Terms

• Active transport• Amphipathic• Aquaporin• Concentration gradient• Co-transport• Diffusion• Electrochemical gradient• Endocytosis• Exocytosis

• Facilitated diffusion• Flaccid• Fluid mosaic model• Gated channel• Glycolipid• Glycoprotein• Hypertonic• Hypotonic• Integral protein• Ion channel

• Isotonic• Ligand• Membrane potential• Osmoregulation• Osmosis• Osmotic potential• Passive transport• Peripheral protein• Phagocytosis• Pinocytosis

• Plasmolysis• Proton pump• Receptor-mediated

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.

• Ex. Root hairs, alveoli cells, villi, microvilli

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 Model• Amphipathic - ?• Phospholipids bilayer

Fluid Mosaic Model

• Proteins and shape determine the function of the cell

Aquaporins

Membrane Proteins • Integral

– Transport channels (ex.)– Receptors for

communication (ex.)– Attachment (ex.)

• Peripheral – temporarily attached • Attachment sites

• Enzymes

• Signaling

• Electron carriers (ETC)

Membrane Carbohydrates

• Glycolipids – oligosaccharides – Attached to lipids – Cell attachment forming tissues– ‘Self’ recognition

• Antigens – A, B, O blood types

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.

?? ?

Plasmolysis

Plant Transport

• Concepts:• Water potential• Active transport

Water Potential Values

Medium Water Potential (MPa)

Air (50% humidity) -100.0

Air (90% humidity) -13.0

Leaf -1.5

Stem - 0.7

Root - 0.4

Soil - 0.1

Saturated soil + 2.0

Water Potential in Plants

• Water moves from soil into root cells because the cells have lower water potential due to the (Ψs)

Soil or root?

• To maintain solute potential in their cells, plants use cotransport

• *Cotransport molecules are proteins

Proton Pumps

Real life scenario: what happens with salt water intrusion or over-fertilization? Cell has More/Less water potential than soil?

Cell Soil

Plasmolysis - cells lose water, become flaccid

Additional Water Absorption by Roots

• SA/V increased by:– Root hairs– Mycorrhizae - 90% of terrestrial plants

• 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

• Plants: proton ‘pump’

Channel protein

Carrier protein

Down the concentration

gradient

Against the concentration

gradient

Exocytosis/Endocytosis

• Exocytosis:– Secretion – Secretory vesicle

• Endocytosis:– Phagocytosis– Pinocytosis – Receptor-mediated

endocytosis

Phagocytosis

• Phagocytosis - Pseudopodia ‘engulf’ food items – macrophages

• Pinocytosis – cell drinking; invagination

Receptor-Mediated Endocytosis

• Receptor proteins on the membrane – Hypercholesterolemia = lack protein to take up LDL’s from

blood; recessive

– Cholesterol molecules as LDL’s

2.B.1

• Cell walls create a structural boundary, as well as a permeability barrier for some substances – Most organisms have cell walls (Plants - cellulose;

fungi - chitin, prokaryotes – peptidoglycans)

Water potential = pressure potential + solute potential

? ? ?

Dialysis Tubing Experiment