Physical TransportMembranes, Materials, and Movement

By Jane Horlings

• Membranes surround a cell and form its organelles

Membranes

• Cellular membranes are essential to the structure and function of a cell

• The plasma (cell) membrane surrounds the cell defining its boundaries and serves as the cell’s interface with the outside environment

• Eukaryotic cells are characterized by a membrane-bound nucleus, and other membranous organelles, including the endomembrane system, mitochondria, and chloroplasts (in plants)

Membranes

Membranes

• Cell membranes are selectively permeable, meaning some things can pass, others cannot

• Most biological membranes are permeable to small or lipid-soluble molecules... Why?

• Water molecules may pass the cell membrane

Membranes

• Gases, small polar molecules and a few other substances may also pass

• Other molecules move through special channels, primarily through membrane transport proteins

Diffusion

• Atoms and molecules above absolute zero (-273° C) exhibit random motion

• In other words, at temperatures experienced by living things, molecules are in motion; and molecules of gases and liquids move the most

• Random motion of particles leads to molecular motion, and hence, diffusion

Diffusion

• Diffusion is a process by which molecules move from an area of higher concentration to lower concentration

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

• So let’s look at:1.Temperature2.Size of the molecules3. (we will skip this) electrical

charges

4. 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

• In this petri dish filled with an agar gel (like jello), 2 diferent dyes are put in two holes in the gel, and left to diffuse for an hour

2 chemical dyes with different molecular weights

Diffusion

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

• Small molecules (yellow/orange dye) move more quickly, making a larger “circle”

Diffusion

• The rate of diffusion increases with a greater concentration gradient

DiffusionDiffusion slows as concentration gradient lessens over time. This shows a time series. At the 12:00 time, a dye is put in the bottom of the beaker. In the first 2 hours, diffusion occurs rapidly, as the concentration gradient (differential) is great. Over the next hours, it slows, but doesn’t stop, as equilibrium hasn’t been reached yet.

Osmosis• Osmosis is the diffusion of water across a

selectively permeable membrane• Think of it as a “special case” of diffusion... the

diffusion of water

Osmosis

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

Osmosis• Iso- means same. Isotonic means having the

same concentration of dissolved substances. • Hyper- means higher. Hypertonic means having

a greater concentration of dissolved substances. • Hypo- means lower. Hypotonic means having a

lower concentration of dissolved substances

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 (can you read the salt concentration upside down?)

OsmosisStudy this!!

Osmosis

• Animal cells placed in a hypertonic solution tend to shrivel and die; these were red blood cells put in a highly salty solution

Osmosis

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

• Why is salt used to kill slugs?

• What does it do to

their cells? • See diagram 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

• This is what you’re going to look at in lab!

Plasmolyzed cells

Normal cells

See how the cell membranes shrink around the chloroplasts?

Plasmolysis

Plasmolyzed cellNormal cell

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 under-watered, resulting in low cellular water pressure

Osmosis

• Wilting can be reversible, or irreversible

• Usually you can’t tell if you’ve brought a plant to a point that is reversible or not

Osmosis

• Wilting is of critical importance in agricultural research

• Water is a valuable resource, and farmers often have to pay for water, if they can get it! They need to understand what the water needs of plants are, and irrigate accordingly

• Many areas of the world are deserts, and water is in very limited supply

• You are going to do a part of this lab relating to what you’ve already learned about cell size– The next 2 slides are from lecture 1.3a, which you have already

studied and learned about

Diffusion

• Cell size– Most cells are smaller than 1 mm in diameter– Surface/volume ratio determines cell size– Surface area equates to the plasma membrane– Volume equates to the cytoplasm

Diffusion

• Cell size– As a cell grows, volume increases more rapidly than surface area

(it’s a complex mathematical relationship!)– Therefore small cells have a greater surface/volume ratio than larger

cells (greater ratio is advantageous)– Nutrients from the environment must cross the surface of the cell to

enter– Cells must be small in order for the surface area to be adequate to

supply nutrients to an active cell

Diffusion

• In the lab this week, you will be investigating the rate and extent of diffusion into agar blocks. This is part D in your lab manual

• You will be cutting agar, which is a gel into 3 sizes of cubes

Diffusion

• You will let them sit in a solution (vinegar) which causes a color change, so you can measure the extent of diffusion of the solution in to the cubes, something like what is seen below

Diffusion

Diffusion; the End

How is diffusion related to these cartoons?