1 Cell Membrane: Structure and Function 6.2-6.3 The cell
membrane is the gateway into the cell.
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2 Key words you should know: Phospholipids SolutionPinocytosis
PolarSolute HydrophilicSolventExocytosis HydrophobicPartially
permeablePhospholipid bilayer Fluid mosaic modelSurface area
GlycoproteinsTurgid PlasmolysisPassive transport
CholesterolPlasmolysed ProteinsSelectively permeable Transport
proteinsActive transport EnzymesCarrier protein Receptor
moleculesBulk transport DiffusionEndocytosis Concentration
gradientPhagocytosis Facilitated diffusionPhagocytes
OsmosisCrenated
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3 Cell Membrane Structure All living cells are surrounded by a
membrane. A cell membrane is also known as plasma membrane. To
understand the function of anything in biology, you must study the
structure first! Nerve cell Cell membrane { } cell membrane Gap
between cells Cell Membranes from Opposing Neurons (TEM
x436,740).
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4 Functions of the Plasma Membrane Protective barrier Regulate
transport in & out of cell Regulate transport in & out of
cell Allow cell-cell recognition Allow cell-cell recognition
Provide anchoring sites for filaments of cytoskeleton Provide
anchoring sites for filaments of cytoskeleton Provide a binding
site for enzymes and hormones Provide a binding site for enzymes
and hormones Interlocking surfaces bind cells together (junctions)
Interlocking surfaces bind cells together (junctions) Contains the
cytoplasm (fluid in cell) Contains the cytoplasm (fluid in
cell)
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5 Phospholipids Cholesterol Proteins Proteins (peripheral and
integral) Carbohydrates Structure of Cell Membrane
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6 PHOSPHOLIPID REVIEW Phospholipids are important structural
components of cell membranes. Phospholipids are modified so that a
phosphate group (PO4-) replaces one of the three fatty acids
normally found on a lipid. phosphate group The addition of this
group makes a polar "head" and two nonpolar "tails".
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7 HYDROPHILIC HEAD At one end of the phospholipid is a
phosphate group and several double bonded oxygens. The electrons at
this end of the molecule are not shared equally. This end of the
molecule has a charge and is attracted to water. It is POLAR
HYDROPHOBIC TAILS The two long chains coming off of the bottom of
this molecule are made up of carbon and hydrogen. Because both of
these elements share their electrons evenly these chains have no
charge. They are NON POLAR. Molecules with no charge are not
attracted to water; as a result water molecules tend to push them
out of the way as they are attracted to each other. This causes
molecules with no charge not to dissolve in water.
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8 Phospholipids can form: BILAYERS -2 layers of phospholipids
with hydrophobic tails protected inside by the hydrophilic heads.
The PHOSPHOLIPID BILAYER is the basic structure of membranes.
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9 hydrophilic Polar heads are hydrophilic water loving
hydrophobic Nonpolar tails are hydrophobic water fearing
Phospholipids make membranes Selectively Permeable- Selectively
Permeable- able to control what crosses
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10 Polar, Hydrophilic head Polar, Hydrophilic head Nonpolar,
Hydrophobic Fatty acid tails
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11 FLUID MOSAIC MODEL Cell membranes also contain proteins
within the phospholipid bilayer. This model for the structure of
the membrane is called the: FLUID MOSAIC MODEL FLUID- because
individual phospholipids and proteins can move around freely within
the layer, like its a liquid. MOSAIC- because of the pattern
produced by the scattered protein molecules when the membrane is
viewed from above.
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12 Proteins Are Critical to Membrane Function
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13 Proteins can float in the membrane or be fixed and also have
hydrophobic and hydrophilic portions. Proteins may be embedded in
the outer layer or in the inner layer and some span the two layers.
Hydrophobic and Hydrophilic parts of the protein molecules sit next
to the Hydrophobic and Hydrophilic portions of the phospholids of
the membrane. This ensures the proteins stay in the membrane.
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14 Some of the proteins have carbohydrates attached to them
GLYCOPROTEINS. Glycoproteins act as chemical id tags. Blood types
are the result different glycoproteins.
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15 The membrane also contains molecules of CHOLESTEROL
Cholesterol regulates the fluidity of the membrane gives mechanical
stability helps to prevent ions from passing through the
membrane.
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16 The fluid mosaic model was developed using Freeze Fracture
Studies. The fracture occurs between the two phospholipid layers.
You can clearly see the exposed proteins sticking out of the two
layers. Individual phospholipids are too small to see.
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17 Cell Membrane Permeability Hydrophobic pass easily
Hydrophilic DO NOT
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18 Semipermeable Membrane Small molecules and larger non-polar
molecules move through easily. e.g. O 2, CO 2, H 2 O
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19 Ions Hydrophilic molecules larger than water Large molecules
such as proteins do not move through the membrane on their own.
Semipermeable Membrane
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20 Types of Transport Across Across Cell Membranes
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21 Two Forms of Transport Across the Membrane
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22 Passive Transport Simple Diffusion Doesnt require cell
energy Doesnt require cell energy Molecules move from high to low
concentration Molecules move from high to low concentration
Example: Oxygen or water diffusing into a cell and carbon dioxide
diffusing out Example: Oxygen or water diffusing into a cell and
carbon dioxide diffusing out.
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23 HIGH to LOW Molecules move from area of HIGH to LOW
concentration, down their concentration gradient.
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24 PASSIVE Diffusion is a PASSIVE process which means no cell
energy is used to make the molecules move, they have a natural
KINETIC ENERGY
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25 Diffusion of Liquids
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26 Diffusion through a Membrane Solute moves DOWN the
concentration gradient from HIGH to LOW concentration Cell
membrane
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27 Diffusion of water across a membrane Moves from HIGH water
potential (low solute) to LOW water potential (high solute)
Osmosis
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28 Osmosis-Diffusion of H 2 O Across A Membrane Low solute
concentrationHigh solute concentration
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29 CELL 10% NaCL 90% H 2 O 10% NaCL 90% H 2 O ENVIRONMENT NO
NET MOVEMENT What is the direction of water movement? The cell s at
_______________. equilibrium
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30 CELL 10% NaCL 90% H 2 O 20% NaCL 80% H 2 O What is the
direction of water movement?
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31 CELL 15% NaCL 85% H 2 O 5% NaCL 95% H 2 O What is the
direction of water movement? ENVIRONMENT
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32 Cells in Solutions
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33 Isotonic Solution NO NET MOVEMENT OF H 2 O (equal amounts
entering & leaving) Hypotonic Solution CYTOLYSIS (water
diffuses into the cell) Hypertonic Solution CRENATION (water
diffuses out of the cell)
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34 Cytolysis & Crenation Cytolysis Crenation
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35 Osmosis in Red Blood Cells In Isotonic Solution In Hypotonic
Solution In Hypertonic Solution
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36 hypotonichypertonic isotonic hypertonicisotonichypotonic
hypertonic Indicate the diffusion gradient of water with an arrow.
(Plasmolyzed cell) (Crenated cell) (Turgid cell)
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37 Channel proteins are embedded in the cell membrane &
have a pore for materials to cross Carrier proteins can change
shape to move material from one side of the membrane to the other
Transport Proteins
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38 Passive Transport Facilitated Diffusion Doesnt require
energy Uses transport proteins to move high to low concentration
Moves materials with the concentration gradient. Examples: Glucose
or amino acids moving from blood into a cell.
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39 Facilitated Diffusion Molecules will randomly move through
the pores in Channel Proteins.
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40 Facilitated Diffusion
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41 Carrier Proteins Other carrier proteins change shape to move
materials across the cell membrane
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42 Active Transport Requires energy or ATP Moves materials from
LOW to HIGH concentration AGAINST concentration gradient
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43 Active Transport Examples: Pumping Na + (sodium ions) out
and K + (potassium ions) in against strong concentration gradients.
Called Na+-K+ Pump
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44 Sodium-Potassium Pump 3 Na+ pumped out for every 2 K+ pumped
in; creates a membrane potential (+) (-)
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45 Moving the Big Stuff Out Molecules are moved out of the cell
by vesicles that fuse with the plasma membrane. Exocytosis
Exocytosis - moving things out. This is how many hormones are
secreted and how nerve cells communicate with one another This is
how many hormones are secreted and how nerve cells communicate with
one another.
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46 Exocytosis Large molecules that are manufactured in the cell
are released through the cell membrane. Inside Cell Cell
environment
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47 Exocytosis Exocytic vesicle immediately after fusion with
plasma membrane.
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48 Moving the Big Stuff In Large molecules move materials into
the cell by one of three forms of endocytosis Large molecules move
materials into the cell by one of three forms of endocytosis.
1.Pinnocytosis 2.Receptor-mediated EndoCytosis 3.Phagocytosis
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49 1. Pinocytosis Most common form of endocytosis Most common
form of endocytosis. Takes in dissolved molecules as a vesicle
Takes in dissolved molecules as a vesicle.
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50 Pinocytosis Cell forms an invagination Materials dissolve in
water to be brought into cell Called Cell Drinking
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51 Example of Pinocytosis pinocytic vesicles forming mature
transport vesicle Transport across a capillary cell (blue).
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52 2. Receptor-Mediated Endocytosis Some integral proteins have
receptors on their surface to recognize & take in hormones,
cholesterol, etc.