Cell Structure: Cytoplasm and Membranes

• A cell comprises an endoplasmic reticulum, cytoplasm, plasma membrane, mitochondria, nucleus, nuclear envelope, cytoskeleton, lysosome, Golgi apparatus, and transport vesicle

Cytoplasm

• The cytoplasm contains organelles, inclusions, and a cell matrix

Cytoplasmic Organelles

• Membranous organelles include mitochondria, Golgi apparatus, lysosomes, endoplasmic reticulum, peroxisomes, and cytoplasmic vesicles
• Non-membranous organelles include ribosomes and the cytoskeleton

Cell Membrane

• The cell membrane has a thickness of 9-10 nm
• Is not visible under a light microscope
• Under an electron microscope, it appears as a thin dense line, trilaminar in shape with three layers
• Composed of lipid molecules, protein molecules, and carbohydrate molecules (cell coat)

Chemical Structure of Cell Membrane

• Lipid molecules include phospholipids and cholesterol.
• Protein molecules are either integral (intrinsic) or peripheral (exterinsic)
• Carbohydrate molecules (cell coat) are glycoproteins and glycolipids

Functions of Cell Membrane

• Functions include exchange of materials, endocytosis, exocytosis, sodium-potassium pump action, functions of cell coat, and membrane modifications

Exchange of Materials

• Passive diffusion allows gases such as O2 and CO2 to pass
• Active transport enables amino acids, glucose, and fatty acids to pass.
• Selective transport enables hormones, drugs, and bacteria to pass

Endocytosis

• Phagocytosis transports solid materials
• Pinocytosis transports fluids

Functions of Cell Coat

• Cell coat has an adhesive function
• Enables the cell to respond to hormones or drugs via receptors
• Cell coat is important for immunity.

Membrane Modifications

• Include microvilli, cilia, and cell junctions

Mitochondria

• Defined as membranous organelles often described as the power house of the cell
• Shape can be rods, granules, or filaments and their number can vary
• Rod-shaped or spherical, covered by two membranes.
• The outer membrane is smooth
• The inner membrane forms cristae
• Has two spaces: inter-membranous space which is between the two membranes, and a matrix space containing DNA, RNA, ribosomes, and proteins
• Provides the cell with ATP and is able to divide to increase their number according to energy needs

Ribosomes

• Non-membranous and concerned with protein synthesis
• Basophilic and contain RNA
• Each ribosome is formed of two subunits, small and large
• Formed in the nucleolus and then released through nuclear pores
• Free ribosomes are linked by messenger RNA to form polysomes
• Attached ribosomes are attached to the membranes of endoplasmic reticulum to form RER
• Function in protein synthesis.

Endoplasmic Reticulum

• Intercommunicating tubules, vesicles, and cisternae that form a network
• Two types: Rough (RER) and Smooth (SER)

Rough Endoplasmic Reticulum (RER)

• Located in protein-synthesizing cells such as plasma cells and pancreatic acinar cells
• Intercommunicating cisternae with attached ribosomes
• Function is protein synthesis that will be excreted outside the cell.

Smooth Endoplasmic Reticulum (SER)

• Intercommunicating tubules with no attached ribosomes
• Functions include the synthesis of lipids and steroid hormones, glycogen metabolism, detoxification of toxins and drugs such as alcohol, and calcium regulation in skeletal muscles.

Golgi Apparatus

• Routine staining (H & E) makes it unseen, it is only a pale, negative Golgi image beside the nucleus but can be viewed via silver staining
• Composed of saccules, transfer vesicles, and secretory vesicles

Saccules

• The saccules form 4-10 stacks
• Each stack has a convex face, which is a forming or cis face, and a concave face, which is a mature or trans face

Transfer Vesicles

• Bud from RER and fuse with the convex face
• Transfer proteins from RER to Golgi

Secretory Vesicles

• Arise from the mature face
• Discharge contents outside or remain to form lysosomes.

Functions of Golgi Apparatus

• Chemical modifications to proteins synthesized by RER
• Packaging of proteins to form secretory vesicles
• Secretion of proteins
• Formation of coated vesicles and lysosomes.

Lysosomes

• Membranous organelles with rounded electron-dense bodies
• Contain hydrolytic enzymes
• Two types: primary and secondary

Types of Lysosomes

• Primary lysosomes are newly formed from Golgi
• Secondary lysosomes are primary lysosomes with foreign vesicles

Functions of Lysosomes

• Phagocytosis of foreign particles and bacteria to digest their contents
• Pinocytosis of fluids to digest their contents
• Lysis of old organelles through autophagic vacuoles

Cytoskeleton

• Composed of microtubules, microfilaments, and intermediate filaments

Microtubules

• 25 µm in diameter

Microfilaments

• Thin 7 nm or thick 12-16 nm in diameter

Intermediate Filaments

• 10 nm in diameter

Microfilaments

• Types include thin filaments and thick filaments

Thin Filaments (Actin Filaments)

• Are contractile filaments, 7 nm in diameter
• In muscles, their function supports contraction
• In microvilli, their function supports and gives shape
• In the cleavage furrow facilitate cell division

Thick Filaments (Myosin)

• Thicker than thin filaments at 12-16 nm in diameter
• In muscle, are associated with actin filaments forming the myofibrils

Intermediate Filaments

• 10 nm in diameter that cannot produce contraction
• Functions include supporting the shape of the cell, transmitting forces in smooth muscles and tumor identification

Microtubules

• Pipe-like structures with a 25 nm diameter
• Formed of tubulin protein
• Support the shape of the cell and transport substances in the cytoplasm
• Form centrioles, cilia, and flagella
• Form mitotic spindle of cell division

Centrioles & Centrosome

• Most cells contain 2 centrioles in an area called the centrosome.
• Each centriole is formed of 9 triplets of microtubules.
• The 2 centrioles are perpendicular to each other.
• Functions include the formation of mitotic spindles for cell division and basal bodies of cilia.

Cilia

• Hair-like processes on the free surface of ciliated cells
• Located in respiratory epithelium of trachea and bronchi and female genital system in uterus and fallopian tube

Cilia Structure

• Formed of basal bodies similar to centrioles
• Have a shaft (axoneme) with 9 peripheral doublets of microtubules and 2 central singlets
• Rootlets that are thin fibers

Cell Inclusions

• Non-living materials stored in the cytoplasm
• Store food such as glycogen and lipids, and pigments

Pigments

• Exogenous pigments include lipochrome pigments such as carotene in vegetables and dust and carbon particles
• Endogenous pigments include hemoglobin, lipofuscin, and melanin

Nucleus

• The nucleus can vary in number from being single (mononucleated) in most cells, binucleated in liver cells, or multinucleated in skeletal muscle cells
• RBCs and blood platelets have no nuclei
• Can be small, medium, or large, with a diameter of 3-14 µm in diameter
• Vary in shape - spherical, oval, kidney-shaped, segmented (lobulated), or horse shoe-shaped
• Basophilic due to its DNA and RNA content, and appearance can be open-face (vesicular) or condensed
• Includes a nuclear envelope (membrane), nucleolus, chromatin, and nuclear sap

Chromatin

• The basophilic material forms chromosomes
• Consists of nucleoprotein (DNA + histone proteins)
• Comes in two types, heterochromatin and euchromatin

Types of Chromatin

• Euchromatin is the extended parts of the chromosomal threads with active genes that directs protein synthesis
• Heterochromatin is the coiled parts of the chromosomal threads with inactive genes that do not direct protein synthesis