Cell Biology: Plasma Membrane and Transport Mechanisms

Introduction to Human Physiology

• Human physiology integrates the functions of all the body's cells, tissues, and organs into a whole.
• Separate organs and systems work together to maintain the body's proper function.
• Feedback controls are crucial for maintaining necessary balances.

Cell Physiology

• The Cell: The smallest, self-replicating unit of integrated physiological function.

  • Specialized cells work together in a complex organism.
  • Functional integration results from interactions between specialized cells.

• Cell Components:

  • Plasma membrane: Serves as a permeability barrier, determining what enters and leaves the cell.
  • Extracellular fluid (ECF) and intracellular fluid (ICF) have different compositions.
  • All cells have the same DNA, but not all genes are expressed equally.
  • The organism is composed of cooperating cell types, each contributing unique functions to the whole.

• Cell Composition:

  • CHOS: ~3% of dry mass
  • Lipids: ~40% of dry mass
  • Proteins: ~50-60% of dry mass
  • Nucleic acids: DNA + RNA

• Cell Functions:

  • Obtain food and oxygen from surroundings
  • Eliminate waste products
  • Perform various chemical reactions
  • Synthesize materials for cellular structure, growth, and function
  • Respond to changes in surroundings
  • Reproduce (most cells, excluding nerve and muscle cells)
  • Move materials in and out of the cell and within the cell to carry out cellular activities.

• Cell Organelles:

  • Non-membrane limited: Chromosomes, Nucleoli, Ribosomes, Microtubules, Microfilaments, Centrioles
  • Membrane limited: Nucleus, Endoplasmic reticulum (ER), Golgi apparatus (GA), Lysosomes, Mitochondria, Peroxisomes

• Plasma Membrane Details:

  • 7.5nm thick
  • Composed of a phospholipid bilayer and protein
  • Selective barrier to ion/molecule movement

• Mitochondria:

  • Rod- or oval-shaped, double-membraned organelles
  • Major site of ATP production, oxygen utilization, and CO2 formation
  • Contain enzymes of the Krebs cycle and oxidative phosphorylation

• Endoplasmic Reticulum:

  • Interconnected cell organelle with two opposing membranes
  • Granular ER: Ribosomal particles bound to ER membrane, synthesizes proteins meant for secretion.
  • Agranular ER: No ribosomes, synthesizes fatty acids and steroid hormones, stores calcium in muscle cells

• Golgi Apparatus:

  • Cup-shaped membranous sacs and vesicles
  • Modifies and concentrates proteins prior to secretion

• Secretory Vesicles:

  • Membrane-bound sacs with concentrated protein solutions
  • Release proteins into the environment

• Lysosomes:

  • Density-staining oval bodies with hydrolytic enzymes
  • Digest engulfed bacteria and damaged cell organelles

• Peroxisomes:

  • Play essential roles in metabolism and ROS (reactive oxygen species) detoxification

• Free Ribosomes:

  • 20nm particles of RNA and protein
  • Assemble amino acids into proteins for intracellular use

• Bound Ribosomes/Filaments:

  • Attached to the granular endoplasmic reticulum
  • Composed of protein threads, involved in protein synthesis for secretion and cell movements
  • Support at cell junctions

• Microtubules:

  • Protein tubules with a hollow core (25nm diameter)
  • Maintain cell shape (cytoskeleton)
  • Associated with cilia, flagella, and the mitotic spindle

• Centrioles:

  • Two small cylindrical bodies with fused microtubules
  • Form the spindle apparatus during cell division
  • Involved in cilia formation and movement

• Granules:

  • Aggregates of chemical substance crystals
  • Store specialized end products of metabolism, glycogen granules most common.

• Fat droplets:

  • Spherical globules of triacylglycerol
  • Store fat

• Nuclear Envelope:

  • Surrounds the nucleus with two membranes
  • Nuclear pores (50-70 nm) control molecule movement.
  • Messenger RNA exits through pores.

• Chromatin:

  • Coiled DNA and protein threads
  • 46 strands per human cell nucleus
  • Stores genetic information
  • Condenses into chromosomes during cell division

• Nucleolus:

  • Coiled filamentous structure containing granules
  • Not membrane bound
  • Site of ribosomal RNA (rRNA) synthesis

Cell Membrane

• Composition:

  • Lipids: Phospholipids and cholesterol
  • Proteins: Peripheral and integral
  • Carbohydrates: Polysaccharides

• Lipids:

  • Phospholipids: Amphipathic molecules, glycerol backbone, hydrophobic tails, hydrophilic head; form a bilayer.

• Proteins:

  • Peripheral: hydrophilic, loosely attached, regulate cell function
  • Integral: partly hydrophillic and hydrophobic, embedded, transport molecules, receptors.

• Carbohydrates:

  • Hydrophilic polysaccharides bound to proteins or lipids ( glycoproteins and glycolipids)

• Ion Channels:

  • Leak: continuously open
  • Voltage-gated: regulated by membrane potential changes
  • Ligand-gated: regulated by chemical signals

Cellular Connections

• Tight junctions (zonula occludens): Intercellular complexes controlling paracellular permeability, forming borders between apical and basolateral domains in polarized epithelia.
• Gap junctions: Clusters of intercellular channels facilitating direct ion and small molecule diffusion between adjacent cells, formed by connections (connexons) of trans-membrane proteins (connexins).
• Desmosomes: Intercellular junctions anchoring intermediate filaments to the plasma membrane, critical for stable cell-cell adhesion in tissues experiencing structural stress.
• Adherence junctions (zonula adherens): Cell-cell adhesion complexes allowing for responses to forces and changes in the microenvironment and providing strong mechanical attachments

Cellular Communication

• Cells detect environmental changes by using membrane-embedded receptors.
• Mechanisms of chemical signaling include autocrine, paracrine, synaptic, endocrine, and neuroendocrine mechanisms

Body Fluid Composition and Electrolyte Balance

• **Body Fluid Compartments:**Plasma membrane separates ICF and ECF
  • ECF (extracellular fluid): fluids in blood vessels, interstitial fluid, and transcellular fluid.
  • ICF (intracellular fluid): fluid inside cells
• Fluid composition: Main cations in ECF = Na+; ICF = K+
  • Na(+)-K(+)-ATPase is crucial in maintaining these different concentrations.
  • Main anions in ECF = Cl-, HCO3-; Anions in ICF = proteins and phosphates.
• Distribution of solutes: based on electrochemical activity and osmotic activity.

Transport Across the Cell Membrane

• Passive transport: Does not require metabolic energy.
  • Simple diffusion: Movement of a substance from higher concentration to lower.
  • Facilitated diffusion: Movements of molecules with assistance from membrane proteins.
  • Osmosis: Passive movement of water across a membrane from high to low concentrations.
  • Filtration: Fluid forced across a membrane due to pressure differences.
• Active transport: Needs metabolic energy.
  • Primary active transport: moving substances against concentration gradients using ATP directly.
    • Examples: Na(+)-K(+)ATPase, Ca(2+)ATPase, H(+)-K(+)ATPase.
  • Secondary active transport: Couples downhill movement of one substance to uphill movement of another.
    • Examples: Na(+)-glucose cotransporter, Na(+)-Ca(2+) exchanger, Na(+)-K(+)-2Cl- cotransporter
• Bulk transport: Transporting large molecules.
  • Endocytosis: engulfing material into the cell through invagination of the membrane.
    • Phagocytosis: Large molecules, such as bacteria, are engulfed
    • Pinocytosis: Liquid is engulfed
  • Exocytosis: releasing material from the cell through fusion of secretory vesicles.

Physiological Adjustment

• Homeostasis: Maintaining a more or less constant internal environment.
  • Intrinsic regulation: cells, tissues, and organs self-regulate based on their local conditions.
  • Extrinsic regulation: nervous and endocrine systems coordinate long-distance adjustments.
• Allostasis: Maintaining stability via acute changes. Adapting to and coping with stress.
• Allostatic load: The cost of prolonged adaptation to stress.
• Heterostasis (Adaptation): Long-term changes to adapt or cope with the environment (e.g., adjusting to high altitude, cold, or heat).