Chapter 5 Flashcards
fluid mosaic model
proteins float in the phospholipid bilayer membrane
Where are most of the proteins in relation to the membrane?
noncovalently embedded in the phospholipid bilayer
What are the functions of membrane proteins?
~move materials through the membrane
~intercellular recognition and adhesion
~receive chemical signals from the cell’s external environment
Where are the membrane carbohydrates found?
attached to lipids or protein molecules
What are the functions of membrane carbohydrates?
recognising specific molecules, such as those on the surface of adjacent cells
phospholipids
the lipids in biological membranes with both hydrophilic and hydrophobic regoins
hydrophilic regions in phospholipids
the phosphorus-containing “head” of a phospholipid is electrically charged and therefore associates with polar water molecules
hydrophobic regions in phospholipids
the long, nonpolar fatty acid “tails” of a phospholipid associate with other nonpolar materials, but they do not dissolve in water or associate with hydrophilic substances
How do phospholipids coexist with water?
They form a bilayer, where the fatty acid “tails” interact with each other and the polar “heads” face the outside, aqueous environment
most common fatty acids in membranes
chains with 16-18 carbon atoms and 0-2 double bonds
saturated fatty acid chains
no double bonds, allow for close packing
unsaturated fatty acid chains
double bonds, “kinks” make for a less dense, more fluid packing
steroid
a family of carbon compounds that have multiple linked rings
cholesterol
a steroid that plays an important role in modulating membrane fluidity
What are the factors that affect membrane fluidity?
lipid composition and temperature (among others)
How does lipid composition affect membrane fluidity?
Saturated fatty acids pack tightly together while unsaturated fatty acids or those with shorter chains increase membrane fluidity. Anaesthetics inserted into cell membranes reduce the fluidity of nerve cell membranes and thereby decrease nerve activity.
How does temperature affect membrane fluidity?
Membrane fluidity declines under cold conditions because molecules move more slowly at lower temperatures. Some organisms change lipid composition of their membranes when it gets cold.
What is the typical ratio between proteins and phospholipids in cell membranes?
1 protein molecule for every 25 phospholipid molecules
What determines how a membrane protein will be positioned relative to the bilayer?
The arrangement of the amino acids in the membrane protein
What are the two general types of membrane proteins?
peripheral and integral
peripheral membrane proteins
lack exposed hydrophobic groups and are not embedded in the bilayer
integral membrane proteins
at least partly embedded in the phospholipid bilayer, and have both hydrophilic and hydrophobic regions
anchored membrane proteins
membrane proteins that have fatty acids or other lipid groups covalently attached to them
transmembrane protein
an integral membrane protein that extends all the way through the phospholipid bilayer and protrudes on both sides
What happens when two cells fuse?
a single continuous membrane forms around both cells, and some proteins from each cell distribute themselves uniformly around this membrane
glycolipid
consists of a carbohydrate covalently bonded to a lipid
glycoprotein
consists of one or more short carbohydrate chains covalently bonded to a protein
proteoglycan
a protein with even more carbohydrate molecules attached to it, and the carbohydrate chains are often longer than in glycoproteins
selective permeability
when some substances can pass through the biological membranes but not others
passive transport
occurs when a substance moves from the side of the membrane where its concentration is higher to the side where its concentration is lower (down the gradient)
active trasnport
requires the input of metabolic energy because it involves the movement of substances against the their concentration gradient
diffusion
the process of random movement toward a state of equilibrium. It is a net movement from regions of greater concentration to regions of lesser concentration.
What does the rate of diffusion depend on?
~diameter of molecules or ions
~temperature of the solution
~concentration gradient in the system
simple diffusion
small molecules pass through the phospholipid bilayer of the membrane
osmosis
a diffusion process in which water molecules pass through specialised channels in membranes
osmotic pressure
the pressure that needs to be applied to a solution to prevent the flow of water across a membrane by osmosis
What is the equation for osmotic pressure due to water?
Π = cRT Π is osmotic pressure c is total solute concentration R is the gas constant T is the absolute temperature
hypertonic solution
has a higher solute concentration than the other solution
isotonic solutions
have equal solute concentrations
hypotonic solution
has a lower solute concentration than the other solution
turgor pressure
the pressure in within a plant cell which prevents further water from entering
channel proteins
integral membrane proteins that form channels across the membrane through which certain substances can pass
carrier proteins
speed up the diffusion of bound substances through the phospholipid bilayer, transporting polar molecules such as sugars and amino acids
facilitated diffusion
diffusion is made easier by channel or carrier prpoteins
ion channels
protein channels that are each for a particular ion, consisting of the same basic structure of a hydrophilic pore
gated channel
opens when a stimulus causes a change in the 3D shape of the channel
ligand
the binding of a chemical signal that can act as a stimuli for a gated channel
aquaporins
specific channels that allow large amounts of water to move down its concentration gradient
active transport
Directional, moves a substance either into or out of a cell organelle. The energy source usually comes from ATP.
primary active transport
involves the direct hydrolysis of ATP, which provides the energy required for transport
secondary active transport
does not use ATP directly, energy is supplied by an ion concentration gradient or an electrical gradient, established by primary active transport
Where is the higher concentration of potassium ions?
inside the cell
Where is the higher concentration of calcium ions?
outside the cell
sodium-potassium pump
an integral membrane glycoprotein that breaks down an ATP molecule into ADP and a phosphate ion, and uses the released energy to bring two K+ ions into the cell and export three Ca+ ions
How does secondary active transport help?
aids in the uptake of amino acids and sugars, which are essential raw materials for cell maintenance and growth
secrete
release to the external environment
How can cells take up or secrete intact large molecules?
via vesicles in processes called exocytosis and endocytosis
exocytosis
the process by which materials packaged in vesicles are secreted with the cell
How does exocytosis occur?
vesicle membrane fuses with the cell membrane, making an opening to the outside, and then releases the contents to the outside
endocytosis
a general term for a group of processes that bring small molecules, macromolecules, large particles, and even small cells into eukaryotic cells
How does endocytosis occur?
cell membrane invaginates, forming a small pocket around materials from the environment, eventually forming a vesicle which then breaks off and migrates into the cell
receptors
proteins that bind to specific molecules and then set off specific cellular responses
What are the receptors for endocytosis?
integral membrane proteins located on the extracellular surface of the cell membrane
What are the types of endocytosis?
phagocytosis, pinocytosis, and receptor endocytosis
phagocytosis
“cellular eating”
receptors in the cell membrane recognise a specific ligand on the surface of a large particle or even an entire cell, and then the phagocytic cell engulfs the particle or other cell
pinocytosis
“cellular drinking”
vesicles bring fluids and dissolved substances, including proteins, into the cell
receptor endocytosis
mechanism for bringing specific large molecules, recognised by specific receptors, into the cell
endosome
a membrane-enclosed compartment with which the vesicle fuses; here the contents of the vesicle are separated and sorted
low-density lipoproteins
where cholesterol is packaged
