Cell membranes and cytoplasm

Cell membranes

The base function of membrane is to separate the cells of external environment (plasma membrane - fig. 1) and to divide the cells in functional and structural compartments (endomembranes - fig. 2). In addition, these membrane represent a barrier for most water-soluble (polar) molecules (1, 2, 3).

In accordance with the fluid-mosaic model, the cells membranes consist of a lipid bilayer, several classes of proteines, nicotinamide adenine dinucleotide phosphate (NADP), mobile ions etc. By analysing the physicochemical properties of molecular components, we can understand the cell membranes properties and functions. Cell membranes have the lipids from four classes: phospholipids, galactosylglycerides, glucocerebrosides and sterols (4). Due to their amphipatic property, they form in water the lipid bilayer and lipid micelle, the structures relatively similar with plasma membrane and cell endomembranes. Phospholipids containing both hydrophilic (''water-loving''-head group) and hydrophobic (''water-hating''-two fatty acid tails) (5). These lipids can move into the lipid bilayer. Membrane proteins are associated with lipid bilayers in many different ways. Considering their position in lipid bilayer, today are described six classes of membrane proteins: peripheral, integral, fatty-acid-linked, prenyl group-linked, phosphatidylinositol-anchored and cholesterol-linked (6). Points of view of their functions, the proteins are classified in several classes: structural proteins, cell adhesion proteins, enzymes, receptors (ex. G-protein couplet receptor), transport and carrier proteins. These proteins, like lipids, can move into the lipid bilayer. Plasma membrane and each endomembranes has a specific system of proteins, in relation with their functions (7).

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In eukaryotic cells, the cytoplasm (cytos = cell, container and plasm = structure) is viscous material located between the plasma membrane and nucleus (8, 9).
Cytoplasm is a colloidal system, in which the dispersion medium (dispersing medium) is water and the dispersed phase consists of different macromolecules (10).
Substances found in the cytoplasm following percentages: water 80,5 - 85 %, proteins 10 - 15 %, lipids 2 - 4 %, polysaccharides 0,1 - 1,5 %, DNA 0,4 %, RNA 0,7 %, small organic molecules 0,4 %, inorganic molecules and ions 1,5 % (11). Cytoplasm is a dynamic cell component with zone structures in accordance with the functions performed. For example, the outer cytoplasm (ectoplasm) is non-granulated and having a role in cell locomotion. The contrary, the inner layer of the cytoplasm (endoplasm), that is granulated, have metabolic role. In addition, the cytosol is divided from endoplasmic reticulum membrane in several compartments (11).

The cytoplasm is structurally divided in three major components: the cytosol, organelles and cytoplasmic inclusions. The cytosol and cytoskeleton form the hyaloplasm.
Cytosol is a mixture of inorganic substances and macromolecules (metabolites, enzymes) dissolved in water. Also, in cytosol were identified the proteins of cell cytoskeleton and the cytoplasmic inclusions. Cytosol is the main cell region of cellular metabolism reactions (metabolic pathway) due to a large number of enzymes (isomerase, aldolase, kinase, mutase, etc). Among the citoplasmic metabolic pathways are: anaerobic glycolysis, pentose phosphate pathways, gluconeogenesis, protein synthesis, etc. Eukaryotic cytoskeleton contains three types of cytoskeletal filaments, which are microfilaments (7 nm), intermediate filaments (10 nm), and microtubules. As regarding cytoplasmic inclusions, we can enumerate: glycogen (ex. hepatocytes) and starch granules (plant cells) (12, 13).

In cytosol is present the phenomenon of macromolecular crowding. The macromolecular crowding phenomenon occurs into cell cytoplasm because the high concentrations of macromolecules reduce the volume of solvent available for other molecules in the solution. As a result, the effective concentration of macromolecules is high, the cytosol is divided in a network of narrow pores and favouring macromolecular interactions (protein - protein, DNA - protein, RNA - protein) (14).

In prokaryotic cells, the cytoplasm don't have Golgi apparatus, endoplasmic reticulum and mitochondrion, ribosomes are smaller and it isn't separated from the nucleolus (15).

Cytoplasm are complex functions: maintaining the shape and consistency of the cell, cell motility and locomotion (flagellum, microvillus) medium for cell organelles, growth and division cell, site of biochemical reactions, store many chemical for life, transport of substances (16, 17).

  6.   Protein folding requires crowd control in a simulated cell
  7.   Plasmolysis in Elodea
  8.   Striated muscle fiber
  9.   Amoebic Endocytosis
10.   Cellular Respiration - Animation
11.   Enzyme - Animation 
12.   FRET

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