Unit 3: Passive Membrane Transport Flashcards
Define
Semi-permeable
Allowing some, but not all, substances to pass through
Describe how nonpolar substances move across the membrane
Directly through phospholipid bilayer
Why can nonpolar substances get through the phospholipid bilayer?
Dissolve into the lipid tails that make up the bilayer
Describe how polar substances move across the membrane
Through channels or carrier proteins (transport proteins)
Why can polar substances not get through the phospholipid bilayer?
Hydrophobic core (lipid tails) repel the polar or charged molecules; particles cannot dissolve in hydrophobic core
Examples of nonpolar substances
CO2, O2
Examples of polar substances
Glucose, ions
Describe
Transport proteins
Chains of amino acids, with polar amino acids on the cytoplasmic and extracellular side of protein, plus through the center to make a canal through which particles can pass;
Nonpolar amino acids make up the exterior portion that gets sandwiched in lipid tails
Compare and contrast
Channel and carrier proteins
Both:
* Transmembrane proteins
* Allow particles to move through membrane
* Specific to one or only a few particles
Differences:
* Channel proteins do not change shape; carrier proteins do
* Channel proteins allow faster passage through membrane; carrier proteins are slower
Define
Aquaporin
A channel protein that is specific to water
What is meant by
Transport protein specificity
Each transport protein (channel or carriers) allow only one or a few specific particles to cross
Cells can have unique combinations of transport proteins to suit their specific needs (and can even change which proteins they have on their membrane at a given time)
Describe
Concentration gradient
A difference in solute concentrations between two regions
Characteristics of
Passive membrane transport
- Does not require cellular energy
- Occurs spontaneously (consequence of random thermal motion)
- Net movement of particle from area of higher to lower concentration (“down” concentration gradient)
- Continues until dynamic equilibrium is reach
Types of
Passive membrane transport
- Simple diffusion
- Facilitated diffusion (including osmosis)
Define
Dynamic Equilibrium
When the net movement of particles is equal in all directions
Individual particles still move
What type of particles move via simple diffusion?
Small & nonpolar
Ex: CO2 and O2
Describe
Simple diffusion
Movement of small, nonpolar substances across the phospholipid bilayer, from an area of high concentration to an area of low concentration
Describe
Facilitated diffusion
Movement of large and/or polar/ionic substances across the membrane via channel or carrier proteins, from an area of high concentration to an area of low concentration
Define
Osmosis
Diffusion of water across a membrane
Define
Tonicity
Concentration of solutes that a membrane is not permeable to (cannot pass through membrane)
Define
Hypotonic
Side of a membrane with lower tonicity / higher free water concentration
Define
Hypertonic
Side of a membrane with higher tonicity / Lower free water concentration
Define
Isotonic
Equal tonicity across a membrane
In what direction does water move during osmosis?
Higher → Lower free water concentration
Hypotonic → Hypertonic
Higher ψ → Lower ψ
What is
Water potential
A water’s tendency to move
Includes pressure and the effects of solutes
Solute potential (ψS) is always…
negative
Ionization (i) of NaCl
Consider formula for solute potential
2
Ionization (i) of sucrose
Consider formula for solute potential
1
When calculating solute potential, temperature must always be in
Kelvin
Water potential is normally expressed in
(what unit?)
bars
What happens to a plant cell in a hypertonic environment?
Water moves out
Plant cell shrinks
May plasmolyze
What happens to an animal cell in a hypertonic environment?
Water moves out
Animal cell shrinks
What happens to a plant cell in an isotonic environment?
Equal movement of water in/out
No turgor pressure
Plant will wilt
What happens to an animal cell in an isotonic environment?
Equal movement of water in/out
No change to cell size
What happens to a plant cell in hypotonic environment?
Water moves into cell
Cell wall prevents bursting
Cell experiences turgor
What happens to an animal cell in hypotonic environment?
Water moves into cell
Prolonged exposure may cause cell to lyse (burst)
The ideal osmotic environment of plant cells
Hypotonic
The ideal osmotic environment of animal cells
Isotonic
Animal cell in hypotonic environment; cell lyses
Animal cell in isotonic environment
Animal cell in hypertonic environment
Plant cell in hypotonic environment; makes plant turgid
Plant cell in isotonic environment; makes plant flaccid
Plant cell in hypertonic environment; can cause plasmolysis
Define
Turgor pressure / Turgid
Outward pressure of a cell against a cell wall
Good for plants… Helps them stay upright!
Define
Flaccid
(in terms of osmosis!)
Wilting
Due to plant’s exposure to isotonic or hypertonic environment
