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Cell Biology: Membrane
structure and function
Membranes consist largely of
phospholipid and protein molecules
Outline
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1. Key concepts
2. Cell Membrane General
3. Movement across cell membrane
4. Conclusions
Key Concepts:
1.
Membranes consist largely of
phospholipid and protein molecules
2.
Organelles are membrane-bound
compartments inside eukaryotic cells
3.
Diffusion is the movement of ions from
a region of higher concentration to one
of lesser concentration
Key Concepts:
4.Osmosis is the movement of water across
a selectively permeable membrane to a
region where its concentration is lower
5.Some membrane proteins function in
passive transport whereas some function
in active transport
Cell Membrane General
 Cell
membrane consists of a bilayer of
phospholipids and embedded proteins
 Transport
proteins span the bilayer
 Open
channels, gated channels, carriers, and
pumps
 Receptor
proteins receive chemical signals
 Recognition proteins are used for
identification
Membrane system
Membrane system
The Nuclear Envelope
1.
2.
Double-membrane system
Pores allow exchange
The Endoplasmic Reticulum
Protein and Lipid Synthesis
Golgi Bodies and Vesicles
Phospholipids bilayer
Phospholipid
Bilayer arrangement of lipids
Plasma Membrane
 Bilayer
of phospholipids
 Hydrophilic
phosphate *head*
 Hydrophobic
*tails* of fatty acids
Fluid Mosaic Model
 Specialized
proteins and enzymes embedded in
the membrane
Movement across cell
membrane
1. Passive transport
a. simple diffusion
b. osmosis
c. facilitated diffusion
2. Energy-requiring transport
a. active transport
b. exocytosis
c. endocytosis
Passive Transport
Movement of substances across a membrane, going
down a gradient of concentration, pressure, or
electrical charge. Does not require the cell to
expend energy.
1. Simple diffusion: Net movement of particles
down a concentration gradient. Small, nonpolar molecules (e.g. O2, CO2) can get through
the phospholipid bilayer by simple diffusion.
Diffusion doesn’t have to have a membrane
(perfume).
Passive Transport
2.
Osmosis: Diffusion of water across a differentially
permeable membrane – that is, a membrane that is
more permeable to water than to dissolved molecules
(e.g. sugars, salt).
(always need a membrane) water moves from an area
with fewer solutes to an area with higher solutes.
3.
Facilitated diffusion: Diffusion of (normally watersoluble) molecules through a channel or carrier
protein. For example, glucose (large non-polar
molecule) across cell membrane by channel protein.
Only down a concentration gradient and no energy
required
How Substances Cross
Cell Membranes
 Diffusion
 Movement
of substance from a region where it is
more concentrated to a region where it is less
concentrated
Factors Influencing the Rate and
Direction of Diffusion
 Concentration
 Until
gradient
equilibrium is reached
 Molecular
 Small
size
molecules move faster
 Temperature
 Faster
 Electric
at higher temperatures
or Pressure gradient
 Electrical
 Pressure
charge difference across membrane
differences
Osmosis
Effect of solute concentration on water movement
Passive Transport
(Facilitated Diffusion)
 Solute
transport
through transport
protein
 Movement
 From
higher to lower
concentration
 No
energy use
Energy-requiring Transport
Movement of substances across a membrane, usually
against a concentration gradient, using cellular
energy.
1. Active transport: movement of small molecules or
charged ions through membrane – carried by
carrier protein using ATP.
Sodium (Na+)-potassium(K+) pump: sodium
pumped out of a cell and potassium pumped into a
cell
Energy-requiring Transport
2. Endocytosis: Movement of large particles, including
large molecules or entire microorganisms, into a cell
by engulfing extracellular material, as the plasma
membrane forms membrane-bound sacs that enter
the cytoplasm.
a. Phagocytosis - “cell eating”, engulf solid materials
b. Pinocytosis – “cell drinking”, liquid substance
3. Exocytosis: Movement of materials out of a cell by
enclosing the material in a membranous sac that
moves to the cell surface, fuses with the plasma
membrane, and opens to the outside, allowing its
contents to diffuse away. (Golgi apparatus  vesicle
 moves out)
Active Transport
 ATP
needed
 Movement
is against
the concentration
gradient
 Sodium-potassium
pump
 Calcium
pump
Active Transport
a. A transport protein, Ca++ and an ATP
b. Transport protein binds ATP and Ca++
c. E from ATP change the shape of protein and moves the ion across the membrane
d. The carrier release the ion and the remnants of the ATP )ADP and P) and back to original shape
Exocytosis and Endocytosis
 Exocytosis
 Vesicle
moves to cell
surface and fuses
with plasma
membrane
 Endocytosis
 Substances
move in
when plasma
membrane balloons
inward
Phagocytosis
 Form
of endocytosis
 Cell
engulfs microbes, large particles, and cellular
debris
 Amoebas and white blood cell
In Conclusion
1. The plasma membrane maintains the cell as
a separate entity
2. Proteins carry out most of cell membrane
functions
3. Membranes divide the cytoplasm into
functional compartments called organelles
4. Cell membrane is a bilayer of lipids with
proteins embedded
In Conclusion
5. Osmosis is the diffusion of water across
a selectively permeable membrane in
response to concentration gradients
6. Small non-polar molecules diffuse across
the membrane’s bilayer
7. Passive transport allows movement
down a concentration gradient
In Conclusion
8.
Active transport pumps a solute across
the membrane against the concentration
gradient using ATP
9.
Exocytosis entails movement of a
vesicle to the plasma membrane and
release of particles
10. Endocytosis entails the plasma
membrane folding in to engulf particles