Plants lack centrioles. Instead, they have microtubule organizing centers (MTOCs). They also have -mitochondria -ribsomes -plastids -ER

Prokaryotes have NO internal membranes. Therefore, they lack mitochondria, ribosomes, plastids, and ER.

Microtubules are part of the cytoskeletal structure and are made of the protein tubulin. The others all consist of selectively permeable plasma membranes.

Countercurrent exchange refers to a mechanism that enhances diffusion, such as increased absorption of oxygen across fish gills. It is not an example of how cells communicate. Contact inhibition is the phenomenon whereby cells can sense that they are in a crowded environment at which point they stop dividing. Quorum sensing relates to the fact that cells, such as bacteria, can sense the size of their own population. cAMP is a common secondary messenger in the cytoplasm of a cell receiving a signal from outside the cell.

Human red blood cells are small. An average cell is about 80 micrometers.

Smooth ER connects the Rough ER to the Golgi, carries out detoxification, and produces lipids like steroids. The nucleolus produces RNA.

An animal cell in a hypertonic solution would shrink because the concentration of water is greater inside the cell than outside the cell. Since water flows down a gradient, it would flow OUT of the cell. Plasmolysis means cell shrinking.

Starch, a polysaccharide, is too lage to diffuse through the plasma membrane.

The uptake of cholesterol by a cell occurs by receptor-mediated endocytosis, which requires energy. The facilitated diffusion of glucose into a cell, countercurrent exchange, and the diffusion of oxygen into as fish's gills are all examples of passive transport

Oxygen is absorbed by diffusion. All the other choices are examples of active transport.

The plasma membrane is ORGANIZED and made of many small particles that move about readily. Hence the name, fluid mosaic. A membrane is a very ACTIVE structure. A cell's activity is limited by how fast plasma membranes can take in and get rid of materials.

Plasmodesmata are functionally like gap junctions in animal cells and are a means of cytoplasmic communication among plant cells. A system of plasmodesmata is called a SYMPLAST.

The nucleus, chloroplasts, and mitochondria all contain DNA.

cAMP is a common secondary messenger in the cytoplasm of a cell, not on the plasma membrane of a cell. It is activated when a lipid-soluble molecule (the first messenger) passes through the plasma membrane and enters the cell. All the others are examples of receptors on the cell surface.

The key word is "initiates." The first thing that must happen before transduction can occur or before a response can result is that a receptor must be activated.

1.) Hydrophilic signaling molecules (acting as the first messenger) bind with receptors on the surface of cell membranes. 2.) Once activated, the receptor catalyzes a reaction within the cytoplasm to produce cAMP, a common second messenger. 3.) The presence of cAMP in the cytoplasm leads to a cascade effect that ultimately causes a response in the cytoplasm or nucleus.

- Hydrophilic signaling molecules dissolve through the plasma membrane and bind with a receptor in the cytoplasm. Hydrophilic signaling molecules, those that bind to a receptor on the cell surface, require a second messenger. An ion channel receptor is an example of a receptor on the surface of a cell.

The most important thing about signal transduction is that a single signal can be amplified many fold.

Tyrosine kinase receptors, G-protein-couple receptors, and ion-channel receptors are all receptors on the cell surface of the plasma membrane. When the receptor is on the surface of the cell, a second messenger (like cAMP) is required.

- One example is RTK, receptor tyrosine kinase. Steroid messengers diffuse directly through the cell membrane and once inside the cell, bind to a second messenger, like cAMP. G-protein-coupled receptors are another example of a receptor on the surface of a cell membrane. Ion-channel receptors are another example of a receptor on the cell surface.