Movement across the Cell Membrane
The diffusion of solutes across a synthetic membrane Molecules of dye WATER Membrane (cross section) Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Net diffusion Net diffusion Equilibrium Equilibrium (b) Diffusion of two solutes
Diffusion: 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Diffusion movement from HIGH LOW concentration
Simple Diffusion: Move from HIGH to LOW concentration passive transport no energy needed movement of water diffusion osmosis
Facilitated Diffusion: Diffusion through protein channels channels move specific molecules across cell membrane no energy needed open channel = fast transport HIGH facilitated = with help LOW The Bouncer
Active Transport: Cells may need to move molecules against concentration gradient conformational shape change transports solute from one side of membrane to other protein pump costs energy = ATP LOW conformational change ATP HIGH The Doorman
Active transport: Many models & mechanisms ATP ATP antiport symport
Getting through cell membrane: Passive Transport Simple diffusion diffusion of nonpolar, hydrophobic molecules lipids HIGH LOW concentration gradient Facilitated transport diffusion of polar, hydrophilic molecules through a protein channel HIGH LOW concentration gradient Active transport diffusion against concentration gradient LOW HIGH uses a protein pump requires ATP ATP
Transport summary: simple diffusion facilitated diffusion active transport ATP
How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = cellular eating pinocytosis = cellular drinking exocytosis exocytosis
Endocytosis: phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by molecular signal Cholesterol (carried by LDL s) is brought in by receptor-mediated endocytosis.
The Special Case of Water: Movement of water across the cell membrane 2007-2008
Osmosis is just diffusion of water: Water is very important to life, so we talk about water separately Diffusion of water from HIGH concentration of water to LOW concentration of water across a semi-permeable membrane
Figure 7.11 Lower concentration of solute (sugar) Higher concentration of solute More similar concentrations of solute Sugar molecule H 2 O Osmosis is just diffusion of water! Selectively permeable membrane Lets see more Detail here! Osmosis
Osmosis: (detail of membrane) Water molecules can pass through pores, but sugar molecules cannot. This side has fewer solute molecules, more free water molecules. Selectively permeable membrane Osmosis Water molecules cluster around sugar molecules. This side has more solute molecules, fewer free water molecules. OK, that s better!
Water Balance of Cells Without Cell Walls: Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water (see beakers) This is Hypotonic relative to the bag contents This is Hypertonic relative to the bag contents This is??? relative to the bag contents
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 hypotonic hypertonic net movement of water
Managing water balance: Osmoregulation Cell survival depends on balancing water uptake & loss through osmoregulation freshwater balanced saltwater
1 Managing water balance: Hypotonic a cell in fresh water high concentration of water around cell problem: cell gains water, swells & can burst KABOOM! example: Paramecium ATP ex: water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP plant cells turgid = full cell wall protects from bursting No problem, here freshwater
Pumping water out: Contractile vacuole in Paramecium ATP
2 Managing water balance: Hypertonic a cell in salt water low concentration of water around cell problem: cell loses water & can die example: shellfish solution: take up water or pump out salt plant cells plasmolysis = wilt can recover I m shrinking, I m shrinking! I will survive! saltwater
3 Managing water balance: Isotonic animal cell immersed in mild salt solution no difference in concentration of water between cell & environment problem: none no net movement of water flows across membrane equally, in both directions cell in equilibrium volume of cell is stable example: blood cells in blood plasma slightly salty IV solution in hospital That s perfect! I could be better balanced
Aquaporins: 1991 2003 Water moves rapidly into & out of cells evidence that there were water channels protein channels allowing flow of water across cell membrane - rate of osmosis Peter Agre John Hopkins Roderick MacKinnon Rockefeller
Do you understand Osmosis.05 M.03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell
Short-Distance Transport of Water Across Plasma Membranes: (see chapter 36, pages 782 785) To survive, plants must balance water uptake and loss Osmosis is the diffusion of water into or out of a cell that is affected by solute concentration and pressure = water potential Ψ = Water s ability to get up and go! This is expressed by the water potential equation: Ψ = Ψ S + Ψ P Water potential is abbreviated as Ψ and measured in a unit of pressure called the (bars), aka. [megapascal (MPa)] Ψ p = 0 (bars), aka. [Mpa] for pure water at sea level and at room temperatures
How Solutes and Pressure Affect Water Potential: Water potential determines the direction of movement of water Water flows from regions of higher water potential to regions of lower water potential Potential refers to water s capacity to perform work Which side of the membrane has the largest water potential? Which side of the membrane will increase in volume? Side A Ψ p = 0 (bars) Side B
This is expressed by the water potential equation: Ψ = Ψ S + Ψ P The solute potential (Ψ S ), aka. (osmotic potential) of a solution is directly proportional to its molarity Solute potential is always expressed as a negative number, eg. a 0.1 M sucrose is -0.23 bars because it has a negative effect on water potential, i.e. reduces water s ability to move and do work. Water molecules can pass through pores, but sugar molecules cannot. This side has fewer solute molecules, more free water molecules. Selectively permeable membrane Osmosis Water molecules cluster around sugar molecules. This side has more solute molecules, fewer free water molecules. Solute Potential Formula: Ψ s = -icrt i = ionization constant (for solute) C = molar solute concentration at equilibrium R = pressure constant (0.0831 liter bar/mole K ) T = temperature K (273 + C ) (see chapter 36, pages 782 785)
Lower concentration of solute (sugar) Higher concentration of solute More similar concentrations of solute Sugar molecule H 2 O Osmosis is just diffusion of water! Selectively permeable membrane Lets see more Detail here! Osmosis
Water Movement Across Plant Cell Membranes Water potential affects uptake and loss of water by plant cells If a flaccid (limp) cell is placed in an environment with a higher solute concentration, the cell loses water and undergo plasmolysis (wilts) Plasmolysis occurs when the membrane shrinks and pulls away from the cell wall Initial flaccid cell: ψ P = 0 ψ S = 0.7 ψ = 0.7 MPa Environment 0.4 M sucrose solution: ψ P = 0 ψ S = 0.9 ψ = 0.9 MPa Final plasmolyzed cell at osmotic equilibrium with its surroundings: ψ P = 0 ψ S = 0.9 ψ = 0.9 MPa (a) Initial conditions: cellular ψ > environmental ψ
Water Movement Across Plant Cell Membranes If a flaccid cell is placed in a solution with a lower solute concentration, the cell gains water and become turgid; swells Turgor loss in plants causes wilting, which can be reversed when the plant is watered Initial flaccid cell: ψ P = 0 ψ S = 0.7 ψ = 0.7 MPa Environment Pure water: ψ P = 0 ψ S = 0 ψ = 0 MPa Final turgid cell at osmotic equilibrium with its surroundings: ψ P = 0.7 ψ S = 0.7 ψ = 0 MPa (b) Initial conditions: cellular ψ < environmental ψ
Wilted Initial flaccid cell: ψ P = 0 ψ S = 0.7 ψ = 0.7 MPa Environment 0.4 M sucrose solution: ψ P = 0 ψ S = 0.9 ψ = 0.9 MPa Final plasmolyzed cell at osmotic equilibrium with its surroundings: ψ P = 0 ψ S = 0.9 ψ = 0.9 MPa (a) Initial conditions: cellular ψ > environmental ψ
Turgid Initial flaccid cell: ψ P = 0 ψ S = 0.7 ψ = 0.7 MPa Environment Pure water: ψ P = 0 ψ S = 0 ψ = 0 MPa Final turgid cell at osmotic equilibrium with its surroundings: ψ P = 0.7 ψ S = 0.7 ψ = 0 MPa (b) Initial conditions: cellular ψ < environmental ψ
Xylem sap Mesophyll cells Outside air ψ = 100.0 MPa Stoma Water molecule Atmosphere Leaf ψ (air spaces) = 7.0 MPa Trunk xylem ψ = 0.8 MPa Trunk xylem ψ = 0.6 MPa Soil ψ = 0.3 MPa Water potential gradient Leaf ψ (cell walls) = 1.0 MPa Transpiration Xylem cells Adhesion by hydrogen bonding Cell wall Cohesion by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Soil particle Water Water uptake from soil
Diffusion through phospholipid bilayer: What molecules can get through directly? fats & other lipids inside cell NH 3 outside cell lipid sugar salt aa H 2 O What molecules can NOT get through directly? polar molecules H 2 O ions (charged) salts, ammonia large molecules starches, proteins
Diffusion across cell membrane: Cell membrane is the boundary between inside & outside separates cell from its environment Can it be an impenetrable boundary? NO! IN food carbohydrates sugars, proteins amino acids lipids salts, O 2, H 2 O IN OUT OUT waste ammonia salts CO 2 H 2 O products cell needs materials in & products or waste out