Cell Types and the Plasma Membrane

• Cells have different types, each specialized for a specific functions.
• Neurons, skeletal muscle cells, absorptive cells with microvilli, bone cells, ova, and sperm are examples of specialized cells.
• Cells may differ in morphology
• Cells share the common features of being bound by a plasma membrane, containing organelles like the nucleus, mitochondria, ER, and Golgi apparatus and having non-membrane bound structures like the cytoskeleton

Plasma Membrane

• Defines the boundaries of organelles within the cell
• Under an electron microscope, shown to have a trilaminar structure
• Structure is due to a phospholipid bilayer
• Phospholipids are amphipathic, having both polar hydrophilic heads and nonpolar hydrophobic tails
• Held together by hydrophobic interactions
• Selectively permeable to lipids and solvents.
• Highly resistant to charged molecules and large compounds
• Lipid components move laterally and rotate on their axis; flip-flop movement is rare.
• Cholesterol controls membrane fluidity by interacting with fatty acid chains.

Fluid Mosaic Model

• Membrane fluidity increases with temperature.
• Phospholipids can move, increasing the distance between units.
• Shorter length: transmembrane proteins can move easier
• Saturated: pack together closely.
• Unsaturated: kinks increase fluidity.

Functions of the Cell Membrane

• Compartmentalizes the cell, defining the boundaries of cells and organelles.
• Allows specialized activities to proceed without external interference and enables the cellular activities to be regulated independently.
• Provides a scaffold for biochemical activities; plasma proteins can act as enzymes.
• Selectively permeable barrier based on types of channels, such as the Na+/K+ channel.
• Contains receptors to detect external signals and allow for cascade reactions.
• Facilitates cell-to-cell contact, adhesion, and communication.

Membrane Bound Organelles

• Organelles in the cell which are bound by the plasma membrane

Nucleus

• Enclosed by a nuclear envelope, which is a double membrane structure.
• Nuclear pores facilitate the exchange of material between the nucleus and cytoplasm.

Endoplasmic Reticulum (ER)

• Two types : rough ER and smooth ER
• Rough ER constitutes more than half of the membranous surfaces within the cytoplasm.
• Rough ER consists of flattened sheets of membranes and tubules with ribosomes bound to the cytosolic surface, which are active in protein synthesis, with proteins released into cisternae of rER
• Well-developed in protein synthesis and secretory cells (exocrine pancreatic cells)
• Smooth ER is free of ribosomes, has more tubular components, and performs different functions like steroid synthesis and androgen in leydig cells of testis, detoxification, lipid metabolism, calcium storage and release

Golgi Apparatus

• Modifies, packs, and distributes proteins and lipids for secretion or internal use.
• Contains stacked membraneous cisternae with vesicles
• Cis face: faces the ER, receiving vesicles
• Proteins progress through the Golgi, undergoing chemical modifications such as glycosylation
• Trans face: proteins and lipids are released as vesicles, with mature proteins released from the last cisterna.
• Lumen: site of protein and lipid modification, glycosylation, and cleavage of longer proteins into smaller ones.

Mitochondria

• Provides energy for the cell via ATP through aerobic respiration.
• Involved in programmed cell death (apoptosis).
• Has a double membrane, with the inner membrane folded into cristae and the outer membrane is smooth
• Contains its own circular DNA that replicates independently of nuclear DNA.

Lysosomes

• Membrane-bound vesicles which contain enzymes that degrade phagocytosed materials
• Recycle and digest organelles
• Enzymes activated under acidic conditions.

Peroxisomes

• Smaller than lysosomes
• Contain enzymes to break down long fatty acids (FA) and amino acids (AA)
• Commonly found in the liver and perform detoxification.
• Produce H2O2 byproduct, which is potentially toxic
• Catalase in peroxisomes neutralizes H2O2.

Nuclear Structures

• Nucleolus and protein synthesis occur in the context of the nucleus

Nucleus

• The information center contains genetic material
• Present in eukaryotic cells
• Components: nuclear envelope, nucleolus, and nucleoplasm
• Largest and most negative components stain purple with H&E
• Nuclear envelope has a double membrane
• Outer nuclear membrane is continuous with the ER membrane.
• Lumen of the nuclear envelop is continuous with the lumen of the ER.
• Nuclear pores formed by the fusion of the inner and outer nuclear membranes, which allow the passive diffusion of small molecules, active import of proteins, and the export of RNA and proteins.

Nucleolus

• Site of rRNA synthesis and contains genes encoding rRNA (5 pairs of chromosomes: chr 13,14,15,21 and 22)
• Processes and assembles ribosomal subunits from rRNA and ribosomal proteins imported from the cytoplasm
• Assembled subunits are transported out of the nucleus and assembled into functional ribosomes in cytoplasm.
• Contains fibrillar center (depot of inactiated RNA genes), dense fibrillar component (nascent pre-rRNA synthesis and processing), and granular component (ribosomal subunits in different stages of assembling)

Nucleoplasm

• Consists of DNA, nucleoproteins, and structural proteins.
• Chromatin is DNA complexed with histone proteins.
• Heterochromatin is darker and more compact, with less active gene transcription.
• Euchromatin is lightly stained, loosely packed DNA, with active transcription.

Cytoskeleton

• Microfilaments, intermediate filaments, and microtubules

Microfilaments

• Smallest of the three types (less than 10 nm)
• Located in the peripheral of the cell which is the layer below the plasma membrane
• Basic unit is G actin (globular).
• Can polymerize to form long helical filaments and F actin (filamentous actin).
• Are polar and growth occurs on both ends, the ends growing faster
• Concentrated in the cortex
• Present in all cells, and have locomotive and structural functions.
• Tight parallel bundles of microfilaments (polymerization of G actin) allow cells to explore their environment and move toward targets.
• Cell cortex,cell cortex is immediately below the cell membrane, which supports cell movements and cytokinesis by interacting w/ myosin to form contractile ring
• Assemble actin into different structures: bundles or networks, through the interaction of different cross-linking proteins.
• Bundles are seen in the core of microvilli and are connected by fimbrin while networks are at the bottom of microvilli and are connected by filamin.

Intermediate Filaments

• Larges diameter than microfilaments
• They are 10 nm
• Dispersed in the cell
• Rod-like protein that twists another monomer to form a dimer
• 2 dimers then interact in an anti-parallel manner to form a tetramer which is them assembled end to end, forming protofilaments.
• Serve as scaffold and support to the entire cytoskeleton and example of this is keratin Forms a strong network within cells, connected by junctional complexes, to create a sheet of cells resistant to mechanical stresses. Without keratin, cells are fragile and prone to rupture, leading to skin blistering.

Microtubules

• Largest of cytoskeleton components containing a diameter of greater than 10 nm
• Extend from the nuclear region
• building blocks are tubulins that consist of dimers (apha and beta subunits)
• Arranged in longitudinal rows to make protofilaments
• 12 protofilaments arranged to form a hollow center
• They are highly dynamic structures in the cell, with microtubules assembled in the MTOC
• Anchors for one end of microtubules seen in centrosomes and basal bodies (cilia, flagella). separation of chromosomes during mitosis, attaching to centromeres and bringing chromosomes to epipolar planes

Functions of Microtubules

• Intracellular transport of organelles where motor proteins - kinesin and dyneins are used
• Kinesin- plus end
• Dynein- minus ends
• Cell locomotion as a core component of cilia and flagella
• Cilia contains nine doublets + 2 microtubules
• Facilitate the beating movement

Agents that Prevent Microtubular Function

• Colchicine- binds to tubulin dimers to prevent polymerization of dimers for assembling ito microtubules
• Taxol - bind to microtubules to disrupt depolymerization of microtubules.
• Cell division is very dynamic and alterations in polymerization or depolymerization can affect cell division.
• Both drugs can act as anti mitotic drugs to inhibit cell proliferation.
• Contribute to spatial organization and maintain cell shape
• Whole cell locomotion
• Process of cytokinesis
• Move cellular organelles such as vesicles
• Mechanical strength to withstand stretching or compression