Membrane structure and function. Key questions What is the structure and function of cellular...
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Transcript of Membrane structure and function. Key questions What is the structure and function of cellular...
Key questions
• What is the structure and function of cellular membranes?
• How do different entities cross the plasma membrane?
The big picture
Cell membrane function:•Physically separate a cell from its environment•Control what goes in and out of the cell
Revisiting the lipid bilayerCell membrane structure:• When placed in water,
phospholipids spontaneously form into two layers because of the amphipathic nature of the molecule– Hydrophilic heads face the
outside of the membrane– Hydrophobic tails are in
the interior
• This is the framework of all membranes
Fluid mosaic model
Fluid because most of the components of the cell membrane move laterally within the membrane
Fluid mosaic model
Mosaic because the membrane contains a mixture of substances including:• proteins •cholesterol
Fluid because most of the components of the cell membrane move laterally within the membrane
Membrane proteins and their functions
There are two types of membrane proteins•Integral-penetrate the hydrophobic core of the lipid bi-layer•Some span the entire membrane
Peripheral•Not embedded in the bilayer•Peripheral appendages
Hydrophobic region
Hydrophilic region
Functions:
•Transporters •Can act as enzymes (peripheral)•Signal transduction •Recognition proteins •Fasten cells to adjacent cells •Attachment
Membrane proteins and their functions
(a) Transport
ATP
(b) Enzymatic activity
Enzymes
(c) Signal transduction
Signaling molecule
Receptor
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to the cytoskeleton and extracellular matrix (ECM)
Membrane permeabilityThe plasma membrane is selectively permeable
What controls the flow of substances that can pass freely across the membrane?
Concentration gradient occurs when there is a different in concentration between two adjacent regions
Region 1 Region 2
What controls the flow of substances that can pass freely across the membrane?
Concentration gradient occurs when there is a different in concentration between two adjacent regions
Region 1 Region 2
What controls the flow of substances that can pass freely across the membrane?
Substances will move down their concentration gradients (from an area of high to low concentration)
This movement is called diffusion
What controls the flow of substances that can pass freely across the membrane?Diffusion-the movement of molecules or ions down a concentration gradient
What controls the flow of substances that can pass freely across the membrane?Diffusion-the movement of molecules or ions down a concentration gradient
Why does diffusion occur?
What controls the flow of substances that can pass freely across the membrane?
Diffusion occurs because:•Molecules are always in motion•Therefore they collide•They will collide more if they are crowded•These collisions tend to spread them out
Diffusion of a substance across a membrane does not require energy….. It is considered passive transport.
What controls the flow of substances that can pass freely across the membrane?
How quickly a substances diffuses depends on several factors:•Size•Temperature•Steepness of the concentration gradient•Charge•Pressure
Some things can pass freely across the membrane while others cannot
How do molecules and ions that cannot freely cross the bilayer on their own enter or exit a cell?
Some things can pass freely across the membrane while others cannot
How do molecules and ions that cannot freely cross the bilayer on their own enter or exit a cell?
•With the help of transport proteins•By exocytosis and endocytosis
Transport protein•Selective permeability depends on membrane structure AND presence of transport proteins •Each type of transport protein moves a certain molecule across a membrane•There are several types of transport proteins
•Channel proteins (hydrophilic channel)•Ion channels allow ions to cross the membrane (many are gated)•Carrier proteins bind with passenger molecules, change shape and then let the passengers out on the other side
How do molecules and ions that cannot freely cross the bilayer on their own enter
and exit a cell?
There are two kinds of transport1.) Facilitated diffusion
•A concentration gradient drives the diffusion of a substance across a membrane with the assistance of a membrane protein. •No energy is required.
•Ie. glucose
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
How do molecules and ions that cannot freely cross the bilayer on their own enter
and exit a cell?
There are two kinds of transport1.) Facilitated diffusion
•A concentration gradient drives the diffusion of a substance across a membrane with the assistance of a membrane protein. •No energy is required.
•Ie. glucose2.) Active transport
•A membrane protein assists in the transportation of a solute against its concentration gradient•Requires energy•Allows cells to maintain different internal conditions
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
How do molecules and ions that cannot freely cross the bilayer on their own enter
and exit a cell?
More on active transport:•Electrogenic pumps create a membrane potential (electrical potential energy due to a separation of opposite charges).•Two forces drive the passage of ions across a membrane: concentration gradient and charge (electrochemical gradient)
How do molecules and ions that cannot freely cross the bilayer on their own enter
and exit a cell?
EXTRACELLULARFLUID
H+
H+
H+
Proton pump
+
+ H+
H+
+
+
H+
– –
– –
ATP
CYTOPLASM –
How do molecules and ions that cannot freely cross the bilayer on their own enter
and exit a cell?More on active transport:CotransportAn ATP-powered pump that transports a specific solute can indirectly drive the active transport of other solute
Proton pump
–
–
–
–
–
+
+
+
+
+
ATP
H+
H+
H+
H+
H+
H+
H+
H+
Diffusionof H+
Sucrose-H+
cotransporter
Sucrose
Review of passive and active transport
Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
Exocytosis•A vesicle moves to the cell’s surface•Its membrane fuses with the plasma membrane•Contents of the vesicle are released outside of the cell
Bulk transport
Endocytosis•Substances near the cell’s surface are taken into the cell•A small section of plasma membrane balloons inward and then pinches off•Contents are now vesicles which are delivered to their destination within the cell
Bulk transport
Endocytosis•Substances near the cell’s surface are taken into the cell•A small section of plasma membrane balloons inward and then pinches off•Contents are now vesicles which are delivered to their destination within the cell
Both endo and exocytosis involve motor proteins and require energy
Bulk transport