BIOL 111 Chapter 7

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Cell Membrane Structure

"Ragged edge of life"

Functions:

1. provides boundary between cell and outer world
2. selective permeability: embedded proteins control what goes in and out

Composition:

• phospholipids — amphipathic properties (hydrophilic and hydrophobic ends)
• proteins
• cholesterol

Old model: sandwich model (1935-1960s)

New model: fluid mosaic (early 1970s) found with freeze-fracture method

• Fluid = lipids
  • moving back and forth laterally ~10⁷ times per second
  • moving top to bottom about once per month
  • Unsaturated fats separate lipids to make it more fluid (found in plants)
  • Saturated fats make membrane more viscous
  • Cholesterol used for spacing in animal cells
• Mosaic = proteins
  • integral (all the way through) and peripheral (on one side or other)

Selective permeability

• in: Food
• out: Waste
• in/out: Water
• in: Oxygen (for aerobic cells)
• out: Carbon Dioxide

Hydrophobic/nonpolar moleculse just diffuse across membrane (O₂, CO₂, hydrocarbons)

Hydrophilic/polar molecules need special specific transport systems (H₂O, nutrients, proteins, waste)

Diffusion

Movement of molecules down a concentration gradient

If along a biological membrane: passive transport (no energy required)

Osmosis

Diffusion (passive transport) of water across a selectively permeable membrane

Tonicity

property of a solution in reference to a particular membrane

isotonic

iso- = "same"

solutions have equal concentration of solute

dynamic equilibrium: same rate of diffusion; no net movement

hypertonic

hyper- = "over" as in hyperactive / over-active

referenced solution has more solute than other

water diffuses into referenced solution

hypotonic

hypo- = "under" as in hypodermic / under-skin

referenced solution has less solute than other

water diffuses out from referenced solution

Movement Across Membranes

simple diffusion (passive transport)

molecules go through membrane by themselves (usually small, hydrophobic molecules)

facilitated diffusion (passive transport)

Transport protein "tubes" allow polar molecules to naturally diffuse across hydrophobic portion of membrane

active transport

uses transport proteins to pump molecule against concentration gradient (requires ATP)

bulk transport

creates packaging vesicles to contain proteins and polysaccharides

requires ATP

exocytosis (expulsion of molecules); endocytosis (engulfment of molecules)

Vocabulary

Be familiar with the following terms for next lecture:

genome

chromosome

haploid

diploid

sister chromatid

centromere

centriole

centrosome

kinetochore

mitosis

cytokinesis