Plasma Membrane and Transport of
molecules
How do things get in and out of the
cell?
I. The Plasma Membrane
A. The fluid mosaic model describes the
structure of the plasma membrane.
B. In this model the membrane is seen as a
bilayer of phospholipids.
PHOSPHOLIPID BILAYER = CELL
MEMBRANE
Structure of the plasma membrane
Phospholipids with embedded proteins (proteins are
for transporting molecules across membrane)
2. Membrane lipids are phospholipids with
polar, water-soluble heads and long,
nonpolar, insoluble tails.
Polar
NonPolar
3 Types of Cellular Transport
1. Simple Diffusion
2. Facilitated Diffusion
3. Active Transport
1ST TYPE OF TRANSPORT
I. Simple Diffusion = NO ENERGY
REQUIRED
• Brownian motion is the continuous, random
motion of molecules.
• Most materials in and around any cell are in
H2O solution. Diffusion is the movement of
particles from areas of high concentration to
areas of low concentration. It is the result of
Brownian motion.
Brownian Motion is continuous motion.
When materials are evenly distributed in
H2O and no further changes in
concentration occur, dynamic equilibrium
exists.
• Dynamic Equilibrium= random movement
continues but there is no change in
concentration. (dynamic = change;
equilibrium = balance) Dynamic
equilibrium is a characteristic of
homeostasis in the cell.
Diffusion depends on concentration gradients.
• Concentration Gradients = difference in
concentration between two areas (ex: Inside of a
cell and outside of a cell).
• Ions and molecules automatically diffuse (move
through a membrane) from an area of high
concentration to an area of low concentration. This
means they move with the gradient.
• Diffusion across a membrane continues until
there is no concentration gradient. Dynamic
equilibrium then exists because the concentration is
the same on both sides of the membrane.
Figure 8.9 The diffusion of solutes across membranes
Selectivity of membrane- only H2O, oxygen, nitrogen,
carbon dioxide molecules, and a few other non-polar
molecules can diffuse directly across the plasma
membrane.
• Charged ions of polar molecules CANNOT automatically
diffuse across the plasma membrane.
Osmosis- Diffusion of water
Osmosis= a type of simple diffusion!
No net change in concentration in an isotonic
solution because the concentration of H2O is the
same on either side of the plasma membrane
(dynamic equilibrium). However, movement
continues (Brownian motion).
Figure 8.10 Osmosis
3 Types of Cellular Transport
1. Simple Diffusion  no energy required
• Osmosis = diffusion of water
– GOAL: reach dynamic equilibrium
2. Facilitated Diffusion
3. Active Transport
Figure 8.11 The water balance of living cells
Plasmolysis Video
Hypertonic, Isotonic, Hypotonic?
What kind of solution is optimal for plant
cells?
Animal cells?
What happens to a red blood cell in a
hypertonic solution?
ND
2
TYPE OF TRANSPORT
II. Facilitated Diffusion = Facilitated
diffusion- transport proteins embedded in the
plasma membrane transport ions and
molecules (that can’t get thru the membrane
on their own) into and out of the cell as
needed.
PASSIVE = NO ENERGY REQUIRED
Uses CHANNEL PROTEINS
Figure 8.13 One model for facilitated diffusion
3 Types of Cellular Transport
1. Simple Diffusion  no energy required
• Osmosis = diffusion of water
– GOAL: reach dynamic equilibrium
– WITH concentration gradient
2. Facilitated Diffusion  no energy required
– USES CHANNEL PROTEINS
– WITH concentration gradient
3. Active Transport
3rd TYPE OF TRANSPORT
III. Active transport- diffusion goes against the
concentration gradient meaning movement
from an area of low concentration to an area of
high concentration.
ENERGY REQUIRED! (ATP)
GOES AGAINST CONCENTRATION
GRADIENT
TRANSPORT PROTEINS
REQUIRED
From low to high
3 Types of Cellular Transport
1. Simple Diffusion  no energy required
• Osmosis = diffusion of water
– GOAL: reach dynamic equilibrium
– WITH concentration gradient (High to low conc.)
2. Facilitated Diffusion  no energy required
– USES channel PROTEINS
– WITH concentration gradient (High to low conc.)
3. Active Transport  ENERGY (ATP) required!
– USES transport PROTEINS
– AGAINST concentration gradient (Low to high conc.)
Active vs. Passive Transport
• Active = ATP req.
– Against concentration gradient
– Transport proteins required
– Ex: H+ ion pump
• Passive = No ATP rep.
– With concentration gradient
– Ex: simple diffusion, osmosis, facilitated
diffusion (req. channel proteins)
Figure 8.15 Review: passive and active transport compared
Transport – 2 Types
1. Passive (NO ENERGY REQUIRED):
A. Simple Diffusion (High to Low concentration)
A. Osmosis is diffusion of water
B. Facilitated Diffusion (Protein help/ facilitate
movement of particles from High to Low
concentration)
2. Active (ENERGY REQUIRED  ATP)
MOVING AGAINST THE CONCENTRATION
GRADIENT
C. Active Transport
1. Endocytosis
2. Exocytosis
Simple vs Facilitated Diffusion
• BOTH with concentration gradient
• BOTH passive
• FACILITATED = Channel proteins are
used
Active vs. Passive Transport
• Active = ATP req.
– Against concentration gradient
– Transport proteins required
– Ex: H+ ion pump
• Passive = No ATP rep.
– With concentration gradient
– Ex: simple diffusion, osmosis, facilitated
diffusion (req. channel proteins)
Plasmolysis Video
Hypertonic, Isotonic, Hypotonic?
What kind of solution is optimal for plant
cells?
Animal cells?
What happens to a red blood cell in a
hypertonic solution?
Figure 8.11 The water balance of living cells
Transport of large particles
2 Type of ACTIVE TRANSPORT: endo- and
exocytosis
Endocytosis- a cell surrounds material and
takes it in from its environment by
enclosing it in a newly formed vacuole.
Exocytosis- vacuole containing what the cell
needs to dump, merges with the plasma
membrane releasing the material outside the
cell.
Figure 8.18 The three types of endocytosis in animal cells
V. Diseases Associated with
Difficulties in Transport across
membranes.
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Diseases resulting from lack of functional
channels/pumps
Motor neuron problems -Na+ channel
Cystic fibrosis - Cl- channel
Bipolar disorder -Na+, K+, ATPase
Heart problems -Na+, K+, ATPase, Na+ channels
Resistance to chemotherapy - peptide transporter, pGlycoprotein, (Multi-Drug Resistance)
Color Blindness, H+ gradient as pump (rhodopsin)
Some Food Poisoning - Ca+ channel