Cell Biology: Plasma Membrane Structure and Function
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Questions and Answers

What occurs after a signaling molecule binds to a G protein-linked receptor?

  • The α-subunit interacts with its target upon dissociation. (correct)
  • Only the β and γ complex interact with their targets.
  • The three subunits do not dissociate but become inactive.
  • The β and γ subunits dissociate from the α-subunit.

Which of the following is NOT a type of G protein mentioned?

  • Gs2 protein (correct)
  • G12/13 protein
  • Gi protein
  • Gq protein

Which second messenger is most commonly associated with G protein signaling?

  • Cyclic adenosine diphosphate (cADP)
  • Cyclic guanosine monophosphate (cGMP)
  • Inosine triphosphate (ITP)
  • Cyclic adenosine monophosphate (cAMP) (correct)

What is a characteristic of heterotrimeric G proteins?

<p>They make seven transmembrane passes. (C)</p> Signup and view all the answers

Which of the following functions is associated with monomeric G proteins?

<p>They are involved in cell attachment to the extracellular matrix. (B)</p> Signup and view all the answers

What is the thickness range of the plasma membrane?

<p>7.5-10 nm (C)</p> Signup and view all the answers

Which statement correctly describes the structure of the plasma membrane?

<p>It is comprised of an inner and an outer leaflet. (C)</p> Signup and view all the answers

What is one of the main functions of the plasma membrane?

<p>It maintains the cell's structural and functional integrity. (C)</p> Signup and view all the answers

What type of molecules can freely permeate the plasma membrane?

<p>Small lipid-soluble nonpolar molecules (D)</p> Signup and view all the answers

Which component of the plasma membrane helps regulate its fluidity?

<p>Cholesterol (A)</p> Signup and view all the answers

Which type of lipid is restricted to the extracellular aspect of the outer leaflet?

<p>Glycolipids (C)</p> Signup and view all the answers

What property do phospholipids possess that contributes to the formation of the bilayer?

<p>Amphipathic (D)</p> Signup and view all the answers

What is the major role of the glycocalyx formed by the polar carbohydrate residues of glycolipids?

<p>To facilitate cellular recognition (D)</p> Signup and view all the answers

What role does cholesterol play in the membrane at lower temperatures?

<p>It prevents tightly packed structures to maintain fluidity. (D)</p> Signup and view all the answers

What are lipid rafts primarily responsible for?

<p>Enhancing cellular communication and protein trafficking. (C)</p> Signup and view all the answers

Which of the following correctly describes integral proteins?

<p>They span the entire thickness of the membrane. (D)</p> Signup and view all the answers

What characterizes transmembrane proteins?

<p>They can pass through the membrane multiple times. (B)</p> Signup and view all the answers

What function do peripheral proteins typically serve in relation to the cell membrane?

<p>They interact with integral proteins or lipid polar groups. (D)</p> Signup and view all the answers

What provides structural stability to the membrane through its less permeable arrangement?

<p>Cholesterol (A)</p> Signup and view all the answers

How do some membrane proteins remain immobile within the lipid bilayer?

<p>Through interactions with cytoskeletal components. (D)</p> Signup and view all the answers

What role do glycoproteins play in the cell membrane?

<p>They assist in cell recognition and communication. (A)</p> Signup and view all the answers

What is the primary characteristic of facilitated diffusion?

<p>It occurs through specific structures for certain molecules. (A)</p> Signup and view all the answers

Which type of channel is primarily responsible for the passive transport of K+ ions across membranes?

<p>K+ leak channels (B)</p> Signup and view all the answers

How do aquaporins facilitate the movement of water across cell membranes?

<p>By flipping water molecules halfway down the channel. (B)</p> Signup and view all the answers

What is the difference between ungated and gated ion channels?

<p>Gated channels open temporarily in response to stimuli. (B)</p> Signup and view all the answers

Which type of transport is characterized as requiring energy to move substances against an electrochemical gradient?

<p>Active transport (B)</p> Signup and view all the answers

Which channel type opens in response to changes in membrane potential?

<p>Voltage-gated channels (B)</p> Signup and view all the answers

What function do carrier proteins serve in facilitated diffusion compared to active transport?

<p>They can undergo reversible conformational changes. (C)</p> Signup and view all the answers

What specific feature do ligand-gated channels share?

<p>They open when a signaling molecule binds. (A)</p> Signup and view all the answers

What is the primary role of hydrophilic signaling molecules?

<p>They bind to and activate cell surface receptors to trigger diverse physiological effects. (A)</p> Signup and view all the answers

What characterizes the structure of membrane receptors?

<p>They are integral membrane glycoproteins embedded in the phospholipid bilayer with three distinct domains. (C)</p> Signup and view all the answers

Which type of membrane receptor directly opens or closes ion channels?

<p>Channel-linked receptors (D)</p> Signup and view all the answers

Which of the following describes catalytic receptors?

<p>They trigger intracellular enzymatic activity upon binding with ligands. (D)</p> Signup and view all the answers

What is the main function of the extracellular domain of membrane receptors?

<p>To serve as a binding site for signaling molecules. (D)</p> Signup and view all the answers

Which of these molecules are examples of lipid-soluble signaling molecules?

<p>Nitric oxide and steroid hormones (B)</p> Signup and view all the answers

How do membrane receptors influence plasmalemma permeability?

<p>By regulating the conformation of ion channel proteins. (C)</p> Signup and view all the answers

What can result from defective catalytic receptors?

<p>Continuous activation of signaling pathways. (A)</p> Signup and view all the answers

What is the primary function of the Na+-K+ pump?

<p>To maintain constant cell volume by regulating ion concentration (B)</p> Signup and view all the answers

What is transported by ABC transporters in eukaryotes?

<p>Toxins and drugs into the extracellular space (B)</p> Signup and view all the answers

Which type of hormone acts on nearby target cells?

<p>Paracrine (B)</p> Signup and view all the answers

Which of the following components of the Na+-K+ pump directly provides the energy needed for ion transport?

<p>Hydrolysis of ATP (C)</p> Signup and view all the answers

What is required for a target cell to receive a signaling molecule?

<p>Cell membrane receptors (B)</p> Signup and view all the answers

What ion movements are involved in the Na+-K+ pump process?

<p>Three Na+ ions exit and two K+ ions enter (C)</p> Signup and view all the answers

Which type of signaling molecule is typically released into synaptic clefts?

<p>Neurotransmitters (D)</p> Signup and view all the answers

What is the result of the Na+-K+ pump reducing intracellular ion concentration?

<p>Decreased osmotic pressure and cell swelling (C)</p> Signup and view all the answers

Flashcards

Plasma Membrane Structure

The cell membrane, also called plasmalemma, is a thin, double-layered structure (phospholipid bilayer) with embedded proteins.

Phospholipid Bilayer

Two layers of phospholipids form the cell membrane. The heads face outwards, and the tails face inwards.

Cell Membrane Function

Controls what enters and leaves the cell, maintains cell shape, identifies the cell to others, and relays external signals into the cell.

Fluid Mosaic Model

The model describing the cell membrane as a fluid structure with various molecules moving around.

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Integral Proteins

Proteins that are embedded within the cell membrane.

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Peripheral Proteins

Proteins that are loosely attached to the cell membrane.

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Glycolipids

Lipids with attached carbohydrate chains, found on the outer surface of the cell membrane.

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Cholesterol in Membrane

A lipid that helps regulate cell membrane fluidity.

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What does cholesterol do to membrane fluidity?

Cholesterol helps maintain membrane fluidity by preventing phospholipids from packing too tightly at lower temperatures, making the membrane more flexible.

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What special structures does cholesterol contribute to?

Cholesterol helps create lipid rafts, which are specialized microdomains in the membrane that concentrate specific proteins and lipids.

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What are the two main types of membrane proteins?

Membrane proteins are categorized as integral proteins and peripheral proteins, based on how they associate with the phospholipid bilayer.

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What is a transmembrane protein?

Transmembrane proteins span the entire width of the cell membrane, acting as channels, receptors, or transporters.

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Why are transmembrane proteins amphipathic?

Transmembrane proteins have both hydrophilic and hydrophobic regions, allowing them to interact with both the water-based environment and the lipid bilayer.

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Where do integral proteins preferentially attach in freeze-fracture preparations?

Integral proteins tend to remain attached to the P-face (the outer surface of the inner leaflet) of the membrane during freeze-fracture.

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How are peripheral proteins anchored to the membrane?

Peripheral proteins can bind to glycolipids, integral proteins, or phospholipid polar groups via covalent or noncovalent interactions.

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What are the functions of peripheral proteins?

Peripheral proteins play diverse roles including electron carriers, signaling, and structural support.

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Facilitated diffusion

The movement of molecules across a membrane using transport proteins, such as ion channels and carrier proteins, providing a faster pathway for ions and large molecules than simple diffusion.

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Ion channels

Multipass transmembrane proteins that form small aqueous pores, allowing specific ions and small molecules to pass down an electrochemical gradient through the membrane.

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Aquaporins

Specialized channels designed to transport water across the cell membrane rapidly, without allowing protons to pass through.

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Carrier proteins

Multipass transmembrane proteins that undergo conformational changes to transport specific molecules across the membrane, functioning in both passive and active transport.

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Ionophores

Molecules that can transport ions across a membrane, similar to ion channels.

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Gated ion channels

Ion channels that only open transiently in response to specific stimuli, such as changes in voltage, mechanical stimuli, or ligand binding.

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Active transport

The movement of molecules against an electrochemical gradient, requiring energy and carrier proteins.

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Electrochemical gradient

The difference in electrical potential and concentration of ions across a membrane.

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Na+-K+ Pump

A carrier protein that actively transports sodium (Na+) ions out of a cell and potassium (K+) ions into the cell, using energy from ATP hydrolysis.

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Antiport

A type of membrane transport where two substances move across the membrane in opposite directions.

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Symport

A type of membrane transport where two substances move across the membrane in the same direction.

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Glucose Transport

The movement of glucose across an epithelial cell membrane, often facilitated by sodium ions and carrier proteins.

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ABC Transporters

Membrane proteins that use ATP energy to transport molecules across the membrane, often expelling substances like toxins.

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Signaling Cell

The cell that produces and releases a signaling molecule.

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Target Cell

The cell that receives a signaling molecule and responds to its message.

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Receptor Molecule

A protein on the target cell that binds to a specific signaling molecule, initiating a cellular response.

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Lipid-soluble signaling molecules

These molecules can pass through the cell membrane and bind to receptors inside the cell, triggering intracellular events.

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Hydrophilic signaling molecules

These molecules cannot pass through the cell membrane directly and bind to receptors on the cell surface, initiating signaling cascades.

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What are the three domains of a membrane receptor?

Membrane receptors have an extracellular domain that binds the signaling molecule, a transmembrane domain that spans the membrane, and an intracellular domain that interacts with intracellular components.

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What is the role of the intracellular domain of a membrane receptor?

The intracellular domain interacts with intracellular components, like peripheral proteins or organelles, to transduce the extracellular signal into an intracellular response.

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Channel-linked receptors

These receptors act like gates, opening or closing to control the passage of ions across the cell membrane in response to a signaling molecule.

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Catalytic receptors

These receptors have an enzymatic activity that is triggered by ligand binding, initiating a signaling cascade within the cell.

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What is the difference between channel-linked and catalytic receptors?

Channel-linked receptors control ion flow, while catalytic receptors activate intracellular enzymes upon ligand binding.

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Defective catalytic receptors

Some oncogenes (cancer-causing genes) can code for defective catalytic receptors that are continuously active, potentially leading to uncontrolled cell growth.

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G protein-linked receptor

A transmembrane protein that binds signaling molecules and activates heterotrimeric G proteins, leading to intracellular signal transduction.

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Heterotrimeric G protein

A protein composed of three subunits (α, β, and γ) that interacts with G protein-linked receptors and regulates intracellular signaling pathways.

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What are the effects of signaling molecule binding to a G protein-linked receptor?

Binding of a signaling molecule to a G protein-linked receptor can cause either dissociation of the α-subunit from the β and γ complex, or activation of the α-subunit and/or the β and γ complex, which leads to activation of intracellular signaling pathways.

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What are some examples of heterotrimeric G proteins?

Examples of heterotrimeric G proteins include Gs (stimulatory), Gi (inhibitory), Gq (activates phospholipase C), Golf (olfactory-specific), Gt (transducin), G0 (regulates ion channels), and G12/13 (regulates cytoskeleton and cell migration).

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What are monomeric G proteins?

Small, single-chain proteins involved in signal transduction pathways that regulate various cellular processes, including cell proliferation, differentiation, and protein synthesis.

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Study Notes

Plasma Membrane: Structure

  • The plasma membrane, also known as the plasmalemma, is approximately 7.5-10 nm thick.
  • It is composed of a phospholipid bilayer with integral and peripheral proteins.
  • The inner leaflet faces the cytoplasm, and the outer leaflet contacts the extracellular environment.
  • Transmission electron microscopy (TEM) reveals a trilaminar structure, often called the unit membrane.

Plasma Membrane: Function

  • Maintains the cell's structural and functional integrity.
  • Acts as a semipermeable membrane, regulating material movement between the cytoplasm and external environment.
  • Aids in recognizing macromolecules and cells; controls cell interactions.
  • Assists in transducing extracellular signals into intracellular events.
  • Maintains a potential difference between the cytoplasmic and extracellular sides.

Cell Membrane Fluid Mosaic Model

  • The phospholipid bilayer comprises phospholipids, sphingolipids, glycolipids, and cholesterol.
  • The bilayer is freely permeable to small, lipid-soluble, nonpolar molecules but impermeable to charged ions and large molecules.
  • Phospholipids are amphipathic, with a polar head and two nonpolar fatty acid tails.
  • The polar heads face the membrane surfaces, while the tails project into the membrane interior.
  • Leaflet tails form weak noncovalent bonds, maintaining leaflet integrity.
  • Phospholipid distribution is asymmetrical in the two leaflets.

Membrane Proteins

  • Membrane proteins are integral or peripheral.
  • Integral proteins are embedded within the phospholipid bilayer, potentially spanning the entire membrane.
  • Peripheral proteins are located on either the cytoplasmic or extracellular aspects of the membrane, without entering the bilayer.
  • Some membrane proteins diffuse laterally; others are immobile, held in place.
  • They can function in membrane receptors, enzymes, cell adhesion molecules, cell recognition proteins, signal transduction, cell-to-cell contact, and transport.
  • Integral proteins are amphipathic, and some are multipass proteins.
  • Some integral proteins are anchored by fatty acyl or prenyl groups on the membrane.

Glycocalyx

  • Glycocalyx is a carbohydrate-rich layer on the outer surface of the cell membrane.
  • It includes polar oligosaccharide side chains and proteoglycans.
  • Functions include cell attachment, antigen and enzyme binding, facilitating cell-cell recognition, and protecting cells from harmful substances.
  • Glycocalyx helps in appropriate cell-cell interactions.

Clinical Considerations (Selected)

  • Cystinuria: A hereditary condition affecting cystine transport.
  • Cystic fibrosis: A disease impacting chloride channels, leading to thick mucus that may interfere with lung function.
  • Siglecs: A group of transmembrane proteins that interact with sialic acid, often involved in immune cell function and cancer cell interactions.
  • MDR proteins: Multidrug-resistance proteins in some cancer cells that can transport cytotoxic drugs out of the cell.

Cell Membrane Transport Processes

  • Passive transport: Movement of substances down a concentration gradient, no energy required.
    • Simple diffusion: The movement of small, nonpolar molecules across the membrane.
    • Facilitated diffusion: The movement of polar molecules (ions/molecules) across the membrane with protein channels or carrier proteins (down concentration gradient).
  • Active transport: Movement of substances against a concentration gradient, requiring energy.
    • The Na+-K+ pump: A carrier protein that moves Na+ and K+ ions in opposite directions, using energy from ATP hydrolysis. -Maintaining cell volume/transmembrane potential
    • Vesicular transport: Movement of substances in/out of a cell via vesicles (endocytosis/exocytosis), requiring energy.

Endocytosis

  • Phagocytosis: Engulfing solid particles by the cell through pseudopods.
  • Pinocytosis: Intake of extracellular fluid by the cell, forming vesicles.
  • Receptor-mediated endocytosis: Ligand binding to receptor triggers vesicle formation, specific to target substance.

Exocytosis

  • Constitutive secretion: Continuous release of newly synthesized materials into the extracellular environment.
  • Regulated secretion: Release of materials in response to a specific signal, a process often requiring Ca2+ ions, happens in vesicles (e.g., neurotransmitter release).

Cell-to-Cell Communication

  • Cells communicate via signaling molecules, intended receiver cells being target cells.
  • Signaling molecules can be neurotransmitters or hormones (endocrine, paracrine, or autocrine). -Lipid-soluble molecules can directly affect gene expression -Hydrophilic molecules require cell surface receptors

Membrane Receptor Types

  • Channel linked: Ion channels that open or close based on signaling molecule binding (permitting or inhibiting ion movement).
  • Catalytic: Receptors with enzyme activity triggering intracellular enzymatic pathways upon ligand binding.
  • G protein-linked: Receptors that activate intracellular messengers such as cAMP, Ca2+, or inositol phospholipids upon ligand binding.
  • Receptor tyrosine kinases (RTKs): Receptors that trigger a cascade of intracellular events, mostly signaling through downstream phosphorylation cascades.

Non-receptor tyrosine kinases

  • Another group of tyrosine kinases (JAK/STAT) frequently signal through phosphorylation cascades without directly binding to a ligand.
  • They operate in receptor-associated signaling complexes.

Receptor Serine/Threonine Kinases

  • A kinase associated signaling pathway involving receptor oligomerization, activation of type I receptors and stimulation of SMAD proteins, as a crucial transcription factor.

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Explore the intricacies of the plasma membrane through this quiz, focusing on its structure, composition, and critical functions. Learn about the fluid mosaic model, the significance of the phospholipid bilayer, and the membrane's role in cellular integrity and signaling.

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