2007-2008AP Biology

The Cell Membrane

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Membrane FunctionMembrane Function

Outer plasma membraneOuter plasma membrane Forms a boundary between a living cell and Forms a boundary between a living cell and

its surroundings (prok and euk cells)its surroundings (prok and euk cells) Exhibits selective permeabilityExhibits selective permeability

Controls traffic of molecules in and outControls traffic of molecules in and out

Internal organelle membranesInternal organelle membranes Form boundary for the organellesForm boundary for the organelles Compartmentalize chemical reactionsCompartmentalize chemical reactions

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Parts of the Plasma Membrane

Glycerol Backbone

Two Fatty Acid Chains

Phosphate Group

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.

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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

NON-POLAR, and water-fearing or repelled by water

hydrophilic

hydrophobic

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Membrane StructureMembrane Structure

All membranes are All membranes are phospholipid bilayersphospholipid bilayers with embedded proteins.with embedded proteins.

Label the:

Hydrophilic heads

Hydrophobic tails

OUTSIDE CELL

INSIDE CELL

PLASMA

MEMBRANE

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A membrane is a mosaicA membrane is a mosaic Proteins and other molecules are embedded in Proteins and other molecules are embedded in

a framework of phospholipidsa framework of phospholipids

A membrane is fluidA membrane is fluid Most protein and phospholipid molecules can Most protein and phospholipid molecules can

move laterallymove laterally

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Glycoproteins and glycolipids are Glycoproteins and glycolipids are proteins and lipids with short chain proteins and lipids with short chain

carbohydrates attached on the carbohydrates attached on the extracellularextracellular side of the membrane. 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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The cholesterol prevents the fatty acid chains of the phospholipids from sticking together.

CholesterolMolecule

PLASMA MEMBRANE

Cholesterol molecules are found throughout the plasma membrane.

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Embedded in the bilayer are proteinsEmbedded in the bilayer are proteins

Most of the membrane’s functions Most of the membrane’s functions are accomplished by the embedded are accomplished by the embedded

proteins. proteins. Integral proteinsIntegral proteins implanted in the implanted in the

membrane- partway or all the way acrossmembrane- partway or all the way across Peripheral proteinsPeripheral proteins are on one side or the are on one side or the

other of the membraneother of the membrane

Parts of the Plasma Membrane

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Peripheral ProteinsPeripheral Proteins

Types of Membrane ProteinsTypes of Membrane Proteins

1.1. Recognition proteinsRecognition proteins2.2. IntegrinsIntegrins3.3. Cell junction proteinsCell junction proteins4.4. Enzymes Enzymes 5.5. Receptor proteins Receptor proteins 6.6. Transport proteinsTransport proteins

– Passive and activePassive and active

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Adhesion ProteinsAdhesion Proteins

- IntegralIntegral

- Help cells of the same type stick together Help cells of the same type stick together in a tissue.in a tissue.

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Communication ProteinsCommunication Proteins

- IntegralIntegral

- Match with an identical protein on another Match with an identical protein on another cell and form a channel, which directly cell and form a channel, which directly connects two cell’s cytoplasmconnects two cell’s cytoplasm

- Chemical signals flow through the Chemical signals flow through the channels.channels.

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Receptor proteinsReceptor proteins

- bind hormones and other substances on - bind hormones and other substances on the outside of the cell.the outside of the cell. Binding triggers a change inside the cell.Binding triggers a change inside the cell.

Called signal transductionCalled signal transduction Example: The binding of insulin to insulin Example: The binding of insulin to insulin

receptors causes the cell to let glucose receptors causes the cell to let glucose transport into the cell.transport into the cell.

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Fig. 5-1cFig. 5-1c

Messenger molecule

Activatedmolecule

Receptor

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Recognition ProteinsRecognition Proteins

- identify type of cell - identify type of cell andand identify a cell as identify a cell as “self” versus foreign“self” versus foreign Most are glycoproteins Most are glycoproteins

Carbohydrate chains vary between species, Carbohydrate chains vary between species, individuals, and even between cell types in a individuals, and even between cell types in a given individual.given individual.

Glycolipids also play a role in cell recognitionGlycolipids also play a role in cell recognition

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

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Transport ProteinsTransport Proteins Passive Transport ProteinsPassive Transport Proteins

allow water soluble substances (small allow water soluble substances (small polar molecules and ions) to pass polar molecules and ions) to pass through the membrane without any through the membrane without any energy costenergy cost

Active Transport ProteinsActive Transport Proteins The cell expends energy to transport The cell expends energy to transport

water soluble substances water soluble substances againstagainst their their concentration gradientconcentration gradient

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Fig. 5-1dFig. 5-1d

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Membrane becomes semi-permeable via protein channels specific channels allow specific material

across cell membrane

inside cell

outside cell

sugaraaH2O

saltNH3