Cell Physiology Study Notes

Cell Physiology

Introduction

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

• References: Human Physiology by S.I. Fox and Human Anatomy by Marieb & Mallatt.

• Focus: Cell Physiology.

The Cell

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

• Living organisms are cellular: amoebas (unicellular) vs. humans, animals, and plants (multicellular).

• Human body contains 50 to 100 trillion cells.

General Structure of a Cell

• Three main regions:

  • Plasma membrane

  • Cytoplasm

  • Nucleus

• Cellular components include:

  • Microvilli

  • Plasma membrane

  • Smooth endoplasmic reticulum

  • Nuclear envelope

  • Mitochondrion

  • Lysosome

  • Centrioles

  • Secretion granule

  • Microtubules

  • Microfilaments

  • Nucleolus

  • Rough endoplasmic reticulum

  • Golgi apparatus

Plasma Membrane (Plasmalemma)

• Outer, thin, and flexible membrane.

• Separates intracellular from extracellular compartments (fluids).

Structure of the Plasma Membrane

• Made of a double layer of lipids:

  • Phospholipids

  • Cholesterol

  • Glycolipids

• Proteins are embedded within the lipid bilayer.

Phospholipids

• Most abundant lipids in the plasma membrane.

• Heads are hydrophilic (attracted to water).

• Tails are hydrophobic (avoid water).

• Heads line inner and outer faces of the membrane; tails align in the center.

Membrane Proteins

• Two types:

  • Integral proteins

  • Peripheral proteins

Integral Proteins

• Most abundant proteins in the membrane.

• Most extend entirely through the membrane (transmembrane).

• Some protrude from one side.

• Can act as receptors.

Peripheral Proteins

• Mainly on the cytoplasmic side.

• Support the cytoplasmic side of the membrane via a network of filaments.

Glycocalyx

• Sugar covering or cell coat.

• Short chain of carbohydrates projected from the external surface of glycoproteins or glycolipids.

• Functions in cell-to-cell binding and recognition.

• Produced by corneal epithelial surface cells.

• Helps bind mucins onto the corneal surface in the tear film.

Functions of the Plasma Membrane

1. External cell barrier against substances and forces outside the cell.

2. Externally facing proteins act as receptors (hormones, neurotransmitters) and in cell-to-cell recognition.

3. Transport of substances into or out of the cell.

   • Selective permeability: Allows some substances to pass between intra- and extracellular fluids, while preventing others.

Movements of Substances Across the Plasma Membrane

1. Passive process:

   • Substances pass freely through the lipid bilayer down their concentration gradient (from high to low concentration).

   • No energy (ATP) is needed.

   • Diffusion: movement of small, uncharged molecules like oxygen, $CO_2$ and fat-soluble molecules.

2. Active process:

   • Substances move against a concentration gradient (from low to high concentration).

   • ATP is needed.

   • Active transport: Larger water-soluble or charged molecules (glucose, amino acids, ions) are transported by pumps or carriers involving integral proteins.

3. Vesicular or bulk transport:

   • Large particles and macromolecules pass through the membrane via exocytosis and endocytosis.

Exocytosis

• Membrane-lined cytoplasmic vesicles fuse with the plasma membrane and release their contents outside the cell.

• Example: mucus and protein secretions from glands.

• vSNAREs (vesicle membrane proteins) bind with tSNAREs (target plasma membrane proteins), causing lipid layers to join.

Endocytosis

• Brings large molecules into the cell through infolding of the plasma membrane to form cytoplasmic vesicles.

• Clathrin protein deforms the membrane.

• Three types: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Phagocytosis (Cell Eating)

• Plasma membrane forms pseudopods that flow around large molecules (bacteria, cellular debris) and engulf them.

• A membranous vesicle called a phagosome is formed.

• Phagosomes fuse with lysosomes for enzymatic breakdown of contents.

• White blood cells exhibit phagocytic activity.

Pinocytosis (Cell Drinking)

• Fluid phase endocytosis.

• Small infolding of plasma membrane surrounds a small quantity of extracellular fluid with dissolved molecules.

• Main function of cells lining the small intestine: nutrient absorption.

Receptor-Mediated Endocytosis

• Selective mechanism.

• Specific molecules (insulin, hormones, enzymes, LDLs) bind to receptors on the membrane before being taken into the cell in a protein-coated vesicle.

• Vesicle contents are released by binding to lysosomes, and receptors are recycled back to the plasma membrane.

• Viruses and some toxins use this mechanism to enter cells.

Familial Hypercholesterolemia

• Inherited disease in which cells lack receptors that bind to cholesterol-binding LDLs.

• Cholesterol cannot enter cells and builds up in the blood, causing hypercholesterolemia and atherosclerosis.

• Can lead to stroke or myocardial infarction.

The Cytoplasm

• Cellular region between the nucleus and plasma membrane.

• Consists of:

  • Cytosol (cytoplasmic matrix): viscous fluid containing water, ions, and enzymes.

  • Inclusions: stored nutrients and pigments.

  • Organelles.

Ribosomes

• Dark-staining granules with no membrane.

• Site of protein production.

• Two subunits: protein and ribosomal RNA (rRNA).

• Free ribosomes: make proteins used in the cytosol.

• Ribosomes on rER: make proteins for cell membrane or export out of the cell.

• Amino acids are linked together to form proteins via translation, dictated by DNA in the nucleus and carried out by messenger RNA (mRNA).

Rough Endoplasmic Reticulum (rER)

• Ribosome-studded system of membrane-walled envelopes (cisternae) in the cytosol.

• Ribosomes on rER make proteins that enter cisternae and are secreted by the cell in vesicles.

• Also makes proteins of the cell membrane.

Smooth Endoplasmic Reticulum (sER)

• Network of membranous sacs and tubules in the cytosol.

• No ribosomes.

• Involved in:

  • Synthesis of lipids and steroids

  • Lipid metabolism

  • Drug detoxification

Golgi Apparatus

• Stack of 3-10 disc-shaped envelopes or cisternae, bound by membrane.

• Cisternae have a cis (convex) and trans (concave) face.

• Sorts products of rER and packs them in membrane-bound vesicles.

• Sends vesicles to proper destination.

• Secretory granules and lysosomes arise from the Golgi apparatus.

Mitochondria

• Rod-like organelles covered by two membranes in the cytoplasm.

• Inner membrane is folded into projections called cristae.

• Main energy generator of the cell.

• Main site of ATP synthesis.

Lysosomes

• Spherical, membrane-walled sacs containing digestive enzymes (acid hydrolases).

• Site of intracellular digestion.

• Destroy (digest) deteriorated organelles and substances brought into cells by vesicles.

• Fuse with phagosomes and empty enzymes into phagosomes to break down contents.

• Phagocytic cells have many lysosomes.

Tay-Sachs Disease

• Inherited disease.

• Infants lack specific enzymes in lysosomes responsible for breaking down certain glycolipids.

• Glycolipids accumulate in cell membranes (especially neurons).

• Results in mental retardation, blindness, spastic movements, and death within 1.5 years from birth.

Gaucher’s Disease

• Lack of the glucocerebrosidase enzyme causes harmful substances to build up in the liver, spleen, bones, and bone marrow.

• Substances prevent cells and organs from working properly.

• Three main subtypes:

  1. Type 1: most common; involves bone disease, anemia, enlarged spleen, and thrombocytopenia; affects children and adults.

  2. Type 2: begins in infancy with severe neurologic involvement; leads to rapid, early death.

  3. Type 3: may cause liver, spleen, and brain problems; patients may live into adulthood.

• Symptoms vary depending on the type of disease but may include:

  • Bone pain and fractures

  • Enlarged spleen

  • Enlarged liver

  • Lung disease

  • Seizures

Peroxisomes

• Membrane-walled, enzyme-containing sacs.

• Contain oxidase and catalase enzymes.

• Oxidases use oxygen to neutralize free radicals by converting them to hydrogen peroxide.

• Catalases convert hydrogen peroxide to oxygen and water, breaking down poisons (alcohol, phenol, formaldehydes).

• Liver and kidney have many peroxisomes.

Cytoskeleton (Cell Skeleton)

• Network of rods running throughout the cytosol to support cellular structure and generate movements.

• Three types: Microtubules, microfilaments, and intermediate filaments.

Microtubules

• Cylindrical structures made of tubulin proteins.

• Radiate out from the centrosome region near the nucleus.

• Give the cell its shape.

• Organize the distribution and transport of organelles within the cytoplasm.

Microfilaments (Actin Filaments)

• Fine filaments of contractile protein called actin.

• Labile.

• Actin interacts with myosin to generate contractile forces within the cell.

• Involved in muscle contraction and other cellular movements like amoeboid movements and extension of pseudopods.

Intermediate Filaments

• Tough, insoluble, and stable protein fibers.

• Resist tension placed on the cell.

Centrosome and Centrioles

• Centrosome: Spherical structure in the cytoplasm near the nucleus.

  • Consists of a matrix (outer cloud of protein) and an inner pair of centrioles.

  • Matrix protein is involved in elongation of microtubules; mitotic spindle of microtubules radiates from it in dividing cells.

• Cytoplasmic inclusions: Impermanent structures in the cytoplasm such as lipid droplets and glycogen-containing glycosomes.

Centrioles

• Paired cylindrical bodies perpendicular to one another, composed of nine triplets of microtubules.

• Organize a microtubule network during mitosis to form the spindle and asters.

• Form the bases of cilia and flagella.