The Plasma
Membrane and
Transport
Since nearly all cell organelles are composed
of membranes, we will refer to the cell’s
surface as the plasma membrane.
Fluid Mosaic Model
Figure 3.2
Fluid Mosaic Model Animation
Fluid Mosaic Components of the Cell
•
•
•
•
Phospolipid bilayer - two lipid (fat) layers arranged
“tail to tail”; hydrophilic “heads” and hydrophobic “tails”
Proteins - integral and peripheral; may be enzymes,
receptors for hormones, involved in cell recognition,
cell adhesion, cytoskeleton attachment, transport, or
generating action potentials
Cholesterol - gives stability to membrane
Glycocalax - “sugar coating”; glycoproteins,
glycolipids; “self” recognition, determine blood type,
immunity to infection, cancer defense, cell-to-cell
interactions, guides embryonic cells
Specializations of Plasma
Membrane
• Microvilli
- tiny
finger-like
projections that
increase surface
area (absorption in
small intestine)
• Membrane
Junctions
Membrane Junctions
•
•
•
Tight junction – impermeable, “leakproof” fusion of
plasma membranes; i.e. between small intestine
Desmosome – anchoring “buttonlike” junction,
prevents cells from being pulled apart; i.e. between
skin cells
Gap junction – a channel of connexons that allows
chemical substances to pass between cells; i.e. in
heart and embryonic cells
Tight Junction
Desmosome
Gap Junction
Membrane Transport
Passive Membrane Transport
• Driven by kinetic energy of solutes, no ATP required
• Diffusion
• Facilitated diffusion
• Osmosis
• Lipid soluble =substances dissolve in lipids
• Water soluble =substances dissolve in water
• Water soluble substances will not dissolve in lipids and vise-versa
• Solute - what is being dissolved
• Solvent - what is doing the dissolving
•
Simple Diffusion
Particles move from an area of high concentration to an area of
low concentration (or down their concentration gradient)
Diffusion
•
Particles diffuse directly through
the plasma membrane
•
•
Lipid-soluble
Some must diffuse through
channel proteins
•
Lipid-insoluble
Figure 3.6a
Facilitated Diffusion
•
Facilitated diffusion – large,
polar molecules such as
simple sugars
•
Combine with protein
carriers (channel proteins)
Figure 3.6b
Facilitated Diffusion
Diffusion - Osmosis
•
•
Diffusion of water across a semipermeable
membrane
Osmolarity – total concentration of solute particles
in a solution
Effect of Membrane Permeability on Diffusion
and Osmosis
Figure 3.7a
Effect of Membrane Permeability on Diffusion
and Osmosis
Figure 3.7b
Filtration
•
•
The passage of water and solutes through a
membrane by hydrostatic pressure
Pressure gradient pushes solute-containing fluid
from a higher-pressure area to a lower-pressure
area (i.e. in the capillaries of the kidneys)
Tonicity
•
•
•
Isotonic – solutions with the same solute concentration as
that of the cytosol
Hypertonic – solutions having greater solute concentration
than that of the cytosol
Hypotonic – solutions having lesser solute concentration than
that of the cytosol
What would happen to the animal
cells in each beaker?
80% H2O
80% H2O
80% H2O
100% Distilled
Water
80% Water
20% Dissolved
Substances
70% Water
30% Dissolved
Substances
Which way will the water move?
80% H2O
100% Distilled
Water
Hypotonic Solution
Why did the cell
get so big?
Which way will the water move?
80% H2O
80% Water
20% Dissolved
Substances
Isotonic Solution
Why did the cell
stay the same
size?
Which way will the water move?
80% H2O
70% Water
30% Dissolved
Substances
Hypertonic Solution
Why did the cell
get so small?
Active Transport
Active Transport
Active Transport
•
•
•
Uses ATP to move solutes across a membrane against
its concentration gradient
Requires carrier proteins
Example: Sodium-Potassium Pump - 3 Na+ out, 2 K+
in; phosphorylation opens pump to outside;
dephosphorylation opens pump to inside
Sodium-Potassium Pump
Animation
Figure 3.9
Figure 3.10
Vesicular or Bulk
Transport
• Transport of large particles and macromolecules
across plasma membranes
•
•
•
Two types:
Exocytosis - “out of the cell”
Endocytosis - “into the cell”
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•
Endocytosis
3 types of endocytosis:
(1) Phagocytosis “cell eating”– pseudopods engulf solids and
bring them into the cell’s interior
Bacterial Phagocytosis by
Macrophage
Vesicular Transport
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•
(2) Bulk-phase endocytosis (formerly pinocytosis “cell drinking”)–
the plasma membrane infolds, bringing extracellular fluid and
solutes into the interior of the cell
(3) Receptor-mediated endocytosis – uses clathrin-coated pits
(clathrin is a protein) for specific uptake of macromolecules
Figure 3.12
•
Examples in Human Body
Hyponatremia
An abnormally low concentration of
sodium in the blood. Too little sodium
can cause cells to malfunction, and can
be fatal.
Symptoms: abnormal mental status,
brain swelling, convulsions, headache,
nausea, muscle spasms or cramps
Causes: burns, diarrhea, diuretics,
kidney disease, sweating, vomiting,
drinking too much water
•
•
•
Examples in Human Body
•Tetrodotoxin
•a potent neurotoxin with no known antidote
(pufferfish, porcupine fish, ocean sunfish,
triggerfish) blocks action potentials in nerves by
binding to sodium channels in nerve cell
membranes, preventing any affected nerve cells
from firing; paralysis and death within 20 min to 8
hours after ingestion