Chapter 07 - Cell Membranes FRQ and Notes

Membrane Proteins and Their Functions

-A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer -Proteins determine most of the membrane's specific functions

Describe how each of the following can affect membrane fluidity: a. decreasing temperature b. phospholipids with unsaturated hydrocarbon chains c. cholesterol

-As temperatures cool, membranes switch from a fluid state to a solid state.• The temperature at which a membrane solidifies depends on the types of lipids -Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids -Membranes must be fluid to work properly; they are usually about as fluid as salad oil -The steroid cholesterol has different effects on membrane fluidity at different temperatures -At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids -At cool temperatures, it maintains fluidity by preventing tight packing

Membrane carbohydrates are important in cell-cell recognition. What are two examples of this?

-Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane -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. -Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Membrane proteins are the mosaic part of the model. Describe each of the two main categories: integral proteins peripheral proteins

-Peripheral proteins are bound to the surface of the membrane -Integral proteins penetrate the hydrophobic core -Integral proteins that span the membrane are called transmembrane proteins -The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

What is meant by membrane fluidity? Describe the movements seen in the fluid membrane.

-Phospholipids in the plasma membrane can move within the bilayer -Most of the lipids, and some proteins, drift laterally -Rarely does a molecule flip-flop transversely across the membrane

Who proposed the fluid mosaic model of membrane structure? When? Describe this model.

-The fluid mosaic model states that a membrane is a fluid structure with a "mosaic" of various proteins embedded in it -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.

The ___________________________ is the boundary that separates the living cell from its surroundings The plasma membrane exhibits ____________ allowing some substances to cross it more easily than others _______________________ are the most abundant lipid in the plasma membrane

-The plasma membrane is the boundary that separates the living cell from its surroundings -The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others -Phospholipids are the most abundant lipid in the plasma membrane

Distinguish between 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

Distinguish between 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

In the 1960s, the Davson-Danielli model of membrane structure was widely accepted. Describe this model and then cite two lines of evidence that were inconsistent with it.

In Davson and Danielli's sandwich model, proposed in 1935 that -The membrane is coated on both sides with hydrophilic proteins, forming a phospholipid bilayer between two layers of globular proteins. -By the late 1960s, however, many cell biologists recognized two problems with the model, particularly the placement of membrane proteins which have hydrophilic and hydrophobic regions. -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

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

Use Figure 7.9 to briefly describe major functions of membrane proteins.

Six major functions of membrane proteins: -Transport -Enzymatic activity -Signal transduction -Cell-cell recognition -Intercellular joining -Attachment to the cytoskeleton and extracellular matrix (ECM)

The large molecules of all living things fall into just four main classes. Name them.

The critically important large molecules of all living things fall into just four main classes: carbohydrates, lipids, proteins and nucleic acids. On the molecular scale, members of three of the four classes-carbohydrates, proteins and nucleic acids-are huge and are therefore called macromolecules.

Are transport proteins specific? Cite an example that supports your response.

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

Explain what is meant when we say a molecule is amphipathic.

amphipathic molecules, contain a hydrophobic and hydrophilic regions like Phospholipids