Cell Membranes

1
Cell Membranes Movement Across Them
2
Cell (plasma) membrane

• Cells need an inside an outside
• separate cell from its environment
• cell membrane is the boundary

IN food sugars proteins fats salts O2 H2O
OUT waste ammonia salts CO2 H2O products
cell needs materials in products or waste out
3
Building a membrane

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

What substance do you know that doesnt mix with
water?

• ? FATS ?
• ? LIPIDS ?
• oil water dont mix!!

4
Lipids of cell membrane

• Membrane is made of phospholipids
• phospholipid bilayer

phosphate
attracted to water
lipid
repelled by water
5
Semi-permeable membrane

• Need to allow passage through the membrane
• But need to control what gets in or out
• membrane needs to be semi-permeable

So what needs to get across the membrane?
aa
H2O
sugar
lipid
salt
NH3
So how do you build a semi-permeable membrane?
6
Phospholipid bilayer

• What molecules can get through directly?

fats other lipids can slip directly through the
phospholipid cell membrane, but what about
other stuff?
lipid
salt
waste
aa
H2O
sugar
7
Permeable cell membrane

• Need to allow more material through
• membrane needs to be permeable to
• all materials a cell needs to bring in
• all waste a cell needs excrete out
• all products a cell needs to export out

inside cell
lipid
sugar
aa
H2O
holes, or channels, in cell membrane allow
material in out
salt
waste
outside cell
8
Semi-permeable cell membrane

• But the cell still needs control
• membrane needs to be semi-permeable
• specific channels allow specific material in
  out

inside cell
sugar
aa
H2O
salt
waste
outside cell
9
How do you build a semi-permeable cell membrane?

• channels are made of proteins
• proteins both like water like lipids

bi-lipid membrane
protein channelsin bi-lipid membrane
10
Protein channels

• Proteins act as doors in the membrane
• channels to move specific molecules through cell
  membrane

11
Movement through the channel

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

?
?
12
Molecules move from high to low

• Diffusion
• move from HIGH to LOW concentration

13
Diffusion

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

14
Simple Diffusion

• Move from HIGH to LOW

fat
fat
fat
Which way will fat move?
inside cell
fat
fat
fat
fat
outside cell
fat
fat
fat
fat
fat
fat
fat
15
Diffusion through a channel

• Move from HIGH to LOW

sugar
sugar
sugar
sugar
inside cell
sugar
sugar
low
Which way will sugar move?
?
high
outside cell
sugar
sugar
sugar
sugar
sugar
sugar
sugar
16
Diffusion

• Move from HIGH to LOW concentration
• through membrane
• simple diffusion
• no energy needed
• through a protein channel
• facilitated diffusion (with help)
• no energy needed

high
low
17
Active transport

• Cells may need molecules to move against
  concentration difference
• need to pump uphill
• from LOW to HIGH using ATP
• protein pump
• requires energy
• ATP

ATP
18
Transport summary
diffusion
facilitateddiffusion
ATP
activetransport
19
OsmosisMovement of Water Across Cell Membrane
20
Osmosis

• 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

21
Keeping water balance

• Cell survival depends on balancing water uptake
  water loss

22
Managing water balance

• Balanced conditions
• no difference between cell environment
• example blood
• problem none
• water flows across membrane equally, in both
  directions
• volume of cell doesnt change

balanced
23
Managing water balance

• Freshwater
• 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

freshwater
24
Controlling water

• Contractile vacuole in Paramecium

25
Managing water balance

• Saltwater
• a cell in salt water
• example shellfish
• problem lose water
• plasmolysis in plants
• shrinking cell
• solution take up water

saltwater
26
Ice Fishing in Barrow
27
Any Questions??