The plasma membraneFeatures: fluidity (lipids can move laterally), asymmetry (lipids, proteins and sugars are arranged asymmetrically on the inner and external side), discontinuity (the membrane structure is interrupted by integral proteins) and selectivity (the membrane is permeable but with strict selection).

Functions: movement and expansion, mantains structural integrity, receives and transmits informations, imports and exports molecules. Separates the intracellular from the extracellular fluid, works as a selective barrier.

- The thickness of membrane is 7.5nanometers (75 A)

-   Made of:

Phospholipids (hydrophilic head and hydrophobic tails), glycrol linked to a phosphate group, the tails are usually fatty acids and can have different length

Phospolipids can be divided into phosphoglycerides (the most abundant, based on glycerol) and sphingolipids (contained in some membrane, based on aminoalcohool sphingosine).

Glycolipids Cholesterol Sugars Filaments of cytoskeleton Proteins

- The proteins and lipids can drift within the layer, usually laterally and rarely flip flopping to the other layer. There is a pattern produced by the scattered proteins. This creates what is called the fluid mosaic model.

- Fluidity means that lipids and proteins can float in the membrane via diffusion. Temperature and presence of unsaturated fatty acids are directly proportional to fluidity. Cholesterol and length of the fatty acid chains are inversely proportional to fluidity.

- The cholesterol is a component of animal membrane, most of the molecule is hydrophobic but there is one hydrophylic end. Cholesterol restricts the movement of phospholipids molecules. It reduces fluidity of the membrane and the permeability of it to hydrophilic particulars, allowing the cell to mantin concentration differences accross their membranes. It also prevents the crystallization of fatty chains of phospholipids.

- The glycocalix is the name given to the carbohydrate rich, mucuous layer around the cell, made of glycolipids and glycoproteins (glycosaminoglycans) and in some cases contains also enzymes (glycocalyx of intestine epithelium). It’s functions are: barrier, cell adhesion, blood groups and enzymatic action.

There are two types of membrane proteins.

Integral proteins - are firmly inserted into the lipid bilayer, are usually transmembrane or even multipass proteins, and have both hydrophobic and hydrofilic regions. They are mainly involved with transport, forming channels and working as carriers for ions. Can work as hormone receptors, such as with insuline, and as adhesion molecules. 

Peripheral/extrinsic proteins - attached loosely to integral proteins or the membrane,  are easily removed, can support the membrane, work with electron transport, work  as enzymes, mechanical functions (changing cell shape during division, etc). Their functions are: cell adhesion, hormone binding, immobilized enzymes, channels for passive transport, pumps for active transport.

Membrane lipid rafts and signaling: Specialized membrane microdomains containing receptors that signal inside the cell for specific functions.

TransportPassive processes:

There is no ATP expenditure.

DIFFUSION - Molecules go from more concentrated to less concentrate, down to their concentration gradient. High temperatures can increase diffusion speed. It can be either simple (non polar and lipid soluble substances difuse directly through the membrane, such as oxygen, carbon dioxide, fat soluble vitamins and alcohol) or facilitated (the molecule passes through the membrane by binding a protein carrier or moving through water filled protein channels). Facilitated might involve carriers (change shape, can be saturated, transports glucose) or channels (can be leakage ones or control ones).

OSMOSIS - Passage of water. They can pass through osmosis in the lipid bilayer because they are small enough to do so despite the polarity, and through water specific channels called aquaporins (AQP). 

Active processes:

Are the ones where there is ATP expenditure. The substance might be too large or incapable of dissolving in the lipid bilayer. 

ACTIVE TRANSPORT - similar to facilitated difusion in the sense that the molecules must specifically and reversibly combined with a carrier protein. They move the ions uphill, against the concentration gradient. Done by globular proteins in membrane, called pump proteins. Na+ K- pump is the best example. Three sodium ions are pumped out of the azon and two potassium ions are pumped in, each time the pump goes round this cycle it uses one ATP. 

EXOCYTOSIS - Vesicles can also be used to release materials from the cells. If it fuses with the membrane, the contents are then outside the membrane and therefore outside the cell, that is, exocytosis occurred. Digestive enzymes are released like that, or waste products and even excess of water. 

ENDOCYTOSIS - Transport made through the creation of vesicles, formed by pinching off a small piece of the plasma membrane cells and it can contain material that was outside the cell, taking it to the inside. This happens in the placenta, proteins from the mothers blood including antibodies, are absorbed by the fetus by endocytosis. 

Clinical drop: role of the ABC transporter protein MDRs in the resistance of tumor cells to chemotherapy.

Endoplasmic reticulum

Tubules and cisternae delimited by cell membrane and intercommunicating 50% of all the cell membranes Varibla shape Extension depending on the metabolic needs of the cell

Membranes are not bound to ribosomes Synthesis of steroids and lipids (lipids produced there are inserted in the lipid

bilayer on the cytoplasmic side, enzymes named FLIPPASE catalyze the flip-flop movement to bring the newly synthesized molecule on the opposite side of the bilayer)

Storage of ions such as calcium (accumulation and release of ions to the cytoplasm, skeletal and cardiac cells contain a lot of those due to muscle contraction)

Involved in detoxifying processes of the cell (contains enzymes that add OH groups to toxic compounds, thus favouring their elimination)

Glycogen metabolism Biogenesis of cell membrane Key role in glycogen metabolism

Rough endoplasmic reticulum - produces protein, made of a membrane system externally studded with ribosomes. Its proteins are mainly incorporated to the cell or to export from the cell. Present in secretory tissues such as liver, antibody-producing plasma cells, etc.

Ribosomes

Free ribosomes: synthesis of proteins for the nucleus, mitochondria, cytoskeleton, peroxisomes and cytoplasmic side of the plasma membrane.

RER ribosomes: synthesis of membrane proteins, secretion proteins and proteins of lysosomes. Abundant in gland cells (secretion), in plasma cells, in neurons.