1- Cell Physiology-1

Overview

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

• Texts: Human Physiology by S.I. Fox; Human Anatomy by Marieb & Mallat

Cells

• Basic Units of Life:

  • Cells are the fundamental structural and functional units of life.

  • Examples include unicellular organisms like amoebas and multicellular organisms like humans, animals, and large plants.

  • Human body contains approximately 50 to 100 trillion cells.

Cell Structure

• Three Main Regions:

  1. Plasma Membrane

  2. Cytoplasm

  3. Nucleus

• Other Components:

  • Centrioles

  • Secretion granules

  • Microtubules

  • Microfilaments

  • Nucleolus

  • Microvilli

  • Rough endoplasmic reticulum (rER)

  • Golgi apparatus

  • Smooth endoplasmic reticulum (sER)

  • Nuclear envelope

  • Mitochondria

  • Lysosomes

Plasma Membrane

• Definition:

  • The outer thin and flexible membrane of the cell, separating intracellular from extracellular compartments.

Structure of the Plasma Membrane

• Composition:

  • Comprised of a double layer of lipids, including:

    • Phospholipids

    • Cholesterol

    • Glycolipids

  • Proteins embedded within the lipid bilayer.

Phospholipids

• Characteristics:

  • Heads are hydrophilic (water-attracting), facing both intracellular and extracellular fluids.

  • Tails are hydrophobic (water-repelling), aligning in the center of the membrane.

Membrane Proteins

• Types of Proteins:

  1. Integral proteins: Extend across the membrane, may act as receptors.

  2. Peripheral proteins: Attach mostly to the cytoplasmic side, providing support through a filament network.

• Glycocalyx:

  • Carbohydrate chains extending from glycoproteins and glycolipids, facilitating cell-to-cell binding and recognition.

Functions of the Plasma Membrane

1. Serves as a barrier against external substances.

2. Proteins on the exterior function as receptors (e.g., for hormones) and facilitate cell recognition.

3. Regulates transport of substances in and out, acting as a selectively permeable barrier.

Transport Mechanisms across the Plasma Membrane

• Passive Processes:

  • Substances move freely down their concentration gradient (e.g., diffusion of O2, CO2).

• Active Processes:

  • Movement against a concentration gradient requiring ATP (e.g., active transport of glucose).

• Vesicular Transport:

  • Bulk transport involving larger molecules through processes like exocytosis and endocytosis.

Exocytosis

• Definition:

  • Cytoplasmic vesicles fuse with plasma membrane to release contents outside the cell.

• Example:

  • Secretion of mucus and proteins by glands.

Endocytosis

• Definition:

  • Brings large molecules into the cell via internal membrane infolding that forms vesicles.

• Types:

  1. Phagocytosis:

     • Engulfing of large particles (e.g., bacteria) forming a phagosome that fuses with lysosomes.

  2. Pinocytosis:

     • Cell drinking involving the uptake of extracellular fluid and dissolved molecules.

  3. Receptor-Mediated Endocytosis:

     • Specific uptake of molecules like insulin following receptor binding and forming protein-coated vesicles.

Familial Hypercholesterolemia

• Definition:

  • An inherited condition preventing cells from taking in LDL cholesterol due to lack of receptors, leading to high blood cholesterol levels and associated health risks.

Cytoplasm

• Definition:

  • Cellular region between the nucleus and plasma membrane, containing:

    • Cytosol (viscous fluid with water, ions, enzymes)

    • Organelles and inclusions for nutrient storage.

Ribosomes

• Characteristics:

  • Granules with no membrane, site of protein synthesis.

  • Composed of two subunits: protein and ribosomal RNA (rRNA).

  • Free ribosomes synthesize proteins for the cytosol; bound ribosomes (on rER) for membranes/excretion.

  • Proteins are synthesized via Translation directed by nuclear DNA and messenger RNA (mRNA).

Endoplasmic Reticulum (ER)

• Rough Endoplasmic Reticulum (rER):

  • Contains ribosomes; synthesizes proteins for secretion and membrane use.

• Smooth Endoplasmic Reticulum (sER):

  • Lacks ribosomes; involved in lipid synthesis, metabolism, and detoxification.

Golgi Apparatus

• Description:

  • Stack of 3-10 disc-shaped membrane-bound cisternae.

• Function:

  • Sorts and packages products from rER into vesicles for transport.

Mitochondria

• Definition:

  • Rod-shaped organelles with two membranes, primarily responsible for ATP synthesis, the main energy source of the cell.

Lysosomes

• Description:

  • Membrane-walled sacs filled with digestive enzymes (acid hydrolases).

• Function:

  • Intracellular digestion, breaking down organelles and vesicle contents.

• Disease Example:

  • Tay-Sachs disease: enzyme deficiency leads to glycolipid accumulation, causing severe neurological impairment.

Gaucher’s Disease

• Description:

  • Caused by lack of glucocerebrosidase enzyme resulting in harmful substance accumulation.

• Types:

  1. Type 1: Most common, affects bone, blood, and spleen.

  2. Type 2: Severe infantile neuronal involvement, rapid progression.

  3. Type 3: Affects multiple organs, patients may survive into adulthood.

Peroxisomes

• Description:

  • Membrane-walled organelles containing enzymes such as oxidases and catalases.

• Function:

  • Neutralize free radicals and convert harmful substances through metabolic processes.

Cytoskeleton

• Definition:

  • Network of protein rods providing structural support and facilitating cell movement.

• Types:

  1. Microtubules: Provide shape and transport organelles.

  2. Microfilaments: Involved in cell contraction and movement.

  3. Intermediate filaments: Provide tensile strength.

Centrosome and Centrioles

• **Definition: **

  • Centrosome is near the nucleus consisting of a protein matrix and centrioles.

• Function:

  • Organizes microtubule networks during cell division and is the basis of cilia and flagella structure.