2. Membrane Structure and Function
Cells must contain a cell membrane, cytoplasm
and genetic material.
The cell membrane is the EDGE, “boundary of
life”, while the cytoplasm is the site of all the
reactions of life and the genetic material is the
information required for life.
SELECTIVELY PERMEABLE means that the cell
membrane allows some substances across more
easily than others… some it helps and some it
inhibits or rejects all together.
3. Molecules need to move across
membranes in cells
Image modiified from: http://www.accessexcellence.org/AB/GG/importProt.html
IN
food
carbohydrates
sugars,
proteins
amino acids
lipids
salts, O2, H2O
OUT
waste
ammonia
salts
CO2
H2O
products
4. Membrane Structure
Development of models as a scientific
process
Models include
Overton – proposed lipid base
Langmuir (phospholipids) – made membrane
Gorter and Grendel (two layers)
Davson and Danielli (sandwich image, protein outside)
Singer and Nicolson = Fluid Mosaic Model
Required electron microscopy
1972
5. MEMBRANE MODELS
1915: Scientists analyzed isolated red blood cell
membranes, found them made up of proteins and lipids
10 years later: E. Gorter and F. Grendel reasoned
membranes are made of a phospholipid bilayer
1935: Hugh Davson and James Danielli
proposed membrane was a “sandwich”
of proteins and phospholipid bilayer
with proteins on outside of phopholipids
DAVSON-DANIELLI
MODEL
6. FLUID MOSAIC MODEL
1972- S.J. Singer and G. Nicolson propose membrane is a
“mosaic” of proteins and phospholipids that are constantly
moving and changing
Animation from: http://www.sp.uconn.edu/~terry/images/anim/fluidmem.gif
7.
8. Membrane Structure
Phospholipids – phosphate and fatty acid tails
Hydrophilic phosphates
Hydrophobic fatty acid tails
Two layers
With tails together
Protein
Embedded throughout - integral
Some just surface – peripheral
Motility
Phospholipid
9.
10.
11. Idea of Fluidity
Cholesterol molecules prevent membrane
from becoming ‘crispy’
Proteins ‘bob’ along surface
Proteins are anchored by fibers in the
cytoskeleton
Phospholipids can even flip flop
Merging of membranes.
12.
13. Membrane Proteins
Proteins determine most of membrane’s specific
functions
cell membrane & organelle membranes each have
unique collections of proteins
Membrane proteins:
peripheral proteins = loosely
bound to surface of membrane
integral proteins = penetrate into
lipid bilayer, often completely
spanning the membrane =
transmembrane protein
14. Membrane Carbohydrates
Attached to proteins (glycoproteins)
or lipids (glycolipids)
Play a key role in cell-cell recognition
ability of a cell to distinguish neighboring cells from
another
important in organ &
tissue development
basis for rejection of
foreign cells by
immune system
17. Other organelles
The cell membrane is
continuous with other
membranes in the cell
Endoplasmic reticulum (ER)
starts/ends at cell membrane
ER makes cell membrane & it
is turned inside out as the
new cell membrane flows out
of the ER
18. Traffic Across the Membrane
Small particles and ions move in both directions;
sugars, amino acids, CO2, NH4 and O2. Also Na+, K+,
Ca+2 and Cl-
Hydrophobic molecules ( like CO2) can dissolve in lipid
layer
Hydrophilic core of cell membrane impedes movement
of ions and polar molecules that are hydrophilic.
Proteins built into membrane facilitate the process
19.
20. Transport Proteins
Span the Cell membrane
Function as a channel
Some also physically move
passengers
Specificity of fit allows for membrane
selectivity.
21. Passive Transport
Molecules have intrinsic kinetic energy
They are in constant motion.
Cytoplasm, extracellular fluid and cell membrane vary
in concentrations and pressure gradients.
Concentration refers to the overall POPULATION of
molecules as well as the RATIO in that location
compared to another.
DYNAMIC EQUILIBRIUM
22. Passive transport includes
Diffusion
Tendency of molecules of any substance to spread out
into the available space. Substances will diffuse
DOWN their concentration gradient.
Osmosis
The diffusion of water molecules across a selectively
permeable membrane.
Hypertonic = solution with higher [conc.] of solutes
Hypotonic = solution with lower [conc.] of solutes
Isotonic = solutions are equal in solute concentration
Facilitative Diffusion
Transport proteins are helping molecules to cross
membrane, but still diffusion (lowering overall free
energy) thus doesn’t require energy from cell.
23. 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
24. Facilitated Diffusion
Carrier Proteins
Grab molecule,
undergo conformational
change, flip to other side
Channel Proteins
Create passageway for
substances to pass
through
May be gated or not:
open in response to
chemical or electrical
signals
25. Active Transport
Movement of solutes, UP
their concentration gradient,
with the help of transport
proteins in a CM.
Requires cell to expend
energy / do work.
Solute becomes more
concentrated or
“stock- piled” in a particular
area.
Example is Sodium-Potassium
Pump. (Na-K)
26.
27. Electrogenic Pumps
In addition to moving ions, some active transport
pump also generate electric current and/ ATP.
An enzyme called ATP
synthase is built into the
membrane and ADP is
phosphorylated as a result
of ions (often H+) passing
through the membrane.
Examples are found in the
Electron Transport Chains
of respiration (inner
mitochondrial membrane)
and photosynthesis
(chloroplasts thylakoid
membrane)
29. EXOCYTOSIS
• Active transport (requires ATP)
• Uses vesicles
• Releases substances to outside
INSULIN being released by pancreas cells using exocytosis