Cell Biology: Membrane Proteins Quiz

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Questions and Answers

What is the primary function of the glycocalyx on cell surfaces?

Enhancing cellular metabolism

Facilitating energy transport

Providing mechanical protection (correct)

Acting as a receptor for signals

Which type of transport does not require energy?

Facilitated diffusion

Endocytosis

Active transport

Passive transport (correct)

What characterizes the structure of multipass membrane proteins?

They interact weakly with solutes.

They have high specificity for a wide range of solutes.

They bind more strongly to specific solutes. (correct)

They form a single pore open on both sides.

Which of the following transports substances against their concentration gradient?

Active transport Signup and view all the answers

Which type of transporter is specifically responsible for moving two different molecules in the same direction?

Symporter Signup and view all the answers

What happens to solutes during passive transport?

They move along the concentration gradient. Signup and view all the answers

What is the role of transport proteins in membrane permeability?

They facilitate the movement of certain solutes across the membrane. Signup and view all the answers

Which ion is mentioned as being actively transported to maintain cellular balance?

Ca2+ Signup and view all the answers

What is a characteristic feature of lipid rafts in the cell membrane?

They are enriched in cholesterol and certain proteins. Signup and view all the answers

Which type of protein is typically integrated throughout the entire membrane?

Integral proteins Signup and view all the answers

What distinguishes the transmembrane portion of proteins?

It typically consists of 20-30 amino acids long alpha-helices. Signup and view all the answers

Which statement is true regarding the hydropathy index?

It identifies hydrophobic segments in a protein sequence. Signup and view all the answers

What is a notable difference between membrane proteins on the cytosolic versus non-cytosolic side?

Sugar residues are found on the extracellular side. Signup and view all the answers

What type of interactions do transmembrane alpha-helices undergo for stability?

Hydrophobic interactions with each other Signup and view all the answers

How are peripheral proteins generally attached to the cell membrane?

Via interactions with the transmembrane proteins Signup and view all the answers

Which structural feature allows the most efficient hydrogen bonding in transmembrane proteins?

Alpha helices Signup and view all the answers

What is the primary function of the Na+/K+ ATPase pump?

To establish electrochemical gradients for ion transport Signup and view all the answers

Which type of active transport utilizes the energy from ATP hydrolysis?

Primary active transport Signup and view all the answers

How do aquaporins facilitate the movement of water?

By providing a passive channel that blocks ions and allows water Signup and view all the answers

What mechanism do coupled transporters utilize to transport substances?

Using the electrochemical gradient of another ion Signup and view all the answers

What is the role of the asparagine selectivity filter in aquaporins?

To block passage of H+ ions and further restrict ion movement Signup and view all the answers

What is the primary function of the plasma membrane?

To define the cell and provide a selective barrier Signup and view all the answers

Which components are essential for the structure of phospholipids?

A polar head group and two non-polar hydrocarbon tails Signup and view all the answers

How does cholesterol affect membrane fluidity at high temperatures?

It decreases fluidity Signup and view all the answers

What role do saturated fatty acids play in membrane fluidity?

They make the membrane less fluid Signup and view all the answers

Which subclass of phospholipids is derived from sphingosine?

Sphingolipids Signup and view all the answers

What characteristic makes phospholipids amphiphilic?

They contain both hydrophilic and hydrophobic parts Signup and view all the answers

What is an effect of cholesterol at low temperatures?

Increases membrane fluidity Signup and view all the answers

What primary role does the cytoskeleton play in relation to the plasma membrane?

To provide shape and strength to the cell Signup and view all the answers

What primarily contributes to maintain the resting membrane potential?

Na+/K+ pump and K+ leak channels Signup and view all the answers

Which statement accurately describes the function of voltage-gated Na+ channels during an action potential?

They open first, causing depolarization in the neuron. Signup and view all the answers

What happens when K+ channels open during an action potential?

Repolarization of the membrane happens. Signup and view all the answers

What is the threshold membrane potential for triggering an action potential?

-55 mV Signup and view all the answers

What ions are primarily involved in establishing the electrochemical gradients across the cell membrane?

Na+ and K+ ions Signup and view all the answers

How does the Na+/K+ pump affect ion concentrations inside and outside the cell?

It pumps K+ into the cell while Na+ is pumped out. Signup and view all the answers

What role do ion channels play in the transport of ions?

They are specific for certain ions, limiting the rate of passage. Signup and view all the answers

What is the resting membrane potential in a typical neuron?

-70 mV Signup and view all the answers

Study Notes

Cell Biology Lesson & Book Notes

The notes cover eukaryotic cell structure and function, endosymbiosis theory, the plasma membrane, phospholipids, sterols, lipid rafts, transmembrane and peripheral proteins, membrane proteins, hydropathy plots, and membrane proteins differing cytosolic vs non-cytosolic side. Also, included are membrane transport, types of transporters, active transporters, transport of water, ion channels, neurons and voltage-gated channels, presynaptic and postsynaptic cells, ABC transporter, neuromuscular transmission, intracellular compartments & protein transport, transport into the nucleus, nuclear import & export, Transport into mitochondria, Transport into the ER, protein synthesis at ER-bound ribosomes, Synthesis of soluble proteins in the ER, What happens to proteins translated in the ER, Proper protein folding is a requirement for leaving the ER, Export & degradation of misfolded ER proteins, Vesicular transport, Transport of vesicles to their target membranes, Protein glycosylation continues in the Golgi, Transport to the cell exterior: Exocytosis, Transport from cell exterior into the cell: Endocytosis, mechanisms of cell signaling, principle of cell signaling, extracellular signal molecules, three classes of receptors in signalling, scaffold proteins, G-protein coupled receptors, signalling of GPCRs via cAMP, GPCRs & Ca2+ signaling, Enzyme-coupled receptors, Receptor tyrosine kinases (RTKs), Ras activates the MAP kinase pathway, PI3K-Akt signalling pathway, JAK-STAT signalling pathway. Also included is a section on the cytoskeleton and extracellular matrix: lesson 5, covering microtubules, intermediate filaments, actin filaments. Also included are motor proteins and the different stages of mitosis and what occurs during each stage, and lastly apoptosis (genetic regulated cell death).

Lesson 1: Cells & Organelles, Membrane Structure

Eukaryotic cells have components and functions described on the next slide.
Animal cells differ from plant cells. Plants have a cell wall, a central vacuole, and chloroplasts but no centrosomes.

Membrane Structure

The plasma membrane defines the cell, provides shape and strength (with the cytoskeleton), and forms a selective barrier.
It consists of lipids, proteins, and sterols (e.g., cholesterol).
It is a fluid lipid bilayer held together by noncovalent interactions.

Phospholipids

Phospholipids are amphiphilic, with a polar head group and nonpolar hydrocarbon tails.
Important classes include phosphoglycerides and sphingolipids.

Sterols (Cholesterol)

Cholesterol stabilizes membrane fluidity at high temperatures and immobilizes fatty acid chains at low temperatures.
This makes the membrane less deformable and less permeable.

Lipid Rafts

Lipid rafts are dynamic structures enriched in cholesterol, sphingolipids, glycolipids, and certain proteins.
They are thicker than other membrane regions due to their composition.
Lipid rafts serve as platforms for protein interactions and signaling.

Transmembrane/Integral Proteins

Transmembrane proteins are integrated within the entire membrane and can be single-helix, multiple-helix, or barrel-shaped.
Peripheral proteins are attached to the outer surface of the membrane, sometimes via a helix or covalently attached to a lipid chain.

Membrane Proteins

Membrane proteins give cells their functional properties and can associate in various ways with the membrane.
They are amphiphilic, containing both hydrophilic and hydrophobic amino acids.

Hydropathy Plots

Hydropathy plots predict transmembrane regions based on amino acid composition.
Positive values in the plot correspond to hydrophobic segments, while negative values correspond to hydrophilic segments.

Membrane Proteins - Cytosolic vs Non-cytosolic side

Most membrane proteins are glycosylated, meaning they have sugar residues added in the ER and Golgi apparatus, usually on the extracellular side of the protein.
Disulfide bonds form between cysteines on the non-cytosolic side, but these bonds cannot form on the cytosolic side due to the reducing environment.
The glycocalyx is a carbohydrate-rich zone on the cell surface and its function is to protect the cell from chemical and mechanical stress and prevent unwanted cell-cell interactions.

Lesson 2: Membrane Transport

Membrane permeability depends on polarity, charge, and size.
Solutes cross membranes via transport proteins such as channels and transporters.
Channels are integral membrane proteins that form pores.
Transporters bind to solutes more strongly and undergo conformational changes to allow transport.

Types of Transporters

Transporters can be divided into uniporters (one solute), symporters (multiple solutes in the same direction), and antiporters (multiple solutes in opposite directions).
Passive transport moves solutes down their concentration gradient, while active transport moves them against their concentration gradient, requiring energy input.

Active Transporters (including Na+/K+ ATPase).

Active transport mechanisms include coupled transport and ATP-driven pumps (e.g., Na+/K+ ATPase pump).
The Na+/K+ ATPase pump creates high Na+ outside the cell and high K+ inside, using ATP.

Transport of Water

Water sometimes needs to be transported rapidly by aquaporins. Water follows the osmotic gradient.
Aquaporins are passive channels that allow water passage.

Ion Channels

Ion channels allow ions to move across membranes in response to electrical, mechanical, and chemical signals.
They are selective for specific ions.

Transport Channels (Voltage-gated, Ligand-gated, Mechanically gated)

Voltage-gated ion channels open and close in response to changes in membrane potential.
Ligand-gated ion channels open and close in response to the binding of a ligand.
Mechanically-gated ion channels open in response to physical forces

Lesson 3: Intracellular Compartments & Protein Transport

Proteins and other macromolecules are transported within cells via gated, transmembrane, and vesicular transport.
Gated transport occurs through nuclear pores.
Transmembrane transport occurs via translocators.
Vesicular transports are packages into vesicles.

Signal Sequences

Cells use signal sequences (amino acid sequences) to direct proteins to their correct cellular destinations.
Signal sequences can determine where a protein belongs.

Transport into the Nucleus (Gated Transport)

The nuclear envelope is formed by two concentric membranes, with nuclear pores that act as selective gates.
Small molecules can diffuse freely, while larger molecules require active transport.
Proteins destined for the nucleus contain nuclear localization signals.

Nuclear Import & Export

Ran-GTP is important for nuclear transport, dissociating from receptors in the cytosol and binding to receptors in the nucleus.

Transport into Mitochondria

Proteins destined for the mitochondrial matrix cross two membranes through protein translocators (TOM and TIM).
Signal sequences in the proteins target them to the mitochondria.

Transport into the ER (Transmembrane Transport)

Synthesis of proteins destined for the ER often happens at ER-bound ribosomes.
Signal recognition particles (SRPs) bind to signal sequences and direct ribosomes to the ER membrane.

Synthesis of Soluble Proteins & Membrane-Bound Proteins in the Endoplasmic Reticulum

Soluble proteins are transported into the ER lumen.
Membrane-bound proteins are partially translocated and remain embedded in the ER membrane.
Start- and stop-transfer sequences help determine which parts of membrane-bound proteins are exposed to the lumen or the cytosol.

Protein Modifications

Protein glycosylation, disulfide bond formation, and lipid anchoring occur in the ER, altering protein structure and function.

Proper Protein Folding in the ER

Proteins must correctly fold, and misfolded proteins are targeted for degradation or export.
Chaperones like calnexin and other proteins assist with correct folding.
Misfolded proteins are exported for degradation by the proteasome.

Export & Degradation of Misfolded Proteins

Protein export proteins (i.e. chaperonins) assist in proper protein folding, but misfolded ones enter the ER-associated degradation (ERAD) pathway.

Vesicular Transport

Membrane proteins are transported within the cell in vesicles.
Different types of coated vesicles (e.g., clathrin-coated, COPI, COPII) transport proteins between compartments.
The secretory pathway (red arrows) moves proteins from the ER to the Golgi and to the plasma membrane.
The endocytic pathway (green arrows) imports materials into the cell, via the Golgi and to the lysosomes.
Material may go from one compartment to another for reuse, storage or recycling

Transport of Vesicles to Their Target Membranes

Vesicle transport is regulated by proteins such as Rab proteins, v-SNAREs, and t-SNAREs.
Complementary v- and t-SNAREs facilitate vesicle docking and fusion.

Protein Glycosylation in the Golgi

Oligosaccharide addition to proteins and lipids continues in the Golgi.
Glycosylation patterns influence protein function.

Transport to the Cell Exterior: Exocytosis

Constitutive exocytosis occurs continuously, while regulated exocytosis occurs in response to specific signals.
Exocytosis releases proteins, lipids, and other molecules into the extracellular space.

Transport from Cell Exterior into the Cell: Endocytosis

Endocytosis imports materials into the cell using vesicles.
Different types of endocytosis (e.g., phagocytosis, pinocytosis, receptor-mediated endocytosis) occur.
Late endosomes fuse with lysosomes.

Apoptosis: Genetic Cell Death Regulation in All Living Tissues

Apoptic cell death is regulated genetically and plays essential roles during embryonic development, normal cell turnover, immunity and other physiological processes.
Morphological characteristic involve shrinkage, condensation, breakdown of the nuclear and cellular envelopes and apoptotic bodies .

Lesson 4: Cell Signaling

Cells communicate with each other via four main types of intercellular signaling: paracrine, synaptic, endocrine, and contact-dependent.

Principle of Cell Signaling

Extracellular signal molecules bind to receptors, initiating intracellular signaling pathways.
Intracellular signaling molecules relay, amplify, integrate, and distribute signals, ultimately affecting effector proteins and cellular responses.

Extracellular Signal Molecules

Different signal molecules elicit varying responses in target cells.

Three Classes of Receptors in Signaling

Ion-channel-coupled receptors, G-protein-coupled receptors, and enzyme-coupled receptors are the major types of receptors.

Mechanism of Signal Transduction

Signaling pathways are often mediated by protein phosphorylation, GTP-binding proteins, and adaptor proteins.

Scaffold Proteins

Scaffold proteins organize signaling complexes.

G-Protein-Coupled Receptors (GPCRs)

GPCRs are the largest family of cell-surface receptors.
They mediate diverse cellular responses.
They mediate most responses to signals like touch, sight, taste, smell.

Signalling of GPCRs via cAMP

Some G-proteins regulate cyclic AMP (cAMP) production.
cAMP acts as a second messenger, activating protein kinase A (PKA).

GPCRs & Ca2+ Signaling

Gq activates phospholipase C (PLC), leading to the production of inositol triphosphate (IP3) and diacylglycerol (DAG).
IP3 triggers calcium release from the endoplasmic reticulum, activating downstream effectors.

Enzyme-Coupled Receptors

Enzyme-coupled receptors often have intrinsic enzymatic activity or associate with enzymes.

Receptor Tyrosine Kinases (RTKs)

RTKs are the most common type of enzyme-coupled receptors.
They are involved in diverse cellular processes, including cell growth, proliferation, and survival.

Ras Activates the MAP Kinase Signalling Pathway

Activation of Ras protein (via growth factors) is a key step in the MAP Kinase pathway, influencing cell proliferation
Ras activates signaling proteins (proteins such as MAP-Kinase Kinase and MAP-Kinase).

PI3K-Akt Signalling Pathway

The PI3K-Akt pathway influences cell survival, growth, and metabolism

JAK-STAT Signalling Pathway

The JAK-STAT pathway regulates gene expression through cytokine signaling.

Lesson 5: Cytoskeleton & Extracellular Matrix

A complex network of protein filaments (actin filaments, intermediate filaments, microtubules) provides structural support and drives various cellular processes such as movement, transport, and mitosis.

Microtubules

Long, hollow cylinders made of tubulin dimers (composed of alpha- and beta-tubulin).
Microtubules grow out of organizing centers, such as centrosomes.
Microtubules have structural polarity (+ and - ends).

Microtubules Bind to GTP

Tubulin can hydrolyze GTP, leading to dynamic instability in microtubule growth and shrinkage.

Motor Proteins (Kinesin and Dynein)

Motor proteins (e.g., kinesin and dynein) use ATP to transport cargo along microtubules.

Actin Filaments (Microfilaments)

Actin filaments are helical polymers of actin.
They are involved in cell shape, movement, cytokinesis, and muscle contraction.

Intermediate Filaments

Rope-like fibers formed by many different proteins.
They provide mechanical strength to cells and tissues.

Lesson 6: Cell Cycle & Apoptosis

The cell cycle describes the series of events that cells go through to divide.
Main phases: G1, S, G2 and M (mitosis).
Checkpoints ensure accurate chromosome duplication and proper execution of cell division.

Cyclins and Cyclin-dependent Kinases (Cdks)

Cdk activity is regulated by cyclins.
Cyclins and Cdks drive different stages of the cell cycle.
Degradation of cyclins is an important mechanism for controlling Cdk activity and cell cycle progression.
Specific Cyclins regulate the G1 phase, S phase, G2 phase, M phase.

Regulators of Cdk Activity (p53)

p53 activity results in DNA repair, cell cycle arrest, or apoptosis.

S-phase: Chromosome Duplication

Ensure precise duplication of the genome occurs.
This involves the pre-replication complex and DNA helicases.

Stages of Mitosis (Prophase, Prometaphase, Metaphase, Anaphase, Telophase, Cytokinesis)

Mitosis is the division of replicated chromosomes into two daughter cells.

Motor Proteins of the Spindle

Motor proteins like kinesin and dynein are involved in chromosome movement during mitosis (specifically, separation and pull apart).

Apoptosis: Genetic regulated cell death

Apoptosis is a programmed cell death critical to normal development and immunity.
Morphological changes happen during apoptosis, including cell shrinkage, chromatin condensation, and formation of apoptotic bodies.
Caspases (proteases) play a central role in the execution phase of apoptosis.

Intrinsic Apoptosis Pathway

The intrinsic pathway involves mitochondrial damage.
Cytochrome C is released into the cytoplasm, leading to apoptosome formation and caspase activation.

Extrinsic Apoptosis Pathway

Extrinsic pathway is triggered by extracellular signals.
Death receptors (e.g., Fas) on the target cell bind to ligands, initiating caspase cascade.

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Description

Test your knowledge on the various functions and structures of membrane proteins in cell biology. This quiz covers topics such as glycocalyx, transport mechanisms, and lipid rafts. Each question is designed to reinforce your understanding of cellular membrane dynamics.