The Cell Membrane
Usman Sumo Friend Tambunan Arli Aditya Parikesit
Bioinformatics Group
Overview
•
Cell membrane separates living cell from
nonliving surroundings
•
thin barrier = 8nm thick•
Controls traffic in & out of the cell
•
selectively permeable•
allows some substances to cross more easily than others• hydrophobic vs hydrophilic
Phospholipids
•
Fatty acid tails• hydrophobic
•
Phosphate group head• hydrophilic
•
Arranged as a bilayerMore than lipids…
•
In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into theMembrane is a collage of proteins & other
molecules embedded in the fluid matrix of the
lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembrane proteins
Filaments of cytoskeleton Peripheral
protein Glycoprotein
Membrane fat composition varies
•
Fat composition affects flexibility• membrane must be fluid & flexible
• about as fluid as thick salad oil
• % unsaturated fatty acids in phospholipids
• keep membrane less viscous
• cold-adapted organisms, like winter wheat
• increase % in autumn
Membrane Proteins
•
Proteins determine membrane’s specific
functions
•
cell membrane & organelle membranes each have unique collections of proteins•
Membrane proteins:
•
peripheral proteins• loosely bound to surface of membrane
• cell surface identity marker (antigens)
•
integral proteinsProteins domains anchor molecule
•
Within membrane• nonpolar amino acids • hydrophobic
• anchors protein into membrane
•
On outer surfaces of membrane• polar amino acids
• hydrophilic
• extend into extracellular fluid & into cytosol
Polar areas of protein
NH2
a-helices in the
cell membrane H+
Porin monomer
b-pleated sheets
Bacterial outer
membrane
proton pump channel
in photosynthetic bacteria
water channel
in bacteria
function through
conformational change
=
shape change
Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter Cell surface
receptor Enzyme
activity
Cell surface
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
The Cell Membrane
Cell Membrane
•
Phospholipid bilayer: hydrophilic heads and
hydrophobic tails
•
Semi-permeability
•
Transport proteins (passive transport channels)
•
Ion pumps (active transport pumps)
•
Receptor proteins (neurons, hormones, immune
Passive Transport
Passive Transport
•
In Passive Transport - Molecules move down the concentration gradient (no energy required)•
1) simple diffusion -molecules are small enough and soluble can pass directly through the lipid bilayer•
2) facilitated diffusion – need transport proteins (molecules are either to large or can’t pass through the lipid bilayerthemselves)
•
3) osmosis – molecules can’t pass through lipid bilayer at all, but water can(movement of water)Passive Diffusion
•
http://www.youtube.com/watch?v=JShwXBWGMyYSolutions
•
A mixture of solid particles (solute) and liquid (solvent)•
Most common solvent is water•
Cells are surrounded by solutionsIsotonic Solutions
•
Particles are equal inside and outside of the cellHypotonic Solutions
•
There are more particles inside than outside•
There is more water outside the cellHypertonic Solutions
•
There are more particles outside•
There is more water insideActive Transport
•
Molecules move against the concentration gradient•
Requires energy•
1. Solute Pumps (Na+-K+-ATPase pump)•
Na+-K+-ATPase pumpTransport Across The Membrane
•
Endocytosis (pinocytosis, phagocytosis): phagocytes (macrophages)- molecules are coming into cells•
Exocytosis: release of proteins, hormones,neurotransmitters- release of proteins, hormones, neurotransmitters
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
How about large molecules?
•
Moving large molecules into & out of cell• through vesicles & vacuoles
• endocytosis
• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
• exocytosis
Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with
lysosome for digestion
non-specific process
The Special Case of Water
Movement of water across
the cell membrane
Osmosis is diffusion of water
•
Water is very important to life, so we talk about water separately•
Diffusion of water fromhigh concentration of water to
low concentration of water
• across a
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
freshwater balanced saltwater
Managing water balance
Managing water balance
•
Isotonic•
animal cell immersed in
mild salt solution
• example:
blood cells in blood plasma
• problem: none
•
no net movement of water • flows across membrane equally, inboth directions
•
volume of cell is stableManaging water balance
•
Hypotonic•
a cell in fresh water
• example: Paramecium
• problem: gains water, swells & can burst
• water continually enters
Paramecium cell
• solution: contractile vacuole
• pumps water out of cell
• ATP
•
plant cells
• turgid freshwater
Water regulation
•
Contractile vacuole in ParameciumManaging water balance
•
Hypertonic• a cell in salt water
• example: shellfish
• problem: lose water & die
• solution: take up water or pump out salt
• plant cells
• plasmolysis = wilt
Aquaporins
•
Water moves rapidly into & out of cells• evidence that there were water channels
1991 | 2003
Peter Agre
John Hopkins
Roderick MacKinnon
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
.05 M .03 M
References
•
http://bioap.wikispaces.com/file/view/Ch%207%20Cell%20Membrane.ppt
•
http://www.xavierhs.org/s/717/images/editor_documents/faculty/tanzmanj/Ce
