B2.1 - membrane structure (3g)

Question 1

what is the basis of cell membranes?

Answer 1

phospholipids and other amphipathic lipids naturally form sheet-like bilayers in water

Question 2

how do lipid bilayers act as barriers?

Answer 2

the hydrophobic hydrocarbon chains that form the core of a membrane have low permeability to large molecules and hydrophilic particles, including ions and polar molecules, so membranes function as effective barriers between aqueous solutions

Question 3

what do lipid bilayers act as a barrier for?

Answer 3

this occurs because the phospholipid molecules are tightly packed meaning:
- large molecules cannot pass through
- smaller, polar or charged molecules do not readily pass through the hydrophobic core of the membrane
- (ie) the hydrophobic core is what keeps water molecules inside cells

Question 4

how do the amphipathic properties of phospholipids play a role in membrane structure?

Answer 4

• phospholipids have both hydrophobic and hydrophilic parts, which are both needed
• this forms a lipid bilayer (formed naturally by phospholipids in water)
• hydrophilic head faces outwards and hydrophobic tails face inwards

Question 5

what is the diversity of proteins associated with membranes?

Answer 5

membrane proteins are diverse in terms of:
1. structures - due to the extremely large number of combinations of amino acid sequences, and resulting 3D shapes

2. locations with respect to the membrane
3. functions - (eg, membrane transport, cell recognition, cell adhesion, hormone receptors, enzymes)

Question 6

what are integral proteins?

Answer 6

integral proteins are embedded in one or both of the lipid layers of a membrane
- transmembrane proteins extend from the fluid on one side of the membrane, through the membrane and into the fluid on the other side

Question 7

what are peripheral proteins?

Answer 7

peripheral proteins are attached to one or other surface of the bilayer

Question 8

what are the roles of peripheral proteins?

Answer 8

• they act as receptors for hormones
• they act as electron carriers

Question 9

what are channel proteins?

Answer 9

channel proteins have pores through which specific molecules can pass, dependent on their size and charge, through the process of facilitated diffusion
- they also act as receptors for neurotransmitters

Question 10

what are pump proteins?

Answer 10

pump proteins use energy from adenosine triphosphate (ATP) to transfer specific particles across membranes.
- pump proteins are specific to one type of molecule, based on the shape and chemical properties
- used in active transport

Question 11

what is the function of cholesterol?

Answer 11

reduces and regulates membrane fluidity
- found in animal membranes only

Question 12

what are aquaporins?

Answer 12

channel proteins that allow water molecules to move through a pore in their structure, facilitating water molecule movement across the membrane
- used in osmosis

Question 13

what are glycoproteins?

Answer 13

glycoproteins are carbohydrate structures linked to proteins

Question 14

what are glycolipids?

Answer 14

glycolipids are carbohydrate structures linked to lipids

Question 15

where are glycoproteins and glycolipids located?

Answer 15

these molecules position carbohydrates on the extracellular side of membranes

Question 16

what are the roles of glycoproteins and glycolipids ?

Answer 16

they have roles in:
- cell recognition - due to the diverse carbohydrate chains, including those that determine tissue type and must be matched for successful organ transplants
- cell adhesion

Question 17

what are the components of the fluid mosaic model of membrane structure, indicating hydrophobic and hydrophilic regions?

Answer 17

the phospholipids are fluid, allowing the movement of proteins within the membrane

hydrophilic regions of membrane components occur where they are exposed to the cytoplasm, extracellular fluid or phospholipid heads
- (eg) hydrophilic regions of proteins
- (eg) the entirety of peripheral membranes
- (eg) the carbohydrate part of glycoproteins
- (eg) the pore of channel proteins

hydrophobic regions of membrane components occur where exposed to the core of the membrane bilayer, where the hydrophobic phospholipid tails are located
- (eg) cholesterol is mainly hydrophobic, so is located amongst the phosopholipid fatty acid tails
- (eg) hydrophobic regions of proteins, such as the embedded regions of integral proteins (and transmembrane proteins)