Chapter 11, Problem 26DAP

(a)

Summary Introduction

To explain:  The reasons of the following statement “When the membranes showed a “railroad track” appearance along with two dark-staining lines that are separated by a light space”.

Introduction:

The fluid mosaic model of plasma membrane was proposed by Singer and Nicolson. He concluded that the structure of plasma membrane is arranged in mosaic pattern. It is composed of phospholipid bilayer, cholesterol, intrinsic as well as extrinsic protein and carbohydrates.

Explanation of Solution

The three models are used to explain the following results:

Model A: This model is supported as it explains that two visible dark-lines are either protein layer or phospholipids heads whereas clear space or empty space is either lipid bilayer or hydrophobic core.

Model B: This model is not supported as it requires uniformly stained band that surrounds the cell.

Model C: This model is also supported as it concludes that two dark lines that appear are phospholipid heads while the clear zone is tails. It helps to drawn an assumption that membrane proteins are not observable as they are not stained with osmium dye.

(b)

Summary Introduction

To explain: The reasons of the following statement “The thickness of membranes in cells fixed and stained in the same way was found to be 5-9nm. The thickness of a “naked” phospholipid bilayer, without proteins was to 4-4.5nm and thickness of single monolayer of protein was about 1nm”.

Introduction:

Plasma membrane is serves as a selectively permeable membrane as it allows the entry and exists of only selected particles to go in and out from the cell. It is made of lipid bilayer in which hydrophobic tail is present inside the lipid membrane while hydrophilic tail is present outside the membrane. It also consists of two types of proteins in which extrinsic protein is present at the surface of lipid bilayer and extrinsic protein is embedded within the lipid bilayer.

Explanation of Solution

The three models are used to explain the following results:

Model A: This model is supported as it indicates a “naked” bilayer of size 4.5 nm and two layers of proteins of 2 nm that is equal to 6.5 nm. This value is under observed range of thickness.

Model B: This model is not supported as it is unable to make any predictions regarding the thickness of membrane.

Model C: This model is unclear as it predicts that the membrane is thicker than “naked bilayer”. It is supported only when substantial amount of protein projects from bilayer.

(c)

Summary Introduction

To explain: The reasons of the following statement “The average amino acid composition of membrane proteins is not distinguishable from that of soluble proteins. In particular, a substantial fraction of residues is hydrophobic”.

Introduction:

Fluid mosaic model explains that the protein molecules are dispersed in between the lipid bilayer of cell membrane. These protein molecules either are associated with the lipid layer or are present freely on the surface of the layer that creates a mosaic. The phospholipids and proteins can move freely within the membrane as lipid molecules contribute to the fluid nature of membrane. The free movement of proteins and phospholipids creates a fluid mosaic pattern.

Explanation of Solution

Three models are designed to explain this statement such as:

Model A: The result of this model is unclear as it concludes that proteins are bound with membrane by ionic interactions. It suggests that membrane protein contains high proportion of charged amino acids. It also explains that protein layer is very thin and there is no space for hydrophobic protein core and hence hydrophobic residues are exposed to solvent.

Model B and Model C: Both the models are supported as they predict that protein is composed of a mixture of hydrophobic and charged residues.

(d)

Summary Introduction

To explain: The reasons of the following statement “The ratio of monolayer area to cell membrane area was about 2.0. At higher pressures-thought to be more likely those found in cells-the ratio was substantially lower”.

Introduction:

The proteins present on the lipid layer of a cell membrane play very important role. They act as a channel which provides the entry to selected substances into the cell from the external environment of the cell. The proteins thus allow cell membrane to be selectively permeable so that only important substances which are required for the cellular functions could enter the cell.

Explanation of Solution

Three models that are used to explain this statement are as follows:

Model A: This model is unclear. It is difficult to predict the result from this model because it provides the ratio of 2.0 and this ratio is very difficult to achieve under physiological pressures.

Model B: This model is also not supported because it fails to make any predictions about the amount of lipid in membrane.

Model C: This model is supported because it concludes that the surface area of membrane is covered with proteins and the ratio obtained is less than 2.0 which is observed under physiological relevant conditions.

(e)

Summary Introduction

To explain: The reasons of the following statement “Circular dichroism spectroscopy uses changes in polarization of UV light to make inferences about the protein secondary structure. On average, this technique showed that membrane proteins have a large amount of a helix and little or no β–sheet. This finding was consistent with most membrane proteins having globular structures”.

Introduction:

A cell membrane is made of different molecules like phospholipids, proteins and cholesterol that contribute for the flexible nature of a cell membrane. A cell membrane is selectively permeable in nature which means it allows selective substances to enter the cell. The movement of molecules could be passive like in osmosis and diffusion, active transport or facilitated transport

Explanation of Solution

There are three models that are used to explain this statement:

Model A: This model is unclear as it predicts that proteins are in its extended confirmation and is supported only when proteins are layered on the surface of helical segments.

Model B and Model C: Both the models are supported as it predicts that the globular proteins mostly contain helical segments.

(f)

Summary Introduction

To explain: The reasons of the following statement “Phospholipase C is an enzyme that removes polar head group from phospholipids. Studies show that treatment of intact membranes with phospholipase C removed about 70% of the head groups without disrupting “railroad track” structure of membrane”.

Introduction:

Plasma membrane serves as a selectively permeable membrane as it allows the entry and exists of only selected particles to go in and out from the cell. It is made of lipid bilayer in which hydrophobic tail is present inside the lipid membrane while hydrophilic tail is present outside the membrane

Explanation of Solution

Three models are used to explain this statement which involves:

Model A: This model is unclear as it fails to explain that head groups of phosphorylamine are protected by protein layer. It is supported if it explains that protein covers the surface of phospholipids which is protected by phospholipase.

Model B and Model C: Both the models are supported as they explain that head groups are accessible to phospholipase.

(g)

Summary Introduction

To explain: The reasons of the following statement “In article of Singer “a glycoprotein of molecular weight is about 31,000 in human red blood cell membranes which is cleaved by tryptic treatment of membranes into soluble glycopeptides of about 10,000 molecular weight, while the remaining portions are quite hydrophobic”

Introduction:

The cell membrane is selectively permeable in nature which means it allows selective substances to enter the cell. Some molecules like polar molecules are transported by active transport as they need energy to be transported inside or outside the cell.

Explanation of Solution

Three models are used to explain this statement which involves:

Model A: It is not supported because it predicts that proteins are easily available for trypsin digestion and undergo the process of multiple cleavages with no protected hydrophobic segments.

Model B: This model is also not supported as all proteins are arranged in a bilayer and not available for trypsin.

Model C: This model is supported as the segments of proteins are able to span lipid bilayer that is protected from trypsin and segments that are exposed at surfaces are cleaved. The trypsin resistant regions have high proportion of hydrophobic residues.