Cell Structures and Functions

Transport that requires energy. Molecules move against their electrochemical gradient.

The transport of large amounts of substances. Substances are transported through vesicles in bulk transport. There are two types of bulk transport.

Bulk transport always occurs through vesicles. These vesicles either come from inside the cell and merge with the plasma membrane and release their contents, or they pinch off from the cell membrane and extracellular contents are absorbed into the cell.

Endocytosis is when the cell absorbs nutrients from outside of the cell by "swallowing" large molecules or large amounts of fluids. Molecules near the plasma membrane (extracellularly) form into a pit in the membrane, which pinches off and forms a vesicle.

Specific Endocytosis is when the cell takes up specific molecules.

Receptor mediated endocytosis is a specific form of endocytosis. The cell has receptors on its surface that's structure allows them to bind a specific molecule. When this specific molecule binds to the receptor, the cell absorbs molecules in the environment, knowing that the specific molecule is present. The vesicle forms pits where these molecules congregate, and when the vesicles pinches off into the cell, the molecule enters the cell.

Receptor mediated endocytosis is essentially specific pinocytosis, as both processes absorb fluids with solutes.

Nonspecific Endocytosis is when the cell randomly absorbs molecules in its environment.

Phagocytosis is a type of endocytosis wherein the cell absorbs large particles. These can be food, other cells, nutrients, harmful substances, etc.

A type of random endocytosis describing the random absorption of fluids with solute for a cell.

This type of bulk transport occurs when a vesicle from inside the cell merges with the plasma membrane and releases its contents to the extracellular region. Exocytosis allows for membrane proteins and macromolecules to exist in the plasma membrane of the cell.

Active transport of molecules often allows for gradients to be maintained. Gradient maintenance allows for the cell to undergo various processes, as gradients are forms of potential energy.

Electrogenic pumps are transport proteins that use ATP to pump an ion against its electrical gradient, restoring voltage potential.

The Na+/K+ transport protein is a pump that moves 3 Na+ out of the cell for every 2 K+ inside of the cell. This occurs due to a shape change facilitated by the addition and removal of phosphate groups, similar to how the H+ pump changes its shape.

Action potentials occur when Na+ rushes into the cell, causing depolarization of the membrane potential. K+ ions then rush out of the cell repolarizing the membrane. However, when Na+ rushes into the cell, it does not diffuse out of the cell because there is still a higher concentration of Na+ outside of the cell, the same is true for potassium that will not rush back into the cell because K+ concentration is still higher in the cell. Moreover, so much K+ has left the cell that the membrane potential is more polarized than is resting membrane potential. The sodium potassium pump restores concentrations of Na+ and K+, so that Na+ and K+ are ready to move down their gradients accordingly when an action potential comes. Without the Na+/K+ pump, action potentials would not be possible, because eventually Na+ and K+ would reach equilibrium and there would be no membrane potential.

The H+ pump is a transport protein that can move hydrogen against its electrochemical gradient. It does this when ATP binds a phosphate to the protein, causing a shape change in the protein as the R groups interact with phosphate, which causes the protein to change shape. Each time a protein changes between its two possible shapes, it moves a hydrogen ion against its gradient. Also, when phosphates are removed from these pumps, that is when they resume their "resting" shape.

When a molecule goes down its electrochemical gradient, it releases energy as the molecule is doing work, as the molecules become more stable as they lose potential energy. This energy is used to actively pump another molecule against its gradient. This process is only possible when a electrochemical gradient is maintained, which is facilitated by the H+ pump and many other types of pumps.

The electrochemical gradient is the gradient that arises because of the influence of charge difference across a membrane, and concentration difference across a membrane.

The electrical gradient refers to the voltage difference across a membrane, and an ions electrostatic attraction to one region of the membrane. For example, an ion that is positively charged will want to move to areas of relative negative charge.

The concentration gradient is the gradient that arises due to differences in concentrations of a molecule across a membrane. Molecules naturally diffuse to down their concentration gradients.

Passive Transport is the transport of molecules without any expenditure of energy. Molecules do not need energy to diffuse DOWN their concentration gradient.

Simple diffusion is the diffusion of small, non-polar molecules. This type of diffusion requires no transport protein, as small nonpolar molecules can fit between the phospholipids and are not repelled by the hydrophobic bilayer.

Small nonpolar molecules are hydrophobic molecules. Some examples are gases like dioxide and carbon dioxide.

Phospholipids are a type of fat. They are comprised of a glycerol, bound to two fatty acid chains. One hydroxyl group on the glycerol binds to a phosphate group.

Facilitated Diffusion is a type of diffusion that requires a transport protein.

Polar/Ionic and Large molecules require transport proteins. This is because they are either both/or hydrophilic and therefore are repelled by the hydrophobic bilayer, or they are too big to fit through phospholipids.

Transport proteins are proteins with an environment that allows for hydrophilic or large molecules to pass through the membrane. The protein can be comprised of a region of hydrophilic amino acids, that do not repel ions and other polar molecules.

Channel proteins are proteins that always deal in passive transport. They simply open and create an environment that allows for the diffusion of molecules down their electrochemical gradient.

Channel proteins can allow for the diffusion of large, nonpolar molecules if the channel creates a large space wherein large molecules can fit. However, keep in mind that there molecules can be both large and polar, I segregated the topics (Large Molecule Channels and Ion/Polar Molecule Channels) so that I could describe ion channels in more detail, without including large, nonpolar molecules like sucrose or nonpolar proteins.

Protein channels that allow for the passive transport of ions.

Some ion channels never close, allowing ions to constantly diffuse down their gradients. An example of this is some of the potassium ion channels in a neuron.

Some ion channels can close for various reasons, restricting diffusion of ions. These ion channels are therefore known as gated ion channels because they open due to some stimulus.

The ion channel opens because of a deformity of the plasma membrane.

A membrane has a voltage difference across it, also known as membrane potential due to the relative concentrations of ions on the inside and outside of a cell. So, when these concentrations change and membrane potential changes, a voltage gated ion channel may open or close.

Ion Channels that open because of the binding of a ligand. The ligand causes a conformational shape change that opens the channel.

Carrier proteins allow for passive transport through a shape change that first binds a molecule, then when the protein changes shape, the molecule can diffuse to the other side of its gradient. Carrier-like proteins can facilitate active transport. For example, the Na+/K+ pump pumps ions against their gradient by conformational shape changes. However these pumps use ATP.

Osmosis is the diffusion of water from an area of high free water concentration, to an area of low free water concentration. Areas are low in free water concentration if the solute concentration is high, as water forms a hydration shell around ionic solute.

Two hydrogens polar-covalently bound to an oxygen.

Water is a large, polar molecule. So, it naturally diffuses slowly across a plasma membrane. Aquaporins are transport proteins that create an environment favorable to water, so that water molecules can diffuse much faster to areas of low free water concentration.

Water balance is the relative concentration of water on the inside of a cell and on the outside of a cell.

Tonicity is a measure of how well a substance can change water balance.

Hypotonic solutions are solutions with relatively lower concentrations of solute.

Hypertonic solutions are solutions with relatively higher concentrations of solute. Water flows towards hypertonic regions.

Isotonic describes a scenario where concentration of solute inside and outside of a cell is equivalent. So, there is no net change in water concentration or water balance between a cell and its environment.