Transport Across Cell Membrane

Fluid Mosaic Model

Flexible and dynamic layer composed of organic molecules

• Phospholipids form bilayer with hydrophylic heads facing outward + hydrophobic tails inward (creates semi-permeable barrier)

• Proteins are attached to bilayer; function in transport, signaling, and structural support

• Glycoproteins + glycolipids are involveed in cell recognition

• Carbohydrates help with recognition, adhesion, and signaling

• Cholesterol helps maintain membrane fluidity and stability

Hydrophobic + Hydrophilic Regions

Hydrophilic

• Head of phospholipid face towards outside and inside of cell towards water

Hydrophobic

• Non polar fatty acid tails that face each other

Passive Transport

Substances move across cell membrane from high to low concentration (with gradient)

Simple Diffusion

• Movement of nonpolar molecules through the semipermeable phospholipid bilayer

• Requires no help of transport proteins

Osmosis

• The diffusion of water across a semipermeable membrane

• Water moves from low concentration to high concentration to balance solute levels

Facilitated Diffusion

• Transport of polar substances or ions across a semipermeable membrane

• Requires transport proteins; requires no energy

Active Transport

Moves substances across cell membrane from low to high (against gradient)

• Used to move very large molecules that wont fit through the membranes ort ransport proteins

• Requires ATP

Endocytosis (in)

Cell engulfs substances by wrapping its membrane around to form a vesicle

• Phagocytosis is when the cell takes in large particles like bacteria

• Pinocytosis is when the cell takes in liquids or dissolved substances in small vesicles

Exocytosis (out)

Opposite of Endocytosis

• Forms a vesicle around unwanted particles and expels it out of cell

Cell Membrane

• Described as selectively permeable because it allows SOME substances to pass, blocking others

• Smaller molecules can pass while larger and polar molecules require channels or cerriers

• Helps maintain homeostasis

Rate of Diffusion Factors

Temperature

• High = increase of KE (diffuses quicker)

• Low = slower diffusion

Molecule Size

• Small = faster difussion b/c easier movement

• Large = slower + requires protein channels

Molecule Charge

• Nonpolar = easy diffusion

• Polar / Ionic = more difficulty; often requires protein assistance

Concentration Gradient

• Steeper = faster rate of diffusion

• Rate slows down as gradient lessens

Pressure Gradient

• High pressure on one side can push molecules across membrane faster