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The Cell Membrane.

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Presentation on theme: "The Cell Membrane."— Presentation transcript:

1 The Cell Membrane

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

4 Cell membrane defines cell
Cell membrane separates cell from aqueous environment Thin = 8nm thick Controls transport in & out of the cell Some substances cross more easily than others hydrophobic (nonpolar) vs. hydrophilic (polar) Small vs. big.

5 Cell membrane must be 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!

6 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

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

8 Why are proteins the perfect molecule to build structures in the cell membrane?

9 nonpolar & hydrophobic
Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic

10 I like the polar ones the best!
Classes of amino acids What do these amino acids have in common? I like the polar ones the best! polar & hydrophilic

11 Protein domains anchor molecule
Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane in fluid polar amino acids hydrophilic extend into extracellular fluid & into cytosol Polar areas of protein Nonpolar areas of protein

12 H+ Examples NH2 H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic) a-helices in the cell membrane aquaporin = water channel in bacteria Porin monomer b-pleated sheets Bacterial outer membrane H2O H+ proton pump channel in photosynthetic bacteria function through conformational change = protein changes shape H2O

13 Many Functions of Membrane Proteins
Outside Plasma membrane Inside “Channel” Transporter Enzyme activity Cell surface receptor 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. “Antigen” Cell surface identity marker Cell adhesion Attachment to the cytoskeleton

14 Membrane Proteins Proteins determine membrane’s specific functions
Every membrane in a cell has a unique collection 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, across whole membrane “transmembrane” protein ex: transport proteins channels, permeases (pumps)

15 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

16 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.

17 Cell Membranes are not static!!
Watch this

18 Any Questions??

19 Movement across the Cell Membrane (Ch. 7)

20 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

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

22 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”

23 conformational change
Active Transport Cells may need to move molecules against concentration gradient conformational shape change transports solute from one side of membrane to other protein “pump” “costs” energy = ATP conformational change LOW HIGH ATP Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport. “The Doorman”

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

25 Transport summary simple diffusion facilitated diffusion
ATP active transport

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

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

28 About Osmosis 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

29 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

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

31 Managing water balance
1 Managing water balance Hypotonic a cell in fresh water high concentration of water around cell problem: cell gains water, swells & can burst example: Paramecium solution: contractile vacuole pumps water out of cell Uses ATP plant cells turgid = full cell wall protects from bursting KABOOM! No problem, here ATP freshwater

32 Pumping water out Contractile vacuole in Paramecium ATP

33 Managing water balance
2 Managing water balance Hypertonic a cell in salt water low concentration of water around cell problem: cell loses water & can die example: shellfish solution: take up water or pump out salt plant cells plasmolysis = wilt can recover I’m shrinking, I’m shrinking! I will survive! saltwater

34 Managing water balance
3 Managing water balance Isotonic animal cell immersed in mild salt solution no difference in concentration of water between cell & environment problem: none no net movement of water cell in equilibrium volume of cell is stable example: blood cells in blood plasma slightly salty IV solution in hospital That’s perfect! I could be better… balanced

35 Aquaporins 1991 | 2003 Water moves rapidly into & out of cells
evidence that there were water channels protein channels allowing flow of water across cell membrane Peter Agre John Hopkins Roderick MacKinnon Rockefeller

36 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

37 Any Questions??

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