Chapter 6
Lipids,
Membranes,
and the First
Cells
Key Concepts
• Phospholipids are amphipathic lipid molecules—they are part
hydrophobic and part hydrophilic. Plasma membranes are made up of bilayers of phospholipids. These bilayers are selectively permeable.
• Ions and molecules diffuse spontaneously from regions of higher
concentration to regions of lower concentration—a process called diffusion. Movement of water across a plasma membrane is a
special case of diffusion called osmosis.
• In cells, membrane proteins are responsible for the passage across membranes of ions and large and/or polar molecules in the
Lipids: What Is a Lipid?
• Lipids are carbon-containing compounds that are found in
organisms and that are largely nonpolar and hydrophobic.
• Hydrocarbons are molecules that contain only carbon and
hydrogen.
• Lipids have a major hydrocarbon component called a fatty acid. A
Three Types of Lipids Found in Cells
• Lipids are defined by solubility rather than by chemical structure,
so their structures vary widely.
• Three types of lipids are the most important found in cells:
Fats are composed of three fatty acids linked to glycerol.
Steroids are a family of lipids distinguished by a four-ring structure. One important steroid in mammals is
cholesterol.
Structure of Phospholipids
Phospholipid
Polar head (hydrophilic)
The Structure of Membrane Lipids
• Membrane-forming lipids are amphipathic, containing both
hydrophobic and hydrophilic regions.
• For example, phospholipids are amphipathic. The “head” region
contains highly polar covalent bonds, as well as positive and negative charges. Phospholipids also have a nonpolar fatty acid “tail” region.
• When placed in solution, the phospholipid heads interact with
Phospholipid Bilayers
• Phospholipid bilayers, or simply lipid bilayers, form when two
sheets of phospholipid molecules align. The hydrophilic heads in each layer face a surrounding solution, while the hydrophobic
tails face one another inside the bilayer.
• Phospholipid bilayers form spontaneously, with no outside input
Phospholipids Form Bilayers in Solution
Lipid micelles Lipid bilayers
Water
No water
Hydrophilic heads interact with water
Selective Permeability of Lipid Bilayers
• The permeability of a structure is its tendency to allow a given
substance to pass across it.
• Phospholipid bilayers have selective permeability. Small or
Selective Permeability of Lipid Bilayers
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+
Selective Permeability of Lipid Bilayers
Permeability scale (cm/sec)
High permeability
Low permeability
O2, CO2
H2O
Glycerol, urea
Glucose
Cl–
K+
Selective Permeability of Lipid Bilayers
Size and charge affect the rate of diffusion across a membrane.
Phospholipid bilayer
O2, CO2, N2
H2O, urea, glycerol
Glucose, sucrose
Types of Lipids Affect Membrane Permeability Differently
• When a double bond exists between two carbons in a hydrocarbon
chain, the chain is said to be unsaturated. Hydrocarbon chains
without double bonds are termed saturated.
• A double bond in an unsaturated lipid causes a bend or “kink” in
the hydrocarbon chain, preventing the close packing of hydrocarbon tails and reducing hydrophobic interactions.
• Phospholipids with unsaturated tails form membranes that are
Temperature Affects Membrane Fluidity and Permeability
• Membrane fluidity decreases with temperature because molecules
in the bilayer move more slowly.
Why Do Solutes Move across Lipid Bilayers?
• Small molecules and ions in solution are called solutes.
• The random movement of solutes due to kinetic energy is known
as diffusion.
• A difference in solute concentrations across a selectively
permeable membrane creates a concentration gradient.
• When a concentration gradient exists, there is a net movement of
Diffusion across a Selectively Permeable Membrane
DIFFUSION ACROSS A LIPID BILAYER
1. Start with different solutes on opposite sides of a lipid bilayer. Both molecules diffuse freely across bilayer.
Lipid bilayer
2. Solutes diffuse
across the membrane— each undergoes a net movement along its own concentration gradient.
Diffusion and Osmosis
• Osmosis occurs when solutions of different concentrations are
separated by a membrane that is permeable to water but not to the solutes. Water spontaneously moves across the membrane toward the solution with the higher solute and lower water concentration.
Osmosis
OSMOSIS
1. Start with more solute on one side of the lipid bilayer than the other, using molecules that cannot cross the selectively permeable membrane.
Diffusion and Osmosis
• If the solution outside a cell has a higher solute concentration than
the interior has, then water will move out of the cell by osmosis, shrinking it. Such a solution is said to be hypertonic relative to the inside of the cell.
• If the solution outside a cell has a lower solute concentration than the interior has, then water will move into the cell, swelling it.
Such a solution is said to be hypotonic to inside of the cell.
• If solute concentrations are equal on the outside and inside of a
Osmosis Can Shrink or Burst Membrane-Bound Vesicles
Hypertonic solution Hypotonic solution Isotonic solution
Net flow of water out of cell; cell shrinks
Net flow of water into cell; cell swells or even bursts
Membrane Proteins
• Although phospholipids provide the basic membrane structure,
plasma membranes contain as much protein as phospholipids.
• The fluid-mosaic model of membrane structure suggests that
some proteins are inserted into the lipid bilayer, making the
membrane a fluid, dynamic mosaic of phospholipids and proteins.
• Some proteins, called integral proteins, are amphipathic and so
The Fluid-Mosaic Model of Membrane Structure
Fluid-mosaic model
Cell exterior
Phospholipid bilayer
Cell interior
How Do Membrane Proteins Affect Ions and Molecules?
• Integral proteins that span the membrane are called
transmembrane proteins. These proteins are involved in the transport of selected ions and molecules across the plasma
membrane. These proteins can therefore affect membrane permeability.
• The transmembrane proteins that transport molecules are called
transport proteins. There are three broad classes of transport proteins:
1. Channels
2. Carrier proteins or transporters
Facilitated Diffusion via Channel Proteins
• Membrane channel proteins circumvent the plasma membrane’s
impermeability to small, charged compounds.
• When ions build up on one side of a plasma membrane, they
establish both a concentration gradient and an electrochemical gradient.
• Molecules and ions always diffuse through channels down their
electrochemical gradients. This passive transport requires no energy expenditure by the cell and decreases the charge and
Facilitated Diffusion via Channel Proteins
• Cells have many different types of channel proteins in their
membranes, each featuring a structure that allows it to admit a particular type of ion or small molecule.
• These channels are responsible for facilitated diffusion: the
Membrane Channels: Highly Selective and Regulated
Water pores allow only water to pass through.
Outside cell
Inside cell
Potassium channels allow only potassium ions to pass through. Outside cell Inside cell Closed Hydrophilic interior Hydrophobic exterior
Potassium ions can enter the
channel, but cannot pass into the cell
Membrane Channels: Highly Selective and Regulated
Water pores allow only water to pass through.
Outside cell
Inside cell
Hydrophilic
interior Hydrophobic
Membrane Channels: Highly Selective and Regulated
Potassium channels allow only potassium ions to pass through.
Outside cell
Inside cell
Closed
Potassium ions can enter the
channel, but cannot pass into the cell
Facilitated Diffusion via Carrier Proteins
• Facilitated diffusion can occur through channels or through
carrier proteins, also called transporters, which change shape during the transport process.
• Facilitated diffusion by transporters occurs only down an
electrochemical gradient, reducing differences between solutions.
• Glucose is a building block for important macromolecules and a
major energy source, but lipid bilayers are only moderately permeable to glucose.
• A glucose transporter named GLUT-1 increases membrane
Active Transport by Pumps
• Cells can also transport molecules or ions against an
electrochemical gradient; this process requires energy in the form of ATP and is called active transport.
• Pumps create a chemical and electrical gradient across the
membrane. Cells use active transport to create an internal environment that significantly differs from the environment outside the cell.
• For example, the sodium-potassium pump, Na+/K+-ATPase, uses
How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
Outside cell
Inside
cell Phosphate
group
1. Three binding sites within the protein have a high affinity for sodium ions.
2. Three sodium ions from the inside of the cell bind to these sites.
3. A phosphate group from ATP binds to the protein. In response, the protein changes shape.
How the Sodium-Potassium Pump Works
5. In this conformation, the protein has binding sites with a high affinity for potassium ions.
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
6. Two potassium ions bind to the pump.
7. The phosphate group drops off the protein. In response, the protein changes back to its original shape.
How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
1. Three binding sites within the protein have a high
affinity for sodium ions.
2. Three sodium ions from the inside of the cell bind to these sites.
How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
3. A phosphate group from ATP binds to the protein. In response, the protein changes shape.
4. The sodium ions leave the protein and diffuse to the exterior of the cell.
How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
5. In this conformation, the protein has binding sites with a high affinity for potassium ions.
Phosphate group
6. Two potassium ions bind to the pump.
How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
7. The phosphate group drops off the protein. In response, the protein changes back to its original shape.
Mechanisms of Membrane Transport: A Summary
Diffusion Facilitated diffusion Active transport
Outside cell
Inside cell
Passive movement of small, uncharged molecules along an electrochemical gradient, through a membrane
