3 - cell membranes

Question 1

why do we need biological membranes?

Answer 1

a cell must be separated from its environment if it is to maintain complex order in a chaotic physical world.

Question 2

what does the plasma membrane do?

Answer 2

the plasma membrane acts as a barrier between the cell and the external environment.
- allows a complex organised system to exist within

different organelles perform specific functions and need to be separated from the cytoplasm.

Question 3

how can select substances enter and leave the cell?

Answer 3

the cell membrane is selectively permeable.

it contains components that can regulate the transport of molecular species.

Question 4

what are the roles of cell membranes?

Answer 4

the roles of cell membranes are both morphological and functional.

cell membranes are flexible, allowing the cell to readily change shape, and self healing.

Question 5

how are cell membranes visiualised?

Answer 5

they cannot be visualised in detail by light microscopy so they are visualised instead by electron microscopy techniques.

Question 6

how thick are cell membranes?

Answer 6

cell membranes have a thickness of 6-8 nm wide and their structure is defined by a lipid bilayer.

Question 7

cell membrane composition

Answer 7

composition of a typical plasma membranes estimates by biochemical analysis:

• 40-45% lipid
• 45-50% protein
• 5-10% carbohydrate

Question 8

what is the most abundant type of lipid?

Answer 8

the most abundant type of lipid is a phospholipid

Question 9

what does each phospholipid consist of?

Answer 9

each phospholipid molecule consists of a hydrophobic fatty acid tail and a hydrophilic phosphate head.

Question 10

what are lipids

Answer 10

lipids are an exception among macromolecules because they do not form polymers.

their common feature is that they have little or no affinity for water (hydrophobic), either wholly or partially:
- amphiphilic lipids have both hydrophobic and hydrophilic (‘water loving’) regions.

Question 11

what does hydrophobicity stem from?

Answer 11

hydrophobicity stems from the domination of non-polar covalent bonds

Question 12

what does hydrophilicity stem from?

Answer 12

hydrophilicity stems from the presence of polar covalent bonds

Question 13

what are the 6 main classes of lipids?

Answer 13

• common fatty acids
• triacylglycerols
• carboxyl group
• phospholipids
• steroids
• glycolipids

Question 14

what do saturated fatty acids contain?

Answer 14

saturated fatty acids contain C-C single bonds with a hydrogen at every position.

SOLID at room temperature e.g. animal fats

Question 15

what do unsaturated fatty acids contain?

Answer 15

unsaturated fatty acids contain C=C double bonds with >2 fewer hydrogen atoms.

LIQUID at room temperature e.g. vegetable oils

Question 16

what is the difference between mono and polyunsaturated lipids?

Answer 16

fatty acids with one double bond are known as monounsaturated; those with two or more double bonds are called polyunsaturated.

Question 17

what do lipids with cis- form tend to be?

Answer 17

lipids with the cis- form tend to be oils; lipids with the trans- form tend to be more solid.

Question 18

what configuration do the atoms around the double bond tend to be?

Answer 18

the atoms around the double bond may be in cis- or trans- configuration.

Question 19

what do phospholipids form?

Answer 19

phospholipids form a major part of cell membranes.

Question 20

what do phospholipids consist of?

Answer 20

phospholipids consist of glycerol attached to two fatty acid chains.

the third hydroxyl group on glycerol combines with phosphoric acid to form a polar phosphate group.

Question 21

what are three common phospholipids?

Answer 21

• phosphatidic acid
• phosphatidyl-ethanolamine
• phosphatidyl-choline

Question 22

what is an amphiphilic molecule?

Answer 22

an amphiphilic (or amphipathic) molecule confers an ability to self-organise.

Question 23

what happens to phospholipids in water?

Answer 23

the hydrophobic tails are repelled by water:
- tail groups cluster together to minimise contact with water; head groups remain in contact with water.
- various structures can result from this.

the head is hydrophilic and the tail is hydrophobic.

Question 24

what are phospholipid bilayers?

Answer 24

phospholipid bilayers (a layer 2 molecules thick) are the basis of cell membranes

the hydrophobic tails pointing inwards, away from both the aqueous interior and the exterior of the cell.

Question 25

what are glycolipids?

Answer 25

glycolipids are amphipathic lipids with a carbohydrate containing head group.

Question 26

what are the biological roles of glycolipids?

Answer 26

biological roles include cellular recognition, attachment and energy provision.

Question 27

what are important glycolipid molecules industrially?

Answer 27

important molecules industrially: used as emulsifiers, in cosmetics, detergents and in drug delivery systems.

Question 28

what are steroids?

Answer 28

steroids are lipids with a carbon skeleton consisting of four fused carbon rings.

Question 29

how are different steroids created?

Answer 29

different steroids are created by varying the functional groups attached to the rings.

Question 30

what is the most important steroid?

Answer 30

the most important steroid is cholesterol, which is a precursor from which other steroids are derived.

Question 31

what are examples of steroids?

Answer 31

- cholesterol is essential for all animal life - involved in many biological processes - an important component of biological membranes. affects membrane "packaging" and "fluidity"

Question 32

what is the fluid-mosaic model of cell membranes?

Answer 32

it was proposed in 1972 by Singer and Nicolson after realising that: membranes must have a complex structure to carry out their many and varied activities. in their model, individual protein molecules move on and within a fluid bilayer of phospholipid. - membranes can be considered as a 2D liquid in which lipid and protein molecules diffuse.

Question 33

what contributes to the mechanical strength of membranes and their functionality?

Answer 33

proteins are highly variable in structure and function but they all contribute to the mechanical strength of membranes and their functionality.

Question 34

what are the two main roles of the cell membrane?

Answer 34

morphological = barrier between a cell (or organelle) and its environment. functional = regulated the exchange of compounds; transmits signals to and from the cell

Question 35

what are the four properties of a lipid bilayer with embedded proteins?

Answer 35

- strong (mechanical support) but flexile (allow for movement) - fluid (enables movement of membrane lipids and proteins) - selectively permeable (to control what gets in and out) - self-sealing (allows processes such as cell division without rupturing)

Question 36

what are integral membrane proteins?

Answer 36

Integral (intrinsic) membrane proteins are permanently attached to the membrane, either completely spanning it (transmembrane) or attaching to one side only

Question 37

what are peripheral membrane proteins?

Answer 37

Peripheral (extrinsic) membrane proteins adhere only temporarily to the membrane, associating with integral membrane proteins or the peripheral regions of the lipid bilayer

Question 38

what do integral membrane proteins act as?

Answer 38

some act as carrier molecules to actively transport substances. other as ore passive routers for the movement of material in and out of the cell. other integral proteins are enzymes, catalysing specific reactions at a particular location in the cell. many membrane proteins are sensors and receptors; the signal is often received at the outer face of the membrane and transduced across the membrane. some proteins on the inner surface attach onto the cytoskeleton to anchor the membrane in place.

Question 39

what do transporters do?

Answer 39

transporters carry a molecule from one side of the plasma membrane to the other.

Question 40

what do receptors do?

Answer 40

receptors can bind an extracellular molecule and this activates an intracellular process.

Question 41

what do enzymes in the membrane do?

Answer 41

enzymes in the membrane can do the same thing they do in the cytoplasm of a cell: transform a molecule into another form/species.

Question 42

what do anchor proteins do?

Answer 42

anchor proteins can physically link intracellular structures with extracellular structures.

Question 43

what are 5 more specific functions enabled by membrane proteins and carbohydrates?

Answer 43

- cell recognition - receptor sites for hormones and neurotransmitters - maintains the ionic asymmetry across the membrane - transmission of nerve impulses - metabolic activities

Question 44

..

Answer 44

Intracellular membranes (membranes inside the cell) have similar properties to that of the cell surface membranes, though can have very different compositions/structures. noted some important features for individual organelles

Question 45

why do almost all cellular processes rely upon intracellular membranes?

Answer 45

almost all cellular processes rely upon intracellular membranes: - acting as a reaction surface - acting as an intracellular transport system - providing separate intracellular compartments (isolating different chemical reactions/functions)