Cell (plasma) Membrane

• Cells need an inside and an outside

  • Separate cell from its environment

  • cell membrane is the boundary

Lipids of Cell Membrane

• Membrance is made of phospholipids

  • phospholipid bilayer

Phospholipid Bilayer

• What molecules can get through directly?

  • fats and other lipids can slip directly through the phospholipid cell membrane

Membrane Proteins

• Proteins determine most of the membrane’s specific functions

  • cell membrane and organelle membranes each have unique collections of proteins

• Membrane Proteins:

  • peripheral proteins = loosely bound to surface of membrane

  • integral proteins = penetrate into lipid bilayer, often completely spanning the membrane = transmembrane proteins

Membrane Protein Types (you do need to know all)

• Channel Proteins - wide open passage

• Ion channels - gated (opening and closing the space)

• Aquaporins - water only, kidney and plant root only

• Carrier Protiens - change shape

• Transport Proteins - require ATP

• Recognition Proteins - glycoproteins

• Adhesion Proteins - anchors

• Receptor Proteins - hormones

Membrane Carbohydrates

• Play a key role in cell-cell recognition

  • Ability of a cell to distinguish neighboring cells from another

  • important in organ & tissue development

  • basis for rejection of foreign cells by immune system

Cholesterol

• Provides stability in animal cells

• replaced with sterols in plant cells

Getting Through A Cell Membrane

• Passive Transport

  • No energy needed

  • move down concentration gradient

  • Non-polar molecules, CO2 and O2

• Active Transport

  • Movement against concentration gradient (low → high)

  • requires ATP

Diffusion

• 2nd Law of Thermodynamics

  • Universe tends towards disorder

• Diffusion

  • movement from high → low concentration

Diffusion of 2 solutes

• Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances

The Special case of water

• Osmosis

  • Movement of water across cell membrance

Concentration of Water

• Direction of osmosis is determined by comparing total solute concentrations

  • Hypertonic - more solute, less water

  • Hypotonic - less solute, more water

  • Isotonic - equal solute, equal water

  • WATER MOVES FROM HYPOTONIC TO HYPERTONIC

Managing Water Balance

• Cell survival depends on balancing water uptake and loss

Hypotonicity

• animal cell in hypotonic solution will gain water, swell & possibly burst (cytolysis)

  • Paramecisum vs. Pond Water

  • Paramecium is hypertonic

  • H20 continually enters cells

  • to solve problem, specialized organelle, contractile vacuole

  • pumps H20 out of cell = ATP

• plant cell

  • turgid (turgor pressure)

  • cell wall

Hypertonicity

• animal cell in hypertonic solution will lose water, shrivel and possibly die

  • salt water organisms are hypotonic compared to their environment

  • they have to take up water and pump out salt

• plant cells

  • plasmolysis

Water Potential (ψ)

• Water moves from a place of greater water potential to a place of lesser water potential (net.)

• As the concentration of a solute increases in a solution, the water potential will decrease accordingly.

Water Potential and Solute Potential

• Ψ = Ψp(pressure) + Ψs(solute)

  • The unit is (-)BARS

    • Note: the water potential will be equal to the solute potential of a solution in an open container since pressure would be zero

• Ψs = -iCRT 

  • i= ionization constant (sucrose = 1.0)

    C = molar concentration

    R = 0.0831 liter bars/mole K

    T = temp in Kelvin (273+C)

Facilitated Diffusion

• Move from HIGH to LOW concentration through protein channel

  • passive transport

  • no energy needed

  • facilitated = with help (from proteins)

Gated Channels

• Some channel proteins open only in presence of stimulus (signal)

  • stimulus usually different from transported molecule

    • ex: ion-gated channels

      • when neurotransmitters bind to a specific gated channels on a nuearon, these channels open = allow Na+ ions to enter nerve cell

    • ex: voltage-gated channels

      • change in electrical charge across nerve cell membrane opens Na+ & K+ channels

Active Transport

• Cells may need molecules to move against concentration situation

  • need to pump against concentration

  • protein pump

  • requires energy

    • ATP

How about large molecules?

• Moving large molecules into & out of cell requires ATP!

  • through vesicles & vacuoles

  • endocytosis

    • phagocytosis = “cellular eating”

    • pinocytosis = “cellular drinking”

    • receptor-mediated
      endocytosis

  • exocytosis