Biology Now, 2e

Engineering Life ■ 63

it works, Devaraj decided to try to build a cell

artificially, using materials not typically found

in nature. “If you want to really understand the

principles by which life operates and evolves,

the best way to do so is to build a cell from the

ground up,” he says.

Scientists suspect that one of the first events at

the beginning of life on Earth was the formation

of a plasma membrane, a barrier separating a

cell from its external environment. A plasma

membrane is made of two layers of phospho-

lipids, organic molecules with a water- loving,

or hydrophilic, head, and a water-fearing, or

hydrophobic, tail. In water, these molecules

form a double layer with heads out and tails

in, a barrier that separates the contents of the

cell from what lies outside the cell. Thus, the

membrane is a phospholipid bilayer, a mostly

impermeable barrier. When a phospholipid

bilayer forms a sphere, or liposome, the fluid

inside the liposome can have a different compo-

sition from the fluid outside (Figure 4.3). The

ability to maintain an internal environment

separate from the external environment is one

of the most critical functions of the plasma

membrane of a cell.

Given a container of phospholipids, anyone

can make a membrane, says Devaraj. “Making

membranes is almost a trivial thing,” he says.

“You take natural or synthetic phospholipids, add

water, and they form membranes.” Yet research-

ers had been unable to form a membrane from

scratch—without using preexisting phospholip-

ids. In nature, new phospholipids are created by

enzymes embedded in the cell.

Instead of trying to engineer new phospho-

lipids, Devaraj wanted to start with something

simpler. He worked with graduate student Itay

Budin, who was then at Harvard University and

is now a postdoctoral researcher at UC Berkeley,

Figure 4.3

Liposomes form when phospholipids and water are shaken together

When you shake a mixture of phospholipids and water, the phospholipid bilayers bend and link together to form spheres called

liposomes. This simple structure is remarkably similar to the basic structure of a cell.

Q1: Why is it important that the phosphate head of a phospholipid is hydrophilic?

Q2: What essential component of a cell do liposomes lack, and why is that omission important?

Q3: Could the tendency of phospholipid bilayers to spontaneously form spheres have played a role in the origin of life?

(Hint: Refer to “The Characteristics of Living Organisms” on page 6 of Chapter 1.)

Phospholipid

heads are

attracted to

water and to

each other.

Phospholipid

tails retreat from

water and pivot

inward, creating

a bilayer.

When shaken in water,

the phospholipids

self-organize into

spheres, or liposomes.

Individual phospholipid Phospholipid bilayer Liposome

Water

Water

Lipid

bilayer

Hydrophilic

phosphate

head

Hydrophobic

fatty acid tails

JNeal Devaraj is a biochemist at UC San Diego who

is working to make an artificial cell from the bottom

up, starting with the membrane and then building

other organelles.

NE AL DEVARAJ