A group of students are researching the structural components of the eukaryotic plasma membrane. They summarise their findings in four statements:

1. The portion of an integrin receptor protein that spans the core of the phospholipid bilayer is primarily composed of amino acids with (1) R-groups.
2. Carbohydrate chains covalently bound to membrane lipids, forming (2), play a major role in cell-cell recognition and stabilizing membrane structure.
3. In animal cell membranes, cholesterol molecules interact with phospholipid fatty acid tails at high temperatures to (3) membrane fluidity.
4. Membrane-spanning proteins that specifically bind to a solute and undergo a conformational change to transfer the solute across the bilayer are classified as (4) proteins.

Which of the following rows correctly identifies the missing terms?