1- Cell Physiology-1

Lecturer Information

• Dr. R. Ahangari, University of Central Florida, Orlando

• Primary texts: Human Physiology by S.I. Fox and Human Anatomy by Marieb & Mallat

Cell Basics

• Cells: Basic structural and functional units of life

• Types of Cells:

  • Unicellular (e.g., amoebas)

  • Multicellular (e.g., humans, animals, large plants)

• Cell Count: Approximately 50 to 100 trillion cells in the human body

General Structure of a Cell

• Three main regions:

  1. Plasma Membrane

  2. Cytoplasm

  3. Nucleus

• Important Cellular Components:

  • Centrioles, secretion granules, microtubules, microfilaments, nucleolus, microvilli, rough endoplasmic reticulum, Golgi apparatus, smooth endoplasmic reticulum, nuclear envelope, mitochondria, lysosomes

Plasma Membrane (Plasmalemma)

• Definition: Thin and flexible membrane surrounding the cell

• Function: Separates intracellular fluid from extracellular fluid

Structure of the Plasma Membrane

• Composition:

  • Double layer of lipids (phospholipids, cholesterol, glycolipids)

  • Embedded proteins

Phospholipids

• Characteristics:

  • Heads: Hydrophilic, facing water (intra- and extracellular)

  • Tails: Hydrophobic, facing inwards, avoiding water

Membrane Proteins

• Types:

  1. Integral Proteins:

     • Extend wholly or partially through the membrane

     • Can act as receptors

  2. Peripheral Proteins:

     • Located mainly on cytoplasmic side

     • Support cytoskeleton via filament networks

• Glycocalyx:

  • Sugar covering for cell identification and binding

Glycocalyx Tear Film

• Function: Helps bind mucins to the corneal surface

Functions of the Plasma Membrane

• Acts as a barrier against external substances

• Functions in cell-to-cell communication via receptors

• Selectively permeable: Regulates movement of substances

Movements of Substances Across Plasma Membrane

• Passive Process:

  • Movement down concentration gradient (diffusion)

  • No ATP required

• Active Process:

  • Movement against concentration gradient, ATP needed

  • Involves pumps or carriers

• Vesicular/Bulk Transport:

  • Mechanism for large particles or macromolecules

  • Types: Exocytosis and endocytosis

Exocytosis

• Mechanism: Cytoplasmic vesicles fuse with the plasma membrane

• Function: Releases contents outside the cell (e.g., mucus, proteins)

• Proteins involved: vSNAREs (vesicle) and tSNAREs (target)

Endocytosis

• Mechanism: Plasma membrane engulfs large molecules to form vesicles

• Involves clathrin for membrane deformation

• Types of Endocytosis:

  1. Phagocytosis (cell eating)

  2. Pinocytosis (cell drinking)

  3. Receptor-mediated endocytosis

Phagocytosis

• Mechanism: Membrane forms pseudopodes to envelop large molecules

• Result: Formation of phagosome, which fuses with lysosomes for digestion

• Example: Activity of white blood cells

Pinocytosis

• Mechanism: Membrane engulfs small quantities of extracellular fluid

• Function: Mainly occurs in intestinal cells for nutrient absorption

Receptor-Mediated Endocytosis

• Mechanism: Specific molecules bind to membrane receptors before being internalized

• Examples: Insulin, hormones, LDL

• Important in virus and toxin entry

Familial Hypercholesterolemia

• Condition: Lack of LDL receptors leads to high blood cholesterol

• Consequences: Increased risk of stroke and myocardial infarction

The Cytoplasm

• Definition: Region between the nucleus and plasma membrane

• Components: Cytosol, inclusions, organelles

Ribosomes

• Structure: Dark granules, no membrane

• Function: Site of protein synthesis

• Types:

  • Free ribosomes: Proteins for cytosol

  • Attached ribosomes: Proteins for membranes or secretion

• Process: Translation guided by mRNA from the nucleus

Rough Endoplasmic Reticulum (rER)

• Structure: Membrane-walled envelopes in cytosol

• Function: Synthesis of proteins

Smooth Endoplasmic Reticulum (sER)

• Structure: Tubular membranous sacs in cytosol, lacks ribosomes

• Function: Lipid and steroid synthesis, detoxification

Golgi Apparatus

• Structure: Stack of membrane-bound cisternae

• Function: Sorting, packaging, and sending proteins from rER

Mitochondria

• Structure: Rod-like organelles with double membranes

• Function: Main energy generator, site of ATP synthesis

Lysosomes

• Structure: Membrane-walled sacs with digestive enzymes

• Function: Intracellular digestion, destruction of worn organelles

• Role in diseases: Tay-Sachs disease (enzyme deficiency)

Gaucher’s Disease

• Condition: Lack of glucocerebrosidase enzyme leading to harmful substance accumulation

• Subtypes: Type 1 (common), Type 2 (infantile), Type 3 (adult onset)

Peroxisomes

• Structure: Membrane-walled sacs containing oxidase and catalase

• Function: Neutralization of free radicals, detoxification

Cytoskeleton

• Definition: Network of rods supporting cell structure and motion

• Types: Microtubules, microfilaments, intermediate filaments

Microtubules

• Structure: Cylindrical made of tubulin proteins

• Function: Maintain cell shape, organelle transport

Microfilaments (Actin Filaments)

• Structure: Fine filaments of actin protein

• Function: Involved in cell movement and muscle contraction

Centrosome and Centrioles

• Definition: Structure near the nucleus with an outer matrix and inner centrioles

• Function: Organizes microtubule network for cell division and forms bases of cilia and flagella