Cells - Lecture 6

Discussion focused on the nucleus and plasma membranes, cytoplasm later in the lecture.

Visibility: Plasma membranes are too thin (7-8 nm) to be observed under a light microscope, necessitating the use of electron microscopy for visualization.
Observation under Electron Microscope: Plasma membranes exhibit a characteristic dark-light-dark pattern in electron micrographs. This is a direct result of the organized arrangement of phospholipids within the membrane, where the hydrophilic heads face the aqueous environment and the hydrophobic tails are oriented inward.

Orientation: The phospholipid bilayer consists of negatively charged heads that face outward towards both the external environment and the cytoplasm, while the fatty acid tails project inward, away from water.
Staining: To visualize the plasma membrane, positively charged stains such as lead ions and uranyl ions are employed, as these ions are attracted to the negatively charged phosphate groups of the phospholipid heads.
Layers: In microscopy, plasma membranes are represented as multiple layers due to their bilayer structure combined with intercellular materials, which protect and maintain cellular integrity.

Definition: The glycocalyx is an extracellular matrix that surrounds the cell, composed predominantly of glycoproteins, glycan chains, and molecules derived from blood plasma, contributing to cellular communication and protection.
Components:
- Glycoproteins: These are proteins with carbohydrate side chains that are either embedded within or anchored to the plasma membrane, playing critical roles in cell recognition and signaling.
- Glycan Chains: Comprising elements like heparan sulfate, hyaluronic acid, and chondroitin sulfate, these chains are chemically similar to mucins and further enhance the cell's protective barrier.
- Blood Plasma Derived Molecules: Various factors, including growth factors and hormones, that integrate into the glycocalyx, modulating cellular responses to their environment.

Staining Technique: The utilization of ruthenium red, which is positively charged, facilitates binding to the negatively charged groups within the glycocalyx, making it an effective tool for observation under fluorescence microscopy.
Study Example: In research involving human umbilical vein endothelial cells, the presence of the glycocalyx was identified in flow-adapted cells—those exposed to consistent blood flow—whereas it was absent in non-flow adapted cells, indicating its role in protecting against physical friction from blood flow.

Protection: The glycocalyx functions as a shield for the plasma membrane, safeguarding it against mechanical injury and chemical damage.
Immune Response: Acts as a cellular signature, allowing immune cells to discern between self and non-self, vital for immune surveillance and response.

Definition: The terminal web constitutes a network of actin filaments located internally within the cytoskeleton. It is essential for maintaining the cellular architecture and shape, providing structural support.

Fluid Mosaic Model: This model illustrates the dynamic nature of the cell membrane, where lipids and proteins are not static but move laterally, enabling membrane fluidity and functionality.
Phospholipids: Each phospholipid is composed of a hydrophilic phosphate head and two hydrophobic fatty acid tails, which create the bilayer structure.
Types of Membrane Proteins:
- Integral Proteins: These proteins span the entire membrane and are crucial for functions like transport of molecules and cellular communication.
- Peripheral Proteins: Though attached to the inner or outer sides of the membrane, they do not penetrate through it and are involved in signaling pathways.
Cholesterol: Embedded within the lipid bilayer, cholesterol molecules help to regulate membrane fluidity and contribute to specialized structures known as lipid rafts, which are involved in signaling and cellular interactions.

Distribution: Different types of phospholipids are localized differently in the outer and inner leaflets of the membrane, which plays a significant role in the membrane potential and signal transduction pathways.
Transporters:
- Flippases: These enzymes transport specific phospholipids from the outer leaflet to the inner leaflet, maintaining asymmetry.
- Floppases: Conversely, they move phospholipids from the inner leaflet to the outer leaflet.
- Scramblases: These facilitate the bidirectional movement of phospholipids without the requirement for ATP, contributing to membrane dynamics during processes like apoptosis.

Phosphatidylserine Exposure: The externalization of phosphatidylserine serves as a signal for phagocytes to recognize and eliminate dying cells, marking a critical step in apoptosis.
Annexin V Staining: This staining technique utilizes fluorescent markers to visualize phosphatidylserine presence, helping identify cells undergoing apoptosis.

Definition: Lipid rafts are specialized microdomains within the plasma membrane that are enriched with cholesterol, saturated fatty acids, and specific proteins, playing a pivotal role in cellular signaling and the entry/exit of pathogens.
Observation Techniques: Advancements in cryo-electron microscopy have allowed for detailed visualization of lipid raft structures, illustrating significant differences in their physical composition compared to surrounding membrane areas.

Structure and Function: The Golgi apparatus is responsible for modifying, sorting, and packaging proteins that are synthesized in the rough endoplasmic reticulum. It plays a key role in post-translational modifications, including glycosylation.
Staining Techniques: Silver staining is particularly useful for highlighting the Golgi apparatus, owing to its unique reducing environment that influences both protein folding and glycosylation processes, enabling efficient protein processing and transport.

Prokaryotic vs. Eukaryotic Structure: Ribosomes differ between prokaryotic and eukaryotic cells in their RNA and protein compositions, with eukaryotic ribosomes being larger and more complex.
Polyribosomes: These structures consist of multiple ribosomes translating a single mRNA strand simultaneously, indicating active protein synthesis and the cell’s metabolic activity.

Methods Used: Techniques such as centrifugation and spectroscopy are employed to analyze ribosome populations and their functional status in tissues.
Comparative Analysis: Studying the differences between normoxia (normal oxygen levels) and hypoxia (low oxygen levels) provides insights into cellular health and the dynamics of protein synthesis under varying oxygen conditions.