Cell Membrane Structure
and Function
Goal: To be able to describe how the
structure of the cell membrane
regulates the flow of materials
into/out of the cell.
Anatomy of a Cell Membrane
Phospholipid, Hydrophilic, hydrophobic, polar, nonpolar, phosphate, glycoprotein, protein
“Selectively Permeable”
• perme = “allow”
• A characteristic of cell membranes that allows it to regulate
the passage of molecules
• Selective permeability depends on the structure of the
membrane
• Not to be confused with semi-permeability
– Dialysis tubing is semi-permeable. What characteristic allows
molecules to travel through the membrane?
• Our cell membranes are “selective” due to:
1.
Proteins embedded within membrane
• decide what can pass: big, small, necessary, unnecessary, charged, etc.
2.
Properties of the phospholipids that make up the layer
•
Nonpolar, hydrophobic
Membrane Proteins
Some molecules may need “help” (of a protein) to get
through the cell membrane, so require help from
transport proteins:
– Large molecules
– Charged molecules or ions (Na+ or Cl- )
– Molecules that may be traveling up their concentration
gradient
Cross-Section of the Cell Membrane
Which molecules will allowed through the lipid bilayer
and which ones may need the help of a protein?
H2O, CO2, Glucose (C6H12O6), Starch (lots of glucoses), Potassium (K+), estrogen, Na+
Why or why not?
Video Clip: Cell Membranes
Recap: Cell Membranes
Characteristics:
– Selectively Permeable
• Regulates what enters/exits cell
• Helps transport certain molecules
across with help of proteins
– “Bilayered”
• Hydrophobic
• Hydrophilic
– Embedded proteins (different
functions)
• Cell identification
• Enzymes
• Transport of molecules
• Etc!
Interchangeable terms
• Cell Membrane
• Phospholipid Bilayer
• Lipid Bilayer
• Plasma Membrane
• Fluid Mosaic Model
Types of Molecular Movement
• Diffusion
• Passive Transport
– Simple diffusion
– Facilitated diffusion
– Osmosis
• Active Transport
high to low concentration
high to low concentration
high to low concentration
high to low concentration
low to high concentration
Passive Transport
Diffusion of solutes across a membrane
Simple Diffusion
Facilitated Diffusion
• Molecules can easily pass
through the membrane
• Molecule characteristics allow it
to pass hydrophobic part of cell
membrane
•
•
•
Diffusion = high  low concentration
•
Molecules have to pass through
membrane proteins to cross
membrane
Molecule characteristics do not allow
it to pass through hydrophobic part
of cell membrane
Diffusion = high  low concentration
Active Transport
• Molecules pass through a protein to cross membrane
• Not diffusion because active implies movement of solutes
against their concentration gradient (i.e., low  high)
• Being “active” requires energy! ATP
• Requires proteins or “pumps” to transport molecules
across the membrane.
Video Clip: Active Transport
In Summary: Active Transport
• Transport through a
membrane
• Requires energy (in the
form of ATP)
• Molecules move from low
to high concentration
With your group, organize the following terms into the
T-chart below and justify their placement there.
facilitated diffusion
high  low concentration
low  high concentration
Passive Transport
protein
osmosis
requires a membrane
simple diffusion
Energy
ATP used
Active Transport
Your Tasks
• Collect Day 2 data
• Clean Up:
– Solutions down sink
– Cups rinsed and brought to cart up front
– All containers rinsed and drying upside down
Homework due Monday
– Analysis Questions
Homework due Tuesday?
• Learn About Cell Membranes! Worksheet
• Interested in extra credit?
– Learn about how soap works! Research micelle formation
and explain, in words and using a few diagrams, how soap
works and how this relates to what we learned today
about the phospholipid bilayer. Submit typed response
with diagrams tomorrow.