Biological Science - Lipids, Membranes, and the First Cells
Biological Science - Lipids, Membranes, and the First Cells
Learning Objectives
By the end of this lecture, you should be able to:
Explain what a lipid is and describe why lipids are non-polar and hydrophobic.
Identify the building blocks of lipids.
Explain the difference between saturated and unsaturated fats.
Identify the composition of steroids and their functions.
Describe the function of a phospholipid and why its unique properties are used to form a semi-permeable membrane.
Describe what factors can affect the fluidity of the membrane.
Explain the fluid mosaic model of the plasma membrane.
Describe the 3 transport mechanisms (diffusion, facilitated diffusion, active transport) for molecules to move across a membrane.
Explain what osmosis is and what happens to a cell submerged in a hypotonic, isotonic, and hypertonic environment.
Define what an amphipathic protein is, and describe the 3 different proteins (carriers, transport proteins, and pumps) associated with the plasma membrane.
Lipids: What Is a Lipid?
Lipids:
Primarily carbon and hydrogen-based compounds (Hydrocarbons).
Largely nonpolar and hydrophobic (insoluble in water) due to C-C and C-H bonds (carbon to carbon and carbon to hydrogen bonds).
Building blocks of many lipids include:
Fatty Acids: hydrocarbon bonded to a carboxyl (-COOH).
Isoprenoid units: repetitive 5 carbon units.
Three Types of Lipids Found in Cells
Three most important types of lipids found in cells:
Triglycerides (fats).
Phospholipids.
Steroids.
The Structure of Fats
Fats: composed of glycerol linked to 3 fatty acids (triglyceride) by an ester linkage.
Formation: Fats form via dehydration reactions.
Fats consist of glycerol linked by ester linkages to three fatty acids.
Saturated and Unsaturated Fatty Acids
Saturated fatty acids chains:
Linear structure.
Unsaturated fatty acid chains:
Contains one or more double bonds (kinks).
Double bonds affect the physical characteristics of lipids.
Characteristics of Saturated and Unsaturated Fatty Acids
Saturated lipids:
Solids at room temperature (linear fatty acid tails stack tightly).
Example: Butter.
Some saturated lipids have extremely long hydrocarbon tails (e.g., waxes), forming stiff solids at room temperature.
Unsaturated lipids:
Liquids at room temperature (kinked fatty acid tails do not stack tightly).
Example: Safflower oil.
Structure of Steroids
Steroids:
Characterized by a four-ring structure, differing by functional groups.
Example: Cholesterol:
A hydrophilic hydroxyl group attached to the top ring and an isoprenoid “tail” attached at the bottom.
Functions in plasma membranes of many organisms and include hormones like testosterone and estrogen, which regulate reproductive cycles.
Structure of Membrane Lipids
Membranes are made primarily of phospholipids (along with some proteins).
Phospholipids are different from fats:
Glycerol attached to 2 hydrophobic fatty acid “tails” (or isoprene tails).
Contains phosphate and charged group “head”.
Phospholipids and Water
Upon contact with water, phospholipids form:
Micelles: Heads face the water and tails face each other.
Lipid bilayers: Two layers of phospholipid molecules align, with hydrophilic heads facing the surrounding solution and hydrophobic tails facing each other inside the bilayer.
Importance of Membranes
Plasma membrane (or cell membrane) serves as a semi-permeable barrier that separates life from nonlife.
Functions of membranes include:
Keeping damaging materials out.
Allowing entry of materials needed by the cell.
Facilitating the chemical reactions necessary for life.
Selective Permeability of Lipid Bilayers
The permeability of a structure is its tendency to allow a given substance to pass across it.
Lipid bilayers are highly selective;
Phospholipid bilayers have characteristics that determine permeability.
Small nonpolar molecules move across phospholipid bilayers quickly, while charged or large polar substances cross slowly, if at all.
Factors Affecting Membrane Permeability
Many factors influence the behavior of the membrane:
Number of double bonds between the carbons in the phospholipid's hydrophobic tail.
Temperature.
Number of cholesterol molecules in the membrane.
Bond Saturation and Membrane Permeability
Unsaturated (kinked) hydrocarbon chains are much more fluid.
Saturated (linear) hydrocarbon chains are much less fluid.
Other Factors That Affect Permeability
Adding cholesterol to membranes increases the density of the hydrophobic section, leading to:
Decreased membrane fluidity.
Causes molecules in the bilayer to move more slowly.
Leads to decreased permeability.
Fluidity of the Membrane
Individual phospholipids can move laterally throughout the lipid bilayer; however, they rarely flip between layers.
Solute Movement across Lipid Bilayers
There are three primary transport mechanisms:
Passive transport: does not require energy.
Example: random movement of molecules from high concentration to low concentration (diffusion).
Facilitated diffusion: occurs across a selectively permeable membrane.
Active transport: requiring energy to move substances against a concentration gradient.
Osmosis in Hypertonic, Hypotonic, and Isotonic Solutions
Isotonic: equal solute concentration inside and outside the cell; no net water movement.
Hypotonic: solution with a lower concentration of solutes than inside the cell; water moves into the cell by osmosis.
Hypertonic: solution with a higher concentration of solutes than inside the cell; water moves out of the cell by osmosis.
The Fluid-Mosaic Model of Membrane Structure
The plasma membrane is best understood as a fluid-mosaic model:
The fluid and dynamic mosaic consists of phospholipids and proteins.
Membrane Proteins
Transmembrane (Integral) proteins: amphipathic proteins that span the membrane and face both the interior and exterior of the cell.
Involved in transporting selected ions and molecules across the plasma membrane and can affect membrane permeability.
Peripheral proteins: found only on one side of the membrane, often attached to integral proteins.
Proteins Can be Amphipathic
Hydrophilic regions located near polar phosphate heads.
Hydrophobic regions located near nonpolar tails.
Membrane Proteins Affect Ions and Molecules
Transport proteins are integral proteins that transport molecules classified into 3 classes:
Channels: selectively allow molecules to pass along the membrane.
Transporters: change shape during the transport process, moving molecules down a concentration gradient.
Pumps: provide active transport of molecules across the membrane against a concentration gradient (requires energy, ATP).
Membrane Channels are Highly Selective
Key residues in membrane channels allow specific molecules to pass while blocking others, including most ions.
Carrier Proteins Undergo Structural Changes to Move Substances
Example of GLUT-1, a transmembrane transport protein facilitating glucose diffusion:
Unbound protein: GLUT-1 with its binding site facing outside the cell.
Glucose binding from outside the cell.
Conformational change to transport glucose inside the cell.
Release of glucose inside the cell.
The Sodium-Potassium Pump Depends on ATP
The sodium-potassium pump process includes:
Unbound protein.
Sodium binding.
Shape change to transport sodium out and potassium in.
ATP provides energy for the pump action.
Summary of Transport Mechanisms of Membrane Transport
Diffusion:
Passive movement of small, uncharged molecules along an electrochemical gradient through a membrane.
Proteins involved: None.
Facilitated diffusion:
Passive movement through membrane proteins.
Utilizes protein transporters.
Active transport:
Active movement of molecules against a concentration gradient using ATP.
Involves ATP and specific transport proteins.