2. draw a phospholipid molecule and label which parts are hydrophobic and which parts are hydrophilic.
3. oxygen is a small, nonpolar molecule. can it pass through the cell membrane on its own

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### Question 3 (Oxygen passing through cell membrane)
# Brief Explanations:
1. Recall cell membrane structure: The cell membrane is a phospholipid bilayer with a hydrophobic interior (fatty acid tails) and hydrophilic exterior (phosphate heads).
2. Analyze oxygen's properties: Oxygen ($O_2$) is small, nonpolar, and uncharged.
3. Relate to membrane permeability: Nonpolar molecules can dissolve in the hydrophobic lipid bilayer. Small, nonpolar molecules like oxygen can pass through the cell membrane via simple diffusion (no energy, no protein needed) because they can interact favorably with the hydrophobic core.
# Answer:
Yes, oxygen can pass through the cell membrane on its own. It is a small, nonpolar molecule that can dissolve in and diffuse through the hydrophobic interior of the phospholipid bilayer (cell membrane) via simple diffusion.

### Question 2 (Drawing a phospholipid and labeling hydrophilic/hydrophobic parts)
# Brief Explanations:
1. Structure of a phospholipid: A phospholipid has two main parts - a **hydrophilic (water - attracting) head** and **hydrophobic (water - repelling) tails**.
2. Head composition: The head consists of a phosphate group ($PO_4^{3 - }$) and a glycerol backbone. The phosphate group is polar, making the head hydrophilic.
3. Tails composition: The tails are two long fatty acid chains. Fatty acid chains are nonpolar, so the tails are hydrophobic.
4. Drawing steps:
    - Draw a circle or a rounded shape for the hydrophilic head (label it "Hydrophilic Head" and note it contains the phosphate group and glycerol).
    - Draw two wavy or straight lines attached to the head for the hydrophobic tails (label them "Hydrophobic Tails" and note they are fatty acid chains).
# Answer:
To draw a phospholipid:
1. Draw the hydrophilic head: A rounded structure (representing glycerol + phosphate group). Label this part "Hydrophilic Head" (polar, water - attracting).
2. Draw two long, linear (or slightly kinked) structures attached to the head: These are the hydrophobic tails (fatty acid chains). Label this part "Hydrophobic Tails" (nonpolar, water - repelling).

(Note: For a more accurate drawing, the head can be depicted with the phosphate group shown as $PO_4$ and glycerol as a 3 - carbon backbone, and the tails as hydrocarbon chains with $ - CH_2 - $ repeats.)

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Question 3 (Oxygen passing through cell membrane)

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Question 2 (Drawing a phospholipid and labeling hydrophilic/hydrophobic parts)