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What are the main compartments of a eukaryotic cell?
Eukaryotic cells are divided into two main compartments: the nucleus and the cytoplasm.
The nucleus contains the cell's genetic material
The cytoplasm is an aqueous environment that houses various organelles and structures.
What does the cytoplasm contain?
The cytoplasm contains:
Organic molecules (e.g., proteins, carbohydrates)
Membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum)
Ions (e.g., Na+, K+, Ca2+)
Cytoskeletal elements (e.g., microtubules, actin filaments)
How does structure relate to function in eukaryotic cells?
The structure of cellular components determines their function.
A loss of structural integrity can lead to a loss of function, impacting overall cell health and activity.
What are the primary functions of the plasma membrane?
The plasma membrane serves several critical functions:
Determination of cell shape: Provides structural support.
Selective transport: Regulates what enters and exits the cell.
Cell recognition: Identifies cells through surface antigens.
Communication: Facilitates signaling through receptors.
Tissue organization: Helps organize cells into tissues via junctions. (rapid communication between cells.
What is the composition of the plasma membrane?
The plasma membrane is primarily composed of a lipid bilayer that includes:
Lipids: Phospholipids and cholesterol
Proteins: Integral and peripheral proteins
The lipid bilayer allows lipids and proteins to diffuse within the membrane, contributing to its fluid nature.
How does temperature affect membrane fluidity?
Membrane fluidity changes with temperature:
Warm temperatures: Fatty acid tails (unsaturated with kinked tails) become less rigid (fluid), allowing for more movement and potentially increasing permeability.
Cool temperatures: Fatty acid tails (saturated with straight tails) become more rigid, decreasing fluidity and permeability.
What are the characteristics of phospholipids?
Phospholipids are amphipathic molecules that:
Have a polar hydrophilic (water-attracting) head.
Contain two nonpolar hydrophobic (water-repelling) tails fatty acyl chains.
Their structure allows them to form a bilayer in an aqueous environment, with heads facing outward and tails facing inward.
Where is phospholipid derived from?
Glycerol, which gives raise to the name, Phosphoglycerides
What determines the bilayer formation of the plasma membrane?
The Amphipathic molecular structure
Hydrophobic head
The carbon chains in the hydrophobic chains lack charged groups, which causes no interaction with water
How does the bilayer's width vary?
The width of the lipid bilayer is determined by the length of the fatty acid side chains.
Additionally, the composition of the two leaflets (outer and inner) can differ, with specific phospholipids like phosphatidylcholine (PC) and sphingomyelin (SM) found in the outer leaflet, while phosphatidylethanolamine (PE) and phosphatidylserine (PS) are generally in the inner leaflet.
How is membrane asymmetry maintained?
Membrane asymmetry is maintained by ATP-dependent transporters embedded into the membrane. They include:
Flippases: Catalyze the movement of phospholipids from the extracellular leaflet to the cytosolic leaflet (flip inward).
Floppases: Mediate the movement from the cytosolic leaflet to the extracellular leaflet (flop outward).
-ABC transporters: Also contribute to the transport of various molecules across the membrane.
What role does cholesterol play in the membrane?
Cholesterol stabilizes the plasma membrane by:
Providing rigidity due to its steroid ring structure.
Affecting membrane permeability: its concentration can increase leading to greater membrane stabilization and greater rigidity or its concentraction can decrease leadiing to less membrane stabilization increasing fluidity.
At warm temperatures, it allows for more movement; at cool temperatures, it restricts movement, thus affecting overall membrane fluidity.
What are the types of membrane proteins and their functions?
Membrane proteins are classified into:
Peripheral proteins: Attach to one surface of the membrane, associated with polar head groups, and do not penetrate the membrane.
Integral proteins: Span the membrane (transmembrane proteins) and function as channels, carriers, or receptors. They are anchored by hydrophilic amino acids on either side.
Transmembrane proteins
They are integral proteins that span the membrane. The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acid, often coiled into alpha helices
How do aquaporins (water channels) function in the membrane?
Aquaporins are specialized water channels that:
Facilitate the passive transport of water into and out of cells.
Allow water to move down its osmotic concentration gradient.
Have a pore size that permits single-file passage of water molecules, creating a hydrophilic pathway through the hydrophobic membrane.
Renal Aquaporin Activation Process
ADH Release: ADH (Antidiuretic Hormone) is a hormone that helps control water balance in your body. It's released into the bloodstream and travels to your kidneys.
ADH Receptor Binding: ADH finds an ADH receptor in the kidney and binds to it.
G Protein Activation: When ADH binds to its receptor, it activates a helper protein attached to it called a G protein. This G protein is like a messenger that carries the signal inside the cell.
Adenylate Cyclase Activation: The G protein then activates an adenylate cyclase enzyme.
cAMP Formation: Adenylate cyclase converts ATP into cAMP (cyclic AMP). cAMP is like a second messenger, carrying the signal further.
Protein Kinase A Activation: cAMP activates another enzyme called Protein Kinase A. This enzyme adds phosphate groups to other proteins, which can change how they work.
Phosphorylation: Protein Kinase A adds a phosphate group to a water channel protein called Aquaporin-2. This process is called phosphorylation, and it's like flipping a switch on the Aquaporin-2.
Aquaporin-2 Activation: When phosphorylated, Aquaporin-2 changes shape and moves to the cell's surface.
Water Flow: When Aquaporin-2 is active and in place, water can flow through these channels into the cell, allowing the kidney to reabsorb water and concentrate urine.
What are ion channels and their characteristics?
Ion channels are integral membrane proteins that:
Are found in all cells and allow specific ions to pass through.
Are classified by selectivity (which ions can pass), conductance (how many ions can pass), and gating (open/closed states).
For example, K+ channels have a tetrameric structure and a voltage-sensor domain that responds to changes in membrane potential.
Voltage gated
Opens in response to change in membrane potential (action potential)
Ligand gated
Opens in response to a specific extracellular signal. Has a confirmation gate
Signal gated
Opens or closes in response to a specific intracellular molecule.
Stretch gated
Opens in response to change membrane stretch e.g muscle
What is the structure of a K+ channel?
The K+ channel has a tetrameric ( 4 proteins) structure, consisting of four subunits. This structure is 2 functionally and independent dormains and it forms an ion conduction pore that allows potassium ions to pass through.
What is the function of the voltage-sensor domain in a K+ channel?
The voltage-sensor domain is positioned at the periphery of the channel and consists of four transmembrane segments. It detects changes in membrane potential and undergoes structural rearrangement, leading to conformational changes in the conduction pore.
How do K+ ions move through the channel?
K+ ions are typically hydrated in solution. As they pass through the channel:
Electronegative oxygens face inward toward the ion, creating charge attraction.
K+ ions lose their hydration shell when entering the selectivity filter.
The channel contains four K+ binding sites.
The entry of cytoplasmic K+ into the inner chamber displaces other ions due to charge repulsion, allowing them to move to the next site.
What are solute carriers and how are they categorized?
Solute carriers are proteins that facilitate the transport of molecules across cell membranes. They are categorized by their mode of transport:
Uniporters: Transport one type of molecule (e.g., GLUT-1 for glucose).
Symporters: Transport two or more molecules in the same direction (e.g., Na+K+Cl- cotransporter.
Antiporters: Exchange one molecule for another in opposite directions (e.g., Na+/H+ exchanger).
How do ATP-dependent transporters function?
ATP-dependent transporters use energy from ATP to move ions and molecules across membranes.
ATPase ion transporters include:
2 subtypes
P-type: Phosphorylated during transport (e.g., Na+/K+ pump)
V-type: Found in organelle membranes (e.g., vacuolar H+/ATPase).
ATP-binding cassette (ABC) transporters: Have amino acid domains that bind ATP and transport various substances.
What is the mechanism of the Na+/K+ pump?
The Na+/K+ pump operates in several steps:
1. ATP binds to the pump, and 3 intracellular Na+ ions bind.
2. ATP is hydrolyzed, leading to phosphorylation of the pump and release of ADP.
3. The pump changes shape, exposing Na+ to the extracellular space, where it is released.
4. The pump then binds 2 extracellular K+ ions.
5. Dephosphorylation occurs, returning the pump to its original state and transporting K+ into the cell.
6. The cycle repeats, maintaining the electrochemical gradient.
What is the role of the V-ATPase pump?
The V-ATPase pump functions to acidify intracellular organelles that are necessary for their activation, such as lysosomes, by pumping H+ ions across membranes. It consists of integral and peripheral subunits, with the peripheral component facing the extracellular fluid and being phosphorylated.
What is an ATP-Binding Cassette (ABC) Transporter?
ABC transporters are a superfamily of membrane proteins that utilize energy from ATP hydrolysis to transport substances across cellular membranes.
What are the main components of ABC transporters?
Nucleotide-Binding Domain (NBD):
Also known as the ATP-binding cassette.
Binds and hydrolyzes ATP to provide energy for transport.
Transmembrane Domain (TMD):
Composed of hydrophobic alpha helices.
Involved in substrate recognition and translocation across the membrane.
What is the function of ABC transporters?
They translocate various substrates, including:
Nutrients
Lipids
Drugs
Metabolic products
Operate in both import and export mechanisms.
How do ABC transporters work?
ATP Binding: Induces conformational changes in the transporter, allowing substrate movement.
ATP Hydrolysis: Resets the transporter to its original state, preparing it for another transport cycle.
How do ABC transporters contribute to drug resistance?
ABC transporters can lead to multi-drug resistance (MDR) in cancer cells:
They pump out chemotherapy drugs, reducing their effectiveness.
Increased expression occurs in response to drugs like vinblastine, epirubicin and paclitaxel.
Overexpression of these transporters correlates with decreased drug efficacy.
What is diffusion
Diffusion is the spontaneous movement of molecules from areas of high concentration to areas of low concentration
How does diffusion work in cells?
It is driven by thermal motion, it allows for the distribution of substances.
In cell membranes, diffusion through the lipid bilayer is often inefficient for larger molecules, which typically use transporters.
Factors affecting diffusion include temperature, molecule size, and membrane fluidity.
What is an electrochemical gradient?
An electrochemical gradient is the combined effect of concentration and electrical charges on ion movement
Electrochemical potential arises from:
Difference of ion concentration on either side of the membrane
Charges of ions
Difference in voltage between the two sides of the membrane
Electrochemical driving force
It is the total forces acting upon ions across a membrane
When the net direction of the force is equal to the sum of the chemical and electrical driving forces
Chemical and electrical forces can act in the same or opposite direction
What is passive transport?
Passive transport is the movement of molecules across a membrane in a predicted direction due to a concentration gradient, without the use of energy.
What are the characteristics of passive transport?
Transport with the Gradient: Molecules move from an area of higher concentration to an area of lower concentration.
No Energy Required: Relies on the natural kinetic energy of molecules.
What are some examples of passive transport?
Water through Aquaporins: Facilitated diffusion of water molecules.
Glucose via GLUT-1: Facilitated diffusion of glucose through glucose transporter 1.
What is active transport?
Active transport is the movement of molecules across a membrane in the opposite direction of the concentration gradient, requiring energy.
What are the characteristics of active transport?
Against the Gradient: Molecules move from an area of lower concentration to an area of higher concentration.
Requires Energy: Energy, often in the form of ATP, is needed to drive the transport process.
What is primary active transport?
Primary active transport is a process where the energy for transporting molecules across a membrane is derived directly from ATP hydrolysis.
What are the characteristics of primary active transport?
Direct Use of ATP: Energy from ATP hydrolysis is used directly to transport molecules against their concentration gradient.
What is secondary active transport?
Secondary active transport is a process where the energy for transporting molecules is derived from the concentration differences of another molecule across the membrane.
What are the characteristics of secondary active transport?
Indirect Use of Energy: Utilizes the energy stored in the form of a concentration gradient of one molecule to drive the transport of another molecule against its gradient.
Carriers Involved: Often involves carriers that transport more than one molecule simultaneously.
How does secondary active transport work with multiple molecules?
Co-transport Mechanism: One molecule is transported down its concentration gradient, providing the energy to transport another molecule against its concentration gradient.
What are extracellular signaling molecules?
Cells release extracellular signaling molecules, such as hormones and neurotransmitters, to communicate with other cells.
What happens when signaling molecules bind to receptors?
Signaling molecules (ligands) bind to specific receptors, triggering an intracellular signal that leads to various cellular functions.
Where can receptors be located within a cell?
Receptors can be located in the cell membrane, cytoplasm, or nucleus, depending on the type of signaling molecule.
What is signal transduction?
Signal transduction is the process by which the interaction between a signaling molecule and its receptor transduces a message inside the cell, leading to a cellular response.
What can the message from signal transduction lead to?
The message may result in the activation or inactivation of intracellular proteins, altering cellular functions and responses.
What are the traits of signal transduction pathways?
Multiple, Hierarchical Steps: Signal transduction involves a series of steps, often organized hierarchically.
Amplification: Binding of a signal molecule to a receptor can amplify the signal, leading to a magnified cellular response.
Activation of Pathways: Can activate numerous pathways, regulating various cellular functions.
Feedback Mechanisms: Antagonism or regulation through feedback mechanisms to maintain balance.
Antagonism by feedback mechanism steps
Initial Signal Reception
A messenger (first messenger) binds to a receptor on the cell surface.
The receptor is coupled to a G protein with α, β, and γ subunits.
G Protein Activation
The binding of the messenger causes a conformational change in the receptor.
This change triggers the dissociation of the α subunit from the βγ complex.
The α subunit exchanges GDP for GTP, becoming activated.
Effector Activation
The activated α subunit binds to and activates phospholipase C (PLC).
PLC cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: Inositol 1,4,5-trisphosphate (IP3) and Diacylglycerol (DAG). The second Messenger Actions
IP3 Pathway:
IP3 binds to receptors on the endoplasmic reticulum.
This binding causes calcium channels to open, releasing Ca2+ into the cytosol.
Increased cytosolic Ca2+ activates calmodulin.
Activated calmodulin then stimulates various protein kinases.
These kinases phosphorylate target proteins, leading to cellular responses such as contraction, metabolism changes, or transport.
DAG Pathway:
DAG activates protein kinase C (PKC).
PKC phosphorylates target proteins by transferring a phosphate group from ATP.
These phosphorylated proteins then mediate specific cellular responses.
What are the traits of signaling molecules?
Variety: Include proteins, amines (e.g., epinephrine), and steroid hormones.
Cell-Type Specific: The effect of signaling molecules is specific to the cell type.
Distance of Action: Can act over long distances (endocrine) or short distances (paracrine or autocrine).
Target Cell Response: Depends on receptor expression, affinity, and specificity.
What is receptor affinity?
Receptor affinity refers to the strength with which a receptor binds to its specific signaling molecule (ligand).
What determines receptor specificity?
Receptor specificity is determined by the structure of the receptor, which allows it to bind selectively to particular signaling molecules.
What is receptor saturation?
Receptor saturation occurs when all available binding sites on receptors are occupied by signaling molecules, indicating a finite number of binding sites.
What are some types of receptors?
Ligand-gated ions
G protein-coupled receptors
Kinase-linked receptors
Nuclear receptors
What role do GTP-binding proteins play in signal transduction?
GTP-binding proteins (G-proteins) hydrolyze GTP to GDP, playing a crucial role in transmitting signals from receptors to target molecules inside the cell.
How are chemical signals transduced into electrical signals?
Chemical signals, such as neurotransmitters, can be transduced into electrical signals through ion channel receptors, which alter the membrane potential of the target cell.
What are G proteins?
G proteins are guanine nucleotide-binding proteins that play a crucial role in signal transduction by binding GTP and GDP. Activation occurs with GTP binding, while inactivation happens through GTP hydrolysis to GDP
How do G proteins function in signal transduction?
G proteins relay signals from activated cell surface receptors to intracellular targets, such as the cytoskeleton, and are involved in enzyme-linked receptor pathways, gene expression regulation, and cell proliferation and differentiation
What is the structure of G Protein-Coupled Receptors (GPCRs)
GPCRs are single polypeptide chains with seven membrane-spanning alpha helical segments. They interact with G proteins to cause conformational changes that allow the alpha subunit to accept different guanine nucleotides
What are heterotrimeric G proteins composed of?
Heterotrimeric G proteins are composed of three subunits: alpha, beta, and gamma.
The beta and gamma subunits are tightly linked and remain together
While the alpha subunit separates from them during normal function
How are G proteins activated and inactivated?
In the absence of a ligand, G proteins are inactive with GDP bound to the alpha subunit. Ligand binding induces a conformational change, releasing GDP and allowing GTP to bind, activating the alpha subunit. Termination occurs when the alpha subunit hydrolyzes GTP to GDP and reassociates with the beta and gamma subunits
How do negative feedback mechanisms prevent harmful effects in signaling pathways?
Activation of membrane-bound receptors can trigger negative feedback mechanisms, which prevent harmful effects due to persistent activation of signaling pathways.
What role do desensitization and endocytic receptor removal play in signal termination?
Desensitization and endocytic receptor removal are termination signals that differ from the re-association of G protein subunits. They help reduce receptor activity and remove receptors from the cell surface.
How do G protein coupled receptor kinases (GRKs )contribute to signal termination?
GRKs, a family of receptor serine/threonine kinases, phosphorylate the intracellular domain of the G protein, which is a key step in signal termination.
What is the function of beta-arrestins in signal termination?
Beta-arrestins bind to phosphorylated receptors, preventing interaction with G proteins, inactivating the receptor, and promoting receptor removal from the plasma membrane.
How do kinase/arrestin interactions lead to desensitization and downregulation?
Kinase/arrestin interactions, along with endocytosis, lead to desensitization and downregulation of cellular responses due to prolonged exposure to hormones.
What role do alpha subunits play in signal transduction?
Alpha subunits of G proteins couple to various effector proteins, initiating downstream signaling pathways.
What is cAMP and its role as a G protein effector?
cAMP (cyclic adenosine monophosphate) is a second messenger produced by the activation of adenylate cyclase, which is coupled to G protein alpha subunits. It plays a critical role in regulating various cellular response
What is DAG and its function as a G protein effector?
DAG (diacylglycerol) is a lipid-derived second messenger produced from the cleavage of PIP2. It activates protein kinase C (PKC), which regulates numerous cellular processes.
What is IP3 and its role as a G protein effector?
IP3 (inositol triphosphate) is a second messenger generated from the cleavage of PIP2. It facilitates the release of calcium ions from intracellular stores, influencing various cellular activities.
What is PIP2 and its significance in signaling?
PIP2 (phosphatidylinositol 4,5-bisphosphate) is a membrane phospholipid that serves as a precursor for the second messengers DAG and IP3, which are crucial in G protein-coupled signaling pathways.
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