Diffusion and Physical Transport Membranes, Materials, and Movement

Membranes Cell membranes are selectively permeable Most biological membranes are permeable to small or lipid-soluble molecules Water molecules may pass the lipid bilayer

Membranes Gases, small polar molecules and a few other substances may also pass Other molecules move through special channels, primarily through membrane transport membranes

Diffusion Diffusion is a process by which molecules move from an area of higher concentration to lower concentration Some substances may diffuse through the cell membrane

Diffusion Random motion of particles leads to diffusion Atoms and molecules above absolute zero exhibit random motion

Diffusion Molecules move from an area of higher concentration to one of lower concentration, ultimately reaching equilibrium

Diffusion The rate of diffusion depends on temperature, the size of the molecules, electrical charges, and the concentration gradient

Diffusion Diffusion rates increase as temperature increases

Diffusion Molecular motion slows at low temperatures, hence diffusion slows

Diffusion Diffusion increases with decreasing molecular size 2 chemical dyes with different molecular weights Diffusion increases with decreasing molecular size

Diffusion Large molecules move more slowly (makes sense!), hence diffuse a smaller distance in the gel

Diffusion The rate of diffusion increases with a greater concentration gradient

Diffusion Diffusion slows as concentration gradient lessens over time

Osmosis Osmosis is the diffusion of water across a selectively permeable membrane

Osmosis Two solutions may be isotonic to each other, or one may be relatively hypertonic and the other relatively hypotonic

Osmosis Human cells are isotonic with a 0.9% sodium chloride (salt) solution Human cells are hypotonic compared to sea water

Osmosis Human cells are hypertonic compared to distilled water How would you state the comparison of distilled water to human cells? Of this IV fluid?

Osmosis Fluids in the human body are approximately 0.9% salts An IV bag is balanced to that salt balance, termed physiological saline The fluids in this bag are isotonic to human body fluids

Osmosis in animal and plant cells

Osmosis Animal cells placed in a hypertonic solution tend to shrivel and die

Osmosis This is seen in the red blood cell at the top; normal red blood cell at bottom

Osmosis Plant cells and others with cell walls placed in a hypertonic solution tend to plasmolyze The cell membrane shrinks around the cell contents

Plasmolyzed cells Normal cells

Plasmolysis

Osmosis Animal cells placed in a hypotonic solution tend to swell and burst How can we still drink distilled water and live?

Osmosis Plant cells placed in a hypotonic solution tend to become turgid (“full”) Turgor pressure is the internal water pressure usually present in cells with walls (plants, fungi)

Osmosis Turgor pressure not only provides pressure to each cell, but gives the plant pressure to stay erect, holding up stems and leaves

Osmosis Turgor pressure is maintained by water contained in the central vacuole of mature plant cells Central Vacuole

Plant Cells and Turgidity

Osmosis Turgor pressure provides structural support in non-woody plants Wilting occurs when plants are underwatered, resulting in low cellular water pressure

Osmosis Wilting can be reversible, or irreversible

Osmosis Wilting is of critical importance in agricultural research

Diffusion; the End