Cell Structures - AP Biology

When secretory proteins are synthesized they localize to the endoplasmic reticulum (ER), specifically the rough ER, for modification. Following modification there, secretory proteins are then packaged in secretory vesicles which go on to interact with the Golgi body, and are then finally released from the the plasma membrane.

Glycolysis (the process of breaking down glucose) takes place in the cytoplasm, or cytosol—the aqueous portion of the cytoplasm. It is in the cytoplasm where the enzymes required for glycolysis are found.

The citric acid cycle takes place in the mitochondrial matrix, and the electron transport chain takes place along the inner mitochondrial membrane in order to pump protons into the intermembrane space.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

A phosphatase removes a phosphate group from its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

Several different types of proteins can change the structure of a ligand, such as isomerases, and ubiquitin ligases add ubiquitin to their ligands.

Ubiquitin ligases add ubiquitin to their ligands. The addition of ubiquitin acts as a signal that a protein has become ineffective and is ready for degradation. When multiple ubiquitin residues have been added to a protein molecule, it is transported to the lysosome in the cell to be digested.

A phosphatase removes a phosphate group from its ligand.

A kinase is an enzyme that phosphorylates—or adds a phosphate group to—its ligand.

The addition and removal of phosphate groups can serve critical functions in the regulation of protein activity. The binding or uncoupling of phosphate groups frequently serves to activate or deactivate proteins.

Several different types of proteins can change the structure of a ligand, such as isomerases.

As the name implies, when activated and induced to undergo a conformational change by a ligand, ions are able to flow through the channel and into the cell. This allows the charge across the membrane tobe manipulated by the cell.

The binding of ligands causes the activation and conformational change in the ion channel to open it. Then, ions are able to flow into the cell. After a short time, the ligand dissociates from the ion gated channel. This causes the channel to deactivate and close.

Local cell signaling in eukaryotic cells refers to the communication between nearby cells. This is done through direct contact between cells, namely via cell junctions and cell-cell recognition. Gap junctions are intercellular connections that allow cytoplasmic transfer in animal cells. The counterpart in plant cells is the plasmodesmata, which are channels penetrating the cell walls of cells, allowing communication. Cell-cell recognition is the ligand-receptor binding between two cells that elicits receptor cell response. Methods of local cell signaling allow nearby cells to communicate with each other and coordinate cellular responses and activities.

Phagocytosis is a form of endocytosis in which a cell takes up solid material through the invagination of the plasma membrane to form intracellular vesicles. In multicellular organisms, the process of phagocytosis is utilized in nutrient uptake, immune system response, and in cell recycling. Cells perform phagocytosis to uptake solid nutrients into the cell. The immune system uses phagocytosis to consume foreign material for eventual degradation. In the continual process of cell recycling, old and dead cell material is taken up and reused by cells.

Pinocytosis is a type of endocytosis. During pinocytosis, the cell takes up extracellular fluid through plasma membrane invagination and vesicle formation. The process is energetically costly and requires many molecules of ATP. Pinocytosis is a non-specific process, meaning that materials are not selectively taken up. In other words, extracellular fluid is engulfed along with any or all particles within it. The fluid taken up by pinocytosis is already digested and degraded; therefore, the process is not accompanied by the action of lysosomes.

In clathrin-mediated endocytosis, the binding of a specific ligand to a receptor triggers intracellular protein recruitment, which includes clathrin. These proteins stabilize the invagination and allow the clathrin pit to pull away from the plasma membrane. After it has separated from the membrane, the proteins and clathrin dissociate from the vesicle, which then fuses with an endosome. The invagination and vesicle formation from the plasma membrane includes the internalization of both the receptor and ligand. Over time the uptake by clathrin-mediated endocytosis decreases as the number of receptors on the cell’s surface. In other words, particle uptake declines due to the internalization of recptors as a result of clathrin-mediated endocytosis.

In clathrin-mediated endocytosis, the binding of a ligand to a receptor triggers plasma membrane invagination and protein recruitment. The recruited proteins, including clathrin, bind to the intracellular domain of the receptor. Clathrin facilitates invagination and endocytosis of the ligand by stabilizing the plasma membrane curvature. Clathrin and other recruited proteins dissociate from the endocytic vesicle once it fully invaginates and pinches off from the plasma membrane.

The endoplasmic reticulum is an organelle that manufactures proteins, steroid molecules, cholesterol, and other lipids. The rough endoplasmic reticulum is embedded with ribosomes to allow polypeptides to be translated into the interior of the organelle. The smooth endoplasmic reticulum is responsible for lipid synthesis and degradation of toxins.

Exocytosis is the release of the contents of vesicles into extracellular space through the fusion of the vesicles with the plasma membrane. The process of exocytosis is a method through which the cell is able to secrete proteins into extracellular space. An example of this is the release of neurotransmitters from a neuron. During the process of exocytosis, intracellular vesicles originating from the Golgi apparatus fuse with the plasma membrane. This fusion increases the surface area of the plasma membrane and incorporates any proteins within the vesicle membrane into the cell’s plasma membrane.

Active transport is the movement of molecules against their concentration gradient (from an area of low concentration to an area of high concentration, requiring energy, usually in the form of ATP. Passive transport is the movement of molecules with their concentration gradient (from an area of high concentration to an area of low concentration), and does not require energy input. Facilitate diffusion is the movement of molecules with their concentration gradient across the cell membrane using transmembrane proteins (carrier proteins or channels), and does not require energy. Osmosis is the movement of a solvent (usually water), from an area with a lower concentration of solute to an area of higher concentration of solute; this process does not require energy.

This is the definition of passive transport. Active transport requires energy for molecules to move. Also, the Na+/K+ pump requires energy, and thus is a form of active transport. Vesicle transport, including endocytosis and exocytosis, also requires energy.

Only small, nonpolar molecules and small, uncharged, polar molecules can freely diffuse across the lipid bilayer of the cell. Glucose is a large molecule that cannot freely diffuse across the lipid bilayer, even if this would be favored by the concentration gradient. Facilitated diffusion is the movement of molecules across the lipid bilayer using carrier proteins or channels, which does not require energy input.

A hypotonic solution is a solution in which there is a lower concentration of solutes in the solution than in the cell. Thus, water (the solvent) will enter the cell, causing the cell to swell.

As the size of a cell increases, the surface area to volume ratio decreases, as surface area is a squared function, while volume is a cubic function. Due to the decreasing surface area to volume ratio, there is less area for the diffusing molecules to actually enter the cell, thus decreasing the rate at which diffusion can occur.

The cell membrane is a phospholipid bilayer, where the hydrophobic heads prevent hydrophilic molecules (such as charged ions) from crossing. Small, uncharged molecules are able to passively diffuse across the cell membrane when favored by the concentration gradient.