Physiology Chapter: Membrane Structure & Function

Physiology

• Physiology is the study of biological function, focusing on how the body works from the cellular to organismal level.
• The emphasis is on mechanisms, answering "how" questions and using cause-and-effect sequences.
• Physiology studies body functions, mechanisms, and regulations in all living organisms.
• Human physiology specifically studies functions in the human body, including cells, organs, and organ systems.
• Medical physiology applies human physiological knowledge to manage dysfunctions and diseases.

Membrane Structure and Function

• In 1972, Singer and Nicolson revised the fluid mosaic model.
• Globular proteins are embedded in the lipid bilayer.
• Nonpolar regions of proteins touch bilayer's interior; polar regions project outward.
• This model portrays a dynamic, fluid membrane, with lateral movement of lipids and proteins.

Membrane Lipids

• Lipids are the major component of cell membranes.
• Phospholipids form a bilayer arrangement, oriented with hydrophilic heads facing water and hydrophobic tails facing inward.
• Fatty acids are important components of lipids, typically containing even numbers of carbon atoms, potentially saturated or unsaturated.

Glycolipids

• Glycolipids contain both carbohydrates and lipids, derived from ceramides.
• Critical components of cell membranes, especially abundant in nerve tissue.
• Play a role in cellular interactions, growth, and development, serving as antigens and receptors.

Prostaglandins and Related Compounds

• Prostaglandins, thromboxanes, and leukotrienes, collectively called eicosanoids, are derived from fatty acids with 20 carbons.
• They act as potent regulators of various physiological and pathologic processes, unlike hormones which act systemically.
  • They act locally, unlike hormones.

Phosphatidylinositol

• Phosphatidylinositol is a precursor to second messengers.
• It's broken down to diacylglycerol and inositol triphosphate.
  • These molecules act as internal signals or second messengers.

Cholesterol

• Cholesterol accounts for about 20% of membrane lipids, and sits among the fatty acid tails.
• It regulates membrane fluidity, stiffening in some areas.

Membrane Proteins

• Membrane proteins, though only about 2% of molecules, constitute 50% by weight.
• Proteins like integral or transmembrane proteins permeate the membrane, with hydrophobic parts in the bilayer and hydrophilic parts interacting with the cytoplasm or extracellular environment.
  • Integral proteins often have oligosaccharide attachments on the extracellular surface.
• Peripheral proteins, anchored to the cytoskeleton.
  • Some, like the anion exchange protein, are linked to a spectrin framework.

Membrane Proteins: Channel Proteins

• Channel proteins are integral proteins with pores that allow water or hydrophilic solutes to pass through.
• Can be opened or closed in response to:
  • Ligand-regulated gates (chemical messengers)
  • Voltage-regulated gates (electrical potential)
  • Mechanically-regulated gates (physical stress)

Membrane Proteins: Pumps and Carriers

• Proteins act as pumps, actively transporting ions across the membrane.
• Others function as carriers, moving substances down electrochemical gradients by facilitated diffusion.
  • These aid in the movement of molecules across the membrane.

Membrane permeability

• Selectively permeable membranes pass some substances readily while blocking others.
• The lipid bilayer is permeable to nonpolar, uncharged molecules (oxygen, carbon dioxide, steroids) but impermeable to most large molecules or ions.

Transport of Substances

• Substances cross membranes by either:
  • diffusion
  • active transport
• Lipid solubility correlates to the rate of diffusion across the lipid layer. Water-soluble compounds need specific channels/carriers.

Simple Diffusion

• Simple diffusion is a passive process, moving substances down an electrochemical gradient without energy input.

Facilitated Diffusion

• Facilitated diffusion is like simple diffusion, but uses a membrane carrier protein.
• It's also passive and subject to saturation and competition for carrier proteins.

Active Transport

• Active transport moves substances against an electrochemical gradient, requiring energy (typically ATP).
• Primary active transport, using ATP directly, and
• Secondary active transport, using electrochemical gradients established by primary active transport.

Na+/K+ Pump

• The Na+/K+ pump, a primary active transport protein, maintains low intracellular sodium concentration.
  • It regulates cell volume, supports secondary active transport, produces heat, and maintains membrane potential.

Secondary Active Transport

• Secondary active transport uses an electrochemical gradient, typically sodium, to move a second solute.
• Cotransport (symport) or countertransport (antiport) are subtypes, based on the relative directions of movement.

Cotransport (Symport) / Countertransport (Antiport)

• Examples include Na+ - glucose and Na+-amino acid transport in the intestines and kidneys.

SGLT2 Inhibitors

• SGLT2 inhibitors lower blood sugar in adults with type 2 diabetes.

Osmolarity

• The total solute concentration in a solution.
• Expressed as osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L).

Osmotic Pressure

• The pressure required to prevent the net movement of water across a semipermeable membrane.
• Proportional to the total solute concentration.

Diffusion of Water Through Aquaporins

• Most water movement across membranes uses aquaporins (pores that permit water passage).

Tonicity

• How a solution affects cell volume:
  • Isotonic: No net water movement.
  • Hypotonic: Water enters the cell, causing swelling.
  • Hypertonic: Water exits the cell, causing shrinkage.

Vesicular Transport

• Large molecules and particles move through the cell membrane in vesicles.
• Endocytosis and exocytosis are prominent types.
• Types of endocytosis include phagocytosis, pinocytosis, and receptor-mediated endocytosis.