Biological Membranes

Question 1

What is the plasma membrane?

Answer 1

Phospholipid bilayer

Question 2

How can the phopholipid bilayer form?

Answer 2

Phospolipids are amphiphatic. They have a hydrophobic fatty acid tail which is non polar and a polar phosphate head. The bilayer forms as the phosphate heads project into the intra and extracellular fluid and the fatty acid tails which bundle together form the inside of the membrane

Question 3

What are the lipid tails made of?

Answer 3

Hydrocarbon chains which are non polar. They can be saturated or unsaturated (depending on carbon to carbon bonds) unsaturated fatty acid chains cant bundle together in the membrane as closely

Question 4

What does the fluidity of the phospholipid bilayer allow for?

Answer 4

• diffusion of lipids and proteins in the lateral plane
• interactions between lipids and proteins
• equal sharing of membranes between daughter cells following division
• allows cells to change shape

Question 5

What determines the fluidity of a membrane?

Answer 5

The composition of a membrane (cholesterol)

Question 6

How does cholesterol affect the membrane?

Answer 6

• Intercalates between membrane phospholipids
• Tightens packing in the bilayer
• Decreases membrane permeability to small molecules

Question 7

What does the plasma membrane do?

Answer 7

• Creates two different environments (intracellular and extracellular)
• Uses proteins to control what can enter and leave the cell
• Uses proteins to allow for cell identification

Question 8

In eukaryotic cells what does the plasma membrane contain?

Answer 8

• Phospholipids
• Glycolipids
• Cholesterol

Question 9

Why do phospholipids form sealed compartments?

Answer 9

Energetically favourable

Question 10

Where are lipid bilayers assembled?

Answer 10

Smooth endoplasmic reticulum

Question 11

Where are fatty acids synthesised?

Answer 11

In the cytesol

Question 12

How are phospholipids formed?

Answer 12

Fatty acids are embedded in the outer cytosolic leaflet of the SER and in steps the glycerol and phosphate groups are added

Question 13

How does phospholipid synthesis occur?

Answer 13

• the enzymes scramblase and flippase flip phospholipids between the outer and inner cytosolic leaflet of the SER membrane allowing them to be distributed evenly
• Vesicles transport the newly synthesised region of the bilayer to membranes where they fuse and the newly formed phospholipids are incorporated into the membrane

Question 14

What do scramblase and flipase do?

Answer 14

catalyse trans-bilayer movement of phospholipids in the SER

scramblase passively and flipase actively

Question 15

What is the glycocalyx

Answer 15

Region on the membrane in the extracellular fluid where glyco-(proteins and lipids) are found

Question 16

What does the glycocalyx do?

Answer 16

Forms a carbohydrate-rich layer which protects against mechanical and chemical damage

Question 17

What are the different types of membrane proteins?

Answer 17

• Integral and peripheral proteins
• Integral proteins can be single or multipass made of alpha helices or beta sheets
• Peripheral proteins can extend intracellularly or extracellularly

Question 18

How can proteins allow a charged protein to pass through the hydrophobic lipid bilayer

Answer 18

Hydrophobic R groups in the integral membrane protein face out into the lipid bilayer and the polar hydrophilic r groups are facing into the created channel allowing the amino acids to pass through the membrane

Question 19

How are Integral membrane proteins inserted into the membrane?

Answer 19

• a 20 amino acid hydrophobic single peptide at the N-terminal directs the polypeptide chain to a translocator in the ER membrane
• Growing polypeptide chain is threaded through the membrane (translocation)
• Single peptide is cleaved off by signal peptidase and the newly synthesised protein is released into the ER lumen

Question 20

How are single-pass Integral membrane proteins inserted into the membrane?

Answer 20

• a 20 amino acid hydrophobic single peptide at the N-terminal directs the polypeptide chain to a translocator in the ER membrane
• Growing polypeptide chain is threaded through the membrane (translocation) until start transfer peptide is read
• When the translocator interacts with the stop-transfer sequence translocation ceases and the protein is released into the bilayer
• Single peptide is cleaved off by signal peptidase

Question 21

How are glyco-proteins inserted into the membrane?

Answer 21

• Glycoproteins are put in place in the phospholipid bilayer of the vesicle which then fuses with the plasma membrane and the glycoproteins are located on the extracellular region of the membrane