Plasma Membrane Structure and Function

These proteins facilitate critical tasks such as transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton or extracellular matrix (ECM).

Carbohydrates and Cholesterol

• Carbohydrates: Located exclusively on the extracellular surface, carbohydrates are essential for cell-cell recognition. They can be covalently bonded to lipids to form glycolipids or, more commonly, to proteins to form glycoproteins.
• Cholesterol: This steroid molecule plays a vital role in regulating membrane fluidity. At warm temperatures, cholesterol restrains the movement of phospholipids to prevent the membrane from becoming too liquid. At cool temperatures, it prevents the phospholipids from packing too tightly, which helps the membrane avoid solidifying.

Membrane Fluidity and Permeability

The physical state of the membrane is highly dependent on its composition. Membranes rich in unsaturated hydrocarbon tails remain fluid at lower temperatures because the "kinks" in these tails prevent tight packing. In contrast, saturated tails pack together more tightly, increasing membrane viscosity.

Selective Permeability

The plasma membrane is selectively permeable, meaning it allows some substances to cross more easily than others.

• Hydrophobic (nonpolar) molecules: Small molecules like hydrocarbons can dissolve in the lipid bilayer and cross easily.
• Polar molecules and Ions: Substances like sugars or ions struggle to pass through the hydrophobic core of the bilayer.

To move hydrophilic substances across the membrane, the cell utilizes transport proteins:

• Channel Proteins: These provide hydrophilic tunnels or corridors through which specific molecules or ions can pass. A key example is aquaporins, which facilitate the rapid passage of water.
• Carrier Proteins: These proteins bind to specific molecules and undergo a change in shape to shuttle the solute across the membrane.

Exam Tip: Remember that while both channel and carrier proteins facilitate transport, channel proteins act like a "tunnel," whereas carrier proteins act like a "shuttle" that changes shape.

Summary of Transport Mechanisms

Passive vs. Active Transport

Passive transport involves the diffusion of substances down their concentration gradient (from high to low concentration) without any energy investment from the cell. This includes osmosis, which is the diffusion of free water across a selectively permeable membrane.

Active transport, however, moves substances against their concentration gradients. This process requires energy, typically in the form of ATP. An example is the sodium-potassium pump, which uses ATP to exchange ions across the membrane to maintain electrochemical gradients.

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Bulk Transport

For very large molecules, such as proteins or polysaccharides, the cell uses bulk transport, which requires energy:

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.
• Endocytosis: The cell takes in materials by forming new vesicles from the plasma membrane. This includes phagocytosis ("eating"), pinocytosis ("drinking"), and receptor-mediated endocytosis, which is a highly specific process used to acquire bulk quantities of specific substances.