Membrane Structure and Function

1
Membrane Structure and Function
2
Membrane Function

• Membranes organize the chemical activities of
  cells.
• The outer plasma membrane
• forms a boundary between a living cell and its
  surroundings
• Exhibits selective permeability
• Controls traffic of molecules in and out

3
Membrane Function

• Internal membranes provide structural order for
  metabolism
• Form the cell's organelles
• Compartmentalize chemical reactions

4
Fluid Mosaic Model of the PM

• A membrane is a mosaic
• Proteins and other molecules are embedded in a
  framework of phospholipids
• A membrane is fluid
• Most protein and phospholipid molecules can move
  laterally

5
Membrane Structure
Phospholipid
Phospholipids are the major structural component
of membranes.
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Membrane Structure

• All membranes are phospholipid bilayers with
  embedded proteins.

Phospholipid Bilayer
Label the Hydrophilic heads Hydrophobic tails
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• Embedded in the bilayer are proteins
• Most of the membranes functions are accomplished
  by the embedded proteins.
• Integral proteins span the membrane
• Peripheral proteins are on one side or the other
  of the membrane

8
Plasma Membrane Components

• Glycoproteins and glycolipids are proteins/lipids
  with short chain carbohydrates attached on the
  extracellular side of the membrane.

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Fig. 5-1a
Carbohydrate of glycoprotein
Glycoprotein
Glycolipid
Integrin
Phospholipid
Microfilaments of cytoskeleton
Cholesterol
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Types of Membrane Proteins

• Cell-cell recognition proteins
• Integrins
• Intercellular junction proteins
• Enzymes
• Signal transduction proteins
• Aka - Receptor proteins
• Transport proteins
• Passive and active

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• Cell-cell recognition proteins - identify type of
  cell and identify a cell as self versus foreign
• Most are glycoproteins
• Carbohydrate chains vary between species,
  individuals, and even between cell types in a
  given individual.
• Glycolipids also play a role in cell recognition

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• Integrins are a type of integral protein
• The cytoskeleton attaches to integrins on the
  cytoplasmic side of the membrane
• Integrins strengthen the membrane
• Intercellular junction proteins - help like cells
  stick together to form tissues

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• Many membrane proteins are enzymes
• This is especially important on the membranes of
  organelles.

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• Signal transduction (receptor) proteins bind
  hormones and other substances on the outside of
  the cell.
• Binding triggers a change inside the cell.
• Called signal transduction
• Example The binding of insulin to insulin
  receptors causes the cell to put glucose
  transport proteins into the membrane.

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Fig. 5-1c
Messenger molecule
Receptor
Activated molecule
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Transport Proteins

• Passive Transport Proteins
• allow water soluble substances (small polar
  molecules and ions) to pass through the membrane
  without any energy cost
• Active Transport Proteins
• The cell expends energy to transport water
  soluble substances against their concentration
  gradient

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Fig. 5-1d
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Transport of Substances Across the Plasma
Membrane (PM)

• Passive Transport
• (Simple) Diffusion (5.3)
• Facilitated diffusion (5.6)
• Osmosis (5.4, 5.5)
• Active Transport (5.8)
• Bulk Flow (5.9)
• Endocytosis
• Exocytosis

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Passive Transport

• In passive transport substances cross the
  membrane by diffusion
• Diffusion - net movement of substances from an
  area of high concentration to low concentration
• no energy required

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Factors Affecting Diffusion Rate

• Steepness of concentration gradient
• Steeper gradient, faster diffusion
• Molecular size
• Smaller molecules, faster diffusion
• Temperature
• Higher temperature, faster diffusion

21
Simple Diffusion

• Nonpolar, hydrophobic molecules diffuse directly
  through the lipid bilayer
• Simple diffusion does not require the use of
  transport proteins.
• Examples O2, CO2, steroids
• Polar, hydrophilic substances cannot pass
  directly through the lipid bilayer
• Examples water, ions, carbohydrates

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Simple Diffusion
Polar molecules (ex. Glucose, water) ions (ex.
H, Na, K)

• small, nonpolar molecules
• (ex. O2, CO2)

LIPID-SOLUBLE
WATER-SOLUBLE
LIPID-SOLUBLE
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Facilitated Diffusion

• In facilitated diffusion small polar molecules
  and ions diffuse through passive transport
  proteins.
• No energy needed
• Most passive transport proteins are solute
  specific
• Example glucose enter/leaves cells through
  facilitated diffusion

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Facilitated Diffusion
Higher concentration of
Passive transport protein
Lower concentration
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Osmosis

• Osmosis diffusion of water across a selectively
  permeable membrane
• Water moves from an area of _______ water
  concentration to an area of _____ water conc.
• Is energy required ?
• Water travels in/out of the cell through
  aquaporins

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Osmosis Terms

• Consider two solutions separated
• by a plasma membrane.
• Hypertonic
• solution with a relatively high concentration of
  solute
• Hypotonic
• solution with a relatively low concentration of
  solute
• Isotonic
• solutions with the same solute concentration

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Lower concentration of solute
Higher concentration of solute
Equal concentration of solute
H2O
Solute molecule
Selectively permeable membrane
Water molecule
Solute molecule with cluster of water molecules
Net flow of water
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Osmosis and Animal Cells
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Osmosis and Plant Cells
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Osmosis

• When a Cell is Placed in a Hypotonic Solution
• Water concentration is _________ the cell.
• Water flows ___________ the cell.

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Osmosis

• When a Cell is Placed in a Hypertonic Solution
• Water concentration is _________ the cell.
• Water flows ___________ the cell.

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Isotonic solution
Hypotonic solution
Hypertonic solution
H2O
H2O
H2O
H2O
Animal cell
(1) Normal
(2) Lysed
(3) Shriveled
Plasma membrane
H2O
H2O
H2O
H2O
Plant cell
(4) Flaccid
(5) Turgid
(6) Shriveled (plasmolyzed)
See page 83
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• Osmosis Summary
• When a cell is placed in a Hypotonic solution
• Cell gains water through osmosis
• Animal cell lyses plant cell becomes turgid
  (firm)
• When a cell is placed a Hypertonic solution
• Cell loses water through osmosis
• Animal cell shrivels plant cell plasmolyzes

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Active Transport

• Active transport proteins move substances across
  the PM against their concentration gradient.
• Requires energy (ATP)
• Active transport proteins are highly selective
• Active transport is needed for proper functioning
  of nerves and muscles

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Active Transport of X

• Active transport proteins span the plasma
  membrane
• They have openings for X on only one side of
  the membrane
• X enters the channel and binds to functional
  groups inside the transport protein.
• Cytoplasmic ATP binds to the transport protein

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Active Transport of X

• A phosphate group is transferred from ATP to the
  transport protein
• protein is energized by the added P.
• The energized transport protein changes shape and
  releases X on the other side of the cell.
• The phosphate group is released from the
  transport protein and it resumes its original
  shape.
• Process repeats.

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Fig. 5-8-1
Transport protein
Solute
Solute binding
1
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Fig. 5-8-2
Transport protein
Solute
Solute binding
Phosphorylation
1
2
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Fig. 5-8-3
Transport protein
Protein changes shape
Solute
Solute binding
Phosphorylation
Transport
1
2
3
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Fig. 5-8-4
Transport protein
Protein changes shape
Phosphate detaches
Solute
Protein reversion
Solute binding
Phosphorylation
Transport
4
1
2
3
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Active Transporttell the story
ATP
P
ADP
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Bulk Flow

• Vesicles are used to transport large particles
  across the PM.
• Requires energy
• Types
• Exocytosis
• Endocytosis
• Phagocytosis, pinocytosis, receptor-mediated

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Exocytosis
Fluid outside cell
Vesicle
Protein
Cytoplasm
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Bulk Flow

• Exocytosis
• Cytoplasmic vesicle merges with the PM and
  releases its contents
• Example
• Golgi body vesicles merge with the PM an release
  their contents
• How nerve cells release neurotransmittors

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Endocytosis
Vesicle forming

• Endocytosis can occur in three ways
• Phagocytosis ("cell eating")
• Pinocytosis ("cell drinking")
• Receptor-mediated endocytosis

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Endocytosis

• Endocytosis
• PM sinks inward, pinches off and forms a vesicle
• Vesicle often merges with Golgi for processing
  and sorting of its contents

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Endocytosis - terms

• Phagocytosis cell eating
• Membrane sinks in and captures solid particles
  for transport into the cell
• Examples
• Solid particles often include bacteria, cell
  debris, or food
• Pinocytosis cell drinking
• Cell brings in a liquid

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Endocytosis - comments

• Phagocytosis and pinocytosis are not selective
• Membrane sinks inward and captures whatever
  particles/fluid present.
• Vesicle forms and merges with the Golgi body

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Receptor Mediated Endocytosis

• Receptor Mediated Endocytosis is a highly
  specific form of endocytosis.
• Receptor proteins on the outside of the cell bind
  specific substances and bring them into the cell
  by endocytosis

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Receptor Mediated Endocytosis

• Receptor proteins on PM bind specific substances
  (vitamins, hormones..)
• Membrane sinks in and forms a pit
• Called a coated pit
• Pit pinches closed to form a vesicle around bound
  substances
• Cytoskeleton aids in pulling in the membrane and
  vesicle formation

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Fig. 5-9c
Plasma membrane
Receptor-mediated endocytosis
Coat protein
Receptor
Coated vesicle
Coated pit
Coated pit
Specific molecule
Material bound to receptor proteins
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Fig. 5-9
Phagocytosis
Food being ingested
EXTRACELLULAR FLUID
CYTOPLASM
Pseudopodium
Food or other particle
Food vacuole
Pinocytosis
Plasma membrane
Vesicle
Plasma membrane
Receptor-mediated endocytosis
Coat protein
Coated vesicle
Receptor
Coated pit
Coated pit
Specific molecule
Material bound to receptor proteins