lOMoARcPSD|266586 02
Lecture 9: Cells
A Plasma Membrane Surrounds the Cell
•
▪ Separates a cell from its environment
•
▪ Is selectively permeable
– Permits movement of some substances into and out of the cell, but blocks others
•
▪ Enables transfer of information between environment and cell
•
▪ Allows the cell to concentrate nutrients and molecules inside.
•
▪ Gives the proper environment for chemical reaction (metabolism)
•
Plasma membrane is a lipid bilayer
o – Phospholipids: polar head and nonpolar tail
o – Cholesterol: makes membrane a bit more rigid
o – Proteins: provide means of transport through
membrane
o – Carbohydrates: recognition patterns for cells and
organisms ▪ Non-rigid
▪
Fluid mosaic
– Many proteins are not anchored in place. Able to move or float around in the
lipid layer. Lipids are also able to float around (Fluid)
Membrane Structure:
Molecules Cross the Plasma Membrane in Several Ways
▪
Passive transport: cell does not need to expend energy for this
– Diffusion
– Osmosis
▪
Active transport: cell must expend energy ▪ Bulk transport
lOMoARcPSD|266586 02
– Involves membranous vesicles to move larger substances
– Endocytosis – Exocytosis
Passive Transport: Principles of Diffusion and Osmosis
•
▪ Passive transport: transports a substance without having to expend energy
•
▪ Passive transport relies on diffusion
•
▪ Diffusion: movement of molecules from a region of high concentration to a region of
low concentration until it is distributed equally
•
▪ High concentration → low concentration
– (difference in concentration between two locations--
Concentration gradient)
•
▪ “Down” the gradient
– Move from high concertation to low concentration
Osmosis: Diffusion of Water
•
▪ Osmosis: the diffusion of water across a selectively permeable membrane
•
▪ Water moves from an area of low solute concentration to an area of high solute
concentration
•
▪ Osmotic pressure: fluid pressure required to exactly oppose osmosis
•
Passive Transport Moves with the Concentration Gradient
▪
Diffusion directly through the lipid bilayer – Small uncharged nonpolar molecules
Active Transport Requires Energy
▪
Active transport moves substances from an area of lower concentration to an area of higher
concentration
– Transported substance moves against the concentration gradient
– Requires a membrane protein (transporter)
– Requires ATP or other energy source ▪ Example: sodium-potassium pump
lOMoARcPSD|266586 02
•
Endocytosis and Exocytosis Move Materials in Bulk
•
▪ Used to move larger molecules
– Endocytosis: brings substances into the cell
•
– As substance enters, it is surrounded by a membrane, forming a membrane-bound
vesicle
•
– Exocytosis: expels substances from the cell
– Substance is contained within a membranous vesicle,
•
which then fuses with the membrane, releasing the substance to the external environment
Information Can Be Transferred Across the Plasma Membrane
•
▪ Receptor proteins span membrane—required for transmission of information to and
from cell
•
▪ Receptor sites (on receptor proteins)—interact specifically with signal molecules
•
▪ A change is triggered within the cell as a result of binding of signal molecule to
receptor site
▪
Different cell types have different receptor proteins
The Sodium–Potassium Pump Helps Maintain Cell Volume
▪
Sodium (Na+)–potassium (K+) pump expels unwanted ions (Na+), stockpiles needed ones
(K+), and maintains cell volume
•
▪ ATP is used to expel three Na+ for every two K+ brought into the cell
•
▪ Increase in cell volume = more water in cytoplasm, accomplished by decreasing
pumping and allowing more sodium inside cell
•
▪ Decrease in cell volume = less water in cytoplasm, accomplished by increasing
pumping and expelling more sodium ions
Isotonic Extracellular Fluid Also Maintains Cell Volume
▪
Tonicity: relative concentration of solutes in two fluids
▪
Isotonic
lOMoARcPSD|266586 02
•
– Extracellular and intracellular solute concentrations are equal
•
– Cells maintain a normal volume in isotonic extracellular fluids
•
– Regulatory mechanisms maintain extracellular fluid that is isotonic with intracellular
fluid
•
Isotonic Extracellular Fluid Also Maintains Cell Volume
▪
Variations in tonicity – Hypertonic
– Extracellular solute concentration higher than intracellular solute concentration
– Water will diffuse out of cell
– Cell may shrink and die – Hypotonic
– Extracellular solute concentration lower than intracellular solute concentration
– Water will diffuse into cell – Cell may swell and burst
