AP Biology 2.3-2.8

Plasma Membrane

  • Consists of a double layer of phospholipids

  • The cell wall surrounds the plasma membrane in some organisms

  • Some molecules can pass through the plasma membrane, and some can’t, depending on size, properties, and charge

Phospholipids

  • Plasma membranes are composed of a double layer of phospholipids

  • Phospholipids contain polar hydrophilic heads that face the watery environment inside or outside the cell

  • Phospholipid contains nonpolar hydrophobic fatty acid tails that face the inside of the membrane away from the watery environment of the cell

Carbohydrate Side Chains

  • Serve as identification tags and help with cells adhering to each other

Amiphatic

  • When a molecule has both a hydrophobic part and a hydrophilic part

  • Ex. Phospholipids

  • Essential for cell membranes

Fluid Mosaic Model

  • The fluid-mosaic model says that the cell membrane is fluid and moving. The lipid bilayer is a mosaic of embedded proteins, cholesterol, and carbs that drift around and perform different jobs

  • Lipid Bilayer=Fluid, flexible, oily tails

  • Mosaic=Proteins and carbs: Integral and peripheral proteins and carbohydrate chains are scattered around the cell membrane

  • Cholesterol stops the membrane from being too rigid or floppy

  • This arrangement lets the membrane be a selective barrier

Integral + Peripheral Proteins

  • Integral proteins are embedded deep in the lipid bilayer and do jobs like transport and signaling

  • Peripheral proteins sit loosely on the membrane surface and acts as enzymes, signalling, and anchors

  • Integral

    • Channels, carriers, pumps, aquaporins

    • Receptors for signals and cell adhesion molecules

    • Almost all transport through cell membrane

  • Peripheral

    • Attach and support: Anchor the membrane to help keep cell shape and hold proteins in place

    • Speed reactions and relay signals

Glycolipids

  • Lipids with short carbohydrate chains attached on the extracellular side of the membrane

  • Help with cell to cell communication

Glycoproteins

  • Used for signaling, adhesion, and protecting/folding proteins

  • Proteins with sugars attached

  • Exposed on the cell surface

Selective Permeability

  • The structure of the cell membrane gives it the property of selective permeability

  • Allows some, but not all, materials to cross

  • Molecules including N2, O2, and CO2, that are small and nonpolar can pass directly through the phospholipid bilayer of the membrane

  • Polar molecules and charged ions require a transport protein

Aquaporins

  • Water-specific channels

  • Integral membrane proteins that form narrow, water-specific pores

  • Allows rapid, passive movement of water molecules in a single file while excluding ions and proteins

  • Let lots of water pass through the membrane quickly

Osmosis

  • Water moves from the side with more water to the side with less water

  • Passive movement of water through an aquaporin or straight through the lipid bilayer (less common and MUCH slower)

  • Moves from low solute to high solute

  • Moves from high solvent to low solvent (water is the solvent)

  • The direction water moves changes cell volume and mass; cells can shrink and grow

Diffusion

  • Diffusion is the passive movement of particles from an area of higher concentration → lower concentration until evenly spread out

Concentration Gradient

  • Diffusion is the passive movement of particles from an area of higher concentration → lower concentration until evenly spread out

Passive Transport (Simple + Facilitated)

  • Any time a substance is moving by diffusion its passive transport because there is no outside energy required

  • Simple Diffusion

    • Movement of small, nonpolar molecules directly through the lipid bilayer from high to low concentrations

    • Passive transport (No ATP)

    • Moves through the lipid part of the membrane (No protein needed)

  • Facilitated Diffusion

    • Movement of polar or charged molecules across the membrane via specific transport from proteins, moving from high to low concentrations

    • Passive transport (No ATP)

    • Ions move via ion channels, glucose via the GLUT carrier, and water mainly via aquaporins

    • Through channel proteins or carrier proteins

Water Potential

  • Water potential is the potenial that water has to move

  • Water flows from a region of high potential to low potential

  • Ψ = Ψp + Ψs

    • Ψp (pressure potential) Physical push on the water, in plant cells, usually positive, in other cells, typically no pressure

    • Ψs (Solute potential): How solutes reduce water’s potential

      • Ψs​=−iCRT

      • i=Ionization constant (1 in sucrose solutions)

      • C=Molar concentration

      • R =Pressure constant (0.0831 bars)

      • T=Temperature in Kelvin (C + 273)

Isotonic

  • The solution inside and outside the cell have the same amount of solute

  • No net water movement

  • No change in size or mass

Hypertonic

  • The solution outside the cell has more solute (less water)

  • Water leaves the cell and goes towards the area with more solute

  • Cell shrinks and shrivels

  • Water travels from high to low concentrations

Hypotonic

  • The solution outside the cell has less solute (more water)

  • Water enters the cell

  • Cell grows and may pop

  • Water travels from high to low concentrations

Active Transport

  • Moving substances against their concentration gradient

  • Requires energy, either directly or indirectly

  • Primary Active Transport

    • A transport protein uses ATP directly to move ions/molecules across the membrane against their gradient

  • Secondary Active Transport

    • One substance moves down its concentration gradient through a transporter, and that free energy is used to carry a second substance against its gradient

    • The transporter itself does not require ATP

    • The downhill movement of one molecule releases free energy that the molecule going upwards uses

Sodium+Potassium Pump

  • A membrane protein that uses energy from the cell to move sodium ions out of the cell and potassium ions into the cell

  • Both moved against the concentration gradient

  • Uses ATP

  • Active transport

Membrane Potential

  • Difference in electrical charge between inside and outside of the cell

  • The cell keeps different ions inside and outside, so the inside ends up more negatively charged than the outside

Exocytosis

  • A cells internal membrane (bubble) fuses with the cell surface and releases its contents outside

  • An internal membrane travels to the plasma membrane, the vesicle fuses with the plasma membrane, and the vesicles contents spill out the cell

  • Moves out of the cell

Endocytosis

  • A cell engulfs materials by folding its plasma membrane around it, forming a vesicle that pinches off and brings the materials inside

  • Brings stuff in

  • Active process; requires ATP

  • Membrane folds in, forms a vesicle, and the vesicle then moves inside and is moved elsewhere

  • Phagocytosis

    • "Cell-eating”

    • Big solids

  • Pinocytosis

    • “Cell-drinking”

    • Fluids and tiny solids

Cholestoral

  • Cholestoral sits between phospholipids tails in the bilayer and tunes membrane fluidity

  • Prevents the membrane from becoming too fluid at high temps and too rigid at low temp