Chapter 6: Lipids, Membranes, and the First Cells – Study Notes

Chapter 6: Lipids, Membranes, and the First Cells

Introduction to Plasma Membranes

The plasma membrane (or cell membrane) is a fundamental structure that separates life from nonlife. It acts as a barrier between the cell's interior and the external environment, maintaining homeostasis and enabling cellular function.

• Functions of membranes:

  • Keep damaging materials out of the cell

  • Allow entry of materials needed by the cell

  • Facilitate chemical reactions necessary for life by bringing reactants into proximity

Functional Groups in Biological Molecules

Functional groups are specific groups of atoms within molecules that are responsible for characteristic chemical reactions.

• Amino groups (-NH2): found in proteins

• Carboxyl groups (-COOH): found in proteins & lipids

• Carbonyl groups (C=O): found in carbohydrates

• Hydroxyl groups (-OH): found in carbohydrates & lipids

• Phosphate groups (OPO32-): found in nucleic acids

• Sulfhydryl groups (-SH): found in proteins

Lipid Structure and Function

What Are Lipids?

Lipids are carbon-containing compounds found in organisms. They are largely nonpolar and hydrophobic, meaning they do not dissolve in water.

• Many lipids are hydrocarbons: molecules containing only carbon and hydrogen

• Hydrocarbons are hydrophobic because electrons are shared equally in C-H bonds

• Lipids function as pigments, scents, vitamins, and sex hormone precursors

• They serve as building blocks for complex lipids

Fatty Acids and Isoprene

• Fatty acids: simple lipids consisting of a hydrocarbon chain bonded to a carboxyl (-COOH) group

  • Typically contain 14–20 carbon atoms

  • Can be saturated (no double bonds) or unsaturated (one or more double bonds)

• Isoprene: a building block for more complex lipids

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: have no double bonds, straight chains, solid at room temperature (e.g., butter)

• Unsaturated fatty acids: have one or more double bonds, kinked chains, liquid at room temperature (e.g., olive oil)

• Hydrogenation can convert unsaturated fats to saturated fats by breaking double bonds and adding hydrogen atoms

Types of Lipids in Cells

• Steroids

• Fats (Triglycerides)

• Phospholipids

These groups are characterized by their insolubility in water rather than a shared chemical structure.

Steroids

• Structure: four fused rings

• Examples: cholesterol, sex hormones (testosterone, progesterone, estrogen)

• Have both polar (hydrophilic) and nonpolar (hydrophobic) regions

Fats (Triglycerides)

• Structure: three fatty acids linked to a glycerol molecule via dehydration reactions (forming ester linkages)

• Examples: butter, lard, cod liver oil, margarine, cow’s milk (myristic acid), palm oil (stearic acid), omega-3 fatty acids

Phospholipids

• Structure: two fatty acids linked to glycerol, which is linked to a phosphate group

• Phospholipids are the building blocks of cell membranes

• Contain hydrophilic (polar) head and hydrophobic (nonpolar) tail regions

Amphipathic Nature of Phospholipids

Phospholipids are amphipathic (dual-sympathy), meaning they have both hydrophilic and hydrophobic regions.

• Hydrophilic head: contains glycerol, a negatively charged phosphate group, and a charged or polar group

• Hydrophobic tail: comprised of two nonpolar fatty acid or isoprene chains

Lipids Spontaneously Form Bilayers

Lipid Micelles and Bilayers

• Lipid micelles: spherical structures with hydrophilic heads facing water and hydrophobic tails interacting with each other

• Lipid bilayers: two sheets of phospholipids align with hydrophilic heads facing out and hydrophobic tails facing in

• Bilayers form spontaneously without outside energy

• Artificial membrane-bound vesicles are called liposomes

Lipids and Chemical Evolution

• Evidence suggests lipids were present during chemical evolution

• Simple lipids (e.g., fatty acids) can be synthesized under early Earth conditions

• Modern meteorites contain lipids

Phospholipid Bilayers and Permeability

Selective Permeability of Lipid Bilayers

Lipid bilayers are selectively permeable, allowing some substances to pass more easily than others.

Type of Molecule	Permeability
Small, nonpolar molecules (O2, CO2, N2)	High
Small, uncharged polar molecules (H2O, glycerol)	Moderate
Large, uncharged polar molecules (glucose, sucrose)	Low
Small ions (Cl-, K+, Na+)	Very low

Factors Affecting Membrane Permeability

• Saturated hydrocarbon chains: longer tails, with cholesterol, and low temperatures decrease permeability

• Unsaturated hydrocarbon chains: shorter tails, no cholesterol, and high temperatures increase permeability

• Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in length, making the membrane less permeable

• Phospholipids move laterally within the bilayer; membrane fluidity decreases as temperature drops

Diffusion and Osmosis

Diffusion

Diffusion is the movement of molecules from regions of high concentration to regions of low concentration, driven by a concentration gradient.

• Increases entropy and is spontaneous

• Equilibrium occurs when molecules are randomly distributed and there is no net change

• Passive transport: substances diffuse without an outside energy source

Osmosis

Osmosis is a special type of diffusion involving water movement across a selectively permeable membrane.

• Water moves from regions of low solute concentration to regions of high solute concentration

• This dilutes the higher concentration of solute and equalizes concentration on both sides of the bilayer

Effects of Solution Concentration on Cells

Solution Type	Solute Concentration	Water Movement	Cell Effect
Hypertonic	Higher outside vs. inside	Water moves out	Cell shrinks
Hypotonic	Lower outside vs. inside	Water moves in	Cell expands
Isotonic	Equal outside and inside	No net movement	Cell size remains the same

Membranes and Chemical Evolution

Protocells and the Origin of Life

• The first lipid bilayers likely provided containers for replicating RNA

• Negatively charged ribonucleotides can cross lipid bilayers and enter lipid-bound vesicles

• Simple vesicle-like structures that harbor nucleic acids are called protocells

• Protocells may have had simple, permeable membranes and represent possible intermediates in the evolution of the cell

Proteins Alter Membrane Structure and Function

Role of Membrane Proteins

• Phospholipids provide the basic membrane structure, but plasma membranes contain as much protein as phospholipid

• Proteins can insert into membranes and be amphipathic (having both hydrophilic and hydrophobic regions)

• Proteins can fold into shapes and integrate into lipid bilayers

Theories of Plasma Membrane Composition

• Sandwich model: membrane proteins coat the exterior surfaces of the phospholipid bilayer

• Fluid-mosaic model: proteins are embedded within the phospholipid bilayer, allowing for lateral movement

Types of Membrane Proteins

Type	Description
Integral (Transmembrane) Proteins	Segments face both interior and exterior surfaces; portions passing through the bilayer have hydrophobic side chains
Peripheral Membrane Proteins	Bind to the membrane without passing through it; may be found on interior or exterior of the cell

Transport Across Membranes

Passive Transport Mechanisms

• Simple diffusion: movement of small, nonpolar molecules directly through the bilayer

• Facilitated diffusion: movement of molecules via channel or carrier proteins

Channel Proteins

• Transmembrane proteins that form pores or openings in the membrane

• Can be gated and tightly controlled

• Enable ions or small polar molecules to move down their concentration gradient (still passive)

• Ion channels: establish electrochemical gradients (concentration and charge)

• Example: Cystic fibrosis is caused by defects in a transmembrane protein for Cl- ions (CFTR)

• Example: Aquaporins permit water to cross the plasma membrane

Carrier Proteins

• Transmembrane proteins that change shape to transport solutes across a membrane

• Transport larger molecules (e.g., glucose)

• Still passive transport

• Example: GLUT carrier protein increases membrane permeability to glucose by changing shape when it binds glucose

Active Transport Mechanisms

• Moves substances against their concentration gradient (from low to high concentration)

• Requires input of energy (often from ATP)

• Pumps: membrane proteins that use energy to move molecules across the membrane

• Example: Sodium-potassium pump transports Na+ and K+ against their gradients using ATP

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion (Channel)	No	Down gradient	Ion channels, aquaporins
Facilitated Diffusion (Carrier)	No	Down gradient	GLUT (glucose)
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump

Key Equations

• Diffusion rate: $J = -D \frac{dC}{dx}$

• Osmosis: $\Delta \Psi = \Psi_s + \Psi_p$ (Water potential = solute potential + pressure potential)

Additional info:

• Membrane fluidity and permeability are crucial for cell signaling, nutrient uptake, and waste removal.

• Protocells are a model for understanding the origin of cellular life.