1.4 Membrane Transport Flashcards
What are 2 key qualities that cellular membranes posses?
they are
- semi-permeable
- selective
What does it mean that a membrane is semi-permeable?
They are semi-permeable (only certain materials may freely cross – large and charged substances are typically blocked)
WHat does it mean that a membrane is selective?
They are selective (membrane proteins may regulate the passage of material that cannot freely cross)
In what 2 ways can movement of material occur across a membrane?
Movement of materials across a biological membrane may occur either ACTIVELY or PASSIVELY
What does passive transport involve?
Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)
Does passive transport require energy? why?
NO
Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)
What are the 3 main types of passive transport?
- simple diffusion
- facilitated diffusion
- osmosis
What molecules are moved in simple diffusion?
movement of small or lipophilic molecules (e.g. O2, CO2, etc.)
What molecules are moved in osmosis? What is osmosis dependent on?
movement of water molecules (dependent on solute concentrations)
What molecules are moved in facilitated diffusion? What helps?
movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)
What does active transport involve?
Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)
Does active transport require energy? why?
Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)
What are the 2 main types of active transport?
primary (direct)
secondary (indirect)
What does primary active transport involve?
Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport
What does secondary active transport involve?
Involves coupling the molecule with another moving along an electrochemical gradient
What is diffusion?
Diffusion is the (random) net movement of molecules from a region of high concentration to a region of low concentration
When will diffusion stop?
This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium)
What 3 factors can influence diffusion?
- temperature
- molecular size
- steepness of gradient
How does temperature affect diffusion?
affects kinetic energy of particles in solution
How does molecular size affect diffusion?
larger particles are subjected to greater resistance within a fluid medium
How does the steepness of gradient affect diffusion?
rate of diffusion will be greater with a higher concentration gradient
What is osmosis?
Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)
What type of solvent is water? What does this mean?
Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes)
How does water help solute transport?
Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions
How is the water concentration affected by a high solute concentration?
At a higher solute concentration there are less free water molecules in solution as water is associated with the solute
Where does osmosis occur and why?
Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration (not associated with other molecules)
What is osmolarity?
Osmolarity is a measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)
Relating to osmolarity, what 3 categories can solutions be put into?
- hypertonic
- hypotonic
- isotonic
What does it mean if a solution is hypertonic?
Solutions with a relatively higher osmolarity are categorised as hypertonic (high solute concentration ⇒ gains water)
What does it mean if a solution is hypotonic?
Solutions with a relatively lower osmolarity are categorised as hypotonic (low solute concentration ⇒ loses water)
What does it mean if a solution is isotonic?
Solutions that have the same osmolarity are categorised as isotonic (same solute concentration ⇒ no net water flow)
How can the osmolarity of a tissue be tested?
by bathing the sample in solutions with known osmolarities
What will happen if the tissue is placed in a hypertonic solution?
The tissue will lose water when placed in hypertonic solutions
What will happen when a tissue is placed in a hypotonic solution?
gain water when placed in hypotonic solutions
How can the change in weight of a tissue be measured?
Water loss or gain may be determined by weighing the sample before and after bathing in solution
How can tissue osmolarity be inferred?
Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)
What type of conditions must tissue/organs used in medical procedures be kept in and why?
Tissues or organs to be used in medical procedures must be kept in solution to prevent cellular dessication
This solution must share the same osmolarity as the tissue / organ (i.e. isotonic) in order to prevent osmosis from occurring
What effect will uncontrolled osmosis have in a hypertonic solution?
animal cells
Uncontrolled osmosis will have negative effects with regards to cell viability:
In hypertonic solutions, water will leave the cell causing it to shrivel (crenation)
What effect will uncontrolled osmosis have in a hypotonic solution?
animal cells
In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis)
How are the effects of uncontrolled osmisis moderated in a plant cell?
In plant tissues, the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall
What effect will uncontrolled osmosis have in a hypertonic solution?
plant cells
In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
What effect will uncontrolled osmosis have in a hypotonic solution?
plant cells
In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)
What is facilitated diffusion?
Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein
What molecules need to use facilitated diffusion?
It is utilised by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)
What proteins mediate facilitated diffusion?
This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins
What are carrier proteins?
Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane
What type of interaction is the interaction between carrier proteins and solutes similar to?
Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
Can carrier proteins move molecules against the concentration gradient?
YES
Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
How does the rate of transport with carrier proteins compare with channel proteins?
Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)
What are channel proteins?
Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
Are channel proteins selective?
YES
Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
Can channel proteins move molecules against a concentration gradient?
NO
Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
How do axons of nerve cells transmit electrical impulses?
The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane
What type of pump is used to translocate ions in nerve cells? What does it do at rest?
At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within
How does the sodium-potassium pump in nerve cells work?
When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels
What are potassium channels?
Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
What parts is a potassium channel composed of?
The channel is comprised of four transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest region that restricts the passage of alternative ions
How are potassium channels selective?
Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage
In which direction do molecules move via active transport?
Active transport uses energy to move molecules against a concentration gradient
In what two ways can energy for active transport be generated?
The direct hydrolysis of ATP (primary active transport)
Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)
What is used to help with active transport?
Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)
In 3 steps, how does a carrier protein help with active transport?
A specific solute will bind to the protein pump on one side of the membrane
The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
The solute molecule is consequently translocated across the membrane (against the gradient) and released
In 3 steps, how does a carrier protein help with active transport?
A specific solute will bind to the protein pump on one side of the membrane
The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
The solute molecule is consequently translocated across the membrane (against the gradient) and released
What is a sodium-potassium pump?
An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)
- What needs to occur for the process to start?
Sodium-Potassium Pump
Three sodium ions bind to intracellular sites on the sodium-potassium pump
- What is transferred to the pump?
Sodium-Potassium Pump
A phosphate group is transferred to the pump via the hydrolysis of ATP
- What change occurs to the pump?
Sodium-Potassium Pump
The pump undergoes a conformational change, translocating sodium across the membrane
- Why does the pump undergo a change?
Sodium-Potassium Pump
The conformational change exposes two potassium binding sites on the extracellular surface of the pump
- How does the pump return to its original shape?
Sodium-Potassium Pump
The phosphate group is released which causes the pump to return to its original conformation
- What do all these steps result in?
Sodium-Potassium Pump
This translocates the potassium across the membrane, completing the ion exchange
What materials are transported in vesicular transport?
Materials destined for secretion are transported around the cell in membranous containers called vesicles
What role does ER have in vesicular transport?
The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials
What role does rough ER have in vesicular transport?
Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use
What role does smooth ER have in vesicular transport?
Smooth ER is involved in lipid synthesis and also plays a role in carbohydrate metabolism
How are materials transported from the ER?
Materials are transported from the ER when the membrane bulges and then buds to create a vesicle surrounding the material
What role does the golgi apparatus play in vesicular transport?
The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex
Where do materials move in a golgi apparatus? (vesicular transport)
Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face
What happens inside the golgi apparatus?
While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.)
How is material transported from the golgi apparatus?
Material sorted within the Golgi apparatus will either be secreted externally or may be transported to the lysosome
Where are vesicles transported to?
Vesicles containing materials destined for extracellular use will be transported to the plasma membrane
How are materials released from the cell?
The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid
In what 2 ways are materials sorted by the golgi apparatus secreted?
Released immediately into the extracellular fluid (constitutive secretion)
Stored within an intracellular vesicle for a delayed-release in response to a cellular signal (regulatory secretion)
How does the fluidity of the membrane help bulk transport?
This weak association allows for membrane fluidity and flexibility, as the phospholipids can move around to some extent
This allows for the spontaneous breaking and reforming of the bilayer, allowing larger materials to enter or leave the cell without having to cross the membrane (this is an active process and requires ATP hydrolysis)
What is endocytosis?
The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane
How does endocytosis occur? 2 steps
An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
The invagination is then sealed off to form an intracellular vesicle containing the material
What are the 2 main types of endocytosis?
- phagocytosis
2. pinocytosis
What is phagocytosis?
The process by which solid substances are ingested (usually to be transported to the lysosome)
What is pinocytosis?
The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)
What is exocytosis?
The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane
How do vesicles help in the process of exocytosis?
Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment
What does the process of exocytosis add to the membrane?
The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis
