Eukaryotic Cell Structure

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Nucleus

  • Contains the cells hereditary material.
  • Controls the cells activities.
  • The nuclear envelope is double membrane.
    Outer membrane is often joined to the endoplasmic reticulum so has ribosomes.Controls the entry and exit of materials to the nucleus.
  • Nuclear poresallows the passage of large molecules, e.g. mRNA.Around 3000 pores per nucleus.
  • Nucleoplasm is a jelly-like material that makes up the majority of the nucleus.
  • Chromosomesare protein-bound, linear DNA.
  • The nucleolusis a spherical region of the nucleoplasm.
    Manufactures ribosomal RNA & assembles ribosomes.

  • Functions:
    • control centre through tRNA and mRNA production.
    • retain genetic material (DNA & chromosomes)
    • manufacture ribosomal RNA & ribosomes
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Mitochondrion

  • Site of aerobic stages of respiration (Krebs cycle  & oxidative phosphorylation pathway).
  • Produce ATP from respiration products.
  • Abundant in active cells.
  • Surrounded by a double membranethat controls movement of material in and out. Inner membrane is folded to form cristae.
  • Cristaeare extensions of the membrane. Provide a large surface area for the attachment of enzymes used in respiration.
  • The matrixcontains proteins, lipids, ribosomes, and DNA that allows the mitochondria to produce some of their own proteins.
    Many enzymes for respiration are found in the matrix.
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Choloroplasts

  • Organelles that carry out photosynthesis.
  • Usually disc-shaped but vary in shape and size.
  • The double membrane is highly selective in what it allows through.

  • The granaare stacks of up to 100 thylakoids.
    Contain chlorophyll. Where the first stage of photosynthesis occurs.

  • The stromais a fluid-filled matrix where sugar synthesis happens.
    Also contains other structures like starch grains.
  • Adapted for the function of harvesting sunlight/photosynthesis in these ways:
    • Granal membranes provide a large surface area for chlorophyll, electron carriers and enzymes to attach for the first stage of photosynthesis.
    • Stroma fluid has all of the enzymes needed for sugar synthesis.
    • Can easily manufacture some proteins needed as they contain DNA and ribosomes.
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Endoplasmic Reticulum

  • 3D system of sheet-like membranes.
  • Spread through the cytoplasm.
  • Continuous with the nuclear membrane.
  • Enclose a network of tubules and flattened sacs (cisternae).
  • Two types:
    • Rough endoplasmic reticulum (RER)
      • has ribosomes on the surface.
      • provides a large surface area for protien and glycoprotein synthesis.
      • provides a pathway for transport of materials through the cell.
    • Smooth endoplasmic reticulum (SER)
      • lacks ribosomes & is more tubular.
      • synthesises, stores and transports lipids.
      • synthesises, stores and transports carbohydrates.
  • Cells that manufacture & store carbohydrates, proteins and lipids will have a very extensive ER.
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Golgi Apparatus

  • Similar to SER in structure but is more compact.
  • Made from flattened sacs (cisternae) with vesicles.
  • The proteins/lipids produced by the ER are passed through the Golgi in a strict sequence and are modified, often by adding non-protein components to them.
  • It also allows them to be sorted and sent to the correct destinations.
  • After being sorted, the proteins and lipids are transported in the vesicles that are regularly pinched from the ends of the apparatus.
    These move to the cell surface where they release their contents to teh outside.
  • Functions are to:
    • Add carbohydrates to proteins (glycoproteins).
    • produce secretory enzymes.
    • secrete carohydrates.
    • transport, modify and store lipids.
    • form lysosomes.
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Lysosomes

  • Formed when vesicles contain proteases and lipases. (Up to 50)
  • Contain lysozymes that hydrolyse the cells walls of some bacteria.
  • Isolate the enzymes from the rest of the cell until their needed.
  • Functions are to:
    • hydrolyse the material ingested by phagocytic cells.
    • release enzymes to the outside of the cells (exocytosis) to destroy material around the cell.
    • digest worn-out organelles so useful chemicals can be re-used.
    • completely break down cells (autolysis).
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Ribosomes

  • Small, cytoplasmic granules found in all cells.
  • May occur freely in the cytoplasm or be associated with the RER.
  • Two types, depending on which cells they are found in:
    • 80Sare found in eukaryotic cells and around 25 nm in diameter.
    • 70Sare found in prokaryotic cells, mitochondria and chloroplasts, and are slightly smaller.
  • Ribosomes have two small subunits, one large and one small. which each contain ribosomal RNA and protein.
  • They account for up to about 25% of cells
  • The site of protein synthesis.
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Cell Wall

  • In plant cells it consists of microfibrils of cellulose embedded in a matrix.
    • Have considerable strength so add to the strength of the whole wall.
  • Consist of a number of polysaccharides.
  • Has a thin layer called the middle lamellawhich cements adjacent walls together.
  • Functions are:
    • to provide mechanical strength to prevent the cell from bursting by the pressure from osmosis.
    • to give mechanical strength to the plant.
    • to allow water to pass along it to contribute to the movement of water through the plants.
  • The cell walls of algae are made from cellulose and glycoproteins.
  • The cell walls of fungi contain chitin.
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Vacuoles

  • Fluid-filled sac surrounded by a single membrane, called the tonoplast.
  • Usually one large central one in plant cells.
  • Contain a solution of sugars, wastes, mineral salts, amino acids, and sometimes pigments such as anthocyanins. 
  • Serve a variety of functions including:
    • The sugars and amino acids may act as a temporary food store. 
    • They support herbaceous plants by making cells turgid.
    • The pigments can colour petals to attract pollunating insects.
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