Chapter 7Membrane Structure

and Function

Artificial Membranes

Phospholipids will self-assemble into bi-layers

Davson & Danelli 1935-1970

They didn’t know that back then.

Current Fluid Mosaic Model

Membrane FluidityCholesterol maintains fluidity of animal cell

membranes

Plant cell membranes have extra unsaturated fatty acids as

in winter wheat

Membrane Proteins Drift About

Membrane Components

Integral (Trans-membrane) Protein

Membranes Have Sides

• Cytoplasmic & extracellular sides differ

• Membrane is recycled• Loss & gain of plasma

membrane is equal• Carbs built by ER &

modified by Golgi

Cell Membrane Proteins

Membrane Carbohydrates

• Carbohydrates on extracellular surface made in ER and modified in Golgi

• Cell-to-Cell recognition

• Oligosaccharides on external side of plasma membrane

• Function as markers

• ABO blood group antigens

• Attached to proteins or lipids

Blood Group Antigens

Movement of Materials Across the Cell Membrane

• 1) Passive Transport - – The cell doesn’t need to expend energy to do

• 2) Active Transport – Cells need to expend energy to do it (ATP)

Particles in Motion

• Particles of all states of matter (s,l,g) in constant motion = Brownian motion

• Particles will move so they are evenly spread out (dynamic equilibrium)

• Particles continue to move due to ambient heat

Passive Transport

• 3 Types:• 1) Diffusion

• 2) Osmosis (Diffusion of Water)

• 3) Facilitated Diffusion

Diffusion• The net movement of

materials from an area of high concentration to an area of low concentration. – Through pores in the

membrane– (Hi to Low) Down its

concentration gradient

Paul Lewis – Simple Diffusion Demonstration

http://hydrodictyon.eeb.uconn.edu/people/plewis/applets/Diffusion/diffusion.html

Click on the simulation to view it.

Permeability of Lipid Bilayer

Hydrophobic molecules can dissolve in the lipid bilayer, and cross easily

Hydrophilic molecules, such as ions and polar molecules cannot easily cross

Concentration Gradients

• The difference between concentrations on either side of a membrane

• If the particles are charged (+ or -), such as H+, Na+, K+, Cl-, the gradient is an electrochemical one

• Gradients have POTENTIAL ENERGY!

Passive Transport is Diffusion Across a Membrane Down a

Concentration Gradient

The cell does not expend energy of its own

Compare the following

DIFFUSION

Dynamic Equilibrium

• Diffusion will continues until particles become evenly spread out

• The concentration on both sides of the membrane become the same

• Some particles may not be able to reach equilibrium

Paul Lewis Simulation

http://hydrodictyon.eeb.uconn.edu/people/plewis/applets/Osmosis/osmosis.html

Click on the simulation to view it.

Osmosis

• Diffusion of water across cell membrane– Water moves from where it is in higher

concentration to an area where it is in lower concentration

– *If you have a High conc. of dissolved substances, you will have a LOW conc. of water!

– “osmos” = Greek word for pushing

OSMOSIS

OSMOSIS

Facilitated Diffusion

• Passive transport of substances across a membrane by means of channel and carrier transport proteins

• Hi to low concentration

• Takes place both directions (In or Out)

• No energy is expended

Facilitated Diffusion

Co-transport

Factors Affecting Rate of Diffusion

• > Temperature = faster diffusion

• > Concentration = faster diffusion

• > Size of particles: smaller particles = faster diffusion

Active Transport

• Requires ATP or another energy source such as an Na+ or H+ gradient

• Always used to concentrate materials against the normal direction of diffusion

• Ex. Roots collect minerals

• Cells build up gradients: Concentration and electrochemical

• Pump mechanisms: Proton, Na+/K+

Active Transport (con’d)

• Uses energy to change shape of membrane proteins to allow substances to pass thru

• Moves materials from Low to High conc.

• One direction only - like turnstiles

• Exocytosis, endocytosis, phagocytosis

Active Transport

Active Transport

Click on picture to see video

Cellular Transport

Comparing Solutions

1. Isotonic

2. Hypotonic

3. Hypertonic

Gotta love the Greeks!

ISOS=equalHYPO=below/underHYPER= above/over

Prefix refers to the amount of solute outside the cell!

ISOTONIC SOLUTIONS

• Concentration of dissolved substances in solution is the same as concentration of dissolved substances inside the cell.

• No net water movement

• Dynamic Equilibrium

Isotonic Importance

• Cells usually exist in isosmotic surroundings (increased salinity in lakes can kill the animals there!)

• Immunizations are isotonic solutions so they do not damage the cells by gain or loss of water.

• I.V. solutions must be isotonic too.

HYPOTONIC SOLUTIONS• Concentration of dissolved substances

is lower in solution outside the cell than concentration inside the cell.

• There is more water outside the cell than inside.

• Water moves into the cell

Cells immersed in hypotonic solutions

• EX: In animal cells, the pressure inside cell increases causing the cells to swell and sometimes burst!

• EX: In plant cells, the rigid cell wall prevents bursting, but the cells become more firm.

Osmotic Pressure (Pressure Potential Ψp)

• Pressure created when water enters a cell pushing against the cell membrane and cell wall

• Animal cells can’t build up a Pressure Potential – They enlarge and burst

• Plant cell walls prevent plant cells from bursting = turgor pressure

• Water potential = 0 bars at equilibrium

Osmotic Pressure

HYPERTONIC SOLUTIONS

• Concentration of dissolved substances outside cell is higher than concentration inside cell.

• There is more water inside cell than outside.

• Water moves out of the cell

Cells immersed in hypertonic solutions

• Ex: In plant cells, membrane and cytoplasm shrink away from cell wall and plant wilts.

• EX: In animal cells, the pressure decreases and the cells shrivel.

What about the big boys?

Endocytosis• The cell can engulf large particles that

are too large to fit through pores in the cell membrane

Exocytosis• Large particles (waste, indigestable

material) are exported from cell

Critical Thinking…..

• How does salting the roads in winter, sometimes kill plants next to the road?

• Why do marine fish constantly drink saltwater?

• Why do freshwater fish produce large quantities of dilute urine?

Water PotentialΨ = Ψp + Ψs

• Ψ is 0 (bars or Mpa) for pure water open to the atmosphere

• Inversely proportional to solute content

• Directly proportional to pressure• Measures the relative tendency of

water to move from one location to another

• Systems move to lower free energy

Ψs = Solute Potential

• Adding solute “decreases” Ψs • Proportional to solute concentration

Ψp = Pressure Potential

• Physical pressure on a solution

• Pressure of the cell wall - Turgor

• Negative pressure is called Tension

• Ψp = 0 for water open to the atmosphere

• F.Y.I: Approximately 1 bar = 1 atm. = 101.3 kPa = .1 MPa

At Dynamic Equilibrium

•Ψ = 0•No free energy•Ψp cancels Ψs

Phagocytosis