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APBioChap7 Osmosis, Water Potential, etc. Cheat Sheet by

Membrane & Transport, Water Potential, osmosis, diffusion, passive transport, etc.

Major Formulas

Calcul­ating Water Potential
Ψ = Ψs + Ψp
Calcul­ating Solute Potential
Ψs = -iRCT
Solute Potential
Pressure Potential
The # of particles the molecule will make in water
Molar Conent­ration (from experi­mental data)
Pressure Constant = 0.0831 liter bar/mole K
Temper­ature in degrees Kelvin = 273 + C° of solution

Membrane Structure

Principal compon­ents:
Lipids (phosph­oli­pids, choles­terol), proteins, carboh­ydrate groups.

Made of glycerol, two fatty acid tails, and a phosph­ate­-linked head group.
→A phosph­olipid bilayer involves two layers of phosph­olipids with their tails pointing inward

Lipid composed of four fused carbon rings
→Found alongside phosph­olipids in the core of the membrane.

Active Transport

Primary Active Transport
Directly uses a source of chemical energy (e.g., ATP) to move molecules across a membrane against their gradient
Secondary Active Transport
Uses an electr­och­emical gradient – generated by active transport – as an energy source to move molecules against their gradient
→Does not directly require a chemical source of energy such as ATP
The Sodium­-Po­tassium Pump Cycle
Moves Na+ out of cells and K+ into them
Electr­ogenic pump
1. The pump is open to the inside of the cell and binds/­takes up 3 Na+ ions
2. Once the Na+ ions bind, the pump is triggered to hydrolize ATP. One P-group is attached to the pump, and then phosph­ory­lated. ADP is released as a by-pro­duct.
3. Phosph­ory­lation causes the pump to change form so that it then faces the exterior of the cell. Like this, the pump no longer has an affinity for Na+ ions, and 3 are released.
4. Facing this direction, the pump now has an affinity for K+ ions. It binds 2 of them which triggers the release of the P-group attached to the pump.
5. With the P-group gone, the cell once again changes form and then faces towards the interior of the cell.
6. The pump is now back to step 1, and the cycle repeats.
The sodium­-po­tassium pump acts primarily by building up a high concen­tration of potassium ions inside the cell, which makes potass­ium’s concen­tration gradient very steep. The gradient is steep enough that potassium ions will move out of the cell (via channels), despite a growing negative charge on the interior. This process continues until the voltage across the membrane is large enough to counte­rba­lance potass­ium’s concen­tration gradient. At this balance point, the inside of the membrane is negative relative to the outside. This voltage will be maintained as long as K+concen­tration in the cell stays high


may be engulfed when no longer needed
cells with genetic damage are replaced
defense against infection
signals trigger caspases to carry out apoptosis

Hypotonic vs Isotonic vs Hypertonic

Isotonic solution: Solute concen­tration is the same as that inside the cell; no net water movement across the plasma membrane

Hypertonic solution: Solute concen­tration is greater than that inside the cell; cell loses water

Hypotonic solution: Solute concen­tration is less than that inside the cell; cell gains water

Cells without cell walls will shrivel in hypertonic solution and lyse (burst) in a hypotonic solution


Phosph­olipids are ampipathic because of their hydrop­hilic, polar heads and hydrop­hobic, nonpolar tails.

The hydrop­hilic heads of phosph­olipids in a membrane bilayer face outward, contacting the watery fluid both inside and outside the cell. Since water is a polar molecule, it readily forms electr­ostatic intera­ctions with the phosph­olipid heads.

Phosph­olipids tuck their fatty acid tails away in the interior of the membrane, where they are shielded from the surrou­nding water.

Selective Permea­bility

The hydrop­hobic core of the plasma membrane helps some materials move through the membrane, while it blocks the movement of others
Polar molecules can easily interact with the outer face of the membrane, where the negatively charged head groups are found, but they have difficulty passing through its hydrop­hobic core
While small ions are the right size to slip through the membrane, their charge prevents them from doing so. Instead, they must be transp­orted by special proteins.
Larger charged and polar molecules, like sugars and amino acids, also need help from proteins


where rRNA & ribosomes are synthe­­sized
protein factories
Endome­mbrane System
regulates protein traffi­­c+­m­e­ta­­bolic functions
holds chromatin, surrounded by nuclear envelope
Rough: makes proteins Smooth: synthe­­sizes lipids, stores Ca++, detoxifies drugs/­­po­isons
Golgi Apparatus
processes, packages, & secretes substances
intrac­­el­lular digestion
powerhouse of the cell
storage & pumping out water
absorbs light & synthesize sugar
maintains cell shape, flow, positi­­oning
Centro­­somes MTOCs
organize spindle fibers (cell division)
Cell Wall
protects, maintains shape, regulates water intake
break down fatty acids to be used for forming membranes and as fuel for respir­ation, transfer hydrogen from compounds to oxygen to create hydrogen peroxide and then convert hydrogen peroxide into water

Osmosis & Tonicity

Osmosis is the movement of water through a semi-p­erm­eable membrane from a region of high concen­tration to a region of low concen­tra­tion, tending to equalise the concen­tra­tions of the water.
Osmosis is passive transport, meaning it does not require energy to be applied.
The ability of an extrac­ellular solution to make water move into or out of a cell by osmosis is know as its tonicity.
A solution's tonicity is related to its osmola­rity, which is the total concen­tration of all solutes in the solution.

Passive Transport

A substance moves from an area of high concen­tration to low concen­tration until its concen­tration becomes equal throughout a space
Facili­tated Diffusion
Molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers
A concen­tration gradient exists for these molecules, so they have the potential to diffuse into (or out of) the cell by moving down it. However, because they are charged or polar, they can't cross the phosph­olipid part of the membrane without help. Facili­tated transport proteins shield these molecules from the hydrop­hobic core of the membrane, providing a route by which they can cross.
Channel proteins span the membrane and make hydrop­hilic tunnels across it, allowing their target molecules to pass through by diffusion
Very selective and will accept only one type of molecule for transport
Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and certain parts of the kidney
Play an important role in electrical transm­ission along membranes (in nerve cells) and in muscle contra­ction (in muscle cells)
Carrier Proteins
Change their shape to move a target molecule from one side of the membrane to the other
Will change shape in response to binding of their target molecule, with the shape change moving the molecule to the opposite side of the membrane
Provide hydrop­hilic molecules with a way to move down an existing concen­tration gradient (rather than acting as pumps)

Types of Cell Commun­­ic­ation

Quorum Sensing
monitors bacteria population density & controls gene expression
Autocrine Signals
produced & used by same cell
Juxtacrine Signals
physically touching cells (gap junctions, plasmo­­de­s­mata)
Paracrine Signals
adjacent (not touching) cells (synapses, growth factors)
Endocrine Signals
for all tissues, long distance (hormones)

Prokaryote vs Eukaryote

no internal membra­nes­/or­gan­elles
membr­­ane­­-bound organelles
circular DNA
DNA forms chromo­somes
smaller ribosomes
larger ribosomes
anaerobic or aerobic metabolism
aerobic metabolism
no cytosk­eleton present
cytosk­eleton present
mainly unicel­lular
mainly multic­ellular


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