Cell Structure and Functions: Membrane Transport

Cell Structure

• A cell has three basic parts: the plasma membrane, the cytoplasm, and the genetic material (DNA or RNA)
• The plasma membrane regulates what enters and leaves the cell
• The cytoplasm is the jelly-like substance inside the cell where metabolic reactions occur
• The genetic material contains the cell's instructions for growth, reproduction, and response to stimuli

Phospholipids in a Watery Environment

• Phospholipids organize into a bilayer in a watery environment due to their hydrophobic tails and hydrophilic heads
• The tails face inward, away from water, and the heads face outward, interacting with water

Membrane Transport

• Membrane transport refers to the movement of substances (solutes, ions, and water) in and out of the cell
• Two factors involved: permeability and size of solutes or substances
• Interstitial fluid is the fluid outside cells, rich in nutrients, sugars, fatty acids, vitamins, hormones, salts, and wastes

Selective Permeability

• The plasma membrane allows only certain substances to enter the cell
• Nutrients enter, and wastes leave the cell through passive or active transport mechanisms

Passive Transport

• No energy (ATP) is needed
• Molecules move down their concentration gradient from high to low concentration
• Types of passive transport: diffusion, osmosis, and facilitated diffusion

Concentration Gradient

• A concentration gradient exists when the solute concentration in a solvent is greater at one point than at another
• The concentration gradient is calculated as the concentration difference between two points divided by the distance between them

Diffusion

• The movement of solutes from an area of higher concentration to an area of lower concentration
• Example: a salt crystal placed in a beaker of water, where salt ions move into the water, eventually reaching equilibrium

Facilitated Diffusion

• Transport proteins (carrier or channel proteins) assist molecules across the membrane
• Examples: glucose, amino acids, H2O, and ions
• Channel proteins form passageways through the plasma membrane, allowing specific ions or molecules to enter or exit the cell
• Carrier proteins bind to molecules, change shape, and ferry them across the membrane

Osmosis

• Refers to the diffusion of water from high to low concentration
• Aquaporins are channel proteins for water passage
• Tonicity refers to the ability of a solution to change the shape or tone of cells by changing water volume

Active Transport

• Energy (ATP) is needed to move molecules against their concentration gradient from low to high concentration
• Types of active transport: primary and secondary active transport

Primary Active Transport

• Directly uses ATP to drive transport
• Examples: Ca2+ pump, H+ pump, and Na+-K+ pump
• The sodium-potassium pump is an example of primary active transport

Secondary Active Transport

• Moves more than one substance at a time
• Types: symport (2 substances moved in the same direction) and antiport (2 substances moved in opposite directions)
• Examples: cotransport of sugars, amino acids, and ions

Vesicular Transport

• Fluid and large particles are transported across membranes in vesicles (sacs)
• Types: exocytosis (eject substances out of the cell) and endocytosis (ingest substances into the cell)

Endocytosis

• Types: phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis
• Examples: white blood cells engulfing bacteria, intestinal cells ingesting fluid with dissolved molecules, and concentration of specific substances (ligands) that bind to receptor proteins