A solute moving down its concentration gradient across a non-permeable barrier is an example of facilitated diffusion. It requires a carrier protein, but no energy. Any particle crossing a non-permeable barrier will require a protein, and cannot pass via diffusion or osmosis. ATP will not be required to transport a particle down its gradient.

If the particle were travelling against its gradient, it would require ATP AND a protein, and active transport would be the correct answer. Simple diffusion and osmosis require no energy or protein.

Integral membrane proteins are proteins that span the entire membrane, whereas peripheral membrane proteins are proteins that associate only with only one side of the membrane (the "periphery").

Plasma membrane channels are proteins that facilitate the exchange of ions and other molecules between the extracellular and intracellular sides of a cell. To accomplish this task, a channel must span through the membrane (phospholipid bilayer); therefore, membrane channels are classified as integral membrane proteins. Recall that the inside of a phospholipid bilayer is extremely hydrophobic. Since like dissolves like, a membrane channel must contain hydrophobic regions that can interact with the interior of the phospholipid bilayer.

Amphipathic molecules contain both polar and nonpolar regions. Integral proteins (and most peripheral proteins) are amphipathic molecules. Phospholipids are also amphipathic molecules because they contain a polar head and a nonpolar tail.

Membrane-bound organelles are a key distinction between eukaryotic cells and prokaryotic cells. Membrane-bound organelles serve diverse purposes, and often have associated protein structures to help carry out enzymatic reactions or other functions.

Cell membranes are almost invariably at least bilayers, however, making choice 2 incorrect. A bilayer functions to sequester the lipid tails common to membranes away from the aqueous cytosol. Incidentally, the apicoplast is surrounded by four membranes, but the effect is the same.

Both prokaryotic and eukaryotic cells have plasma membranes that separates the cell's contents from the external environment. Prokaryotic cells frequently have more external layers (cell wall and cell capsule), since they are generally more exposed and require additional protection from the environment, but these layers will always lie one top of the fundamental cell membrane.

Prokaryotes have 70S ribosomes, while eukaryotes have 80S ribosomes. Eukaryotic cells contain membrane-bound organelles, such as the nucleus, while prokaryotes lack such organelles. Prokaryotic organisms are always unicellular, while eukaryotic organisms can be either unicellular (protists) or multicellular.

Water molecules are small and can travel through the membrane via simple diffusion. The rate of simple diffusion, however, is extremely slow due to the polarity of water molecules and their reluctance to enter the hydrophobic core of the membrane. Faster transportation of water molecules occurs via facilitated diffusion by specialized membrane channels called aquaporins; therefore, water can be transported via simple diffusion and facilitated diffusion.

Hydrophobic regions on membrane channels are essential to span the hydrophobic portions of the phospholipid bilayer. Although they are hydrophobic, lipids are not the main components of the hydrophobic regions. The hydrophobic regions in a membrane channel consist of hydrophobic amino acids that contain nonpolar side chains.

Remember that facilitated diffusion and simple diffusion are both forms of passive transport; therefore, these processes do not require energy and transport molecules from a region of high concentration to low concentration. They don’t move molecules against their electrochemical gradient.

Facilitated diffusion occurs at a much higher rate than simple diffusion. Facilitated diffusion uses membrane channels to transport molecules; therefore, it is much easier for molecules to traverse through a channel (facilitated diffusion) than the phospholipid bilayer (simple diffusion).

Albumin, glucose, and potassium ions are all examples of NON-permeable solutes. To be permeable, a solute must be small and nonpolar. Albumin, the main osmoregulatory protein, is bulky and too large to cross the membrane freely. Glucose is also large, as well as polar, and cannot cross the membrane. Potassium cannot freely cross due to the positive charge on the ion. All of these will require facilitated means of entering the cell.

Testosterone is a steroid hormone, and as such is small and nonpolar. All steroid hormones have intracellular receptors, and are able to enter a cell freely. Of the four choices, it is the only solute that can permeate the cell membrane.

Active transport most correctly describes this type of movement, as it uses ATP as an energy source. In contrast, the other four choices are all different types of passive transport, constituting types of movement where no energy source is needed. Diffusion is simply the net movement of particles down their concentration gradient. Facilitated diffusion is a similar concept, but uses specialized transport proteins. Osmosis describes the movement of water, and lastly, filtration is the movement of both solute and water molecules.

Nonpolar molecules and very small polar molecules can freely pass through the lipid bilayer, while large, polar molecules and ions need to be aided by transport proteins. Sodium and potassium are both charged ions that would not be able to cross the membrane. Glucose and citrate are too large, and also contain polar regions.

Carbon dioxide is the only answer choice that is both small and nonpolar enough to simply diffuse across the membrane.

Each phospholipid consists of a polar phosphate head and two non-polar lipid tails. The phospholipid bilayer consists of polar heads facing the inside and outside of the cell, which interact with polar, aqueous environments. The non-polar hydrocarbon tails are packed on the inside of the bilayer, as far away from the polar environments as possible.

To answer this question you need to know the directionality of the sodium-potassium pump and the number of ions pumped each cycle. Remember that each cycle of the sodium-potassium pump moves three sodium ions to the outside of the cell and two potassium ions to the inside of the cell. The amount of sodium ions outside the cell will increase by three and the amount of potassium ions inside the cell will increase by two.

The final result after one cycle of the sodium-potassium pump will be 33 sodium ions outside the cell and 22 potassium ions inside the cell.