Membrane Transport

• • Biological levels of organization include atoms, molecules, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystems, biosphere
  • Physiology stops at organisms
  • Physiological processes are shaped by evolution, obey physical and chemical laws, are usually regulated
  • Physiology is integrative
• Animals

  • Eukaryotes
  • Share key traits:
  • Multicellularity, with cells that lack cell walls and have an extensive extracellular matrix
  • Heterotrophy - they obtain carbon compounds from other organisms (ingesting food instead of absorbing it like plants with photosynthesis)
  • Motility - they move on their own at some point in their lives
• Physiology

  • Our study of animal physiology: molecule, organelle, cell, tissue, organ, organ system
  • A relationship between form and function begins at the molecular stage - protein shape correlates with their roles, such as membrane proteins with hydrophilic exteriors and hydrophobic interiors
  • This is also true at the molecular level and for cell shapes, how they look correlates to their function
• Actin filaments (microfilaments) - made of actin

  • Maintain cell shape by resisting tension (pull)
  • Moves cell via cell crawling or muscle contraction
  • Divide animal cells in two
  • Moves organelles and cytoplasm in plants, animals, and fungi
• Intermediate filaments - made of keratins or lam-ins

  • Maintain cell shape by resisting tension (pull)
  • Anchors nucleus and some other organelles
• Microtubules - made of alpha- and beta-tubulin dimers

  • Maintain cell shape by resisting compression (push)
  • Have a positive and negative end
  • Moves cells via flagella or cilia
  • Moves chromosomes during cell division
  • Provides tracks for intracellular transport
• Plasma Membrane

  • Separates life from non-life
  • Consists of a phospholipid bilayer
  • Has proteins that either span bilayer or attach to one side
  • Creates distinct internal environment
  • All contents inside cell are the cytoplasm besides nucleus
  • Keeps damaging materials out of the cell
  • Allows entry of materials needed by the cell
  • Facilitates the chemical reactions necessary for life
• Fluid-Mosaic Model

  • Proteins spanning the membrane are integral membrane proteins or transmembrane proteins
  • Have segments facing both interior and exterior surfaces + portion passing through hydrophobic tails in bilayer (these have hydrophobic side chains)
  • Peripheral membrane proteins bind to the membrane without passing through it - may be found on interior or exterior of cell
  • Model suggests some proteins are inserted into the lipid bilayer, and that the membrane is a fluid, dynamic mosaic of phospholipids and proteins
• Phospholipid Bilayers

  • Two sheets of phospholipid molecules align
  • Hydrophilic heads face outward
  • Hydrophobic tails face each other on inside
  • Have selective permeability
  • Small or non polar molecules move across quickly
  • Charged or large polar substances cross slowly, if at all
  • Provide basic membrane structure
  • Plasma membranes contain as much protein as phospholipid
  • Proteins can insert into a membrane
• Diffusion

  • Passive transport
  • A concentration gradient is created by a difference in solute concentrations - net movement from high-concentration to low-concentration regions
  • Diffusion along a concentration gradient is spontaneous and increases entropy
• Facilitated Diffusion

  • Passive transport - movement of substances through channels doesn’t need energy
  • Transmembrane assisting passive transport
  • Two types of membranes possible: channel proteins from pores that selectively admits certain ions, and carrier proteins that undergo a conformational change to transport specific molecules across the membrane
• Primary Active Transport

  • Pumps are membrane proteins that provide active transport of molecules across the membrane
  • Sodium-potassium pump - uses ATP to transport Na+ and K+ against their concentration gradients
• Secondary Active Transport

  • Cotransporters:
  • Symporters - transport solutes against concentration gradient, using energy released when another solute moves in the same direction along its electrochemical gradient
  • Antiporters -  similar to symporters but the actively transported solute moves in opposite direction
• • Ion channels are specialized membrane proteins
  • Form pores, or openings, in a membrane
  • Ions diffuse through
• Electrochemical gradients occur when ions build up on one side of a plasma membrane

  • They establish both a concentration gradient and charge gradient
  • Ions diffuse down their electrochemical gradients
• Electrochemical Gradients

  • When membrane potential exists, ions on both sides have potential energy
  • Ions move across membranes in response to concentration gradients as well as charge gradients
  • Combination of electrochemical gradient and a concentration gradient is an electrochemical gradient