Cell Biology Part IV: Cellular Structure: Membrane and Framework

Cell Biology Part IV: Cellular Structure: Membrane and Framework

Basic Membrane Structure

• Membrane serves as a flexible boundary separating a cell and its environment, and also as a boundary for different compartments (organelles) within a cell.

Functions of Membranes

• All organelles are surrounded by a membrane (either one or two layers).

• Functions of the plasma membrane:

  • Receiving Information:

    • Receiving chemical signals via receptor proteins.

    • Sensing environmental changes with specialized sensing cells.

  • Import and Export Molecules:

    • Nutrient input and waste release through passive diffusion or specialized channels.

  • Movement:

    • The fluidity of the plasma membrane allows the cytoplasm to flow in certain directions, resulting in movement.

  • Protection:

    • Has little to no function in protection.

    • A cell's compartments/organelles are surrounded by membranes with one or two layers.

    • One layer of phospholipid bilayer: Vesicles, Lysosomes, Peroxisomes

    • Two layers of phospholipid bilayer: Mitochondria, Nuclei, Chloroplasts (in plants)

Basic Structure of Plasma Membrane

1. Phospholipid Bilayer

2. Transmembrane Proteins (varying amounts depending on the membrane):

   • Function:

     • Cross-membrane transportation.

     • Communication.

     • Connection.

3. Modification on the proteins, such as polysaccharides

Phospholipid Bilayer

• Composed of lipids and proteins.

• Hydrophilic and Hydrophobic Components:

  • Phospholipid molecule divided into two components:

    • Hydrophilic head:

      • Composed of polar chemical groups (including phosphate).

      • Can be dissolved in water.

    • Hydrophobic tail:

      • Composed of nonpolar long carbon chain, similar to lipid molecules.

      • Cannot be dissolved in water.

• Amphipathic:

  • The characteristic of having both hydrophilic and hydrophobic components.

  • Common detergents also have amphipathic properties, allowing them to dissolve oils and lipids.

  • Detergents work by having their hydrophobic tails interact with oil's long carbon chains and hydrophilic heads interact with water, helping oil and water to mix.

• Different Phospholipids:

  • Numerous types of phospholipids exist.

  • The major difference is in the hydrophilic heads; hydrophobic tails are similar.

• Fluidity of Phospholipids:

  • Phospholipid naturally forms a sealed compartment.

  • Phospholipid molecules cannot exist independently in an aqueous environment; they aggregate to form a bilayer or vesicle.

  • Hydrophobic tails cannot be exposed to water.

  • The fluidity of the phospholipid membrane allows the molecules to constantly move.

  • This property allows for higher permeability.

  • Allows membranes to fuse with each other.

  • Vesicles (wrapped with membrane) may form from an organelle and fuse with another organelle due to phospholipid fluidity.

  • New phospholipid molecules are always made in the cytosol and then redistributed to the other side.

  • Fluidity helps this redistribution.

  • Flippases actively transfer certain phospholipids from the cytosolic side to the non-cytosolic side (outside).

  • Double bonds influence phospholipid molecule angles.

  • The double bond on one of the tails makes an un-rotatable angle, resulting in a kink on the tail.

  • This kink is important for maintaining space between molecules, allowing phospholipids to spread out.

  • Steroids Modulate Membrane Fluidity

  • Steroids (cholesterol) are often present in the plasma membrane in large amounts.

  • They fill the space between phospholipid tails, reducing fluidity.

  • Too much cholesterol stiffens the membrane and reduces other functions like permeability, making transportation difficult.

Membrane Lipids

• Glycolipids mainly locate on the non-cytosolic side (exterior side) of the plasma membrane.

• Form a layer of carbohydrate lubrication to protect against friction, similar to glycoproteins and proteoglycans.

• Different kinds of phospholipids locate unevenly in either the cytosolic (interior) or non-cytosolic side (exterior) based on their function.

Membrane Proteins

• Membrane proteins, or membrane-bound proteins, are located in the plasma membrane.

• Functions:

  • Transporters: Examples include electron transport chains, H^+ pumps, and water channels.

  • Anchors: Examples include electron transport chains and photocenters; they anchor themselves into the membrane.

  • Receptors: Examples include neuron receptors.

  • Enzymes: Examples include ATP synthase.

• Different Ways for Membrane Proteins to Associate with the Membrane:

  • Transmembrane: Proteins typically use\alpha-helices to insert through the membrane.

  • Membrane-associated: Located entirely on one side of the membrane, but a section of its peptide anchors in one layer of the membrane.

  • Lipid-linked: Some proteins are linked with lipid groups, replacing the phosphate on the phospholipid, and this becomes an anchor inserted into the membrane.

  • Protein-attached: Some proteins cannot insert their peptide into the membrane but link with another protein that can.

  • Transmembrane Proteins with Multiple\alpha-Helices:

    • Hydrophobic side chains allow the protein to be embedded in the membrane.

    • Hydrophilic side chains are exposed to a central pore that allows H_2O to pass through.

    • The size of the pore can be changed depending on the relationship between each helix.

    • This type of channel controls passage by the property of target chemicals, allowing hydrophilic substances to pass but not hydrophobic ones.

  • Transmembrane Protein Formed by a\beta-Barrel:

    • Composed of 16\beta-sheets curved around each other to form a "barrel."

    • The barrel has a certain diameter (size doesn’t change) due to the degree that\beta-sheets can bend.

    • This barrel becomes a channel that allows water or nutrients to come in but prevents larger molecules (such as toxins) from entering.

    • This type of channel controls passage by the size of target chemicals, allowing smaller molecules to pass but not bigger ones.

    • Example: Porin (a type of channel).

Cell Cortex

• Reinforces the cell surface.

• Cell Cortex: A membrane surface protein meshwork that determines the shape of the cells.

• Composed of surface proteins, mainly Spectrin (though not the only cortex protein), a long, thin, flexible protein that connects with other proteins to form a meshwork cell cortex.

Cell Surface Carbohydrates

• Cell surface is often coated with carbohydrates:

  • Glycoprotein: Proteins covalently linked with oligosaccharides (short sugar chain).

  • Proteoglycan: Proteins covalently linked with polysaccharides (long sugar chain).

  • Glycolipid: Phospholipids covalently linked with oligosaccharides (short sugar chain).

  • All three are located on the non-cytosolic side (outside) of the membrane and form a carbohydrate surface to protect the membrane by lubricating the cell membrane.

Membrane Mobility

• Membrane proteins are not stationary. Experiment:

  1. Initially, all proteins are located on their respective sides of the fusion cell.

  2. Over time, they mix together, demonstrating that the proteins are mobile, not stationary.

Membrane Structure and Transport

Basic Membrane Structure

• The cell membrane is crucial for:

  • Transporting molecules across the membrane.

  • Facilitating transport within the cell.

  • Providing a structural framework within the cell.

Membrane Transport Overview

• Chapter 12: Membrane Transport

• Diffusion is the simplest form of transport.

• Membranes allow selective passage of molecules.

• Various transport systems exist, including:

  • Channel Proteins

  • Carrier Proteins

  • Passive Transport

  • Active Transport

  • Symport

  • Antiport

  • Osmosis

• Channel opening can be triggered by various stimuli.

• Neuronal signaling relies on membrane transport, including:

  • Action Potential

  • Synapses

  • Psychoactive drugs

Transport Mechanisms

• Transport across the membrane occurs via:

  • Direct diffusion through the phospholipid membrane (without a transporter).

  • Transport through specialized membrane proteins (transporters).

• Two main types of transporters:

  • Carrier Proteins

  • Channel Proteins

• Small, non-charged hydrophobic molecules can diffuse directly through the membrane by dissolving in the phospholipids.

• Small, non-charged hydrophilic molecules can also diffuse through the membrane.

• Diffusion: Movement of particles from an area of higher concentration to an area of lower concentration.

  • Driven solely by the concentration gradient (no energy input required).

• Larger molecules and charged ions cannot diffuse directly through the membrane; they require special channels or carriers.

• In a pure phospholipid bilayer (without proteins or channels), some particles can still pass through via simple diffusion.

• Diffusion relies on the concentration gradient.

• The concentration gradient can be considered a driving force.

• Molecules that rely on simple diffusion:

  • Small

  • Non-charged

Carrier Proteins vs. Channel Proteins

• Carrier Proteins: Transport molecules by binding to a specific binding site. This binding is highly specific, similar to enzyme-substrate interactions.

• Channel Proteins: Select molecules based on size and electric charge. If a molecule is small enough and has the appropriate charge, it can pass through when the channel is open.

• Key difference: The method of solute selection, which affects specificity.

  • Carrier proteins require binding.

  • Channel proteins do not require binding.

• Carrier proteins transport molecules via binding and conformational changes:

  1. Molecules bind to a specific binding site on the carrier protein.

  2. The carrier protein undergoes a conformational change, opening on the other side of the membrane.

  3. Molecules are released and move into the cell, following the concentration gradient.

  • Carrier proteins can transport molecules along or against the concentration gradient.

Passive vs. Active Transport

• Passive Transport: Does not require energy; molecules move across the membrane based on the concentration gradient.

  • Can occur via carrier proteins, channel proteins, or simple diffusion.

• Active Transport: Requires energy to pump molecules against the concentration gradient.

  • Only performed by carrier proteins.

• Active transport relies on different energy sources:

  • Coupled Transporters: One solute moves along its concentration gradient, providing the energy for another solute to move against its gradient.

  • ATP-driven pumps: Use energy from ATP hydrolysis to pump molecules.

  • Light-driven pumps: Use light energy (e.g., Bacteriorhodopsin).

Coupled Transporters

• Couple the transport of two solutes using one carrier protein.

  • Antiport: Two solutes are transported in opposite directions.

  • Symport: Two solutes are transported in the same direction.

Example of Coupled Transport

• State A: When Na^+ binds to the protein, it increases the protein's affinity for glucose, leading to glucose binding.

• State B: When Na^+ leaves the protein, the protein loses its affinity for glucose, causing glucose to disengage.

• Since Na^+ concentration is higher in the extracellular space, it readily enters the cell, driving the entry of glucose as well.

• Coupled transport can involve one active and one passive transport process.

• Active transport can facilitate passive transport without direct ATP or energy input.

Glucose Transport in Intestinal Epithelium

1. A Na^+-driven symport pumps glucose into cells, resulting in high intracellular glucose concentration.

2. Glucose is released to the other side of the epithelium through a passive uniport.

• Active transport moves molecules against their concentration gradient and requires energy.

• Passive transport moves molecules along their concentration gradient and does not require energy.

ATP-Driven Transporters

• Can function as symports or antiports.

• Both solutes rely on ATP for energy and do not depend on each other's concentration gradients.

Osmosis

• The diffusion of water across a membrane, moving from an area of higher water concentration to lower water concentration.

• Passive process (does not require ATP).

Osmosis in Single-Celled Organisms

• Typically, the extracellular water concentration is higher, causing water to diffuse into the cell via osmosis.

• Uncontrolled water influx can cause cells to burst.

• Mechanisms to remove excess water are necessary:

  • Animal cells: Maintain intracellular solute concentration by pumping out ions to balance the concentration across the membrane.

  • Plant cells: Pump excess water into a central vacuole.

  • Protozoan cells: Pump excess water out using contractile vacuoles.

Ion Channels

• Selectively allow certain ions to pass through the membrane.

• Selection is less specific than in carrier proteins.

Triggering Channel Opening

• Once a channel is open, molecules flow through, driven by the concentration gradient.

• Voltage-gated channels: Open and close in response to changes in voltage across the membrane.

• Ligand-gated channels: Open and close in response to ligand binding.

Neuronal Signaling

• Neurons consist of:

  • Cell body

  • Dendrites

  • Axon

• Neurons send and receive signals.

• Dendrites: Receive signals from other neurons.

• Axon: Transmits signals to the neuron terminal.

Action Potential

• Achieved by depolarization of the neuron's membrane (axon).

• When the gate opens, ions flow in (resting potential).

• Depolarization: Normally, the membrane is positively charged outside and negatively charged inside. When a neuronal signal arrives, Na^+ ions flux in, neutralizing the polarity.

• Resting membrane potential: V = -70 mV

Synapses

• Synapses transmit signals to the next neuron.

• When an action potential reaches the terminus, it triggers the opening of Ca^{2+} channels, causing an influx of Ca^{2+} into the nerve terminal.

• Neurotransmitter receptors are also channels that open upon neurotransmitter binding.

• The influx of Ca^{2+} triggers the release of neurotransmitters, which bind to neurotransmitter receptors.

• Neurotransmitter receptors are ion channels that open upon binding with neurotransmitters, allowing ions to flow in and initiate another action potential.

Psychoactive Drugs

• Application of understanding membrane transport in the context of psychoactive drugs.

Intracellular Compartments and Transport

• Cells are filled with organelles that perform different functions.

• Proteins need to be transported within a cell to reach their designated compartments.

• Cells have compartments.

• A signal sequence (signal peptide) determines the destination of proteins.

Translocation for Different Compartments

• Nuclei

• Mitochondria

• Endoplasmic Reticulum (ER)

• Membrane-bound proteins

Other Transport Mechanisms involving Membranes

• Golgi Apparatus

• Vesicles: Clathrin-coated and COP-coated

• Membrane protein modification

• Chaperones

• Endocytosis and Exocytosis

• Lysosomes

Protein Transport Overview

• After being synthesized, proteins must be transported to the compartments where they are needed.

• Proteins are transported to their designated compartments after synthesis.

Protein Structure Review

• Complete functional proteins are long polypeptides in 3D structures, achieved after folding into their tertiary structure.

• Polypeptides are synthesized by connecting amino acids together, from the NH2 group (N-terminus) to the COOH group (C-terminus).

• Proteins are translated from mRNA, which is transcribed from DNA.

Signal Sequence (Signal Peptide)

• The N-terminal (front) section of a polypeptide often determines its destination; this section is the signal sequence or signal peptide.

• The signal peptide sequence tells the cell where to send the protein, acting like a tag.

• The N-terminus often contains a signal sequence (or signal peptide).

• This short section (about 10 amino acids) indicates the protein's destination, guiding the protein translocation process.

• The signal peptide is not part of the functional protein and is cleaved off after the protein has been transported to its destination.

Nuclear Protein Import

• Nucleus proteins are synthesized in the cytosol and then transported into the nucleus through nuclear pores.

• Nuclear pores are complex structures allowing nuclear proteins to enter and mature mRNA to exit, while preventing incompletely spliced messenger RNA from exiting or unwanted substances from entering.

Nuclear Protein Transport Mechanism

• Nuclear proteins have a special signal sequence recognized and bound by a “nuclear transport receptor.”

• The bound complex is transported through the nuclear pore.

• This process requires energy input from GTP hydrolysis.

• After entering the nucleus, the nuclear receptor dissociates and exits back to the cytosol to transport the next protein.

• A receptor is always involved in transport to an organelle.

Protein Transport to Mitochondria

• The signal sequence is recognized and bound by a receptor protein associated with a translocator.

• The protein-translocator complex diffuses to a contact site, where two channels align, allowing the new protein to enter.

• The signal sequence is cleaved off after translocation is complete.

Protein Biosynthesis in the Cytosol and ER

• Cytosolic proteins are translated in the cytosol.

• The same ribosomal components can be used to make ER ribosomes.

• ER proteins are translated on the surface of the ER.

Soluble Protein Transport to ER

• When a soluble protein is synthesized, the signal sequence on the N-terminus is made first.

• The signal sequence is recognized by the signal recognition particle (SRP), which directs the peptide-ribosome complex to the SRP receptor on the ER membrane.

• Then, the SRP releases the peptide-ribosome complex to a translocation channel.

• The newly made peptide enters the ER through the translocation channel and is folded into the correct conformation.

• The signal sequence is cleaved off by signal peptidase.

• After entering, the protein is folded into its correct conformation.

Protein Transport to ER (Membrane-Bound Proteins)

• For membrane-bound proteins with an internal transmembrane domain:

  • The signal sequence is cleaved off by signal peptidase as usual.

  • When the transmembrane domain (a special sequence) goes through the translocation channel, the channel discharges the domain to the membrane, where it remains bound.

Chaperones

• Misfolded proteins (resulting from denaturation) are refolded by chaperones (proteins responsible for repair) before exiting the ER.

• After repair, a vesicle budding process transports the correctly folded protein to its destination.

Golgi Apparatus and Protein Translocation

• After transport to the ER, proteins are transported to the Golgi Apparatus via budding and fusion of vesicles.

• Membrane-bound proteins remain bound in the membrane at this stage.

Vesicle Budding (Clathrin-Coated)

1. Cargoes are bound by cargo receptors (which recognize transport signals), and this complex is then bound by adaptin.

2. Adaptin is then bound by the clathrin coat, forming a vesicle.

3. Dynamin clips the vesicle off.

4. Once completed, the coating is removed, and the naked vesicle is released.

Exocytosis through Secretory Vesicles

• Constitutive secretion releases newly made proteins constantly.

• Regulated secretion waits for a signal to release newly made proteins.

Endocytosis

• Pinocytosis: the ingestion of fluid (including small particles), a way of obtaining food.

• Phagocytosis: the ingestion of large particles/cells, performed by specialized phagocytic cells.

• During phagocytosis, target particles/cells must bind and activate surface receptors, requiring specific recognition.

• Once activated, phagocytes extend sheet-like pseudopods to engulf the target cell/particle.

Cytoskeleton: Intermediate Filaments, Microtubules, and Actin Filaments

Intermediate Filaments

• Composed of α-helix proteins.

• Provide mechanical strength to cells.

• Keratin reinforces epidermal cells.

• Lamina: Forms a lining supporting the nuclear membrane.

• Two intermediate filament molecules twist to form a coiled-coil dimer.

• Globular domains connect molecules end-to-end.

• Every monomer forms an α-helix.

• Help cells withstand mechanical stress; extend through epithelial cells to prevent rupture.

• Keratins in different cells are connected via pores at tight junctions, forming keratin filaments.

• Connect to other protein fibers like microtubules with plectin.

Nuclear Lamina

• A special type of intermediate filament that supports the nuclear membrane.

• Forms a lining underneath the nuclear membrane.

• Disassemble during mitosis (controlled by phosphorylation) and reassemble after mitosis to reform the nuclear membrane.

Microtubules

• Long, hollow tubes made of tubulin proteins.

• Major component of mitotic fibers.

• Provide transport tracks within the cell.

• Composed of α-tubulin and β-tubulin dimers.

• Polarity: α-tubulin end is the minus end; β-tubulin end is the plus end.

• Tubulins extend in the direction of the plus end (β-tubulin).

• Polarity determines the direction of transport along the microtubule.

• γ-tubulin serves as the base for microtubules to grow from the centrosome.

Assembly and Disassembly

• αβ-tubulin dimer binds to GTP, gaining energy to bind to the elongating tube.

• If GTP is hydrolyzed to GDP before the next dimer binds, the GDP-containing dimer falls off.

• Assembly and disassembly occur constantly, depending on GTP availability.

• Extension and shortening are random.

• Can be stabilized by capping proteins or chromosomes at the plus end, preventing disassembly.

• Organize organelles by constructing a network in the cytosol for anchoring.

Motor Proteins

• Dynein and Kinesin are motor proteins for polar transport along microtubules.

• Each has a motor head and a cargo tail.

• Cargo tails carry cargo, and motor heads slide along microtubules.

• Kinesin moves toward the plus end, and dynein moves toward the minus end.

• Transport directions are crucial in polarized cells.

• Microtubules serve as tracks for motor proteins.

• Example: Transport along axons.

Flagella

• Made of 9+2 bundles of microtubules.

• Each bundle contains two microtubules, with one attached to the dynein cargo tail.

• Dynein arms push neighboring bundles, causing them to pass each other.

Actin Filaments

• Composed of actin molecules (subunits).

• Myosin attaches and walks on actin filaments.

• Reinforce the cell surface via the cell cortex network.

• All actin molecules (subunits) are connected in the same direction.

• Each subunit is identical (unlike microtubules).

• Two strands of actin filaments combine to form a two-stranded helix.

Polymerization

• ATP binds to actin, which incorporates into the growing strand.

• When ATP is hydrolyzed to ADP, stability decreases, and subunits fall apart.

• Capping proteins attach to protect the plus end, stopping polymerization.

• ARP complex attaches to existing actin filaments, providing a site for new filament branching.

• Hydrolyzed ADP attached to actin promotes depolymerization via depolymerizing protein.

Function

• Polymerization elongates filaments, pushing the membrane and causing the cell to crawl.

• Actin cortex performs cellular movement.

Myosin

• Myosin "walks" on actin filaments (Myosin I) for cellular transport.

• Uses binding-releasing actions and conformational changes.

• Walks from the minus end to the plus end.

• One-way walking ensures precise directional movement of myosin-associated organelles.

• Myosin action in muscle (Myosin II): Head contains ATPase, which hydrolyzes ATP for energy to pull actin.

Muscle Contraction

• Head is locked to the actin filament at rest.

• When the head releases ADP, it pulls actin back.

• At the end of the cycle, the head is locked to actin again.

• When ATP binds to the head, the head releases actin.

• When the head hydrolyzes ATP, it moves forward along the actin filament.

• When the head releases phosphate, it binds to a location further ahead.

• Muscular cells are filled with actin and myosin fibers called myofibrils.

Contraction Mechanism

• Muscular cells have sarcomeres with Ca^{2+} channels on their membrane.

• When channels open, Ca^{2+} flows into the cytoplasm to trigger contraction.

• Tropomyosin normally covers myosin-binding sites on actin.

• Ca^{2+} ions bind to troponin, which is associated with tropomyosin.

• This removes tropomyosin from actin, freeing the binding sites for myosin.