Lecture 8 Review

Cell membranes are primarily composed of a phospholipid bilayer, a double layer of phospholipid molecules. These molecules have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. The hydrophilic heads face outwards, interacting with the watery environment inside and outside the cell, while the hydrophobic tails face inwards, creating a barrier to water-soluble molecules.

Cell membranes act as selective barriers, controlling the movement of molecules in and out of the cell. They allow some molecules to pass through freely while others require specialized transport mechanisms. This selective permeability is crucial for maintaining the cell's internal environment and carrying out essential functions.

Membranes are dynamic structures that are constantly changing and adapting to the cell's needs. They are fluid, meaning the phospholipids can move within the bilayer, allowing for flexibility and reorganizing. They are also selectively permeable, controlling what molecules can cross them, crucial for maintaining cellular homeostasis.

Membrane proteins are embedded within the phospholipid bilayer, contributing to the structure and function of membranes. They play various roles, including transporting molecules, acting as receptors for signaling molecules, and anchoring the membrane to other cellular structures.

The phospholipid bilayer is the fundamental building block of cell membranes, providing a barrier between the watery environment inside and outside the cell. Its structure, with hydrophilic heads facing outwards and hydrophobic tails facing inwards, prevents water-soluble molecules from passing through freely, regulating the passage of molecules in and out of the cell.

The presence of internal membranes, in addition to the plasma membrane, leading to compartmentalization within the cell.

They act as a barrier, regulating what enters and exits the cell. They also provide a surface for interactions with the environment, allowing for signaling, communication, and cell growth.

Membranes are primarily formed by lipids and proteins, with lipids like phospholipids forming a bilayer structure.

Phospholipids are amphipathic molecules, meaning they have both a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. This structure allows them to form stable bilayers in water.

Most membranes in cells have a net negative charge due to the predominantly negatively charged head groups of phospholipids.

Cell membranes are primarily constructed from a phospholipid bilayer. This bilayer consists of two layers of phospholipid molecules arranged with their hydrophilic heads facing outwards and their hydrophobic tails facing inwards.

The phospholipid bilayer's structure creates a barrier that regulates the passage of molecules into and out of the cell. This barrier is selectively permeable, allowing some molecules to pass through freely while others require specialized transport mechanisms.

Membrane fluidity describes the lateral movement of lipids and proteins within the plane of the membrane. This fluidity allows for flexibility and dynamic changes within the membrane.

Membranes with saturated lipids are less fluid compared to those containing unsaturated lipids. Saturated lipids have straight chains that pack tightly, while unsaturated lipids have kinks due to double bonds, reducing packing density and increasing fluidity.

Besides saturation, the length of fatty acid tails, the ratio of saturated to unsaturated lipids, and the presence of cholesterol all affect membrane fluidity.

Cholesterol helps regulate membrane fluidity. It stiffens the membrane at higher temperatures, reducing permeability, but at lower temperatures, it increases fluidity.

Membrane asymmetry arises from the specific distribution of phospholipids within the bilayer, with different types concentrated on each leaflet. This asymmetry is achieved through the action of enzymes like scramblase and flippase.

Phospholipids are primarily synthesized in the endoplasmic reticulum (ER), with some synthesis also occurring in mitochondria.

The Golgi apparatus plays a crucial role in maintaining and modifying the asymmetry of membranes. It uses enzymes like flippase to move specific phospholipids between the leaflets of the bilayer.

Membrane proteins can be classified into integral proteins, which span the membrane, and peripheral proteins, which associate with the membrane surface.

Integral membrane proteins often cross the membrane with an α-helix structure. This structure allows for both hydrophobic and hydrophilic interactions with the membrane.

Multiple transmembrane α-helices can assemble to form hydrophilic pores through the membrane, allowing the passage of water-soluble molecules.

Fluorescence Recovery After Photobleaching (FRAP) is a technique used to measure the diffusion rate of membrane proteins. It involves bleaching a specific region of the membrane and then monitoring the recovery of fluorescence as unbleached proteins diffuse into the bleached area.

Membrane proteins play a variety of crucial roles, including transport of molecules, communication through signaling pathways, enzymatic activity, cell-cell recognition, and anchoring to cytoskeletal structures.