Structure and Function of Plasma Membranes

Integral Membrane Proteins

Integral membrane proteins can possess one or more alpha-helices or beta-sheets that traverse the membrane, playing crucial roles in various cell functions.

Peripheral Proteins

Peripheral proteins are located on the membrane's exterior and interior surfaces, attaching to integral proteins or phospholipids. They participate in enzymatic processes, cytoskeleton attachment, and serve as recognition sites for the immune system, distinguishing self from non-self proteins.

Carbohydrates and the Glycocalyx

Carbohydrates are key components of the plasma membrane, found exclusively on the exterior surface, bound to proteins (glycoproteins) or lipids (glycolipids). These carbohydrate chains, composed of 2-60 monosaccharides, enable cell recognition and are collectively known as the glycocalyx. This "sugar coating" attracts water, facilitating cell interaction with its environment and is crucial for tissue formation and immune recognition. Viral adaptation is evident, as pathogens like HIV display changing glycoproteins to evade immune detection.

Sterols and Membrane Fluidity

Sterols, such as cholesterol, contribute to membrane fluidity by dampening temperature variations that can affect membrane stability. They help maintain fluidity across a range of temperatures and organize transmembrane proteins into lipid rafts.

Selective Permeability

Plasma membranes exhibit selective permeability, crucial for cellular function. They allow certain substances to pass while restricting others, vital for cell survival. This selectivity is influenced by the structure of phospholipids, which have both hydrophilic and hydrophobic regions, impacting how molecules cross the membrane. Nonpolar substances, like fat-soluble vitamins and gases, pass easily, while polar molecules and ions require specialized transport mechanisms.

Diffusion

Diffusion is the movement of substances from areas of higher to lower concentration until equilibrium is reached. This process is energy-efficient, relying on concentration gradients rather than active cell energy. Factors influencing diffusion include concentration gradient extent, molecular mass, temperature, and solvent density. Understanding these dynamics is key in various biological processes, including cellular transport mechanisms.