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endocytosis• Selectively permeable• Sodium-potassium pump• Tonicity• Transport protein• Turgid
• Surface area-to-volume ratios affect a cell’s ability to exchange materials. – As cells increase in volume, the relative surface area
decreases and demand for material resources increases; more cellular structures are necessary to adequately exchange materials and energy with the environment. Limits cell size.
2.A.3 – Organisms Must Exchange Matter With the Environment to Grow, Reproduce
and Maintain Organization.
• The surface area of the plasma membrane must be large enough to adequately exchange materials; smaller cells have a more favorable surface-area-to-volume ratio for exchange of materials with the environment.
SA/V Practice Problems
• Simple cuboidal epithelial cells lines the ducts of certain exocrine glands. Various materials are transported into or out of the cells by diffusion. The formula for the surface area of a cube is 6s2 and the formula for volume is s3 where s = length of the side of the cube. Which of the following cube-shaped cells would be most efficient in removing wastes by diffusion?
10µm 20µm 30µm40µm
• Cells lining the kidneys are cuboidal. What is the SA/V of a kidney cell with a side length of 3.5µm?
• What would be the SA/V if cell (b) had a side that is 2.7µm?
• Which cell (a or b) would have an easier time with diffusion?
• What is the SA/V of a spherical liver cell with a diameter of 9.2µm?
2.B.1 – Cell Membranes Are Selectively Permeable Due to Their Structure
• Cell membranes separate the internal from the external environment
• The fluid mosaic model explains selective permeability of the membrane– Cell membranes consist of phospholipids, proteins,
cholesterol, glycoproteins and glycolipids
– Phospholipids have both hydrophobic and hydrophilic regions; fatty acids are oriented towards the middle and phosphate portions are oriented to the outsides
• 2.B.1 continued:
– Embedded proteins can be hydrophilic with charged and polar side groups, or hydrophobic with nonpolar side groups.
– Small, uncharged molecules (N2, O2,) and small hydrophobic molecules pass freely across the membrane; hydrophilic and ions move across through embedded channel and transport proteins. Water moves across through the membrane and through aquaporin proteins.
Semi or Selectively Permeable
• CO2, O2, steroid hormones enter cells easily; conclusion?
• The membrane must be mostly made of _______• Ions (Na+, Cl-, Ca++) proteins and larger molecules
(glucose) move more slowly or not at all; conclusion?
1. Cells must not need those molecules or ions
2. The membrane must have (?) that enables that stuff to get in/out
Membrane Carbohydrates• Glycoproteins – oligosaccharides attached to proteins
– Antibodies (MHC)
– Mucin
– Collagen
– Hormones – ex. FSH
Kinetic Energy
• Molecules are in constant motion• Kinetic energy is ‘free’ energy (usable)• The greater the kinetic (free) energy, the ___
molecules can move.• Molecules move ___ a concentration ___.
Movement Across the Membrane
• Passive Transport – molecules have enough free energy– Diffusion
– Osmosis
– Facilitated diffusion
– Hydrostatic pressure/dialysis
• Active transport – against a concentration gradient– Pumps (proteins)– Endo/exocytosis
2.B.2 – Growth and Dynamic Homeostasis Are Maintained By the Constant Movement
of Molecules Across the Membrane.
• Passive transport requires no cellular energy; movement of molecules from high to low concentration – Facilitated diffusion through proteins
• Ex. Glucose, Na+/K+
– Hypertonic, hypotonic, isotonic
Diffusion
• Kinetic (free) energy of molecules – Down a concentration gradient until equilibrium
– Higher kinetic (free) energy = faster movement
• Gases; small, uncharged molecules– In solution**- membranes moist
– SA/V of lungs is ?
Osmosis
• Diffusion of water through a semi-permeable membrane
• Cells are a solution, in a solution• Compare solutions:
– Hypertonic/hyperosmotic
– Hypotonic/hypoosmotic
– Isotonic/isoosmotic
• **Important to understand concentration gradient • Water moves from hypotonic to hypertonic
Time
Water Potential
Measurement of the Potential of Water to Move
Through a MembraneUseful for Mathematically Predicting
Which Way Water Will Flow
Water Potential
• What is potential ?• Water Potential = ?• Water flows from high water potential to low water
potential till _____(?)***• Water potential is expressed as Psi (Ψ) • Psi is measured in MPa, atm, or bar
– Car tire = 32 psi, 0.2 Mpa– Sea level = 14.5 psi, 0.0MPa, 1 atm, 1 bar, 760mm Hg
Water Potential
Water Water MovementMovement
Force
Down a hill
Garden hose
Fresh to salty
Straw
Water Potential = Pressure Potential + Solute Potential
• Pressure potential: (p ) – Positive pressure, pushing like a hose
– Negative pressure; sucking like a straw Major factor moving water through plants
• Solute potential: (s) – Reduction in water potential due to the presence
of dissolved solutes • Solutes take up space in the water (dilutes pure water)
• Solutions have lower water potential than pure water
Water Potential
• Water potential (Ψ) = Ψp + Ψs
– Ψp – pressure potential (atmospheric pressure)
– Ψs – solute potential (osmotic pressure)
• The Ψp of atmosphere at sea level = 0 MPa
• The Ψs of pure water = 0 MPa
– Pure water at sea level = 0 MPa
Solute Potential
• Solute Potential (Ψs ) = - iCRT– i – ionization constant– C – Concentration in Moles– R – pressure constant (0.0831 literbars/mole-K)– T – temperature in Kelvin (273 + oC)
• I = number of ions that will ionize– Glucose = 1
– NaCl = 2 (Na+, Cl-)
Calculating Solute Potential (s)
s = - iCRT• Ex. A 1.0 M sugar solution @ 22° C under standard
atmospheric conditions:
s = -(1)(1.0M)(0.0821 L · bar )(295K) M · K
s = -24.22 bars
• Adding solute to water lowers its water potential– Solute molecules take up space – Ex. 0.1 M solution = - 0.23 MPa– A 0.1 M solution at sea level:
0 MPa (Ψp)
+ - 0.23 MPa (Ψs)
- 0.23 MPa = Ψ
Ψp = 0+Ψs = 0
Problem:
• A student calculates that the water potential of a solution inside a bag is: s = -6.25 bar, p = 0 bar
• And the water potential of the solution surrounding the bag is s = -3.25 bar, p = 0 bar.
– In which direction will the water flow? • Inside = - 6.25 bar; outside = - 3.25 bar• Water will flow into the bag. This occurs because
there are more solute molecules inside the bag (therefore a value further away from zero) than outside in the solution.
• Increase rate of water uptake– Integral proteins in the membrane– Congenital diabetes insipidus (?) – mutation
Aquaporins
Facilitated Diffusion
• Glucose moves faster through membranes than diffusion can account for (?)
• Diffusion through proteins – May require a receptor
– Insulin/glucose – what is diabetes?
highered.mcgraw-hill.com
Hydrostatic Pressure
• Pressure created by blood - (Ψp)
• Glomerulus of the kidney - dialysis
Active Transport
Moving Molecules Against a GradientIons
Large Molecules
Cell Membrane
• Ions, polar molecules, large molecules move slowly or not at all
• Integral proteins enable movement of specific molecules across the membrane– Shape determines function– Protein shape is sensitive to change (homeostasis)
• 2.B.2• Active transport requires free energy (ATP)
– Establish and maintain concentration gradients
– Moves molecules and ions
– Needs membrane proteins
• Endocytosis and exocytosis move large molecules (use of vesicles)
Active Transport
• Nerve cells:– Na+ K+ ion pump– Membrane potential - difference in electrical charge
across a membrane– Electrochemical gradient– Costs the cells ___(?)
Co-Transport
• Passing of molecules against their concentration gradient using energy from another molecule’s energy