1. Lipids, Membranes, and the First Cells
Chapter 6
Lipids, Membranes, and the First Cells
Biological Science, Third Edition
– Scott Freeman
Lectures by Cheryl Ingram-Smith

2. 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 processes of facilitated diffusion and active transport.

3. 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 fatty acid is a hydrocarbon chain bonded to a carboxyl (COOH) functional group. Hydrocarbons are the reason that lipids do not dissolve in water.

4. 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.
Phospholipids consist of a glycerol that is linked to a phosphate group (PO42-) and to either two chains of isoprene or two fatty acids.
For the Cell Biology Video Space Filling Model of Cholesterol, go to Animation and Video Files
For the Cell Biology Video Stick Model of Cholesterol, go to Animation and Video Files

5. Structure of Phospholipids
Polar head
(hydrophilic)
Nonpolar tail
(hydrophobic)

6. 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 water while the tails do not, allowing these lipids to form membranes.
For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files

7. 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 of energy required.

8. Phospholipids Form Bilayers in Solution
Lipid micelles
Lipid bilayers
Water
No water
Hydrophilic heads interact with water
Hydrophobic tails interact with each other
Hydrophilic heads interact with water

9. 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 nonpolar molecules move across phospholipid bilayers quickly, but charged or large polar substances cross slowly, if at all.

10. Selective Permeability of Lipid Bilayers
Permeability scale (cm/sec)
Size and charge affect the rate of diffusion across a membrane.
Phospholipid bilayer
High permeability
O2, CO2, N2
O2, CO2
H2O
H2O, urea,
glycerol
Glycerol, urea
Glucose
Glucose, sucrose
Cl– K+
Cl– , K+, Na+
Low permeability
Na+

11. Selective Permeability of Lipid Bilayers
Permeability scale (cm/sec)
High permeability
O2, CO2
H2O
Glycerol, urea
Glucose
Cl– K+
Low permeability
Na+

12. 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
Cl– , K+, Na+

13. 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 much more permeable than those formed by phospholipids with saturated tails.

14. Temperature Affects Membrane Fluidity and Permeability
Membrane fluidity decreases with temperature because molecules in the bilayer move more slowly.
Decreased membrane fluidity causes decreased permeability.

15. 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 solutes from regions of high concentration across the membrane to regions of lower concentration.

16. Diffusion across a Selectively Permeable Membrane
DIFFUSION ACROSS A LIPID BILAYER
Lipid
bilayer
1. Start with different
solutes on opposite
sides of a lipid bilayer.
Both molecules diffuse
freely across bilayer.
2. Solutes diffuse
across the membrane—
each undergoes a net
movement along its own
concentration gradient.
3. Equilibrium is
established. Solutes
continue to move back
and forth across the
membrane but at equal
rates.

17. 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.
Animation: Diffusion and Osmosis

18. Osmosis OSMOSIS 1. Start with more solute 2. Water undergoes a net
on one side of the lipid
bilayer than the other,
using molecules that
cannot cross the
selectively permeable
membrane.
2. Water undergoes a net
movement from the region
of low concentration of
solute to the region of
high concentration.

19. 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 cell, the cell will stay the same size. In such a case, the outside solution is said to be isotonic to the solution inside the cell.
For the Cell Biology Video Plasmolysis of Plant Cells, go to Animation and Video Files

20. 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
No change

21. 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 can span a membrane, with segments facing both its interior and exterior surfaces. Other proteins, called peripheral proteins, are found only on one membrane side.

22. The Fluid-Mosaic Model of Membrane Structure
Cell exterior
Phospholipid bilayer
Cell interior
Membrane proteins

23. 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:
Channels
Carrier proteins or transporters
Pumps

24. 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 concentration differences between the cell’s exterior and interior.

25. 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 passive transport of substances that would not otherwise cross the membrane.
For the Cell Biology Video Water Movement Through an Aquaporin, go to Animation and Video Files

26. Membrane Channels: Highly Selective and Regulated
Water pores allow only water to pass through.
Hydrophilic
interior
Hydrophobic
exterior
Outside cell
Inside cell
Potassium channels allow only potassium ions to pass
through.
Potassium ions can enter the
channel, but cannot pass into the cell
Outside cell
Inside cell
Closed
When a change in electrical charge occurs
outside the membrane, the protein changes
shape and allows the ions to pass through
Open

27. Membrane Channels: Highly Selective and Regulated
Water pores allow only water to pass through.
Hydrophilic
interior
Hydrophobic
exterior
Outside cell
Inside cell

28. Membrane Channels: Highly Selective and Regulated
Potassium channels allow only potassium ions to pass
through.
Potassium ions can enter the
channel, but cannot pass into the cell
Outside cell
Inside cell
Closed
When a change in electrical charge occurs
outside the membrane, the protein changes
shape and allows the ions to pass through
Open

29. 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 permeability to glucose.

30. 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 ATP to transport Na+ and K+.
For the Cell Biology Video Na+/K+ ATPase Cycle, go to Animation and Video Files

31. 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.
4. The sodium ions leave
the protein and diffuse to
the exterior of the cell.

32. 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.
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.
8. The potassium ions leave
the protein and diffuse to the
interior of the cell. These 8
steps repeat.

33. How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
Outside
cell
Inside
cell
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.

34. How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
Phosphate
group
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.

35. How the Sodium-Potassium Pump Works
HOW THE SODIUM-POTASSIUM PUMP (Na+/K+- ATPase) WORKS
Phosphate
group
Phosphate
group
5. In this conformation, the
protein has binding sites with
a high affinity for potassium
ions.
6. Two potassium ions bind
to the pump.

36. 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.
8. The potassium ions leave
the protein and diffuse to the
interior of the cell. These 8
steps repeat.

37. 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
Passive movement of …
Active movement of …