Cellular Membranes and Transport

Question 1

What did Gorter discover?

Answer 1

• first to experimentally show that membranes have a bi-layer structure by comparing the surface area produced by membrane lipids in a mono layer structure to the surface area of the cell
• electron microscopy confirmed the bi-layer structure in 1950’s

Question 2

What are the functions of a cell membrane

Answer 2

• plasma membrane enclosed the cell to form a contained working system (unit of life)
• maintains the essential differences in terms of composition and concentration between the cytosol and external environment
• interior membranes: compartmentalises cells, enabling different part of cell to develop specific functions
• membranes for a particularly impermeable barrier to control access to the cytoplasm
• membranes establish uni gradients between the 2 sides by the activity of the membrane associated transports

Question 3

What is the importance of ion gradients created by cell membranes?

Answer 3

• the synthesis of ATP
• the control movement of selected solutes
• the production and transmittance of chemical and electrical signals

Question 4

What functions do membrane associated proteins have?

Answer 4

• act as receptors and sensors of external physical and chemical signals
• are involved in cell recognition e.g. Immunohistocompatibility
• are involved in cell adhesion
• have a role in resistance to pathogens

Question 5

Why are phospholipids amphipathic?

Answer 5

• the head is electrically charges and hydrophilic (water soluble)
• the tail is electrically neutral and hydrophobic (water insoluble)

Question 6

Describe the composition of a phospholipid

Answer 6

• the polar head group consists of a chlorine linked by a phosphate group to glycerol (C3)
• the hydrophobic tail has 2 fatty acid chains forming the hydrocarbon tails which can be 12-34 carbons in length attached to glycerol
• on fatty acid chain is fully saturated (straight) and the other is unsaturated with 1 or more cis double bonds (giving fatty acid a linked profile and allowing rotation- the more unsaturations a chain has, the higher the flexibility and therefore fluidity of the membrane)

Question 7

How do phospholipids act in water?

Answer 7

They form a bi-layer with hydrophilic heads facing outward and hydrophobic tails facing inwards

-the bi-layer forms a vesicle called liposome with an internal compartment (cytosol) and external compartment (extracellular space)

Question 8

What phospholipids can be found on the outer side of the membrane?

Answer 8

• glycolipids
• phosphatidylcholine
• sphingomyelin

Question 9

What phospholipids can be found on the inner (more negative) side of the membrane?

Answer 9

• phosphatidyethanolamine
• phosphatidyelserine
• phosphatidyelinositol

Question 10

Describe the role of sugars in the plasma membrane

Answer 10

• sugar lipid molecules make up around 5% of the plasma membrane
• gangliosides and glycosphingolipids are the most complex
• the sugars are orientated to the outer side of the cell - important for surface of nerve cells

Question 11

Describe the role of cholesterol in plasma membranes

Answer 11

• the eukaryote plasma membrane contains a large amount of cholesterol making the membrane less permeable
• cholesterol molecules orientate themselves in the bi-layer with hydroxyl head close to the polar groups of the lipids, partially immobilising phospholipid molecules - making the membrane less deformable. It also acts as a space preventing ‘phase transitions’ (freezing)

Question 12

Describe the role of proteins in the plasma membrane

Answer 12

Integral proteins (intrinsic)

• have membranes spanning hydrophobic domains that interact with phospholipids
• might have hydrophilic domain that extends out of the phospholipid bi-layer

Peripheral proteins (extrinsic) 
-are hydrophilic and interact with phospholipids and/or intrinsic membrane proteins

Question 13

Name the ways in which proteins are associated with the cell membrane

Answer 13

• attached with a single helix planning the membrane
• attached with multiple helices spanning the membrane
• attached with a covalent bond
• attached via phosphatidyl inositol to external side
• attached non covalently to an integral membrane protein on the internal side

Question 14

What are the functions of the associated membrane proteins?

Answer 14

• transport: of water, minerals and nutrients
• signalling and recognition: perception of environmental signals and transduction
• enzyme activities: many proteins associated with membranes have enzyme activity
• cell adhesion
• anchorage of the cytoskeleton to the plasma membrane

Question 15

What is ‘transduction’?

Answer 15

A basic process in molecular cell biology involving the conversion of a signal from outside the cell to a functional charge within the cell

Question 16

Explain membrane permeability

Answer 16

• protein free lipid bi-layers are highly impermeable to ions
• the smaller the molecule the more soluble it is in oil-the more rapidly it will diffuse across the lipid bi-layer
• smaller non polar gases such as O2 and CO2 rapidly diffuses across the lipid bi-layer

Question 17

Explain the process of diffusion across the plasma membrane

Answer 17

• uncharged polar molecules can also diffuse quickly if they are small enough (e.g. Water)
• large polar molecules (e.g. Glucose) can hardly diffuse at all
• membranes are virtually impermeable to charged molecules, irrespective of size (e.g. Ions)
• proteins are in membranes therefore facilitate movement of molecules that cannot passively diffuse through the membrane

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Question 18

How do channel proteins work?

Answer 18

They form water filled pored across the lipid bi-layer through which ions can diffuse. This path way can be open and shut by a variety of different mechanisms. Channels open under the effect of hormonal signals, mechanical signals (e.g. Stretching) and electrical signals

Question 19

How do carrier proteins work?

Answer 19

Carrier proteins alternate between 2 conformations. Meaning the binding site is subsequently available on one side of the membrane then the other

Question 20

Describe plasma membrane fluidity

Answer 20

• cell membranes are plastic and deformable
• lateral movement of lipids and proteins: the plasma membrane is a likened to a fluid mosaic where the lipids are able to move freely about in the plane of the membrane at a speed of 2μm per second
• it is rare for components to flip flop (to swap layers) between layers (this is usually mediated by phospholipid translocators in the ER)
• the phospholipids have the ability to fully rotate which gives the membrane fluidity