Cell Biology: Eukaryotic Cells and Membranes

Eukaryotic Cells

• Eukaryotic cells have membranes, organelles, and cytosol.
• Cell shape is defined by the cytoskeleton, a network of protein filaments.
• The cytoskeleton is made up of three types of filaments.
• The cytoskeleton provides structural support to the cell membrane.
• Various proteins attach to and rely on the cytoskeleton for function.

Biomembranes

• Membrane lipid composition affects membrane shape, protein anchorage, and protein activity.
• Phospholipids spontaneously form a lipid bilayer with a hydrophobic core.
• The hydrophobic core of the phospholipid bilayer acts as a barrier preventing water-soluble substances from passing through.
• The core provides membrane stability due to hydrophobic interactions.
• These interactions are not disrupted by changes in pH or ionic strength.

Membrane Orientation and Adaptation

• One side of the membrane faces the cytosol, while the other faces the extracellular environment.
• Cell membranes can be smooth (e.g., red blood cells) or have projections (e.g., cilia in respiratory epithelium).
• Nerve axons have multiple membrane layers that form myelin sheaths.

Types of Lipids in Biomembranes

• Phosphoglycerides (PGs) are common lipids, including phosphatidylcholine.
• Sphingolipids (SLs) contain sphingosine and may include choline, sugars (cerebrosides/gangliosides).
• Steroids are hydrophobic and need to mix with phospholipids to form a bilayer.
• Lipids move laterally within the membrane.

Lipid Composition Affects Membrane Properties

• Phosphoglycerides (PGs) and sphingolipids (SLs) are synthesized in the endoplasmic reticulum and Golgi body, respectively.
• Membranes rich in sphingomyelin can withstand stress.
• Shorter and less saturated fatty acids create kinks and increase membrane fluidity.
• Cholesterol intercalates into the lipid bilayer, increasing thickness and reducing fluidity.
• Sphingolipids form thicker bilayers.
• Membrane curvature is determined by the relative size of the polar head to the tail.

Membrane Microdomains

• Lipids are not randomly distributed in the membrane.
• Cholesterol and sphingolipids cluster into less fluid areas called lipid rafts.
• Lipid rafts resist detergent extraction and are disrupted by cholesterol depletion, like antibiotics.
• Lipid rafts are rich in receptors and signaling proteins.

Membrane Protein Components

• Membrane proteins function varies depending on the protein itself.
• Proteins may be buried in the lipid bilayer and attached to the cytosolic or exoplasmic leaflets.
• Membrane proteins on the extracellular side bind to molecules (hormones, metabolites etc).
• Protein domains form channels/pores for intra- and extracellular molecule transport.
• Proteins on the cytosolic side may anchor cytoskeletal proteins or act as signaling molecules.

Membrane-Protein Interaction

• Integral membrane proteins span the lipid bilayer, with cytosolic and exoplasmic domains.
• Integral proteins have polar amino acids in their cytosolic/exoplasmic domains and hydrophobic amino acids in their transmembrane domains.
• Exoplasmic domains can be bound to sugar residues.
• Lipid-anchored proteins are covalently attached to membrane lipids.
• Peripheral membrane proteins don't enter the bilayer.
• Peripheral proteins interact with integral proteins and polar lipid heads.

Membrane Glycoproteins

• Many transmembrane proteins are linked to carbohydrates (O-linked or N-linked).
• Carbohydrate chains are oriented towards the exterior of the cell.
• Glycoproteins form antigens, which determine blood groups.

Common Functions of Plasma Membrane

• The plasma membrane acts as a barrier.
• Transport proteins control nutrient and waste flow.
• The membrane maintains cell ionic composition.
• Membrane proteins/glycoproteins form junctions connecting cells, strengthening tissues, and facilitating communication.
• Membrane proteins anchor the cytoskeleton and act as receptors.
• Membranes surrounding organelles contain proteins specific to those functions.

Explore the intricate structures of eukaryotic cells, focusing on the roles of the cytoskeleton and biomembranes. Understand how membrane lipid composition influences cell shape and function, and learn about membrane orientation and adaptation. This quiz covers key concepts in cell biology and their relevance in cellular processes.