Cellular Transport: Diffusion and Osmosis

• Materials are moved into and out of cells
• Module 1 focuses on cells as the basis of life
• Cells coordinate activities within their internal and external environments
• A practical investigation to model diffusion and osmosis is conducted
• Active transport, endocytosis, and exocytosis are examined

Cellular Transport

• Materials are transported into and out of cells passively or actively

Passive and Active Movement

• Passive involves diffusion and osmosis, which require no energy input
• Active transport requires cellular energy to move molecules

Passive Transport: Diffusion

• Diffusion is the movement of any molecules from high to low concentration until equilibrium with no energy
• Concentration gradient is the difference in substance concentration in two areas, possibly divided by a barrier

Diffusion Demonstration

• First, a colored sugar cube is placed in a large volume of clear water
• Second, the dissolving cube establishes a steep concentration gradient for sugar and dye
• Sugar and dye concentrations are high near the cube and negligible elsewhere
• Third, sugar and dye molecules spread through the solution as time passes
• Eventually, the sugar and dye molecules are evenly distributed throughout the solution

Brownian Motion

• Diffusion results from random motion due to kinetic energy
• This was first discovered by Robert Brown
• Diffusion involves the movement of solid, liquid, or gas molecules through another solid, liquid, or gas

Diffusion Rate Factors

• Diffusion rate depends on the concentration gradient
  • A greater concentration difference creates a steeper gradient and faster diffusion
• Diffusion may speed up or slow down, depending on the temperature
  • Heat increases the rate because particle kinetic energy speeds up
• Higher temperature results in faster diffusion
• Larger surface area results in faster diffusion
• Higher concentration gradient results in faster diffusion
• Smaller particles result in faster diffusion
• Gas results in fastest diffusion, Liquid results in faster diffusion than solid, and Solid being the slowest diffusion

Facilitated Diffusion

• Some larger and electrically charged molecules move too slowly by simple diffusion to satisfy needs
• Proteins in the cell membrane act as membrane transporters, accelerating molecule movement across the membrane
• The movement is along a concentration gradient with no energy input
  • This is facilitated diffusion

Facilitated Diffusion: Protein Assistance

• Facilitated diffusion involves the movement of molecules being assisted by carrier proteins in the membrane
• A protein that acts as a membrane-transporter is specific to one or several solutes
• Facilitated diffusion allows larger or charged molecule movement more rapidly than simple diffusion

Channels and Carriers

• A protein may act as a channel or a carrier protein
• Channels are the fastest form of transport
• These work like gates and have open and closed states, determined by electrical or physical signals

Carrier Proteins

• A carrier protein binds with a solute
• The protein then changes shape/conformation to move the solute to the other side
• Once released to on the other side, the carrier returns to its original shape

Passive Transport: Osmosis

• Osmosis is the movement of water from high to low water concentration
• Transport occurs through a selectively permeable membrane and does not require energy

Aquaporins

• Water moves through protein channels called aquaporins
• Movement of water across membranes occurs much faster than expected
• Tissues with high-water permeability have more aquaporins in membranes

Osmosis Rate Factors

• The rate of water movement depends on two things
  • Solute concentration, the concentration including the dissolved substances in the solution
  • Opposing physical pressures or tension exerted on the water

Osmosis Effects in Animal Cells

• Water moves according the the concentration inside and outside the cell in animal cells

Osmosis Effects in Plant Cells

• Water moves according to concentration inside and outside cells, causing plasmolyzed, flaccid, and turgid forms

Comparing Diffusion and Osmosis

• Both diffusion and osmosis are examples of passive transport
• Passive transport is characterized by the movement of a substance down a concentration gradient with no cellular energy

Passive Transport Practical Investigation

• The aim is to observe passive transport through dialysis tubing
• Set up the following: create a dialysis tube by tying a string at one end, filling with starch, and tying another end
• Create a 250mL beaker with enough iodine solution
• The tube is now put into the beaker
• Leave the apparatus overnight (away from direct sunlight)
• Record any observations occur

Osmosis First Hand Investigation

• The aim is to observe the process of osmosis by using potatoes
• First, set up distilled and salt water in separate bowls
• Leave the potatoes in each overnight
• Observe and note any changes

Active Transport

• Active transport requires a carrier protein to move chemicals from low to high concentration, using cellular energy

Active Transport: Larger Particles

• In addition to small ions and molecules, cells need to remove and take in larger molecules and particles
• Some cells are capable of engulfing entire unicellular microorganisms
• There are two primary mechanisms for transporting these particles
  • Endocytosis
  • Exocytosis

Active Transport: Endocytosis

• Endocytosis is the active transport that moves particles into a cell
• There are variations of endocytosis, but all share common characteristics
  • The material is surrounded by a plasma membrane area, which buds off inside the cell, forming a pocket
  • The pocket pinches off
  • The particle is contained in a newly created intracellular vesicle formed from the plasma membrane

Endocytosis Types

• There are three main types of endocytosis
  • Phagocytosis
  • Pinocytosis
  • Receptor-Mediated Endocytosis

Endocytosis: Phagocytosis

• Phagocytosis (cell eating) is the process by which larger particles, like cells or relatively large particles, are taken in
• When microorganisms invade the body, neutrophils remove invaders by phagocytosis
  • The neutrophil surrounds and engulfs microorganisms, then destroys them

Phagocytosis Process

• The vesicle merges with a lysosome for material breakdown in the newly formed compartment (endosome)
• Accessible nutrients are extracted from the degradation of the vesicular content
• The newly formed endosome merges with the plasma membrane and releases it contents into the extracellular fluid
• Endosomal membrane becomes part of the plasma membrane again

Endocytosis: Pinocytosis

• A variation of endocytosis is pinocytosis (cell drinking)
• Molecules, including water, needed from the extracellular fluid are taken in
• Pinocytosis is a non-specific process
• The cell takes in the solute that are dissolved in the liquid it envelops

Differences in Pinocytosis

• Pinocytosis results in a much smaller vesicle that does not merge with a lysosome

Receptor-Mediated Endocytosis

• Receptor-mediated endocytosis is an extremely selective importing process
• Specificity occurs by receptor proteins located on depressed areas of the cell membrane called coated pits
• The cytosolic surface of coated pits is covered by coat proteins

Receptor-Mediated Endocytosis Process

• A cell only takes in an extracellular molecule if it binds to the specific receptor protein on the cell's surface
• Once bound, the coated pit pinches in to form a coated vesicle
• The coated vesicle fuses with a lysosome to digest the engulfed material which is released into the cytosol

Active Transport: Exocytosis

• Exocytosis is the process of cell secretion or excretion
• Its purpose is to expel material from the cell into the extracellular fluid
• Waste material is enveloped in a membrane and fuses to the interior of the plasma membrane
• This fusion opens the membranous envelope on the exterior
• Waste material is expelled into the extracellular space

Summary of Cellular Transport

• Diffusion/facilitated diffusion
  • High concentration of particles to low concentration of particles
  • Movement is along he concentration gradient
  • The movement of any type of molecule from high to low concentration until equilibrium is reached
• Osmosis
  • Has a selectively permeable membrane
  • High concentration of water molecules to low concentration of water molecules
  • Movement along concentration gradient
  • Water movement from high to low occurs through a selectively permeable membrane
• Active Transport
  • Has a selectively permeable membrane
  • Movement against concentration gradient
  • Solute moves from low concentration to high through a selectively permeable membrane

Cellular Transport Comparison

• Active transport: occurs through carrier proteins
  • Fast rate of movement
  • Low to high concentration
  • Requires energy
  • Transports sugars and amino acids
• Passive transport: occurs through carrier proteins via facilitated diffusion or between phospholipids via osmosis and diffusion
  • Slow rate of movement
  • High to low concentration
  • No energy required
  • Transports water, oxygen, carbon dioxide, sugars, and amino acids