6. The Membrane

Biological Membranes

Cell Membranes: The Phospholipid Bilayer | A-level Biology | OCR, AQA, Edexcel

 The fluid aspect of the membrane is due to the behavior of phospholipids, and the mosaic aspect is due to the presence of carbohydrates. 

 No. The fluid aspect is due to phospholipid behavior, but the mosaic is not due to the presence of carbohydrate. 

 The fluid aspect of the membrane describes its structure at normal temperatures, and the mosaic aspect describes the behavior of the membrane as the temperature is lowered. 

 No. The fluid mosaic model describes membrane structure at normal temperatures, wherein some components are more fluid and some are less fluid, and also describes how some components form mosaic structures. 

 The mosaic aspect of the membrane is due to the glycosylation of phospholipids on the cytoplasmic side of the membrane. 

 No. The carbohydrate chains are on the outer surface of the membrane and are not the reason that the plasma membrane is referred to as being mosaic. 

 Only phospholipids are capable of moving in the membrane. 

 No. Proteins can also move in the membrane, but they are far less mobile than phospholipids. 

 The fluid aspect of the membrane is due to the mobility of phospholipids, and embedded proteins account for the mosaic aspect. 

Consider what contributes to the fluid aspect of the model and what contributes to the mosaic aspect of membrane structure.

The plasma membrane is referred to as a "fluid mosaic" structure. Which of the following statements about that model is true?

 Parts of proteins that are exposed on the cytoplasmic side of the endoplasmic reticulum are also exposed on the cytoplasmic side of the plasma membrane. 

 No. This choice is true, but there is a better response. 

 The asymmetrical distribution of membrane proteins, lipids, and carbohydrates across the plasma membrane is determined as the membrane is being constructed. 

 Every integral membrane protein has a specific orientation in the plasma membrane. 

 The two lipid layers may differ in specific lipid composition. 

 All of the listed responses are correct. 

Consider how asymmetry may affect structure and function.

Which statement about the sidedness of the plasma membrane is correct?

According to the fluid mosaic model of membrane structure, proteins of the membrane are mostly
a. spread in a continuous layer over the inner and outer surfaces of the membrane.
b. confined to the hydrophobic interior of the membrane.
c. embedded in a lipid bilayer.
d. randomly oriented in the membrane, with no fixed inside-outside polarity.

Animation: Membrane in Motion

 Carbohydrates are found associated with the membranes of prokaryotic cells only. 

 No. Although carbohydrate is an important component of bacterial cell walls, carbohydrate is also found associated with the membranes of eukaryotic cells. 

 The carbohydrate composition of most eukaryotic plasma membranes is quite similar. 

 No. Variation in carbohydrate composition of the plasma membrane is critical in cellular recognition. 

 Carbohydrates on the plasma membrane are typically long, complex chains of several dozen monosaccharides. 

 No. The carbohydrates on cell surfaces are generally branched chains of fewer than 15 sugars. 

 Carbohydrates associated with the plasma membrane are located on both surfaces of the membrane. 

 No. The primary location of carbohydrates is on the outer surface of the plasma membrane, where they can contribute to cell-cell recognition. 

 Membrane carbohydrates function primarily in cell-cell recognition. 

Consider the structure and function of carbohydrates located on the plasma membrane.

Which of the following statements concerning carbohydrates associated with the plasma membrane is correct?

Consist of a single layer of phospholipids and proteins.

Consist of a phospholipid bilayer composed of a variety of fatty acids.

Consist of protein molecules embedded in a dynamic bilayer of phospholipids.

Consist of a phospholipid bilayer between two layers of hydrophilic proteins.

The fluid mosaic model of the membrane proposed that membranes:

 On the outside (external) surface of the membrane 

 No. The outside surface of the plasma membrane is associated with an aqueous solution. 

 No. The inside surface of the plasma membrane is associated with an aqueous solution. 

 In the interior and on the inside surface but not on the outside surface 

 On either surface of the membrane but not in the interior of the membrane 

 No. Cholesterol is hydrophobic and will be found in the interior of the bilayer membrane. 

Consider the chemical properties of cholesterol.

Consider the currently accepted fluid mosaic model of the plasma membrane. Where in the plasma membrane would cholesterol most likely be found?

Animation: Functions of the Plasma Membrane

Animation: Structure of the Plasma Membrane

Biological Membranes Example 1

 Phospholipids are completely unable to interact with water. 

 No. Phospholipids have a polar, hydrophilic head and a nonpolar, hydrophobic tail. 

 Phospholipids form a single sheet in water. 

 No. In water, phospholipids form a two-layered structure. 

 Phospholipids form a structure in which the hydrophobic portion faces outward. 

 No. In water, the hydrophilic heads face outward and the hydrophobic tails meet inside the bilayer. 

 They are triacylglycerols, which are commonly available in foods. 

 No. Phospholipids are diglycerides, with two fatty acids attached to glycerol. Triacylglycerols (fats and oils) have three fatty acids attached to glycerol. 

 Phospholipids form a selectively permeable structure. 

Recall that phospholipids are amphipathic and that molecules with like properties tend to associate.

Which of the following statements about the role of phospholipids in the structure and function of biological membranes is correct?

BioFlix: Membranes (extracellular matrix)

Cell membranes are way more complicated than you think - Nazzy Pakpour

Inside the Cell Membrane

Cell Membrane Structure and Function

 Cell membranes must be composed of polar molecules in order to interact with the aqueous solutions inside cells. 

 No. Hydrophobic molecules are nonpolar. They should not be able to interact strongly with aqueous solutions inside the cell because the cell membrane would break down. 

 In order to maintain a proper pH, cell membranes must be hydrophobic. 

 No. The hydrophobicity of the molecules does not affect pH. 

 Cell membranes must be hydrophobic in order to allow polar and charged molecules to enter and leave the cell. 

 No. Polar and charged molecules do not interact with hydrophobic molecules, so they do not readily cross cell membranes. 

 In order to perform their function of separating the aqueous solutions outside cells from the aqueous solutions inside cells, cell membranes cannot be soluble in water. 

Consider the meaning of the suffix "-phobic." What substance are cells primarily composed of?

Why are cell membranes composed primarily of hydrophobic molecules?

Fluid mosaic model | Cells | Biology | FuseSchool

 No. The interior of the plasma membrane is hydrophobic. 

 On the inside (cytoplasmic) surface of the membrane 

 No. Although this surface is hydrophilic, carbohydrates are not attached here. 

 On both hydrophilic surfaces of the membrane but not in the hydrophobic interior 

 No. Carbohydrates are found on the external surface of the plasma membrane as a means of identifying cell types. 

 Carbohydrates are rarely associated with plasma membranes. 

 No. Carbohydrates are almost always associated with the outer surfaces of the plasma membrane as a means of cell identification. 

Based on the functions of membrane carbohydrates, consider where the carbohydrates need to be to fulfill this function.

Consider the currently accepted fluid mosaic model of the plasma membrane. Where in the membrane would carbohydrates most likely be found?

The text claims that the evolution of an oxygen-rich atmosphere paved the way for increasingly efficient cellular respiration and higher growth rates in organisms. Explain.

Animation: Selective Permeability of Membranes

Which of the following factors would tend to increase membrane fluidity?
a. a greater proportion of unsaturated phospholipids
b. a greater proportion of saturated phospholipids
c. a lower temperature
d. a relatively high protein content in the membrane

Biological Membrane and Fluid Mosaic Model

Biological Physics   Fluid Mosaic Model   Facilitated Diffusion   Biological Membranes

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