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Membrane Structure and Function
A Cellular Membrane
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Is a mosaic of
proteins and carbohydrates
floating in a fluid bi-layer
of phospholipids.
Cellular Membrane
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Proteins are
embedded in the bilayer
or attached to the surface.
Cellular Membrane
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Carbohydrates are
linked to the proteins and
linked to the lipids.
Selective Permeability
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A property of biological membranes
Allows some substances to cross the
cell membrane more easily than others.
Selective Permeability
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small nutrient
waste molecules
respiratory gases
inorganic ions.
The plasma membrane regulates the passage
of these substances.
Passive Transport
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Passive transport is diffusion across a
membrane
Diffusion
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Diffusion is the spontaneous movement
of a substance down its concentration
gradient.
From a more concentrated to a less
concentrated area.
Osmosis
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Osmosis is the passive transport of
water.
Water flows across a membrane from
the side with a lesser concentration of
solute (hypotonic) to the side with the
greater solute concentration
(hypertonic).
Cell Survival Depends on
Balancing Water Uptake and Loss
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Cells lacking walls (as in animals and some
protists) are either isotonic with their
environments or else have adaptations for
osmoregulation.
Plants, prokaryotes, fungi, and some protists
have an elastic wall around their cells, which
keeps the cells from bursting in a hypotonic
environment. Under such conditions, these
cells are turgid.
Specific Proteins Facilitate the Passive
Transport of Selected Solutes
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In facilitated diffusion, transport
proteins hasten and help the movement
of certain substances across a
membrane.
Diffusion, osmosis, and facilitated
diffusion are all passive transport
processes that do not require the input
of energy from the cell.
Active Transport Is The Pumping of
Solutes Against Their Gradients
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Energy, usually in the form of ATP, is
harnessed by specific membrane proteins that
perform the active transport.
Some ion pumps generate voltage across
membranes.
In co-transport, a membrane protein couples
the transport of one solute to another.
Exocytosis and Endocytosis
Transport Large Molecules
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Exocytosis- intracellular vesicles
migrate to the plasma membrane, fuse
with it, and release their contents.
Endocytosis- large molecules within
vesicles pinch inward from the plasma
membrane.
Endocytosis-Transport of Large
Molecules into the cell.
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Three types of endocytosis are:
Phagocytosis- the ingestion of large particles
or whole cells;
Pinocytosis- the intake of tiny droplets of
extracellular fluid with all its contained
solutes; and
receptor–mediated endocytosis- the ingestion
of specific substances that bind to receptor
proteins on the membrane.
Specialized membrane
proteins
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Specialized membrane proteins transmit
extracellular signals to the inside of the
cell.
Websites
www.bmb.psu.edu/courses/bisci004a/
cells/cellstruc.htm