10BIO 111_2025.09.15 (Class 10) AUDIO
Membrane Structure and Fluid Mosaic Model
Membrane Fluidity and the Fluid Mosaic Model
Membranes are not rigid structures like a wall; they are flexible and dynamic.
The fluid mosaic model describes the membrane, emphasizing its fluidity and the mosaic arrangement of components.
Membrane fluidity is crucial for proper cell function, allowing processes like cell growth, movement, and division.
Held together primarily by hydrophobicity of the fatty acid tails of phospholipids.
Analogy: A bathtub filled with water (represents the aqueous environment) with ping-pong balls (phospholipid heads) floating on the surface in a single layer. Sloshing the water creates waves that move the ping-pong balls, illustrating the fluid nature. Adding a tennis ball (membrane-bound protein) also shows its movement within this fluid layer.
The fluidity is influenced by temperature and the composition of phospholipids and cholesterol.
Phospholipid Structure and its Influence on Fluidity
Phosphate head: Charged and hydrophilic, it interacts with the aqueous environment.
Hydrocarbon (fatty acid) tails: Nonpolar and hydrophobic, they cluster together away from water.
Saturated vs. Unsaturated Tails: The structure of the fatty acid tails significantly impacts membrane fluidity.
Saturated tails are straight and can pack tighter, leading to a less fluid membrane.
Unsaturated tails have kinks (bends) due to double bonds, preventing tight packing and resulting in a more fluid membrane.
The balance between saturated and unsaturated tails is crucial for maintaining appropriate fluidity.
Higher temperatures increase fluidity, while lower temperatures decrease it.
In water, phospholipids naturally cluster due to hydrophobicity, forming bilayers that create a boundary, partitioning water into separate compartments.
Modified Phospholipids
Phospholipids can be modified by linking other molecules.
Glycolipids: Phospholipids with sugars attached.
Specialized Phospholipids: Certain modifications, like linking amino acids (e.g., phosphatidylserine), create unique phospholipids embedded in membranes, often associated with specialized cellular responses like inflammation.
Cholesterol: Structure, Function, and Health Implications
Cholesterol is a type of lipid (not a protein) found within membranes.
Influence on Membrane Rigidity: The amount of cholesterol directly affects membrane rigidity. At higher temperatures (>37^ ext{o}C), cholesterol 'tames' fluidity by restricting phospholipid movement. At lower temperatures (<37^ ext{o}C), it 'maintains' fluidity by preventing tight packing of phospholipids, acting as a fluidity buffer.
More cholesterol = more rigid membrane.
Less cholesterol = less rigid membrane.
Certain areas of the membrane may have higher cholesterol to create more rigid structures called lipid rafts, which act as platforms for specific cellular processes.
Presence in Organisms: Cholesterol is found only in animal cells.
It is not found in plant cells, prokaryotes, or fungi. (This explains why plant-based foods like spinach do not contain cholesterol).
Dietary and Health Implications
Cholesterol is a hydrophobic (nonpolar) molecule and does not break down easily in water.
Excess dietary cholesterol can accumulate, clustering to itself and forming plaque in blood vessels.
Plaque buildup can decrease blood flow, leading to serious cardiovascular issues:
Heart attack: Complete blockage in the heart.
Stroke: Complete blockage in the brain.
Coronary bypass surgery: A medical procedure to reroute blood flow around severely blocked arteries, often using a vein from the leg.
Essential Biological Roles
While high levels are problematic, cholesterol is essential for animal life.
It is a precursor molecule for many hormones:
Sex hormones: Testosterone, estrogen.
Stress hormone: Cortisol.
Transport of Cholesterol: Due to its extreme hydrophobicity, cholesterol cannot freely cross membranes to enter cells for hormone synthesis.
It is transported by specialized proteins called lipoproteins.
High-density lipoproteins (HDLs) are often termed 'good' cholesterol because they transport excess cholesterol from the body's tissues back to the liver for excretion or reuse, thus preventing plaque buildup. Low-density lipoproteins (LDLs) are 'bad' cholesterol as they transport cholesterol from the liver to cells, and high levels can lead to cholesterol deposition in arteries, contributing to plaque formation.
HDLs and LDLs are measured in blood tests and indicate cholesterol transport and risk.
Dietary Sources: Foods known to be high in cholesterol include shrimp and lobster.
Membrane Proteins: Diverse Functions
Proteins embedded in or associated with membranes perform a wide variety of functions, sometimes even multiple functions per protein.
Transporters: Because the hydrophobic membrane acts as a barrier to polar and charged molecules (e.g., glucose), transporter proteins facilitate their movement across the membrane.
These can be channel proteins (forming a pore for continuous passage) or carrier proteins (binding molecules and undergoing conformational changes to move them across).
Enzymes: Speed up specific biochemical reactions without being consumed.
General rule for identifying enzymes: Their names often end in "-ase" (e.g., sucrase breaks down sucrose).
Membrane-bound enzymes can work on substances outside or inside the cell.
Cell Surface Receptors: Proteins on the cell surface that bind to specific signaling molecules (ligands) to receive and transmit signals into the cell.
Analogy: Like a baseball glove catching a baseball or a satellite dish receiving a signal.
Crucial for cell communication (a topic for a later chapter).
Signal transduction often involves conformational changes in the receptor upon ligand binding.
Cell Surface Identity Markers: Unique molecular patterns on the cell surface that allow cells to recognize each other and distinguish "self" from "non-self."
These can be sugars or glycolipids.
Example: Cat bite (immune system recognizing foreign cells), kidney transplants (matching tissue types; close matches require immunosuppressive drugs to prevent rejection).
Cell to Cell Adhesion: Proteins that help cells stick together to form tissues and organs, maintaining structural integrity (e.g., stomach cells held together).
Attachment to the Cytoskeleton: Membrane proteins link the membrane to the internal cytoskeleton, providing structural support and anchoring.
Cytoskeletal proteins (from Chapter 4):
Actin filaments: Smallest, often found at the cell periphery.
Intermediate filaments: Medium-sized, can span the membrane.
Microtubules: Largest.
Proteins like integrins help span the membrane.
Tight junctions (involving intermediate filaments) are crucial for holding cells so tightly that even liquid cannot pass between them, preventing leakage (e.g., preventing strong stomach acid, pH 1, from dissolving stomach lining cells).
Protein Structure within Membranes
Regions of membrane proteins that face the aqueous environment (outside or inside the cell) are typically polar, while regions embedded in the hydrophobic core of the membrane are nonpolar.
Proteins can form specific secondary structures within the membrane, such as alpha helices or beta sheets.
Hydrophilic Channels: These structures can be arranged to create channels through the hydrophobic membrane, allowing polar substances to pass.
A beta sheet can form a barrel-like structure with polar side chains facing inwards.
Multiple alpha helices can associate to form a central hydrophilic pore.
Examples of Transmembrane (Integral) Proteins:
Single-pass: A single polypeptide chain (with one N-terminal and one C-terminal) passes through the membrane once, typically as an alpha helix.
The single alpha helix in single-pass proteins is stable within the hydrophobic lipid bilayer due to its nonpolar amino acid side chains.
For example, the N-terminal might be extracellular, and the C-terminal cytoplasmic.
Multi-pass: A single polypeptide chain weaves through the membrane multiple times, often forming several alpha helices connected by loops.
These multi-pass proteins are critical for many cellular processes, including signal transduction and ion transport, as they can form complex channels.
For example, a three-pass transmembrane protein or a common seven-pass transmembrane protein (also known as a serpentine protein due to its snake-like path).
Beta-barrel: Several beta sheets form a barrel-like structure that spans the membrane.
Peripheral Proteins: Proteins that are associated with only one side of the membrane, not embedded within it.
Glycoproteins: Sugars linked to proteins, often serving as receptors or identity markers.
Virus Entry: A Receptor-Mediated Process
Viruses are large, charged nucleic acid structures encased in a protein coat, making direct membrane passage impossible.
Mechanism: Viruses exploit cell surface receptors.
A specific viral protein binds to a complementary cell surface receptor (e.g., the CD4 receptor). This binding is highly specific due to the 3D shapes of the interacting molecules.
This binding activates the transmembrane receptor, triggering a cellular response.
The cell then performs phagocytosis (engulfing the virus), bringing the large, polar viral particle into the cell, bypassing the membrane barrier.
(Side note): The seven-pass transmembrane protein shape is very common. An example is a small roundworm whose genome dedicates 5\% to making these types of transmembrane proteins.
Passive Transport: Movement Without Energy
Definition: The movement of substances across a membrane without the expenditure of cellular energy (ATP).
Driving Force: Occurs down a concentration gradient—molecules move from an area of high concentration to an area of low concentration.
Analogy: Opening a bottle of perfume; the scent (molecules) spreads from the high concentration at the bottle opening to the lower concentration throughout the room.
Substances that can cross the membrane via simple diffusion:
Nonpolar molecules: The hydrophobic membrane is not a barrier. Examples: oxygen (O_2) diffuses readily.
Very small polar molecules: Despite being polar, their small size allows them to diffuse across the membrane. Example: water (H_2O).
Facilitated Diffusion:
Movement of molecules (e.g., glucose, ions, amino acids) across the membrane with the help of transporter proteins (channels or carriers) down their concentration gradient. This still does not require ATP or cellular energy.
Osmosis:
The specific diffusion of water (H_2O) across a selectively permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration), often through aquaporins (channel proteins).
Substances that struggle or cannot cross the membrane via simple diffusion:
Larger polar molecules: Unable to easily pass through the hydrophobic core. However, these molecules can cross via facilitated diffusion if specific transporter proteins are present.
Ions (charged particles): Even the smallest ions, like hydrogen ions (H^+) (a single proton), cannot cross the membrane directly due to their charge, despite being smaller than a water molecule.
This highlights that charge is a more significant barrier than size for membrane permeability.
These also rely on facilitated diffusion through specific channel or carrier proteins.
Determining Permeability Based on Properties:
Nonpolar compounds: Pass easily.
Small, uncharged polar compounds: Can pass (e.g., water, urea). Water often uses aquaporins for faster passage (facilitated diffusion).
Large, uncharged polar compounds: Cannot pass easily or at all via simple diffusion.
Ions (charged): Cannot pass via simple diffusion.
Example: Steroid Hormones
Estrogen and Testosterone: These are large, nonpolar steroid hormones, synthesized from cholesterol. They can cross the cell membrane because they are primarily nonpolar.
Cholesterol itself: Is extremely hydrophobic and tends to stick in the membrane rather than crossing it freely. Therefore, it requires active transport into the cell via lipoproteins (HDLs and LDLs) to be used for hormone synthesis.
Upcoming Exam Information
Date: Wednesday.
Content: Chapters 1 to 4 .
Important Advice:
Your personal self-worth is not defined by an exam score.
The average score is expected to be a "C"; if this is your first "C," do not be discouraged.
Anticipate that likely no more than 5 or 6\% of students will get an "A" on this first exam.
Avoid studying all night before the exam; prioritize rest.
Pre-Exam Offering: A brief prayer will be offered before the exam for those who wish to attend (starting by 10:00) to help calm nerves. Attendance is optional and respects different faith traditions.