Category 6 Flashcards
Intracellular Dynamics, the Cytoskeleton and Protein Sorting
Why do cells need a cytoskeleton, given their natural default shape?
To deviate from their default spherical shape, enabling them to perform specific functions such as movement, adhesion, and division.
Describe the basic building blocks of intermediate filaments, and how these subunits assemble into the final structure.
Built from protein monomers with an alpha-helical structure that assemble into dimers, then tetramers.
These tetramers connect end-to-end to form long, strong filaments resembling braided ropes, providing resistance to tensile forces.
Explain how keratin filaments and their associated structures provide mechanical strength to skin.
In skin, keratinocytes attach to the basement membrane via hemidesmosomes and to each other via desmosomes, creating a mechanically coupled network. Keratin filaments distribute force across multiple cells within this network, preventing localised damage and providing overall strength to the epithelial layers.
What is Epidermolysis Bullosa Simplex (EBS) and how does it relate to intermediate filaments?
A genetic disease caused by mutations in keratin genes, weakening the structural integrity of the skin. Minor mechanical stress can cause cells to tear, leading to blister formation.
Explain the concept of “treadmilling” in actin filaments.
A dynamic equilibrium where actin monomers are added to the plus end of an actin filament while being removed from the minus end.
Describe how actin filaments and spectrin contribute to the unique shape of erythrocytes and the consequence of mutations in spectrin.
Actin filaments and spectrin form a network beneath the erythrocyte membrane, anchored by transmembrane proteins, supporting its biconcave shape. Genetic defects in spectrin lead to spherocytosis, where erythrocytes become spherical, leading to anaemia due to their reduced lifespan.
How do actin filaments, along with cross-linking proteins, mediate membrane protrusion?
Cross-linking proteins, like filamin, connect actin filaments into bundles, increasing their mechanical strength. These bundles orient with their plus ends pointing towards the plasma membrane, which leads to deformation and protrusion of the membrane.
Explain the importance of the GTP cap in microtubule dynamics.
The GTP cap is critical for microtubule dynamics because GTP-bound β-tubulin stabilises the plus end, promoting polymerisation. When GTP is hydrolysed to GDP, the microtubule becomes unstable, leading to depolymerisation and shrinkage.
Describe the role of the centrosome in microtubule organisation within the cell.
The centrosome, located near the nucleus, serves as the microtubule-organising center (MTOC). It contains γ-tubulin, which nucleates microtubule growth and anchors the minus ends, allowing microtubules to extend towards the cell periphery.
How do motor proteins (kinesins and dyneins) utilise microtubules for intracellular transport, and what direction does each type of motor protein move cargo?
Kinesins move cargo along microtubules towards the plus end, while dyneins move cargo towards the minus end. This allows for the directional transport of various cellular components, such as vesicles and organelles, within the cell.
What is the key difference between prokaryotic and eukaryotic cells in terms of internal organisation?
Prokaryotic cells lack internal membrane-bound organelles, while eukaryotic cells have a complex system of organelles, including a nucleus. This compartmentalisation in eukaryotes allows for the separation of biochemical reactions and specialised functions.
Explain the importance of cellular compartmentalisation in eukaryotic cells, giving a specific example.
Cellular compartmentalisation in eukaryotes separates biochemical reactions, preventing interference and damage; lysosomes, for example, contain destructive enzymes and a low pH, which must be contained to prevent damage to the cell’s DNA in the nucleus.
Describe the two main domains of a polarised epithelial cell and their significance.
Polarised epithelial cells have an apical domain facing the external environment or a lumen, and a basolateral domain facing the underlying tissue. This polarisation allows for controlled uptake and release of substances in specific directions, facilitated by tight junctions.
How does the shape of a red blood cell contribute to its primary function?
Red blood cells have a biconcave shape, which maximises the surface area for gas exchange and enables them to efficiently carry oxygen and remove carbon dioxide. They are also anucleate, which means they have no nucleus and more space for haemoglobin.
Explain how the saturation level of fatty acid tails affects membrane thickness and protein interactions.
Unsaturated fatty acid tails, with their kinks, create thinner regions in the membrane compared to saturated fatty acid tails, which are straight and create thicker regions. This difference in thickness affects the ability of different proteins to integrate within the membrane, affecting the overall membrane function.
Describe the three types of movement phospholipids can undergo within a membrane. Which one is least likely to occur without the assistance of a protein?
Lateral diffusion within the membrane.
Rotation within the membrane.
Flexion within the membrane.
Flip-flop is the least likely to occur without the assistance of a protein due to the hydrophilic head group needing to pass through the hydrophobic core of the membrane.
What are lipid rafts, and how do they contribute to membrane function?
Lipid rafts are microdomains within the cell membrane that are enriched in cholesterol and saturated fatty acids, creating thicker, more ordered regions. These rafts serve as platforms for specific proteins and signalling molecules, concentrating them in particular areas for functional activity.
Describe how GFP is used to study organelle dynamics in living cells.
GFP, or Green Fluorescent Protein, is genetically fused to a protein of interest. The location and movement of that protein, and thus the organelle it resides in, can be visualised in living cells using fluorescence microscopy.
Name the three basic components of the cytoskeleton and their approximate sizes.
The three basic components of the cytoskeleton are actin microfilaments (approximately 7 nm), intermediate filaments (intermediate size), and microtubules (approximately 25 nm). These filaments provide structural support, enable cell movement, and facilitate intracellular transport.
How do motor proteins facilitate intracellular transport?
Motor proteins, such as kinesins and dyneins, bind to cargo like organelles or vesicles and use ATP hydrolysis to “walk” along cytoskeletal filaments, typically microtubules. This energy-dependent movement allows for the directed transport of materials within the cell over long distances.
What is the default location for a protein that lacks any sorting signals, and why?
The default location for a protein without sorting signals is the cytosol. This is because there are no specific targeting sequences to direct it to any particular organelle or pathway, so it remains where it was synthesized.
Distinguish between co-translational and post-translational protein targeting, providing an example of an organelle that utilizes each mechanism.
Co-translational targeting occurs while the protein is being translated on the ribosome, with the ribosome docking at the target organelle (e.g., ER). Post-translational targeting happens after the protein is fully synthesized and then transported to its destination (e.g., nucleus, mitochondria).
Describe the role of the ER signal sequence and the Signal Recognition Particle (SRP) in initiating the import of a protein into the endoplasmic reticulum.
The ER signal sequence, a hydrophobic sequence at the N-terminus, acts as a recognition signal. The Signal Recognition Particle (SRP) binds to this sequence, pausing translation and escorting the ribosome-mRNA complex to the ER membrane.
Explain how the SRP facilitates the delivery of a ribosome synthesizing a secretory protein to the ER membrane.
The SRP-ribosome complex binds to the SRP receptor on the ER membrane. This interaction facilitates the transfer of the ribosome to the translocon, a protein channel, allowing translation to resume with the nascent polypeptide entering the ER.
