Introduction to Lipids and Cell Membranes
General Characteristics and Nature of Lipids
Macromolecule Classification: Lipids are identified as one of the four major macromolecules, but they are unique compared to carbohydrates, proteins, and nucleic acids.
Non-Polymeric Nature: Unlike other macromolecules, lipids are not considered "true" polymers because they do not consist of repetitive monomer subunits.
Bonding and Association: Lipids do not typically form strong chemical bonds (like covalent bonds) with one another to create a chain. Instead, they associate with each other by staying in close proximity due to their similar chemical properties and charge interactions.
Physical State of the Cell: Contrary to the view of a cell being a rigid structure like a building, it is more accurately described as a "ball of jelly." The cell wall (membrane) is composed of lipids that are not tied together but stay close simply because they are lipids together.
Biological Functions of Lipids
Insulation and Temperature Regulation: Lipids are exceptional at insulating the body and retaining heat. * A higher concentration of lipids, specifically in the hypodermis layer of the skin, improves the body's ability to maintain its internal temperature.
Organ Protection and Visceral Fat: All internal organs are wrapped in fat for protection from physical impact. * Visceral Fat: This refers to fat that encases the organs. While necessary for protection, an excess of visceral fat is dangerous. * The "Skinny Fat" Phenomenon: Research has shown that individuals who appear thin externally can still be unhealthy if they have high levels of visceral fat. In some cases, fat can literally encase the heart, leading to heart attacks despite a low external body fat appearance. * DEXA Scan: This is a medical imaging tool used to show fat composition and identify exactly where fat is located in the body. * BMI and Patient Care: A patient might have a high Body Mass Index (BMI) but still be healthy if their fat is not the specific type that wraps around the organs.
Long-term Energy Storage: Lipids serve as a primary site for energy storage. The body stores energy in fat for as long as possible and only breaks it down when it is absolutely necessary.
Hormone Production: Fats, specifically cholesterol, are the precursors for several essential hormones, including: * Testosterone * Estrogen * Progesterone * Steroids * Cortisol
Vitamin Absorption: The body cannot absorb specific fat-soluble vitamins without the presence of lipids in the diet and the intestines. These vitamins include: * * * * * If fat is absent, these vitamins cannot be dissolved and will simply be excreted from the body ("come out the other end").
Chemical Structure: Triglycerides and Saturation
Triglyceride Components: The standard fat structure consists of two main parts: * Glycerol: The backbone of the molecule. * Fatty Acid Tails: These are long chains consisting of carbon-hydrogen bonds.
Fat Saturation Defined: Saturation refers to the presence or absence of double bonds within the carbon-hydrogen chains of the fatty acid tails.
Saturated Fats: * Structure: Contain no double bonds in the fatty acid tails. * Physical Properties: Because the tails are straight (no bends), the lipids can pack together very tightly. * State at Room Temperature: Solid. Examples include the white fat on a steak or a stick of butter. * Health and Flavor: Saturated fats are often considered more tasty but have a strong correlation with negative health outcomes if consumed in high amounts.
Unsaturated Fats: * Structure: Contain at least one double bond in the fatty acid tails. * Cis Bonds: Naturally occurring unsaturated fats typically have "cis" double bonds, which cause a "kink" or a bend in the tail. * Physical Properties: Due to the kinked tails, the lipids cannot pack tightly together, which prevents them from solidifying. * State at Room Temperature: Liquid. Examples include olive oil. * Health Benefits: Associated with better health outcomes. * Omega-3s: Found in oily fish. These fats specifically help build the membranes in the brain and are vital for brain health.
Trans Fats and Arterial Health
Artificial Nature: Trans fats are "fake" or synthetic; they do not exist naturally in any significant form.
Industrial Process: In a trans fat, the hydrogen atoms around the double bond are flipped to opposite sides (the "trans" configuration), whereas naturally occurring fats are in the "cis" configuration (same side).
Properties: Trans fats are liquids that have been modified to taste like high-quality saturated fats (like beef tallow or butter). They are used in frying foods like hamburgers and french fries because they have an extremely long shelf life and do not spoil easily.
Health Hazards and Arteriosclerosis: * Hydrophobicity: Because fat is hydrophobic (water-hating), it cannot be dissolved by water in the blood. It tends to collect in the arteries. * Body Processing: The body has mechanisms to clean out natural fats, but it has no mechanism to process trans fats because they are not real substances. * Accumulation: Trans fats collect and build up in the arteries over a lifetime. This leads to Arteriosclerosis (damage to the arteries). * Catastrophic Events: If a small clot of this accumulated fat dislodges, it can travel to a smaller artery in the brain or heart, stopping oxygen flow and causing a heart attack or stroke. * Age Factor: Individuals above the age of have likely been exposed to trans fats and may already have significant arterial damage. While major countries (including the US) have largely phased out or banned trans fats in the last to years, the damage remains as the body cannot easily remove them.
Lipoproteins: HDL vs. LDL
Misconception: While often called "good cholesterol" (HDL) and "bad cholesterol" (LDL), these are actually lipoproteins (a mixture of lipids and proteins), not pure cholesterol.
HDL (High-Density Lipoprotein): Known as "good" cholesterol. It circulates in the blood vessels, "scoops out" fat collecting in the arteries, and delivers it to be broken down and converted into useful substances like estrogen and progesterone.
LDL (Low-Density Lipoprotein): Known as "bad" cholesterol. Its job is to deposit fat/cholesterol in the body where it is needed (such as for cell membranes).
The Balance: LDL is essential for delivering cholesterol to cells, but having a higher level of LDL than HDL means more fat is being deposited than removed. This results in arterial damage and high blood pressure over time.
Steroids and Cell Membranes
Steroid Structure: Steroids are a form of lipid that look different from triglycerides because they do not have fatty acid tails. Instead, they are composed of four fused rings of carbon atoms.
Common Steroids: Estrogen, progesterone, testosterone, cortisol, and cholesterol all share this four-ring structure.
Phospholipids: These are the specific fats that form the cell membrane (the wall around the cells). * Composition: A glycerol molecule, two fatty acid tails, and a phosphate group. * Amphipathic Nature: * Hydrophobic portion: The fatty acid tails hate water. * Hydrophilic portion: The phosphate head is charged and likes water. * Bilayer Arrangement: Because of the charged phosphate group, the cell can exist in a water-based environment. The hydrophilic heads point outward toward the water inside and outside the cell, while the hydrophobic tails point inward, away from the water.
Questions & Discussion
Question from student: Can you explain the phospholipid concept again and how it interacts with water?
Response: Traditionally, lipids hate water. However, the addition of a charged phosphate group to the lipid "head" allows it to form hydrogen bonds and associate with water. This makes the head hydrophilic (water-loving). This allows a ball of lipids to exist within a water-filled body.
Question regarding water transport: If the inner tails of the membrane are hydrophobic, can water pass through the membrane?
Response: No, water cannot simply pass through the hydrophobic tails. However, during processes like osmosis, water moves through the membrane via specific channels that are hydrophilic. This topic of transport will be covered in more detail on Monday.
Logistics: The session ended at . The class was instructed to meet in the computer lab at for further work.