Molecular Biology Wk 4

Question 1

what is a cell

Answer 1

A living cell is a self-reproducing system of molecules held inside a container. That container is the plasma membrane—a protein-studded, fatty film so thin that it cannot be seen directly in the light microscope

Question 2

what is the plasma membrane

Answer 2

Plasma membrane consists of a two-ply sheet of lipid molecules about 5 nm thick.

Cell membranes act as selective barriers. The plasma membrane separates a cell from its surroundings, enabling the molecular composition of a cell to differ from that of its environment.

Question 3

what are the membrane-bound organelles of a cell

Answer 3

ER
Transport vesicle
nucleus
peroxisome
lysosome
Golgi apparatus
mitochondria

Both nucleus and mitochondria are enclosed by two membranes

Question 4

Functions of the plasma membrane

Answer 4

receptor proteins in the PM act as sensors that enable the cell to receive information about changes in its environment and respond to them

the flexibility of the membrane and its capacity for expansion allow cell growth and movement

if a cell is to survive and grow, nutrients must pass inward across the PM and waste products must pass out. the highly selective channels and pumps protein molecules allow specific substances to be imported and others to be exported

Question 5

what is cell signalling

Answer 5

In a typical communication between cells, the signaling cell produces a particular type of extracellular signal molecule that is detected by the target cell.

extracellular signal molecules bind either to the cell surface receptors or to intracellular enzyme or receptors

Question 6

explain the process of exocytosis

Answer 6

In all eukaryotic cells, a steady stream of vesicles buds from the trans Golgi network and fuses with the plasma membrane in the process of exocytosis. There are two pathways : constitutive and regulated. In secretory cells, the regulated and constitutive pathways of exocytosis diverge in the trans Golgi network.

Constitutive exocytosis pathway supplies the plasma membrane with newly made lipids and proteins.

Regulated exocytosis pathway operates only in cells that are specialized for secretion.

Question 7

what is regulated secretion

Answer 7

An increase in blood glucose signals insulin-producing endocrine cells in the pancreas to secrete the hormone. Secretory vesicles store insulin in a pancreatic β cell. The insulin in each secretory vesicle is stored in a highly concentrated, aggregated form. After secretion, the insulin aggregates dissolve rapidly in the blood.

Question 8

what is endocytosis

Answer 8

Eukaryotic cells are continually taking up fluid, as well as large and small molecules, by the process of endocytosis. Two main types of endocytosis are distinguished on the basis of the size of the endocytic vesicles formed.

Pinocytosis (“cellular drinking”) involves the ingestion of fluid and molecules via small pinocytic vesicles (250 nm in diameter). Whereas all eukaryotic cells are continually ingesting fluid and molecules by pinocytosis, large particles are ingested mainly by specialized phagocytic cells.

Macrophages, for example, ingest more than 1011 of your worn-out red blood cells each day

Question 8

what is endocytosis

Answer 8

Eukaryotic cells are continually taking up fluid, as well as large and small molecules, by the process of endocytosis. Two main types of endocytosis are distinguished on the basis of the size of the endocytic vesicles formed.

Pinocytosis (“cellular drinking”) involves the ingestion of fluid and molecules via small pinocytic vesicles (250 nm in diameter). Whereas all eukaryotic cells are continually ingesting fluid and molecules by pinocytosis, large particles are ingested mainly by specialized phagocytic cells.

Macrophages, for example, ingest more than 1011 of your worn-out red blood cells each day

Question 9

what is pinocytosis

Answer 9

Pinocytosis, a type of endocytosis. An invagination of the plasma membrane encapsulates many water-soluble solutes ranging in size from salts to macromolecules.
➢ Pinocytosis is, in some cases, considered to be a constitutive process, while in others it is receptor-mediated and highly regulated. Pinocytosis is carried out mainly by the clathrin-coated pits and vesicles.

Question 10

what is the lipid bilayer

Answer 10

Cells are filled with—and surrounded by—water, so the structure of cell membranes is determined by the way membrane lipids behave in a watery (aqueous) environment.

Question 11

what are phospholipids

Answer 11

A typical membrane lipid molecule has a hydrophilic head and two hydrophobic tails.

for phosphatidylcholine the tail is kinked

all membrane lipids are amphipathic

Question 12

what is amphipathic

Answer 12

Amphipathic molecules are subject to two conflicting forces: the hydrophilic head is attracted to water, while the hydrophobic tails shun water and seek to aggregate with other hydrophobic molecules.

Question 13

Phospholipid bilayers spontaneously close in on themselves to form sealed compartments.

describe this statement

Answer 13

Amphipathic molecules such as phospholipids necessarily assemble into self-sealing containers that define closed compartments.
●This remarkable behavior, fundamental to the creation of a living cell, is simply a result of the property that each molecule is hydrophilic at one end and hydrophobic at the other.
●The closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water, which would be energetically unfavorable.

Question 14

how can the fluidity of bilayers be studied

Answer 14

using synthetic lipid bilayers, which are easily produced by the spontaneous aggregation of amphipathic lipid molecules in water. Like fluidity, flexibility is important for membrane function, and it sets a lower limit of about 25 nm to the size of vesicle that cell membranes can form. Pure phospholipids, for example, can form closed, spherical liposomes, when added to water; they vary in size from about 25 nm to 1 mm in diameter.

Question 15

describe the types of movement of membrane phospholipid molecules

Answer 15

Membrane phospholipids are motile.

Without proteins to facilitate the process, it is estimated that this event, called “flip- flop,” occurs less than once a month for any individual lipid molecule under conditions similar to those in a cell. On the other hand, as the result of random thermal motions, lipid molecules continuously exchange places with their neighbors in the same monolayer. This exchange leads to rapid lateral diffusion of lipid molecules within the plane of each monolayer.

flexion and rotation

Because of these movements, the bilayer behaves as a twodimensional fluid, in which the individual lipid molecules are able to move in their own monolayer.

Question 16

The Fluidity of a Lipid Bilayer Depends on Its Composition

describe this

Answer 16

Two major properties of hydrocarbon tails affect how tightly they pack in the bilayer: their length and the number of double bonds they contain: 1.A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another and therefore increases the fluidity of the bilayer.
2. Lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid .

Question 17

what is the function of cholesterol

Answer 17

In animal cells, membrane fluidity is modulated by the inclusion of the sterol cholesterol. Cholesterol tends to stiffen cell membranes. Because cholesterol molecules are short and rigid, they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails

Question 18

describe membrane assembly

Answer 18

In eukaryotic cells, new phospholipids
are manufactured by enzymes bound to the cytosolic surface of the endoplasmic reticulum.

Using free fatty acids as substrates, the enzymes deposit the newly made phospholipids exclusively in the cytosolic half of the bilayer.

Newly synthesized phospholipids are added to the cytosolic side of the ER membrane and then redistributed by enzymes that transfer them from one half of the lipid bilayer to the other.

Question 19

Certain Phospholipids Are Confined to One Side of the Membrane

describe this

Answer 19

Newly synthesized phospholipids are added to the cytosolic side of the ER membrane and then redistributed by enzymes that transfer them from one half of the lipid bilayer to the other. When membranes leave the ER and are incorporated in the Golgi, they encounter enzymes called flippases, which selectively remove phosphatidylserine (light green) and phosphatidylethanolamine (yellow) from the noncytosolic monolayer and flip them to the cytosolic side.

This transfer leaves phosphatidylcholine (red) and sphingomyelin (brown) concentrated in the noncytosolic monolayer. The resulting curvature of the membrane may actually help drive subsequent vesicle budding.

Question 20

describe the distribution of phospholipids

Answer 20

Phospholipids and glycolipids are distributed asymmetrically in the lipid bilayer of a eukaryotic plasma membrane.

Phosphatidylcholine (red) and sphingomyelin (brown) are concentrated in the noncytosolic monolayer
phosphatidylserine (light green), and phosphatidylethanolamine (yellow) are found mainly on the cytosolic side.
cholesterol (green) is distributed almost equally in both monolayers.
phosphatidylinositols (dark green), are shown in the cytosolic monolayer, where they participate in cell signaling.

Membranes are transported by a process of vesicle budding and fusing. Membranes retain their orientation during transfer between cell compartments.

Question 21

What are the functions of membrane proteins

Answer 21

Some transport particular nutrients, metabolites, and ions across the lipid bilayer.
➢Others anchor the membrane to macromolecules on either side.
➢Still others function as receptors that detect chemical signals in the cell’s environment and relay them into the cell interior, or
➢Work as enzymes to catalyze specific reactions at the membrane.
Each type of cell membrane contains a different set of proteins, reflecting the specialized functions of the particular membrane.

Question 22

What are the two types

Answer 22

Integral membrane proteins - proteins are directly attached to the lipid bilayer- can be removed by only disrupted the bilayer with detergents

Peripheral membrane proteins- the remaining membrane proteins which can be released from the membrane by more gentle extraction procedures that interfere with the protein

Question 23

Membrane proteins can associate with the lipid bilayer in different ways.
Describe the first way

Answer 23

Transmembrane

these transmembrane proteins are amphipathic, having both hydrophobic and hydrophilic regions. Their hydrophobic regions lie in the interior of the bilayer, nestled against the hydrophobic tails of the lipid molecules. Their hydrophilic regions are exposed to the aqueous environment on either side of the membrane.

Question 24

Second way

Answer 24

Mono layer associated alpha helix

Other membrane proteins are located almost entirely in the cytosol and are associated with the cytosolic half of the lipid bilayer by an amphipathic α helix exposed on the surface of the protein

Question 25

Third way

Answer 25

lipid linked
Some proteins lie entirely outside the bilayer, on one side or the other, attached to the membrane only by one or more covalently attached lipid groups

Question 26

Fourth way

Answer 26

Protein attached

Yet other proteins are bound indirectly to one or the other face of the membrane, held in place only by their interactions with other membrane proteins

Question 27

What is the cell cortex

Answer 27

Stabilises the plasma membrane

Spectrin mesh work forms the cell cortex in human rbcs

Question 28

What are proteoglycans

Answer 28

One or more long polysaccharide chains

Question 29

How is the glycocalyx formed

Answer 29

All of the carbohydrate on the glycoproteins, proteoglycans, and glycolipids is located on the outside of the plasma membrane, where it forms a sugar coating called the carbohydrate layer or glycocalyx.

Question 30

What is the function of the glycocalyx

Answer 30

This layer of carbohydrate helps protect the cell surface from mechanical damage. As the oligosaccharides and polysaccharides adsorb water, they also give the cell a slimy surface, which helps motile cells such as white blood cells squeeze through narrow spaces and prevents blood cells from sticking to one another or to the walls of blood vessels.

Question 31

The recognition of the cell-surface carbohydrate on neutrophils is the first stage of their migration out of the blood at sites of infection

Answer 31

Specialised transmembrane proteins called lectins.

These proteins recognize particular sugar groups carried by glycolipids and glycoproteins on the surface of neutrophils circulating in the blood

The neutrophils consequently stick to the endothelial cells that line the blood vessel wall. This association is not very strong, but it leads to another, much stronger protein–protein interaction (not shown) that helps the neutrophil slip between the endothelial cells, so it can migrate out of the bloodstream and into the tissue at the site of infection.