Chapter 6: Lipids, Membranes, and the First Cells

Overview of the Plasma Membrane

The plasma membrane, also known as the cell membrane, is a fundamental structure that separates life from nonlife. It acts as a selective barrier, allowing entry of essential materials, excluding harmful substances, and facilitating chemical reactions necessary for life by sequestering appropriate chemicals.

6.1 Lipid Structure and Function

Lipids are carbon-containing compounds that are insoluble in water due to their high proportion of nonpolar bonds. Hydrocarbons, which are nonpolar molecules containing only carbon and hydrogen, are hydrophobic because their electrons are shared equally.

• Isoprenoids: Hydrocarbon chains functioning as pigments, scents, vitamins, and sex hormone precursors. They are building blocks for more complex lipids.

• Fatty Acids: Hydrocarbon chains bonded to a carboxyl (–COOH) functional group. They contain 14–20 carbon atoms and can be saturated or unsaturated.

• Saturated Fatty Acids: Only single bonds between carbons, maximizing hydrogen atoms.

• Unsaturated Fatty Acids: One or more double bonds, causing kinks in the chain. Polyunsaturated chains have many double bonds.

Saturation affects physical state: Highly saturated lipids are solid at room temperature, while highly unsaturated lipids are liquid.

A Look at Three Types of Lipids Found in Cells

Lipids do not possess a shared chemical structure; their structure varies widely. The three most important types of lipids found in cells are:

• Steroids: Distinguished by a bulky, four-ring structure. Examples include hormones (estrogen, testosterone) and cholesterol.

• Fats (Triacylglycerols/Triglycerides): Composed of three fatty acids linked to glycerol. Their primary role is energy storage, as they contain many high-energy bonds.

• Phospholipids: Consist of glycerol linked to a phosphate group and two hydrocarbon chains. Fatty acid tails are found in Bacteria and Eukarya, isoprenoid tails in Archaea. Their primary role is to form cell membranes.

How Membrane Lipids Interact with Water

Lipids have a wide array of functions, including energy storage, acting as pigments, serving as signals, forming waterproof coatings, and acting as vitamins. Membrane lipids contain both hydrophobic and hydrophilic regions:

• Hydrophilic Head: Contains glycerol, a negatively charged phosphate group, and a charged or polar group.

• Hydrophobic Tail: Nonpolar and hydrophobic; water cannot form hydrogen bonds with the tail.

• Amphipathic: Substances with both hydrophilic and hydrophobic regions (e.g., cholesterol).

6.2 Phospholipid Bilayers

Amphipathic lipids do not dissolve in water. Their hydrophilic heads interact with water, while hydrophobic tails do not. Amphipathic lipids form micelles (tiny spherical aggregates) or lipid bilayers (paired sheets).

Phospholipid bilayers form spontaneously in water, decreasing entropy at the level of lipid organization. Artificial membrane-bound vesicles (liposomes) can be formed in the lab.

Planar bilayers are constructed across holes in glass or plastic walls, separating two aqueous solutions. They are used to measure permeability of lipid bilayers.

Selective Permeability of Lipid Bilayers

Phospholipid bilayers exhibit selective permeability:

• Small or nonpolar molecules move across quickly.

• Charged or large polar substances cross slowly, if at all.

• Examples: Oxygen (small, nonpolar) moves quickly; glucose (large, polar) moves slowly.

How Does Lipid Structure Affect Membrane Permeability?

Membrane permeability is influenced by:

• Length of hydrocarbon tails

• Saturation state of hydrocarbon tails

• Presence of cholesterol molecules

Unsaturated tails (with double bonds) create kinks, weakening the barrier to solutes. Saturated tails have fewer spaces and stronger hydrophobic interactions, making the membrane denser and less permeable.

Cholesterol increases the density of the hydrophobic section, reducing membrane permeability by forcing phospholipid tails closer together.

How Does Temperature Affect the Fluidity and Permeability of Membranes?

Phospholipids move laterally within the bilayer. Membrane fluidity decreases as temperature drops, causing decreased permeability.

6.3 How Substances Move Across Lipid Bilayers: Diffusion and Osmosis

Dissolved solutes are in constant random motion due to thermal energy. Diffusion is the spontaneous movement of molecules and ions along a concentration gradient, increasing entropy.

• Equilibrium: Molecules are randomly distributed, with no net movement.

• Passive Transport: Diffusion across a membrane without an outside energy source.

Osmosis

Osmosis is a special case of diffusion involving water across selectively permeable membranes. Water moves from regions of low solute concentration to high solute concentration, diluting the higher concentration and equalizing concentrations.

Hypertonic solutions cause water to move out of the cell, shrinking it. Hypotonic solutions cause water to move into the cell, swelling it. Isotonic solutions result in no net water movement.

Membranes and Chemical Evolution

The first lipid bilayers likely provided containers for replicating molecules such as RNA. Protocells are simple vesicle-like structures that harbor nucleic acids and may have been intermediates in the evolution of cells.

6.4 Proteins Alter Membrane Structure and Function

Phospholipids provide the basic membrane structure, but plasma membranes contain as much protein as phospholipids. Proteins can insert into the membrane due to their amphipathic nature, forming passageways across the lipid bilayer.

Development of the Fluid-Mosaic Model

The fluid-mosaic model suggests that some proteins span the membrane, making it a dynamic mosaic of phospholipids and proteins. This model replaced the earlier sandwich model.

Freeze-fracture techniques and electron microscopy provided evidence for the fluid-mosaic model.

• Integral (Transmembrane) Proteins: Span the membrane, with segments facing both interior and exterior surfaces.

• Peripheral Membrane Proteins: Bind to membrane lipids without passing through, found on interior or exterior.

The Fluidity of Membranes

Membranes are held together mainly by weak hydrophobic interactions. Most lipids and some proteins can move sideways within the membrane, but flip-flop across the membrane is rare.

FRAP (Fluorescence Recovery After Photobleaching)

FRAP experiments demonstrate the lateral mobility of membrane proteins and lipids, supporting the fluid-mosaic model.

Membrane Protein Mobility

Experiments mixing mouse and human membrane proteins show that membrane proteins can move sideways within the plasma membrane.

Systems for Studying Membrane Proteins

Membrane proteins can be separated from membranes using detergents, which are small amphipathic molecules that form micelles and are water soluble. Detergents allow identification of membrane proteins affecting permeability, such as channels, carriers, and pumps.

Channel Proteins Facilitate Diffusion

Ion channels are specialized membrane proteins that form pores, allowing ions to cross membranes. Electrochemical gradients occur when ions build up on one side, establishing both concentration and charge gradients.

Channel proteins are selective, permitting only particular ions or small molecules to pass. Aquaporins allow water to cross the plasma membrane.

Gated Channels

Gated channels open or close in response to signals, such as binding of molecules or changes in electrical voltage. Movement through channel proteins is passive and does not require energy.

Cystic Fibrosis and Ion Channels

Cystic fibrosis is caused by defects in the CFTR transmembrane protein, which allows passage of chloride ions. Defective CFTR prevents chloride ions from leaving cells, leading to concentrated secretions.

Carrier Proteins Facilitate Diffusion

Carrier proteins assist passive transport by selectively picking up solutes on one side of the membrane and dropping them on the other. The best-studied carrier protein is GLUT-1, which increases membrane permeability to glucose by changing shape when it binds glucose.

Pumps Perform Active Transport

Facilitated diffusion is passive, but active transport moves substances against their gradient and requires energy, often provided by ATP. The sodium–potassium pump (Na+/K+–ATPase) uses ATP to transport Na+ and K+ ions against their concentration gradients.

Secondary Active Transport (Co-transport)

Pumps set up electrochemical gradients, which represent potential energy. Secondary active transport uses these gradients to power movement of another molecule against its gradient, without directly using ATP.

Plasma Membranes Define the Intracellular Environment

Biological membranes allow the cell's internal environment to differ from the outside environment through selective permeability and specificity of proteins. Evolution of membrane proteins enabled early cells to create internal environments conducive to life, favoring cells with efficient and selective membranes.

Summary Table: Passive and Active Mechanisms of Membrane Transport