slide 7 Flashcards
Cell membranes are selectively
permeable
– they allow some
substances to cross more easily
than other substances.
Although only two phospholipid
molecules thick the cell membrane
regulates all differences between
the internal and external
environments.
Relative to the size of other macromolecule
structures of critical
importance, the thickness of the phospholipid bilayer is very small.
An amphipathic
molecule is one that has both
a hydrophilic region and a hydrophobic region
fluid mosaic
The phospholipids and
proteins comprise a
fluid mosaic
Freeze-fracture preparation of cell plasma
membranes for electron microscopy confirms that
the proteins are imbedded in the membrane
A membrane is a complex collage
of different proteins
embedded in the lipid bilayer and connected to both the
cytoplasm and extracellular matrix
The membrane is held together primarily by hydrophobic
interactions
- phospholipid molecules can move within it
Proteins can
also move within the phospholipid bilayer
Tracking of proteins in the cell membranes
of a mouse cell
and a human cell after experimental fusion of these cells
demonstrates the movement of proteins in the membranes.
The fluidity of membranes changes with:
- Temperature
- Structure of phospholipid tails
- Cholesterol
Phospholipids form the main
structure of the cell membrane
but the proteins determine
most of its functions.
Peripheral proteins are those
that are bound only to one
surface of the membrane
(a) Transport
Proteins that span the membrane can provide passage for hydrophilic molecules. Some proteins actively pump substances from one side of the membrane to the other.
Water is critical for cells -
- but water does not pass easily
through biological membranes.
Aquaporins are transmembrane proteins
through which
water can pass the hydrophobic region of the membrane
(b) Enzymatic Activity
Enzymes can exist embedded
in the membrane where they
act upon substances in the
adjacent solution.
(c) Signal Transduction
Some transmembrane proteins change shape when they bind specific substances such as hormones. This change in shape can relay a chemical message to the inside of the cell by binding to a cytoplasmic protein.
(d) Cell Recognition
Some glycoproteins serve a identification tags that are specifically recognized by membrane proteins of other cells.
(e) Intercellular Joining
Membrane proteins play a role in forming connections between cells. Gap junctions and tight junctions are established through proteins.
Attachment to ECM
Elements of the cytoskeleton can be bound to membrane proteins to stabilize their position. Membrane proteins can bind to the extracellular matrix to stabilize the position of cells and coordinate cellular changes.
An emergent property derived
from the organization of
molecules in the endomembrane system is the ability to
regulate transport into, within, and out of the cell.
How do substances cross
the plasma membrane?
It depends on the
chemistry of the
molecules.
Hydrophobic molecules are lipid soluble and
pass through the phospholipid bilayer rapidly.
Polar molecules do not pass through the
phosholipid bilayer rapidly.
Passive transport is
movement of a substance
across a membrane with no energy investment
Active transport
uses energy to move
substances across a membrane
Cell membranes use both methods
passive transport
active transport
Cell membranes use both methods
passive transport
active transport
diffusion.
The passive movement of molecules from where they are more
concentrated to where they are less concentrated is called
diffusion.
concentration gradient
This is movement down a concentration gradient
In the process of osmosis
water moves across a membrane from where it is more concentrated (relative to hydrophilic solutes) to where it is less concentrated.
In the process of osmosis
water moves across a membrane from where it is more concentrated (relative to hydrophilic solutes) to where it is less concentrated.
Hypotonic solution:
solutes are less
concentrated in the
solution than in cell
Isotonic solution
solutes have the same
concentration inside
and outside cell
Hypertonic solution:
solutes are more
concentrated in the
solution than in cell
facilitated diffusion.
Movement of solutes down their
concentration gradient through
transmembrane proteins
Channel proteins
provide
corridors in the membrane
Carrier proteins
undergo
shape change to pass solutes
The nonpolar center of the cell
membrane impedes
passage of
polar molecules and atoms.
Active transport
is the use of
energy to move solutes against
their concentration gradients
ATP Powers Cell Activity
Energy can be transferred within a cell through formation and breaking of highenergy bonds between phosphate groups.
An ion
is an atom or group of atoms that has gained or lost
one or more electrons and thereby acquired a charge.
A membrane potential
is the unequal distribution of ions
across a membrane that establishes an electrical voltage.
electrochemical gradient
The combined forces of the concentration gradient and the
membrane potential is the electrochemical gradient.
A proton pump
is a transmembrane protein that moves
H+ out of the cell by active transport to build up an
electrochemical gradient of H+ across the membrane.
In cotransport
the movement of ions down their
electrochemical gradient can power the
movement of another substance up its gradient.
Bulk Transport
Very large molecules (e.g. proteins or polysaccharides)
cannot cross the membrane via transport proteins but must
be transported within vesicles
Bulk transport across the plasma
membrane occurs by
exocytosis (out of the cell) and endocytosis (in to the cell).
Exocytosis
In exocytosis transport vesicles migrate to the plasma membrane, fuse with it, and release their contents
Endocytosis
In endocytosis the cell takes in macromolecules by
forming new vesicles from the plasma membrane
Three types of endocytosis are used
- phagocytosis
- pinocytosis
- receptor-mediated endocytosis
In phagocytosis a cell
engulfs a particle by
wrapping pseudopodia
around it and pulling it
into a space enclosed
by phospholipid bilayer.
Pinocytosis
Pinocytosis is the
transport of extracellular
fluid into the cell by
formation of vesicles.
Receptor-Mediated Endocytosis
Receptors on the extracellular side of the membrane can
bind specific molecules to be transported into vesicles.
