Biology IB HL - 1.4 Membrane Transport Part 2

Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction

YES

Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)

Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)

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

NO

| Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)

The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane

At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within

When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels

Integral proteins with a hydrophilic inner pore via which potassium ions may be transported

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

Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage

Active transport uses energy to move molecules against a concentration gradient

The direct hydrolysis of ATP (primary active transport)

Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)

Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)

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

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

An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)

Three sodium ions bind to intracellular sites on the sodium-potassium pump

A phosphate group is transferred to the pump via the hydrolysis of ATP

The pump undergoes a conformational change, translocating sodium across the membrane

The conformational change exposes two potassium binding sites on the extracellular surface of the pump

The phosphate group is released which causes the pump to return to its original conformation

This translocates the potassium across the membrane, completing the ion exchange

Materials destined for secretion are transported around the cell in membranous containers called vesicles

The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials

Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use

Smooth ER is involved in lipid synthesis and also plays a role in carbohydrate metabolism

Materials are transported from the ER when the membrane bulges and then buds to create a vesicle surrounding the material

The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex

Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face

While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.)

Material sorted within the Golgi apparatus will either be secreted externally or may be transported to the lysosome

Vesicles containing materials destined for extracellular use will be transported to the plasma membrane

The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid

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)

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)

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

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

phagocytosis

| 2. pinocytosis

The process by which solid substances are ingested (usually to be transported to the lysosome)

The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane

Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment

The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis