Plasma Membrane
Outermost cell component
Maintains cellular integrity
Controls what gets into and out of cell
Gives cell identity: recognize “self”
Membranes found throughout cell
Right now: outermost
Plasma Membrane
Phospholipid bilayer
 Phospholipids have both hydrophilic and hydrophobic regions
 Hydrophilic phosphate “head” seeks water
 Hydrophobic fatty-acid “tails” avoids water
Plasma Membrane
Phospholipid bilayer
 In water, phospholipids will form bilayers
 Two layers of phospholipids
 Hydrophobic tails point inward
 Hydrophilic heads point outward in both directions
 In contact with water on inner (intracellular) and outer (extracellular)
surfaces
The Plasma Membrane
Phospholipid bilayer
 Determines which molecules will pass through
 Bulk of thickness is hydrophobic
 Hydrophobic molecules pass through readily
 e.g., Fatty acids, steroid hormones, etc.
 Movement of hydrophilic molecules is restricted
 e.g., Ions, amino acids, sugars, etc.
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The Plasma Membrane
Cholesterol
 Nestled between phospholipid molecules
 Restricts movement through membrane
 Maintains membrane fluidity
 Not too rigid in cold temperatures
 Not too fluid in high temperatures
The Plasma Membrane
Proteins
 Some are embedded within the membrane
 Some lie on a surface of the membrane
 Various roles
 Structural support
 Recognition
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Communication
Transport
Etc.
The Plasma Membrane
Proteins
 Structural support
 Some proteins on the cytoplasmic surface are attached to cytoskeleton
 Stabilize the cell
 Give animal cells their characteristic shape
The Plasma Membrane
Proteins
 Recognition
 Cells in the immune system can discern between self molecules and foreign
molecules
 This discernment is mediated by cell-surface proteins
The Plasma Membrane
Proteins
 Communication
 Cells communicate with one another in various ways
 Signals sent to neighboring and distant cells
 Various types of signaling molecules
 Signals are received through receptor proteins
 Each receptor has a binding site that “fits” with a specific signaling
molecule
 e.g., Insulin receptor, etc.
The Plasma Membrane
Proteins
 Transport
 Many materials cannot simply pass through the plasma membrane
 Transport proteins move specific molecules across the membrane
 e.g., Ca2+ channels, Na+/K+ pump, etc.
The Plasma Membrane
Glycocalyx
 Short, branched carbohydrate chains jut from the extracellular face of the
membrane
 Attached to membrane lipids and proteins
 Various functions
 Binding sites for signaling molecules
 Lubricate cells
 Adhesion to adjacent cells
Diffusion, Gradients, Osmosis
 All molecules and ions are in a constant state of random motion
Temperature is a measure of the degree of motion
 There is no motion at absolute zero
 -273 oC
 Movement exists at all temperatures above absolute zero
Diffusion, Gradients, Osmosis
 Molecules will tend to move from a region of high concentration to a region of
lower concentration
 Diffusion
 Movement of molecules down their concentration gradient
 Movement of molecules from an ordered state to a disordered state
 Dictated by Laws of Thermodynamics
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Diffusion, Gradients, Osmosis
 A membrane can be permeable to some molecules and impermeable to others
 Such a membrane is often permeable to water (the solvent), but
impermeable to some molecule (the solute) dissolved in the water
 Molecules able to move across the membrane will move down their
concentration gradients
Diffusion, Gradients, Osmosis
 If a membrane permeable to water but impermeable to salt separates salt
water from distilled water, what will happen?
 Water will move across the membrane in both directions
 More water will move into the salt solution than will move from the salt
solution
Diffusion, Gradients, Osmosis
 The plasma membrane is a semipermeable membrane
 Somewhat permeable to water and lipids
 Impermeable to larger charged substances
Osmosis
 Osmosis: movement of water along its concentration gradient across a semi-
permeable membrane
 Key words:
 Water
 Semipermeable
 Concentration gradient
Diffusion, Gradients, Osmosis
 Osmosis takes place across the plasma membrane
 e.g., Fluid uptake by plants
 e.g., Various metabolic processes in animals
 e.g., Return of fluid to blood vessels
 Etc.
Diffusion, Gradients, Osmosis
 Two solutions with identical solute and solvent concentrations are termed
isotonic
 If two solutions have different solute concentrations
 The solution with more solute is termed hypertonic
 The solution with less solute is termed hypotonic
Diffusion, Gradients, Osmosis
 What happens to animal and plant cells when placed in an isotonic solution?
 Hypertonic?
 Hypotonic?
Moving Small Stuff In and Out
 Molecules can move across a plasma membrane in a variety of ways
Directly across the phospholipid bilayer
 Via proteins embedded within the bilayer
 Without the expenditure of energy
 With the expenditure of energy
Moving Small Stuff In and Out
 Movement of molecules across a membrane without the expenditure of energy
is termed passive transport
 Movement directly through the bilayer
 Movement facilitated by membrane proteins
 Movement of molecules across a membrane with the expenditure of energy is
termed active transport
 “Paid” movement via membrane proteins
Moving Small Stuff In and Out
Passive Transport: Simple Diffusion
 Molecules such as O2, CO2, and steroid hormones move down their
concentration gradients directly across the bilayer
 No protein channel is required
 No energy expenditure is required
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Moving Small Stuff In and Out
Passive Transport: Facilitated Diffusion
 Molecules such as sugars, amino acids, and various ions cannot directly
traverse the bilayer
 These molecules can move down their concentration gradients across the
membrane through special protein channels
 No energy expenditure is required
Moving Small Stuff In and Out
Active Transport
 Molecules can move across membranes against their concentration gradients
 Special protein channels are required
 Energy expenditure is required
Moving Big Stuff In and Out
 Relatively small molecules can cross membranes in the ways we have
discussed
 Sometimes it is necessary to move large molecules across membranes
 Large molecules cannot be moved by the same mechanisms as small
molecules
 Channels and pumps are too small
 Movement employs vesicles
Moving Big Stuff In and Out
Movement Out: Exocytosis
 Movement of materials out of a cell by fusing a vesicle with the plasma
membrane
 Vesicle’s contents released into extracellular fluid
 e.g., Vesicles budding from Golgi complex fuse with plasma membrane
to export proteins
 e.g., Waste products released
Moving Big Stuff In and Out
Movement In: Endocytosis
 Movement of large materials into a cell
 e.g., Ingestion of an entire bacterial cell
 Accomplished by enclosing them within vesicles derived from the plasma
membrane
 Essentially, exocytosis in reverse
 Three forms
 Pinocytosis
 Receptor-mediate endocytosis
 Phagocytosis
Moving Big Stuff In and Out
Movement In: Endocytosis
 Pinocytosis (“cell drinking”)
 Cell folds inward
 “Harbor” is formed
 Membrane fuses, harbor pinches off
 Vesicle is formed
 Material within harbor brought into cell
 Small volume of extracellular fluid
 Materials dissolved in fluid
Moving Big Stuff In and Out
Movement In: Endocytosis
 Receptor-mediated endocytosis
 Groups of receptors congregate in depression in cell membrane
“Coated pit”
Receptors specific for a particular molecule
 e.g., Cholesterol
 Pit deepens and pinches off
 Vesicle is formed
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Moving Big Stuff In and Out
Movement In: Endocytosis
 Phagocytosis (“cell eating”)
 Can bring even larger materials into the cell
 e.g., Amoeba and various white blood cells engulf entire cells
 Cell sends out pseudopodia
 “False feet”
 Pseudopodia surround food, then fuse
 Vesicle is formed
 Material often digested following fusion of vesicle with lysosome
Moving Big Stuff In and Out
Endosymbiosis
 Bacteria ancestral to mitochondria were engulfed by phagocytosis
 Chloroplasts arose in the same manner
 Bacterium now surrounded by two membranes
 Outer membrane of host (eukaryote) origin
 Inner membrane of bacterial origin
 “Vesicle” maintained rather than digested
Moving Big Stuff In and Out
 The plasma membrane is constantly being “remade”
 Membrane is lost via endocytosis
 Membrane is gained via exocytosis
Application
 Blood and tissue typing
 Familial hypercholesteremia
 Cystic fibrosis
Cystic Fibrosis
 Hereditary
 Impacts all cells in body
 Progressive disability, early death
 Difficulty breathing, immune system impairment
 Sinus infection, failure to thrive, diarrhea
 1 in 3900 kids in US
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CF
1 in 22 of European descent are carriers
Failure to produce protein found in outer cell membrane
Channel membrane move Cl- into cell
+ ions attracted to – ion, don’t move into cell
Sweat glands, pancreas, lung
Cell Video
Inner workings of a cell