cell membrane in this lesson: cell membranes and transport
TRANSCRIPT
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Cell MembraneIn This Lesson:
Cell Membranes and Transport
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Today’s Agenda
• The Cell Membrane– Structure– Function• You know…osmosis? Diffusion?
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By the end of this lesson…
• You should be able to describe in detail the structure of the cell membrane and link it to its functions.
• You should be able to predict the outcome of an osmotic process.
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So now then…
• Okay, you giant piles of cells you…• Where do we go from here?– Cells are going to be turning into cell membranes
and transport – it’s what allows multicellular organisms to be so darn interesting.• And yes, it’s how cells interact with their
environments.
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Cell Membrane Structure Overview
• The cell membrane is around 8 nm thick.– For perspective, the thickness of human hair is around 99,000
nm.• It’s composed of:– Lipids
• Mainly phospholipids and some cholesterol.
– Carbohydrates• Signal molecules attached to…
– Proteins• Embedded in the membrane.
• How was it discovered?• TED: Ethan Perlstein – Insights into Cell Membranes Via Dish Detergent
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The Phospholipid Bilayer• The cell membrane is primarily
composed of phospholipids.• The cell is sitting in a water-
based environment, therefore:– Each phospholipid has a polar
(hydrophilic) head…• It’s a phosphate group.
– …and a non-polar (hydrophobic) pair of tails.• They’re fatty acids.
• Because it has hydrophilic and hydrophobic parts, it’s amphipathic.
Polar
Non-polar
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The Phospholipid Bilayer• These phospholipids are
then arranged in a bilayer.– Key: Don’t confuse a bilayer
for a “double membrane.”– A bilayer is one membrane
with two layers.
Polar
Non-polar
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The Phospholipid Bilayer• The bilayer acts as a semi-permeable barrier.• Polar molecules can’t get in or out.
Sugar Salt
WasteLipids
H2O
Non-PolarTails
PolarHeads
PolarHeads
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Quick Note: Permeability
• Some things are “impermeable:”– Raincoats, balloons, brick walls.
• Some things are “permeable:”– Air, water.
• Some things are “semi-permeable:”– Nets, gates, cell membranes.• Semi-permeability is sometimes called selective
permeability.
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Back to the Phospholipid Bilayer
• Importantly, the composition of the bilayer is not constant.
• A certain percentage is composed of phospholipids with unsaturated fatty acid tails; the rest with saturated tails.– Unsaturated hydrocarbons lead to
increased fluidity.• The lower the temperature, the more
unsaturated the membrane needs to be to prevent freezing.
– Cholesterol is also in the membrane and acts to increase viscosity except at low temperatures.
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Another View
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Membrane Proteins
• Membrane proteins provide the bulk of the cell-specific (or organelle-specific) functions.
• There are two main types:– Peripheral Proteins• They’re stuck to the outside of the cell.• Example: Antigens (cell markers)
– Integral Proteins• They’re stuck within and usually span the membrane.• Example: Transport Proteins
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So why proteins?
• What do you see in the picture to the right?
• What are the blue things with two tails?– Phospholipids
• What’s the yellow thing wedged in there?– Cholesterol
• What are the red squiggly lines?
Polar areasof protein
Non-polar areas of protein
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So why proteins?
• amino acids (which are the building blocks of proteins) can be polar or non-polar
• That makes proteins (also amphipathic) a great candidate for transmembrane proteins.
Polar areasof protein
Non-polar areas of protein
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Cell Surface Proteins
• Cell surface proteins play a key role in recognition between cells.– This aids in development of
organs and tissues.• Antigens are proteins on the
cell surface that cause a response from the immune system.– They’re how the body “rejects”
cells that are foreign.
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Cell Surface Proteins
• Take a look at the image to the right. See those two orangey things?– They’re carbohydrate chains.
• One’s coming from a lipid, making it a glycolipid.
• The other is coming from a protein, making it a glycoprotein.
• These carbohydrate chains make the cell identifiable to other cells.
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4 Functions of Cell (Plasma) Membranes:
• Cell signaling• Selective transport• Excretion of wastes• Structural support