Membrane Lipids and Proteins

Question 1

What is a membrane?

Answer 1

Two Dimensional sea of mobile lipid in which proteins diffuse (or float)

Question 2

Give 2 main examples of cell membranes?

Answer 2

• plasma membrane

- internal membranes ( e.g. endoplasmic reticulum , Golgi apparatus)

Question 3

what do membranes contain?

Answer 3

• lipids
• proteins
• carbohydrates ( glyco-proteins , - lipids)

N.B. the ratio of proteins and lipids is based on function - eg myelin sheath - will have a higher proportion of lipids , and mitochondria and chrloroplast will have a higher proportion of proteins.

Question 4

What main property do membrane lipids have?

Answer 4

they are responsible for barrier properties

Question 5

major membrane lipids include? (4)

Answer 5

• phospholipid
• steroids
• neutral fats
• glycolipid

Question 6

what are the properties of phospholipids?

Answer 6

• amphipathic - have hydorphillic head and hydrophobic tails)
• polar head group ( charge varies), the head group may have a specific function ( eg intracellular signaling )
• fatty acid ( R) chains vary in length (C12 to C22) and degree pf unsaturation ( number of double bonds ).
  each C=C bond introduces a rigid kink in the R tail.

Question 7

give an example of a steroid and its properties?

Answer 7

• cholesterol and its esters
• amphipathic
• relatively large C-H portion

Question 8

give an example of a neutral fat?

Answer 8

• glycerides

Question 9

what are some properties of a glycolipid?

Answer 9

• minor

- includes some antigens (e.g blood groups - A and B)

Question 10

Discuss the membrane as a bilayer?

Answer 10

• amphpathic lipids at oil- water interface:-
• hydrophilic ‘Polar’ head - water
• hydrophobic tail - oil
• phospholipids spontaneously form bilayers in aqueus solutions
• hydrophobic interactions cause tails to aggregate and excluse water from a core
• structure stablised by tail-tail and head- head interactions

Question 11

what are the three types of membrane proteins?

Answer 11

• integral ( intrinsic)
• peripheral ( exofacial/ endofacial )
• transmembrane ( which includes single span , multispan and multimeric protein complexes )

Question 12

what is an integral ( intrinsic ) protein?

Answer 12

• one or more regions embedded in lipid bilayer . most are infact TRANSMEMBRANE proteins!!!

Question 13

What is a peripheral ( exofacial / endofacial ) protein?

Answer 13

• it is attached to membranes through integral membrane proteins or membrane lipids
• on surface

Question 14

what is a transmembrane protein ? and what are the three types?

Answer 14

Transmembrane proteins arranged asymmetrically in membrane orientation established during biosynthesis.

The three types are :

• single span - anchoring , receptor - transducer function
• multiple spans - may form an aqueus ‘pore’ in the membrane ( channels , transporters)
• multimeric protein complexes - with central ‘pores’

Question 15

What are the building blocks of proteins?

Answer 15

Amino acids

Question 16

Do Amino acids have the same charge?

Answer 16

No

• some are neutral (Zwitterionic)- ‘polar’ depending on side groups ( e.g OH , C=O)
• Cationic (i.e lysine , arginine , ‘basic’
• anionic (i.e. glutamate, aspartate , ‘acidic’

Question 17

What is a Hydropathy index?

Answer 17

the number on an amino acid representing the hydrophobic or hydrophillic properties of its sidechain.

( it reflects its charge)

Question 18

What does electrical Charge distribution on a protein depend on?

Answer 18

primary structure

• high charge- (lots of +/- residues)- hydrophillic ( surface of membrane )
• low charge - Hydrophobic ( inside membrane )

(transmembrane domains likely to regions rich in hydrophobic AA residues anchored by interactions with phosopholipid tails)

Question 19

How do you predict protein structure using a ‘hydropathy plot’?

Answer 19

it shows hydrophobicity/ hydrophiilicity of amino acid/protein.

Question 20

why is Lactose Permease important ?

Answer 20

• symporter
• membrane protein
• found in bacteria
• uses proton gradient ( in direction of cell) to bring in lactose into the cell
• alpha helix’s ( shield H-bonds within helix)- tend to be polar
• Beta sheets- form outwith membrane - tend to be charged.

Question 21

Evidence for Fluid mosaic Model ( structure ) ?

Answer 21

• Lipid Area ( unimolecular film ) = 2x membrane area
• Xray and neutron diffraction ( regular variation in densty of ( eg myelin sheath ) )

—— phosphate and cholestrol scatter xrays /// esters scatter neutrons !!

• transmission electron microscopy ( trilaminar ‘railroad’) ( you see the double membrane !!!)
• freeze etching ( reveals embedded proteins in fracture faces) ( electricity and freezing exposes the particles and shows proteins can move laterally!

Question 22

Evidence for Fluid mosaic model ( dynamics)?

Answer 22

• Electron Spin resonance (ESR) ; spectroscopy 9 use spin- labelled phosopholipids )
• phase - transition ( membrane ‘melts’ from gel ( crystalline) to sol (fluid ) as temperature increases)- reducing melting point because fatty acids favor fluid condiitons…..
• protein tagging ( fluoresent label/ antibody) - these observe specific protein movement (e.g. cell fusion, electric field , FRAP- fluoresence recovery after photobleaching )

N.B. some proteins are immobile !

Question 23

What is another function of the cell membrane?

Answer 23

• a barrier and exchange surface between cell and the environment

Question 24

what mechanisms can substances cross the cell membrane ?

Answer 24

• diffusion , osmosis, transport - facilitative /active

Question 25

What is Diffusion?

Answer 25

Downhill ( down a concentration ) gradient - simple diffusion is across a bilayer or channel.

the process of diffucion can be sped up by having transport proteins - where the protein has a confromational change as solute is transported across the membrane.

Question 26

What is active primary transport?

Answer 26

transport against a concentration gradient - energy gives it the ability to do so , e,g, Na/Glucose transport , and calcium pump