The Plasma Membrane IB Biology HL E. McIntyre

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The Plasma MembraneIB Biology HLE. McIntyre
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History of the Plasma Membrane

• 1665 Robert Hooke
• 1895 Charles Overton - composed of lipids
• 1900-1920s must be a phospholipid
• 1925 E. Gorter and G. Grendel - phospholipid
  bilayer
• 1935 J.R. Danielli and H. Davson proteins also
  part, proposed the Sandwich Model
• 1950s J.D. Robertson proposed the Unit
  Membrane Model
• 1972 S.J. Singer and G.L. Nicolson proposed
  Fluid Mosaic Model

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Plasma Membrane is made of Phospholipids

• Gorter Grendel
• Red Blood Cells analyzed
• Enough for Phospholipid bilayer
• Polar heads face out and
  Nonpolar tails face in
• Does not explain why some
  nonlipids are permeable

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Plasma Membrane Models

• Sandwich Model
• (Danielli Davson)
• 2 layers of globular proteins with phospholipid
  inside to make a layer and then join 2 layers
  together to make a channel for molecules to pass
• Unit Membrane Model
• (Robertson)
• Outer layer of protein with phospholipid bilayer
  inside, believed all cells same composition, does
  not explain how some molecules pass through or
  the use of proteins with nonpolar parts, used
  transmission electron microscopy
• Fluid Mosaic Model
• (Singer Nicolson)
• Phospholipid bilayer with proteins partially or
  fully imbedded, electron micrographs of
  freeze-fractured membrane

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Which membrane model is correct?

• 1) Rapidly freeze specimen
• 2) Use special knife to cut membrane in half
• 3) Apply a carbon platinum coating to the
  surface
• 4) Use scanning electron microscope to see the
  surface
• According to the electron micrograph which
  membrane model is correct?
• Why?
• Fluid-Mosaic Model

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Fluid-Mosaic Model

• Fluid the plasma membrane is the consistency of
  olive oil at body temperature, due to unsaturated
  phospholipids. (cells differ in the amount of
  unsaturated to saturated fatty acid tails)
• Most of the lipids and some proteins drift
  laterally on either side. Phospholipids do not
  switch from one layer to the next.
• Cholesterol affects fluidity at body
  temperature it lessens fluidity by restraining
  the movement of phospholipids, at colder
  temperatures it adds fluidity by not allowing
  phospholipids to pack close together.
• Mosaic membrane proteins form a collage that
  differs on either side of the membrane and from
  cell to cell (greater than 50 types of proteins),
  proteins span the membrane with hydrophilic
  portions facing out and hydrophobic portions
  facing in. Provides the functions of the membrane

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Structure of the Plasma Membrane
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Structure of the Plasma Membrane

• Phospholipid bilayer
• Phospholipid
• Hydrophilic head
• Hydrophobic tails
• Cholesterol
• Proteins
• Transmembrane/
• Intrinsic/Integral
• Peripheral/Extrinsic
• Cytoskeletal filaments
• Carbohydrate chain
• Glycoproteins

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Label the Blank Diagram of the Plasma Membrane

• Phospholipid bilayer
• Phospholipid
• Hydrophilic head
• Hydrophobic tails
• Cholesterol
• Proteins
• Transmembrane/
• Intrinsic/Integral
• Peripheral/Extrinsic
• Cytoskeletal filaments
• Carbohydrate chain
• Glycoproteins

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Proteins of the Plasma Membrane Provide 6
Membrane Functions

• 1) Transport Proteins
• 2) Receptor Proteins
• 3) Enzymatic Proteins
• 4) Cell Recognition Proteins
• 5) Attachment Proteins
• 6) Intercellular Junction
• Proteins

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1) Transport Proteins

• Channel Proteins channel for lipid insoluble
  molecules and ions to pass freely through
• Carrier Proteins bind to a substance and carry
  it across membrane, change shape in process

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2) Receptor Proteins

• Bind to chemical messengers (Ex. hormones)
  which sends a message into the cell causing
  cellular reaction

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3) Enzymatic Proteins

• Carry out enzymatic reactions right at the
  membrane when a substrate binds to the active
  site

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4) Cell Recognition Proteins

• Glycoproteins (and glycolipids) on
  extracellular surface serve as ID tags (which
  species, type of cell, individual). Carbohydrates
  are short branched chains of less than 15 sugars

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5) Attachment Proteins

• Attach to cytoskeleton (to maintain cell shape
  and stabilize proteins) and/or the extracellular
  matrix (integrins connect to both).
• Extracellular Matrix protein fibers and
  carbohydrates secreted by cells and fills the
  spaces between cells and supports cells in a
  tissue.
• Extracellular matrix can influence activity
  inside the cell and coordinate the behavior of
  all the cells in a tissue.

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6) Intercellular Junction Proteins

• Bind cells together
• Tight junctions
• Gap junctions

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How do materials move into and out of the cell?

• Materials must move in and out of the cell
  through the plasma membrane.
• Some materials move between the phospholipids.
• Some materials move through the proteins.

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Plasma Membrane Transport

• Molecules move across the plasma membrane by

Active Transport
Passive Transport
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What are three types of passive transport?
Passive Transport

• Diffusion
• Facilitated Diffusion
• Osmosis

ATP energy is not needed to move the molecules
through.
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Passive Transport 1 Diffusion

• Molecules can move directly through the
  phospholipids of the plasma membrane
• This is called

DIFFUSION
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What is Diffusion?

• Diffusion is the net movement of molecules from a
  high concentration to a low concentration until
  equally distributed.
• Diffusion rate is related to temperature,
  pressure, state of matter, size of concentration
  gradient, and surface area of membrane.

http//www.biologycorner.com/resources/diffusion-a
nimated.gif
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What molecules pass through the plasma membrane
by diffusion?

• Gases (oxygen, carbon dioxide)
• Water molecules (rate slow due to polarity)
• Lipids (steroid hormones)
• Lipid soluble molecules (hydrocarbons, alcohols,
  some vitamins)
• Small noncharged molecules (NH3)

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Why is diffusion important to cells and humans?

• Cell respiration
• Alveoli of lungs
• Capillaries
• Red Blood Cells
• Medications time-release capsules

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Passive Transport 2 Facilitated Diffusion

• Molecules can move through the plasma membrane
  with the aid of transport proteins
• This is called

FACILITATED DIFFUSION
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What is Facilitated Diffusion?

• Facilitated diffusion is the net movement of
  molecules from a high concentration to a low
  concentration with the aid of channel or carrier
  proteins.

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What molecules move through the plasma membrane
by facilitated diffusion?

• Ions
• (Na, K, Cl-)
• Sugars (Glucose)
• Amino Acids
• Small water soluble molecules
• Water (faster rate)

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How do molecules move through the plasma membrane
by facilitated diffusion?

• Channel and Carrier proteins are specific
• Channel Proteins allow ions, small solutes, and
  water to pass
• Carrier Proteins move glucose and amino acids
• Facilitated diffusion is rate limited, by the
  number of proteins channels/carriers present in
  the membrane.

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Specific Types of Facilitated Diffusion

• Counter Transport the transport of two
  substances at the same time in opposite
  directions, without ATP. Protein carriers are
  called Antiports.
• Co-transport the transport of two substances at
  the same time in the same direction, without ATP.
  Protein carriers are called Symports.
• Gated Channels receptors combined with channel
  proteins. When a chemical messenger binds to a
  receptor, a gate opens to allow ions to flow
  through the channel.

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Why is facilitated diffusion important to cells
and humans?

• Cells obtain food for cell respiration
• Neurons communicate
• Small intestine cells transport food to
  bloodstream
• Muscle cells contract

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Passive Transport 3 Osmosis

• Water Molecules can move directly through the
  phospholipids of the plasma membrane
• This is called

OSMOSIS
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What is Osmosis?

• Osmosis is the diffusion of water through a
  semipermeable membrane. Water molecules bound to
  solutes cannot pass due to size, only unbound
  molecules. Free water molecules collide, bump
  into the membrane, and pass through.

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Osmosis in action

• What will happen in the U-tube if water freely
  moves through the membrane but glucose can not
  pass?
• Water moves from side with high concentration of
  water to side with lower concentration of water.
  Movement stops when osmotic pressure equals
  hydrostatic pressure.

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Why is osmosis important to cells and humans?

• Cells remove water produced by cell respiration.
• Large intestine cells transport water to
  bloodstream
• Kidney cells form urine

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Osmosis and Tonicity

• Tonicity refers to the total solute concentration
  of the solution outside the cell.
• What are the three types of tonicity?
• Isotonic
• Hypotonic
• Hypertonic

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Isotonic

• Solutions that have the same concentration of
  solutes as the suspended cell.
• What will happen to a cell placed in an Isotonic
  solution?
• The cell will have no net movement of water and
  will stay the same size.
• Ex. Blood plasma has high concentration of
  albumin molecules to make it isotonic to tissues.

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Hypotonic

• Solutions that have a lower solute concentration
  than the suspended cell.
• What will happen to a cell placed in a Hypotonic
  solution?
• The cell will gain water and swell.
• If the cell bursts, then we call this lysis. (Red
  blood cells hemolysis)
• In plant cells with rigid cell walls, this
  creates turgor pressure.

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Hypertonic

• Solutions that have a higher solute concentration
  than a suspended cell.
• What will happen to a cell placed in a Hypertonic
  solution?
• The cell will lose water and shrink. (Red blood
  cells crenation)
• In plant cells, the central vacuole will shrink
  and the plasma membrane will pull away from the
  cell wall causing the cytoplasm to shrink called
  plasmolysis.

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Review Passive Transport

• Diffusion O2 moves in and CO2 moves out during
  cell respiration
• Facilitated Diffusion glucose and amino acids
  enter cell for cell respiration
• Osmosis cell removal or addition of water

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Review Tonicity

• What will happen to a red blood cell in a
  hypertonic solution?
• What will happen to a red blood cell in an
  isotonic solution?
• What will happen to a red blood cell in a
  hypotonic solution?

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What are three types of Active transport?

• 1) Active Transport
• 2) Exocytosis
• 3) Endocytosis
• Phagocytosis
• Pinocytosis
• Receptor-Mediated endocytosis

Active Transport
ATP energy is required to move the molecules
through.
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Active Transport

• Molecules move from areas of low concentration to
  areas of high concentration with the aid of ATP
  energy.
• Requires protein carriers called Pumps.

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The Importance of Active Transport

• Bring in essential molecules ions, amino acids,
  glucose, nucleotides
• Rid cell of unwanted molecules (Ex. sodium from
  urine in kidneys)
• Maintain internal conditions different from the
  environment
• Regulate the volume of cells by controlling
  osmotic potential
• Control cellular pH
• Re-establish concentration gradients to run
  facilitated diffusion. (Ex. Sodium-Potassium
  pump and Proton pumps)

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The Sodium-Potassium Pump

• 3 Sodium ions move out of the cell and then 2
  Potassium ions move into the cell.
• Driven by the splitting of ATP to provide energy
  and conformational change to proteins by adding
  and then taking away a phosphate group.
• Used to establish an electrochemical gradient
  across neuron cell membranes.

http//www.biologie.uni-hamburg.de/b-online/librar
y/biology107/bi107vc/fa99/terry/images/ATPpumA.gif
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Active Transport 2 Exocytosis

• Movement of large molecules bound in vesicles out
  of the cell with the aid of ATP energy. Vesicle
  fuses with the plasma membrane to eject
  macromolecules.
• Ex. Proteins, polysaccharides, polynucleotides,
  whole cells, hormones, mucus, neurotransmitters,
  waste

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Active Transport 3 Endocytosis

• Movement of large molecules into the cell by
  engulfing them in vesicles, using ATP energy.
• Three types of Endocytosis
• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis

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Phagocytosis

• Cellular Eating engulfing large molecules,
  whole cells, bacteria
• Ex. Macrophages ingesting bacteria or worn out
  red blood cells.
• Ex. Unicellular organisms engulfing food
  particles.

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Pinocytosis

• Cellular Drinking engulfing liquids and small
  molecules dissolved in liquids unspecific what
  enters.
• Ex. Intestinal cells, Kidney cells, Plant root
  cells

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Receptor-Mediated Endocytosis

• Movement of very specific molecules into the cell
  with the use of vesicles coated with the protein
  clathrin.
• Coated pits are specific locations coated with
  clathrin and receptors. When specific molecules
  (ligands) bind to the receptors, then this
  stimulates the molecules to be engulfed into a
  coated vesicle.
• Ex. Uptake of cholesterol (LDL) by animal cells

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Review Types of Endocytosis

• What is phagocytosis?
• What is pinocytosis?
• What is receptor-mediated endocytosis?

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Types of Cell Junctions

• In Animal Cells
• Tight Junctions
• Desmosomes
• Gap Junctions

• In Plant Cells
• Plasmodesmata

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Tight Junctions

• Transmembrane Proteins of opposite cells attach
  in a tight zipper-like fashion
• No leakage
• Ex. Intestine, Kidneys, Epithelium of skin

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Desmosomes

• Cytoplasmic plaques of two cells bind with the
  aid of intermediate filaments of keratin
• Allows for stretching
• Ex. Stomach, Bladder, Heart

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Gap Junctions

• Channel proteins of opposite cells join together
  providing channels for ions, sugars, amino acids,
  and other small molecules to pass.
• Allows communication between cells.
• Ex. Heart muscle, animal embryos

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Plasmodesmata

• Channels between the cell walls of plant cells
  that are lined with the plasma membranes of
  adjacent cells and smooth ER runs through.
• Allows for the exchange of cytosol between
  adjacent cells moving water, small solutes,
  sugar, and amino acids.
• Ex. Xylem and Phloem in Plants

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Review Types of Cell Junctions

• What is the difference between a plasmodesmata,
  tight junction, gap junction, and desmosome?

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References

• Campbell, Neil A., Jane B. Reece, and Lawrence G.
  Mitchell. (1999). Biology. Reading Addison
  Wesley Longman, Inc.
• Mader, Sylvia S. (1996). Biology. Boston
  Times Mirror Higher Education Group, Inc.
• Raven, Peter H. and Johnson, George B. (1989).
  Biology. Boston TimesMirror/Mosby College
  Publishing.