B.2.2 Flashcards
(13 cards)
Endomembrane system
The endomembrane system is composed of the different membranes that are suspended in the cytoplasm within an eukaryotic cell. These membranes divide the cell into functional and structural compartments called organelles.
A system of compartmentalised sacs within the eukaryotic cell that work together to modify, process and ship molecules in and out of the cell.
These are nucleus, vesicles, apparatus, Golgi apparatus, rER
Membranes in different organelles
In eukaryotic cells different organelles can either have no membrane, a single membrane or a double membrane. This membrane being a phospholipid bilayer.
- No membrane
- Ribosomes
- Centrioles
- Microtubules
- Nucleoli
- Single membrane
- Vesicles and vacuoles
- rER
- sER
- Golgi apparatus
- Lysosomes
- Double membrane
- Nuceli
- Mitochondria
- Chloroplast
Adaptations of the mitochodrion
Different adaptation in the mitochondria go as follows:
- Cristae: these are folds of the inner membrane which highly increase the SA:V ratio inside the mitochondria, therefore increasing the area available for oxidative phosphorylation (the production of ATP)
- Inner membrane: Is where electron transport chains and ATP synthases are found
- Outer membrane: Separates the contents of the mitochondria from the rest of the cell, creating a compartment with perfect conditions for aerobic respiration.
- Intermembrane space: The volume between the 2 membranes is very small, therefore there is a high concentration of protons that can be reached very quick.
- Ribosome 70s and naked loop of DNA: These are essential for the mitochondria to create its own proteins.
- Matrix: Contains enzymes for the link reaction and the Krebs cycle (transforming glucose to ATP). The matric has a smaller volume than the cytoplasm so enzymes and substrates are concentrated making the reactions happen more rapidly
Golgi apparatus
The golgi apparatus is made of a multitude of flattened sacs that are called cisternae, these lay between the rER and the cell membrane. Its main focus is to process polypeptides coming from the rER.
The Golgi body contains enzymes responsible form protein chemical modifications and maturation:
- Adding carbohydrates to make glycoproteins
- Adding phosphate or sulphate groups
- Assembling polypeptides and other subunits to form the quaternary structure in proteins
- In plant cells most of the cell wall material during cytokinesis is formed by the Golgi body
- Glycolipids cell membrane proteins lysosome proteins are made there
- Materials that have extracellular functions are packaged into vesicles and secreted
- If directly secreted it’s called constitutive secretion
- If stored and released by chemical signal it is called regulatory secretion
Proteins in the Golgi body are processed in a specific way, polypeptides move from the nearest side of the rER (the cis side) to the opposite side (the trans side)
- Vesicle transport model, the cisternae don’t move and vesicles move proteins between the sacs
- Cisternae maturation model, vesicles from the rER fuse on the cis side forming new cisternae which gradually move to the trans side where they break up into vesicles.
Adaptations of chloroplasts
Photosynthesis consists of 2 different parts:
- Light dependent reaction: which takes place in the thylakoid membrane and produces ATP and NADPH
- Light independent reaction: which takes place in the stroma and depends on ATP and NADPH
Inside the chloroplasts there are different structures:
- Thylakoids that are internal membrane structures which contain chlorophyll and other photosynthetic pigments.
- Many grana and thylakoids give a large SA:V which increases area for light absorption
- Thylakoids contain in the membrane the pigments that electron carriers and ATP synthase use to produce ATP and NADPH that need to be used for light independent reaction.
- To produce ATP a proton gradient must be created between the thylakoid space and the stroma, for this thylakoids have a small internal volume to maximise hydrogen gradients quickly increasing the concentration gradient.
- Grana is a group of thylakoids stacked one on top of the other
- Stroma is the fluid inside the chloroplast that contains enzymes and substrates for the Calvin cycle
- Stroma has suitable pH and enzymes for the Calvin cycle, the small volume of the stroma increases the chances of collision between the enzymes and substrate.
- It has a double membrane
- Lamellae are membrane structures that connect grana
- These connect and separate grana to maximise photosynthetic efficiency.
- Pigments and chlorophyll are arranged into photosystems allowing maximum absorption of light energy.
vesicle function and formation
Vesicles are singular membrane structures that enclose material inside to be transported inside or outside the cell. It is commonly used to transport molecules that are too big inside (endocytosis) or outside (exocytosis) the cell.
One way of engulfing large molecules by endocytosis is with the the help of clarthin proteins to make vesicles:
- Clarithin are three legged molecules
- These clarithin can bind to membrane ligand mediated by another protein called adaption
- When the vesicle is forming, clarithin molecules can bind one to each other forming a lattice of pentagons or hexagons.
- These proteins support the formation of a rounded vesicle with a cage of clarithin proteins around it
- Once the vesicle is formed clarithin is no longer needed therefore it uncoats.
Other functions of vesicles are:
- The transport of materials outside the cell (exocytosis)
- Secretory vesicles, transport proteins and lipids from inside the cell to the cell membrane (exocytosis)
- Endocytic vesicles, formed by invagination of the cell membrane around an extracellular substance during endocytosis.
- Vesicle fusion is the merging of a vesicle with another organelle or with a part of a cell
rER
From the rER, the polypeptides from ribosome protein synthesis can be transported elsewhere in the cell by a vesicle that buds off the rER. The typical destinations of a vesicle from the rER is the Golgi apparatus.
Centrifugation
The process of centrifugation consists of separating organelles according to their size and density. This is done by applying different speeds and duration. Larger organelles will sink to the bottom at a faster rate and lower centrifugation than smaller ones.
Before that the organelles must be separated by a process called fractionation:
- Cells are mixed with ice-cold extraction buffer, the ice slows down degeneration and the buffer helps avoid pH problems.
- The mix is gently blended to separate the fragments.
- The mix is centrifugated. What is located at the bottom (pellet) contains the denser particles (organelles). The remaining liquid (supernatant) contains only the buffer and is discarded.
The pellet is separated and the supernatant is discarded. The pellet is mixed with another liquid and centrifugated again, this process is repeated several times until all the organelles are separated.
Separation of the nucleus and cytoplasm
Separating the DNA from the rest of the cytoplasm safeguards it from the enzymes in the rest of the cell, this is why in eukaryotic cells the DNA is inside the nucleus. This allows from transcription to happen in the nucleus separated from the translation that happens in the cytoplasm.
However in prokaryotes the DNA is in the cytoplasm allowing translation to start before transcription is even finished.
- Transcription is the conversion of portions of DNA into RNA
- Translation is the translation of mRNA into polypeptides in the ribosomes.
In eukaryotic cells translation cannot start until the mRNA has exited the nucleus through the pores. This lets DNA be modified before being translated, this being called posttranscriptional modifications.
- Posttranscriptional modifications help stabilize the DNA and help remove non-coding sequences. This creates a more efficient regulation of the process.
Advantages of compartmentalisation
Eukaryotic cells are divided by compartmentalisation by membrane-bound organelles in the cytoplasm. As the membrane is semipermeable it isolates what is inside the compartments from the rest of the rest of the cell.
Advantages of this are:
- As the volume is smaller inside the organelles compared to the cytoplasm enzymes and substrates can be much more concentrated and have a higher change of collision.
- Substances that can damage the cell are isolated from the rest of the cytoplasm
- Lysosomes and phagocytic vacuoles contain hydrolytic enzymes that digest pathogens and other particles.
- Enzymes that digest proteins, carbohydrates, and lipids that also exist in the cell need to be isolated because if they are not they can kill the cell.
- The pH condition inside the organelle can be ideal for specific processes that differ from the rest of the cytoplasm
- The hydrolytic enzymes in the lysosome work at pH 5 while the rest of the cytoplasm is at pH 7.
- Organelles can freely move around the cell.
Nuclear pores and double membrane in nucleus
The nucleus is the organelle that holds DNA in form of chromatin, protecting and helping to regulate gene expression. The semifluid matrix found inside the nucleus is called the nucleoplasm. The nucleus also contains one or more nucleoli that synthesize ribosomes.
The nucleus is surrounded by a double membrane called the nuclear envelope that has pores called nucleopores.
- The outer membrane is connected to the rER. This way the proteins synthesized in the rER can easily access the nucleus without being transported inside a vesicle.
- The pores are made of integral proteins which allow only specific molecules to pass.
- The presence of pores allows big particles to move in and out the nucleus, this creates a selective passage way.
- There are 50 to 100 proteins compositing the pores
- Histones must enter the nucleus to form chromosomes and chromatin.
- Proteins that regulate gene expression must enter
- mRNA and tRNA must exist the nucleus to work in the cytoplasm
- Ribosomes are synthesized in the nucleolus and are big structures that must exit.
Ribosomes
Ribosomes is where protein synthesis takes place. In ribosomes there are 2 subunits:
- A small subunit that has a binding site for mRNA.
- A large subunit that contains the binding site for tRNA (EPA) and where the condensation reaction takes place
- E site (exit site): Occupied by empty tRNA that leaves the ribosome
- P site (peptidyl site): Holds the tRNA carrying the growing polypeptide chain
- A site (aminoacyl site): Occupied by incoming tRNA with the next amino acid
There are 2 different types of ribosomes:
- Free ribosomes, these are found in the cytoplasm and are used in the mitochondria, chloroplasts, and cytoplasm
- Bounded ribosomes are proteins released in the lumen of the rER from there they are packaged and send to an organelle. If the protein has to be secreted it will be sent to the Golgi body where after modification will be sent to the membrane to be released.
Nuclear envelopes in mitosis and meiosis
The nuclear membrane also plays an important role during mitosis and meiosis. During these processes the double membrane breaks down into vesicles, this is only possible because of the double membrane structure
During telophase I and II these vesicles can rejoin again to make the new nuclear envelope.
- The vesicles bind to the chromosomes
- The vesicles fuse around each single chromosome forming a double membrane
- Then the vesicles fuse with each other forming the complete nuclear envelope.
