Chapter 6: Lipids, Membranes, and the First Cells

What is the plasma membrane?

A barrier that separates the inside of the cell from the outside and controls what enters and leaves.

What are the main functions of the cell membrane?

Acts as a selective barrier and allows chemical reactions by organizing molecules.

What are lipids?

Carbon-based molecules that are insoluble in water due to nonpolar bonds.

Why are lipids hydrophobic?

They contain nonpolar hydrocarbon chains that do not interact with water.

What are hydrocarbons?

Nonpolar molecules made only of carbon and hydrogen.

What is a fatty acid?

A hydrocarbon chain with a carboxyl group at one end.

What is the difference between saturated and unsaturated fatty acids?

Saturated = single bonds only; Unsaturated = one or more double bonds (causes bends/kinks).

What are polyunsaturated fatty acids?

Fatty acids with multiple double bonds.

How does saturation affect lipid state?

Saturated = solid; Unsaturated = liquid at room temperature.

What are the three major types of lipids?

Steroids, fats, and phospholipids.

What are steroids?

Lipids with four fused rings; include hormones like estrogen and cholesterol.

What are fats (triglycerides)?

Lipids made of glycerol and three fatty acids used for energy storage.

How are fats formed?

By dehydration reactions forming ester linkages between glycerol and fatty acids.

Why are fats good for energy storage?

They contain many high-energy bonds and store more energy than carbohydrates.

What are phospholipids?

Lipids with a glycerol, phosphate group, and two fatty acid tails; main component of membranes.

What does amphipathic mean?

Having both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

What parts of a phospholipid are hydrophilic and hydrophobic?

Head = hydrophilic; tails = hydrophobic.

What structures do phospholipids form in water?

Micelles and bilayers.

What is a phospholipid bilayer?

A double layer of phospholipids that forms the basic structure of cell membranes.

Why do bilayers form spontaneously?

Because hydrophobic tails avoid water and hydrophilic heads interact with water.

What are liposomes?

Artificial membrane-bound vesicles used in experiments.

What is selective permeability?

The ability of membranes to allow some substances to pass while blocking others.

Which molecules pass easily through membranes?

Small, nonpolar molecules (like oxygen).

Which molecules cross membranes slowly?

Large or charged polar molecules (like glucose).

How does saturation affect membrane permeability?

Unsaturated = more fluid/permeable; saturated = tighter and less permeable.

How does cholesterol affect membranes?

Increases density and reduces permeability.

How does temperature affect membrane fluidity?

Lower temperature = less fluid, tighter packing, less permeability.

What is diffusion?

Movement of molecules from high concentration to low concentration.

What is equilibrium?

When molecules are evenly distributed with no net movement.

What is passive transport?

Movement across a membrane without energy.

What is osmosis?

Movement of water across a membrane from low solute to high solute concentration.

What is a hypertonic solution?

Higher solute outside → water leaves cell → cell shrinks.

What is a hypotonic solution?

Lower solute outside → water enters cell → cell swells.

What is an isotonic solution?

Equal solute concentrations → no net water movement.

What are protocells?

Simple membrane-bound structures that may have been early life forms.

What is the fluid-mosaic model?

Membrane is a flexible mix of lipids and proteins.

What are integral (transmembrane) proteins?

Proteins that span the membrane.

What are peripheral proteins?

Proteins attached to the surface of the membrane.

What are ion channels?

Proteins that form pores to allow ions to cross membranes.

What is an electrochemical gradient?

A combination of concentration and charge differences across a membrane.

What are aquaporins?

Channel proteins that allow water to cross membranes.

What are gated channels?

Channels that open/close in response to signals.

What is facilitated diffusion?

Passive transport using proteins to move molecules across membranes.

What are carrier proteins?

Proteins that change shape to move substances across membranes.

What is active transport?

Movement of substances against their gradient using energy (ATP).

What does the sodium-potassium pump do?

Uses ATP to move sodium and potassium ions against their gradients.

What is secondary active transport?

Uses energy from gradients (not ATP directly) to move molecules.

Why are membranes important for life?

They create a controlled internal environment and allow selective transport.