B2.2 : Organelles and Compartmentalisation

organelle

a discrete structure in a cell that is adapted to perform a specific function

advantages of compartmentalisation list

• localised conditions

• concentration of enzymes/ substrates are adapted

• isolation of substances can prevent damage

• adaptation of numbers and locations od organelles

• surface area

advantages of compartmentalisation localised conditions

Conditions such as pH can be maintained at an ideal level for a particular process, which may be different from the levels needed for other processes inside the cell

advantages of compartmentalisation concentration of enzymes

Enzymes and substrates for a particular process can be much more concentrated than if they were spread throughout the cytoplasm. This increases the chance for collision between active site and substrate and thus increases the rate of reaction

advantages of compartmentalisation isolation of substances

Substances that could cause damage to the cell can be kept inside the membrane of an organelle e.g. digestive enzymes of a lysosome could digest and kill the cell, if they were not safely stored inside a lysosome membrane protecting remaining structures from degradation.

adaptation regarding location and numbers of organelles

Organelles with their content can be moved around within a cell and specific organelles can be increased in number depending on the cell’s needs without the whole cell having to divide.

surface area

There is a larger area of membranes available for processes that happen within or across membranes. Large areas of membrane can become dense with proteins for a specific process

structure inside cells that aren’t organelles

• the cytoskeleton: network of proteins which allow for the cell to have a distinct shape and be able to move. Not organelles bc not enclosed by membrane nor involved in metabolic processes

• cell wall protects against mechanical stresses but is outside cell and not involved in metabolic processes

• the cytoplasm is necesary to the function of the cell, but it isn’t a discrete part of it

advantages of compartmentalisation lysosomes

lysosomes are membrane bound organelles which contain enzymes which digest and recycle waste materials inside of the cell. the optimum ph for these enzymes is very low, so the lysosomes are filled with hydrochloric acid.

These enzymes and this PH could potentially be very harmful to other organelles, so compartmentalisation here is key

in such a small area the enzymes are more concentrated, so the process is more efficient

advantages of compartmentalisation macrophages

Macrophages use phagocytic vacuoles full of digestive enzymes to break down pathogens and thus create an immune response.

These enzymes and this PH could potentially be very harmful to other organelles, so compartmentalisation here is key

advantages of compartmentalisation of nucleus

• post transcriptional modification can occur as there is a separation between the transcription and translation step

• this isnt possible in prokaryotic organisms as they lack a nucleus and thus these two processes aren’t separated

• physical protection of DNA as it is very sensitive to environmental changes

“to prevent protein synthesis before post transcriptional modification of mRNA”

how are nuclei reformed during mitosis

• membrane bound vesicles which contain protein and lipids specific for the nuclear membrane from the ER bind to individual chromosomes

• eventually, the vesicles fuse together together to form the double membrane of the nucleus with the chromosomes inside

potential disadvantages of cell compartementalisation

slowed down response to environment

structure of mitochondrion + parts

Cristae

folds in inner membrane of mitochondria, outside of inner membrane. increases surface area, allowing for more space for the enzymes needed for ATP production

inner membrane

folded to increase surface area

outer membrane of mitochondria features

• is semi permeable

• has many porins which allow ion and other larger molecules to enter

mitochondria’s matrix

fold inside inner membrane. contains many enzymes and molecules in high concentration needed in Krebs cycle

intermembrane space inside of mitochondria

rather small, so has a high concentration of molecules. a concentration gradient is formed, and that is used to generate ATP

structure of chloroplasts

outer membrane of chloroplasts

• permeable to small molecules

inner membrane of chloroplasts

• less permeable

• most transport requires transport proteins

stoma what is it + purpose

• inside of the chloroplast but not in thyakoid

• has all enzymes required for the Calvin cycle, is where it takes place

• efficiency is maximised because all required molecules are in stoma and thus have a high concentration

thyakoids how is it’s structure good for it’s purpose ?

• this compartmentalisation is good as certain molecules inside of it might be harmfull to other parts of the cell

• it’s disk like structure maximises surface area and increases the amount of chlorophyll inside on the membrane

where light dependent reactions of photosynthesis take place

stack of thylakoids

granums: maximises amount of light absorbed

what holds granums together ?

Lamellae

what side of the RER are bound ribosomes bound to ?

to the cytosolic side

for what purpose do bound ribosomes produce proteins ?

for extracellular uses

what is the specisitivity of mRNA codes which go to the ER ?

they have an ER signal sequence. This tells the ribosome they are bound to to go to the Rough endoplamic

what is the inside of the endoplasmic reticulum called ?

the ER lumen

what occurs inside the of RER

it has enzymes to modify the cells

eg. adding carbohydrates (glycoproteins , adding phosphate groups, assisting in folding…

after they are transported by vesicles

how are proteins transportes by vesicles to the golgi apparatus

• a piece of the RER membrane breaks of with the protein inside

• that piece becomes the vesicle

• instead of leaving the cell as would happen in exocytosis, the vesicle travels to the golgi apparatus

parts of the golgi apparatus

• cis compartments

• median compartments

• trans compartments

cis compartment of the RER

first step

• vesicle fuses with the cis membrane and releases proteins inside golgi apparatus

what proteins are sent to the median part of the golgi apparatus

proteins destined for use inside the cell( lysosomes, plasma membranes or vesicles )

what proteins are sent to the trans part of the golgi apparatus

proteins intended for extracellular use

examples of vesicules

• Transport vesicles

• Secretory vesicles

• Lysosomes

• Peroxisomes

transport vesicles

these vesicles transport materials from one part of the cell to another. An example is the transport of proteins from the rough endoplasmic reticulum to the Golgi apparatus.

secretory vesicles

these vesicles store and transport molecules to be secreted outside the cell, such as hormones, neurotransmitters and digestive enzymes.

lysosomes

these vesicles contain hydrolytic enzymes that can break down macromolecules such as proteins, carbohydrates and lipids. They play a key role in the degradation of cellular waste products and in removing damaged or aged organelles.

peroxisomes

these vesicles are similar to lysosomes but have a different set of enzymes that are involved in the detoxification of harmful compounds and lipid metabolism.

clathrin role in vesicle formation

forms a cage like structure around the vesicle, polymerises, and acts as a scaffold, helping the vesicle form appropriatly. afterwards, it breaks through hydrolisis

clathrin coated vesicles appear in what procceses ?

• endocytosis

• phagocytosis

• the transport of cargo from the Golgi apparatus to the plasma membrane

• the formation of lysosomes.