biology EXAM #3 pt.1 Flashcards
the plasma membrane
what is it
exhibits
it allows
transports
the boundary that separates the living cell from its surroundings
Exhibits selective permeability
It allows some substances to cross it more easily than others
Transport proteins are often responsible for controlling passage across cellular membranes
The fluid mosaic model of membrane structure
States that a membrane is a fluid structure of lipids with a “mosaic” of various proteins embedded in it
Phospholipids
Are the most abundant
Are… , containing
The _______________ of the phospholipids are….
Are the most abundant lipid in the plasma membrane
Are amphipathic, containing both hydrophobic and hydrophilic regions
The hydrophobic tails of the phospholipids are sheltered inside the membrane, while the hydrophilic heads are exposed to water on either side
In the fluid mosaic model,
Proteins are not
the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
randomly distributed in the membrane
Phospholipids in the plasma membrane held together by ____________________.
Lipids and proteins can
Rarely, a lipid may
weak hydrophobic interactions
move sideways within the bilayer.
flip-flop across the membrane, from one phospholipid layer to the other
Membranes must be….; fluidity affects:
Membranes that are too
Organisms living in extreme temperatures have
fluid to work properly; fluidity affects both permeability and movement of transport proteins
fluid cannot support protein function
adaptive differences in membrane lipid composition
The type of hydrocarbon tails in phospholipids affects
______________ also affect the fluidity
the fluidity of the plasma membrane, the more unsaturated, the more fluidity of the membrane.
Temperature
The steroid cholesterol has different effects (temperature buffer) on membrane fluidity at different temperatures:
–At moderate ºT :
–At low ºT :
Plants use different but related…
-it reduces fluidity by reducing mobility of phospholipids,
-it prevents solidifications by disrupting regular packaging of phospholipids
steroid lipids to buffer membrane fluidity
Variations in lipid composition of cell membranes of many species appear to be
Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary
example, cell membranes have a high proportion of unsaturated hydrocarbon tails in fish that live in extreme cold
example, in winter wheat, the percentage of unsaturated phospholipids increases in autumn to prevent membrane solidification during winter
adaptations to specific environmental conditions
A membrane
Proteins determine most of the
is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
membrane’s functions
INTEGRAL PROTEINS
Penetrate the
Are often
hydrophobic core of the lipid bilayer
transmembrane proteins, completely spanning the membrane
Peripheral proteins:
Are
Some proteins:
are attached to __________, others attached to________
The hydrophobic regions of an integral protein consist of
are appendages loosely bound to the surface of the membrane
-cytoskeleton; fibers of the ECM
one or more stretches of nonpolar amino acids, often coiled into α helices
Cell-surface membranes can carry out several functions:
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
TRANSPORT
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.
ENZYMATIC ACTIVITY
A protein built into the membrane
may be an enzyme with its active site 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.
SIGNAL TRANSDUCTION
A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
CEL-CELL RECOGNITION
Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
INTERCELLULAR JOINING
Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions
ATTACHMENT TO THE CYTOSKELETON AND THE EXTRACELLULAR MATRIX
Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and
intracellular changes
____________ proteins are important in the medical field
For example, HIV must bind to the immune cell-surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell
HIV cannot enter the cells of resistant individuals who lack CCR5
Drugs are now being developed to mask the CCR5 protein
Cell-surface
Cells recognize each other by
Membrane carbohydrates may be covalently bonded to
Carbohydrates on the extracellular side of the plasma membrane vary
binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins)
among species, individuals, and even cell types in an individual
Proteins in the plasma membrane can ____________________, others are ____________
drift within the bilayer, inmobile due to their attachment to the cytoskeleton
Cell-cell recognition:
Is a cell’s ability to :
Membrane carbohydrates:
Interact with the ….
Use ______ and ________
distinguish one type of neighboring cell from another
surface molecules of other cells, facilitating cell-cell recognition
glycoproteins, glycolipids
Membranes have distinct
This affects the movement of proteins synthesized in
The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is
inside and outside faces
the endomembrane system
determined when the membrane is built by the ER and Golgi apparatus
Membrane proteins and lipids
Are synthesized in the ER and Golgi apparatus
A cell must exchange materials with its surroundings, a process controlled by the
plasma membrane
Hydrophobic molecules examples:
Are…
Hydrophilic molecules including ions and polar (like glucose) molecules do not…
Proteins built into the membrane play key roles in
(hydrocarbons, oxygen, carbon dioxide)
lipid soluble and can pass through the membrane rapidly
cross the membrane easily
regulating transport
Transport proteins allow
Some transport proteins, called ____________, have a:
Channel proteins called _________ greatly facilitate the:
They are composed of
Overall, up to
passage of hydrophilic substances across the membrane
channel proteins: hydrophilic channel that certain molecules or ions can use as a tunnel
aquaporins: passage of water molecules
four polypeptide subunits that each form a channel for the passage of water
3 billion water molecules pass through per second
Other transport proteins, called ____________, bind to
A transport protein is specific for the
carrier proteins: molecules and change shape to shuttle them across the membrane
substance it moves
Molecules transport by means of
passive (diffusion, osmosis, facilitated by channel proteins) and active transport (carrier proteins, exocytosis and endocytosis).
Passive transport is
diffusion of a substance across a membrane with no energy investment
Diffusion
Is
Each substances diffuse down its own concentration gradient, unaffected by the concentration of other substances.
It diffuses because of the
the movement of molecules from a region of higher concentration to a region of lower concentration.
difference in concentration of the substance from one area to another, from high to low concentration until it reaches equilibrium.
Diffusion of one solute.
The membrane has pores large enough for molecules
of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated. This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.
Diffusion of two solutes.
Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concentration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side.
Substances diffuse down their own concentration gradient, the region along which
No work must be done to move substances down the concentration gradient.
The diffusion of a substance across a biological membrane is
the density of a chemical substance increases or decreases.
passive transport because no energy is expended by the cell to make it happen.
Osmosis
Is
Free water molecules diffuse across
Water keeps moving until the solute concentration is equal on both sides
the movement of water (water molecules not clustered around another substance) across a selectively permeable membrane
a membrane from the region of lower solute concentration to the region of higher solute concentration
Tonicity
Is the ability of a
The tonicity of a solution depends on
Has a great impact on
solution to cause a cell to gain or lose water
its concentration of solutes that cannot cross the membrane relative to that inside the cell
cells without walls
If a solution is isotonic
The concentration of solutes is
There will be no
The volume of a cell without a cell wall is
the same as it is inside the cell
net movement of water
stable in an isotonic solution
If a solution is hypertonic
The concentration of solutes is
The cell will
Net diffusion of water is from
Cells without cell walls will
Cells without cell walls will ________ in hypertonic solution
greater than it is inside the cell
lose water
inside the cell to the surrounding solution
lose water, shrivel, and likely die in hypertonic solution
shrivel
If a solution is hypotonic
The concentration of solutes is
Net diffusion of water is from
The cell will
Cells without cell walls will _______ in a hypotonic solution
less than it is inside the cell
the surrounding solution to the inside of the cell
gain water
lyse (burst)
Cell walls:
Help maintain
If a plant cell is turgid: -It is in a _______ environment
-It is very firm, a healthy state in most plants
If a plant cell is flaccid
-It is in an ________________ environment
water balance
-hypotonic
-isotonic or hypertonic
Hypotonic or hypertonic environments cause problems for cells without walls because
Organisms that live in such environments require a method of
For example, Paramecium live in a hypotonic environment; they have a contractile vacuole to pump excess water out of the cell
they cannot tolerate excessive water loss or uptake
osmoregulation, control of solute concentration and water balance
Bacteria and archaea living in hypersaline (excessively salty) environments have mechanisms to ensure that water does not leave the cell
A plant cell in a hypotonic solution
If a plant cell and its surroundings are isotonic, there is
swells until the wall opposes uptake; the cell is now turgid (firm)
no net movement of water into the cell; the cell becomes flaccid (limp)
In a hypertonic environment,
What happens?
lethal effect called
plant cells lose water
The membrane pulls away from the cell wall, causing the plant to wilt, a potentially lethal effect called plasmolysis
Transport proteins:
Allow passage of
In facilitated diffusion (Passive Transport Aided by Proteins)
Transport proteins…
hydrophilic substances across the membrane
speed the movement of molecules across the plasma membrane
Channel proteins provide
Aquaporins facilitate
Ion channels facilitate
Some ion channels, called __________,
For example, in nerve cells, ion channels open in response to electrical stimulus
Other gated channels open in response to chemical stimulus—binding of a specific substance to the protein
corridors that allow a specific molecule or ion to cross the membrane
the diffusion of water
the transport of ions
gated channels, open or close in response to a stimulus
Carrier proteins undergo a
This can be triggered by
Carrier proteins involved in facilitated diffusion move
subtle shape change that moves the solute-binding site across the membrane
the binding and release of the transported molecule
substances down their concentration gradients; no energy input is required
Channel proteins
Provide corridors that allow a specific molecule or ion to cross the membrane
Active transport:
Moves substances
Requires
All proteins involved in active transport are
against their concentration gradient
energy, usually in the form of ATP hydrolysis, to move substances against their concentration gradients
carrier proteins
nada
mada
Active transport allows cells to
For example, an animal cell has a much higher potassium (K+) and a much lower sodium (Na+) concentration compared to its surroundings
This is controlled by the sodium-potassium pump, a transport protein that is energized by transfer of a phosphate group from the hydrolysis of ATP
maintain concentration gradients that differ from their surroundings
The sodium-potassium pump
buscar video or sumn
Membrane potential is
Voltage is created by
The inside of the cell is
the voltage across a membrane
differences in the distribution of positive and negative ions across a membrane
negative in charge relative to the outside, favoring passive transport of cations into and anions out of the cell
Two combined forces, collectively called the _________ _________, drive the :
the forces
-__________
-____________
An ion diffuses…
electrochemical gradient, drive the diffusion of ions across a membrane
A chemical force
An electrical force
down its electrochemical gradient
An electrogenic pump is a
The main electrogenic pump differs between
In animals,
The main electrogenic pump of plants, fungi, and bacteria is a _______ _______, which actively transports
transport protein that generates voltage across a membrane, storing energy that can be used for cellular work
plants and animals
it is the sodium-potassium pump
proton pump, transports hydrogen ions (H+) out of the cell
“electrogenic pump topic”
_______________ is the major electrogenic pump of animal cells
Example the H+ gradient generated in ETS in aerobic cellular respiration and the light reaction of photosynthesis, also the Na+ K+ pump.
Electrogenic pumps help
The sodium-potassium pump
store energy that can be used for cellular work
Cotransport
Occurs when
The diffusion of an actively transported solute down its concentration gradient is coupled with the
Cotransport:
active transport of a specific solute indirectly drives the active transport of another solute
transport of a second substance against its own concentration gradient
active transport driven by a concentration gradient
Plant cells use proton pumps to generate
A cotransporter couples the movement of
This is how plants
an H+ gradient across the cell membrane
H+ back down its concentration gradient to the active transport of sucrose into the cell
load sucrose into their veins for transport around the plant body
Animal cells use a
Sodium potassium pumps actively transport
Normally, sodium in waste is reabsorbed in __ ______ to:
similar cotransporter to couple the active transport of glucose to the diffusion of Na+ into cells lining the intestine
Na+ out of the cell to maintain the electrochemical gradient
the colon to maintain a constant level in the body
When a person has diarrhea,
A rapid drop in ________ can be life-threatening
Drinking a concentrated ______ and _______ solution enables :
waste is expelled too fast for reabsorption, causing sodium levels to drop
sodium
salt (N a C l) and glucose, enables uptake through the N a+/glucose transporters in the intestine
Bulk transport across the plasma membrane occurs by
Large proteins like ___________ and __________,
Cross:
They require :
exocytosis and endocytosis
polysaccharides and proteins, Cross the membrane in bulk via vesicles and by different mechanisms
energy
In endocytosis, the cell takes in
Endocytosis is a reversal of
There are three types of endocytosis:
macromolecules by forming vesicles from the plasma membrane
exocytosis, involving different proteins
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis
In Phagocytosis, a cell
The vacuole fuses with a _________ to :
engulfs a particle by extending pseudopodia around it and packing it in a membranous sac called a food vacuole
lysosome to digest the particle
In Pinocytosis, molecules are
Pinocytosis is nonspecific for ….
any and all solutes are
Parts of the plasma membrane that form vesicles are
taken up when extracellular fluid is “gulped” into tiny vesicles
the substances it transports
taken into the cell
lined on the inner side with coat proteins, forming coated vesicles
In receptor-mediated endocytosis, vesicle formation is
Receptor proteins bound to specific solutes from the extracellular fluid are
Emptied receptors are
triggered by solute binding to receptors
clustered in coated pits that form coated vesicles
recycled to the plasma membrane by the same vesicle
Human cells use receptor-mediated endocytosis to:
, which is carried in particles called:
Individuals with familial hypercholesterolemia have missing or defective L D L receptor proteins
Cholesterol accumulates in the blood, building up lipids and narrowing the space in the blood vessels, resulting in potential heart damage or stroke
take in cholesterol
low-density lipoproteins (LDLs)
In exocytosis transport,
Many secretory cells use exocytosis to
For example, cells in the pancreas secrete insulin by exocytosis
vesicles migrate to the plasma membrane, fuse with it, and release their contents outside the cell
export their products
The Davson-Danielli sandwich model of membrane structure
Stated that the membrane was
model was supported by
made up of a phospholipid bilayer sandwiched between two protein layers
electron microscope pictures of membranes
