Plasma Membrane

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Plasma Membrane
THE FLUID MOSAIC MODEL

• What is it?
• What is it made of?
• What is its function?

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Structure of the Cell Membrane
Outside of cell
Carbohydrate chains
Proteins
Lipid Bilayer
Transport Protein
Phospholipids
Inside of cell (cytoplasm)
Animations of membrane structure
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Membrane structure A membrane is a fluid mosaic
of lipids, proteins and carbohydrate.
A membrane is a fluid mosaic The membrane is a
mosaic of protein molecules bobbing in a fluid
bilayer of phospholipids.
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Fluid Mosaic Model of the cell membrane
Polar heads love water dissolve.
Membrane movement animation
Non-polar tails hide from water.
Carbohydrate cell markers
Proteins
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HOW DOES THE MEMBRANE MOVE?

• A membrane is held in together by weak
  hydrophobic interactions.

• Most membrane lipids and some proteins can drift
  laterally within the membrane

• Molecules rarely flip transversely (flip-flop)
  across the membrane, because hydrophilic parts
  would have to cross the membranes hydrophobic
  core.

Fig. 8.4a
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About Cell Membranes

• All cells have a cell membrane
• Functions
• Controls what enters and exits the cell to
  maintain an internal balance called homeostasis
• Provides protection and support for the cell

TEM picture of a real cell membrane.
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About Cell Membranes (continued)

• Structure of cell membrane
• Lipid Bilayer -2 layers of phospholipids
• Phosphate head is polar (water loving)
• Fatty acid tails non-polar (water fearing)
• Proteins embedded in membrane

Phospholipid
Lipid Bilayer
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About Cell Membranes (continued)

• 4. Cell membranes have pores (holes) in it
• Selectively permeable Allows some molecules in
  and keeps other molecules out
• The structure helps it be selective!

Pores
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Membranes are mosaics of structure and function

• A membrane is a mosaic of different proteins
  embedded and dispersed in the phospholipid
  bilayer. These proteins vary in both structure
  and function, and they occur in two spatial
  arrangements

1- Integral proteins 2- Peripheral proteins
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TYPE OF INTEGRAL PROTEINSin the plasma membrane
may provide a variety of.
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FUNCTIONS OF INTEGRAL PROTEINS Channel Protein
Allow a substance to move across the membrane (EX
all hydrogen ions to flow across membrane of
electron transport chain) Carrier Protein
Selectively interacts with specific molecules or
ions so it can cross membrane (EX Sodium and
Potassium pump) Cell Recognition Protein Called
glycoproteins, allow cell to be recognized by
bodys immune system Receptor Protein
Specifically shaped to a specific molecule (EX
liver stores glucose after insulin binds to cell
receptor) Enzymatic Protein Catalyze specific
reactions (EX ATP metabolism)
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FUNCTIONS OF PERIPHERAL PROTEINS To provide
structural stability and shape to the plasma
membrane
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• What else can we find in this membrane?
• Cholesterol helps control membrane fluidity by
  making the membrane
• Less fluid at warm temperatures (e.g. 37 C body
  temperature) by restraining the phospholipid
  movement.
• More fluid at lower (cool) temperatures by
  preventing close packing of phospholipids.

• Cells may alter membrane lipid concentration in
  response to changes in temperature

Fig. 8.4c

• Many cold tolerant plants (e.g. winter wheat)
  increase the unsaturated phospholipid
  concentration in autumn, which prevents the
  plasma membranes from solidifying in winter.

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• Lastly, why is fluidity important??
• Membrane must be fluid to work properly.
  Solidification may result in permeability changes
  and enzyme deactivation.

• Unsaturated hydrocarbon tails enhance membrane
  fluidity because kinks at the carbon-to-carbon
  double bonds hinder close packing of
  phospholipids.