Cell organelles

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Cell organelles

Nucleus:

  • stores the cell's genetic material (DNA) and coordinates the cell's activities e.g. growth or protein synthesis
  • only present in eukaryotes
  • occupies around 10% of cell's volume
  • surrounded by a nuclear envelope
  • contains one or more nucleoli (synthesize proteins)

Rough endoplasmic reticulum:

  • surface is covered with ribosomes
  • involved mainly with the production and processing of proteins that will be secreted from the cell
  • ribosomes assemble amino acids into protein units, which are then transported into RER for further processing
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Cell organelles

Lysosomes:

  • serves as digestive compartments for cellular materials that have exceeded their lifetime or are no longer useful
  • recycle the cell's organic material in a process called autophagy
  • break down cellular waste products, fats, carbohydrates, proteins and other macromolecules into simple compounds
  • speherical organelles contained by a single layer membrane
  • membrane protects cell from digestive enzymes that would otherwise cause damage
  • found in all animal cells, especially white blood cells

Mitochondria:

  • rod-shaped organelles, convert oxygen and nutrients into adenosine triphosphate (ATP) for aerobic respiration
  • found in nearly all eukaryotes
  • generally oblong in shape, range in size between 1 and 10 micrometers in length
  • has its own circular DNA and produces it independently
  • two specialised membranes encircle each mitochondrion
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Cell organelles

Cell surface membrane/plasma membrane:

  • permeable to specific molecules, allowing nutrients and other essential elements to enter the cell and waste materials to leave the cell
  • composed of a double layer of lipids
  • within the phospholipid bilayer, diverse proteins are embedded, while other proteins adhere to the surface of the bilayer

Centrioles:

  • only found in animal cells
  • typically located near the nucleus in the centrosome
  • positioned so they are at right angles to each other
  • each cnetriole is made of nine bundles of microtubules
  • play a role in cell division. During interphase, the centrioles are duplicated and the original centrosome divides and pairs are split up so that one set of centrioles in each of the new microtubule-organising centres
  • new centres radiate microtubules in clusters called asters
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Cell organelles

Golgi apparatus:

  • comprised of 5-8 cup-shaped membrane-covered sacs called cisternae 
  • modifies proteins and lipids that have been built in the endplasmic reticulum and prepares them for export out of the cell. Usually located close to the nucelus 
  • each golgi stack has two distinct ends, or faces - the trans face is where they exit in vesicles
  • the cis face of the golgi stack is the end of the organelle where substances enter from the ER
  • also sorts the macromolecules produced by the cell and targets them for distribution to their proper location

Ribosomes:

  • found bound to the rough endoplasmic reticulum as well as freely scattered in the cytoplasm
  • eukaryotic ribosomes are produced and assembled in the nucleolus
  • composed of approximately 60% ribosomal RNA and 40% protein
  • a single ribosome in a eukaryotic cell can add 2 amino acids to a protein chain every second - in prokaryotes, ribosomes can add 20 to a polypeotide every second
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Cell organelles

Microfilaments:

  • range from 5-9 nanometers in diameter
  • help to generate the forces used in cellular contraction and basic cell movements
  • also allow dividing cells to pinch off into two cells and are involved in amoeboid movements of certain types of cell
  • solid rods made of actin - appears in globular form
  • each microfilament has a polar charge, affecting their growth
  • typically nucleated at the plasma membrane
  • considered part of the cell cortex, which regulates the shape and movement of the cell's surface

Smooth endoplasmic reticulum:

  • most proteins are transported from here in vesicles
  • involved with the production of cellular products such as hormones and lipids, and the detoxification of drugs and poisons
  • also stores calcium and is involved in calcium metabolism
  • in muscle cells, SER releases calcium to trigger muscle contraction
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Cell organelles

Cilia and flagella:

  • in multicellular organisms, cilia move a cell or group of cells to help transport fluid or materials past them. Flagella have a more smooth movement
  • also generate water currents to carry food and oxygen past the gills and clams and transport food through digestive systems of snails
  • flagella are primarily found on gametes, but create water currents necessary for respiration and circulation in sponges and coelenterates
  • in eukaryotes they contain dynein (a motor protein) and microtubules, composed of tubulin
  • the core of each of the structures is called the axoneme, and contains two central microtubules that are surrounded by an outer ring of nine doublet microtubules
  • dynein molecules are located around the circumference of the axoneme at regular intervals along its length where they bridge the gaps between adjacent microtubule doublets
  • axoneme complex is surrounded by a plasma membrane, which is attached to the cell at the basal body
  • cilia are usually shorter and occur in much larger numbers than flagella
  • in both, movement is generated by the activation of dynein and the resultant bending of the axoneme
  • adjacent cilia move almost simultaneously, wave-like patterns occur in cilia
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