Lipids and Membranes

This chapter explores how the plasma membrane, life's defining barrier, is constructed from lipids and proteins.
Key topics include:
- Lipid structure and function.
- How lipids spontaneously form bilayers.
- Mechanisms of substance movement across bilayers (diffusion and osmosis).
- The role of membrane proteins.

Plasma Membrane (Cell Membrane):
- Separates life from non-life.
- Serves as a selective barrier:
  - Allows the exchange of necessary materials between the cell and its environment.
  - Keeps damaging materials out of the cell.
Membranes around Organelles:
- Facilitate chemical reactions essential for life by sequestering appropriate chemicals.
Lipids are one of the major components in all membranes.

A diverse class of molecules primarily composed of carbon and hydrogen atoms (hydrocarbons) with few oxygen-containing functional groups.
Lack a shared chemical structure:
- Their structure varies widely; they are not considered polymers.
- The hydrocarbon skeleton can be arranged in many different ways.
Nonpolar and Hydrophobic:
- Electrons are shared equally in C-H bonds, making lipids nonpolar and thus hydrophobic (water-fearing).
Important Lipid Types in Organisms:
- Triglycerides (fats and oils): Primarily used for long-term energy storage.
- Phospholipids: Essential components of cellular membranes.
- Steroids: Function as hormones and cholesterol; also components in membranes.

Structure:
- A large lipid formed from two smaller types of molecules:
  - Glycerol: A sugar alcohol.
  - Fatty acids: Hydrocarbon chains bonded to a carboxyl functional group (- ext{COOH}).
- Triglycerides are composed of three fatty acids linked to a glycerol molecule via dehydration reactions.
Primary Role: Long-term energy storage.
Difference between Fats and Oils: Primarily determined by the degree of saturation of their fatty acid chains.

Saturated Hydrocarbon Chains:
- Consist of only single bonds between carbon atoms.
- Examples of saturated fats (e.g., coconut oil, butter) are typically solid at room temperature.
- Saturated lipids with extremely long hydrocarbon tails form waxes, which are particularly stiff solids at room temperature.
Unsaturated Hydrocarbon Chains:
- Contain one or more double bonds between carbon atoms.
- Monounsaturated: Possess one double bond (e.g., olive oil).
- Polyunsaturated: Possess more than one double bond (e.g., corn oil, sunflower oil).
- Unsaturated fats are typically liquid at room temperature.
- The more double bonds present, the more fluid the lipid.
- Unsaturated fats oxidize (become rancid) more easily.
Examples of Saturation Levels (as mixtures):
- Mostly Saturated: Animal fats (butter, beef tallow, lard) and tropical oils (coconut oil, palm oil).
- Rich in Monounsaturated: Olive oil, canola oil, peanut oil, safflower oil.
- Rich in Polyunsaturated: Flaxseed oil, walnut oil, sunflower oil, corn oil, soybean oil, cottonseed oil.

Fats store significantly more energy than carbohydrates.
- A fatty acid (component of fat) contains higher potential energy ( 9 ext{ kcal/g} ) compared to carbohydrates ( 4 ext{ kcal/g} ).
- The extensive C-H bonds in fatty acids contribute to this higher energy density.
Note on Saturated vs. Unsaturated Energy:
- Saturated fats store slightly more total energy.
- However, unsaturated fats are more easily broken down and absorbed, leading to a higher amount of metabolizable energy.
- Unsaturated fats do not pack tightly, making them more fluid, more easily transported across cell membranes, and more accessible to digestive enzymes.

Isomerism in Unsaturated Regions: The hydrogen atoms at the unsaturated (double bond) regions can be arranged in different positions:
- Cis: Hydrogens are on the same side of the carbon chain; this is the fatty acid structure typically found in nature.
- Trans: Hydrogens are on opposite sides of the chain; these are primarily produced during food processing.
Hydrogenation:
- The process of adding hydrogen atoms to unsaturated fatty acids.
- Converts liquid fats (oils) into a semi-solid (spreadable) or solid form (e.g., creating margarine from plant oil).
- Often results in the creation of trans fatty acids, which have negative health effects.
- Trans fats may be listed on food labels as