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Lecture 3 Outline (Ch. 7)
I.
Membrane Structure
II.
Membrane Proteins
III. Permeability
IV. Transport Across Membranes
A. Passive
B. Facilitated
C. Active
D. Bulk
V. Lecture Concepts
Membrane structure
1915, knew membrane made of lipids and proteins
• Reasoned that membrane = bilayer
Where to place proteins?
Lipid layer 1
Proteins
Lipid layer 2
Membrane structure
Experiment to determine membrane fluidity:
• marked membrane proteins mixed in hybrid cell
Membrane structure
Membrane fluidity
• phospholipid f.a. “tails”: saturation affects fluidity
• cholesterol buffers
temperature changes
Membrane structure
“fluid mosaic model” – 1970s
• fluid – phospholipids move around
• mosaic – proteins embedded in membrane
Membrane structure
• freeze fracture
• proteins intact,
one layer or other
• two layers look different
Membrane structure
• cell membrane – amphipathic - hydrophilic & hydrophobic
hydrophilic
hydrophobic
hydrophilic
• membrane proteins inserted, also amphipathic
Membrane Proteins
Membrane proteins:
Integral: inserted in membrane
- transmembrane – span mem.
Peripheral: next to membrane
- inside or outside
Membrane Proteins
Selectively Permeable Membranes
Cell membranes only allow some molecules across w/out help:
• Small, non-polar
molecules OK
ex. hydrocarbons,
O2, CO2
• No charged, polar,
or large molecules
ex. sugars, ions,
water*
Transport Across Membranes
Types of transport:
A. Passive transport
B. Facilitated diffusion
C. Active transport
D. Bulk transport
• Energy Required?
• Directionality?
Passive Transport - Simple Diffusion
• NO ENERGY required
• DOWN concentration gradient
• molecules equally
distribute across
available area
- non-polar molecules
(hydrocarbons, O2, CO2)
Passive Transport - Osmosis
• osmosis – movement of water across cell membrane
• water moves via
special channels
• moves into/out of cell
until solute concentration
is balanced
Passive Transport - Osmosis
• tonicity – # solutes in solution in relation to cell
- hypotonic – fewer
solutes in solution
- isotonic – equal
solutes in solution
animal cell
- hypertonic – more
solutes in solution
plant cell
Passive Transport - Osmosis
Paramecium example
• regulate water balance
• pond water hypotonic
• water into contractile
vacuole
– water
expelled
Facilitated Diffusion
• NO ENERGY required
• DOWN concentration gradient
- Large, charged, polar
(sugar, ions, water)
• Use transport proteins - channel or carrier proteins
Active Transport
• ENERGY IS required
- Usually ions or large molecules
(Na+, K+, glucose)
• UP/AGAINST concentration gradient
• transport carrier proteins
a. ion pumps
b. co-transporters
• Ex. Na-K ion pump
- Na+ ions: inside to out
- K+ ions: outside to in
- net: high Na+
outside, high K+
inside cell
Active Transport
• Ex. proton (H+) pump
• ATP used pump H+ ions out
• against concentration and charge gradients
*gradients – used by cell for energy potential
Bulk Transport
• ENERGY IS required
• Several or large molecules
• Molecules moved IN
- endocytosis
• phagocytosis – “food” in
• pinocytosis – water in
• receptor-mediated
endocytosis
– proteins bind molecules,
vesicles inside
• Molecules moved OUT
- exocytosis
Self-Check
Type of
transport
Energy
required?
Movement
direction?
Examples:
Simple diffusion
no
Down conc. gradient
O2, CO2, nonpolar molecules
Osmosis
Facilitated
diffusion
Active transport
Bulk transport
Lecture 3 concepts
-
Describe the ‘fluid mosaic model’ including meaning and
experimental evidence
-
List and describe types of membrane proteins
-
Define amphipathic & explain application to cell membranes
-
Discuss what is meant by a selectively permeable membrane and
how this applies to cell membranes
-
Name types of transport across cell membranes and describe
each one
-
Given a tonicity of solution, determine direction of water
movement and hypothesize result on a cell
-
Write out a list of new terminology and provide descriptions