The Cell Membrane

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The Cell Membrane
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Membrane Function

• Outer plasma membrane
• Forms a boundary between a living cell and its
  surroundings (prok and euk cells)
• Exhibits selective permeability
• Controls traffic of molecules in and out
• Internal organelle membranes
• Form boundary for the organelles
• Compartmentalize chemical reactions

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Summary Section 2 pages 175-178
Parts of the Plasma Membrane
Phospholipids are the main component of a plasma
membrane. They are lipids made of a phosphate
molecule, a glycerol molecule, and two fatty acid
chains.
Phosphate Group
Glycerol Backbone
Two Fatty Acid Chains
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Summary Section 2 pages 175-178
The head of the phospholipid molecule is
attracted to water, whereas, the glycerol and two
fatty acid tails are repelled by water.
POLAR, and water-loving or attracted to water
hydrophilic
NON-POLAR, and water-fearing or repelled by water
hydrophobic
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Membrane Structure

• All membranes are phospholipid bilayers with
  embedded proteins.

OUTSIDE CELL
Label the Hydrophilic heads Hydrophobic tails
PLASMA MEMBRANE
INSIDE CELL
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• A membrane is a mosaic
• Proteins and other molecules are embedded in a
  framework of phospholipids
• A membrane is fluid
• Most protein and phospholipid molecules can move
  laterally

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• Glycoproteins and glycolipids are proteins and
  lipids with short chain carbohydrates attached on
  the extracellular side of the membrane.

These carbohydrate chains act as antennae,
receiving chemical messages from other cells.
They are also markers identifiers that identify
the cell to other cells.
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Summary Section 2 pages 175-178
PLASMA MEMBRANE
Cholesterol molecules are found throughout the
plasma membrane.
Cholesterol Molecule
The cholesterol prevents the fatty acid chains
of the phospholipids from sticking together.
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Parts of the Plasma Membrane

• Embedded in the bilayer are proteins
• Most of the membranes functions are accomplished
  by the embedded proteins.
• Integral proteins implanted in the membrane-
  partway or all the way across
• Peripheral proteins are on one side or the other
  of the membrane

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Peripheral Proteins

• Types of Membrane Proteins
• Recognition proteins
• Integrins
• Cell junction proteins
• Enzymes
• Receptor proteins
• Transport proteins
• Passive and active

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• Adhesion Proteins
• Integral
• Help cells of the same type stick together in a
  tissue.

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• Communication Proteins
• Integral
• Match with an identical protein on another cell
  and form a channel, which directly connects two
  cells cytoplasm
• Chemical signals flow through the channels.

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• Receptor proteins
• - bind hormones and other substances on the
  outside of the cell.
• Binding triggers a change inside the cell.
• Called signal transduction
• Example The binding of insulin to insulin
  receptors causes the cell to let glucose
  transport into the cell.

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Fig. 5-1c
Messenger molecule
Receptor
Activated molecule
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• Recognition Proteins
• - identify type of cell and identify a cell as
  self versus foreign
• Most are glycoproteins
• Carbohydrate chains vary between species,
  individuals, and even between cell types in a
  given individual.
• Glycolipids also play a role in cell recognition

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• Many membrane proteins are enzymes
• This is especially important
• on the membranes of organelles.

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Transport Proteins

• Passive Transport Proteins
• allow water soluble substances (small polar
  molecules and ions) to pass through the membrane
  without any energy cost
• Active Transport Proteins
• The cell expends energy to transport water
  soluble substances against their concentration
  gradient

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Fig. 5-1d
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• Membrane becomes semi-permeable via protein
  channels
• specific channels allow specific material across
  cell membrane

inside cell
sugar
aa
H2O
salt
outside cell
NH3