Membrane Transport

• Plasma membranes are selectively permeable• Some molecules easily pass through the

membrane; others do not

Types of Membrane Transport

• Passive processes– No cellular energy (ATP) required– Substance moves down its concentration gradient

• Active processes– Energy (ATP) required– Occurs only in living cell membranes

Passive Processes

• What determines whether or not a substance can passively permeate a membrane?

1. Lipid solubility of substance2. Channels of appropriate size3. Carrier proteins

PLAY Animation: Membrane Permeability

Passive Processes

• Simple diffusion• Carrier-mediated facilitated diffusion• Channel-mediated facilitated diffusion• Osmosis

Passive Processes: Simple Diffusion

• Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer

PLAY Animation: Diffusion

Figure 3.7a

Extracellular fluid

Lipid-solublesolutes

Cytoplasm

(a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer

Passive Processes: Facilitated Diffusion

• Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:– Exhibit specificity (selectivity)– Are saturable; rate is determined by number of

carriers or channels– Can be regulated in terms of activity and quantity

Facilitated Diffusion Using Carrier Proteins

• Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids)

• Binding of substrate causes shape change in carrier

Figure 3.7b

Lipid-insoluble solutes (such as sugars or amino acids)

(b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein

Facilitated Diffusion Using Channel Proteins

• Aqueous channels formed by transmembrane proteins selectively transport ions or water

• Two types:– Leakage channels

• Always open

– Gated channels• Controlled by chemical or electrical signals

Figure 3.7c

Small lipid-insoluble solutes

(c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge

Passive Processes: Osmosis

• Movement of solvent (water) across a selectively permeable membrane

• Water diffuses through plasma membranes:– Through the lipid bilayer– Through water channels called aquaporins (AQPs)

Figure 3.7d

Watermolecules

Lipidbillayer

Aquaporin

(d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer

Passive Processes: Osmosis

• Water concentration is determined by solute concentration because solute particles displace water molecules

• Osmolarity: The measure of total concentration of solute particles

• When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

Figure 3.8a

(a) Membrane permeable to both solutes and water

Solute and water molecules move down their concentration gradientsin opposite directions. Fluid volume remains the same in both compartments.

Leftcompartment:Solution withlower osmolarity

Rightcompartment:Solution with greater osmolarity

Membrane

H2O

Solute

Solutemolecules(sugar)

Both solutions have thesame osmolarity: volumeunchanged

Figure 3.8b

(b) Membrane permeable to water, impermeable to solutes

Both solutions have identicalosmolarity, but volume of thesolution on the right is greaterbecause only water is free to move

Solute molecules are prevented from moving but water moves by osmosis.Volume increases in the compartment with the higher osmolarity.

Leftcompartment

Rightcompartment

Membrane

Solutemolecules(sugar)

H2O

Importance of Osmosis

• When osmosis occurs, water enters or leaves a cell

• Change in cell volume disrupts cell function

PLAY Animation: Osmosis

Tonicity

• Tonicity: The ability of a solution to cause a cell to shrink or swell

• Isotonic: A solution with the same solute concentration as that of the cytosol

• Hypertonic: A solution having greater solute concentration than that of the cytosol

• Hypotonic: A solution having lesser solute concentration than that of the cytosol

Figure 3.9

Cells retain their normal size andshape in isotonic solutions (same

solute/water concentration as insidecells; water moves in and out).

Cells lose water by osmosis and shrink in a hypertonic solution

(contains a higher concentration of solutes than are present inside

the cells).

(a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions

Cells take on water by osmosis untilthey become bloated and burst (lyse)

in a hypotonic solution (contains alower concentration of solutes than

are present in cells).

Summary of Passive Processes

• Also see Table 3.1

Process Energy Source

Example

Simple diffusion

Kinetic energy

Movement of O2 through phospholipid bilayer

Facilitated diffusion

Kinetic energy

Movement of glucose into cells

Osmosis Kinetic energy

Movement of H2O through phospholipid bilayer or AQPs