Chapter 7: Plasma membrane function Flashcards
Plasma Membrane (PM)
is the boundary that separates living cell from its surroundings.
PM exhibits selective permeability,
allowing some substances to cross it more easily than others.
Variations in LIPID composition of PM of many species…
are adaptations to specific environmental conditions.
Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary.
Lipids are hydrophobic because they consist mostly of hydrocarbons, which form nonpolar covalent bond
Phospholipids are the most
abundant lipid in the PM
Phospholipids are
amphipathic molecules, containing hydrophobic and hydrophilic regions
Fluid mosaic model
states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
Phospholipids in the PM can move within the bilayer
– Most of the lipids, and some proteins, drift laterally
– Unlikely for a molecule to flip-flop transversely across PM
As temperatures cool, membranes switch from a fluid state to a solid state
– Temperature at which a membrane solidifies depends on the types of lipids & steroid cholesterol the PM contains
– At moderate temperatures, cholesterol reduces membrane fluidity; at low temperatures, it hinders solidification
Membranes rich in unsaturated fatty acids are more fluid than
those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about
as fluid as salad oil
A membrane is a collage of different proteins,
embedded in fluid matrix of lipid bilayer
There are different types of
– Integral membrane proteins
– Peripheral proteins
Proteins determine most of the
membrane’s specific functions
Major functions of membrane proteins
– Transport; Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM)
Membranes have
distinct inside and outside faces
The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the PM is determined
when membrane is built by products made in the ER and Golgi apparatus
A process PM controls
A cell must exchange materials with its surroundings
As PM are selectively permeable, they regulate cell’s molecular traffic
- Hydrophobic (nonpolar) molecules, and gases can dissolve in the lipid bilayer and pass through the membrane rapidly
- Polar molecules (such as sugars) do not cross membrane easily
- Hence, small/uncharged/nonpolar/lipid-soluble molecules pass most easily thru the lipid core of a PM
Passive Transport
is diffusion of a substance across a membrane with no energy investment
DIFFUSION
is the tendency for molecules to spread out evenly into the available space
Although each molecule moves randomly,
diffusion of a population of molecules may be directional
At dynamic equilibrium,
molecules cross the membrane at the same time.
Substances diffuse down their concentration gradient
(the region along which the density of a chemical substance increases or decreases)
– No work must be done to move substances down the concentration gradient
– Diffusion of a substance across PM is passive transport because
no energy is expended by cell to make it happen
OSMOSIS
is the diffusion of water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of
higher solute concentration until the solute concentration is equal on both sides
Tonicity
is the ability of a surrounding solution to cause a cell to gain or lose water
Isotonic solution:
Solute concentration is same as that inside cell; no net water movement across PM
Hypertonic solution
Solute concentration is greater than that inside cell; cell loses water
Hypotonic solution
Solute concentration is less than that inside cell; cell gains water
Hypertonic or hypotonic environments
create osmotic problems for organisms
Osmoregulation
ontrol of solute concentrations and water balance, is a necessary adaptation
Protist Paramecium
is hypertonic to the pond water it lives in (a hypotonic environment) has a contractile vacuole that acts as a pump to actively transport water out
Plant cell walls help maintain water balance
– A plant cell in a hypotonic solution swells until the wall opposes uptake;
the cell is now turgid (firm)
– If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may plasmolyze (wilt)
Transport Proteins
allow passage of hydrophilic substances across the membrane
A transport protein is specific for the substance it moves across the membrane
channel proteins, and carrier proteins are transport proteins.
channel proteins
They have a hydrophilic channel that certain molecules
or ions can use as a tunnel
-provide corridors that allow a specific molecule or ion to cross PM
-Aquaporins
-Ion channels
carrier proteins
They bind to molecules and change shape to shuttle the molecules across the membrane
facilitated diffusion
transport proteins speed passive movement of molecules across PM
Facilitated diffusion is passive because
solute moves down its concentration gradient & the transport requires no energy
Aquaporins
for facilitated diffusion of water
Ion channels
that open or close in response to a stimulus (gated channels)
