1. Cell membrane

2. Aaaah, one of those structure–function
Phospholipids
Phosphate
“attracted to water”
Phosphate head
hydrophilic
Fatty acid tails
hydrophobic
Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah, one of those structure–function
examples

3. Arranged as a Phospholipid bilayer
Serves as a cellular barrier / border
sugar
H2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
lipids

8. Cell membrane defines cell
Cell membrane separates living cell from aqueous environment
thin barrier = 8nm thick
Controls traffic in & out of the cell
allows some substances to cross more easily than others
hydrophobic (nonpolar) vs. hydrophilic (polar)

9. Permeability to polar molecules?
Membrane becomes semi-permeable via protein channels
specific channels allow specific material across cell membrane
inside cell
H2O aa
sugar
salt
outside cell
NH3

10. Cell membrane is more than lipids…
Transmembrane proteins embedded in phospholipid bilayer
create semi-permeabe channels
lipid bilayer
membrane
protein channels in lipid bilyer membrane

11. Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme activity
Cell surface receptor
“Antigen”
Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.
Cell surface identity marker
Cell adhesion
Attachment to the cytoskeleton

12. Membrane Proteins Proteins determine membrane’s specific functions
cell membrane & organelle membranes each have unique collections of proteins
Classes of membrane proteins:
peripheral proteins
loosely bound to surface of membrane
ex: cell surface identity marker (antigens)
integral proteins
penetrate lipid bilayer, usually across whole membrane
transmembrane protein
ex: transport proteins
channels, permeases (pumps)

13. Filaments of cytoskeleton
Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Transmembrane
proteins
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
Phospholipids
Filaments of cytoskeleton
Cholesterol
Peripheral
protein
Cytoplasm
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

14. Membrane carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish one cell from another
antigens
important in organ & tissue development
basis for rejection of foreign cells by immune system
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

15. Movement across the Cell Membrane

16. Diffusion 2nd Law of Thermodynamics governs biological systems
universe tends towards disorder (entropy)
Movement from high concentration of that substance to low concentration of that substance.
Diffusion
movement from HIGH  LOW concentration

17. Simple Diffusion Move from HIGH to LOW concentration movement of water
“passive transport”
no energy needed
movement of water
diffusion
osmosis

18. Facilitated Diffusion
Diffusion through protein channels
channels move specific molecules across cell membrane
no energy needed
facilitated = with help
open channel = fast transport
HIGH
LOW
Donuts!
Each transport protein is specific as to the substances that it will translocate (move).
For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.
Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.
These channel proteins allow fast transport.
For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion.
“The Bouncer”

19. Getting through cell membrane
Passive Transport
Simple diffusion
diffusion of nonpolar, hydrophobic molecules
lipids
HIGH  LOW concentration gradient
Facilitated transport
diffusion of polar, hydrophilic molecules
through a protein channel
Active transport
diffusion against concentration gradient
LOW  HIGH
uses a protein pump
requires ATP
ATP

20. Transport summary simple diffusion facilitated diffusion
ATP
active transport

21. How about large molecules?
Moving large molecules into & out of cell
through vesicles & vacuoles
endocytosis
phagocytosis = “cellular eating”
pinocytosis = “cellular drinking”
exocytosis
exocytosis

22. Endocytosis fuse with lysosome for digestion phagocytosis
non-specific process
pinocytosis
triggered by molecular signal
receptor-mediated endocytosis

23. Osmosis is just diffusion of water
Water is very important to life, so we talk about water separately
Diffusion of water from HIGH concentration of water to LOW concentration of water
across a semi-permeable membrane

24. Concentration of water
Direction of osmosis is determined by comparing total solute concentrations
Hypertonic - more solute, less water
Hypotonic - less solute, more water
Isotonic - equal solute, equal water
hypotonic
hypertonic
water
net movement of water

25. Managing water balance
Cell survival depends on balancing water uptake & loss
freshwater
balanced
saltwater

26. Do you understand Osmosis…
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell