Ribosomes

General Characteristics

3

1. contain RNA

2. they are intensely basophilic

3. the cytoplasm of cells rich in ribosomes stain with basophilic dyes hematoxylin

Ribosomes

 Structure

4

1. formed in the nucleolus

2. consists of a large and a small subunit

3. initially the rRNA is transcribed as a single chain and then cut into three smaller fragments

4. function of the proteins is to stabilize the structure of the RNA

Ribosome

Function

4

1. responsible for the translation of mRNA into protein

2. acts as a ribozyme, RNA acting as an enzyme 

3. mRNA is bound to the ribosome by the capped sequence synthesis is begun at initiation sequence

4. protein synthesis continues until a stop sequence

polyribosomes or polysomes

2

1. consist of a single mRNA molecule associated with a number of ribosomes along its length

2. the longer the mRNA, the greater the number of ribosomes

Types of Ribosomes

2

 1. Free Ribosomes or Polysomes- in the cytoplasm synthesize soluble proteins for the cytoplasm and nucleus.

2. Membrane-Bound Ribosomes-associated with R.E.R.

Membrane bound ribosomes Make what types of proteins?

4

1. secreted proteins,

2. membrane proteins

3. those targeted for specific organelles

4. the interaction with membrane is determined by the mRNA

Mechanism for Ribosome attaching to E.R.

6

1. the initial mRNA codes for a Signal Sequence, stretch of 20 to 25 hydrophobic amino acids 

2. signal sequence is recognized and bound by (SRP) which blocks further elongation until the mRNA is attached to the RER 

3. SRP-ribosome complex is bound by Docking Proteins present on the RER

4. SRP is released allowing translation to continue

5. protein translocators on the RER bind to the ribosomes and form a channel through which the newly synthesized proteins pass  Vectorial Discharge

6.  once the protein has entered the endoplasmic reticulum, the signal sequence is clipped off by enzymes

E.R. General Characteristics

5

1. only visible by EM 

2. composed of a single continuous membrane which encloses a single sac or ER lumen 

3. lumen is separated from the rest of the cytoplasm

4. the ER lumen may be10% of the total cell volume

5. two major types: Rough and Smooth ER

S.E.R. Stucture

4

1. membrane lacking ribosomes

2. continuous with the rough ER 

3. often in the form of fine tubules

4. prominent in cells, which carry out lipid metabolism secrete steroid hormones

S.E.R functions

4

1. Breakdown of glycogen, Carbohydrate metabolism

2. Detoxification of drugs.

3. Regulation of muscle contraction (hold/release Ca)

4. Synthesis of phospholipids and cholesterol

R.E.R Strucutre

4

1. ribosomes give it a rough appearance under EM 

2. ribosomes always on cytoplasmic side,

3. often in the form of flattened sacs called cisternae 

4. rough ER is particularly prominent in protein secreting cells.

R.E.R Function

8

 1. involved in the synthesis, processing and segregation of membrane proteins, proteins targeted for organelles, secretion

2. vesicles containing various proteins pinch off

3. transfer (transport) vesicles termed coated or uncoated depending the presence or absence of an electron dense layer on the surface 

4. coated vesicles from the ER are surrounded by protein  COPII

which can polymerize to form a polygonal structure -

5.  clathrin and COPI surround coated vesicles from other organelles

6. these proteins are responsible for the initial formation of the vesicles but are then lost from these vesicles

7. the surfaces of the transfer vesicles have proteins v-SNAREs on their surfaces which determine what they will fuse with

 8. the orientation of membrane proteins is maintained throughout their further processing.  

Golgi Complex General Characteristics

3

1. present in most if not all nucleated cells

2. generally located near the nucleus 

3. anywhere from 1 to 100 Golgi complexes per cell depending upon the cell type.

Golgi Structure

5

1. consists of a series of  cisternae arranged in a stacks 

2.  about 6 cisternae per stack but this  may vary

3. the stack is polarized

4. Cis, Forming Face  closely associated with transitional portion of the ER

5. Trans, Maturing Face  directed towards the plasma membrane  associated with condensing and secretory vesicles

Golgi Function

2

1. Biochemical modification of biological material

2. Director of macromolecular traffic

Golgi function

Biochemical modification of biological material

6

1. carbohydrate core of glycoproteins is elaborated upon by enzymes in the Golgi

2. sulfation of glycosaminoglycans

3. cleavage of peptide chain of pro-proteins 

4. different enzymes are in different cisterna so that modifications can be done in the correct sequence 

5. according to the cisternal maturation model, new cis cisternae continually form and migrate to the trans-face

Golgi functions

Director of macromolecular traffic

5

1. once  the modifications on the proteins have been completed, they are then packaged for export 

2. proteins destined for a particular destination are concentrated in Condensing vacuoles in a specific region of the Golgi termed the trans Golgi network this mechanism  is unknown

3. the vacuoles buds off the membrane to form a secretory vesicle (a type of transport vesicle) 

4. the cytoplasmic side of the transport vesicles probably have docking markers which determine their final destination.

Lysosome

General Characteristics

4

1. present in all eukaryotic cells

2. arise from the trans Golgi network

3. proteins that are destined to be part of the lysosome contain , mannose-6-phosphate 

4. this is recognized by proteins in the trans Golgi network that packages these proteins into transport vesicles that give rise to lysosomes

Lysosome

General Structure

4

1. comes in a variety of shapes and sizes

2. contains a large number of acid hydrolases, break down a variety of molecules andhave an optimal enzymatic activity at pH 5

3. this protects cytoplasmic structures if the enzymes are accidentally released

4. the membrane contains a proton pump which maintains the internal pH around 5

Types of Lysosomes

4

1. Primary Lysosomes - newly formed as buddings of the trans Golgi network  appears to be homogeneous under the electron microscope.

2. Secondary Lysosomes -  contain ingested substances small fragments of digested material enters the cytoplasm and can be reused

3. Residual bodies in older lysosomes containing the remnants of digested material 

4. Lipofuscin when there is alarge quantities of residual bodies 

Types of Secondary Lysosomes

5

1. Digestive vacuoles -resulting from the phagocytosis of large particles such as bacteria.

2. Early endosome cells take up material through the process of endocytosis (NOT A LYSOSOME)

3. Late endosome from early endosome  when it takes on lysosomal enzymes

4. Autophagic vacuoles -contain intracellular organelles

5. Multivesicular bodies - contain numerous vesicles.

Lysosome fucntion

2

1. degrade substances within the cell

2. the lysosome fuses with the target material and then digests it with the hydrolytic enzymes

Peroxisomes (microbodies)

General Characteristics

3

1. present in most cells

2. proteins are selectively imported from the cytoplasm

3. thought to be a very primitive organelle predating mitochondria

peroxisome (micorbodies)

Structure

1. small packets of oxidative enzymes

2. surrounded by a single membrane

Peroxisomes (Micorbodies)

Function

3

1. carry out the oxidation of various substances

2. contains a series of enzymes that produce H₂0₂ which is utilized by the enzyme Catalase to oxidize various substances

3. acts to detoxify substances (ethanol) and breakdown fatty acids

Mitochondira General Characteristics

5

1. powerhouse of the cell

2. numbers vary most numerous in cells requiring lots of energy

3. evolved from energy producing bacterium (purple photosynthetic bacteria)  took up residence in cytoplasm of  primitive eukaryotic cell

4. contain everything needed to do protein synthesis (DNA. mRNA. tRNA, ribosomes) which is very similar to Bacteria

5. mitochondria divide by fission.

Mitochondira Structure

2

1. most often in the form of a cylinder, but size and shape vary 

 2. contains Outer membrane Inner membrane, intermembrane space, matrix

1. contains channel-forming proteins which make the membrane permeable to small molecules 

2.  similar to the endoplasmic reticulum

 chemically equivalent to cytosol with respect to small molecules

 Mitochondria Inner Membrane

7

1. contains special lipids that make it relatively impermeable to ions and small molecules  

2.  highly folded to form Cristae to increase the surface area 

3. shape and number of cristae vary depending upon the cell

4. contains the following elements: 5) Transport proteins -to make the membrane selectively permeable to certain substances (e.g. ATP). 

6) Proteins of Electron Transport System (Respiratory Chain).

7) Elementary (or F1) Particle knob-like structures - contain ATP synthetase activity

Mitocondiral Matix

5

1. highly concentrated material inside the inner membrane

2. contains various metabolic enzymes, Citric Acid Cycle enzymes

3. contain the material necessary to carry out protein synthesis (DNA, mRNA, tRNA and ribosomes) 

4. small fraction of the mitoc. proteins are made in the matrix 

5. dense granules composed of precipitated calcium salts

Production of Energy By the Mitochondira

2

1.  this is done through the Chemiosmotic Mechanism,

2. Oxidative Phosphorylation  requires  0₂ NADH produced by various metabolic reactions is converted into ATP

The citric acid cycle results in the production of NADH in the matrix

1. NADH donates an electron to the electron transport system  which is located in the inner mitochondrial membrane

2. through a series of steps, the electron is eventually combined with 0₂  to form H₂0

3.  in this process protons (H⁺) are pumped out of the matrix into the intermembrane space

Since the inner mitochondrial membrane is impermeable to H^+, a proton gradient is formed between the matrix and the intermembrane space

Oxidative Phosphorylation step 6

3

1. The H+ can enter the matrix by passing through a hole in the F1 particle

2. this passage causes the head group to rotate (converting chemical energy into mechanical energy)

3. this mechanical energy is then used by the complex to catalyzes the formation of ATP from ADP and P (mechanical energy converted back to chemical energy).

Oxidative Phosphorylation step 7

3

1. The ATP can then leave the matrix through a specific membrane carrier and enter the cytoplasm where it is used

2. a single molecule of ATP/ADP may go in and out of the mitochondria several time a minute

3. the level of ATP is maintained at approximately 10 times that of ADP

Cytoplasmic Inclusions

Pigment Granules

2

1. Melanosomes  - found in the epidermis - when initially formed, they are surrounded by a membrane - as they mature, the outer membrane may be lost.

2. Lipofuscin (Fig. 21) - secondary lysosomes (see section on lysosomes).

Cytoplasmic Inclusions

Glygogen General Characteristics

4

1. large, branched polymer of glucose 

2. serves as a storage for glucose

3. large amounts are found in the liver and muscles 

4. often closely associated with the smooth ER.

Cytoplasmic Inclusions

Glygogen Histochemical appearance

3

1. light microscopy, glycogen can be stained using the periodic acid-Schiff reagent.

2. Under the electron microscope (after staining with lead), glycogen appears as small, electron-dense granules that have an irregular shape

3. they are not directly covered with a membrane

Cytoplasmic Inclusions

Lipids General Characteristics

1. Serve as local storage of stores of energy

2. particularly abundant in cells that secrete lipid hormones and adipose cells

3. often associated with the smooth ER and mitochondria.

Cytoplasmic Inclusions

Lipids

Histochemical appearance

6

1. Often lost from tissue during routine preparation

2. lipid soluble in organic solvents used in processing of tissue.

3. Can be stained in frozen sections by a fat soluble dye

4. Fixation with osmium preserves the lipid

5. under the electron microscope there is a gradient of density from the outside to the inside reflecting the diffusion of the osmium

6. most droplets are not surrounded by a unit membrane