Membrane Structure and Function

Ch. 7

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

Cell Membranes are Fluid Cell Membranes are Fluid Mosaics of Lipids & ProteinsMosaics of Lipids & Proteins

*Phospholipids are amphipathic-both hydrophobic & hydrophilic

The structure & arrangement of the phospholipid bilayer is a result of the molecular properties of phospholipids

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

The Permeability of the Lipid Bilayer

Small nonpolar (hydrophobic) molecules can pass freely across the membrane

Polar molecules pass through slowly or not at all; they require the aid 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?

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.

Passive Transport is Diffusion of a Substance Across a Membrane With No Energy Requirement

O2 can diffuse into a cell from blood & CO2 can diffuse out and get carried back to lungs to be exhaled

Effects of Osmosis on Water Balance

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.

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 Think

List all the types of passive transport you can think of.

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

gradient

Doesn’t require expenditure of energy

Requires energy

No assistance, 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 voltage across them due to a separation of opposite charges.

The The membrane potentialmembrane potential is a result of a higher is a result of a 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

The The electrochemical gradientelectrochemical gradient can cause positive ions to can cause positive ions to diffuse into the cell diffuse into the cell

Creating Membrane Potential- Creating Membrane Potential- Two ExamplesTwo Examples

Ion pump=Sodium-Potassium Pump

Electrogenic 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 “sucks” in the other

in sorta “sucks” in the other

desired molecule

desired molecule

Bulk Transport Across the Plasma Bulk Transport 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 lysosome

Pinocytosis: “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?