Lecture 7 - Cellular Form and Function

Question 1

What is the Extracellular Matrix?

Answer 1

Specific to Animal Cell - specialized material outside of the cell

Question 2

What are Lysosomes?

Answer 2

Specific to Animal Cell - degradation of cellular components that are no longer needed

Question 3

What are the two type of vacoules?

Answer 3

Only in the plant cell - degradation and water storage

Question 4

What is the cytoplasm?

Answer 4

Contents of the cel outside of the nucleus (includes organelles and cytosol)

Question 5

What is Cytosol?

Answer 5

Aqueous part of the cytoplasm (does not include membrane bound organelles) - includes ribosomes and cytoskeleton

Question 6

What is the lumen?

Answer 6

Inside the organelles (except the nucleus)

Question 7

Why is it called the Fluid Mosaic Model of the Membrane?

Answer 7

Fluid - due to the mobility of lipids and some of the proteins, stuff moves around but is still stable
Mosaic - many different lipids and proteins
Membrane = 1 lipid bilayer = 2 layers/leaflets

Question 8

What’s the basic structure of a phospholipid?

Answer 8

Hydrophilic Head - group + phosphate + glycerol
Hydrophobic Tail - 2 hydrocarbon chains

Question 9

What type of molecules is a phospholipid?

Answer 9

Amphipathic - both hydrophilic and hydrophobic parts
Different biochemical/biophysical properties

Question 10

What type of lipids are present in the membrane?

Answer 10

Phospholipids, Sterols (cholesterol) and Glycolipids (has a sugar group)

Question 11

Whats the name of a phospholipid with a glycerol group?

Answer 11

Phosphoglycerides

Question 12

Whats the average length of a hydrocarbon chain?

Answer 12

14-24 carbons + unsaturated/saturated

Question 13

In an aqueous environment how do phospholipids orient themselves?

Answer 13

Spontaneously self-associate into a bilayer (polar - water, and hydrophobic tail - other hydrophobic tail)

Question 14

What are artificial sealed phospholipid bilayers called?

Answer 14

Liposomes

Question 15

How does the bilayer orient its self to become energetically favourable?

Answer 15

Naturally forms a sphere to prevent the tails from facing the aqueous side

Question 16

What are the three uses of liposomes?

Answer 16

• Study lipid properties
• Membrane protein properties
• Drug delivery into cells

Question 17

Why are membranes fluid?

Answer 17

A membrane can be deformed without causing damage - laser tweezer pull on it like skin (lipids move around to adapt to mechanical disruption)

Question 18

What are the three rapid movements of phospholipids?

Answer 18

• Diffuse laterally (deeper into the plane)
• Rotate (spins)
• Flex (wobble side to side)

Question 19

What phospholipid movement rarely occurs on its own?

Answer 19

Flip flip - movement from one leaflet to other (needs help to happen)

Question 20

What’s the important of cell membrane fluidity?

Answer 20

It is carefully regulated as it is important for function (membrane proteins depend on it)

Question 21

What are two things membrane fluidity is affected by?

Answer 21

Temperature and composition (maintain fluidity)

Question 22

What happens at low temperatures?

Answer 22

The membrane becomes more viscous and less fluid, molecules don’t move, don’t want this to happen!

Question 23

Composition: Phospholipid Saturation

Answer 23

Increase fluidity - make unsaturated (add cis-double bonds at lower temps to reduce tight packing due to kinks)

Question 24

Composition: Phospholipid tail length

Answer 24

Increase fluidity: shorter tails (fewer interactions between the lipid tails and makes more space)

Question 25

Composition: Lipid Composition

Answer 25

Decrease mobility - add cholesterol (stiffen membrane, less permeable to water and only in animal cells)

Question 26

What is cholesterol's affect on mobility?

Answer 26

Decreases cell membrane mobility - insert themselves to fill spaces - slow movement (1:1 ratio of cholesterol and phospholipids)

Question 27

What is structure of sterols?

Answer 27

polar head group + hydrophobic ring top + ONE hydrophobic tail

Question 28

Animal vs Plant Sterols:

Answer 28

Animals - mainly cholesterol Plants - plant sterol and some cholesterol (plasma membrane is less permeable to polar molecules)

Question 29

What promotes Lipid movement from one leaflet to another?

Answer 29

Scramblase - Enzyme in the ER (phospholipic translocator)

Question 30

What is the role of scramblase?

Answer 30

Promote flip flop - phospholipids are made in the cytosolic leaflet of ER, this helps balance the two leaflets of the bilayer

Question 31

What's the role of Flippases?

Answer 31

Catalyze membrane flips in the golgi, specific phospholipid movement only!

Question 32

Asymmetry of Bilayer - Glycoproteins and Glycolipids

Answer 32

- formed by adding sugar groups ot the lipids/proteins of the laminal face of golgi - end up on plasma membrane (inside organelles and non cytosolic face) - protect membrane from harsh environments

Question 33

Integral Membrane Proteins

Answer 33

Proteins directly attached to lipid bilayer, extraction methods use detergents --- destroy lipid bilayer

Question 34

What are the 3 types of integral membrane proteins?

Answer 34

Transmembrane, Monolayer and lipid linked

Question 35

What are peripheral membrane proteins?

Answer 35

They do not insert into the membrane, located on either face of the membrane

Question 36

What are peripheral membrane protein properties?

Answer 36

Can be bound to other proteins that are transmembrane, bound to lipids, or attached by non covalent interactions (use gentle extraction methods that keep the bilayer intact)

Question 37

How do covalent interactions differ between the two types?

Answer 37

Integral - covalent Peripheral - noncovalent

Question 38

Transmembrane proteins

Answer 38

Amphipathic which hydrophilic (outside) and hydrophobic (inside)

Question 39

Things to remember about alpha helices and transmembrane domains

Answer 39

1) not all alpha helices pass through the membrane 2) not all transmembrane domains are alpha helices

Question 40

What are the three types of transmembrane proteins?

Answer 40

1) Single a-helix: single pass, has one hydrophobic domain 2) Multi a-helices: multipass, many hydrophobic domains 3) B-barrel - multipass but not a-helices

Question 41

How many hydrophobic amino acids are present in a membrane spanning a-helix?

Answer 41

20-30

Question 42

Which two transmembrane proteins serve as channels?

Answer 42

multiple a-helices and b-barrel (inside is hydrophillic, outside is hydrophobic) - based on amino acid side chains

Question 43

What is essential for transmembrane protein in terms of function?

Answer 43

Specific orientation - proteins don't flip-flop so it cant be fixed

Question 44

Techniques for identifying these structures?

Answer 44

1) X-ray crystallization - determines 3D structure 2) Hydrophobicity plots - segments of 20-30 hydrophobic amino acids that span bilayer

Question 45

Hydrophobicity plots

Answer 45

(-) --> hydrophillic (+) --> hydrophobic 0 --> N-terminus 100 --> C-terminus Calculation depends on the neighbouring amino acids, can identify number of passes

Question 46

Monolayer Associated Membrane Proteins

Answer 46

Amphipathic - anchored on the cytosolic face ex. Sar 1 - membrane bending - vesicle bending at the ER

Question 47

Lipid-linked Membrane Protiens

Answer 47

1) Protein with GPI anchor (made in ER lumen end up on cell surface) 2) Protein with another lipid anchor (cytosolic enzymes add an anchor and directs it to cytosolic face)

Question 48

Extraction of Membrane Proteins

Answer 48

Also amphipathic (detergents) - formation of micelles and complex (tails connected to tails etc)

Question 49

Studying the Properties

Answer 49

Use detergents to isolate, then purify, remove detergent, addition of phospholipids ---> protein incorporated into an artificial bilayer

Question 50

Lateral Diffusion of Membrane Proteins

Answer 50

lateral diffusion within leaflet (no flip-flop), FRAP, protein is fuse to Green fluorescent protein.

Question 51

Fluorescence Recovery after Photobleaching (FRAP)

Answer 51

- Photo bleach an area after labelling - proteins move quickly --> cover up --> fast diffusion

Question 52

Rate of Fluorescence recovery

Answer 52

time taken for neighbouring unbleached fluorescent protein to moving into bleached area