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Membrane Structures and Functions - Coggle Diagram
Membrane Structures and Functions
Cellular Membrane are Fluid Mosaics
Lipids and proteins are the main components of membrane, but carbohydrates are also important
The Fluidity of Membranes
the membranes are held together by weak hydrophobic interactions. Lipids may move across the membrane from one phospholipids layer to other
when the temperatures are cool, membranes switch from a
fluid
state to a solid
state
. The temperature depends on the types of lipids. Membranes that are
rich in unsaturated fatty acids
are
more fluid
than those rich in saturated fatty acids.
Membranes must be fluid to work properly
!!!!!
Cholesterol is a membrane component in animal cells that has variable effects on membrane fluidity at different temperatures.
-Warm Temperatures: cholesterol restrain movement of phospholipids
-Cooler Temperatures: maintains fluidity by preventing tight packing
Differences in Membrane Lipids Composition
Lipids composition of the cell membrane appears to be adapted to environmental conditions in many species:
EX
: cell membranes have high proportion of unsaturated hydrocarbon tails in fish that live in extreme cold
Organisms living in variable temperature conditions are able to change lipids composition in response to changing temperature:
EX
: in winter wheat, the percentage of unsaturated phospholipids increases in autumn to prevent membrane solidification during winter
Amphipathic
= molecules containing
hydrophobic
(water-fearing) AND
hydrophilic
(water-loving)
Hydrophilic
regions of the protein are oriented towards the cytosol and extracellular fluid inside and outside the membrane
Hydrophobic
regions are embedded in the bilayer
membranes are composed of mainly phospholipids. Phospholipids form a bilayer with hydrophobic tails inside the membrane, and hydrophilic heads exposed to water on either side
Fluid Mosaic Model=
membrane structure depicts the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. Proteins are often form groups that carry out common functions
Membrane in Selective Permeability
The plasma membrane controls the exchange of materials between the cell and its surroundings.
Selective Permeability=
some substance cross more easily than others
The fluid mosaic model explains how membranes regulate molecular traffic across the membrane
Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules dissolve in the lipids bilayer and pass through the membrane rapidly.
EX
: Hydrocarbons, CO2 and O2 pass easily through the membrane
Hydrophobic interior of the membrane impedes the passage of hydrophilic (polar) molecules.
EX: sugars, water, and ions pass through slowly
Transport Proteins
Hydrophilic substances cross membrane more quickly by passing through transport proteins.
Channel Proteins
have a hydrophilic channel that certain molecules or ions can use as a tunnel
Channel proteins called
Aquaporins
greatly increase the rate of passage of water molecules. They are composed of four polypeptides subunits that each form a channel for the passage of water. up to 3 billion water molecules pass through per second.
Carrier Proteins
bind to molecules and change shape to shuttle them across the membrane
Transport proteins move only specific substance.
Ex
: glucose carrier proteins only transport glucose; they will not transport fructose, a structural isomer of glucose
The selective permeability of a membrane is dependent on both the lipids bilayer and the specific transport proteins it contains
Active Transport
Membrane potential
is the voltage across a membrane. Voltage 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 anion out of the cell
Some transport proteins use energy to move solutes against their concentration gradient. Active transport requires energy usually in the form of ATP hydrolysis, to move substance against their concentration gradients
Electrochemical gradient= two combined forces, drives the diffusion of ions across a membrane.
-A chemical forces (the ion's concentration gradient)
-An electrical force (the effect of the membrane potential on the ion's movement)
Electrogenic
Pump is a transport protein that generates voltage across a membrane, storing energy that can be used for cellular work.
Cotransport
occurs when active transport of a solute indirectly drives transport of other substance
Bulk Transport
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, molecules are taken up when extracellular fluid is "gulped" into tiny vesicles
Small molecules
and water enter or leave the cell through the lipid's bilayer or via transport proteins.
Large molecules,
such as polysaccharides and proteins, cross the membrane in bulk inside vesicles
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
, 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
-Phagocytosis (cellular eating)
-Pinocytosis (cellular drinking)
-Receptor=mediated endocytosis
Receptor-mediated Endocytosis
, vesicle formation is triggered by solute binding to receptor. Receptor proteins bound to specific solutes from the extracellular fluid are clustered in coated pits that form coated vesicles
Human cells use receptor-mediated endocytosis to take in cholesterol, which is carried in particles called low-density lipoproteins (LDLs) Individuals with familial hypercholesterolemia have missing or defective LDL receptors proteins
Passive Transport
Tonicity
is the ability of a surrounding solution to cause a cell to gain or lose water. It depends on the concentration of solutes in the solution that cannot cross the membrane, relative to that inside the cell.
Diffusion
= the movement of particles of many substances so that they spread out evenly into the available space.
Each molecule move randomly, diffusion of a population of molecules may be directional
Dynamic equilibrium, as many molecules cross the membrane in one direction as on the other
Concentration Gradient
: the region along which the density is own concentration gradient unaffected by the concentration of other substance
The diffusion of substance across a biological membrane is
passive transport
because no energy is expended by the cell. Concentration gradient represents potential energy that drives diffusion. But the rate of diffusion also depends on membrane permeability to the specific substance
Osmosis
is the diffusion of free water (water molecules not clustered around another substance) across selectively permeable membrane.
Free water molecules diffuse across a membrane from the region of lower solute concentration to the region of higher solution concentration