Cell Biology Membrane Transport
Note: please remember that these are abbreviated lecture notes and that I often cover
additional material in class. Also, the notes provided are not meant to replace your
reading the text.
Membrane Transport
- Regulation of traffic of nutrients, wastes, ions
- Membranes are selectively permeable
o Sugars, amino acids, nucleotides, ions
 Too polar/charged to pass
 Require transport proteins
o O2, CO2 and H2O
 Freely pass
Types of Transport
- Simple Diffusion
- Passive Transport (facilitated diffusion)
o Requires transport protein
- Active Transport
o Requires transport protein + energy
Simple Diffusion
- high concentration to low concentration
- no energy required
- small molecules; O2, CO2 and H2O
Osmosis
- movement of H2O across a membrane from a region of low to high
[solute]
- hypotonic solution – lower solute conc. than inside
- hypertonic solution – higher solute conc. than inside
- isotonic solution – same
Osmoregulation (control of water balance)
- most animals cells live in a hypotonic solution
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o  intracellular solutes e.g. negatively charged ions/organic
molecules
- water tends to flow in; can burst cell
- Animal cells
o Continuously pump out inorganic ions (e.g. Na+)
o Primary purpose of Na+/K+ pump
- Plants (and algae, fungi, bacteria)
o cell wall keeps from bursting
o cells become turgid (firm)
o turgor pressure forces out as much water that comes in
o in hypertonic solution, plants wilt (plasmolysis)
Transfer of Uncharged Solutes
- determined only by concentration gradient
- by diffusion or transport proteins
- e.g. glucose
Cellular Ion Concentrations
- [ion] in cytosol differs from extracellular fluid
- cytosol is more negative charged (more anions)
- maintained by ion specific transport mechanisms
Inside Cell
Outside Cell
Ca2+


Na2+


K+


Membrane Potential
- difference in electrical charge (voltage) across membrane
- cytoplasmic side is more negative
- favors passage of cations into cell; anions out of cell
Transfer of Charged Solutes
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- electrochemical gradient
o concentration gradient + membrane potential
o can work in same direction
 Na+
o Can work in opposite direction; little movement

K+
Passive Transport
- movement of substance down its concentration or electrochemical
gradient
- requires ion channel or carrier protein
Passive transport via Ion channels
- Multipass membrane proteins
- Hydrophilic pore
- Selective
o Na+, K+, Cl-, Ca2+
o Size and charge
- Direction determined by electrochemical gradient
- Fast (1000x faster than transport by carrier proteins)
- Open or closed depending on conformation
o Ligand-gated
o Stress-activated (mech. Force)
 auditory hair cells
o Voltage-gated – nerve and muscle cells; venus fly trap
Passive Transport via Carrier Proteins
- Multipass transmembrane proteins
- Selective – ions, sugars, amino acids, nucleotides
- Binding triggers conformational change
- Direction according to conc. or electrochemical gradient
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- Types
o Uniports – 1 molecule in one direction
 Passive glucose carriers
 Determined by conc. gradient
o Symports – 2 different molecules in same direction
o Antiports – 2 different molecules in opposite direction
Active Transport
- movement of substance against its concentration or
electrochemical gradient
- requires carrier protein
- requires energy input
o ATP hydrolysis
o Existing ion gradient
Na+-K+ pump (Na+-K+ ATPases)
- Animal cells
- Antiport carrier protein
- Pumps out 3 Na+; pumps in 2 K+
- Uses ATP
- Uses approximately 1/3 of energy in most cells!
- Function
o Maintains osmotic balance; keeps cells from bursting
o Extracellular Na+ gradient drives in other molecules
Active Transport of Glucose into Animal cells
- high glucose in cytosol; low glucose in extracellular fluid
- Na+-K+ pump creates high extracellular [Na+]
- Glucose driven in via glucose-Na+ symport
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Active Transport of Sucrose into Plant Cells
- plants and bacteria do not have Na+-K+ pumps
- use H+-ATPases (H+-pumps)
o H+ gradient used to drive in sucrose via sucrose-H+ symport
H+-ATPases
- also used to regulate pH in animal and plant cells
- e.g. lysosomes
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