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Cell Membranes &

Movement Across Them

2006-2007

*  Cells need an inside & an outside… *  separate cell from its environment

* cell membrane is the boundary

Cell (plasma) membrane

IN  food    -­‐  sugars    -­‐  proteins    -­‐  fats  salts  O2  H2O  

OUT  waste    -­‐  ammonia    -­‐  salts    -­‐  CO2    -­‐  H2O    products    -­‐  proteins  

cell  needs  materials  in  &  products  or  waste  out  

*  How do you build a barrier that keeps the watery contents of the cell separate from the watery environment?

Building a membrane

What  substance  do  you  know  that  doesn’t  mix  with  water?  

→  FATS  ←  →  LIPIDS  ←    

Remember:  oil  &  water    don’t  mix!!  

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* Membrane is made of special kind of lipid *  phospholipids *  “split personality” * Membrane is a double layer *  phospholipid bilayer

Lipids of cell membrane

inside  cell  

outside  cell  

lipid  

“repelled  by  water”  

“attracted  to  water”  

phosphate  

The  Fluid  Mosaic  

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*  We  have  learned:  *  Proteins  and  substances  such  as  cholesterol  become  

embedded  in  the  bilayer,  giving  the  membrane  the  look  of  a  mosaic.    

*  Because  the  plasma  membrane  has  the  consistency  of  vegetable  oil  at  body  temperature,  the  proteins  and  other  substances  are  able  to  move  across  it.    

*  That’s  why  the  plasma  membrane  is  described  using  the  fluid-­‐mosaic  model.  

The  Fluid-­‐Mosaic  Model  

The  Fluid-­‐Mosaic  Model  

*  Channels are made of proteins *  proteins both “like” water & “like” lipids

How do you build a semi-permeable cell membrane?

bi-­‐lipid  membrane  

protein  channels  in  bi-­‐lipid  membrane  

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*  Cell membrane controls what gets in or out *  Need to allow some materials — but not all — to pass

through the membrane *  semi-permeable

*  only some material can get in or out

Semi-permeable membrane

aa   H2O  sugar   lipids   salt   waste  

So  what  needs  to  get  across  the  membrane?  

O2  

*  What molecules can get through the cell membrane directly? *  fats and oils can pass directly through

Crossing the cell membrane

inside  cell  

outside  cell  

lipid  salt  

aa   H2O  sugar  

waste  

but…  what  about  other  stuff?

*  Need to make “doors” through the membrane *  protein channels allow substances in & out

*  specific channels allow specific material in & out *  H2O channel, salt channel, sugar channel, etc.

Cell membrane channels

inside  cell  

outside  cell  

sugar  aa  H2O  

salt  waste  

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*  Proteins act as “doors” in the membrane *  channels to move specific molecules through cell

membrane

Protein channels

HIGH  

LOW  

*  Why do molecules move through membrane if you give them a channel?

Movement through the channel

?  

?  

HIGH  

LOW  

*  Diffusion *  move from HIGH to LOW concentration

Molecules move from high to low

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*  Move from HIGH to LOW concentration *  passive transport *  no energy needed

Diffusion

diffusion   osmosis  

diffusion  of  water  

*  Move from HIGH to LOW

Simple Diffusion

inside  cell  

outside  cell  

Which  way  will  fat  move?  

fat  

fat  

fat  

fat  

fat  

fat   fat  

fat  fat  

fat  

fat  

fat  

fat  

fat  

LOW  

HIGH  

*  Move from HIGH to LOW through a channel

Facilitated Diffusion

inside  cell  

outside  cell  

sugar  

sugar  

sugar  

sugar  

sugar  sugar  

sugar   sugar  

sugar  sugar  sugar  

Which  way  will  sugar  move?  

sugar  sugar  

LOW  

HIGH  

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*  Move from HIGH to LOW concentration *  directly through membrane

*  simple diffusion *  no energy needed

*  help through a protein channel *  facilitated diffusion (with help) *  no energy needed

Diffusion

HIGH  

LOW  

Simple vs. facilitated diffusion

inside  cell  

outside  cell  

lipid  inside  cell  

outside  cell  

H2O  

simple  diffusion   facilitated  diffusion  

H2O  

protein  channel  

Transport summary simple  diffusion  

facilitated  diffusion  

active  transport  

ATP  

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* Cells may need molecules to move against concentration “hill” *  need to pump “uphill”

*  from LOW to HIGH using energy *  protein pump *  requires energy * ATP

Active transport

ATP  

* Cells may need molecules to move against concentration “hill” *  need to pump “uphill”

*  from LOW to HIGH using energy *  protein pump *  requires energy * ATP

Active transport

ATP  

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*  What  if  we  want  to  get  something  large  inside  or  out  of  the  cell?  *  endocytosis  is  the  engulfing  of  substances  outside  of  

the  cell,  forming  a  vesicle  that  brings  the  substances  inside  the  cell    

*  exocytosis  is  the  discharge  of  substances  from  vesicles  at  the  cell  surface    

Passage  into  and  out  of  Cells    

*  Phagocytosis  is  endocytosis  of  particulate  (solid)  matter    

     *  Pinocytosis  is  endocytosis  of  liquid  matter    

Forms  of  Endocytosis  

*  What  organelle  did  we  create?  *  Vesicles  

*  What  organelle  would  be  used  for  this?  *  Vesicles  

Exocytosis  

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*  Transport  of  specific  molecules  into  the  cell  involves  receptor-­‐mediated  endocytosis    *  molecules  to  be  transported  must  first  bind  to  specific  

receptors  in  the  plasma  membrane    

Receptor-­‐mediated  endocytosis  

Receptor-­‐mediated  endocytosis    

*  With  your  tables  discuss  the  following  demonstration.  

*  What  is  happening?  *  Why  does  this  happen?  *  What  is  the  process  called?  

Water  +  Food  Coloring  

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What  have  we  learned?  

Osmosis Movement of Water Across

Cell Membrane

2006-2007

*  Water is very important, so we talk about water separately *  Osmosis *  diffusion of water from HIGH concentration of water to LOW

concentration of water *  across a semi-permeable membrane

Osmosis

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*  Cell survival depends on balancing water uptake & water loss

Keeping water balance

freshwater balanced saltwater

* Freshwater *  a cell in fresh water *  high concentration of water around

cell *  cell gains water *  example: Paramecium *  problem: cells gain water,

swell & can burst *  water continually enters Paramecium cell

*  solution: contractile vacuole *  pumps water out of cell

Keeping right amount of water in cell freshwater

No problem, here

KABOOM!

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* Saltwater *  a cell in salt water *  low concentration of water around cell *  cell loses water

*  example: shellfish *  problem: cell loses water *  in plants: plasmolysis *  in animals: shrinking cell

*  solution: take up water

Keeping right amount of water in cell saltwater

I will survive!

I’m shrinking, I’m shrinking!

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Controlling water § Contractile vacuole in Paramecium

*  Contractile vacuole in Paramecium

Controlling water

http://www.youtube.com/watch?v=9Ynm5ZOW59Q

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Introduction  of  Hypertonic  Solution  

* Balanced conditions *  no difference in concentration of water

between cell & environment * cell in equilibrium

* example: blood * problem: none * water flows across membrane

equally, in both directions * volume of cell doesn’t change

Keeping right amount of water in cell balanced

I could be better…

That’s better!

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Ice Fishing in Barrow

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