Light microscopy of lipid droplets inside of diatom cells. Transmission electron microscopy of mouse brown adipocyte with large lipid droplets (L).

Fluorescence microscopy of yeast cells. Left: nucleus, middle: lipid droplet; right: plasma membrane. Transmission electron micrograph of Brassica seed.

oil bodies

Storage protein

1.4 Lipid

Learning objectives

• Chemical composition, characters and function of lipid

• Differentiate between triglyceride and lipoid

• Structure and chemical characters of triglyceride

• Phospholipid and steroid as lipoids

Lipid 脂类• Also formed by carbon, hydrogen and

oxygen atoms

• Ratio of H : O is not 2 : 1 (more H, less O)

• May also contain nitrogen (N) and phosphorus (P)

• Examples: waxes 蜡, sterols 类固醇, fat-soluble vitamins 脂溶性维生素 (such as vitamins A, D, E, and K), triglycerides 脂肪, hormones 激素such as adrenocorticoid 肾上腺皮质素, phospholipids etc.

• Two major groups of lipids:• Triglyceride• Lipoid

A giant unilamellar vesicle (GUV), designed to mimic rafts of lipid segregation in cell plasma membranes, was examined under multiphoton fluorescence microscopy.

Importance of lipid• storing energy

• contains twice the energy-rich (C-H) bonds as glucose and stores twice as much energy as glucose

• signaling

• structural components of cell membranes

• Provide insulation (heat, nerve cells) and

• Source of fat-soluble vitamins (A, D, E, K)

• Protection

Solubility of lipid

• Soluble in organic solvents 有机溶剂 such as ether 乙醚 and benzene 苯, acetone 丙酮

• Insoluble in water

Left top: Oil is insoluble in water.Left middle: An emulsion 乳浊液 of water and oil in the presence of ethanol.Left bottom: Oil is soluble in chloroform, an organic solvent.

Triglyceride 脂肪、三甘油酯• Important for energy storage

• Formed by condensation of three fatty acids and one glycerol

Structure of fatty acid in triglyceride

• Long chains of carbon and hydrogen atoms.

• Each carbon atom has 4 bonds: 2 to C atoms, 2 to H atoms.

Quiz

• How many water molecule(s) is released for every formation of one triglyceride?

a. 0

b. 1

c. 2

d. 3

e. 4

Double bonds in fatty acids

• Sometime, only one atom attached to a carbon

• The carbon atom has an extra valence electron

• A double bond will formed between two carbon with extra electron

"saturated" with hydrogen

Saturated and unsaturated lipids

Food source example for triglycerides• Unsaturated lipids tend to

have lower melting points than saturated lipids.

• At room temperature, unsaturated triglycerides exists as liquid, while saturated triglycerides exists as solid.

• Docosalhexaenoic acid (DHA) is a highly unsaturated lipids that are commonly infused into our food.

Quiz

• Fish, safflower oil, sunflower oil, and corn oil are sources of

a. unsaturated fat.

b. saturated fat.

c. steroids.

Lipoid 类脂

• A large group of molecules with similar solubility with triglyceride

• Example• Phophoslipid

• steroid

Phospholipid 磷脂

• Similar to triglyceride

• two fatty acids and a phosphate group (may contain nitrogen and other elements) attached to the glycerol backbone

亲水

疏水

Importance of phospholipid

• Construction of cell membrane (phospholipid bilayer 磷脂双分子层) and other vesicles in the cell

• Insulator on nerve cells in the brain to speed up nerve impulses

• Precursor 前体 to other important compounds such as Platelet-activating factor (for platelet aggregation 聚集血小板)

Self-organization of phospholipids.

Quiz

• All lipids are similar because they

a. do not mix with oil

b. do not mix with water

c. are hydrophilic

d. do not mix with oxygen

Steroid 类固醇

• Structurally very different to triglyceride

• four rings arranged in a specific configuration

• But have similar solubility characters to triglyceride, i.e. hydrophobic

The cholesterol molecule follows the pattern of the four-fused-ring steroid skeleton .

胆固醇

Example of steroid

• Vitamin D

• Cholesterol 胆固醇

• Hormone: • Sex steroids (testosterone 雄激素,

progesterone 孕激素, estrogen 雌激素)

• Ecdysone 昆虫蜕皮素

• Adrenocorticoids 肾上腺皮质素

Quiz

• The class of lipids that does not contain fatty acids is

a. triglycerides

b. steroids

c. phospholipids

Quiz

• The basic units or building blocks of triglycerides are

a. simple sugars (monosaccharides).

b. double sugars (disaccharides).

c. amino acids.

d. glycerol and fatty acids.

e. nucleotides.

Quiz

• A ____________ fatty acid has one double covalent bond between carbon atoms.

a. cholesterol

b. monounsaturated

c. phospholipid

d. polyunsaturated

e. saturated

Conclusion

• Lipid is formed by carbon, hydrogen and oxygen atoms, and its ratio of H : O is not 2 : 1. It may also contain nitrogen and phosphorus.

• Lipid is a hydrophobic molecule but it is soluble in organic solvents.

• Triglyceride is formed by condensation of three fatty acids and a glycerol.

• At room temperature, saturated fat exists in solid state while unsaturated fat exists in liquid state.

• Lipoid includes phospholipids and steroids.

• Steroids do not resemble triglyceride but have similar dissolution characters to triglyceride.

Quiz

• The molecules that stored energy in animal cells are_____.

A. Cellulose, Starch C. Starch, Triglyceride

B. Glycogen, Triglyceride D. Glycogen, Cholesterol

Short answer question

• Triglyceride formed by what substances?

• Triglyceride is made up of three molecules of fatty acids and one molecule of glycerol.

Short answer question

• Drawing examples to illustrate your answer, describe the functions of lipids in organisms.

1. In seeds，Lipids can be a long-term energy stores because it is compact an insoluble in water.

2. In adipose tissues below dermis lipids works as heat insulator.

3. Around the delicate internal organs like kidney, lipid works for protection.

Short answer question

• Why do many organisms store lipids rather than carbohydrates?

• Lipids releases approximately twice as other much energy as one molecule of carbohydrate functions.