CellMembraneNotesNonAP

The Cell Membrane – Structure and Function : 

2007-2008 The Cell Membrane – Structure and Function

Overview: 

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 Made of phospholipids , proteins & other macromolecules

Phospholipid bilayer: 

Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads

More than lipids… : 

More than lipids… In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model !

Membrane lipids – What do they do?: 

Membrane lipids – What do they do? Fat composition affects flexibility membrane must be fluid & flexible about as fluid as thick salad oil unsaturated fatty acids keep membrane less viscous cholesterol in membrane Stabilizes the membrane

Membrane Proteins – What do they do?: 

Membrane Proteins – What do they do? Membrane proteins: peripheral proteins on surface of membrane cell surface identity marker ( antigens ) integral proteins across whole membrane transport proteins channels, pumps

Many Functions of Membrane Proteins: 

Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Cell surface receptor Enzyme activity Cell surface identity marker Attachment to the cytoskeleton Cell adhesion

Membrane carbohydrates – What do they do? : 

Membrane carbohydrates – What do they do? Play a key role in cell-cell recognition One cell recognizes another antigens basis for rejection of foreign cells by immune system

Diffusion across cell membrane: 

Diffusion across cell membrane Cell membrane is the boundary between inside & outside… separates cell from its environment IN food carbohydrates sugars, proteins amino acids lipids salts, O 2 , H 2 O OUT waste ammonia salts CO 2 H 2 O products cell needs materials in & products or waste out IN OUT Can it be an impenetrable boundary? NO!

Diffusion: 

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

Diffusion through phospholipid bilayer: 

Diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids inside cell outside cell lipid salt aa H 2 O sugar NH 3 What molecules can NOT get through directly? polar molecules H 2 O ions salts, ammonia large molecules starches, proteins

Facilitated Diffusion: 

Facilitated Diffusion Diffusion through protein channels channels move specific molecules across cell membrane no energy needed “The Bouncer” open channel = fast transport facilitated = with help high low

Active Transport: 

Active Transport “The Doorman” conformational change Cells may need to move molecules against concentration gradient shape change moves molecule from one side of membrane to other protein “pump” “costs” energy = ATP ATP low high

Getting through cell membrane: 

Getting through cell membrane Passive Transport Simple diffusion movement of nonpolar (noncharged), hydrophobic molecules lipids high  low concentration gradient Facilitated transport movement of polar (charged), hydrophilic molecules through a protein channel high  low concentration gradient Active transport diffusion against concentration gradient low  high uses a protein pump requires ATP ATP

How about large molecules?: 

How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles Endocytosis = Taking material in phagocytosis = “cellular eating” pinocytosis = “cellular drinking” Exocytosis = Pushing material out exocytosis

Osmosis is diffusion of water: 

Osmosis is 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

Concentration of water: 

Concentration of water Direction of osmosis is determined by comparing solute (dissolved molecule) concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water hypotonic hypertonic water net movement of water

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 .05 M .03 M Osmosis…

Managing water balance: 

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

Slide 20: 

Any Questions??