Chapter 5 Membranes and Transport

Cell Membrane

• Function: To control passage of substances

• Selectively permeable: Some substances and chemicals can pass through, some can’t– Based on size and charge of molecule

Fluid mosaic model

• Membrane includes three parts– Phospholipid bilayer (fluid)– Proteins (mosaic)– Cholesterol

Phospholipid bilayer

• Made of phospholipids• Phospholipid– Glycerol with phosphate heads– Fatty acid tails

Proteins

• Functions of membrane proteins– Enzyme activity– Cell to cell signaling (glycoprotein)– Cell recognition (glycoprotein)– Transport of materials• Channel proteins• Pump proteins

Glycoproteins

• A Protein with a carbohydrate chain attached• Used for cell recognition and cell signaling• Used to recognize tissue types in transplant

patients

Membrane Proteins

• Transport Proteins

Cholesterol

• Helps maintain fluidity level of membrane– Too much=stiff– Too little=flimsy

Selectively permeable

• Can freely pass through lipid bilayer– Small non-polar molecules (not charged)• Oxygen gas• Carbon dioxide

Selectively permeable

• Cannot pass through freely– Charged molecules (polar)• Water• Ions Na+, K+, H+

– Large molecules• Amino acids• Proteins• Glucose

Concentration Gradient

• The gradual difference in the concentration of solutes in a solution between two regions

• The membrane separates the two regions

Concentration Gradient

• Since membrane is selectively permeable a concentration gradient can build up– Solute will collect on one side and then move

through membrane to reach equilibrium• Equilibrium: The state in which the

concentrations of the diffusing substance in the two compartments become equal.

Passive transport

• Moving materials across a membrane, down the concentration gradient without cellular energy

• Molecules are bouncing off of each other: referred to as kinetic energy

Types of Passive Transport

• Diffusion: Movement of materials through the lipid bilayer down a concentration gradient

• Examples:– Diffusion of Oxygen into blood from lungs– Diffusion of Carbon dioxide out of blood to lungs

Types of Passive Transport

• Osmosis: Movement of water through aquaporins down the concentration gradient– Movement of water from high water

concentrations to low water concentrations

Hypertonic

• Solution is higher in solute or lower in water concentration relative to cell

• Water will move from high water to low water

Hypotonic

• Solution is lower in solute or higher in water concentration relative to cell

• Water will move from high water to low water

Isotonic

• Solution outside and inside the cell are at equilibrium

• Water will move back and forth equally

What happens if…

• The cell is placed in a hypertonic environment?

• The cell is placed in a hypotonic environment?

• The cell is placed in an isotonic environment?

Types of Passive Transport

• Facilitated Diffusion: Movement of materials down a concentration gradient through channel proteins– For large or charged molecules– Example: Glucose or amino acids

Passive Transport Comments

• All types of passive transport:– Movement from high to low concentrations– Trying to reach equilibrium

Active Transport

• Movement of materials independent of concentration gradient using cellular energy– Usually movement from low to high

concentrations– Requires the cell to use ATP (energy) to move

materials

Active Transport

• Uses Transport proteins called pumps– Proton pumps– Sodium-Potassium pumps (required for nerve

impulses)

Active Transport Example

• We need Sodium and Potassium on certain sides of the membrane– Cannot rely on diffusion so cell must pump them

to the right side• Independent of concentration gradient

Active Transport

Exocytosis

• Type of active transport when the cell exports materials where a vesicle fuses with the cell membrane

• Used for very large molecules• Example: proteins

Endocytosis

• Type of active transport where the membrane fuses with the cell membrane to import materials from the outside

• Cell brings in food or other proteins