Cellular Structures and Organelles Laura Fraser Cotlin, Ph.D. Fundamentals 1 – Dental/Optometry August 9, 2011

A Lymphocyte and a Neuron

Organelle Classification Membrane Bound Plasma membrane Nucleus RER SER Golgi Apparatus Lysosomes Endosomes Peroxisomes Mitochondria Non-membrane Bound Ribosomes Proteasome Microtubules Actin Filaments Intermediate Filaments Centrioles and Basal Bodies Cilia, Flagella Inclusions

Schematic of a Polarized Cell

Fluid Mosaic Model of the Cell Membrane

Functions of Cell Membranes Maintains structural integrity of the cell Controls movement of substances in and out of cell Regulates cell-cell interactions Serves as a recognition board via receptors Establishes transport system for specific molecules Transduces extracellular physical and chemical signals into intracellular events

Molecular Composition of Cell Membranes The Lipid Component Phospholipids Glycolipids Cholesterol Physical property of lipids forms a bimolecular leaflet. (hydrophobic and hydrophilic components - amphipathic) The Protein Component Integral proteins, transmembrane proteins, multipass proteins Peripheral proteins - extracellular and cytosolic.

(cross section of microvilli) EM of Cell Membrane (cross section of microvilli)

Two important concepts about membranes Movements of proteins in the membrane Arrangements of proteins in the membrane

Functions of Integral Membrane Proteins

Channel Proteins -allow passage of one particular ion -can be: Voltage-gated proteins Ligand-gated proteins Mechanically-gated proteins other Pumps and Carrier Proteins -bind and transport ions and molecules -examples: Na+-K+ pump Ca++ channels glucose transporter

Surface Receptors -involved in receptor-mediated endocytosis -serve as signaling molecules which are kinases themselves, or are associated with kinases -serve as signaling molecules which are coupled to G- proteins -cytokine and steroid receptors Linkers and Structural Proteins -involved in cell-cell attachment; cell-matrix attachment -serve as scaffolds for cytoskeleton

The Cell Nucleus

Structure of the Nucleus

EM of the Cell Nucleus

Nucleus and Nuclear Envelope

Structure of the Nuclear Envelope The nuclear envelope separates the contents of the nucleus from the cytoplasm, thus separating the genetic material from the rest of the cellular milieu. The nuclear envelope consists is a double membrane (inner and outer) which are joined at the nuclear pore complexes. The inner membrane associates with a nuclear lamina, the outer membrane becomes contiguous with the endoplasmic reticulum

Diagram of the NPC

The Nuclear Pore Complex (NPC) The NPC is a large macromolecule consisting of more than 100 proteins which provides the only route for molecules to travel in and out of the nucleus. The nuclear pores and associated transport proteins control all ingress and egress of substances to the nucleus. Small molecules can diffuse freely; macromolecules are selectively transported and require energy. Larger molecules require a nuclear localization signal (amino acid sequence) to enter nucleus. This signal contains a stretch of several basic amino acids such as lysine and arginine. RNAs are transported through the NPC as ribonucleoproteins. This transport requires energy provided by ATP hydrolysis Nuclear shuttle or transport proteins bring substances to the nuclear pore, and tether the substance at the pore. Entry requires energy expenditure.

Diagram of the NPC

EM of the Nuclear Pore Complex

The chromatin material Heterochromatin: constitutive and facultative: constitutive chromatin remains transcriptionally inactive whereas the facultative form can be de-condensed and transcriptionally active, depending on cell type. Euchromatin distribution and function. Transcriptionally active DNA, and has some “open” areas that are cell specific. The interphase nucleus contains both transcriptionally inactive heterochromatin as well as decondensed, transcriptionally active euchromatin. Major types of proteins located in the nucleus are: histones, fibrous proteins, nuclear matrix, ribosomal proteins, transcription factors.

Nucleolus

The Nucleolus The most prominent substructure in the nucleus is the nucleolus. It is the site of rRNA synthesis and ribosome assembly. The primary transcript of the rRNA gene is 45S pre- rRNA which is processed to yield the three mature forms of RNA for ribosome construction 18S, 5S and 28S rRNAs. Small ribosomal proteins (49 for the large subunit and 33 for the small) are made in the cytoplasm and transported to the nucleolus through the nuclear pore complex

The functional unit of euchromatin The functional unit of euchromatin is called the nucleosome. The nucleosome consists of histone proteins, 2 copies of H2A, H2B, H3 and H4 around which is wrapped 166 DNA base pairs. An additional 48- bp segments form the link between adjacent nucleosomes, and another histone (H1 or H5) binds to this DNA.

The Nucleosome

Schematic of Chromosome Packaging

Another cartoon depicting DNA packaging

Ribosomes Giant macromolecular machines that bring together mRNA and aa- tRNA to synthesize a polypeptide. Consist of a small subunit and a large subunit that bind together during translation of mRNA. Each subunit consists of one or more ribosomal RNA (rRNA) molecules and many distinct proteins. In eukaryotes, the large subunit contains ~49 proteins and three different sizes of RNAs ,whereas the small subunit contains ~33 proteins and one RNA molecule. Ribosomes can be used many times to read mRNA and translate proteins. Ribosomes can associate with ER membranes if the nascent polypeptide contains a signal sequence. As many as 11-15 ribosomes can be on one mRNA. This structure is called a polyribosome.

Organization of Ribosomes

Location of Protein Synthesis

The Secretory Pathway

The Endoplasmic Reticulum A continuous system of flattened membrane stacks and tubules Site of synthesis of membrane and secretory proteins and lipids of the cell Continuous with the outer half of the nuclear membrane ER pumps and channel proteins regulate cytoplasmic calcium concentrations (Ca++ concentration is kept high in the ER.) ER enzymes metabolize drugs Ribosomes attach to ER membrane if nascent polypeptide bears a signal sequence ER is very dynamic. Continuous bidirectional traffic moves small vesicles between ER and Golgi. ER is of two types, Rough ER and Smooth ER

Rough Endoplasmic Reticulum

The ER and Golgi Apparatus

The Golgi Complex

Activity in the Golgi Golgi complex has 3 compartments: cis, medial and trans Proteins are terminally glycosylated here Proteins are directed to different cellular compartments using specialized targeting signals All proteins destined for secretion or as membrane proteins pass through the 3 compartments and are posttranslational modified and “packaged” into delivery vesicles that bud from from the trans-golgi network (TGN). There is a bidirectional flow of vesicles in the Golgi

ER and Golgi

The Lysosome Contains degradative enzymes separated from other cellular components by an impermeable membrane Lysosomal enzymes are synthesized at the RER, transported to Golgi, and contain a phosphorylated mannose on the protein surface which targets them for transport to the lysosome. Vesicular transport guided by phosphomannose delivers enzyme to lumen of lysosome. Plasma membrane vesicles called endosomes or phagosomes deliver ingested microorganism to lysosome for destruction. Deficiencies in lysosomal enzymes cause many congenital disease.

Lysosomes

Lysosomes

Endocytosis and Endosomes

Phagocytosis and Phagosomes

Peroxisomes Membrane-bound organelle containing enzymes that participate in oxidative reactions. Oxidize fatty acids but energy generated is not used to synthesize ATP Peroxisomal proteins are synthesized in the cytoplasm and transported into the organelle using a specialized targeting amino acid sequence. Genetic defects in peroxisomal biogenesis cause several forms of mental retardation.

Peroxisomes

Mitochondria Double membrane structures containing enzymes that convert energy released from breakdown of nutrients into the synthesis of ATP. Mitochondria cluster near sites of high ATP utilization (sperm tails, contractile apparatus of muscles, active transport of membranes, nerve terminals). Mitochondria play key role in response to toxic stimuli by releasing a toxic cocktail of proteins that bring about the death of a cell (apoptosis). Mitochondria form differently from other organelles, by pinching in two. They do not divide synchronously with cell division. Mitochondria have their own DNA and protein synthesizing apparatus and make some of their own proteins

Mitochondria

Organization of Mitochondria

Overview of Mitochondrial Functions

Mitochondria