Cell Membrane

Question 1

What are the features of cell membrane?

Answer 1

thin, pliable, elastic structure only 7.5 to 10 nanometers thick.

Question 2

What is the nature of permeability of cell membrane ?

Answer 2

Semi permeable ( allows free exchange of certain substances between ECF and ICF)

Question 3

What are compositions of cell membrane?

Answer 3

1. Proteins (55%)
2. Lipids (40%)
3. Carbohydrates (5%).

Question 4

How did Fluid musaic model describe proteins of plasma membrane?

Answer 4

the proteins are found to float in the lipid layer instead of forming the layers of the sandwich-type model.

Question 5

By electron microscope what are the layers of cell membrane?

Answer 5

_one central electron-lucent layer
_ two electron- dense layers.

Question 6

What are the differences between electron lucent layer and electron dense layers?

Answer 6

-The electron lucent layer is single central layer
While the electron dense layer are two layers placed one on each side of central layer.

-The electron lucent layer is lipid formed layer, While dense electron layers are protein formed layers.

Question 7

What is the characteristic feature of lipid layer?

Answer 7

It is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell

Question 8

What are the major lipids layer compositions?

Answer 8

1. Phospholipids
2. Cholesterol.

Question 9

What are the component of phospholipids layer of the cell membrane?

Answer 9

phosphorus and fatty acids:

-Aminophospholipids -sphingomyelins -phosphatidylcholine -Phosphatidyletholamine
-phosphatidylglycerol -phosphatidylserine
-phospha tidylinositol

Question 10

Describe the phospholipid molecules

Answer 10

Phospholipid molecules are arranged in two layers. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail portion.

Question 11

What are the differences between head portion and tail portion?

Answer 11

-Head portion is the polar end

Tail portion is the non-polar end.

-Head portion is soluble in water and has strong affinity for water (hydrophilic).

Tail portion is insoluble in water and repelled by water (hydrophobic).

-Hydrophilic head portions of outer layer face the ECF and those of the inner layer face ICF (cytoplasm).

hydrophobic tail portions meet in the center of the membrane.

Question 12

What are the benefit of cholesterol for cell membrane?

Answer 12

1-cholesterol is responsible for the structural integrity of lipid layer of the cell membrane.

2-Cholesterol molecules help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids.

3-Cholesterol controls much of the fluidity of the membrane as well

Question 13

Talk about permeability of cell membrane?

Answer 13

Since it is semi permeable membrane, So it allows only fat soluble substances to pass (O2,CO2, and alcohol)

while, water-soluble substances such as glucose, urea and electrolytes cannot pass through this layer.

Question 14

The most common protein substances that presen in cell membrane…

Answer 14

Glycoproteins

Question 15

What are the classification of protein molecules in cell membrane?

Answer 15

1. Integral proteins or transmembrane proteins.
2. Peripheral proteins or peripheral membrane proteins.

Question 16

What are the integral proteins?

Answer 16

proteins that pass through entire thickness of cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane.

Question 17

What are the benefits from integral proteins?

Answer 17

1-provide structural channels (or pores) through which water molecules and water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids.

2-Other integral proteins act as carrier proteins for transporting substances that otherwise could not penetrate the lipid bilayer.

3-serve as receptors for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane.

Question 18

What is the active transport?

Answer 18

Transport substances in the direction opposite to their natural direction of diffusion

Question 19

How integral proteins spanning the cell membrane provide information about the environment to the cell interior.

Answer 19

1-Interaction of cell membrane receptors with specific ligands that bind to the receptor causes conformational changes in the receptor protein.

2-enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as second messengers

3- relaying the signal from the extracellular part of the receptor to the interior of the cell.

Question 20

What are the peripheral proteins?

Answer 20

proteins which are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane.

Question 21

What is the relation between integral proteins and peripheral proteins of cell membrane?

Answer 21

Peripheral proteins are loosely bound with integral proteins or lipid layer of cell membrane. So, these protein molecules dissociate readily from the cell membrane.

Question 22

What is the function of peripheral proteins?

Answer 22

almost entirely as enzymes or as controllers of transport of substances through the cell membrane (pores).

Question 23

Receptor proteins…

Answer 23

serve as the receptor sites for hormones and neurotransmitters

Question 24

Antigens..?

Answer 24

Some proteins act as antigens and induce the process of antibody formation

Question 25

Which proteins are responsible for attachment of cells to their neighbors or to basal lamina.

Answer 25

integral proteins

Question 26

Main component of carbohydrate in cell membrane

Answer 26

proteoglycans

Question 27

What are the proteoglycans?

Answer 27

carbohydrate substances that bound to small protein core , they are loosely attached to the outer surface of the cell as well.

Question 28

What is glycocalyx

Answer 28

loose carbohydrate coat found on outside surface of cell

Question 29

What are the function of carbohydrate moieties in the cell membrane ?

Answer 29

1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negative objects. 2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another. 3. Many of the carbohydrates act as receptor substances for binding hormones, such as insulin; when bound, this combination activates attached internal proteinsthat, in turn, activate a cascade of intracellular enzymes. 4. Some carbohydrate moieties enter into immune reactions.

Question 30

What are the functions of cell membrane?

Answer 30

1. Protective function: Cell membrane protects the cytoplasm and the organelles present in the cytoplasm 2. Selective permeability: Cell membrane acts as a semipermeable membrane, which allows only some substances to pass through it and acts as a barrier for other substances 3. Absorptive function: Nutrients are absorbed into the cell through the cell membrane 4. Excretory function: Metabolites and other waste products from the cell are excreted out through the cell membrane 5. Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane 6. Maintenance of shape and size of the cell: Cell membrane is responsible for the maintenance of shape and size of the cell.

Question 31

What are the types of lipid movement within the cell membrane?

Answer 31

1-Fast lateral diffusion 2-Flip-flop movement

Question 32

What is the Fast lateral diffusion of lipid?

Answer 32

Movement of membrane lipids and some proteins laterally within the membrane.

Question 33

What is Flip-flop movement of lipid?

Answer 33

Movement of lipid molecules from one half of the bilayer to the other on a one for one exchange basis.

Question 34

Why lipid molecules rarely flip transversely across the membrane?

Answer 34

because hydrophilic parts would have to cross the membrane's hydrophobic core.

Question 35

What is the membrane Fluidity?

Answer 35

Fluidity of a phospholipid bilayer means that cells are pliable. Fluidity of the membrane prevents it from solidifying as external temperatures drop (cold)

Question 36

What are the factors that effect on the membrane fluidity?

Answer 36

1-Temperature 2-Lipid composition

Question 37

How can lipid composition effect on fluidity?

Answer 37

Saturated fatty acids in phospholipid tails are saturated with bound hydrogen atoms. There are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight. unsaturated fatty acids do not contain a maximal number of hydrogen atoms, but they do contain some double bonds between adjacent carbon atoms; a double bond results in a bend in the string of carbons of approximately 30 degree. Saturated fatty acids, with straight tails, are compressed by decreasing temperatures, and they will press in on each other, making a dense and fairly rigid membrane unsaturated fatty acids are compressed, the “kinked” tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules

Question 38

What is elbow room?

Answer 38

It helps to maintain fluidity in the membrane at temperatures at which membranes with high concentrations of saturated fatty acid tails would “freeze” or solidify.

Question 39

What is the effect of cold environment on cell membrane?

Answer 39

cold environment tends to compress membranes composed largely of saturated fatty acids, making them less fluid and more susceptible to rupturing.

Question 40

What is the role of cholesterol in membrane fluidity?

Answer 40

1-At 37°C, cholesterol makes the membrane less fluid by restraining phospholipid movement. 2-Cholesterol hinders the close packing of phospholipids; it lowers the temperature required for the membrane to solidify.