Lipids, membranes and cellular transport

1
Lipids, membranes and cellular transport
2
Functions

• Lipids serve as
• Energy stores
• Catabolised in ?-oxidation to produce ATP
• Structural molecules
• cell membranes

From Tortora GJ Grabowski SR (2000) Principles
of Anatomy and Physiology (9th Edition). New
York John Wiley Sons, p62
3
Structure and behaviour

• General lipid structure is amphipathic
• Polar, hydrophilic head
• Hydrophobic tail

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.298.
4
Fatty acids

• Fatty acids - simplest lipid structure
• Hydrophilic carboylate group
• Hydrophobic hydrocarbon tail
• Exist in ionised form at physiological pH

Stearate ion From Matthews, CK van Holde KE
(1990) Biochemistry. Redwood CityBenjamin
Cummings p.299.
5
Hydrophobia

• Hydrocarbon tail hydrophobic because
• hydrogen and carbon outer electron shells half
  full
• Non-polar structure
• Will not form H bonds with water
• Will only allow other molecules to approach as
  close as van der Waals radius

Water molecule (polar) From Matthews, CK van
Holde KE (1990) Biochemistry. Redwood
CityBenjamin Cummings p.33.
6
Solubility

• Dependent on ability to form H bonds
• Soluble
• surrounded with hydration shells and dissolve
• Insoluble
• Surrounded by clathrate structures provides some
  solubility
• If too heavy drop out of solution (insoluble)

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings p.39.
From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings p.38.
7
Saturated and unsaturated fatty acids

• Saturated fatty acid
• Carbons in tail saturated with hydrogen
• Unsaturated fatty acid
• Carbons in tail contain one or more double bonds
• Creates bend in tail
• Molecules cannot pack as tightly
• Increases fluidity

Oleate ion From Matthews, CK van Holde KE
(1990) Biochemistry. Redwood CityBenjamin
Cummings p.299.
8
Triacylglycerols

• Large quantities of free fatty acids not found in
  cells
• Cells store fatty acids as triacylglycerols
• Glycerol 3 fatty acids
• Esterification with glycerol reduces hydrophilic
  nature of fatty acid
• not solulble
• Fatty acids usually a mix of saturated and
  unsaturated

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.301.
9
Membrane lipids - phospholipids

• Glycerophospholipids are major membrane lipids
• Two fatty acids (R1, R2) and polar head group
  (R3)
• Structure of R3 group varies

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.303.
10
Membrane lipids - phospholipids

• Glycerophospholipid double fatty acid tail
  confers cylindrical shape
• Allows packing into sheets

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.303.
11
Membrane lipids - cholesterol

• Cholesterol is a steroid
• ring structure makes bulky and rigid
• alters membrane fluidity by interfering with
  close packing of hydrocarbon tails of membrane
  phospholipids

From Elliott WH Elliott DC. (1997)
Biochemistry and Molecular Biology. New
YorkOxford University Press, p45.
From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.303.
12
Membrane structure

• Fluid mosaic model
• Phospholipid bilayer
• Two leafs of bilayer usually different in lipid
  composition
• Integral and peripheral proteins
• Integral usually form transport channels
• Lipids and proteins in constant motion
• Bands of protein partition areas of membrane to
  limit lateral diffusion

From Tortora GJ Grabowski SR (2000) Principles
of Anatomy and Physiology (9th Edition). New
York John Wiley Sons, p62
13
Membrane transport

• Membranes selectively permeable
• Three transport mechanisms
• Passive transport
• Diffusion down concentration gradient
• Non-saturable
• Although polar, water moves fast enough passively
  in most cells
• Some membranes contain aquaporin
• Facilitated transport
• Protein channels speed diffusion down
  concentration gradient
• Channels present where passive diffusion too slow
• Some channels gated
• Saturable process
• Active transport
• Movement against gradient
• Consumes free energy (from hydrolysis of ATP)
• Primary - consumes directly
• Secondary - consumes indirectly

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.324.
14
Primary active transport - sodium potassium pump

• Maintains sodium and potassium gradients across
  cell membranes
• Four protein complex
• Two subunits enzymes which hydrolyse ATP
• Free energy drives transport

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.326.
15
Secondary active transport - intestinal glucose
transport

• Sodium potassium pump generates gradient across
  membrane
• Far from equilibrium
• Potential energy to drive transport

From Matthews, CK van Holde KE (1990)
Biochemistry. Redwood CityBenjamin Cummings
p.328.