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Cell Structure and Cell Organelles

The Cell Membrane

The cell or plasma membrane is a double layer of lipids with proteins both on its surface and embedded within. It surrounds the cell and closes the cell off from its surroundings.

The cell membrane

Thin boundary of the cytoplasm that surrounds the living cell.

Consists of a lipid bilayer and may carry or contain various proteins.

Membranes allow for the compartmentalization of the cell, resulting in the formation of different reaction spaces (compartments) in the cell. The membrane also controls the metabolism of the cell.

All cells are divided by membranes into closed reaction spaces (compartmentalization). The cell membrane consists of a lipid bilayer that has various proteins and some cholesterol molecules embedded within it. The membrane thus primarily forms a protective barrier against water-soluble or polar substances, while lipophilic or nonpolar substances can easily pass through.

Structure of the cell membrane

Structure of the membrane: A. Lipid bilayer around a cell. B. Enlarged section with embedded protein and cholesterol - polar substances bind to the surface of the membrane, amphiphilic substances align themselves within the membrane according to their structure, and nonpolar molecules accumulate within the membrane. C. Enlargement of a membrane lipid with its outward facing polar end and inwardly directed nonpolar alkyl groups.

This basic behavior of the lipid bilayer is modulated by numerous membrane proteins. Receptors, ion channels, and transport or ion channel proteins are particularly important. Receptors are proteins that have binding sites for one or more different ligands. Binding of the ligand, which is often a hormone or neurotransmitter, results in a change to the protein structure that either opens an ion channel or initiates a metabolic cycle within the cell by means of the G-proteins. (second messenger).

Types of membrane protein

Types of membrane proteins: Membrane-bound proteins modulate the permeability of the cell membrane, which is otherwise exclusively based on osmosis and lipophilicity. Ion channels allow for the controlled influx and efflux of ions along the concentration gradient. Receptors react to the presence of ligands and initiate intracellular synthetic mechanisms or open and close ion channels. In addition, transport proteins and ion pumps allow for the energy-consuming transport of molecules and ions against a concentration gradient.

Ion channels are transmembrane proteins that enable the influx and efflux of ions along the concentration gradient. There are voltage-, receptor-, and ligand-gated ion channels. Porins are analogous to ion channels and enable the exchange of uncharged substances along a concentration gradient. Some porins only differentiate molecules by size while others only allow certain classes of substances to pass through. Finally, there are also proteins that actively transport molecules and ions in or out against the concentration gradient.

Membrane transport processes

Links and Literature

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