Cell Physiology

• Introduction to Cells

  • Cells are the building blocks of all organisms
  • Function of cell depends upon specific structural properties
  • All cells come from the division of preexisting cells
  • Cells are the smallest units that perform all vital physiological functions
  • Each cell maintains homeostasis at the cellular level
• Subdivisions of the cell (4 major subdivisions)

  - Plasma Membrane

  • Encloses the cell

  - Cytoplasm

  • Gelatinous liquid that fills the inside of the cell

  - Organelles

  • Structures with specific functions (e.g., nucleus, mitochondria

  - Nucleus

  • Houses genetic material
• PLASMA MEMBRANE FUNCTIONS

  - Physical isolation 

  • Barrier

  - Regulation of exchange with the environment

  • Ions and nutrients enter

  • Wastes eliminated and cellular products released

  - Sensitivity to the environment

  • Extracellular fluid composition and chemical signals

  - Structural support

  • Anchors cells and tissues
• Plasma membrane

  - Phospholipid bilayer

  • Hydrophilic heads - face outward on both sides, toward watery environments

  • Hydrophobic fatty - acid tails - inside membrane

  • Barrier to ions and water - soluble compounds
• Membrane proteins (and channels)

  - Integral proteins

  • Within the membrane

  - Peripheral proteins

  • Bound to inner or outer surface of the membrane

  - Anchoring proteins (stabilizers)

  • Attach to inside or outside structures

  - Recognition proteins (identifiers)

   • Label cells as normal or abnormal

  - Enzymes

  • Catalyze reactions
• Membrane proteins (and channels)

  - Receptor proteins

  • Bind and respond to ligands (ions, hormones)

  - Carrier proteins

  • Transport specific solutes through membrane

  - Channels

  • Regulate water flow and solutes passing through membrane

  • Gated channels open or close to regulate passage of substances
• Membrane carbohydrates

  - Proteoglycans, glycoproteins, and glycolipids

  • Form sticky “sugarcoat” (glycocalyx)

  • Extend outside cell membrane

  - Functions of the glycocalyx

  • Lubrication and protection

  • Anchoring and locomotion

  • Specificity in binding (function as receptors)

   • Recognition (immune response)
• Organelles (in cell and cytoplasm)

  - Cytoplasm

  • All materials inside the cell, outside of the nucleus

  - Cytosol (intracellular fluid)

  • Contains dissolved materials

  - Nutrients, ions, proteins, and waste products

  • High protein and potassium levels

  • Low carbohydrate, lipid, amino acid, and sodium levels

  - Organelles

  • Structures with specific functions (e.g. nucleus, mitochondria)
• Organelles

  - Non-membranous organelles

  • No membrane

  • Direct contact with cytosol

  • Include the cytoskeleton, centrioles, ribosomes, proteasomes, microvilli, cilia, and flagellaI

  - Membranous organelles

  • Isolated from cytosol by a plasma membrane

  • Nucleus, Endoplasmic reticulum (ER), the Golgi apparatus, lysosomes, peroxisomes, and mitochondria
• Non-membranous organelles

  - Cytoplasmic Inclusions

  • Masses of insoluble materials in cells

  • Some consist of glycogen or lipid droplets

  - Cytoskeleton

  • Structural proteins for shape and strength

  • Microfilaments

  • Intermediate filaments

    • Microtubules
• Cytoskeleton

  - Microfilaments — thin filaments composed of the protein actin

  • Provide mechanical strength

  • Interact with other proteins to adjust consistency of cytosol

   • Interact with thick filaments of myosin for muscle contraction

  - Intermediate filaments — mid-sized between microfilaments and microtubules

   • Durable

  • Strengthen the cell and maintain its shape

  • Stabilize position of organelles

  • Stabilize cell position
• Cytoskeleton

  - Microtubules — large, hollow tubes of tubulin proteins

  • Attach to centrosome

  • Strengthen cell and anchor organelles

  • Change cell shape

  • Move organelles within the cell with the help of motor proteins(kinesin and dynein)

  • Form spindle apparatus to distribute chromosomes

  • Form centrioles and cilia of organelles
• Organelles

  - Microvilli

  • Increase surface area for absorption

   • Attach to cytoskeleton

  - Centrioles

  • Form spindle apparatus during cell division

  • Centrosome — cytoplasm next to the nucleus that surrounds centrioles
• Organelles

  - Cilia (singular, cilium)

  • Slender extensions of plasma membrane

   • Move fluids across the cell surface

  • A primary cilium is nonmotile

  - Found on a variety of cells

  - Senses environmental stimuli

  • Motile cilia are found on cells lining the respiratory and reproductive tracts

  - Microtubules in cilia are anchored to a basal body

  - Flagellum is whip-like extension of cell membrane
• Organelles

  - Ribosomes — organelles that synthesize proteins 

  • Composed of small and large ribosomal subunits

  - Contain ribosomal RNA (rRNA)

  • Free ribosomes in cytoplasm

  - Manufacture proteins that enter cytosol directly

  • Fixed ribosomes are attached to ER

  -  Manufacture proteins that enter ER for packaging

  - Proteasomes

  • Organelles that contain enzymes (proteases)

  • Disassemble damaged proteins for recycling
• Endoplasmic reticulum

  • Contains storage chambers known as cisternae

   • Functions

  1. Synthesis of proteins, carbohydrates, and lipids
  2. Storage of synthesized molecules and materials
  3. Transport of materials within the ER
  4. Detoxification of drugs or toxins
• Endoplasmic Reticulum

  - Smooth endoplasmic reticulum (SER)

   • No attached ribosomes

  • Synthesizes

  - Phospholipids and cholesterol (for membranes)

  - Steroid hormones (for reproductive system)

  - Glycerides (for storage in liver and fat cells)

  - Glycogen (for storage in muscle and liver cells)

  - Rough endoplasmic reticulum (RER)

   • Surface covered with ribosomes

  - Active in protein and glycoprotein synthesis

  - Folds proteins into secondary and tertiary structures

  - Encloses products in transport vesicles for delivery to Golgi apparatus
• Golgi apparatus (post office)

  - Functions

  1. Modifies and packages secretions for release from cell (hormones, enzymes)
  2. Adds or removes carbohydrates to or from proteins
  3. Renews or modifies the plasma membrane
• Lysosomes

  - Powerful enzyme - containing vesicles produced by Golgi apparatus

  - Primary lysosomes

  • Contain inactive enzymes

  - Secondary lysosomes

  • Formed when primary lysosomes fuse with damaged organelles and enzymes are activated

  - Function to destroy bacteria, break down molecules, and recycle damaged organelles
• Peroxisomes

  • Small, enzyme - containing vesicles
  • Produced by division of existing peroxisomes
  • Break down organic compounds such as fatty acids
• Mitochondria

  - Smooth outer membrane

  - Inner membrane has numerous folds (cristae)

  • Cristae surround fluid contents (matrix)
• Mitochondria

  - Take chemical energy from food (glucose)

  • Produce the energy molecule ATP (uses oxygen)

  • Cellular Metabolism

    - Catabolism - Breakdown of complex molecules into simple ones 

  – Anabolism - Synthesis of molecules to build/support organs andtissues

  - Happens simultaneously

  • "A B C D" : Anabolism = Biosynthesis ; Catabolism = Degradation
• Recipe for ATP

  1. Glycolysis

  ▪ Glucose to pyruvic acid (in cytosol)

  ▪ Mitochondria absorb pyruvate molecules

  2. Citric acid cycle (Krebs cycle, tricarboxylic acid cycle, or TCA cycle)

  ▪ Occurs in mitochondrial matrix

  ▪ Breaks down pyruvate

  3. Electron transport chain

  ▪ Inner mitochondrial membrane, changes electrical stability for influx and efflux of produces

  - 3 steps occur simultaneously
• Aerobic vs Anaerobic

  - glucose brought in, out goes ATP absorbs oxygen and short carbon chains, pyruvate and generate carbon dioxide, ATP and water

  VS

  - at times of inadequate oxygen, pyruvate is used to turn pyruvic acid to lactic acid for ATP

  • exercising, build up of lactic acid in cells cause side effects of cramping
• Transporting across a plasma membrane

  - Plasma membrane is selectively permeable (semi-permeable)

  • Allows some materials to move freely

  • Restricts other materials based on their:

  - Size

  - Electrical charge (creating membrane potential?)

  - Molecular shape

  - Lipid solubility
• Transport through plasma membrane

  - Transport through plasma membrane can involve passive processes (no energy required) or active processes (requiring energy)

  • Passive: Diffusion, osmosis, facilitated diffusion and sometimes carrier-mediated transport

  • Active: Vesicular and sometimes carrier-mediated transport
• Passive transport

  - Diffusion

  • Net movement of a substance across membrane from area of higher concentration to area of lower concentration

  • Ions and molecules are constantly in motion to maintain specific environment

  • Concentration gradient

  - Substances move from area of HIGH concentration to area of LOW concentration
• Diffusion

  - Factors influencing diffusion

  • Distance the particle has to move

  • Ion and molecule size

  - Smaller = faster diffusion

  • Temperature

  - More heat = faster diffusion

   • Concentration gradient

  - Steeper gradient = faster diffusion

   • Electrical forces

  - Opposites attract, like charges repel
• Diffusion across plasma membrane

  - Simple diffusion (smaller products, quicker easier)

  • Lipid-soluble compounds (alcohols, fatty acids, and steroids)

  • Dissolved gases (oxygen and carbon dioxide)

   • Water molecules

  - Channel-mediated diffusion (larger)

   • Water-soluble compounds and ions

   • Affected by size, charge, and interaction with channel walls
• Osmosis

  - Diffusion of water across a selectively permeable membrane

  - Water molecules diffuse across a membrane toward the solution with more solutes

  - At equilibrium, the solute concentrations on the two sides of the membrane are equal.

  - The volume of solution that has more solutes, increased at the expense of that of solution that has less solutes, changing solubility of solution
• Osmotic pressure

  - Osmosis can be prevented by resisting the change in volume.

  - If the osmotic pressure of solution that has more solubles is equal to the amount of hydrostatic pressure required to stop the osmotic flow:

  • The osmotic flow from solution with less solubles is stopped from going to solution with more.
• Osmotic terms

  - Hydrostatic (fluid) pressure

  • Pressure of standing fluid (water) on membrane

  - Osmotic pressure

  • The force with which pure water moves into a solution as a result of solute concentration

  • Hydrostatic pressure is the pressure needed to block osmosis

  - Water moves until osmotic pressure = hydrostatic (fluid) pressure
• Osmotic terms

  - Osmosis occurs more rapidly than solute diffusion

  • Because water can cross a membrane through abundant water channels (aquaporins)

  • Aquaporins outnumber solute channels

  - Osmolarity (osmotic concentration)

    • Total solute concentration in a solution
• Tonicity & Osmolarity

  - The amount of solution and concentration of solute in the solution will cause the cell to gain or lose water, changing cell volume & shape

  - Osmolarity describes the concentration of solute in a volume of solution

   - Tonicity describes how that solution affects cells
• Tonicity & Osmolarity

  • Depends on the nature of the solutes

  • Isotonic solution (iso = same, tonos = tension)

    - Does not cause osmotic flow!

  • Hypotonic solution (hypo = below)

    - Lower solute concentration than the cell

  • Hypertonic solution (hyper = above) 

    - Higher solute concentration than the cell
• Tonicity & Osmolarity

  - The number of solute particles per volume of solvent

  - When considering both tonicity and osmolarity...

  • A cell in an isotonic solution = no osmotic flow, cell stays same size and shape

  • A cell in a hypotonic solution = cell gains water which may lead to cell rupture (hemolysis)

  • A cell in a hypertonic solution = cell loses water and may shrink (cremation)
• Carrier mediated transport (across membrane)

  - Proteins transport ions or organic substrates across plasma membrane, depends on:

  - Specificity

  • One transport protein, one set of substrates

  - Saturation limits

  • Rate depends on availability of transport proteins and substrates

  - Regulation

  • Cofactors such as hormones affect activity of carriers
• Carrier Mediated Transport direction

  - Symport (cotransport)

  • Two substances move in the same direction at the same time

  - Antiport (counter-transport)

  • One substance moves in while another moves out
• Carrier Mediated Transport

  - Facilitated diffusion

  - Passive

  - Across concentration gradients

  - Carrier proteins transport molecules too large to fit through channel proteins (glucose, amino acids)

  • Molecule binds to receptor site on carrier protein

  • Protein changes shape, molecule passes through

  • Receptor site is specific to certain molecules

  - Limited by # of binding sites
• Carrier Mediated Transport

  - Active transport

  - Proteins move substrates against concentration gradients

  - Requires energy, such as ATP

  • Ion pumps move ions (Na+, K+, Ca2+, Mg2+)

  • Exchange pumps move two ions in opposite directions at the same time

  - E. g. , Iodine needs to move from blood (low conc. ) into thyroid gland (high conc.)

  - ATP needed to work against concentration gradient