Cell Membranes

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Cell Membranes Movement Across Them
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Cell (plasma) membrane

• 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
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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!!

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Lipids of cell membrane

• Membrane is made of phospholipids
• phospholipid bilayer

phosphate
attracted to water
lipid
repelled by water
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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?
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Phospholipid bilayer

• What molecules can get through directly?
• small molecules

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

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

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• Most cell membranes are composed principally of
• DNA and ATP
• proteins and lipids
• chitin and starch
• nucleotides and amino acids

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• Most cell membranes are composed principally of
• DNA and ATP
• proteins and lipids
• chitin and starch
• nucleotides and amino acids

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Movement through the channel

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

?
?
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Molecules move from high to low

• Diffusion
• move from HIGH to LOW concentration

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Diffusion

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

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

• Osmosis
• diffusion of water from HIGH concentration of
  water to LOW concentration of water
• across a semi-permeable membrane

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• Normally, in the process of osmosis, the net flow
  of water molecules into or out of the cell
  depends upon differences in the
• 1. concentration of water molecules inside and
  outside the cell
• 2. concentration of enzymes on either side of the
  cell membrane
• 3. rate of molecular motion on either side of the
  cell membrane
• 4. rate of movement of insoluble molecules inside
  the cell

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Keeping water balance

• Cell survival depends on balancing water uptake
  water loss

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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
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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
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Controlling water

• Contractile vacuole in Paramecium

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Managing water balance

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

saltwater
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