Prokaryotic Cells
There are two major cell types, prokaryotic and eukaryotic cells.
Prokaryotes (or bacteria) are very small, their cells are between 1 and 5 um in diameter. Bacteria appear in a variety of different shapes: Spherical (coccus), rodlike (bacillus), curved rod (vibrios) and spiral (spirilli).

In addition there are flat, triangular, starlike and square shapes, which are however, less frequent. Bacterial cells are surrounded by a semirigid cell wall , which consists of a polysaccharide matrix, crosslinked by polypeptide chains. Based on their cell wall composition bacteria can be divided into two groups: Gram positive and Gram negative.
Eukaryotic Cells

Eukaryotic cells are much more complex than prokaryotic cells. Animals cells range from 10 to 30 um in diameter and plant cells about 10um. Eukaryotic cell may be spherical, flat or extremely elongated (nerve cells).

Eukaryotic cells have two distinct compartments, the nucleus and the cytoplasm . The nucleus which contains the chromosomal DNA is surrounded by two membranes, called the nuclear envelope. The cytoplasm consists of a number of membrane bound structures collectively called organelles. They carry out very specialized functions. In addition to organelles, ribosomes are suspended in the cytoplasm image or are attached to membranous sacs, called rough endoplasmic reticulum or RER. The RER is the site of the synthesis of enzymes destined for the degradation of certain nutrients, metabolites or spent organelles, protein destined for export from the cell (secretion), or proteins be to incorporated into the plasma membrane. Before the proteins reach their final destination, they are structurally modified in a neighboring membrane bound organelle, called Golgi complex. From there certain hydrolytic enzymes are packaged into lysosomes , which are capable of breaking down biological molecules. Mitochondrion , an organelle surrounded by two membranes is the cell's powerhouse, i.e. it is continuously producing the high energy molecule ATP. In animal cells peroxisomes carry out special oxidation reactions in which hydrogen peroxide is produced and neutralized. In plants the conversion of lipids into sugar is carried out in Glyoxisomes. Chloroplasts in green plants are the site of photosynthesis, producing glucose and glyceraldyde. Animal cells and plant cells differ from each other in several components and assembly, check your textbook for details. In addition the eukaryotic cell contains a network of filaments and microtubules known as cytoskeleton, which provides mechanical support and helps in the guided transport of small vesicles and organelles.

The Endoplasmic Reticulum

The region near the nucleus of a eukaryotic cell contains two types of interconnected membraneous structures, the smooth endoplasmic reticulum (SER) and the rough endoplasmic reticulum (RER) . The SER plays an important role in the carbohydrate metabolism, synthesis of fats and phospholipids and in the detoxification drugs. The RER is very important for protein processing. The space inclosed by the RER membrane is called lumen. The ER is called rough, because its membrane appears rough under the microscope. The membrane is actually studded with The protein is them discharged into the cytoplasm and will remain there or it may diffuse to and subsequently into an organelle. However, the later destinations require that the peptides have a destination signal

As mentioned above the RER contains ribosomes, however they are not attached permanently to the RER membrane. Let's assume a protein synthesis begins on a free ribosome and a particular sequence of very hydrophobic amino acids emerges as shown below

This hydrophobic sequence called signal peptide binds to a so called signal recognition particle, which prevents further synthesis. The entire complex migrates to the RER attaches itself to a receptor protein called a docking protein. The signal particle dissociates off and protein synthesis continues , with the protein discharged into the lumen of the RER.

Certain enzymes inside the lumen, so called chaperones help the newly synthesized proteins to fold into their proper conformations.

Some proteins are not completely discharged into the lumen and thus become peripheral proteins (membrane bound). Depending on certain signals embedded in the amino acid sequence, other proteins are glycosylated (addition of sugars) as shown below

Following these reactions the proteins are removed from the RER through a process of membrane invagination and then they are packaged in membranous transitionary vesicles. These vesicles travel to a neighboring organelle, the Golgi complex.

Golgi Comples

The Golgi complex consist of three distinct membranous sacs, the cis, medial and trans Golgi. The incoming transitionary vesicles fuse with the cis Golgi membrane and the content is released into the interior. Vesicles with membrane bound proteins will also fuse with the Golgi membrane and the bound protein becomes also exposed to the interior.

Enzymes in the lumen of the Golgi are specialized to modify the polysaccharide chain of the glycoproteins. Some of the glycoproteins modified in the Golgi are destined to be incorporated into the plasma membrane of the red blood cells. Blood group specificity is in part caused by these modified polysaccharide chains of the glycoproteins. Another important modification is the attachment of a mannose-6-phosphate to certain glycoproteins. Proteins labeled with a mannose-6-phosphate tag are destined to become lyososomal enzymes . After their modification, the glycoproteins and all other proteins are packaged into small vesicles , budding off from the trans Golgi

There are a three main destinations for vesicles released from the trans Golgi
1. Proteins destined for secretion (export from the cell)
2. Proteins destined to be incorporated into the plasma membrane surface
3. Enzymes destined for the lysosomes .

1. Proteins destined for secretion are concentrated in the trans Golgi network and packaged into clathrin coated vesicles.

These vesicle migrate to the plasma membrane, where they either fused immediately with the membrane and discharge their content to the exterior ( constitutive secretion or they are stored near the membrane until a signal such as a neurotransmitter or hormone causes them to fuse with the membrane and release the enclosed content ( regulated secretion ).The fusion of the vesicle with the membrane and discharge its content is called exocytosis.

Insulin, a peptide hormone, which is released into the blood when the glucose concentration is too high, is secreted form the beta cells of the pancreas in a regulated secretion pathway. (Insulin will migrate through the blood stream and bind to specific receptors on liver cells, causing them to remove glucose from the blood).
Here are some examples of Secretory Proteins
Constitutive (continuous) Secretion

• Serum proteins
  - Albumin (most abundant protein in blood) secreted from liver cells into blood
  - Immunoglobulins (antibody response) secreted from lymphosites into blood
• Extracellular matrix
  - Collagen (fibrous protein in connective tissue) secreted from fibroblasts ( connective cell tissue)
  proteoglycans (cell coat) from Fibroblasts

• insulin secreted from pancreatic beta cells
• glucagon secreted from pancreatic alpha cells
• endorphins (inhibitory neurohormones acting on neurons that transmit pain impulses) secreted from neurosecretory cells
• ACTH (growth hormone) from anterior pituitary
• Trypsin and Chymotrypsin (hydrolases) secreted from the pancrease into the stomach
• casein, lactalbumin (milk proteins) secreted from mammary glands

3. Lysosomal enzymes are concentrated in the trans Golgi through a strong interaction with specific mannose-6-phosphate receptors. After being packaged into clathrin coated vesicle they fuse with larger vesicles, so called early endosomes . The lumen of these endosomes is very acidic ( they contain a proton pump , think about how it may work), causing the mannose-6-phosphate tagged hydrolases to dissociate from their receptors and become active. The receptors are eventually returned in vesicles to the trans Golgi. The early endosomes can fuse with other vesicles coming into the cell by endocytosis such as a phagosome (contains food particles or invading microorganism) or an autophagosome (contains spent organelles) digesting the biopolymers and releasing amino acids, lipids, sugars and nucleic acid bases into the cytoplasm for further use.

Problems with the processing of lysosomal hydrolases can cause severe and deadly diseases. Many of these diseases are caused by the deficiency of specific lysosomal hydrolases, causing the accumulation of excess amounts of especially lipids. Tay Sachs disease leads to the accumulation of certain lipids called gangliosides, causing mental retardation and early death. Patients with Hurler and Hunter syndrome cannot degrade glycoaminoglycan.
It is also possible that lysosomal enzymes are not packaged correctly, because the mannose 6-phosphate receptors are either not produced or are not functioning. In such a case the hydrolases may be packaged into secretory vesicles and discharged to the exterior, where the can damage any biological material they encounter. In another senario the lysosomal membrane may rupture and the content is spilled into the cytoplasm. Both defects are lethal in infants.
In certain instances such a mechanism may be used to destroy cells. In the development of individual fingers from initially webbed hands in human embryos, the webbed tissue is degraded by hydrolases released from lysosomes inside the cytoplasm.

Problems:

1. Do cytoplasmic enzymes have signal sequences?
2. Why are RER and Golgi in close proximity
3. Why are lysosomal enzymes not synthesized on free ribosomes
4. Because of a mutation the cell does not recognize signal peptide sequences, concequently the mechanism of transporting ribosomes to the RER is inhibited. What are the concequences of such a mutation?
5. What is the difference between constitutive and regulated secretion
6. What is causing lysosomal diseases?

1. enzymes which catalyze reactions in the cytoplasm have no signal peptide, they are released from free ribosomes into the cytoplasm.
2. Golgi receives proteins from the RER for further modification (mostly glycoproteins are affected)
3. If they would be synthesized on free ribosomes, they will be released into the cytoplasm an dcould not be packaged into lysosomes.
4. Proteins could no longer be synthesized into the lumen of the RER. Protein synthesis would countinue on free ribosomes and the proteins would be released into the cytoplasm, including secretory proteins, lysosomal proteins and membrane bound proteins such as receptor proteins and tansporter protein. No of these proteins would be functional.
5. in constitutive secretion vesicles migrate from the trans Golgi to the inside of the plasma membrane and discharge their content to the exterior by exocytosis. In regulatory secretion the vesicles "wait" at the plasma membrane for an extracellular signal (e.g. a hormone). Attachment of the hormone to its cellular receptor causing the secretory vesicles to fuse with the membrane and discharge their content to the outside.
6. mostly inactive hydrolases which lead to the accumulation of cellular debris.