Cell Transport Across Membranes

• Cells transport across membranes using osmosis, diffusion, and passive/active transport
• This semester is Spring 2025, with Dr. Maximilian Lyon on MWF from 12:00-12:50 pm in ISE 211

Housekeeping

• Office hours and SI schedule:
  • Sunday: Katie 6-7 pm in Ritter 323
  • Monday: Dr. Lyon 10-11 am in Macelwane 100; Shikara 4:45-5:45 pm in Xavier G14
• Sign into iClicker for attendance

Learning Objectives

• Explain why biological membranes are semipermeable, based on the structure and properties of phospholipids and membrane proteins
• Differentiate active and passive transport mechanisms
• Identify different types of plasma membrane transporters in a cell model
• Ascertain water's net movement across a membrane due to osmosis

Diffusion

• Diffusion is the random movement of particles
• Solutes move from areas of high concentration to areas of low concentration
• Equilibrium is when things are randomly distributed

Cell Membranes

• Phospholipid bilayers block the diffusion of most substances
• Passive transport across them doesn't require energy
• Nonpolar or very small, simple uncharged molecules pass through
• 1 Na⁺ moves as fast as 1,000 glucose molecules, 1,000,000,000 H2O molecules, or 100,000,000,000 O₂ molecules

Plasma Membranes

• Plasma membranes contain lots of proteins
• Some proteins are used as surface for reactions, others transmit matter or information
• Carbohydrates are attached to the outer surface of proteins (forming glycoproteins) or lipids (forming glycolipids)

Osmosis

• Osmosis is the "diffusion of water", and doesn't require energy, but does require proteins
• Water moves across a selectively-permeable membrane
• Water moves from low solute to high solute, until concentration reaches equilibrium
• Osmosis can be facilitated by aquaporins ("water hole" proteins)

Tonicity

• Tonicity describes relative solute concentration on either side of a membrane
• Direction depends on total amount of dissolved "stuff"
• Isotonic is equivalent solute concentration, and hypotonic is when the solutes are dilute outside, while hypertonic is where solutes are concentrated outside

Diffusion

• Simple diffusion requires no energy or proteins
• Small, uncharged particles move directly through a membrane
• Net movement is from high concentration side to low concentration side (down the concentration gradient)
• At equilibrium, movement in = movement out

Diffusion Facilitation

• Facilitated diffusion requires proteins, but no energy
• Movement of normally impermeable (large/polar) particles occurs across a membrane
• Substances move from high to low concentration, but requires a protein channel or carrier, both increase "effective permeability"

Channels

• Channel proteins have the form of a tunnel (pore) through a membrane and are usually always open
• A specific size, shape, and amino acids filter for specific materials
• Some have gates that need to be "unlocked" and opened

Carrier Proteins

• Carrier proteins act as a "revolving door" through the membrane
• Specific size, shape, and amino acids filter for specific materials
• Binding their target causes a shape change

Active Transport

• Active transport requires energy and proteins
• Energy comes from one of two sources:
  • Primary: Breaks down ATP to release energy
  • Secondary: Uses a pre-existing concentration gradient of another substance
• Individual proteins can be cotransporters, moving two substances in the same direction (symporters) or opposite directions (antiporters)
• Active transport is how concentration gradients form inside the body

Na+/K+ antiporter (Na+/K+ ATPase)

• Breaks ATP (adenine triphosphate) into ADP (adenine diphosphate) and P; (inorganic phosphate)
• Transports 3 Na+ ions out, 2 K+ ions in, both against their concentration gradients
• Uses 1/3 of the energy in cells (2/3 in neurons)

Na+/glucose cotransporter

• Using the flow of one particle down its concentration gradient to drive another up its concentration gradient
• Binds Na+ and glucose on the same side of the membrane
• Na⁺ moves down its gradient, and pulls glucose up its gradient
• This type of transport symporters (both move the same direction)
• The initial gradient has to be made by a primary active transporter using ATP

Cystic Fibrosis

• Cystic fibrosis is the most common genetic disease in humans of Northern European descent
• Caused by mutations to CFTR, a Cl- ion channel
• As a result, Cl- builds up in the cell, and drives osmosis in
• Mucus outside of the cell gets thick and stiff

Assignments

• Complete the reading quiz, due Monday at 12:00 pm
• Read Chapters 2.3, 2.4, and 5.1
• Follow the highlights from the link
• Complete the group sheet assignment
• Connect to iClicker for attendance