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Lecture 3

Lipids and Cell Membranes

}  Lipids }  Saturated versus unsaturated fat }  Hydrogenation process – trans fats }  Essential fatty acids }  Steroids }  Phospholipids }  Cell membranes }  Fluid mosaic model

Macromolecules: All living organisms contain large molecules: - carbohydrates, proteins, and nucleic acids, and lipids - living organisms are composed of cells

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Lipids carbon-containing compounds that are largely….

Fats: Large molecules assembled from smaller molecules (dehydration)

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functions

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Hydrogenated Fat

Hydrogenation: -  converts unsaturated C=C double bonds into C-C single bonds

-  adds ……… to unsaturated fats, eliminating double bonds and making them into partially or completely saturated fats

-  complete hydrogenation converts unsaturated or partially saturated fat into completely saturated "hard" fat.

Trans Fats

hydrogenation: - a by-product of hydrogenation is also the creation of trans fat: some of the cis double bonds are converted to trans double bonds (associated with atherosclerosis)

-  build up in walls of arteries and make arteries more rigid -  form plaque, clogging arteries -  cause insulin resistance -  contribute to type 11 diabetes -  lower HDL and increase LDL

-  Conclusion from article: Higher trans fat intake is associated

with an increased risk of all-cause mortality.

-  interferes with enzymes that convert EFA to signal molecules that are important for cell function

-  lowers efficiency of immune system

Kiage, et al., American Journal of Clinical Nutrition, 2013

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Fats/Lipids In Our Diet: Naturally-occurring Trans Fats

Trans'' fats occur naturally: - when rumen bacteria in the stomachs of ruminant animals (cows, sheep) digest grass, the result is the formation of trans-rumenic and trans-vaccenic acid (by hydrogenation of polyunsaturated fats in the grass)

-  conjugated linoleic acid, or CLA, is a trans-rumenic acid commonly found in grass-fed animals and dairy products

-  natural trans fats from animal products are not known to be associated with increased risk of heart disease

}  Essential

}  Nonessential ◦  available in the diet or readily synthesized in amounts

sufficient to meet the organism’s physiological needs

role of omega-3 and omega-6 polyunsaturated fatty acids -  constitute fundamental and functional components of cell

membranes -  affect fluidity and flexibility of membranes -  affect permeability and activity of membrane-bound enzymes -  influence production of hormones/signal molecules that are

key messengers affecting blood pressure, clotting, inflammation, immune function

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DHA (Docosahexaenoic acid) and EPA (Eicosapentaenoic acid) critically important for human health

-  DHA predominates in cell membranes of our brains

and eyes (important roles in vision and nervous system function)

-  critical for brain development

Obtained from: Plant foods – Aquatic animals – Omega-6 EFA Obtained from: Importance of omega-3 and omega-6 EFA: you have to

have the correct ratio

}  Present “western” diet is dominated by omega-6 FA and under represented by omega-3 FA

}  Omega-6 EFA dominate our modern diet:

abundant in cooking oils, meats and prepared foods

*** Addressing the imbalance

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}  Addressing the imbalance: increase omega-3 EFA and decrease omega-6 EFA

NA diet (on average): 15-35 parts omega-6:1 part omega-3 Hunter-gatherer ancestors: 1:1 Optimal diet: 1-3 parts omega-6:1 part omega-3

Long Chain Omega-3 Fatty Acids in Human Health. www.vitalchoice.com Holub, J.H. 2002. JAMC 166:608-615.

}  American Heart Association guidelines: recommends people eat fish 2-3x/week or supplement with fish oil

}  Current mean intakes (adults) of EPA and DHA are

130mg/day }  Target intakes: 900-1200mg of EPA+DHA/day

}  Trials in the US have indicated that intakes of fish-derived omega-3 fa (up to 665mg/d) over 10.5 years were associated with progressive reduction in mortality related to coronary heart disease

Holub, J.H. 2002. JAMC 166:608-615.

family of lipids distinguished by a four-ring structure (one important steroid in mammals is cholesterol) Cholesterol is the molecule from which other steroids (sex hormones) are synthesized

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Cholesterol:

Main constituents of cell membranes

Phospholipids differ in: -  fatty acid chain length

-  degree of saturation or unsaturation in fatty acids

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Lipid micelles Lipid bilayersWater

No water

Hydrophilic heads interact with water

Hydrophobic tails interact with each other

Hydrophilic heads interact with water

lipid bilayers, form when two sheets of phospholipid molecules align

Lipids and Cell Membranes

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Membrane Permeability

Permeability scale (cm/sec) Size and charge affect the rate of diffusion across a membrane.

Phospholipid bilayer

O2, CO2, N2

H2O, urea, glycerol

Glucose, sucrose

Cl– , K+, Na+

High permeability

Low permeability

O2, CO2

H2O

Glycerol, urea

Glucose

Cl–

K+

Na+

The permeability of a structure is its tendency to allow a given substance to pass across it; phospholipid bilayers have selective permeability

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Cells and Cell Membranes

Sandwich Model

Where are Membrane Proteins Found in the Lipid Bilayer?

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Membrane Proteins

Phospholipid Bilayer

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- membrane is a fluid dynamic mosaic of phospholipids and proteins

•  located on outer surface of membrane •  cell to cell recognition

•  glycolipids •  glycoproteins

Water pores allow only water to pass through.

Outside cell

Inside cell

Potassium channels allow only potassium ions to pass through.

Outside cell

Inside cellClosed

Open

Hydrophilic interior Hydrophobic

exterior

Potassium ions can enter the channel, but cannot pass into the cell

When a change in electrical charge occurs outside the membrane, the protein changes shape and allows the ions to pass through

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Fluid-mosaic model

Cell exterior

Phospholipid bilayer

Cell interior Membrane proteins

lipids: physical integrity of membrane, barrier proteins/ion channels: movement of materials carbohydrates: attached to lipids or proteins – recognition water pores: passage of water cholesterol: important to membrane integrity

All living organisms contain large molecules: carbohydrates, lipids, proteins, and nucleic acids Lipids: fats, steroids, phospholipids – different form-

different function Essential fatty acids – important in health All organisms consist of cells; cells are surrounded by cell membranes Cell membranes (phospholipids, proteins, cholesterol, glycoproteins, glycolipids) are fluid and dynamic