IB TOPIC 2.4 MEMBRANES Passive Transport (pages 38-40)

Passive and Active Transport • There are two general types of cellular transport:

• Passive transport• Does not require energy (in the form of ATP)• Occurs in situations where there are areas of difference concentrations

of a particular substance• Movement occurs from areas of high concentration to low concentration • Movement is said to be along a concentration gradient

• Active transport • Does require ATP energy• Movement is against a concentration gradient• We’ll come back to this concept later in the week

Diffusion • One type of passive transport• Particles move from a region of high concentration to low

concentration• Example below: dye molecules moving from area of high to low

concentration

• In a living system, diffusion often involves a membrane• Example: Oxygen gas moves from outside a cell to inside

Facilitated Diffusion • A type of diffusion involving a membrane with specific

carrier proteins that are capable of combining with the substance to aid its movement

• The carrier changes shape to accomplish this task• No energy is needed (still passive transport)• Facilitated diffusion is very specific and depends on the

carrier protein • Note: the rate of facilitated diffusion will level off when

total saturation of available carriers occurs

Rate of facilitated diffusion

Osmosis • Passive transport• Movement along concentration gradient• Involves only the passive movement of water molecules

across a partially permeable membrane • Partially permeable membrane – only allows certain substances to

pass through• Also called a semi-permeable membrane • Permeable would allow everything to go through

• The concentration gradient of water that allows the movement to occur is the result of a difference between solute concentrations on either side of a semi-permeable membrane

Osmosis

Osmotic solutions • A hyperosmotic solution has a higher concentration of

total solutes than a hypo-osmotic solution• Water therefore moves from hypo-osmotic solution to a

hyperosmotic solution• If iso-osmotic solutions occur on either side of a semi-

permeable membrane, no net movement of water is evident

Equilibrium • Passive transport continues until there is an equal

concentration of the substance in both areas involved• This is called equilibrium

Recap • Table summarized diffusion and osmosis across cellular

membranes

Type of movement Definition

Simple diffusion Substances other than water move between phospholipid molecules or through proteins which possess channels

Facilitated diffusion Non-channel protein carriers change shape to allow movement of substances other than water

Osmosis Only water moves through the membrane using aquaporins which are proteins with specialized channels for water movement

Size and Charge • Substances that move across a membrane passively are

influenced by two major factors: • Size and charge

• Substances that are small in size and non-polar move with ease• Ex of non-polar substances are gases like: oxygen, carbon dioxide,

and nitrogen

• Substances that are polar, large in size, or both do not move with ease• Ions such as chloride, potassium, and sodium• Glucose and sucrose

• Water and glycerol are examples of small, uncharged polar molecules that can fairly easily cross membranes

Question• What kinds of molecules pass through a cell membrane

most easily?

• A. Monosaccharides such as glucose• B. Sodium and potassium• C. Large and polar• D. Small and non-polar

• Answer is D

Question • Which of the following would likely move through the lipid

bilayer of a plasma membrane most rapidly?

• A. Starch• B. Carbon dioxide • C. An amino acid• D. Glucose

• Answer is B

Turn and talk • If someone comes into an enclosed room wearing a

distinctive perfume, what is the process that allows the odor to be distributed throughout the room in a relatively short period of time?

• Diffusion • Movement of molecules from an area of high

concentration to low concentration until they are evenly distributed (equilibrium)

Turn and talk • While walking through the produce section of a grocery

store, it is common to see misters at work spraying water over many of the vegetables. What cellular transport process is being used to maintain healthy-looking veggies?

• Mist provides a layer of water• Outer surface of veggie acts as a semi-permeable

membrane • Higher concentration of water molecules outside the

veggie surface, water will move into the veggie

Membrane Permeability Lab Tomorrow

• See procedure/data table (handout) • Groups of 4 • What you will turn in …

• Answer #1, 2, and 3 page 28 (homework tonight) • Re-draw data table (use a ruler!) from page 29

• So it’s bigger for you to write in

• Complete data table• Write a 2 paragraph conclusion based on your data/results • All of this should be done on the same sheet of paper; include a

title and proper heading • Due: Monday, December 3