Active and Passive Transport
2.A.3 – Organisms must exchange matter with the environment to grow, reproduce and maintain organization
1. Surface area-to-volume ratios affect a cell’s ability to exchange materials.
Evidence of student learning is a demonstrated understanding of each of the following:
1. 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. These limitations restrict cell size.
To foster student understanding of this concept, instructors can choose an illustrative example such as:
Root hairs, alveoli cells, villi, microvilli
2. 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
2.B.1: Cell membranes are selectively permeable due to their structure.
A. Cell membranes separate the internal environment of the cell from the external environment.
B. Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model.
Evidence of student learning is a demonstrated understanding of each of the following:
1. Cell membranes consist of a structural framework of phospholipid molecules, embedded proteins,
cholesterol, glycoproteins and glycolipids.
2. Phospholipids give the membrane both hydrophilic and hydrophobic properties. The hydrophilic
phosphate portions of the phospholipids are oriented toward the aqueous external or internal
environments, while the hydrophobic fatty acid portions face each other within the interior of the
membrane itself.
3. Embedded proteins can be hydrophilic, with charged and polar side groups, or hydrophobic, with nonpolar side groups.
4. Small, uncharged polar molecules and small non-polar molecules, such as N2, freely pass across the
membrane. Hydrophilic substances such as large polar molecules and ions move across the membrane
through embedded channel and transport proteins. Water moves across membranes and through channel
proteins called aquaporins.
C. Cell walls provide a structural boundary, as well as a permeability barrier for some substances to the internal
environments.
1. Plant cell walls are made of cellulose and are external to the cell membrane.
2. Other examples are cells walls of prokaryotes and fungi.
Essential knowledge 2.B.2: Growth and dynamic homeostasis are maintained by the constant movement of
molecules across membranes.
A. Passive transport does not require the input of metabolic energy; the net movement of molecules is from high
concentration to low concentration.
Evidence of student learning is a demonstrated understanding of each of the following:
1. Passive transport plays a primary role in the import of resources and the export of wastes.
2. Membrane proteins play a role in facilitated diffusion of charged and polar molecules through a
membrane.
To foster student understanding, instructors can choose an illustrative example such as:
• Glucose transport
• Na+/K+ transport
X There is no particular membrane protein that is required for teaching this concept.
3. External environments can be hypotonic, hypertonic or isotonic to internal environments of cells.
B. Active transport requires free energy to move molecules from regions of low concentration to regions of high
concentration.
Evidence of student learning is a demonstrated understanding of each of the following:
1. Active transport is a process where free energy (often provided by ATP) is used by proteins embedded in
the membrane to “move” molecules and/or ions across the membrane and to establish and maintain
concentration gradients.
2. Membrane proteins are necessary for active transport.
C. The processes of endocytosis and exocytosis move large molecules from the external environment to the internal
environment and vice versa, respectively.
Evidence of student learning is a demonstrated understanding of each of the following:
1. In exocytosis, internal vesicles fuse with the plasma membrane to secrete large macromolecules out of the
cell.
2. In endocytosis, the cell takes in macromolecules and particulate matter by forming new vesicles derived
from the plasma membrane.
Latin
Amphi - dual
Aqua - water
Co - with
Endo – inside
Terms
Active transport
Amphipathic
Aquaporin
Concentration gradient
Co-transport
Diffusion
Electrochemical gradient
Endocytosis
Exocytosis
Facilitated diffusion
Flaccid
Fluid mosaic model
Exo - outside
Hyper- higher
Hypo - low
Iso - equal
Phago – to eat
Pino – to drink
Plasm - fluid
-tonus - pressure
Trans – across;
Gated channel
Glycolipid
Glycoprotein
Hypertonic
Hypotonic
Integral protein
Ion channel
Isotonic
Membrane potential
Osmoregulation
Osmosis
Osmotic potential
Passive transport
Peripheral protein
Phagocytosis
Pinocytosis
Plasmolysis
Proton pump
Receptor-mediated endocytosis
Selectively permeable
Sodium-potassium pump
Transport protein
Turgid
Power Point Notes:
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: ______________
_______________________________________________________. The membrane must have ______________ that
enables other substance to get in/out.
Fluid Mosaic Model – amphipathic - ____________________________________________________________________.
Phospholipid molecules are constantly moving (fluid) and have numerous proteins floating within (mosaic). Which
proteins and how many of each protein along with the shape of the membrane helps determine the function of the cell.
Lipids in the membrane: phospholipids and steroids. Phospholipids are amphipathic. The hydrophobic fatty acids ‘tails’
face each other in the interior and the hydrophilic ‘heads’ face towards the water (outside and inside the cell). So the
inner part of the membrane is ‘oily’ and this prevents molecules that dissolve in water (polar) from passing through;
lipid-soluble molecules (non-polar, like steroids) can move through the membrane easily. So the membrane regulates
what substances enter the cell because of the
phospholipids. Small, uncharged molecules
(ie. oxygen, carbon dioxide, nitrogen gases)
move easily between the phospholipid
molecules. Polar, hydrophilic or large
molecules (proteins, glucose, etc.) can only
enter through channel molecules. Proteins
that are embedded in the membrane have
hydrophobic and hydrophilic portions which
arrange themselves into hydrophobic or
hydrophilic portions of the membrane which
holds them in place.
Cholesterol is a steroid that connects phospholipid tails together moderating the effect of high or low temperature and
serving to hold protein ‘rafts’ in a particular location within the fluid, always changing membrane.
Membrane Proteins - proteins embedded in the membrane; Integral proteins function as channels or as receptors for
signaling molecules like insulin. Some are just channels and some are gated and may require ATP and/or a signal to
open/close. Peripheral proteins are attached to the membrane only on one side. Many of them act as signal receptors
or enzymes.
Integral
Transport channels
Receptors for communication
Attachment
Peripheral
Attachment sites
Enzymes (such as cyclase)
Signaling
Electron carriers (ETC)
Carbohydrates in the membrane – oligosaccharides (short chains of monosaccharides like glucose) are used for cell
identification and cell/cell attachment to form tissues.
Glycolipids – oligosaccharides attached to _____________________. Glycoproteins – oligosaccharides attached to
_______________ in the membrane. A, B, O blood types are examples of cell/cell recognition created by glycoproteins.
Cell recognition is important in two ways: in immunity, the identification of foreign antigens from non-self cells and
during development, the cell signaling that causes a cell to differentiate from its neighbor(s).
Movement of materials across the membrane:
Kinetic Energy - molecules are in constant motion; Kinetic energy = ‘free’ energy; the greater the kinetic energy, the
___________________ molecules move. Molecules move __________________ a concentration __________________.
Passive Transport – molecules have free energy and move ‘down their concentration gradient’ from high to low
concentration without energy from the cell. Types: diffusion, osmosis, facilitated diffusion.
 Dialysis is a type of movement that occurs in the kidneys. Blood vessels carrying
wastes come into the kidney and get much smaller putting the blood under
pressure. This pressure ‘forces’ larger molecules (ie. urea, amino acids, etc.)
through the vessel membranes and into a collecting tube. This is a type of
passive transport because the cells don’t spend any ATP but molecules are not
really moving ‘down’ a concentration gradient.
 Diffusion - Gases; small, uncharged molecules. In solution**- membranes moist.
SA/V of lungs is very high.
 Osmosis - Diffusion of water through a semi-permeable membrane. Cells are solutions of water surrounded by
a solution (cells that are not surrounded by water are dead, ie. skin). Water moves down its concentration
gradient from where there is more water to where there is less water.
Three terms describe the solution based on how much solute is dissolved in the water when compare to another
solution: hypertonic, hypotonic, and isotonic.
Hypertonic – solution that has a higher amount of solutes and less water. Hypotonic. - solution that has less
solutes but more water. Isotonic - the two solutions have equal amounts of solute dissolved in water.
Osmoregulation - control of water balance. This is mostly because chemistry can only happen when the
reactants are dissolved but for many other reasons as well (ie. substances only move through the membrane in
dissolved form, thermoregulation, pH/salinity balance, etc.).
The outside solution is compared to the solution inside the cell. Typically, you have to be able to
describe/explain what will happen if a cell is placed in a hyper/hypotonic solution. An easy way to remember
what happens is that water moves down its concentration gradient from hypotonic to hypertonic. Hypotonic
means more water, hypertonic means less water, so water will diffuse down its concentration gradient from
hypo into hyper. Over time then, if you place a cell in a hypotonic solution, water will diffuse into the cell
causing it to swell and become turgid (full). Plant cells prefer to be turgid, but animal cells will burst (no cell
wall). Animal cells do well in isotonic solutions. Plasmolysis – the cell will shrink if you place either cell into a
hypertonic solution. Aquaporins – transport proteins that enable water to pass quickly through cell
membranes; faster than just osmosis by itself.

Water Potential – a mathematical measurement of the potential of water to move through a
membrane. Math is used to predict whether water will move into or out of the cell. Water flows from
high water potential to low water potential until it reaches ___________________________***
https://youtu.be/nDZud2g1RVY - solute potential; Bozeman
Water potential is expressed as Psi (Ψ); Psi is measured in MPa, atm, bar or mm.
Ex. Sea level = 14.5 psi, 0.0 MPa, 1 atm, 1 bar, or 760mm Hg
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) – water with no solutes dissolved in it has 0 solute potential.
Water potential goes down when solutes are added because solutes
take up space in the solution. Therefore: a solution has a lower water
potential than pure water
Water potential (Ψ) = Ψp + Ψs
Ψp of atmosphere at sea level = 0 Mpa
Ψs of pure water = 0 MPa
Pure water at sea level = ___ MPa
Solute Potential (Ψs ) = - iCRT
i – ionization constant – (the number of ions that will form when dissolved in water)
Glucose = 1;
NaCl = 2 (Na+and Cl-)
C – Concentration (in Moles)
R – pressure constant (0.0831 liter-bars/mole-K)
T – temperature in Kelvin (273 + oC)
Example:
Calculating Solute Potential (s); s = - iCRT
A 1.0 M sugar solution @ 22° C under standard atmospheric conditions:
Adding solute to water lowers its water potential (solute molecules take up space)**********
Example Problem:
A student measured the solute potential (s) inside of a ‘cell’ to be - 6.25 bar and the pressure potential (p) to be
0.0 bar. The solute potential (s) surrounding the bag was found to be -3.25 bar, and the pressure potential (p)
was 0.0 bar. Identify the direction water will flow and justify your answer.
Is it possible for p to be greater than s? Describe how this is possible and what would be expected results?
Facilitated Diffusion – diffusion of larger/polar, charged molecules through transport proteins (may require a receptor);
Insulin/glucose – insulin is the signaling molecule there are two types of diabetes.
Active transport – molecules are moved against a concentration gradient. This requires ATP from the cell and a ‘pump’
molecule (protein). Cells must establish and maintain concentration gradients (like chemiosmosis). Integral proteins
embedded in the membrane enable movement of specific molecules across the membrane. The shape of the integral
protein is sensitive to change, another reason cells can only tolerate certain ranges of environmental factors.
Nerve cells use a Na+/K+ ion ‘pump’, to create and maintain membrane potential the difference in electrical charge
across its membrane.
Co-Transport - movement of molecules against their concentration gradient using energy from another molecule’s
energy; A cell ‘pumps H+ outside the membrane, a concentration gradient builds up and a larger, less permeable
molecule ‘rides’ in with the H+. Plants use proton ‘pumps’ to move sucrose and create hyperosmotic conditions in the
root hair cells. Explain how co-transport effects osmosis.
Endocytosis and exocytosis move large molecules using vesicles.
Exocytosis:
Endocytosis
Secretion
Phagocytosis
Pinocytosis
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 Transport in Plants
Transpiration – the movement of water through the plant from the roots to the leaves. Water is absorbed by roots.
Explain why root cells have root hairs.
__________________________________________.
The plant ‘pumps’ solutes (sugars, etc.) into the root
hair cells, which lowers water potential. The soil then
has greater water potential and water diffuses in by
osmosis.
https://youtu.be/MxwI63rQubU - plant transport
Mycorrhizae – a fungus that forms a symbiotic
relationship with plants. Mycorrhizae extend filaments
into the soil and around root cell resulting in additional
surface area to absorb water.
The plant pumps solutes into root hair cells, lowering water potential.