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Ch.7 : Membrane Structure & Function - Coggle Diagram
Ch.7 : Membrane Structure & Function
Passive transport
Passive transport
is diffusion of
a substance across a membrane with no energy
investment
Diffusion
Substances diffuse down their
concentration
gradient
, the region along which the density of a
chemical substance increases or decreases
The concentration gradient represents potential
energy that drives diffusion
Rate of diffusion also depends on
membrane permeability to the specific substance
Diffusion
is the movement of particles of any
substance so that they spread out evenly into the
available space
Effects of Osmosis on Water Balance
Osmosis
is the diffusion of free water
across a selectively permeable membrane
Free water molecules diffuse across a membrane
from the region of lower solute concentration to the
region of higher solute concentration
Water always follows the higher concentration of solute.
Water Balance of Cells with Cell Walls
Plants, prokaryotes, fungi and some
protists have cell walls
Hypotonic solution
A plant cell in a hypotonic solution takes up water
and swells until the inelastic wall exerts back a
pressure on the cell, called turgor pressure
the cell is
turgid (very firm)
, the
healthy state for most plant cells
Water Balance of Cells with Cell Walls
Isotonic solution
Plant cells become
flaccid (limp)
plant wilts
Hypertonic solution
Plant cells lose water
The cell shrivels and the membrane pulls away
from the cell wall in multiple locations, a
phenomenon called
plasmolysis
Plant will wilt and may die
Walled cells of bacteria and fungi also plasmolyze in hypertonic environments
Water Balance of Cells Without Cell Walls
Bacteria and archaea living in hypersaline
(excessively salty) environments have mechanisms
to ensure that water does not leave the cell
Organisms that live in such environments require a
method of
osmoregulation
, control of solute
concentration and water balance
Ex: Paramecium live in a hypotonic
environment; they have a contractile vacuole to
pump excess water out of the cell
Hypotonic
Cells without cell walls will gain water, swell and
lyse (burst)
A solution is
hypotonic
if the solute concentration
is less than that inside the cell
Hypertonic
Cells will lose water,
shrivel (crenate)
, and
likely die in hypertonic solution
A solution is
hypertonic
if the solute concentration
is greater than that inside the cell
Isotonic
Animal cells prefer isotonic environments
Water diffuses across the membrane at the same
rate in both directions; there is no net movement of
water across the membrane
A solution is
isotonic
if its solute concentration is
the same as that inside the cell
Tonicity
is the ability of a surrounding solution to
cause a cell to gain or lose water
Facilitated Diffusion
Passive Transport Aided
by Proteins
In
facilitated diffusio
n, transport proteins speed
the passive movement of molecules across the
plasma membrane
Transport proteins include channel proteins and
carrier proteins
Channel proteins
provide corridors that allow a
specific molecule or ion to cross the membrane
Aquaporins facilitate the diffusion of water
Ion channels
facilitate the transport of ions
Some ion channels, called
gated channel
s, open
or close in response to a stimulus
Ex: in nerve cells, potassium 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
Carrier proteins
move substances down their concentration
gradients; no energy input is required
Membranes exhibit Selective Permeability
Membranes exhibit
selective permeability
; some
substances cross more easily than others
The fluid mosaic model explains how membranes
regulate molecular traffic across the membrane
The Permeability of the Lipid Bilayer
What passes through
Hydrophobic (nonpolar) molecules dissolve in the
lipid bilayer and pass through the membrane
rapidly
Ex: hydrocarbons, CO2 and O2 pass easily
through the membrane
Small, non polar, & uncharged
What struggles/ cannot go through
The hydrophobic interior of the membrane impedes
the passage of hydrophilic (polar) molecules
Ex: sugars, water and ions pass through
slowly, if at all
Transport Proteins
Hydrophilic substances cross membranes more
quickly by passing through
transport proteins
Type of Transport Proteins
Channel proteins
have a hydrophilic channel that
certain molecules or ions can use as a tunnel
Carrier proteins,
bind to molecules and change
shape to shuttle them across the membrane
Channel proteins called
aquaporins
greatly
increase the rate of passage of water molecules
composed of four polypeptide subunits
that each form a channel for the passage of water
up to 3 billion water molecules pass
through per second
Transport proteins move only specific substances
Ex: glucose carrier proteins only transport
glucose; they will not transport fructose, a structural
isomer of glucose
Cellular Membranes are Fluid Mosaics
Fluidity of Membranes
Held together mainly by weak
hydrophobic interactions
Membranes rich in unsaturated fatty acids are more
fluid than those rich in saturated fatty acids
Must be fluid to work properly
Factors that affect membrane fluidity
Unsaturated Hydrocarbon tails
kinks in the tails where double bonds are located
prevent packing
enhanced fluidity
Saturated Hydrocarbon Tails
tails pack together
increased membrane viscosity
Cholesterol
in the animal cell membrane
At warm temperatures (such as 37ºC), cholesterol
restrains movement of phospholipids
At cool temperatures, it maintains fluidity by
preventing tight packing
Plants use different but related steroid lipids to
buffer membrane fluidity
As temperatures cool, membranes switch from a
fluid state to a solid state
The temperature at which a membrane solidifies
depends on the types of lipids
too much fluidity cannot support
protein function
Evolution of Differences in Membrane Lipid Composition
lipid composition of the cell membrane
appears to be adapted to environmental conditions
in many species
Ex: cell membranes have a high proportion
of unsaturated hydrocarbon tails in fish that live in
extreme cold
Ex: At the other extreme, some bacteria and archaea thrive at temperatures greater than in thermal hot springs and geysers. Their membranes include unusual lipids that may prevent excessive fluidity at such high temperatures
Organisms living in variable temperature conditions
are able to change lipid composition in response to
changing temperature
Ex: in winter wheat, the percentage of
unsaturated phospholipids increases in autumn to
prevent membrane solidification during winter
The
fluid mosaic model
of membrane structure
depicts the membrane as a mosaic of protein
molecules bobbing in a fluid bilayer of
phospholipids
Think Lava Lamp/ Tile Mosaic
Membrane Proteins and Their Functions
Phospholipids form the main fabric of the
membrane, but proteins determine most of the
membrane’s functions
Two major types of membrane proteins
Peripheral proteins
are bound to the surface of the membrane
Integral proteins
penetrate the hydrophobic core
Transmembrane proteins
are integral proteins
that span the membrane
Cell-surface membrane proteins
can carry out
several functions
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
The Role of Membrane Carbohydrates in Cell-
Cell Recognition
Cells recognize each other by binding to molecules
on the surface of the membrane
Many of these surface molecules are bonded to
short, branched chains of carbohydrates
Glycolipids
are carbohydrates bonded to lipids
Glycoproteins
are carbohydrates bonded to proteins
Components of
membranes
Lipids and proteins are the main components
carbohydrates are also important
Membranes are composed mainly of phospholipids
Phospholipids
are amphipathic molecules,
containing hydrophobic (“water-fearing”) and
hydrophilic (“water-loving”) regions
Phospholipids form a bilayer with hydrophobic tails
inside the membrane, and hydrophilic heads
exposed to water on either side
Bulk Transport
Large molecules, such as polysaccharides and
proteins, cross the membrane in bulk inside
vesicles
Bulk transport across the
plasma membrane occurs by exocytosis and
endocytosis
Exocytosis
In
exocytosis
, transport vesicles migrate to the
membrane, fuse with it, and release their contents
outside the cell
Many secretory cells use exocytosis to export
their products
Ex: cells in the pancreas secrete insulin by
exocytosis
Endocytosis
In
endocytosis
, macromolecules are taken into the
cell in vesicles
The membrane forms a pocket that deepens and
pinches off forming a vesicle around the material
for transport
Three types of endocytosis
Phagocytosis (“cellular eating”
)
In phagocytosis, a cell engulfs a particle by
extending pseudopodia around it and packing it in a
membranous sac called a food vacuole
The vacuole fuses with a lysosome to digest the
particle
Pinocytosis (“cellular drinking”)
In pinocytosis, molecules are taken up when
extracellular fluid is “gulped” into tiny vesicles
is nonspecific for the substances it
transports; any and all solutes are taken into the
cell
Parts of the plasma membrane that form vesicles
are lined on the inner side with coat proteins,
forming coated vesicles
Receptor-mediated endocytosis
In receptor-mediated endocytosis, vesicle
formation is triggered by solute binding to receptors
Receptor proteins bound to specific solutes from
the extracellular fluid are clustered in coated pits
that form coated vesicles
Emptied receptors are recycled to the plasma
membrane by the same vesicle
EX: Human cells use receptor-mediated endocytosis to
take in cholesterol, which is carried in particles
called low-density lipoproteins (LDLs)
Active Transport
Active transport
requires energy, usually in the
form of
ATP
hydrolysis, to move substances against
their concentration gradients
Enables cells to maintain solute
concentrations that differ from the environment
the concentration of potassium ions
(K+) is higher and the concentration of sodium ions
(Na+) is lower inside animal cells than their
surroundings
Transfer of a phosphate group from ATP to the
sodium-potassium pump energizes the transport
of K+ into the cell and Na+ out of the cell
All proteins involved in active transport are carrier
proteins
Ion Pumps Maintain Membrane Potential
Membrane potential
is the voltage across a
membrane
is created by differences in the distribution
of positive and negative ions across a membrane
The inside of the cell is 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
electrochemical gradient
, drive the diffusion of
ions across a membrane
A chemical force (the ion’s concentration gradient)
An electrical force (the effect of the membrane
potential on the ion’s movement)
An ion diffuses down its electrochemical gradient
Electrogenic Pump
An
electrogenic pump
is a transport protein that
generates voltage across a membrane, storing
energy that can be used for cellular work
Main Types
In animals, it is the sodium-potassium pump
In plants, fungi, and bacteria, it is the
proton pump,
which actively transports hydrogen ions (H+) out of
the cell
Cotransport
Coupled Transport by a Membrane
Protein
Cotransport
occurs when active transport of a
solute indirectly drives transport of other
substances
The “downhill” diffusion of solute is coupled to the
“uphill” transport of a second substance against its
own concentration gradient
Ex: Cotransport result in plants loading sucrose into their veins for
transport around the plant body
Ex: In humans
When a person has diarrhea, waste is expelled too
fast for reabsorption, causing sodium levels to drop
Drinking a concentrated salt (NaCl) and glucose
solution enables uptake through the Na+/glucose
transporters in the intestine