Structure of Eukaryotic cells

Eukaryotic cells feature membrane delimited nucleii containing two or more linear chromosomes; numerous membrane-bound cytoplasmic organelles: mitochondria, RER, SER, lysosomes, vacuoles, chloroplasts; ribosomes and a cytoskeleton. Also, plants, fungi, and some protists have a cell wall.

A.Structure of the nucleus

The nucleus is the round object in the cell that holds the genetic information (DNA) of the cell. It is surrounded by a nuclear envelope and has a nucleolus inside.

  1. Nuclear envelope: The nuclear envelope is a double-layered plasma membrane8 like the cell membrane, although without membrane proteins. To allow some chemicals to enter the nucleus, the nuclear envelope has structures called Nuclear pores. The nuclear envelope is continuous with the endoplasmic reticulum. 
  2. Nucleolus: The nucleolus appears in a microscope as a small dark area within the nucleus. The nucleolus is the area where there is a high amount of DNA transcription taking place.

B. Chromatin

Chromosomes consist of chromatin. This is made up of strings of DNA, which typically measure centimeters in length if stretched out. This DNA is wound around a histone core and organized into nucleosomes.

The chromatin must be uncoiled for gene expression15 and replication16. Chromosome micrograph

C. Endoplasmic reticulum

The endoplasmic reticulum is a cellular organelle made up of a series of extended folded intracellular membranes. It is continuous with the nuclear membane. There are two main types of endoplasmic reticulum:
  • RER: rough endoplasmic reticulum (site of protein synthesis) associated with ribosomes 
  • SER: smooth endoplasmic reticulum (site of lipid synthesis)
1. Rough Endoplasmic Reticulum 

Proteins are directed to the RER by a signal sequence of a growing polypeptide21s on the ribosome. This is recognised by a signal recognition particle which brings the ribosome/ polypeptide complex to a channel on the RER called a translocon. At the translocon, the signal sequence and ribosome/polypeptide complex interact with the translocon to open it. The signal sequence becomes attached to the translocon. The ribosome can continue to translate the polypeptide into the lumen of the RER. As synthesis continues, 2 processes can happen.

  1. If the protein is destined to become a membrane bound protein then the protein synthesis will continue until termination. The ribosome can then dissociate, allowing protein folding within the RER lumen to occur and continuation to the golgi apparatus for processing of the polypeptide. 
  2. If the protein is destined for storage for later secretion after stimulation or for continuous secretion then a protease-enzyme which cuts proteins at the peptide bond-can cut the signal sequence from the growing polypeptide. Continuation to the golgi etc. can then occur.
When produced, proteins are then exported to one of several locations. The proteins are either modified for extracellular membrane insertion or secretion. Note, this is in contrast with ribosomes which do not associate with the RER and produce proteins which will become cytosolic enzymes for example.

2. Smooth Endoplasmic Reticulum 

Smooth endoplasmic reticulum produces enzymes for lipid and carbohydrate biosynthesis and detoxification RER

3. Sarcoplasmic Reticulum 

This is a specialised form of endoplasmic reticulum found in some muscle cell typesparticularly striated, skeletal muscle. Its main function is different from the other 2 types in that is mainly acts as a storage of calcium. This reticulum has voltage gated channels which respond to signals from 'motor neurones' to open and release calcium into the cytoplasm. This can then bring about the next part in muscle contraction.

Structure of Eukaryotic cells
Figure :Image of nucleus, endoplasmic reticulum and Golgi apparatus. 1. Nucleus. 2. Nuclear pore. 3. Rough endoplasmic reticulum (RER). 4. Smooth endoplasmic reticulum (SER). 5. Ribosome on the rough ER. 6. Proteins that are transported. 7. Transport vesicle. 8. Golgi apparatus. 9. Cis face of the Golgi apparatus. 10. Trans face of the Golgi apparatus. 11. Cisternae of the Golgi apparatus.

 4. The Golgi apparatus

The golgi apparatus is made up of multiple stacks of bilipid membranes.
  • Proteins made on the RER are modified and then sorted 
  • Formation of secretory vesicles 
  • Formation of lysosomes (intracellular digestion) 
Other membrane-bound cytoplasmic organelles include:
  • Microbodies (generic term) 
  • Glyoxysome (transforms fat into carbohydrate in plants) 
  • Peroxisome (uses oxidative metabolism to form hydrogen peroxide and is destroyed by catalase) 
5. Ribosomes

Ribosomes are the site of protein synthesis. Ribosomes themselves are synthesized in the cell nucleoli28 and are structured as two subunits, the large and the small. These parts are composed of RNA and protein.

Prokaryotic and eukaryotic ribosomes are different, the eukaryotic ones being larger and more complicated

6. DNA-containing organelles

  • Double membrane 
  • Aerobic metabolism, internal membrane 
  • DNA, ribosomes 
  • Give rise to new mitochondria 
  • Double membrane 
  • Photosynthesis, internal membrane
  • DNA, ribosomes 
  • Give rise to new chloroplasts 
  • Microtubule organizing centers 
    • Animal cells and many protists 
    • Pair constitutes the centrosome 
    • Give rise to flagellum during spermatogenesis 
  • Consist of 9 triplet microtubules 
  •  Mitosis, meiosis
7. Cytoskeleton

Cytoskeleton is a collective term for different filaments of proteins that can give physical shape within the cell and are responsible for the 'roads' which organelles can be carried along.
  • Gives the cell shape 
  • Anchors other organelles 
  • Vital to intracellular transport of large molecules 
The cytoskeleton is composed of 3 main types of filaments:
  • Actin filaments (7 nm) 
  • Microtubules: (25 nm) polymer of tubulin; 13/ring. 
  • Intermediate Filaments 
Both actin and microtubules can have associated motor proteins.

1. Intermediate Filaments

These are rope like filaments, 8-10nm in diameter and tend to give the structural stability to cells. Examples inculude Vimentin, neurofilaments and keratin. It is keratin which priniciply makes up hair, nails and horns.

2. Actin Filaments

These filaments are 2-stranded and composed of dimeric subunits called G-Actin. They contain a GTP molecule in order to bind (polymerise). As GTP is hydrolysed then the structure becomes unstable and depolymerisation occurs. The growth of actin filaments is concentration dependant-that is, the higher the concentration of free G-actin, the greater the polymerisation. The are also polar, having a + and a - end (not related to charge) and polymerisation tends to happen faster at the + end.

Cilia and flagella are threads of microtubules that extend from the exterior of cells and used to move single celled organisms as well as move substances away from the surface of the cell. motor proteins-move, wave motion

sorce: mipa science unram