Cytology: Cell Structure and Function

• Cytology is the study of cells.
• The cell is the organism's structural and functional unit, limited by a cell membrane (plasmalemma).
• Protoplasm, the cell's living substance, divides into the cytoplasm and the nucleus.

Cytoplasm Components:

• Cytosol: a semifluid cytoplasmic matrix.
• Cytoplasmic organelles: living structures performing essential functions.
• Cytoskeleton: tubules and filaments maintaining cell shape.
• Cytoplasmic inclusions: storage forms of non-essential substances.

Nucleus Components:

• Nuclear envelope
• Nucleolus
• Chromatin
• Nuclear matrix

Cell Membrane

• All cells are surrounded by a cell membrane (plasma-membrane or plasmalemma).
• Intracytoplasmic membranes surround some organelles and are similar to the cell membrane.
• Both cell and intracytoplasmic membranes are called unit membranes.
• Cell membranes are not visible with a light microscope (L/M).
• Electron microscope (E/M) reveals specific structures of the cell membrane.

Electron Microscope Observations:

• Low magnification: appears as a dense thin line 8.5-10 nm thick.
• High magnification: shows two electron-dense lines (2.5-3 nm each) separated by an electron-lucent zone (3.5-4 nm).
• The trilaminar appearance is called the unit membrane.

Molecular Structure (Fluid-Mosaic Model)

• The Fluid-Mosaic Model of Singer and Nicholson is the most accepted model.
• The Fluid-Mosaic Model consists of a phospholipids bilayer, cholesterol, and proteins.

Phospholipids Bilayer:

• The backbone of the cell membrane consists of a double layer of phospholipids molecules.
• Phospholipid molecules have polar hydrophilic phosphate heads and non-polar hydrophobic tails of long fatty acid chains.
• In aqueous solution, they form a bilayer with hydrophilic heads outward and hydrophobic tails inward.
• Weak intermolecular forces allow phospholipid molecules to move freely.

Cholesterol Molecules:

• Cholesterol molecules are present in the lipid bilayer.
• The ratio between cholesterol and phospholipids is one to one.
• Cholesterol fill the gaps between the fatty acid tails.

Proteins:

• There are two types of proteins.
• The two types of proteins are integral (intrinsic) and peripheral (extrinsic).

Integral (Intrinsic) Proteins:

• Present among lipid molecules.
• Some are partially embedded in the lipid bilayer.
• Transmembrane proteins- proteins extend across the lipid bilayer and protrude from both membrane surfaces.
• Transmembrane proteins form channels for water-soluble molecule passage (e.g., ions).

Integral Membrane Proteins Functions:

• Pumps

• Channels

• Receptors

• Linkers

• Enzymes

• Structural

• Integral membrane proteins can move within the lipid bilayer.

Peripheral (Extrinsic) Proteins:

• Associated with the surfaces of the bilayer.
• Loosely attached to the membrane surfaces.
• The fluid-mosaic model is due to the mosaic arrangement of membrane proteins within the fluid lipid bilayer.

Cell Coat (Glycocalyx):

• Associated with the external surface of the cell membrane.
• Consists of oligosaccharides conjugated mainly with membrane proteins (glycoproteins) and some membrane lipids (glycolipids).
• Varies in thickness in different cell types.
• Appears as a fine filamentous material when viewed with an electron microscope.
• Important in cell recognition, protection, and intercellular adhesions.

Functions of the Cell Membrane

• Maintains cell structure.
• Controls the passage of materials in and out of the cell.
• Can recognize foreign bodies.

Passage of Materials:

• There are two ways to transport materials in and out of the cell through the cell membrane.
• These are transport of Ions and small molecules and vesicle-mediated transport (mass transfer).

I) Ions and Small Molecules:

a) Passive Transport:

• Molecule passage depends on concentration gradients.
• No energy is needed.
• Passive transport includes simple and facilitated diffusion.

1- Simple Diffusion:

• Dissolved gases (e.g., O2 & N2) and low molecular weight substances (e.g., H2O) enter or leave the cell from high to low concentration.

2- Facilitated Diffusion:

• Facilitated diffusion needs channel proteins or carrier proteins.
• Channel proteins do not form a bond with the molecules.
• Carrier proteins form a bond with the molecules.

b) Active transport:

• Certain ions and molecules are transported against their concentration gradient, from low to high concentration (e.g., sodium-potassium pump).
• Energy is required.
• The process is carried out by carrier proteins.

II) Vesicle-Mediated Transport (Mass Transfer):

• Involves vesicles or vacuoles that form from or fuse with the cell membrane.
• Includes endocytosis (getting substances into the cell) and exocytosis (getting substances out of the cell).

1- Endocytosis:

A) Fluid-Phase Pinocytosis (Cell Drinking):

• Non-selective process in all cell types.
• Small invagination of the cell membrane containing extracellular fluid or substances in solution forms a pinocytotic vesicle.
• Pinocytotic vesicles separate from the cell membrane and either fuse with lysosomes OR the opposite cell surface, releasing contents (trans-cellular transport, e.g., endothelial cells).

B) Phagocytosis (Cell Eating):

• The process of engulfing large particles (e.g., bacteria, microorganisms, cell fragments, and dead cells).
• Performed by specialized cells known as phagocytes, like neutrophils and macrophages.
• Initiated by binding a bacterium or other particles to receptors on the cell surface.
• Stimulates actin and myosin mobilization in the cytoplasm.
• Cell processes (pseudopodia) form and extend, surrounding the bacterium.
• The edges of these processes fuse, enclosing the bacterium in an intracellular phagocytic vacuole.

C) Receptor-Mediated Endocytosis:

• Highly selective process requiring receptors and ligands.
• Ligands (e.g., proteins or hormones) bind to specific receptors at the cell surface.
• Ligand-receptor complexes aggregate in shallow invaginations of the cell membrane called coated pits.
• The cytoplasmic surface of the pit is coated with a spiny fibrous cytoplasmic protein called clathrin.
• Coated pits become deep and pinch off the cell membrane, forming coated vesicles.
• Coated vesicles soon lose their clathrin coat.
• Then, they fuse with early endosomes and go deeper into the cytoplasm.
• Its acidic interior dissociates the ligand from its receptor molecule.
• Receptors move back to the cell membrane to be used again (recycling).
• After the ligands have performed their functions inside the cell, they might return back to the extracellular space to be used again, or go to late endosomes, which transport them to lysosomes for degradation.

2- Exocytosis:

• Refers to the release of cell products into the extracellular compartment.
• Vesicles containing the product fuse with the cell membrane, allowing outflow of their contents.
• There are two general pathways of exocytosis: constitutive and regulated secretory pathways.

A) Constitutive Pathway (Continuous Process):

• Proteins leaving the cell are secreted immediately after their synthesis.
• An example is the secretion of immunoglobulins by plasma cells.

B) Regulated Secretory Pathway (Discontinuous Process):

• Proteins are concentrated and stored in secretory granules.
• In response to stimulation by neurotransmitters or hormones, the secretory granules fuse with the cell membrane to release their contents outside the cell.
• An example is the release of zymogen granules of pancreatic acinar cells.

Membrane Trafficking:

• Means repeated recycling of the cell membrane.
• During endocytosis, portions of the cell membrane are used as endocytotic vesicles.
• During exocytosis, the lost portions are compensated by other membranes.