Sugar, specifically glucose, crosses the cell membrane primarily through specialized transporter proteins embedded in the membrane because glucose cannot diffuse directly through the lipid bilayer due to its size and hydrophilic nature. The main mechanisms are:
- Facilitated diffusion via glucose transporter proteins (GLUTs): These proteins allow glucose to move from a higher concentration outside the cell to a lower concentration inside without using energy. Glucose binds to these transporter proteins, which undergo a conformational change to shuttle glucose across the membrane.
- Secondary active transport via sodium-glucose linked transporters (SGLTs): These cotransporters use the energy from sodium ions moving down their concentration gradient to bring glucose into the cell against its concentration gradient, which requires energy indirectly through the sodium gradient maintained by ATP-driven pumps.
Thus, sugar enters cells through these transporter proteins that assist in glucose crossing the hydrophobic lipid bilayer by either passive facilitated diffusion or active cotransport with sodium ions.
