Cell Membranes and Cell Types

Cell Membrane

• A cell membrane is the barrier between a cell and its environment. It's made up of a variety of components, but primarily lipids, proteins, and carbohydrates.

Prokaryotes and Eukaryotes

• Prokaryotes:
  • Bacteria are prokaryotes
  • Generally very small and unicellular.
  • Earliest and most abundant life forms, evolved about 4 billion years ago.
  • Some species are highly evolved pathogens.
• Eukaryotes:
  • Include animals, plants, and fungi.
  • Some are unicellular and some are multicellular.
  • Evolved about 1 billion years ago.
  • Size ranges differ from prokaryotes.
  • Contain a nucleus (not present in prokaryotes).
• Diagrams of both prokaryotic and eukaryotic cells with their component labels are presented.

Eukaryotic Cell Membranes

• Plasma membrane: Forms the outer barrier between the cell and its environment.
• Internal membranes: Separate the subcellular organelles from the cytoplasm.
• The thickness of subcellular membranes is 6-10nm.

Cell (Biological) Membrane Components

• Lipids (40%):
  • Lipid bilayer structure, mostly composed of phospholipids.
  • Includes sphingomyelin, lecithin, cephalin, and cholesterol.
  • Cholesterol is found only in eukaryotes.
• Proteins (60%):
  • Integrated into the lipid bilayer or on the membrane surface.
  • Includes integral and peripheral proteins.
  • Also includes glycoproteins and glycolipids.
• Carbohydrates (less than 1%):
  • Usually branched oligosaccharides that are primarily bonded to lipids (glycolipids) or proteins (glycoproteins).

Membrane Lipid Components

• Phospholipids: Major components of cell membranes; have a glycerol backbone.
• Glycerolphosphatides: Subtype of phospholipid with a glycerol backbone.
• Saturated vs Unsaturated fatty acids: Different types of fatty acids in phospholipids affect the fluidity of membrane
• Amphipathic molecules: Phospholipids have a hydrophobic (water-fearing) tail and a hydrophilic (water-loving) head.
• Cholesterol: A steroid lipid that's found in animal cell membranes and interspersed among phospholipid tails; it affects membrane fluidity.
• Sphingolipids:
  • Sphingosine (18-carbon amino alcohol) with an unsaturated hydrocarbon chain.
  • Ceramides: Sphingosine bonded to fatty acid (amide bond).
  • Sphingomyelin: Ceramide and phosphate groups joined (ester bond).

Membrane Protein Types

• Integral proteins:
  • Transmembrane proteins with hydrophobic regions spanning the membrane interior.
  • Hydrophilic ends are exposed to the intracellular and extracellular environments.
  • Can serve as ion channels.
  • Often relatively mobile in the membrane and can be immobile from cytoskeleton binding.
• Peripheral proteins:
  • Not embedded in the lipid bilayer, loosely bound to the membrane surface or to integral proteins.
  • Often play static roles in the membrane, such as receptors.

Membrane Carbohydrate Components

• Vary among species, individuals within species, cell types, etc.
• Function as markers that distinguish one cell from another.
• Involved in cell-cell interactions, immune responses, and binding to other molecules.
• Includes monosaccharides (Glucose, Mannose, Galactose, Fucose), and oligosaccharides.
• Many are covalently bonded to lipids (glycolipids) or proteins (glycoproteins) to form more complex structures.

Movement of Membrane Lipids

• Rotation: Lipids rotate freely within their monolayer.
• Lateral diffusion: Lipids move laterally within their monolayer.
• Transversal diffusion ("flip-flop"): Lipids transfer from one monolayer to the other. Energy-dependent and uses specific proteins (flippase, floppase, scramblase).

Membrane Fluidity

• Membrane fluidity depends on several factors that affect fatty acid composition and cholesterol content.
• Temperature, the proportion of unsaturated fatty acids and amount of cholesterol are important considerations in determining fluidity.

Lipid Rafts

• Cholesterol & sphingolipids form microdomains called rafts, which are more ordered regions affecting cellular process like transmembrane transport, signal transduction, and neurotransmission.

Importance and Function of the Biological Membrane

• Cell membranes are selective barriers that regulate the transport of molecules across them, control the environments of compartments, and define the metabolism of those compartments.

Membrane Assymetry

• Two lipid layers of the cell membrane differ from each other in phospholipid type which leads to the asymmetrical structure.
• Carbohydrates are primarily found on the extracellular face of the membrane.
• Membrane proteins and lipids each have a directional orientation, meaning they have an outside and inside face, too.

This quiz explores the fundamental concepts of cell membranes, including their composition and functions. It also delves into the characteristics of prokaryotic and eukaryotic cells, highlighting their differences and the evolution of these cell types. Understanding these concepts is crucial for grasping the basics of biology.