Unit 2 (Week 4) Flashcards
What is any membrane made by living cells; can be the plasma membrane or an internal membrane that surrounds an organelle?
Biological membrane (all 5-10 nm) 5,000-10,000 of these stacked on top of each other to be as thick as paper.
What are the two primary components of a membrane?
Phospholipids that form the basic matrix of the membrane and proteins which are embedded in the membrane or loosely attached to its surface.
Carbohydrates may or may not be attached to the membrane lipids and proteins.
What is the phospholipid bilayer?
The basic framework of a biological membrane, consisting of two layers of phospholipids.
What is a leaflet and what are the two in the phospholipid bilayer?
Leaflets are one half of the bilayer and faces different regions. There are two which are called the cytosolic leaflet and the extracellular leaflet.
T/F The bilayers are symmetrical.
False.
Lipid composition may be more abundant in one leaflet compared to the other. Glycolipids are found primarily in the extracellular leaflet of the plasma membrane. The carbohydrate portion protrudes into the extracellular medium.
What is a protein that has one or more regions that are physically embedded in the hydrophobic region of a membrane’s phospholipid bilayer?
Transmembrane protein
They are folded into “a helix.” The nonpolar amino acids interact favorably with the nonpolar lipid tails.
What is a type of integral membrane protein that is attached to the membrane via a lipid molecule?
Lipid-anchored protein
What are proteins such as a transmembrane protein or a lipid-anchored protein that cannot be released from the membrane unless the membrane is dissolved with an organic solvent or detergent called?
Integral membrane proteins
What is a protein that is noncovalently bound to a region of an integral membrane protein that projects out from the membrane or noncovalently bound to the polar head group of a phospholipids?
Peripheral membrane proteins (hydrogen and/or ionic bonds)
Roughly, how many medications today exert their effects by binding to membrane proteins?
70%
What is a property of biological membranes in which individual molecules remain in close association yet have the ability to move rotationally or laterally within the plane of the membrane? Membranes are semifluid.
Fluidity
What is a property of biological membranes in which movement of membrane components occurs only in two dimensions?
Semifluid
Fluid would be movement in 3 dimensions.
What enzyme allows the polar region of a phospholipid to flip to the other leaflet and does not happen spontaneously like rotational or lateral movements?
Flippase (energetically unfavorable since energy input in the form of ATP is needed)
How fast can a lipid traverse a bacterial cell? An animal cell?
1 second and 10 to 20 seconds.
What is in a membrane, a group of lipids, sometimes including associated proteins, that float together as a unit in a larger sea of lipids?
Lipid raft
Typically have a high amount of cholesterol and/or unique set of transmembrane and lipid-anchored proteins.
What is a key property that affects the fluidity of the phospholipid bilayer?
The length of lipid tails. Ranges from 14-24 carbon atoms with 16-18 being the most common. Shorter tails are less likely to interact with each other, which makes the membrane more fluid.
What is another property in phospholipid tails that help it remain more fluid in the context of carbon-hydrogen relationship?
The double bonds between carbon and hydrogen. (Remember: saturated fats are solid at room temperature versus olive oil that remains more fluid. Olive oil have double bonds while saturated fat is saturated with hydrogen bonds.
T/F Cholesterol is found in plant and animal cells.
False.
Plants have phytosterols that resembles cholesterol’s chemical structure.
What is the third factor that helps plasma membrane’s retain their fluidity and why?
Cholesterol, a short and rigid molecule.
It stabilizes cell membranes based off of lipid composition and temperature.
How does the change in temperature affect cholesterol in the membrane?
At high temperatures, cholesterol makes the membrane less fluid.
At lower temperatures, the opposite happens and prevents it from freezing.
This phenomenon helps animals cope with changing temperatures to make sure their cells operate optimally.
Why might transmembrane proteins move around the phospholipid bilayer?
They could be bound to components of the cytoskeleton or attached to molecules outside the cell like the extracellular matrix (ECM) of animal cells
Discuss how transmembrane proteins are important in the binding of cells to each other and the binding of cells to the extracellular matrix.
Transmembrane proteins called cell adhesion molecules bind to each other to promote cell-to-cell adhesion. In addition, they can bind to filaments in the extracellular matrix, such as collagen fibers, thereby causing a cell to adhere to the extracellular matrix.
Where does the synthesis of most lipids by the cytosol and endomembrane system occur?
The cytosolic leaflet of the smooth ER membrane.
[Synthesis of membrane phospholipids at the ER membrane] What are the steps needed for fatty acids to enter the ER? (5)
- Fatty acids are activated with the attachment of a CoA molecule.
- This fatty acids are then bonded to glycerol-phosphate and inserted into the cytosolic leaflet of the ER membrane via acyl transferase.
- The phosphate is removed by phosphatase enzyme.
- A choline already linked to phosphate is attached via choline phosphotransferase.
- Flippases transfer some of the phospholipids to the other leaflet into the ER lumen.
What are the variety of mechanisms where lipids made in the ER membrane are transferred to other membranes?
- Lipids are transported via vesicles.
- Phospholipids can diffuse laterally around the membrane.
- Via lipid-exchange proteins where a protein extracts a lipid from one membrane, diffuses through the cell, and inserts the lipid into another membrane.
What do most transmembrane proteins contain that direct them to the ER membrane?
ER signal sequence
What conditions of the protein must be met in order for a region to become a transmembrane segment?
If the polypeptide contains a stretch of amino acids that are mostly hydrophobic and form “a helix”
What are the steps for the transmembrane protein to enter the plasma membrane of the ER Lumen?
- The protein is synthesized by a ribosome and begins synthesis into the ER. The ER signal sequence is cleaved by signal peptidase.
- Synthesis continues and a hydrophobic transmembrane segment is made as the polypeptide is threaded through the channel.
- Synthesis is complete and the transmembrane segment remains in the hydrophobic region of the phospholipid bilayer.
What structural feature of a polypeptide causes a region of it to form a transmembrane segment?
The most common feature causing a transmembrane segment to form is a stretch (about 20) of amino acids that mostly have hydrophobic (nonpolar) side chains.
What is the covalent attachment of a carbohydrate to a protein or lipid, producing a glycoprotein or glycolipid, respectively?
Glycosylation (glycolipids and glycoproteins)
What is thought of as the reason why carbohydrates are attached to lipids and proteins on the extracellular side of a plasma membrane?
Proper migration of individual cells and cell layers relies on the recognition of cell types via the carbohydrates on their cell surfaces.
They also have a protection effect.
What two types of protein glycosylation occur within eukaryotes?
N-linked and O-linked
What is N-linked glycosylation?
The carbohydrate is attached to a nitrogen atom of the asparagine side chain.
This involves 14 sugar molecules, called a carbohydrate tree, is first built onto a lipid found in the ER membrane.
An enzyme in the ER, oligosaccharide transferase, transfers the carbohydrate tree from the lipid to an asparagine in the polypeptide. This commonly occurs on the membrane proteins that are transported to the cell surface.
What is O-linked glycosylation?
This only occurs in the Golgi apparatus.
This involves a string of sugars to the oxygen atom of a serine or threonine side chain of a polypeptide.
This is important in animals for the production of proteglycans, which are highly glycosylated proteins that are secreted from cells and help organize the extracellular matrix that surrounds the cells.
Proteoglycans are also a component of mucus, a slimy material that coats many cell surfaces and is secreted into fluids such as saliva.
High concentrations of carbohydrates give mucus its slimy texture.
What is the movement of ions or molecules across a biological membrane?
Membrane transport
What is the property of membranes that allows the passage of certain ions or molecules but not others?
Selective permeability
What are the three ways substances can move directly across a membrane?
Simple diffusion, facilitated diffusion, and active transport.
What is when a substance moves across a membrane from a region of high concentration to a region of lower concentration by passing directly through the phospholipid bilayer?
Simple diffusion
This is an example of passive transport. The diffusion of a solute across a membrane in a process that is energetically favorable and does not require an input of energy.
What is a mechanism of passive transport in which a transport protein provides a passageway for a substance to cross a membrane from an area of higher concentration to one of lower concentration?
Facilitated diffusion.
Another example of passive transport.
What is the transport of a substance across a membrane from an area of low concentration to one of higher concentration with the aid of a transport protein; requires an input of energy?
Active Transport (against the gradient)
What do you call ions and polor molecules in relation to water?
Solutes, because they dissolve in water which then makes a solute; which is what water is when solvents are dissolved by it. A solution is the final product.
What are the four factors that affect the ability of solutes to pass through a phospholipid bilayer?
Size, polarity, charge, and concentration.
What does transmembrane gradient or concentration gradient refer to (definition)?
A situation in which the concentration of a solute is higher on one side of a membrane than on the other.
A universal feature of all living cells. For example, right after you eat a meal containing carbohydrates, a higher concentration of glucose is found outside your cells than inside; this is an example of a chemical gradient.
What are the two components involving ions?
Electrical and chemical
What is the combined effect of both an electrical and a chemical gradient across a membrane; this determines the direction in which ions will move?
Electrochemical gradient
For example, a net positive charge outside the cell and larger amount of Na+ outside the cell compared to within a cell creates a Na+ electrochemical gradient.
What type of transport typically dissolves a pre-existing gradient?
Passive transport
What is the condition in which the solute concentrations on both sides of a plasma membrane are equal, which does not cause a cell to shrink or swell?
Isotonic
What is the difference between hypertonic and hypotonic in relation to outside the cell?
Outside the cell becomes hypertonic when the gradient is greater than the cytosolic side of a cell.
Hypotonic condition exists outside the cell when its gradient is lower than the inside of the cell
If solutes cannot readily move across the membrane, what happens in order to balance solute concentrations?
Water will move across the membrane from the hypotonic compartment (with a lower solute concentration) into the hypertonic compartment (with a higher solute concentration)
This process is called OSMOSIS.
What senses changes in cell volume and allows for necessary movements across the membrane to prevent osmotic changes and maintain normal cell shape?
Transport proteins
What phenomenon occurs when cells, in extreme cases, takes up too much water and ruptures? (when placed in a hypotonic solution)
Osmotic lysis
What is the process called when a cell is placed into a hypertonic solution where the cell will exert water and shrink in an effort to stabilize solute concentrations?
Crenation
Let’s suppose the inside of a cell has a solute concentration of 0.3 M and the outside has a concentration of 0.2 M. If the membrane is impermeable to solutes, in which direction will water move?
Water will move from outside to inside, from the lower to the higher concentration.
What prevents organisms like bacteria, fungi, algae, and plant cells from osmotic lysis?
The rigidity of the cell wall prevents the cell from taking up more water than it can.
What is the shrinkage of algal or plant cytoplasm that occurs when water leaves the cell by osmosis, with the result that the plasma membrane no longer presses on the cell wall?
Plasmolysis
What do contractile vacuoles do in microorganisms, like amoebae and paramecia, that live in extracellular environments that are extremely hypotonic?
Water has a great tendency to move into these cells via osmosis, and when the water does, one or more contractile vacuoles prevent osmotic lysis by taking up water and then periodically discharging it by fusing with the plasma membrane.
What is a transmembrane protein that provides a passageway for the movement of ions and hydrophilic molecules across the phospholipid bilayer?
Transport proteins
These play a central role in the selective permeability of biological membranes.
What are the two categories of transport proteins that move solutes across the membrane?
Channels and transporters.
What is a transmembrane protein that forms an open passageway for the facilitated diffusion of ions or molecules across a membrane?
A channel
When this channel is open, the movement of solutes can be extremely quick, up to 100 million ions or molecules per second.
What is the property of many channels that allows them to open and close to control the movement of solutes across a membrane?
Gated
How are these gated channels controlled?
The noncovalent bonding of small molecules such as ligands.
What are ligands?
Ligands are molecules such as hormones and neurotransmitters.
They are often important in the cell transmission of signals between neurons and muscle cells or between two neurons.
How did Peter Ager first prepare his experiment to test if protein, CHIP28, was responsible for the channel within red blood and liver cells? Try to remember the other steps as well. (3) Steps Total
- They isolated the gene or nucleotides of the CHIP28 in a test tube and added an enzyme, RNA polymerase, to synthesize many copies of CHIP28 mRNA.
- CHIP28 mRNA was inserted into frog eggs where its ribosomes create CHIP28 proteins. The CHIP28 proteins then are inserted into the plasma membrane to become a transmembrane protein.
- As a control measure, frog eggs without the CHIP28 protein are measured under microscopy as well as the experimental group while in a hypotonic medium.
Result: The experimental group ruptured via osmotic lysis
BONUS: What does CHIP28 stand for?
Channel-forming integral membrane protein with a molecular mass of 28,000 daltons. It was renamed aquaporin to indicate its newly identified function of allowing water to diffuse through a channel in the membrane.
What observations about particular cell types in the human body led to the experimental strategy of Peter Ager’s experiment?
Most cells allow movement of water across the cell membrane by passive diffusion. However, it was noted that certain cell types had a much higher rate of water movement, indicating that something different was occurring in these cells. (Red blood cells; kidney and bladder cells that regulate water balance in animals)
What were the characteristics of CHIP28 that made Agre and associates speculate that it may transport water? In your own words, briefly explain how they tested the hypothesis that CHIP28 has this function.
The researchers identified water channels by characterizing proteins that are present in red blood cells and kidney cells but not other types of cells. Red blood cells and kidney cells have a faster rate of water movement across the membrane than other cell types. These cells are more likely to have water channels. By identifying proteins that are found in both of these types of cells but not in other cells, the researchers were identifying possible candidate proteins that function as water channels. In addition, CHIP28 had a structure that resembled other known channel proteins. Agre and his associates experimentally created multiple copies of the gene that produces the CHIP28 protein and then artificially transcribed the genes to produce many mRNAs. The mRNAs were injected into frog oocytes where they could be translated to make the CHIP28 proteins. After altering the frog oocytes by introducing the CHIP28 mRNAs, the researchers compared the rate of water transport in the altered oocytes versus normal frog oocytes. This procedure allowed them to introduce the candidate protein into a cell type that normally does not have the protein present.
Explain how the results of the experiment support the proposed hypothesis.
After artificially introducing the candidate protein into the frog oocytes, the researchers found that the experimental oocytes took up water at a much faster rate in a hypotonic solution as compared to the control oocytes. The results indicated that the presence of the CHIP28 protein did increase water transport into cells.
What is a transmembrane protein that binds a solute and undergoes a conformational change to allow the movement of the solute across a membrane; also called a carrier?
Transporters
They are bound in a hydrophilic pocket where the change switches exposure of the pocket from one side of the membrane to the other side.
Since providing the principle pathway for cellular uptake, what does transporters help the cells accomplish?
Uptake of organic molecules such as sugars, amino acids, and nucleotides to include some hormones and neurotransmitters.
In addition to the above, they also help in exporting waste. Ex. transporter remove lactic acid, a by-product of muscle cells during exercise.
Others regulate pH level and controlling cell volume.
T/F Transporters are much faster than channels.
False. They’re rate of transport is typically 100 to 1,99
How are transporters named?
The number of solutes they bind and the direction in which they transport those solutes.
Three Types of Transporters
What is a type of transporter that binds a single ion or molecule and transports it across the membrane?
What is a type of transporter that binds two or more ions or molecules and transports them in the same direction across a membrane; also called a cotransporter?
What is a type of transporter that binds two or more ions or molecules and transports them in opposite directions across a membrane?
Uniporter
Symporter
Antiporter
What is the difference between a channel and a transporter?
When its gate is open, a channel provides a direct passageway for the movement of a solute across a membrane.
A transporter does not provide a direct passageway for such movement. Instead, a solute must first enter a hydrophilic pocket on one side of the membrane.
The transporter then undergoes a conformational change that exposes the pocket on the other side of the membrane, where the solute is released.
What is the movement from a region of lower concentration to one of higher concentration? (in other words)
The movement of a solute across a membrane against its concentration gradient, which is what active transport accomplishes.
This process is energetically unfavorable and requires an input of energy.
What is a type of transport that involves pumps that directly use energy to transport a solute against a gradient?
Primary active transport
What is a transporter that directly couples its conformational changes to an energy source, such as ATP hydrolysis?
A pump (aka primary active transport)
What is a type of membrane transport that involves the utilization of a pre-existing gradient to drive the active transport of another solute?
Secondary active transport
Ex. In animals, Na+/solute symporters are prevalent in animal cells. The Na+ electrochemical gradient functions as normal, while the another solute “hitches” a ride with the Na+. In this case, the solute is the only actively transported substance.
H+/sucrose symporters are more common in bacteria, fungi, algae, and plant cells.
What was discovered about the phenomenon of active transport in the 1940s, where it was a study conducted of the transport of sodium ions (Na+) and potassium ions (K+)?
In animal cells, the concentration of Na+ is lower inside the cell than outside of the cell.
Whereas K+ is higher inside the cell than outside.
After analyzing the movement of these ions across the plasma membrane of muscle cells, neurons, and red blood cells, researchers determined that the export of Na+ is coupled to the import of K+.
What was the pump pertaining to Na+/K+ named?
Na+/Ka+ - ATPase.
Every time ATP is hydrolyzed for a Na+/K+ - ATPase pump, how many ions of Na+ move into the extracellular environment and how man K+ ions move into the cytosol?
3 Na+ ions and 2 K+ ions
What attachment and detachment of a molecules switches a Na+/K+ - ATPase antiporter from the E2 (transport of Na+) protein conformation to the E1 (transport of K+) protein conformation?
A phosphate molecule created from the hydrolysis of ATP
ATP —-> ADP + Phosphate
What is a pump that generates an electrical gradient across a membrane?
Electrogenic pump
Ex. [Na+/K+ - ATPase antiporter pump, since one cycle of pumping results in the net export of one positive charge, the Na+/K+ - ATPase also produces an electrical gradient across the membrane.
Against the gradient = low to high concentration
What happens to the protein (for E1 and E2 conformations) when phosphate attaches to the transmembrane pump?
It changes the structure of the protein which also changes its affinity towards certain ions like Na+ and K+
What do you call the mechanism when phosphate is bound to a pump covalently?
Phosphorylation
[Begin Exocytosis and Endocytosis] What are the two other mechanisms in which eukaryotes transport larger molecules, like proteins and polysaccharides across a plasma membrane?
Exocytosis and endocytosis
What are two examples of exocytosis?
Secretion of hormones (polypeptides) such as insulin from beta cells of the pancreas
Digestive enzymes from the exocrine cells in the lumen of the small intestine are secreted for digestion.
What are three examples of endocytosis?
Insoluble important nutrients are bound to proteins in the blood like lipids and iron and taken into cells.
Macrophages, cells of the immune system, engulf and destroy bacteria via phagocytosis
What is a process in which material inside a cell is packaged into vesicles and excreted into the extracellular environment? (typically by the Golgi apparatus)
Exocytosis
What is the purpose of the protein coat in exocytosis?
The coat allows the budding process at the surface of the Golgi membrane to form a vesicle.
What is a process in which the plasma membrane invaginates, or folds inward, to form a vesicle that brings substances into the cell?
Endocytosis
What are the three types of endocytosis?
Receptor-mediated endocytosis
Pinocytosis
Phagocytosis
What is a common type of endocytosis in which a receptor in the membrane is specific for a given cargo?
Receptor-mediated endocytosis
What is a type of endocytosis that involves the formation of membrane vesicles from the plasma membrane as a way for cells to internalize the extracellular fluid?
Pinocytosis (from the Greek word meaning, cell drinking) (if the material is a liquid)
This allows a cell to sample the extracellular solutes. Particularly important with cells actively involved in nutrient absorption.
What is a type of endocytosis that involves the formation of a membrane vesicle, called a phagosome, or phagocytic vacuole, which engulfs a particle such as a bacterium?
Phagocytosis. (particulate, like a bacterial cell or organic fragment)
Macrophages kill bacteria this way. Once inside the Macrophage, the phagosome (cargo) fuses with a lysosome, and the digestive enzymes within the lysosome destroy the bacterium.
