Cellular Membranes• Two main roles

• Allow cells to isolate themselves from the environment, giving them control of intracellular conditions

• Help cells organize intracellular pathways into discrete subcellular compartment, including organelles

Membrane Structure

Figure 3.20

Lipid Profile• Lipid bi-layer: phospholipids, primarily

phosphoglycerides

• Other lipids

• Sphingolipids: alter electrical properties

• Glycolipids: communication between cells

• Cholesterol: increase fluidity while decreasing permeability

Cholesterol reduces permeability while enhancing fluidity.

Membrane Fluidity• Environmental conditions can affect membrane

fluidity, e.g., low temperature increases the number of van der Waals forces between lipids and restrict movement

Membrane Fluidity, Cont.• Homeoviscous adaptation – keep membrane fluidity

constant by altering the lipid profile• Three strategies: all reduce the number of van der

Waals forces – Replace long chain fatty acids with shorter chains

– Introduce double bonds

– Introduce phospholipids with higher mobility of polar head groups

Figure 3.23

Membrane Proteins• Can be more than half of the membrane mass

• Two main types– Integral membrane proteins – tightly bound to the

membrane, either embedded in the bilayer or spanning the entire membrane

– Peripheral proteins – weaker association with the lipid bilayer

Figure 3.24

Membrane Transport• Three main types• Passive diffusion• Facilitated diffusion• Active transport

• Distinguished by direction of transport, nature of the carriers, and the role of energy

Figure 3.25

Passive Diffusion• Lipid-soluble molecules

• No specific transporters are needed

• No energy is needed

• Depends on concentration gradient– High low– Steeper gradient results in higher rates

Facilitated Diffusion

• Hydrophilic molecules

• Protein transporter is needed

• No energy is needed

• Depends on concentration gradient

Facilitated Diffusion, Cont.• Three main types of proteins

– Ion channels – form pores– Porins – like ion channels, but for larger molecules– Permeases – function more like an enzyme

Figure 3.25

Ion Channels

Figure 3.26

Active Transport

• Protein transporter is needed

• Energy is required

• Molecules can move from low to high concentration

Active Transport, Cont.• Two main types: distinguished by the source

of energy– Primary active transport – uses an exergonic

reaction– Secondary active transport – couples the

movement of one molecule to the movement of a second molecule

Primary Active Transport• Hydrolysis of ATP provides energy

• Three types– P-type: pump specific ions, e.g., Na+, K+, Ca2+

– F- and V-type: pump H+ – ABC type: carry large organic molecules, e.g.,

toxins

Figure 3.27

Secondary Active Transport• Use energy held in the electrochemical gradient of

one molecule to drive another molecules against its gradient

• Antiport or exchanger carrier: molecules move in opposite directions

• Symport or cotransporter carrier: molecules move in the same direction

Electrical Gradients• All transport processes affect chemical gradients

• Some transport processes affect the electrical gradient

• Electroneutral carriers: transport uncharged molecules or exchange an equal number of charged particles

• Electrogenic carriers: transfer a charge, e.g., Na+/K+ATPase exchanges 3Na+ for 2K+

Membrane Potential• Difference in charge inside and outside the

cell; electrochemical gradient

• Active transporters help establish this gradient

• Can be determined by Nernst equation and Goldman equation

• Two main functions– Provide cell with energy for membrane transport– Allow for changes in membrane potential used by

cells in cell-to-cell signaling

Depolarization and Hyperpolarization

Figure 3.29