Add labels to the figure that follows, which illustrates some molecules that can and cannot pass through cell membranes unaided.

Verified step by step guidance

1. Step 1: Identify the molecules shown in the first image. The small molecules with red and white spheres represent water (H₂O), which can pass through the cell membrane unaided. The larger molecule shown in the inset represents a polysaccharide, which cannot pass through the membrane unaided due to its size and polarity.
2. Step 2: Label the green arrows in the first image as 'Water molecules passing through the membrane unaided.' This indicates that water can diffuse across the lipid bilayer due to its small size and polarity.
3. Step 3: Label the red arrow pointing to the polysaccharide in the first image as 'Large molecules like polysaccharides cannot pass through the membrane unaided.' This highlights the need for transport mechanisms for larger molecules.
4. Step 4: In the second image, label the small red spheres as 'Oxygen (O₂)' and the small white spheres as 'Water (H₂O).' These molecules can diffuse across the membrane unaided due to their small size and non-polar or polar nature.
5. Step 5: Label the protein channels and carriers in the second image as 'Facilitated transport for larger or charged molecules.' This indicates that molecules like glucose or ions require specific transport proteins to cross the membrane.

Key Concepts

Cell Membrane Structure

The cell membrane, also known as the plasma membrane, is composed of a phospholipid bilayer with embedded proteins. This structure is crucial for maintaining the integrity of the cell and regulating the movement of substances in and out. The hydrophobic interior of the bilayer acts as a barrier to most polar molecules, while allowing nonpolar molecules to pass more freely.

Selective Permeability

Selective permeability refers to the ability of the cell membrane to allow certain substances to pass while blocking others. This property is essential for maintaining homeostasis within the cell, as it controls the internal environment by regulating the entry and exit of ions, nutrients, and waste products. Molecules that are small and nonpolar can often diffuse through the membrane, while larger or charged molecules require specific transport mechanisms.

Transport Mechanisms

Transport mechanisms are processes that facilitate the movement of substances across the cell membrane. These include passive transport, such as diffusion and osmosis, which do not require energy, and active transport, which requires energy to move substances against their concentration gradient. Understanding these mechanisms is vital for interpreting how different molecules interact with the cell membrane, as illustrated in the provided figure.