Chapter 7 Membrane Structure and Function Flashcards
Explain the term “amphipathic”.
Amphipathic molecules have both a hydrophilic and a hydrophobic region.
Describe the Davson-Danielli model of membrane structure.
In Davson and Danielli’s sandwich model, proposed in 1935, the membrane is coated on both sides with hydrophilic proteins, forming a phospholipid bilayer between two layers of proteins. By the late 1960s, however, many cell biologists recognized two problems with the model. First, inspection of a variety of membranes revealed that membranes with different functions differ in structure and chemical composition. Secondly, unlike proteins dissolved in the cytosol, membrane proteins are not very soluble in water because they are amphipathic. If such proteins were layered on the surface of the membrane, their hydrophobic parts would be in aqueous surroundings.
Describe the fluid mosaic model of membrane structure.
In 1972, Singer and Nicolson proposed that membrane proteins reside in the phospholipid bilayer with their hydrophilic regions protruding. This molecular arrangement maximizes contact between the hydrophilic regions of proteins and phospholipids with water in the cytosol and extracellular fluid, while providing their hydrophobic parts with a non- aqueous environment. In this fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.
What is meant by membrane fluidity?
Membranes are not static sheets of molecules locked rigidly in place. Most of the lipids and some of the proteins can shift about laterally. It is quite rare for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to the other.
Describe how the following factors can affect membrane fluidity. Temperature? Cholesterol?
A membrane remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies. The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon chains. Because of kinks in the tails where double bonds are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, making the membrane more fluid. The steroid cholesterol, which is wedged between phospholipid molecules in the plasma membranes of animal cells, makes the membrane less fluid at high temperatures by restraining phospholipid movement and lowers the temperature required for the membrane to solidify by hindering the close packing of phospholipids. Thus, cholesterol can be thought of as a “fluidity buffer” for the membrane.
Describe the two types of membrane proteins.
Integral proteins penetrate the hydrophobic interior of the lipid bilayer. Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins.
How can membrane proteins transport molecules?
protein that spans membrane may provide hydrophilic channel across membrane; others shuttle substances from one side to another by changing shape; some hydrolyze ATP as an energy source to actively pump substances across membrane
How can membrane proteins impact enzymatic activity?
protein built into membrane may be an enzyme with active site exposed to substances in adjacent solution; several enzymes may be organized as a team carrying out sequential steps of a metabolic pathway
How can membrane proteins impact signal transduction?
membrane protein (receptor) may have binding site with specific shape that fits shape of chemical messenger, such as hormone; external messenger (signaling molecule) may cause protein to change shape, allowing it to relay message to inside of cell
How can membrane protein participate in cell-cell recognition?
some glycoproteins serve as ID tags specifically recognized by membrane proteins of other cells
How can membrane proteins function in intercellular joining?
membrane proteins of adjacent cells may hook together in various kinds of junctions; longer-lasting than cell-cell recognition
How do membrane proteins participate in attachment to cytoskeleton and ECM?
microfilaments, other elements of cytoskeleton may noncovalently bind to membrane proteins; helps maintain cell shape, stabilizes location of certain membrane proteins; proteins bound to ECM molecules can coordinate extracellular, intercellular changes
What are two examples of cell-cell recognition?
Cell-cell recognition is important in the sorting of cells into tissues and organs in an animal embryo and the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrates.
What macromolecule is important for cell-cell recognition?
Membrane carbohydrates are important in cell-cell recognition
Compare and contrast glycolipids and glycoproteins.
Glycolipids are membrane carbohydrates (short, branched chains of fewer than 15 sugar units) covalently bonded to lipids. However, most are covalently bonded to proteins, forming glycoproteins.
Compare and contrast channel proteins and carrier proteins.
Channel proteins such as aquaporins function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. Carrier proteins hold onto their passengers and change shape in a way that shuttles them across the membrane.
Are transport proteins specific?
Transport proteins are specific for the substances they translocate. For example, a specific carrier protein in the plasma membrane of red blood cells transports glucose across the membrane 50,000 times faster than glucose can pass through on its own. This “glucose transporter” is so selective that it even rejects fructose.
Peter Agre received the Nobel Prize in 2003 for the discovery of aquaporins. What are they?
Passage of water molecules through the membranes of certain cells is greatly facilitated by channel proteins known as aquaporins. Each aquaporin allows entry of up to 3 billion water molecules per second, passing single file through its central channel, which fits ten at a time.
How might the following materials cross the membrane? CO2? Glucose? H+? O2? H2O?
CO2- simple diffusion glucose-transport proteins H+-transport proteins O2- simple diffusion H2O- simple diffusion and protein channels (aquaporins)
Why doesn’ t a plant cell burst?
The plant cell is protected from lysis by the cell wall.
What is diffusion?
Diffusion is the movement of molecules of any substance so that they spread out evenly into the available space. In the absence of other forces, a substance will diffuse down its concentration gradient, i.e. from where it is more concentrated to where it is less concentrated.
What is passive transport?
Passive transport does not need to be induced by the cell through expenditure of energy.
What is osmosis?
The diffusion of free water across a selectively permeable membrane, whether artificial or cellular,
What will happen to a cell immersed in an isotonic, hypertonic, and hypotonic solution?
If a cell without a wall is immersed in an environment that is isotonic to the cell, there will be no net movement of water across the plasma membrane. In a hypertonic solution, the cell will lose water, shrivel, and probably die. In a hypotonic solution, water will enter the cell faster than it leaves, and the cell will swell and lyse (burst).
