BC3- lipids, biological membranes

lipids

-highly reduced carbon-rich substances including the fats, oils, sterols that are soluble in nonpolar organic solvents (e.g. ether, benzene)

-hydrophobic (nonpolar) or amphipathic (containing both nonpolar and polar regions)

-most of the lipids in food are in the form of triacylglycerols, cholesterol, and phospholipids

common types of lipids: fatty-acids

-Fatty acids are carboxylic acids with a long hydrocarbon chain; they are key constituents of lipids

fatty acid-containing lipids: triacylglycerols, glycerophospholipids, sphingolipids

common types of lipids: non-fatty acid-containing lipids

sterols: have four fused rings (steroid nucleus) and a hydroxyl group

lipid functions

Energy storage (e.g. triacylglycerol)

Membrane structure (e.g. glycerophospholipids, sphingolipids, sterols)

Electron carriers (e.g. ubiquinone’s)

Emulsifying agents (e.g. bile salts)

Hormones (e.g. steroid hormones)

fatty acids

carboxylic acids with hydrocarbon chains

Standard nomenclature assigns the number 1 to the carboxyl carbon

Carbon atoms 2 and 3 are also referred to as α and β, respectively

saturated fatty acid

no C-C double bonds

unsaturated fatty acid

at least one C-C double bond

monounsaturated fatty acid

only one C-C double bond

polyunsaturated fatty acid

two or more C-C double bonds

fatty acids: common name

oleic acid

fatty acids: nomenclature

specifies the chain length (counting starts from the carboxyl carbon) and number of double bonds

fatty acids: position of double bonds indicated by (nomenclature)

delta(n)

n indicates lower numbered carbon of each pair

fatty acid nomenclature: total # C:

# double bonds, delta double bond positions

a 16-carbon saturated fatty acid is abbreviated as

16:0

an 18-carbon fatty acid with one double bond between C-9 and C-10 is designated

18:1^delta9

polyunsaturated fatty acids can be numbered from the

methyl carbon (carbon most distant from the carboxyl group) which is called the omega carbon

positions of the double bonds are indicated relative to the omega carbon

polyunsaturated fatty acids with a double bond between C3 and C-4 are called

omega-3 fatty acids

omega-3 fatty acids

essential nutrients

humans cannot synthesize them

include alpha-linolenic acid (ALA), docosahexaenoic acid (DHA), and eicosatetraenoic acid (EPA)

essential fatty acids

cannot be synthesized by the body, must be obtained from diet

melting points of fatty acids

determined by the length and degree of unsaturation of the hydrocarbon chain

the longer the fatty acid chain, the higher the melting point

the fewer the double bonds in a fatty acid, the higher the melting point

the effect of the double bond on the physical properties of the fatty acid is due to the

conformation of the lipid that is caused by the double bond

in saturated fatty acids, the most stable arrangement is

very close packing of the side chains of the lipids

lipids assume an almost crystalline array

can tight packing of fatty acid chains take place?

no, due to the kink that results from cis double bonds

since the interactions between these arrays are less extensive, it takes less energy to disrupt them, resulting in a lower melting point

triacylglycerols

contain 3 fatty acid molecules esterified to the three hydroxyl groups of glycerol

primary storage form of body fat, many fatty acids in biological systems exist this way

very nonpolar, insoluble in water (since carboxylates are esterified and no longer bear a negative charge)

lipases

catalyze the hydrolysis of the ester bond of triacylglycerols in order for the fatty acid to be used for fuel and other purposes

oils

triacylglycerols rich in unsaturated fatty acids generally liquid at room temperature

fats

triacylglycerols rich in saturated fatty acids, generally semisolids or solids at room temperature

two major classifications of lipids

storage lipids (triacylglycerols)

structural lipids providing structure for membranes

storage lipids (triacylglycerols)

used primarily to store energy, neutral and nonpolar

structural lipids

contain both nonpolar and polar groups, provide structure for membranes

two major types of structural lipids

glycerophospholipids and sphingolipids

glycerophospholipids

primary constituents of cell membranes

differ from triacylglycerols in that only two of the hydroxyl groups of glycerol are esterified to fatty acids.

third hydroxyl group contains a phosphate group that is connected to a given “head group” via a phosphodiester bond

sphingolipids

The most distinct difference between sphingolipids and glycerophospholipids is that the backbone is sphingosine, NOT glycerol

contain sphingosine, which is a long-chain amino alcohol. A fatty acid is joined to sphingosine via an amide linkage rather than an ester linkage as seen in glycerol

sterols

contain the steroid nucleus and a hydroxyl group

steroid nucleus: 17-C four-fused ring structure, three of the rings are six-membered, and one is 5-membered

sterols can be present in the membranes of most eukaryotes (like cholesterol)

fatty acids: summary

[R-COOH (R=hydrocarbon chain)] are components of

triacylglycerols, glycerophospholipids, sphingolipids

Unsaturated fatty acids have lower melting points than their saturated

counterparts; the greater the degree of unsaturation, the lower the

melting point

Fatty acids are important metabolic fuels

Fatty acids are stored as neutral lipids called triaclyglycerols

triacylglycerols: summary

are composed of 3 fatty acyl residues esterified to a glycerol

Glycerophospholipids and sphingolipids are the major structural components of membranes

Sterols contain four fused rings (steroid nucleus) and a hydroxyl group e.g. cholesterol

lipid bilayers

structural basis for all biological membranes

noncovalent interactions among lipids make them flexible and self-sealing

polar head groups contain aqueous medium

nonpolar tails point to interior

biological membranes

define the external boundaries of cells and separate cell compartments

consist of proteins embedded in or associated with a lipid bilayer

functions of membranes

selective import and export of waste/nutrients

retain metabolites and ions in the cell

sense signals, transmit info to the cell

provide compartmentalization

separate energy-producing reactions from energy-consuming ones

keep proteolytic enzymes away from important cell proteins

store energy as a proton gradient

support ATP synthesis

integral membrane proteins

also called trans-membrane proteins, contain hydrophobic regions embedded in the hydrophobic lipid bilayer

penetrate or span the bilayer

mediate movement of ions and polar molecules across membrane

can generate proton gradients for ATP production

peripheral membrane proteins

associated with membrane through charge-charge or hydrogen bonding interactions to integral proteins or membrane lipids

more readily dissociated from membranes than integral proteins

proteins on peripheral membrane proteins respond to extracellular signals and communicate them to the cell interior (e.g. insulin receptor)

Hydrophobic (fat soluble) molecules can pass through a membrane’s hydrophobic interior by

simple diffusion down their concentration gradient

e.g. the diffusion of O2 and CO2 across membranes

cell membranes are impermeable to ions and large polar molecules. how do they move across the membrane?

require membrane transporter proteins to form passages through the hydrophobic barrier

three types of integral membrane transporter proteins

channels

passive transporters

active transporters

channels

transmembrane proteins with aqueous pores that extend across membrane

when open, pores allow inorganic ions of appropriate size and charge to pass

ions move down concentration gradient

requires no energy

much faster than transport mediated by transporters

usually not saturable

passive transporters

transmembrane proteins

bind the specific solute to be transported and undergo a series of conformational changes to transfer the solute

solute moves down its concentration gradient

requires no energy

carrier proteins are saturable with solute

passive transport (facilitated diffusion) overview

• Passive transport (facilitated diffusion) does not require an energy source

• The solute is to be transported move down its concentration gradient across the membrane

• The solute is moved through a specific channel

active transport

requires energy, moves a solute up its concentration gradient (low concentration to high concentration)

can be powered by a direct source of energy as ATP, electron transport, or light

saturable with solute

active transport example

The cytochrome b6f complex involved in photosynthesis uses light energy to move H+ across the thylakoid membrane against their concentration gradient

simple diffusion: protein carrier? saturable with solute? movement relative to concentration gradient? energy input required?

protein carrier: no

saturable with solute: no

movement relative to concentration gradient: down

energy input required: no

facilitated diffusion: channel: protein carrier? saturable with solute? movement relative to concentration gradient? energy input required?

protein carrier: channel

saturable with solute: no

movement relative to concentration gradient: down

energy input required: no

facilitated diffusion: passive transporter: protein carrier? saturable with solute? movement relative to concentration gradient? energy input required?

protein carrier: passive transporter

saturable with solute: yes

movement relative to concentration gradient: down

energy input required: no

active transport: protein carrier? saturable with solute? movement relative to concentration gradient? energy input required?

protein carrier: active transporter

saturable with solute: yes

movement relative to concentration gradient: up

energy input required: yes

summary: biological membranes

Lipid bilayers are the structural basis for all biological membranes

Membrane

1) Lipids

2) Proteins

3) Carbohydrates

Membrane proteins

1) Integral membrane proteins

2) Peripheral membrane proteins

summary: biological membranes

Membrane protein functions:

1) transport molecules across membrane (channels, transporters)

2) receive and transduce extracellular signals (receptors)

In passive transport, solutes move across membranes down their concentration gradient

In active transport, solutes move across membranes against their concentration gradient by using energy

Membrane receptors