Cellular Organization of the Human Body

• Lecture focuses on the organization of the human body at the cellular level.
• Includes cell membrane, mitochondria, cytoskeleton, centrosomes, cilia & flagella, and cell junctions.
• After the lecture, students should be able to identify the structure and functions of cell organelles and plasma membrane components.
• They should be able to differentiate between members of the cellular skeleton.
• They should be able to discuss the composition of centrosomes, cilia, and flagella.

Cell Plasma Membrane

• The currently accepted model for the plasma membrane structure is the fluid mosaic model, proposed in 1972.
• Separates the living cell from its surroundings (Intracellular and Extracellular spaces).
• Exhibits selective permeability, allowing certain substances to cross more easily than others.
• Appears as a tri-laminar membrane with two parallel dark lines separated by a light zone when viewed with an electron microscope.
• Molecular composition includes a lipid bilayer, membrane proteins, and carbohydrates, as described by the fluid mosaic model.

Lipid Bilayer:

• Mainly composed of phospholipid molecules, which are amphipathic, having polar (hydrophilic) heads and non-polar (hydrophobic) tails made of fatty acid chains.
• Contains cholesterol molecules that maintain optimal fluidity.
• Cholesterol restricts solidification at low temperatures and restricts movement, preventing weakening at high temperatures.
• Exhibits self-sealing properties by resisting free ends and forming closed vesicles, can fuse with other membranes during exocytosis or endocytosis.

Membrane Proteins:

• Integral membrane proteins penetrate the hydrophobic interior of the lipid bilayer, and some extend partway inside.
• Most are transmembrane proteins and are firmly bound within the membrane.
• Peripheral membrane proteins are not embedded in the lipid bilayer.
• They are loosely associated and located on the inner or outer surfaces of the membrane.

Functions of Membrane Proteins:

• Facilitate intercellular joining by hooking together in cell junctions.
• Act as channel proteins to allow passage of ions or specific molecules.
• Function as pumps or carriers for active or passive transport.
• Some function as embedded enzymes that catalyze chemical reactions.
• Function in signal transduction, transmitting signals into the cell (hormones).
• Some serve as identification tags in cell-cell recognition (glycoproteins).
• Mediate cell-ECM adhesion, like with Integrins proteins attaching the ECM to the cytoskeleton.

Membrane Carbohydrates:

• They form the cell coat or glycocalyx, which protects cells.
• These are short-branched polysaccharide chains with fewer than 15 sugar units.
• A majority attach to membrane proteins (glycoproteins) and some to the lipid bilayer (glycolipid).
• Function as markers distinguishing one cell from another.
• The four human blood groups designated A, B, AB, and O are examples where they reflect a variation in the carbohydrate part of glycoproteins on red blood cells.

Mitochondria:

• Known as the powerhouse of the cell.
• It is a self-replicating organelle capable of growing and dividing.
• The number increases in active cells like liver (800-2000) and muscle cells (hundreds to thousands).
• The increase is due to their energy-demand nature, depending on muscle type and activity level.
• Acts as the site of cellular respiration, which uses nutrients and oxygen to produce energy in the form of ATP.
• Cellular respiration stages include glycolysis, the citric acid cycle, and oxidative phosphorylation.
• Contains ribosomes and circular DNA.
• Enclosed by two membranes: a smooth outer membrane and a folded inner membrane called cristae.
• Cristae increase surface area, enhance cellular respiration.
• The inner membrane divides the mitochondrion into two internal compartments.
• The inter-membrane space is a the narrow region between the inner and outer membranes.
• The mitochondrial matrix is enclosed by the inner membrane.
• Enzymes that make ATP are built into the inner mitochondrial membrane.

Cytoskeleton:

• The cytoskeleton is a network of protein fibers throughout the cytoplasm.
• Plays a role in division, shape, structure, movement, support, transport, and organization of the cell.
• Composed of microtubules, microfilaments, and intermediate filaments.

Microtubules:

• Hollow cylinders.
• Consist of 13 columns of tubulin molecules formed of dimer protein subunits (α- and ß-tubulins).
• Undergo assembling and disassembling.
• Maintain cell shape.
• Facilitate cell motility (forms the core of motile cilia and flagella).
• Forms mitotic spindle thread to help with chromosome movements during division.
• Form tracks for organelle movement.

Microfilaments:

• Composed of actin filament protein subunits and are flexible.
• Exist as two intertwined strands of bead-like actin molecules (polymer).
• Form bundles or networks.
• Undergo assembling and disassembling.
• Maintain cell shape.
• Generate movement with myosin in muscles.
• Form cleavage furrow in cell division.
• Form the core of bundles in microvilli.

Intermediate Filaments:

• Considered stable structures.
• They are tough fibers formed of keratin fibrous protein.
• Assembling is irreversible.
• They provide structural support (support cell junctions under stress).
• Stabilize cell shape by anchoring organelles.
• Help with disorganization and reorganization of the nucleus in cell division.
• Form nuclear lamina under the nuclear envelope.

Cytoskeleton Summary:

• Microtubules shape are hollow tubules, composed of α and β tubulins, flexible and are easily assembled and disassembled.
• These compose of Cilia, Flagellum, and Centrosomes.
• Microfilaments shape are two strings of beadlike filaments, made of actin filaments.
• These are flexible fibers that are also reversible and compose of Microvilli, and Steriocilia.
• Intermediate filaments are solid filaments, made of Keratin proteins.
• These are tough and irreversible and compose of hair, nails, and the nuclear lamina.

Centrosome:

• Organelle located near the nucleus.
• Formed of two centrioles at a right angle to each other.
• Each centriole contains nine triplets of MT i.e. hollow cylinder (9 x 3).
• Microtubules are arranged in a ring.

Motile Cilia and Flagella:

• Locomotory organs of unicellular eukaryotes.
• Locomotory organs of sperm.
• Extend from the cell plasma membrane in a cylinder shape.
• Core is a formation of 9 doublets and two that are single (9x2 + 2 MT).
• Originate from basal bodies (9x3 MT).

Cell Junctions:

• Cells in animals or plants are organized into tissues, organs, and organ systems.
• Neighboring cells adhere, interact, and communicate through direct physical contact.
• Types of cell junctions include tight junctions and gap junctions.

Tight Junctions (Occludens type):

• Plasma membranes of neighboring cells are tightly pressed together.
• Bound by specific integral proteins.
• Prevent leakage of extracellular fluid across epithelial cell layers.
• Example: skin cells.

Desmosomes:

• Are anchoring junctions between cells that function like rivets.
• They fastening cells together into strong sheets.
• Attach muscle cells to each other in a muscle.

Gap Junctions:

• Are communicating junctions between cells.
• They provide cytoplasmic channels between adjacent cells.
• Necessary for communication in heart muscle and animal embryos.