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Membrane Structure and Function
https://www.youtube.com/watch?v=ePnbkNVdPio
Objective: Build a 3-D model of the plasma membrane and use it to describe:1.The fluid mosaic model of the plasma membrane2.The components of cell membranes
Membrane StructureMembrane Structure & Function & Function
The selectively permeable plasma membrane acts as traffic control for the cell, allowing only certain things in at any given time
Today’s Objective Describe the structure and function of the
plasma membrane Explain why we say it is a fluid mosaic Identify the primary roles of membrane proteins Explain why we say the membrane is selectively
permeable
What is the membrane made of?
Cell Membranes are Fluid Cell Membranes are Fluid Mosaics of Lipids & ProteinsMosaics of Lipids & Proteins
*Phospholipids are amphipathic-both hydrophobic & hydrophilic
Bilayer of phospholipids
Cholesterol (a type of steroid – a lipid) helps with membrane fluidity
Membrane Proteins Integral proteins –
span the entire width of the phospholipid bilayer
Peripheral proteins – loosely bound to the surface of the membrane
Membrane Structure Results Membrane Structure Results in Selective Permeabilityin Selective Permeability
The fluid mosaic model explains how membranes regulate cellular traffic--it explains how form fits function
Fibers ofextracellularmatrix
Cytoskeleton Cytoplasm
Attachment tocytoskeleton andextracellularmatrix
Cell signaling
Enzymatic activity
Transport
Intercellularjoining Cell-cell
recognition
Cytoplasm
So…
How is the plasma membrane selectively
permeable?
The Permeability of the Lipid Bilayer
Small nonpolar (hydrophobic) molecules can pass freely across the membrane
http://www.sumanasinc.com/webcontent/animations/content/diffusion.html
Larger or polar substances need the help of transport proteins
Transport ProteinsTransport proteins are just as selective as the lipid bilayer.
Two types of transport proteins:1. Channel Proteins- a hydrophilic tunnel through which hydrophilic molecules & ions can pass through
2. Carrier Proteins- Hold onto molecules and change shape in a way that shuttles them across the membrane
What kind of molecules will the transport proteins help moved across the membrane? Why?
So…
• Explain why we say the membrane is selectively permeable
The varied roles of membrane proteins
Various Membrane Protein Functions
Cell-to-cell recognition Intercellular junctions (gap
junctions, tight junctions) Receptor for a chemical
signal
Transport Enzymes Attachment to the cytoskeleton
Carbohydrates are important in cell-to-cell recognition
Glycolipids – polysaccharide bonded to lipid Glycoproteins – polysaccharide bonded to protein
So…
• Identify the primary roles of membrane proteins
Putting it all together Read the case study on cystic fibrosis and
answer the accompanying questions.
Membrane TransportPassive Transport
Passive Transport is Diffusion of a Substance Across a Membrane With No Energy Requirement
Molecules (above 0 K) are always in motion
Diffusion is the tendency for molecules to move down their concentration gradient until a dynamic equilibrium is reached.
Osmosis
Osmosis is the diffusion of water molecules from areas of high (free) water concentration to areas of lower concentration
Quick Think Oxygen & carbon dioxide can cross
the lipid bilayer without help from membrane proteins. What properties allow this to occur?
Water sometimes needs the help of a transport protein (aquaporins) to cross the membrane. Why?
= solute
Water Balance of Cells Water Balance of Cells WithoutWithout Walls e.g. Walls e.g. There are 3 states of tonicity when comparing two solutions:
1. Isotonic: No net movement of water across the cell membrane
Iso= equal
Animal CellsAnimal Cells
=solute
Water Balance of Cells Water Balance of Cells WithoutWithout Walls e.g. Animal CellsWalls e.g. Animal Cells
There are 3 states of tonicity when comparing two solutions:2. Hypertonic: A solution that is relatively hypertonic contains more non-penetrating solutes than the solution that it is being compared to
Hyper = More
Water Balance of Cells Water Balance of Cells WithoutWithout Walls e.g. Animal CellsWalls e.g. Animal Cells
There are 3 states of tonicity when comparing two solutions:3. Hypotonic: A solution that is relatively hypotonic contains fewer non-penetrating solutes than the solution that it is being compared to
Hypo = less
=solute
Quick Think Where will water move and why?1. A cell placed in a 10% salt solution (that’s very
salty).
2. A cell placed in distilled water.
3. A cell placed in an isotonic solution.
The inside of the cell is ________ compared to the outside of the cell because it has more/fewer/same solutes dissolved in solution so water will move _____________________
Water Balance of Cells Water Balance of Cells WithoutWithout Walls e.g. Animal CellsWalls e.g. Animal Cells
Osmoregulation- The control of water balance within a cell.Various methods of osmoregulation are present in organisms that are specially adapted to living in hypertonic or hypotonic environments:
Contractile Vacuole
Quick Think If a Paramecium were to swim from a hypotonic
environment to an isotonic one, would the activity of its contractile vacuole increase or decrease? Why?
Water Balance of Cells Water Balance of Cells WithWith Walls e.g. Walls e.g. Plant CellsPlant Cells
…Prokaryotes & Fungi Too!Hypotonic solutions, like rain, cause turgor (firmness) in plants because the cell wall pushes back against the intake of additional water after a certain point:
Turgor= nice, healthy plantWhen plant cells are isotonic with theirenvironment, they become flaccid
Water Balance of Cells Water Balance of Cells WithWith Walls e.g. PlantsWalls e.g. Plants
In hypertonic solutions, cells with walls experience plasmolysis- they lose water to their environment until the cell membrane shrivels & pulls away from the cell wall. The result is a wilted & possibly dead plant (or other organism)
= solute
Facilitated Diffusion: Passive Facilitated Diffusion: Passive Transport Aided by ProteinsTransport Aided by Proteins
Facilitated diffusion is the use of transport proteins, like channel & carrier proteins, to speed up the diffusion of molecules across a membrane.
A specific type of channel proteins are ion channels which are often gated channels-the presence of a stimulus causes them to open/close
Example- the presence of a certain neuro-transmitter, like acetylcholine, would cause the gated channels of a nerve cell to open and let Na+ into the cell
Quick ThinkList all the types of passive
transport you can think of.
Conclusion Read the case studies and answer the
corresponding questions
Membrane TransportActive Transport
Active Transport Uses Energy to Move Active Transport Uses Energy to Move Solutes Solutes AgainstAgainst Their Gradients Their Gradients
Passive Passive TransportTransport
Includes osmosis,diffusion, & facilitated
diffusion
Active Active TransportTransport
Molecules move down concentration gradient
Molecules are moved against concentration
gradientDoesn’t require energy Requires energy
No help, channel proteins &/or carrier
proteins
Carrier proteins
Active TransportActive TransportATP energy can
power active transport by
binding one of its phosphates to the transport protein
Maintenance of Membrane Maintenance of Membrane Potential by Ion PumpsPotential by Ion Pumps
All cell membranes have a charge across them due to a difference in ion concentration on either side of the membrane
This create This create membrane potentialmembrane potential - higher - higher negative negative ion ion concentration inside the cell relative to the outside of the cellconcentration inside the cell relative to the outside of the cell
This This electrochemical gradientelectrochemical gradient can help move ions into the cell can help move ions into the cell
Creating Membrane Potential- Creating Membrane Potential- Two ExamplesTwo Examples
Ion pump=Sodium-Potassium Pump
Proton Pump
Cotransport: Coupled Transport Cotransport: Coupled Transport by a Membrane Proteinby a Membrane Protein
Cotransport is when the action of one proton pump creates the electrochemical gradient to power another transport protein
The natural flow of H ions back
The natural flow of H ions back
in sorta in sorta ““suckssucks”” in the other
in the other
desired molecule
desired molecule
Bulk Transport Bulk Transport Across the Plasma Across the Plasma Membrane Occurs by Exocytosis & Membrane Occurs by Exocytosis &
EndocytosisEndocytosisExocytosis: Transport vesicles from golgi apparatus move to cell membrane. The vesicle membrane fuses with the cell Membrane, and the contents is expelled out of the cell
Endocytosis: A section of the plasma membrane sinks inward & pinches off to form a vesicle that transports materials into the cell; Three kinds:
Phagocytosis: “Cell eating”Pseudopodia of the cell membrane “reach out” & engulf particles. These particles are then digested when a vacuole fuses with a lysosomePinocytosis: “Cell drinking”Part of the cell membrane caves in and gulps extracellular fluid & the molecules dissolved in it
Receptor-Mediated Endocytosis:Allows for the bulk transport of specific extracellular particles. The vesicle forms only when certain ligands bind to receptor proteins on the cell membrane.
Quick Think The carbohydrates attached to some of
the proteins and lipids of the cell membrane are added as the membrane is made and refined in the ER and Golgi. The new membrane then forms transport vesicles that travel to the cell surface. On which side of the vesicle membrane are the carbohydrates?
http://www.wiley.com/college/pratt/0471393878/student/animations/membrane_transport/index.html
http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html
Websites to check out Websites to check out
Quick Write What is the function of the plasma membrane
and how does the structure of the plasma membrane help it with this job?