To answer this question, we need to complete the table by determining the method of transport for each molecule or ion provided. We must also summarize observations based on how different substances cross cell membranes.
Let's complete the table first:
\begin{tabular}{|l|c|c|c|c|c|}
\hline
Molecule or Ion & \begin{tabular}{c}
Concentration \\
Outside \\
the Cell
\end{tabular} & \begin{tabular}{c}
Concentration \\
Inside \\
the Cell
\end{tabular} & size & \begin{tabular}{c}
Polar, \\
Nonpolar \\
Ions
\end{tabular} & Method of Transport \\
\hline oxygen & high & low & small & nonpolar & diffusion \\
\hline carbon dioxide & high & low & small & nonpolar & diffusion \\
\hline \begin{tabular}{l}
fructose \\
(monosaccharide)
\end{tabular} & high & low & large & polar & facilitated diffusion \\
\hline \begin{tabular}{l}
glutamine \\
(amino acid)
\end{tabular} & low & high & large & polar & active transport \\
\hline sodium ion & low & high & small & ion & active transport \\
\hline potassium ion & low & high & small & ion & active transport \\
\hline protein molecule & high & low & very large & \begin{tabular}{l}
polar or \\
nonpolar
\end{tabular} & endocytosis \\
\hline
\end{tabular}
### Summary of Observations:
1. Nonpolar Molecules:
- Small nonpolar molecules, such as oxygen and carbon dioxide, cross the cell membrane through diffusion. This process occurs because these molecules can easily move through the lipid bilayer of the cell membrane without the need for energy input or transport proteins.
2. Polar Molecules and Ions:
- Large polar molecules, such as fructose (monosaccharide), cross the cell membrane through facilitated diffusion. This process requires specific transport proteins to help these molecules traverse the lipid bilayer, but it does not require energy (ATP).
- Polar molecules that are essential for cellular function, like glutamine (amino acid), and ions such as sodium and potassium, cross the membrane by active transport. This process requires energy (ATP) to move these substances against their concentration gradients through specialized transport proteins (pumps).
3. Very Large Molecules:
- Very large molecules, such as proteins, utilize endocytosis to cross the membrane. This process involves the cell membrane enveloping the molecule to bring it inside the cell, often forming a vesicle. Endocytosis is an energy-consuming process, typically used for transporting large, complex molecules.
By analyzing how different substances cross cell membranes, we can conclude that the method of transport is closely related to the size and polarity of the molecule or ion as well as the concentration gradient across the cell membrane. On the whole, nonpolar molecules can pass through more freely while polar molecules and ions typically require specific facilitated mechanisms.
