The plasma membrane functions
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Transcript of The plasma membrane functions
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The plasma membrane functions
The functions of the plasma membrane include: Isolation Regulation of exchange with the environment Sensitivity to the environment Structural support
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Plasma Membrane Physical barrier - separates intracellular fluids from
extracellular fluids Helps in maintaining homeostasis Plays a dynamic role in cellular activity – selectively
permeable
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Fluid Mosaic Model
Double bilayer of phospholipids
Phospholipids have hydrophobic tails and hydrophilic heads
CH2
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CH3N+
OO O
–POCH2
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C O C OO O
PhosphategroupHydrophilic head
Hydrophobic tails
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The plasma membrane includes proteins
Integral proteins
Within the membrane
Peripheral proteins
Bound to inner or outer surface of the
membrane
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The plasma membrane includes proteins Anchoring proteins (stabilizers)
Attach to inside or outside structures Recognition proteins (identifiers)
Label cells as normal or abnormal Enzymes
Catalyze reactions Receptor proteins
Bind and respond to ligands (ions, hormones) Carrier proteins
Transport specific solutes through membrane Channels
Regulate water flow and solutes through membrane
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Structures on the plasma membrane surfaces
Microvilli, Cilia,Stereocilia
Specialized junctions
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Features of Apical Surface of Epithelium - Microvilli
Projections that increase surface area Folding of the plasma membrane
http://cellbio.utmb.edu/microanatomy/epithelia/epith_lec.htm
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Features of Apical Surface of Epithelium - Cilia
These structures are designed for motility. Epithelia that need to move substances across their
surface (like mucous in the air passages) have cilia. Each cilium or flagellum has a basal body located at its
base. Basal bodies anchor the cilia or flagella and are
thought to be responsible for their formation. They look like centrioles and are believed to be
derived from them
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Flagella: (ex) spermatoza
Extra long cilia Moves cell
http://www.lbl.gov/Science-Articles/Archive/sabl/2006/Jul/02.html
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Cell junctions – 3 groups
Tight junction designed to restrict the movement of material
between the cells they link Gap junction
create cytoplasmatic communication bridges between cells
Anchoring junction attach cells to one another or to extracellular matrix
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http://www.phschool.com/science/biology_place/biocoach/biomembrane2/junctions.html
Membrane Junctions
Tight junction
Anchoring junction
Gap junction
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Tight Junctions An intercellular junction between
cells in which the outer layers of the cell membranes fuse,
reducing the ability of larger molecules and water to pass between the cells.
Tight junctions prevent the free movement of molecules between cells in the intestine and allow the intestinal cell to control absorption
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Gap junctions Example – intercalated discs in the heart, electrical
synapses
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Cell transport mechanisms - How things enter and leave the cell
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2 groups of movement Passive transport – no energy is needed
Diffusion Carrier-mediated
Active transport – requires ATTP Pumps Vesicular transport
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Characteristics of selectivelypermeable membranes EXTRACELLULAR
FLUID
CYTOPLASM
Materials may crossthe plasma membrane
through active orpassive mechanisms.
Passive mechanismsdo not require ATP.
Active mechanismsrequire ATP.
Diffusion ismovement drivenby concentration
differences.
Carrier-mediatedtransport involves
carrier proteins, andthe movement maybe passive or active.
Vesicular transportinvolves theformation ofintracellular
vesicles; this is anactive process.
Plasmamembrane
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Passive transport All molecules in the body are in constant motion
regardless of the presence of a membrane (kinetic energy)
Motion stops only at absolute zero By international agreement, it is defined as 0K on the
Kelvin scale, −273.15°C on the Celsius scale and −459.67°F on the Fahrenheit scale
When a membrane is present the movement in a certain direction can be limited or changed
A molecule will move in a certain direction until collide with another molecule. When this happens, the direction of the movement will change
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Diffusion Depends on a concentration gradient. (What is a
concentration? A concentration gradient?) The driving force is kinetic energy Slow in air and water but important over small distances
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Factors Affecting Diffusion Distance (inversely related) Molecule size (inversely related) Temperature (directly related) Gradient size (directly related) Electrical forces
Attraction of opposite charges (+,–) Repulsion of like charges (+,+ or –,–)
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Diffusion
The movement of molecules will happen in ALL directions
What is usually important is the net rate of diffusion in a certain direction
The net movement will be from high to low concentration until equilibrium is reached
At equilibrium, the net movement is equal in all directions
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When a membrane is present Membrane can be:
Freely permeable (this does not apply to plasma membrane) – allows passage of all substances
Selectively permeable – permits passage of some materials and prevents passage of others
Impermeable – cells can be impermeable to specific substances, but no living cell has a completely impermeable membrane
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Permeability characteristics of membranes
Freely permeable membranes Selectively permeable membranes Impermeable membranes
Freely permeable membranesallow any substance to pass withoutdifficulty.
Selectively permeable membranes,such as plasma membranes, permit thepassage of some materials and preventthe passage of others.
Nothing can pass through impermeablemembranes. Cells may be impermeableto specific substances, but no living cellhas an impermeable membrane.
Protein Protein Protein
Lipids Lipids Lipids
Ions Ions Ions
— — —Water Water Water
Carbohydrates Carbohydrates Carbohydrates
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Selectively permeable membranes Selective based on:
1. Characteristics of material to pass Size Electrical charge Molecular shape Lipid solubility
2. Characteristics of membrane What lipids and proteins present How components are arranged
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Diffusion through cell membrane Diffusion is divided into 2 types: 1. Simple diffusion – the movement of particles through the
membrane with no assistance Nonpolar / lipid-soluble substances that diffuse directly
through the lipid bilayer Gases readily diffuse through lipid bilayer. (Ex.
movement of oxygen inside cells and CO2 outside)
Diffusion of water and other lipid-insoluble molecules happens via protein channels
The channels are highly selective as a result of the diameter, shape, charge and chemical bonds
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Diffusion of lipid-soluble materials
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Diffusion of lipid-insoluble materials
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Diffusion through cell membrane 2. facilitated diffusion - Assisted by carrier protein
Materials are bound to specific proteins and move through water-filled protein channels (big polar molecules; ex. – glucose)
The facilitated diffusion rate depends on the rate in which the carrier protein molecule can undergo changes that allow passage
Carrier Proteins Are integral transmembrane proteins Show specificity for certain polar molecules Their number will influence the amount that can be
transferred through the membrane
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Osmosis Osmosis is a simple diffusion of water.
It occurs through a selectively permeable membrane Occurs when the concentration of a water is different on
opposite sides of a membrane Membrane must be freely permeable to water, selectively
permeable to solutes
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Osmosis – osmolality, osmolarity and osmotic pressure
Osmolality (molecular weight) - One osmole is 1 gram molecular weight
Osmolarity (concentration) - One osmole in one liter Osmotic pressure – defined by the concentration of
solute particles in a solution Is defined by the number of particles, not their size
or nature Each particle in a solution, regardless of its
mass, exerts the same pressure against the membrane
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Effects of Solutions of Varying Tonicity
Tonicity – description of how the solution affects a cell Isotonic – solutions with the same solute
concentration as that of the cytosol Hypertonic – solutions having greater solute
concentration than that of the cytosol Hypotonic – solutions having lesser solute
concentration than that of the cytosol
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Passive Membrane Transport: Filtration
The passage of water and solutes through a membrane by hydrostatic pressure
Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area
Depending on the size of the membrane pores only solutes of a certain size may pass through it.
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Transport that uses ATP
A movement that can be against concentration gradient Uses ATP to move solutes across a membrane Two types:
Active transport - use of carrier proteins Vesicular transport
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Types of Active Transport 2 types according to the source of energy used for the
transport Primary active transport
The energy for the transport derived directly from a high energy molecule – ATP
The hydrolysis of ATP causes phosphorylation of a transport protein that in turn changes its shape.
That change “promotes” the passage of materials (ex. Sodium-potassium pump)
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Types of Active Transport
Secondary active transports – one ATP-powered pump can drive secondary transport of other solutes.
The energy is derived from the energy stored in creating the concentration gradient
This concentration difference was created by the primary active transport that used ATP
Secondary transport, like the primary, depends on carrier proteins, but without the need of energy
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Active transport Symport system – two substances are moved
across a membrane in the same direction Antiport system – two substances are moved
across a membrane in opposite directions (Na/K)
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Vesicular Transport Transport of large particles and macromolecules across plasma
membrane using vesicles and ATP Endocytosis – enables large particles and macromolecules to enter the
cell. Few types: Receptor-mediated endocytosis – selective process that depends
on the binding of extracellular material to a specific receptor This binding initiates the endocytosis
Phagocytosis – “cell eating”; endocytosis of solid objects pseudopods engulf solids and bring them into the cell’s interior Happens in specialized cells Pinocytosis – “cell drinking”; endocytosis of liquids.
This is not a selective process and does not involve receptor
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Vesicular Transport Exocytosis – moves
substance from the cell interior to the extracellular space
Transcytosis – moving substances into, across, and then out of a cell
Vesicular trafficking – moving substances from one area in the cell to another
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Passive Membrane Transport – Review
Process Energy Source Example
Simple diffusion Kinetic energy Movement of O2 through membrane
Facilitated diffusion Kinetic energy Movement of glucose into
cells
Osmosis Kinetic energy Movement of H2O in & out of cells
Filtration Hydrostatic pressure Formation of kidney filtrate
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Active Membrane Transport – ReviewProcess Energy Source Example
Active transport of solutes ATP Movement of ions across membranes
Exocytosis ATP Neurotransmitter secretion
Endocytosis ATP White blood cell phagocytosis