Transcript The Cell Membrane

The Cell Membrane
AP Biology
2007-2008
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
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 Form boundary for the organelles
 Compartmentalize chemical reactions
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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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:
PLASMA
Hydrophilic heads
Hydrophobic tails
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INSIDE CELL
MEMBRANE
 A membrane is a mosaic

Proteins and other molecules are
embedded in a framework of
phospholipids
 A membrane is fluid

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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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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 membrane’s functions
are accomplished by the
embedded proteins.

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Integral proteins implanted in the
membrane- partway or all the way
Peripheral Proteins
Types of Membrane Proteins
1.
2.
3.
4.
5.
6.
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 cell’s 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

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The cell expends energy to transport
water soluble substances against their
concentration gradient
Fig. 5-1d
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 Membrane becomes semi-permeable via
protein channels

specific channels allow specific material
across cell membrane
inside cell
NH
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Biology
3
salt
H 2O
aa
sugar
outside cell