🪺Eukaryotic cell structure 🪺

• Eukaryotic cells make up multicellular organisms

  They have much more complicated internal structures than prokaryotic cells

  They have a membrane bound nucleus and a cytoplasm which contains many membrane bound cellular components
• The ultrastructure of a cell is the features that can only be seen with an electron microscope
• Different sets of reactions take place in different regions of the ultrastructure of the cell
• Reactions take place in the cytoplasm which is separated from the external environment by a cell-surface membrane
• Organelles are membrane bound compartments that the cytoplasm is divided into

  They provide distinct environments that maintain the specific conditions for the different cellular reactions
• Membranes in the cell are selectively permeable and control the movement of substances into and out of the cell and organelles

  They are effective barriers but are fragile
• NUCLEUS

  • contains DNA molecules- DNA directs the synthesis of all proteins required by the cell so it controls metabolic activities because the essential enzymes involved are proteins
  • DNA is contained within a double membrane called the nuclear envelope to protect it from damage in the cytoplasm
  • the nuclear envelope contains nuclear pores that allow molecules to move into and out of the nucleus
• DNA associates with proteins called histones to form a complex called chromatin

  Chromatin coils and condenses to form chromosomes that only become visible when the cell is preparing to divide
• NUCLEOLOUS

  • the nucleolus is an area in the nucleus responsible for producing ribosomes
  • it is composed of proteins and RNA
  • the RNA is used to produce rRNA which is then combined with proteins to form the ribosomes necessary fro protein synthesis
• MITOCHONDRIA

  • Mitochondria are the site of the final stages of cellular respiration
  • mitochondria have a double membrane: the inner membrane is highly folded, forming structures called cristae and the fluid interior is called the matrix
  • the membrane that forms the cristae contains the enzymes used in aerobic respiration
  • mitochondria also have a small amount of DNA - mitochondrial(mt)DNA
  • they can produce their own enzymes and reproduce themselves
• Cellular respiration is where the energy stored in the bonds of organic molecules is made available for the cell to use by the production of the molecule ATP
• VESICLES AND LYOSOMES

  • vesicles are membranous sacs that have storage and transport roles - they consist simply of a single membrane with fluid inside
  • vesicles are used to transport materials inside the cell
  • lyosomes are specialised vesicles that contain hydrolytic enzymes
  • lyosomes are responsible for breaking down waste material in cells
  • lyosomes play an important role in the immune system as the break down pathogens ingested by phagocytic cells
  • lyosomes also play an important role in programmed cell death/apoptosis
• THE CYTOSKELETON

  • it is present throughout the cytoplasm of all eukaryotic cells
  • its a network of fibres necessary for the shape and stability of a cell
  • it holds organelles in place
  • it controls cell movement
  • it controls the movement of organelles within the cell
• THE THREE COMPONENTS OF THE CYTOSKELETON:

  • Microfilaments are contractile fibres formed from the protein actin. They are responsible for cell movement and cell contraction during cytokinesis
  • Microtubules are made of globular tubulin proteins that polymerise to form tubes that form a scaffold-like structure that determines the shape of the cell. They also act as tracks for the movement of organelles, including vesicles
  • Intermediate fibres give mechanical strength to cells and help maintain their integrity
• Spindle fibres are composed of microtubules
• CENTRIOLES

  • centrioles are part of the cytoskeleton
  • they are present in most eukaryotic cells appart from flowering plants and fungi
  • they are composed of microtubules
  • two associated centrioles form the centrosome which is involved in the assembly and organisation of the spindle fibres during cell division
  • in organisms with flagella and cilia, centrioles are thought to play a role in the positioning of these structures
• Flagella and cilia are extensions that protrude from some cell types
• Flagella are longer than cilia
• Cilia are usually present in much greater numbers than flagella
• Flagella are mostly used to enable mobility of the cell
• In some cells, flagella are used as a sensory organelle to detect chemical changes in the cell's environment
• Cilia can be mobile or stationary
• Stationary cilia have an important function in sensory organs like the nose
• Mobile cilia beat in a rhythmic manner, creating a current
• Mobile cilia cause fluids/objects adjacent to the cell to move, for example, in the trachea to move mucus away from lungs
• Ribosomes, the endoplasmic reticulum, and the Golgi apparatus are all closely linked and coordinate the production of proteins. The cytoskeleton plays a key role in coordinating protein synthesis
• ENDOPLASMIC RETICULUM

  The endoplasmic reticulum is a network of membranes enclosing flattened sacs called cisternae

  It is connected to the outer membrane of the nucleus

  There are two types of endoplasmic reticulum:

  • SMOOTH endoplasmic reticulum - responsible for lipid and carbohydrate synthesis, and storage
  • ROUGH endoplasmic reticulum - has ribosomes bound to the surface and is responsible for the synthesis and transport of proteins
• RIBOSOMES

  • they can be free floating in the cytoplasm or on the rough endoplasmic reticulum
  • they are not surrounded by a membrane
  • They are constructed of RNA molecules made in the nucleolus
  • they are the site of protein synthesis
  • mitochondria and chloroplasts also contain ribosomes, as do prokaryotic cells
• GOLGI APPARATUS

  • similar to the smooth endoplasmic reticulum in structure
  • it has a compact structure formed of cisternae and does not contain ribosomes
  • it has a role in modifying proteins and 'packaging' them into vesicles: these can be secretory vesicles if the proteins are destined to leave the cell, or lyosomes which stay in the cell
• Each cilium has two central microtubules surrounded by nine pairs of microtubules arranged like a wheel. This is called the 9+2 arrangement. Pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion