Transcript Slide 1
Cell Membrane/Plasma Membrane
• plasma membrane is the boundary that separates the living cell from its surroundings • • functions: • 1.
integrity of the cell
– size and shape • 2.
controls transport
= “selectively permeable” • 3.
excludes
unwanted materials from entering the cell • forms a physical barrier with the external environment • allows the cell to create different environments outside and inside • 4.
maintains
the ionic concentration of the cell & osmotic pressure of the cytosol • allows for the creation of gradients – electrical and chemical • 5.
forms contacts
with neighboring cells = tissue • 6.
sensitivity
first part of cell that is affected by changes in the extracellular environment
Fibers of extra cellular matrix (ECM) Glyco protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Cholesterol Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE
• • • •
Cell Membrane/Plasma Membrane
comprised of a phospholipid bilayer phospholipids are the most abundant lipid in the plasma membrane – about 75% of lipids
Hydrophilic head Hydrophobic tail phospholipid:
– – glycerol + 2 fatty acids addition of a phosphate group phospholipids are
amphipathic molecules
– containing hydrophobic and hydrophilic regions – – – hydrophobic fatty acid “tails” hydrophilic phosphate “head group” makes the phospholipid both non-polar and polar
WATER WATER
http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf
• • • • •
Plasma Membrane Function – Cell Integrity & Size
the amount of plasma membrane determines the size of the cell metabolic requirements of the cell set upper limits on its size the surface area to volume ratio of a cell is critical as the surface area increases by a factor of n 2 , the volume increases by a factor of n 3
small cells have a greater surface area relative to volume
–
i.e. 6 vs. 1.2
1 5 Surface area increases while total volume remains constant 1 Total surface area [sum of the surface areas of all box sides
number of boxes] SA = height x width 6 cm 2 150 cm 2 25x bigger 750 cm 2 125x bigger Total volume [height
width
length number of boxes] 1 cm 3 125 cm 3 125x larger 125 cm 3 125x larger Surface-to-volume (S-to-V) ratio [surface area
volume] 6 1.2
6
Tissues
Plasma Membrane Models: Scientific Inquiry
• • • scientists began building models of plasma membranes before they were seen with electron microscopes RBC membranes were chemically analyzed in 1915 – found to be made of proteins and lipids 10 yrs later – hypothesis that membranes were composed of phospholipid bilayers – with the hydrophobic tails facing inward away from the aqueous environment • BUT – where were the proteins located in the membrane?
Plasma Membrane Models: Scientific Inquiry
• • • HYPOTHESIS: membrane is a phospholipid bilayer “sandwiched” between two layers of proteins PROBLEM: membrane proteins are not very water soluble because they are amphipathic (just like phospholipids) – made up of hydrophilic and hydrophobic regions 1972 – Singer and Nicolson proposed the model seen below
Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein
• • • freeze-fracturing microtome splits the frozen cell along the hydrophobic interior SEM used to image the cell layers
TECHNIQUE Extracellular layer Proteins Knife Plasma membrane RESULTS Cytoplasmic layer Inside of extracellular layer Inside of cytoplasmic layer
• • • • •
Plasma Membrane Fluidity
plasma membrane also contains embedded proteins many lipids and proteins are also modified through the addition of carbohydrates –
glycoproteins
–
glycolipids
these proteins must be able to change shape to function some of them even laterally diffuse through the PM so the membrane must be fluid
• •
Plasma Membrane Fluidity
fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it membrane fluidity is due to several factors – temperature - lipids move around more with increased temp – lipid packing – lipids with shorter fatty acid tails are less stiff – saturation of fatty acids – more C=C bonds (more unsaturated) increase fluidity – cholesterol – decreases fluidity at warmer temps; increases fluidity at lower temps
Lateral movement occurs
10 7 times per second.
Flip-flopping across the membrane is rare (
once per month).
Scientific Inquiry: Do membrane proteins move?
• • Frye and Eddin – fluorescently labelled membrane proteins of mouse and human cells and fused the cells production of “hybrid” cells with a homogenous mixture of proteins proved that proteins do move within the plasma membrane
RESULTS Membrane proteins Mouse cell Human cell Mixed proteins after 1 hour Hybrid cell
Evolution of Differences in Membrane Lipid Composition
• • Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary © 2011 Pearson Education, Inc.
Membrane Proteins and Their Functions
• • • membrane proteins determine most of the membrane’s specific functions proteins link on the extracellular side to an extracellular matrix of proteins – support the cells within a tissue proteins link on the cytoplasmic side to the cytoskeleton – via adaptor proteins
Fibers of extra cellular matrix (ECM) Glyco protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Cholesterol Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE
Membrane Proteins and Their Functions
• • • two kinds of membrane proteins:
1. Extrinsic or Peripheral:
bound to the surface of the membrane – function mainly as enzymes 2. Intrinsic or Integral: penetrate the hydrophobic core – those that span the membrane are called transmembrane proteins
N-terminus
helix C-terminus EXTRACELLULAR SIDE CYTOPLASMIC SIDE
•
6 major functions of integral membrane proteins:
– 1. transport – channel proteins & transporters –
2. enzymatic activity
– 3. signal transduction – receptor proteins – 4. cell-cell recognition – cell identity markers – 5. intercellular joining - linkers – 6. attachment- to the cytoskeleton and extracellular matrix (ECM)
Enzymes Enzymatic activity Glyco protein Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)
The Role of Membrane Carbohydrates in Cell-Cell
• • • •
Recognition
• cell surface carbohydrates – found on the extracellular surface of the plasma membrane these carbohydrates are covalently bonded to lipids (glycolipids) or to proteins (glycoproteins) on the external side of the PM - carbohydrates vary among species, individuals, and even cell types in an individual often used to identify a type of cell can be exploited by viruses – gain entry to a host cell
HIV Receptor (CD4) Co-receptor (CCR5) HIV can infect a cell that has CCR5 on its surface, as in most people.
Receptor (CD4) but no CCR5 HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals.
Plasma membrane
Synthesis and Sidedness of Membranes
• • membranes have distinct inside and outside faces asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is
determined when the membrane is built by the ER and Golgi apparatus ER ER lumen Transmembrane glycoproteins Secretory protein Golgi apparatus Vesicle Glycolipid Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Secreted protein Membrane glycolipid
Membrane structure results in selective permeability
• • a cell must
exchange
materials with its surroundings –
controlled by the plasma membrane
plasma membranes are
selectively permeable
• permeability = property that determines the effectiveness of the PM as a barrier • • • permeability varies depending on the organization and characterization of the membrane lipids and proteins hydrophobic (nonpolar) molecules dissolve in the lipid bilayer and pass through the membrane rapidly polar molecules do not cross the membrane easily – require transport mechanisms – provided by transport proteins
Transport Proteins
• • • • • transport proteins allow passage of hydrophilic substances across the membrane – specific for the substance it moves numerous types:
1. channel proteins
membrane – have a hydrophilic channel that certain molecules or ions can use as a tunnel to move across the
2. carrier proteins
- bind to molecules and change shape to shuttle them across the membrane
3. pumps
– carrier proteins that require the hydrolysis of ATP (or GTP) to move substances
What determines the direction of transport??
• • •
two basic things 1. concentration of what is being moved 2. available energy
Membrane Permeability and Transport
•transport across the membrane may be:
passive or active
passive transport active transport
diffusion osmosis facilitated primary AT secondary AT endocytosis exocytosis
http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html
A. Diffusion = movement of materials from [high] to [low] -random movement, no energy needs to be synthesized -the movement is driven by the inherent kinetic energy of the particles moving
down their concentration gradient
-
three ways to diffuse:
1. through the lipid bilayer: lipid soluble (non-polar), alcohol, gases, ammonia, fat-soluble vitamins 2. through a channel: charged ions or small polar molecules -some channels are “gated” – open and close 3. facilitated diffusion: larger, polar molecules too big for channels -uses a transporter protein
B. Osmosis
= diffusion of water from [high] to [low] OR
movement of water from [low solute] to [high solute]
-in osmosis – the membrane is permeable to water and NOT to the solutes -BUT it is the concentration of solutes that
causes the water to move
-the solutes are surround by a hydration shell of water molecules
Lower concentration of solute (sugar) Higher concentration of solute Sugar molecule H 2 O Selectively permeable membrane
-this decreases the amount of free water molecules available to move -so increased solute concentration decreases the concentration of free water molecules -water movement is affected by this drop in free water molecule concentration
Same concentration of solute Osmosis
B. Osmosis = diffusion of water from [high] to [low] OR
movement of water from [low solute] to [high solute]
-EXPERIMENT: U shaped tube divided by a membrane permeable to water only -increase the solute concentration in the right half of the tube -decrease free water concentration on the right side also
-water rises on the side of the solutes – due to osmosis
-counteract this rise in water by applying pressure = osmotic pressure (OP) -therefore increasing solute concentration increases osmotic pressure -water will move toward that area to decrease this OP
• • •
Water Balance of Cells Without Walls
How do cells behave in solutions?
must consider solute concentration and membrane permeability the two things combine to create = TONICITY • • • • Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water – degree to which a the concentration of a specific solute surrounding a cell causes water to enter or leave the cell Isotonic solution: Solute concentration is the same as that inside the cell = no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell = cell loses
water
Hypotonic solution: Solute concentration is less than that inside the cell = cell gains water
Water Balance of Cells Without Walls
(a) Animal cell Hypotonic solution H 2 O H 2 O Isotonic solution H 2 O Hypertonic solution H 2 O (b) Plant cell H 2 O Lysed Cell wall H 2 O Normal H 2 O Shriveled H 2 O Turgid (normal) Flaccid Plasmolyzed
• • •
Osmosis
Isotonic solution: Solute concentration is the same as that inside the cell = no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell =
cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell = cell
gains water
• • • • hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation = control of solute concentrations and water balance the protist Paramecium- hypertonic to its pond water environment – has a contractile vacuole that gets rid of the excess water it takes on due to osmosis in medicine – hypertonic IVs are given to induce the movement of water out of a tissue – make the blood plasma hypertonic – increased OP results – osmosis will drive water from the tissue into the plasma – low solute to high solute – used as a treatment for edema
• • •
Water Balance of Cells with Walls: Plants
cell walls also help maintain water balance plant cell in a hypotonic solution swells until the wall opposes uptake – the cell is said to be turgid (firm) if a plant cell and its surroundings are isotonic, there is no net movement of water into the cell – the cell becomes flaccid (limp) - the plant may wilt
(a) Animal cell Hypotonic solution H 2 O H 2 O Isotonic solution H 2 O Hypertonic solution H 2 O (b) Plant cell H 2 O Lysed Cell wall H 2 O Normal H 2 O Shriveled H 2 O
Video: Turgid
Elodea
Turgid (normal) Flaccid Plasmolyzed
• • • in plant cells in a hypertonic environment – cells lose
water
the membrane will pull away from the wall usually lethal effect called plasmolysis Video: Plasmolysis
Facilitated transport = movement of a molecule aided by a protein - movement of a solute from [high] to [low] either: 1. through a channel protein e.g aquaporins- for facilitated diffusion of water e.g. Ion channels that open or close in response to a stimulus (gated channels) 2. through a carrier protein
EXTRACELLULAR FLUID (a) A channel protein Channel protein CYTOPLASM Solute
Carrier proteins and Facilitated Diffusion
-solute binds to the carrier protein located on the plasma membrane -transported across the membrane by the carrier protein’s change in shape
-no energy required – still moving high to low concentration
-but there is a limit to the amount of facilitated diffusion the cells can undergo - has to do with the # of carrier proteins on the PM
EXTRACELLULAR FLUID Carrier protein CYTOPLASM (b) A carrier protein Solute
Active transport uses energy to move solutes against their gradients
• • • some transport proteins can move solutes against their
concentration gradients
Active transport moves substances against their concentration gradients – requires energy, usually in the form of ATP – is performed by specific proteins embedded in the membranes called pumps two kinds of active transport: –
Primary
–
Secondary
A. Primary Active transport
http://highered.mcgraw hill.com/sites/0072437316/student_view0/chapter6/a nimations.html# • •
requires a protein carrier called a pump – a pump that hydrolyzes ATP (ATPase)
e.g. sodium/potassium ATPase pump : maintains a specific concentration of Na+ and K+ in and out of the cell – three Na+ are pumped out of a cell and 2 K+ are pumped into the cell • • • • • • • 3 Na+ ions bind to the pump ATP binds to the pump and is hydrolyzed a phosphate group remains bound to the pump = phosphorylation phosphorylation changes the activity of the pump by altering its shape Na+ is expelled out of the cell – against its concentration gradient 2 K+ ions then bind the pump and causes the release of the P group another shape change by the pump - releases the K+ into the cell
Selective Permeability: Membrane Gradients
• • • • selective permeability of the plasma membrane PLUS the action of pumps like the Na/K ATPase results in a distinct distribution of
positive and negative ions inside and outside the cell
– typically the inside of the cell is more negatively charged this difference in electrical charge and chemical composition between the inside and outside produces a potential energy =
electrochemical gradient
because it occurs across the PM – we call this potential energy =
membrane potential MEMBRANE GRADIENTS ARE THE UNDERLYING REASON DRIVING SECONDARY ACTIVE TRANSPORT
B. Secondary Active transport = ATPase pumps are used to establish a concentration gradient
of one ion -
-i.e. the energy stored in a concentration gradient is used to drive the transport of other materials
which then drives the movement of another ion or ions
-the pump that creates this gradient = electrogenic pump -S.A.T is usually is driven by the establishment of either a Na+ or proton gradient -movement of the additional ions occurs through a membrane protein called a co-transporter
-two kinds: symporters and anti-porters
http://highered.mcgraw hill.com/sites/00724373 16/student_view0/chapt er6/animations.html# diffusion diffusion Na pump diffusion diffusion
e.g. Na/Ca antiporter – opposite direction for Na and Ca movement – the Na+ concentration gradient creates potential energy which is used by an
antiporter protein
- as Na+ leaks back in through this antiporter – the potential energy is converted into kinetic energy which drives the movement of a Ca ion against its gradient e.g. Na/glucose symporter -Na+ diffusion coupled to movement of glucose in the same direction
ATP
H
H
H
Proton pump H
H
Sucrose-H
cotransporter
H
H
Diffusion of H
H
Sucrose
Sucrose
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
• • • small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins large molecules (e.g. polysaccharides and proteins) cross the membrane in bulk via vesicles –
this bulk transport requires energy
two kinds of bulk transport: – –
1. Exocytosis – known as secretion 2. Endocytosis – known as internalization
Exocytosis
http://highered.mcgraw hill.com/sites/0072437316/student_view0/chapter6/animations.html#
• exocytosis= migration of transport vesicles to the plasma membrane – followed by fusion and release of contents – transport vesicles bud from the Golgi apparatus • m any secretory cells use exocytosis to export their products
Endocytosis
• • • endocytosis = internalization following the formation of a vesicle – vesicles form by invaginating from the plasma membrane reversal of exocytosis - involving different proteins three types of endocytosis: –
Phagocytosis (“cellular eating”)
– –
Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis
1. pinocytosis
= “cell drinking”
2. phagocytosis
= “cell eating” • • molecules are taken up when extracellular fluid is “gulped” into tiny pinocytic
vesicles
fusion with a lysosome and digestion into component • • cell engulfs a particle into a vacuole
called a phagosome
fusion with a lysosome to digest the particle
3. receptor-mediated endocytosis = internalization of specific substances at specific locations in the plasma membrane -binding of a ligand with its receptor internalization into the cell -internalization requires a protein called clathrin -receptor-ligand complexes internalized into clathrin-coated pits
clathrin coated vesicle
-CC vesicle fuses with endosomes – eventually fuse to the lysosome for processing http://sumanasinc.com/webcontent/animations/co ntent/endocytosis.html
Extracellular components and connections between cells help coordinate cellular activities • • most cells synthesize and secrete materials that are external to the plasma membrane these extracellular materials include: –
the extracellular matrix (ECM) of animal cells
–
intercellular junctions
–
cell walls of plants
Cell Junctions – Joining Plasma membranes
• • • neighboring cells can interact, and communicate through direct physical contact one kind of contact is through the formation of intercellular junctions several types of intercellular junctions –
Tight junctions - animals cells
– –
Desmosomes – animal cells Gap junctions – animal cells
–
Plasmodesmata – plants cells Tight junctions prevent fluid from moving across a layer of cells Tight junction Tight junction Intermediate filaments Desmosome Gap junction Ions or small molecules Space between cells Plasma membranes of adjacent cells Extracellular matrix TEM TEM 0.5
m 1 0.1
m m
Tight Junctions, Desmosomes, and Gap Junctions – Intercellular Junctions in Animal Cells
• 1. tight junctions: anchoring junction PMs are bound together by interlocking membrane proteins – forms a very tight union – can prevent passage of water and solutes between the two cells and between the two cells – also gives the tissue strength
-complex of numerous proteins:
-these protein complexes interact with the actin of the cells cytoskeleton
Tight Junctions, Desmosomes, and Gap Junctions – Intercellular Junctions in Animal Cells
• • • • •
2. Desmosomes: anchoring junction
fasten cells together into strong sheets comprised of
cellular adhesion molecules/CAMs
and
proteoglycans
adhesion plaque links to components of the cytoskeleton (keratin) desmosomes that link the cell to the underlying basement membrane = hemidesmosome
Tight Junctions, Desmosomes, and Gap Junctions – Intercellular Junctions in Animal Cells
• • • • Gap junctions = communicating junction provides cytoplasmic channels between adjacent cells made of protein complexes called
connexons
capable of opening and closing and connecting the two cell interiors
Cell Walls of Plants
• • • cell wall = extracellular structure that distinguishes plant cells from animal cells – prokaryotes, fungi, and some protists have cell walls functions: –
1. protects the plant cell
–
2. maintains its shape
–
3. prevents excessive uptake of water
made of cellulose fibers embedded in a mixture of polysaccharides and protein
Cell Walls of Plants
• may have multiple layers: – primary cell wall: thin and flexible portion of the wall • made of
cellulose
called pectin and a sugar – middle lamella: thin layer between primary walls of adjacent cells – secondary cell wall –found in some cells • found between the plasma membrane and the primary cell wall • impregnated with lignin for strength
Secondary cell wall Primary cell wall Middle lamella 1
m Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata
Plasmodesmata in Plant Cells
• • Plasmodesmata are channels that perforate plant cell walls – like gap junctions between two plant cells – – plasmodesma (singular) clusters of plasmodesmata found in pit fields through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell
Cell walls Interior of cell Interior of cell 0.5
m Plasmodesmata Plasma membranes