Chapter 5
Biology & Chemistry Bi105 with Godrick at Boston University
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phospholipids
form the lipid layer of the foundation of a cell's membrane
phospholipid bilayer
provides a flexible matrix and imposes a barrier to permeability
transmembrane proteins
float in the lipid bilayer
have a variety of functions, including transport and communication across the membrane
interior protein network
membranes use networks of other proteins to control the lateral movements of some key membrane proteins, anchoring them to specific sites
cell-surface markers
membrane sections assemble in the endoplasmic reticulum, transfer to the Golgi apparatus, and then are transported to the plasma membrane
glycoproteins and glycolipids
glycoproteins
the ER adds chains of sugar molecules to membrane proteins
glycolipids
the ER adds chains of sugar molecules to membrane proteins and lipids
six classes of membrane protein
transporters
enzymes
cell-surface receptors
cell-surface identity markers
cell-to-cell adhesion proteins
attachments to the cytoskeleton
transporters
membranes are very selective, allowing only certain solutes to enter or leave the cell, either through channels or carriers composed of proteins.
enzymes
cells carry out many chemical reactions on the interior surface of the plasma membrane, using enzymes attached to the membrane
cell-surface receptors
membranes are exquisitely sensitive to chemical messages, which are detected by receptor proteins on their surfaces
cell-surface identity markers
membranes carry cell-surface markers that identify them to other cells. most cell types carry their own ID tags, specific combinations of cell-surface proteins and protein complexes such as glycoproteins that are characteristic of that cell type
cell-to-cell adhesion proteins
cells use specific proteins to glue themselves to one another. some act by forming temporary interactions, and others form a more permanent bond
attachments to the cytoskeleton
surface proteins that interact with other cells are often anchored to the cytoskeleton by linking proteins
transmembrane domain
each membrane-spanning domain
composed of hydrophobic amino acids usually arranged into alpha-helices
halophilic
salt-loving
beta-barrel
a polar environment in the interior of the beta-sheets spanning the membrane
open on both ends
is a common feature of the porin class of proteins that are found within the outer membrane of some bacteria
passive transport
many substances can move in and out of the cell without the cell's having to expend energy
concentration gradient
a difference between the concentration on the inside of the membrane and that on the outside
diffusion
a net movement of molecules and ions from regions of high concentration to regions of lower concentration
facilitated diffusion
molecules can still enter the cell by diffusion through specific channel proteins or carrier proteins embedded in the plasma membrane, provided there is a higher concentration of the molecule outside the cell than inside
channel proteins
have a hydrophobic interior that provides an aqueous channel through which polar molecules can pass when the channel is open
carrier proteins
bind specifically to the molecule they assist, much like an enzyme binds to its substrate
selectively permeable
channels and carriers are usually selective for one type of molecule
ion channels
possess a hydrated interior that spans the membrane
gated channels
can be opened or closed in response to a stimulus
membrane potential
an electrical potential difference across the membrane
saturation
at high enough concentrations all carriers will be occupied, and the rate of transport will be constant
aqueous solution
the mixture of ions and molecules and water
solvent
water
solutes
substances dissolved in the water
osmosis
the net diffusion of water across a membrane toward a higher solute concentration
osmotic concentration
determined by the concentration of all solutes in a solution
hypertonic
the solution with the higher concentration in the case that two solutions have unequal osmotic concentrations
hypotonic
the solution with the lower concentration
isotonic
two solutions have the same osmotic concentration
aquaporins
specialized channels for water
osmotic pressure
the force needed to stop osmotic flow
extrusion
by contracting rhythmically, the vacuole pumps out (extrudes) through this pore the water that is continuously drawn into the cell by osmotic forces
isosmotic regulation
some organisms that live in the ocean adjust their internal concentration of solutes to match that of the surrounding seawater
many terrestrial animals circulate a fluid through their bodies that bathes cells in an isotonic solutiont
turgor pressure
internal hydrostatic pressure
presses the plasma membrane firmly against the interior of the cell wall, making the cell rigid
active transport
cells can actively move substances across a cell membrane up their concentration gradients
requires the expenditure of energy, typically from ATP
uniporters
carrier proteins that transport a single type of molecule
symporters
transport two molecules in the same direction
antiporters
transport two molecules in opposite directions
sodium-potassium pump
carrier protein that uses the energy stored in ATP to move NA+ ions and K+ ions
step 1 of Na-K pump
three Na+ bind to the cytoplasmic side of the protein, causing the protein to change its conformation
step 2 of Na-K pump
the protein binds a molecule of ATP and cleaves into adenosine diphosphate (ADP) and phosphate (Pi)
ADP is released, but the phosphate group is covalently linked to the protein
the protein is phosphorylated
step 3 of Na-K pump
the phosphorylation of the protein induces a second conformational change in the protein
this change translocates the three Na+ across the membrane, so they now face the exterior
in this new conformation, the protein has a low affinity for Na+, and the three bound Na+ break away from the protein and diffuse into the extracellular fluid
step 4 of Na-K pump
the new conformation has a high affinity for K+, two of which bind to the extracellular side of the protein as soon as it is free of the Na+the
step 5 of Na-K pump
the binding of the K+ causes another conformational change in the protein, this time resulting in the hydrolysis of the bound phosphate group
step 6 of Na-K pump
the protein reverts to its original shape, exposing the two K+ to the cytoplasm
this conformation has a low affinity for K+, so the two bound K+ dissociate from the protein and diffuse into the interior of the cell
the original conformation has a high affinity for Na+
when these ions bind, they initiate another cycle
coupled transport
the energy released as one molecule moves down its concentration gradient, is captured and used to move a different molecule against its gradient
bulk transport
endocytosis
exocytosis
requires energy expenditure
endocytosis
the plasma membrane envelops food particles and fluids
phagocytosis, pinocytosis, and receptor-mediated endocytosis
phagocytosis
if the material the cell takes in is particulate, such as an organism or some other fragment of organic matter
pinocytosis
if the material the cell takes in is liquid
common among animal cells
receptor-mediated endocytosis
molecules first bind to specific receptors in the plasma membrane
difference cell types contain a characteristic battery of receptor types, each for a different kind of molecule in their membranes
clathrin
the portion of the receptor molecule that lies inside the membrane is trapped in an indented pit coated on the cytoplasmic side with this protein
exocytosis
the reverse of endocytosis
the discharge of material from vesicles at the cell surface
plants: exporting the materials needed to construct the cell wall through the plasma membrane
protists: contractile vacuole discharge
animals: a mechanism for secreting many hormones, neurotransmitters, digestive enzymes, and other substances
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