Chapter 4 Flashcards
What do cells and organelles need to separate internal and external contents?
Barrier
What are the qualities the barrier must have:
Impermeable to most molecules and ions (semi-permeable)
Ability to exchange specific molecules/ions between compartments
Insoluble in water
Permeable to water
What is the cellular membrane?
· A permeability barrier that consists of:
· Phospholipids (fabric of membrane), glycolipids
▪ Sterols (except in bacteria):
- Cholesterol (animals)
- Ergosterols (fungi)
- Phytosterols - no cholesterol (plants)
- Membrane protein
What are membrane proteins?
Integral and peripheral membrane porteins
What are Integral proteins?
transmembrane, embedded in the membrane
What are Peripheral membrane proteins
not embedded in the membrane
Are membranes rigid?
No
What do membranes consist of?
Fluid lipid bilayer in which proteins are embedded and float freely (they can move around)
What is the membrane structure according to the fluid mosaic model?
The model proposes that integral proteins are suspended individually in a fluid lipid bilayer. Peripheral proteins are attached to integral proteins or membrane lipids mostly on the cytoplasmic side of the membrane (shown only on the inner surface in the figure). Carbohydrate groups of membrane glycoproteins and glycolipids face the cell exterior.
What is the Fluid Mosaic Model supported by and what does it demonstrate?
Fluid Mosaic Model is Supported by Experimental Evidence: Membranes are Fluid
Experimental Research: The Frye–Edidin experiment demonstrates that the phospholipid bilayer is fluid
What is involved in cell chemistry review?
· Properties of water
- Water as a solvent
- Polarity
· What does Hydrophilic mean?
· What does Hydrophobic mean?
Are water molecules polar?
Yes
What is polarity?
Polarity is an uneven distribution of charge
Electronegative oxygen atom associates with electropositive hydrogen atoms of adjacent water molecules to form hydrogen bonds
Polar/charged molecules form hydrogen/ionic bonds with water molecules
What is a solvent?
a fluid in which another substance, called a solute, can be dissolved
Is water an excellent solvent?
Yes
Polar/charged molecules form hydrogen/ionic bonds with water molecules
What does hydrophilic mean?
(“water-loving”): sugars, DNA, RNA, organic acids, and some of the amino acids
What does hydrophobic mean?
(“water-fearing”): Do not dissolve in water, lipids (hydrocarbons), or some amino acids (e.g. those in integral membrane proteins that associate with lipids)
· No polar regions
· Do not interact electrostatically with water
· Disrupt hydrogen-bonded structure of water
· Tend to coalesce with each other in the water
· Water molecules tend to exclude molecules that disrupt hydrogen bonding
· Hydrophobic interactions are a major driving force in the folding of molecules (like proteins), assembly of cellular structures and membrane organization.
What does amphipathic mean and are phospholipids amphipathic
having both hydrophilic and hydrophobic parts.
Yes
What happens to phospholipids in aqueous solutions?
In an aqueous environment, phospholipids self-assemble into micelles, liposomes, or bilayers (the hydrophobic effect)
What happens to phospholipids in aqueous solutions?
In an aqueous environment, phospholipids self-assemble into micelles, liposomes, or bilayers (the hydrophobic effect)
Phospholipid structure
(a) Chemical formula of phosphatidylcholine. The polar head group consists of glycerol (shown in pink) linked to the organic molecule choline (shown in blue) by a phosphate group (shown in yellow). In addition, glycerol is linked to two fatty acids, each 18 carbons long. The structure of phospholipids is also often represented as space-filling models (b) and as an icon (c). As shown in the space-filling model, the presence of a carbon-carbon double bond (denoted by the arrow in (a)) imparts a bend to one of the fatty acids.
Why is the fluidity of a membrane important?
for its function
- Fluidity is dependent on how densely individual lipid molecules can pack together
What are the two major factors that influence fluidity?
- Composition of lipid molecules
- Degree of unsaturation of fatty acid tails
- Presence of sterols
2. Temperature
What do the lipid molecule compositions affect?
how closely the molecules interact. Lipid molecules that contain saturated hydrocarbon tails are closely packed (a), whereas unsaturated hydrocarbon tails have kinks that prevent lipid molecules from packing closely together (b).
What happens to phospholipids when the temperature drops low? enzymes and proteins?
phospholipid molecules become closely packed, and the membrane forms a highly viscous semisolid gel
· At low temperatures, enzymes and proteins cannot function if fluidity is not maintained
What happens at high temperatures?
At high temperatures, too fluid, get leakage
What are phospholipids composed of?
Phospholipids are composed of saturated fatty acids
- Each carbon is bound to the max number of hydrogens (all single bonds between C’s)
- Straight shape
- Tighter packing
- Double-bonds between carbons introduce kinks
- Less dense packing
How can proper fluidity be maintained?
over a broad range of temperatures by adjusting fatty acid composition of the phospholipids
What are desaturases?
enzymes that produce unsaturated fatty acids during fatty acid synthesis
· Regulation of desaturases allows for organisms to closely regulate the amount of unsaturated fatty acids & membrane fluidity
Can organisms regulate the degree of fatty acid unsaturation?
Yes
(a) Desaturases are a class of enzymes that introduce carbon-carbon double bonds into fatty acids, thereby altering the degree of unsaturation. (b) Graph showing relative amounts of desaturase transcript amounts (mRNA abundance) in relation to growth temperature in a bacterium.
Why are organisms never at 0?
always need some amount of unsaturated fatty acids
More RNA=?
MORE RNA = MORE mRNA = MORE ENZYMES = MORE DESTAURASES = MORE UNSATURATED FATTY ACIDS = MORE FLUID IN MEMBRANE
How can organisms modify the lipid composition of their membranes to optimize fluidity in response to different temperatures?
By changing:
- Fatty acid desaturation
- Cholesterol content
What are the key functions of membrane proteins?
transport
enzymatic activity
signal transduction
attachment/Recognition
Are some membrane proteins just enzymes?
Yes
What are integral membrane proteins composed of?
predominantly nonpolar amino acids usually coiled into alpha helices
(20 amino acids in helix)
What are amphipathic proteins?
Nonpolar amino acids (often coiled into alpha helices) cluster together in regions that interact with the hydrophobic core of the bilayer
Polar amino acids are mostly found in regions that extend into the aqueous compartment on either side of the membrane (hydrophilic domains)
What is the structure of membrane proteins?
A typical integral membrane protein showing the membrane-spanning alpha-helical segments (blue cylinders) connected by flexible loops of the amino acid chain at the membrane surfaces.
Figure 4.11: Transmembrane proteins can be identified by the presence of stretches of amino acids that are primarily non-polar. These regions of the protein interact with the hydrophobic regions of the membrane. Usually between 17 and 20 amino acids are needed to span the membrane once. For clarity, this model shows only five non-polar amino acids spanning the membrane.
Peripheral membrane porteins?
On the surface of the membrane
Do not interact with hydrophobic core (interact with hydrophilic head)
Held together by a noncovalent bond
Many on the cytoplasmic side of the membrane
Made up of a mixture of polar and non-polar amino acids
Are membranes selectively permeable?
Yes
(some can get across, some can’t)
What is CFTR?
CFTR is a chloride (Cl-) channel in the plasma membrane, whose function is disrupted by a genetic mutation that causes Cystic Fibrosis
What is passive membrane transport?
Movement of molecules across a membrane without the need to expend chemical energy such as ATP
The hydrophobic nature of the membrane restricts the free movement of many molecules and substances essential for life
What are passive transport and diffusion?
Passive transport is driven by diffusion
What is diffusion?
movement of molecules from regions of high concentration to areas of low concentration. It is driven by the increase in entropy associated with energy becoming more dispersed
Driving force is an increase in entropy
The bigger the gradient, the faster the diffusion rate
Primary mechanism of solute movement within a cell
Rate of diffusion depends on the concentration difference or concentration gradient
What do the size and charge of a molecule affect?
the rate of diffusion across a membrane
What is aquaporin?
Transport proteins for facilitated diffusion
What is K+ Voltage-Gated Channel?
Most proteins that carry out facilitated diffusion of ions are controlled by “gates” that open or close their transport channels
Gates can be opened or closed in response to various stimuli, such as voltage across the membrane or the presence of signal molecules
What are carrier proteins?
Bind a specific single solute and transport it across the lipid bilayer (uniport transport)
Undergo conformational changes that move the solute-binding site from one side of the membrane to the other
Can become saturated when there are too few transport proteins to handle all the solute molecule
Compared with simple diffusion, facilitated diffusion leads to …. rates of transport and displays …. kinetics.
higher rather and saturation kinetics
Hypertonic, hypotonic and isotonic?
The diagrams show what happens when a cellophane bag filled with a 2 M sucrose solution is placed in (a) a hypotonic, (b) a hypertonic, or (c) an isotonic solution. The cellophane is permeable to water but not to sucrose molecules. The width of the arrows shows the amount of water movement. In the first beaker, the distilled water is hypotonic to the solution in the bag; the net movement of water is into the bag. In the second beaker, the 10 M solutions is hypertonic to the solution in the bag; the net movement of water is out of the bag. In the third beaker, the solutions inside and outside the bag are isotonic; there is no net movement of water into or out of the bag. The animal cell micrographs show the corresponding effects on red blood cells placed in hypotonic, hypertonic, or isotonic solutions.
What are the characteristics of transport mechanisms?
What is primary active transport?
The same protein that transports the molecules also hydrolyzes ATP to power transport directly
Moves positively charged ions across membranes
H+ pumps (proton pumps)
Cells lining the stomach
Ca2+ pump
Maintain low intracellular Ca2+ concentration
Na+/K+ pump
3 Na+ out, 2 K+ in for every pump cycle
Creates negative membrane potential
Electrochemical gradient across the membrane
What is the Na+/K+ pump?
The sodium-potassium pump is an active transport protein in the plasma membrane. Energy from the protein’s hydrolysis of ATP transports Na+ out of the cell and K+ into the cell, each against its concentration gradient. The pump moves three Na+ out and two K+ in for each ATP molecule hydrolyzed. Model for how a primary active transport pump operates.
What is symport?
Cotransporter solute moves through membrane channel in same direction as driving ion
What is antiport?
Driving ion moves through membrane channel in one direction, providing energy for active transport of another molecule in opposite direction
What is exocytosis?
Secretory vesicle carries secreted materials
Moves through cytoplasm and contacts the plasma membrane
The vesicle membrane fuses with the plasma membrane, releasing contents to the cell exterior
What is pinocytosis?
- Solute and H20 molecules outside the plasma membrane
- Membrane folds inward, enclosing solute and H20 molecules
- Vesicle pinches off as endocytic vesicle
What is receptor-mediated endocytosis?
- Molecules bind to receptor proteins on the outer cell surface
- After binding, receptors collect into a depression in PM called a coated pit
- Network of proteins, called clathrin, on the cytoplasmic side - Pit pinches off PM to form an endocytic vesicle
What phagocytosis?
Phagocytosis, in which lobes of the cytoplasm extend outward and surround a cell targeted as prey. The micrograph shows the protistan Chaos carolinense preparing to engulf a single-celled alga (Pandorina) by phagocytosis; white blood cells called phagocytes carry out a similar process in mammals.
· White blood cells · Protists – Amoeba proteus · Can take in - Large aggregates of molecules - Cell parts - Whole cells - prey
Reception by a cell-surface receptor?
· Signal molecules (usually polar molecules, can’t penetrate through
plasma membrane, can only be received by receptors on the surface of cell)
- Hormones
- Neurotransmitters
· Receptors
- Integral membrane glycoproteins
· Responses usually rapid, short-lived events
(regulate enzyme activity)
Reception by a Receptor Within Cell?
· Signal molecules (nom polar, can get across and interact with plasma membrane, interact with hydrophobic core, does not need receptor on the surface, receptor is inside)
- Steroid hormones
- testosterone, estrogen
· Intracellular receptors
- Steroid hormone receptor
- Hormone-binding domain & DNA-binding domain
· Responses typically occur over a longer
time due to changes in gene expression
What are the three stages of signal transduction?
reception
transduction
response
common to all signaling systems— although they vary greatly in detail
(shown for a system using a surface receptor- integral protein)
Cell signalling?
What is the activation of a surface receptor?
The mechanism by which a surface receptor responds when it binds a signal molecule
Receptor is reversible
What is phosphorylation?
a key reaction in many signalling pathways
Protein kinases often act in a chain, catalyzing a series of phosphorylation reactions called a phosphorylation cascade to pass along a signal
Phosphorylation diagram?
Amplification?
As a result, just a few extracellular signal molecules binding to their receptors can produce a full internal response
Example: Stimulation of glycogen breakdown in liver cells by epinephrine
hormone is first messenger leads to formation of second messenger
- Involves phosphorylation by kinases - leads to activation of enzyme for conversion of glycogen to glucose
