Biology by Bradford
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The subtopic in the middle and four tasks round it: label a diagram, complete a table and fill 9 gaps. Every box rests on a learning objective of the topic in the AP Biology course and exam description. Type into the blanks and check them, or print it on A4, Letter or A3 with an answer key.
| Substance | Crosses the bilayer unaided? | Why |
|---|---|---|
| Oxygen, carbon dioxide | small and nonpolar | |
| Ethanol and steroid hormones | yes | nonpolar or lipid soluble |
| Glucose | large and polar: needs a transport protein | |
| Na⁺, K⁺, Cl⁻ | no | : repelled by the hydrophobic core |
| Water | slowly | small but polar; moves faster through aquaporins |
Membrane proteins with a carbohydrate attached are . They act as markers, so that cells can recognize one another and the immune system can tell self from foreign. Other proteins are receptors, which bind signal molecules, or enzymes. Differences in a cell's membrane proteins and lipids therefore change both what it lets across and how it responds to its environment.
The membrane lets some substances cross easily and blocks others, so it is . What decides is the size and polarity of the molecule: small nonpolar molecules cross the hydrophobic core directly, while charged and large polar molecules need specific transport proteins. A cell controls which proteins are in its membrane, so it controls what it takes in.
| Substance | Crosses the bilayer unaided? | Why |
|---|---|---|
| Oxygen, carbon dioxide | yes | small and nonpolar |
| Ethanol and steroid hormones | yes | nonpolar or lipid soluble |
| Glucose | no | large and polar: needs a transport protein |
| Na⁺, K⁺, Cl⁻ | no | charged: repelled by the hydrophobic core |
| Water | slowly | small but polar; moves faster through aquaporins |
The membrane lets some substances cross easily and blocks others, so it is selectively permeable. What decides is the size and polarity of the molecule: small nonpolar molecules cross the hydrophobic core directly, while charged and large polar molecules need specific transport proteins. A cell controls which proteins are in its membrane, so it controls what it takes in.
Membrane proteins with a carbohydrate attached are glycoproteins. They act as markers, so that cells can recognize one another and the immune system can tell self from foreign. Other proteins are receptors, which bind signal molecules, or enzymes. Differences in a cell's membrane proteins and lipids therefore change both what it lets across and how it responds to its environment.