Cell Biology: Plasma Membrane Structure

Study Notes

The cell is the basic structural and functional unit of an organism. It contains the fundamental constituents of life.
Cells are made up of a plasma membrane and cytoplasm.
The plasma membrane is a physico-biological membrane separating the cell's interior from its external environment. It's composed of phospholipids, proteins, cholesterol, and carbohydrates.
The plasma membrane has a fluid mosaic structure.

Lipids: Phospholipids are most abundant; each has a hydrophilic head and hydrophobic tails. Examples include phosphatidylcholine, phosphatidylserine. Lipid rafts are small, dynamic, sterol- and sphingolipid-rich domains that compartmentalize cellular processes. Caveolae are flask-shaped or cup-shaped invaginations of the membrane.
Proteins: Integral proteins span the membrane, acting as transporters, receptors, and enzymes. Peripheral proteins are associated with one leaflet of the phospholipid bilayer.

Cell attachment and support.
Cellular signaling (lipid rafts form scaffolds for signaling molecules and receptors that relay information).
Regulation of cellular homeostasis (lipid rafts regulate lateral diffusion of proteins in response to stimuli).
Vesicle budding and fusion (important in exocytosis/endocytosis coupling).
Membrane remodeling is carried out by caveolae.
Cellular/membrane transport.

Cytoplasm: A gelatinous solution, containing cytoskeleton and 75-95% water, carbohydrates, fats, and proteins.
Cytosol: The liquid portion of the cytoplasm.
Organelles: Specialized structures with characteristic shapes and specific functions.

A dynamic membrane-bound organelle that extends from the nuclear envelope.
Synthesizes, transports, and controls quality of components in the cell.
Made of flattened sacs and branching tubules.
Rough ER has ribosomes on its surface, involved in protein synthesis.
Smooth ER lacks ribosomes, involved in lipid metabolism.
Transitional ER is also part of the ER, involved in transporting secretory proteins to the Golgi complex.

A central station for protein modification, sorting, and secretion (or degradation via the lysosomal pathway).
Comprised of membrane-bound compartments (cisternae) stacked like a plate.
Vesicles transport molecules between the ER and Golgi complex, and from the Golgi to destinations in the cell.
Proteins and lipids are modified sequentially inside the cisternae, and molecular labels/tags are added by resident enzymes.
The Golgi sends the final products to various parts of the cell via trans face vesicles. Some proteins might be targeted to lysosomes.

Roughly spherical, single membrane-bound organelles that mediate various biosynthetic and biodegradative reactions.
They produce key components of the cell.
They contain enzymes required for the synthesis of ether phospholipids, bile acids, and docosahexaenoic acid (DHA).

Acidic and dynamic organelles that degrade extracellular and intracellular particles.
Cleave materials to produce amino acids, monosaccharides, free fatty acids and nucleotides.
Helps recycle components from worn-out cells.
Maintains an acidic pH (~5.0) within the organelle.
Defective lysosomal enzymes cause storage diseases. Leakage causes cellular autodigestion (autolysis).

Complex oblong organelles found in all eukaryotic cells.
They are the "powerhouses" of the cell, converting oxygen and nutrients into energy.
The major site of ATP synthesis in aerobic cells.
Possess their own DNA (mtDNA) and machinery.
Contain the enzymes of the citric acid cycle and electron transport chain.
Possess intra-organelle spaces (matrix and intermembrane space).

Highly specialized and dynamic membrane-bound organelle.
Houses genetic material (DNA and RNA).
Contains the nucleolus, a region involved in ribosome synthesis.
The nuclear membrane regulates the exchange of materials between the cytoplasm and nucleoplasm.
Contains chromosomes (DNA-protein complex) in a non-dividing cell, and chromatin in dividing cells.
Important for intermediary metabolism, protein synthesis, and RNA synthesis.

Small granular organelles that are part of the macromolecular machinery for translating genetic code into functional proteins.
Two types exist in mammalian cells: cytoribosomes and mitoribosomes.
Cytoribosomes are found on the rough ER; mitoribosomes are found in the mitochondria.
Composed of large and small subunits.

Dense area of cytoplasm, near the nucleus, a membrane-less organelle consisting of a pair of centrioles.
Surrounded by pericentriolar material.
Centrosome is the microtubule organizing center of the cell, involved in cell division and other functions.

Dynamic network of filamentous polymers (intermediate filaments, microtubules, microfilaments, septs) and regulatory proteins.
Located throughout the cytosol.
Involved in maintaining cell shape, structural support, cellular transport, cell movement, cell division, and other processes.

Small membrane- enclosed transport units used to transfer molecules between different cellular compartments.
Include clathrin-coated and coat protein complex-I-coated vesicles.
Often involved in transport to and from the Golgi complex.

Bilipid Layer Model: Phospholipids form a bilayer; heads face outward, tails inward.
Fluid Mosaic Model: Proteins embedded within the lipid bilayer; membrane is fluid, and components move freely.
Dynamic Fluid-Mosaic Model: Nonrandom distribution of membrane proteins and lipids that form clusters; emphasizes fluidity and mosaicism aspects simultaneously.