Cell Membrane Structure and Function

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Cell Membrane Structure and Function

• Nils Nodland
• and
• Taylor Huebner

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Concept 7.1

• Cellular membranes are fluid mosaics of lipids
  and proteins.
• Membrane Models
• Fluidity of Membranes
• Membrane Proteins and Their Functions
• Role of Membrane Carbohydrates in Cell-Cell
  Recognition
• Synthesis and Sidedness of Membranes

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Membrane Models

• Phospholipid Bilayer with hydrophilic head and
  hydrophobic tail.
• Imbedded in the bilayer are proteins which have
  hydrophilic regions on their heads and tails and
  hydrophobic centers

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The Fluidity of Membranes

• The fluidity of membranes is determined by the
  structure of the hydrophobic tails
• Unsaturated tails means more fluidity
• Saturated tails have more viscosity
• Phospholipids move laterally in a membrane, but
  rarely they flip-flop across the membrane
• Cholesterol reduces the fluidity when its in
  animal cells at moderate temperatures, but al
  lower temperatures hinders solidification by
  disrupting the regular packing of the lipids.

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Membrane Proteins and Their Functions
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Membrane Proteins and Their Functions (cont.)

• Transmembrane proteins have specific orientations
  in the membrane and have alpha-helical structures
• Integral proteins penetrate the hydrophobic core
  of the lipid bilayer
• Peripheral proteins arent in the bilayer
  theyre attached to an integral protein

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Membrane Proteins and Their Functions (cont.)

• Transportation active and passive transportation
  through membrane
• Enzymatic Activity Work like a regular enzyme
  but are embedded in the bilayer
• Signal Transduction like the lock and key model,
  used for chemical signals (hormones)
• Cell-cell Recognition glycoproteins are used as
  ID tags
• Intercellular Joining membrane proteins hook
  together to close gaps
• Attachment to cytoskeleton and ECM
  microphilaments bond to membrane proteins to
  maintain structure

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cont
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Role of Membrane Carbohydrates in Cell-Cell
Recognition

• A cells ability to distinguish one type of
  neighboring cell from another is determined by
  the bindings attached to the plasma membrane,
  which are often carbohydrates
• Glycolipids- carbohydrates that are covalently
  bonded to lipids.
• Glycoproteins- carbohydrates that are covalently
  bonded to proteins
• The type of carbs that are present of the plasma
  membrane vary from cell to cell
• Ex. Blood types (A, B, AB, O) carbs on surface
  of red blood cells

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Synthesis and Sidedness of membranes

• Proteins and lipids are synthesized in ER.
  Carbohydrates then added to proteins and modified
• 2) In Golgi App. glycoproteins undergo
  carbohydrate modifications and lipids acquire
  carbs.
• 3) Transmembrane proteins, membrane
  glycoproteins, secretory proteins are transported
  in vesicles
• 4) Vesicles fuse with membrane and release
  secretory proteins from cell

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Synthesis of Membranes
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7.2 Membrane structure results in selective
permeability

• The permeability of the lipid bilayer
• Transport proteins

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The permeability of the lipid bilayer

• Hydrophobic, non-polar molecules (hydrocarbons,
  CO2, and O2) can cross lipid bilayer
• Hydrophobic core of membrane impedes direct
  passage of ions and polar molecules through
  membrane
• Polar molecules (glucose, sugars) pass slowly
  through lipid bilayer

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Transport proteins

• Cell membranes are permeable to specific ions and
  polar molecules
• Hydrophilic substances pass through transport
  proteins
• Transport proteins function by having a
  hydrophilic channel that molecules/ions use as a
  tunnel
• Ex Aquaporins facilitate passage of water
  molecules in certain cells

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7.3 Passive transport is diffusion of a substance
across a membrane with no energy investment

• Effects of Osmosis on water balance
• water balance of cells without walls
• Water balance of cells with walls
• Facilitated diffusion passive transport aided by
  proteins

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Diffusion

• The tendency of molecules of any substance to
  spread out evenly into available space
• In the absence of other forces, substances
  diffuse from high concentrations to low
  concentrations, aka its concentration gradient
• Diffusion of substances across a biological
  membrane is called passive transport
• Passive transport requires no energy

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Effects of Osmosis on water balance

• Osmosis diffusion of water across a selectively
  permeable membrane

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Water balance of cells without walls

• Tonicity ability of solution to cause cells to
  gain or lose water
• Tonicity of solution depends on concentration of
  solutes that cant cross membrane
• Isotonic Solution where no net movement of water
  molecules occurs
• Hypertonic Solution where cell loses water to
  environment and shrivels
• Hypotonic Water enters cell from solution and
  cell swells

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cont
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Water balance (cont)

• Osmoregulation control organisms have for water
  balance, used for cells without cell walls
• Some organisms have evolved to control their
  water balance better, even if theyre not in
  isotonic solutions
• Ex Paramecium live in pond water which is
  hypotonic to it

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Water balance of cells with walls

• Walls maintain water balance of cells
• Turgid Cell wall only expands so much before it
  exerts backpressure that opposes water uptake
  (means very firm)
• Flaccid Cells placed in an isotonic solution
  (means limp)
• Plasmolysis When water escapes cells to solution
  (cells shrivel)

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continued
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Facilitated diffusion passive transport aided by
proteins

• Facilitated diffusion the process of ions and
  polar molecules passing through lipid bilayer
  with aid of transport proteins
• Channel proteins and carrier proteins
• Ion Channels Protein channels for ions
• Gated channels channels which are activated by a
  stimulus

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Transport proteins
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7.4 Active transport uses energy to move solutes
against their gradients

• The need for energy in active transport
• Maintenance of membrane potential by ion pumps
• Cotransport coupled transport by a membrane
  protein

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The need for energy in active transport

• Active transport requires the cell to expend
  energy.
• Active transport allows cells to maintain
  internal concentrations of small molecules that
  differ from its environment
• Gets energy from ATP by transferring its terminal
  phosphate group directly to the transport protein
• Ex. Sodium potassium pump

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Sodium potassium pump
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Maintenance of membrane potential by ion pumps

• All cells have voltages across their plasma
  membrane, which is electrical potential energy- a
  separation of opposite charges
• Cytoplasm of cell is negative in charge
• Voltage across a membrane is called membrane
  potential
• Two forces drive the diffusion of ions across a
  membrane chemical force and an electrical force

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Continued

• The combination of these two forced acting on an
  ion is called the electrochemical gradient.
• Ions do not move across their concentration
  gradient but across their electrochemical
  gradient
• Transport protein that generates voltage across a
  membrane is called an electrogenic pump.

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continued

• The main electrogenic pump is a proton pump,
  which actively transports hydrogen ions out of
  the cell.
• This pumping of H ions transfers positive charge
  from the cytoplasm to the outside solution
• Electrogenic pumps then store energy that can be
  tapped for cellular work.

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Cotransport

• A transporter that takes the energy from another
  source and uses it to drive the transport of
  another solute
• Ex. sucrose

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7.5 concepts

• Exocytosis
• Endocytosis
• Types of endocytosis

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Exocytosis

• Exocytosis the secretion of macromolecules by
  the fusion of vesicles with the plasma membrane.
• Vesicle from the golgi moves along microtubules
  of cytoskeleton to plasma membrane
• When vesicle and membrane come in contact they
  rearrange themselves so that the vesicle membrane
  becomes part of the plasma membrane
• Many secretory cells in exocytosis are used to
  transport products to other cells and also to get
  rid of waste
• Ex. secretion of hormone insulin into blood
  stream to be transported to other cells

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Endocytosis

• The cell taking in macromolecules and particulate
  matter by forming new vesicles in the plasma
  membrane
• Endocytosis is opposite of exocytosis
• Cells also use receptors in endocytosis to take
  in specific molecules.

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Types of Endocytosis

• The three main types of endocytosis are
  phagocytosis, pinocytosis, and receptor mediated
  endocytosis.

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Continued

• Phagocytosis known as cell eating cell engulfs
  a particle by wrapping pseudopodium around it and
  then packaging it
• Pinocytosis known as cell drinking cell gulps
  droplets of extracellular fluid into tiny
  vesicles. Very non specific in what it
  transports. It is not the fluid that is needed
  rather the molecules dissolved in it.
• Receptor-mediated endocytosis enables cell to
  acquire bulk quantities of specific substances,
  even though they may not be very concentrated in
  the extracellular fluid. When binding occurs on
  the receptors the coated pit forms a vesicle
  containing ligand molecules

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Quiz

• In the picture above, which side is hydrophobic?
• Top
• Bottom
• Center
• Top and bottom
• Which hydrocarbon tails are most fluid when in a
  membrane?
• a) unsaturated
• b) saturated
• c) cholesterol
• d) phospholipids

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• 3. Which of these is NOT a function of a cell
  membrane protein?
• a) signal transduction
• b) intercellular joining
• c) membrane plasmolysis
• d) cell recognition
• 4. Which of the following processes includes all
  others?
• a) osmosis
• b) passive transport
• c) facilitated diffusion
• d) diffusion of a solute across a membrane

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• 5. Which of these is a transport protein?
• a) ion channels
• b) gated channels
• c) carrier proteins
• d) facilitated diffusion
• 6. An environment that loses water to a cell is
  said to be
• a) isotonic
• b) hypertonic
• c) tonic
• d) hypotonic
• 7. A plant cell that is in a hypotonic solution
  is
• a) turgid
• b) plasmolyzed
• c) flaccid
• d) lysed

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• 8. Which of the following is NOT an example of
  active transport?
• a) electrogenic pump
• b) proton pump
• c) sodium-potassium pump
• d) ATP pump
• 9. What is phagocytosis a type of?
• a) endocytosis
• b) pinocytosis
• c) exocytosis
• d) gyrocytosis

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• 10. What type of endocytosis is shown in the
  adjacent picture?
• a) phagocytosis
• b) receptor-mediated
• c) pinocytosis
• d) pornocytosis