Cells & MembranesCells & Membranes

Prokaryotic vs. Eukaryotic Cells

• Why would it benefit a cell have to membrane-bound organelles?– Different “compartments” for specific reactions– Allows incompatible reactions to occur at the

same time

Limits to Cell Size

• Surface Area to Volume Ratio– As a cell increases in size, the volume increases

faster than the surface area (SA:V ratio also becomes smaller)

• Why is this significant?– If ratio is too small flow of nutrients (in) &

flow of wastes (out) aren’t fast enough to keep up with metabolic activities of the cell

– Larger cells also have more trouble moving materials through cytoplasm

Protein Traffic in a Cell

• Secretory Protein = protein that is going to leave the cell

• Pathway:– Ribosome Rough ER

vesicle Golgi Apparatus vesicle Plasma (Cell) Membrane

Movement within a cell

• Involves specialized proteins “walking” across microtubules or other proteins

• Requires ATP as an energy source

• Examples include:– Vesicle movement within a cell– Cilia and flagella movement– Muscle contraction

Membrane Properties

• fluid = phospholipids move a lot laterally but not much flip-flopping

• Membranes need to stay fluid to function properly. Fluidity of the membrane can be adjusted by– Temperature– Amount of cholesterol

Fluidity of the membrane• Effect of temperature

– the higher the temperature, the more fluid the membrane will be

– At a certain temperature, each membrane will become more solidified (less fluid)• This depends on the types of fatty acids in the

membrane (saturated or unsaturated)

Fluidity of the membrane

• Effect of cholesterol– At medium temperatures

- reduces fluidity

- prevents lateral movement of phospholipids– At low temperatures

- prevents solidification -- lowers the temperature at which a membrane solidifies

- Prevents phospholipids from being packed closely

Membrane Properties• Mosaic = proteins, carbohydrates

embedded in membrane• Integral/Transmembrane proteins

often have both hydrophilic and hydrophobic regions.

• Membrane proteins have many different functions.

• Carbohydrates are often involved in cell-cell recognition.

hydrophobichydrophilic

Why must a plasma membrane be selectively permeable?

• To control what enters the cell

• Maintain a certain environment within the cell (Dynamic Homeostasis– things constantly moving in and out to maintain certain concentrations, conditions)

Passive Transport• Require NO energy

• Small, nonpolar molecules, O2, CO2

• Moves with the concentration gradient– From high to low concentrations

• Osmosis (diffusion of water)– occurs across the membrane and

through aquaporin (channel proteins)

Passive Transport

• Facilitated Diffusion– Channel Proteins– Carrier Proteins

• What would you expect the amino acid composition to be on the channel portion of the protein?– Polar and hydrophilic (so

that polar/hydrophilic molecules can get across)

Active Transport

• Requires energy• Ions, larger molecules• Moves against the

gradient – From low to high

concentrations

• Ex. Sodium-Potassium pump– Requires ATP to work

Transport using vesicles

• Endocytosis– Cell “eating”– taking

molecules in

• Exocytosis– Expelling molecules from a

cell