Cell Membrane Transport Mechanisms

• Cell transport is made possible through phospholipids (cell membrane), concentration gradient, selective permeability and membrane bound proteins

Selective Permeability

• Selective permeability is property of biological membranes
• Allows some substances to cross the membrane more easily than others

Concentration Gradient

• Concentration gradient references the difference in the amount of solution on each side of a cell membrane

Phospholipids

• Phospholipids have a head that loves water, making it hydrophilic and polar
• Phospholipids have a tail that hates water, making it hydrophobic and non-polar
• The cell membrane forms a bilayer with the heads facing out and tails facing in

Types of Cellular Transport

• Passive transport includes diffusion and facilitated diffusion
  • Facilitated diffusion needs channel proteins
  • Passive transport does not require ATP
• Active transport needs carrier proteins and requires ATP
  • Endocytosis and exocytosis are types of active transport

Passive Transport

• Diffusion is the tendency of a substance to move from an area of high concentration to low concentration across a membrane
• Diffusion does not require the cell expend any energy
• Facilitated diffusion transports molecules with channel proteins during diffusion
  • It requires no energy output

Active Transport

• Active transport moves molecules against a concentration gradient, from low concentration to high, concentration
• Active transport uses carrier proteins in the cell membrane with energy from ATP

Types of Active Transport

• Exocytosis releases of materials out of the cell
  • The vesicle merges with the cell membrane
• Endocytosis intakes of materials from outside the cell
  • The vesicle is formed from the membrane

Cell Size and Diffusion

• Cells remain small to maximize diffusion
• As cell volume increases, efficiency decreases
  • Prokaryotes are limited by efficient metabolism
  • Animal (Eukaryotic) cells are limited by surface area to volume ratio
• The surface area of cells must be proportionally larger than its size/volume

Fluid Mosaic Model

• The fluid mosaic model describes the cell membrane as flexible and made up of different macromolecules
• Cholesterol provides structure and stability to the membrane by interacting with the phospholipids
  • It also prevents the membrane from becoming too flexible or fluid
• Carbohydrate chains are bound to lipids or proteins within the membrane
  • They act as identifiers for the cell
  • These chains also aid in immune system response and cell protection
• Glycolipids provide structure to the membrane and bind to carbohydrate chains

Osmosis

• Osmosis is the diffusion of water and other solvents across a semipermeable membrane
• Solvents move from high to low concentration

Solutions

• Solvents is a liquid that dissolves the solute e.g. water or alcohol
• Solute is anything dissolved in a solvent
• Together they make a solution

Salt

• Salt on the inside or outside of the cell draws water in its direction
• The same applies for all other solutes

Solutions and Cells

• All solutions strive for equal balance on both sides of the membrane for homeostasis
• The substances dissolved in solutions are described using specific vocabulary

Hypertonic

• The concentration of a solute outside the cell is higher than the concentration inside
• Water diffuses out of the cell until equilibrium is established
• The cell will shrink and lose mass

Hypotonic

• Concentration of solute outside the cell is lower than the concentration inside
• Water diffuses into the cell until equilibrium is established
• The cell will get bigger and gain mass

Isotonic

• The concentration of solute outside and inside the cell is equal
• Water diffuses in and out at equal rates
• There is no net change in cell size or mass