low temperatures
-membranes with bent, unsaturated tails will remain fluid at lower temperatures because they cannot be backed closely together
-cholesterol in animal cells resists changes in plasma membrane fluidity caused by change in temperature

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integral proteins

penetrate the hydrophobic interior of the lipid bilayer
-hydrophobic part contains stretches of nonpolar amino acids
-hydrophilic parts are exposed to aqueous solutions

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peripheral proteins

appendages loosely bound to the surface of a membrane

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Functions of Proteins

1. Transport
2. Enzymatic Activity
3. Signal transduction
4. Cell-cell recognition
5. Intercellular joining
6. attachment of cytoskeleton and ECM

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Proteins and transport

may provide hydrophilic channel across membrane or shuttle a substance across by changing shape

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proteins and enzymatic activity

may be enzyme with active sight exposed to substances in adjacent solution

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proteins and signal transduction

may have binding sight that fits the shape of a chemical messenger, protein changes shape and relays message

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proteins and cell-cell recognition

some proteins serve as ID tags that are recognized by membrane proteins of other cells

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proteins and intercellular joining

membrane proteins of adjacent cells hook together

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proteins and attachment of cytoskeleton and ECM

elements of both bound to membrane proteins

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The Role of Membrane Carbohydrates in cell-cell recognition

-cells recognize other cells by minding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
-diversity of molecules allow them to serve as markers

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glycolipids

carbohydrates bonded to lipids

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glycoproteins

carbohydrates bonded to proteins

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The permeability of the lipid bilayer

-nonpolar, hydrophobic molecules can dissolve in the lipid bilayer and easily cross the membrane
-polar molecules pass through very slowly

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transport proteins

allow hydrophobic substances to pass through
-types: channel proteins and carrier proteins

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channel proteins

function by having a hydrophilic channel that certain molecules or ions use as a tunnel through the membrane

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aquaporins

allows water to pass through plasma membrane rapidly

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carrier proteins

hold on to passengers and shuttle them across the membrane by changing shape
-specific for the substance it transports

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passive transport

diffusion of a substance across a membrane with no energy investment

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diffusion

the movement of molecules of any substance so that they spread out evenly into the available space
-a substance will naturally diffuse from where it is more concentrated to where it is less concentrated

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dynamic equilibrium

reached when molecules cross a membrane at equal rates in both directions

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concentration gradient

the region along which the density of a chemical substance increases or decreases

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osmosis

diffusion of free water molecules across a selectively permeable membrane
-water diffuses from region of lower solute concentration (more water molecules) to the region of higher solute concentration until both concentrations are equal

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tonicity

the ability of a surrounding solution to cause a cell to gain or lose water
-depends in part on concentrations of solutes that cannot cross the membrane

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isotonic

an environment in which water will diffuse across a cell membranes at the same rate in both directions. Volume of animal cell remains stable

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hypertonic

more solute outside of cell; cell will lose water and shrivel

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hypotonic

loss solute outside, cell will swell and burst

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osmoregulation

the control of solute concentrations and water balance
-some organisms have less water permeable membranes or have vacuoles that pump out water

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turgid

health, firm state of a plant cell that occurs when it is in a hypotonic solution

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flaccid

limp plant cells in isotonic solutions

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plasmolysis

in a hypertonic environment, a plant cell lose water, shrivels, and its plasma membrane pulls away from the wall

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facilitated diffusion

many polar molecules and ions diffuse passively with the help of transport proteins that span the membrane
-channel proteins allow quick diffusion of some hydrophilic substances
-carrier proteins also carry substances down concentration gradient

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ion channels

channel proteins that transport ions

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gated channels

open or close in response to a stimulus (electric stimulus or binding of substances)

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active transport

uses energy to move solutes against their gradients
-uses carrier proteins
-enables a cell to maintain internal concentrations of small solutes that differ from concentrations in the environment
-ATP can transfer its terminal phosphate group directly to the protein, inducing the protein to change shape

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sodium-potassium pump

exchanges sodium ions for potassium ions across the plasma membrane (using active transport) of animal cells

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membrane potential

the voltage across a membrane
-typically -50 to -200V
-the inside of the cell is negative relative to the outside
-influences traffic, favors passive transport of cations (+) into the cell and anions out of the cell
-some proteins protons, such as NA-K pump, contribute to membrane potential

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electrochemical gradient

the combination of the chemical gradient and electrical force acting on an ion
-ion diffuses down its electrochemical gradient

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electrogenic pump

a transport protein that generates voltage across a membrane
-help store energy that can be tapped for cellular work

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proton pump

an electrogenic pump that actively transports protons (H+) out of a plant cell

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contransport

a mechanism that occurs when an ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes
-couples the diffusion of a previously pumped substance with the transport of a second substance against its gradient.

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exocytosis

transport vesicle from the Golgi moves to the plasma membrane. Proteins rearrange lipids so that the vesicle membrane and plasma membranes fuse. Contents of vesicle are spilled outside the cell
-used by secretory cells to export products

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endocytosis

the cell takes in biological molecules and matter by forming new vesicles from the plasma membrane

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phagocytosis

cell wraps psuedopodia around a particle and packaging it in a food vacuole

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pinocytosis

cell gulps droplets of extracellular fluid into tiny vesicles. Nonspecific in the substance it transports

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receptor-mediated endocytosis

specific substances bind to receptor proteins on the membrane. Proteins cluster in coated pits (which are lined by coat proteins) and form a vesicles. Vesicles is emptied and receptors are recycled to plasma membrane by the same vesicle

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ligands

any molecule that binds specifically to a receptor site on another molecule

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apoplast

in plants, consists of everything external to the plasma membrane of living cells
-includes cell walls, extracellular space, and dead cells

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symplast

consists of the entire mass of cytosol of all living cells in a plant as well as the plasmodes (the cytoplasmic channels that interconnect cells)

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three routes of transport within a plant tissue or organ

1. Apoplastic route
2. Symplastic route
3. transmembrane route

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apoplastic route

water and solutes move along the continuum of cell walls and extracellular space

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symplastic route

water and solutes move among continuum of cytosol,
-substance must pass a plasma membrane once then can move from cell to cell with the plasmodesmata

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transmembrane route

movement out of one cell, across cell wall, and into neighboring cell

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H+ rather than NA+ plays the primary role in basic transport processes in plant cells

-h+ established membrane potential
-H+ most often contransported in plants, helps with absorption of neutral solutes (like sucrose) and movement of ions

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water potential

predicts the direction in which water will flow. Includes the effects of solute concentration and physical pressure
-free water moves from higher to lower water potential
-potential refers to waters capacity to perform work
-equal 0 when pure water in an open containers is at standard conditions

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solute potential

directly proportional to molarity
-expressed as a negative number as the greater the solutes, the fewer free water molecules and the less water potential

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water potential equation

solute potential+ physical pressure= water potential

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pressure potential

physical pressure on a solution. Can be + or - relative to atmospheric pressure

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protoplast

the living part of the cell, which includes the plasma membrane

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Long Distance transport

diffusion is too slow to function in long-distance transport within a plant
-bulk flow plays large role

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bulk flow

the movement of a liquid in response to pressure gradient
-moves from higher to lower pressure
-independent of solute concentration