Cell Membrane: Structure and Function

• The cell membrane, also known as the plasma membrane, is a flexible barrier that separates the cell from its environment.
• It regulates what enters and leaves the cell through selective permeability.
• The membrane creates stable internal conditions different from the external environment.
• It is composed of two layers of phospholipids, along with proteins, carbohydrates, and cholesterol.

Functions

• Maintaining homeostasis
• Regulating transport of materials
• Cell movement and adhesion
• Cell division

Parts of the Cell Membrane

• The basic structure includes two layers of phospholipids, called the phospholipid bilayer.
• The structure is described by the fluid mosaic model.
• The fluid mosaic model explains that the components are fluid and drift laterally.
• The model explains that the membrane is flexible and able to move with the cell.
• The cell membrane is a mosaic, made of different macromolecules.

Lipids

• Includes phospholipids, and steroid lipids like cholesterol.
• Cholesterol helps keep the membrane flexible and fluid.

Phospholipids

• They are amphipathic lipids that make up the cell membrane.
• They have a hydrophobic tail made of two fatty acid chains.
• They have a hydrophilic head made of a phosphate group.
• The head and tail are connected with a glycerol molecule.
• Their arrangement in a bilayer in the membrane is due to their amphipathic nature.
• In the bilayer, the tails are arranged in the inside of the membrane.
• The hydrophilic heads face outwards towards the aqueous environment and inwards towards the cytoplasm.

Proteins

• There are integral and peripheral proteins in the cell membrane.
• Integral proteins span the cell membrane and exist both outside and in it.
• Integral proteins have a hydrophobic core that allows them to interact with the hydrophobic tails of the phospholipids.
• Peripheral proteins are located on the outside of the cell membrane.
• Peripheral proteins are hydrophilic and attach to the phospholipid heads.

Function of Proteins

• Cell signaling
• Anchoring cells to each other and the extracellular matrix
• Cell movement
• Transport of materials
• Enzymatic reactions
• Transport proteins move materials across the membrane.
• Transport proteins act like a pump, binding to the materials to be moved and spending energy
• Transport proteins act as a channel.
• Channel proteins have a large pore that allows certain types of molecules to pass through and diffuse down their concentration gradient.
• Channel proteins can be regulated by the cell and only opened during certain times.
• Proteins act like enzymes and carry out chemical reactions.
• Membrane proteins when activated carry out phosphorylation reactions, that add phosphate groups to molecules inside the cell leading to changes in cell behavior or protein synthesis.

Carbohydrates

• They are macromolecules that are attached to the outside of the cell membrane.
• They act like a nametag for the cell, allowing other cells in a multicellular organism to identify it as part of the whole.
• They play an important role in helping cells attach to the extracellular matrix in cellular adhesion.
• The collection of carbohydrates on the outside of the cell is called the glycocalyx.
• The glycocalyx helps to cushion the plasma membrane and is involved in cell adhesion and recognition.

Cell Membrane Function

• The function of the cell membrane is to act as a physical barrier and to regulate what enters and leaves the cell.
• The main function is to separate the cell from the external environment, allowing the cell to maintain homeostasis.
• It is selectively permeable, only letting certain molecules in and out.
• Phospholipids prevent diffusion of large or charged molecules across the membrane.
• Transport proteins help these types of molecules move into and out of the cell with tight regulation.
• The pancreas produces insulin, which causes cells to move GLUT4 transport proteins to the cell surface to allow glucose to diffuse down its concentration gradient into the cell.
• The cell membrane acts like a transducer, converting chemical signals in the environment to changes inside the cell.
• Signaling molecules outside the cell bind to proteins, which cause changes in protein activation inside the cell that can lead to changes in cell behavior and trigger protein synthesis.
• Growth hormone binds to receptors on the outside of the cell which cause changes in signal transduction that ultimately create changes in gene transcription which promote cell growth and division.
• The cell membrane is important for cell movement and adhesion to the extracellular matrix and other cells.
• Integral proteins are embedded in the membrane that bind to both internal components of the cell, such as the cytoskeleton, and to external components like the extracellular matrix.
• These connections can be remodeled as cells need to grow, divide and move throughout the body.