Organelles

Organelles

Nucleus
1. Structure
  1. A large organelle surrounded by a nuclear envolope which contains many pores. The nucleus contains chromatin and often a nucleolus.
    1. Nuclear envolope: Double membrane surrounding nucleus, continuous with the ER. Controls entry and exit of materials in and out of the nucleus and contains the reactions taking place within it
    2. Nuclear pores: Allow the passage of large molecules, such as RNA, out of the nucleus.
    3. Nucleoplasm: Granular and jelly like, makes up the bulk of the nucleus
    4. Chromatin: DNA found in the nucleoplasm. This is the diffuse form that chromosomes take when the cell isnt dividing. This controls the cells activities.
2. Function
  1. Act as the control centre of the cell through the production of mRNA and hence protein synthesis
  2. Retain genetic information
  3. Manufacture ribosomal RNA and ribosomes
Mitochondrion
1. Structure
  1. Usually oval shaped. They have a double membrane - the inner one is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration.
    1. Double membrane: Outer one controls entry and exit of material. Inner membrane forms cristae.
    2. Cristae: Shelf like extensions of the inner membrane. Some extend across the whole width of the mitochondrion. Provide large surface area for the attachment of enzymes involved in respiration.
    3. Matrix: Makes up the remainder of the mitochondrion. Semi rigid, and contains protein, lipids and traces of DNA that allow the mitochondria to control the production of their own proteins. Enzymes involved in respiration are found here.
2. Function
  1. Sites of certain stages of respiration
    1. Krebs cycle
      1. A series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats and proteins into carbon dioxide and chemical energy in the form of adenosine triphosphate (ATP).
    2. Oxidative phosphorylation pathway
      1. Oxidative phosphorylation (or OXPHOS in short) is the metabolic pathway in which the mitochondria in cells use their structure, enzymes, and energy released by the oxidation of nutrients to reform ATP.
  2. Responsible for the production of ATP
    1. More mitochondria in cells which have a high level of metabolic activity.
Endoplasmic Reticulum
1. 2 Types of ER: Smooth and rough
  1. Smooth.
    1. Structure
      1. System of membranes enclosing fluid filled space.
    2. Function
      1. Synthesise, store and transport lipids
      2. Synthesise, store and transport carbohydrates
  2. Rough
    1. Structure
      1. Similar to smooth, but covered in ribosomes
    2. Function
      1. Provide a larger surface area for the synthesis of proteins and glycoproteins
      2. Provide a pathway for transport of materials
2. Description
  1. An elaborate, 3D system of sheet-like membranes, spreading through the cytoplasm of the cells. It is continuous with the nuclear membrane. The membranes enclose flattened sacs called cisternae. There are two types of ER.
Golgi Apperatus
1. Structure
  1. A group of fluid filled flattened sacs. The golgi apperatus occurs in almost all eukaryotic cells and has a similar structure to the SER except it is more compact.
    1. Cisternae: A stack of membranes make up flattened sacs called cisternae, with small rounded hollow structures called vesicles.
2. Function
  1. Proteins and lipids produced in the ER are passed through the Golgi apperatus in a strict sequence. The Golgi modifies proteins often adding non-protein components. It also 'labels' them, allowing them to be accurately sorted and sent to their correct destinations. Once sorted, the modified proteins and lipids are transported in vesicles which are regularly pinched off from the ends of the Golgi cisternae. The vesicles move to the cell surface where they fuse with the membrane and release their contents to the outside.
    1. Add carbohydrate to proteins to form glycoproteins
    2. Produce secretory enzymes, such as those secreted by the pancreas
    3. Transport, modify and store lipids
    4. Form lysosomes
Lysosomes
1. Structure
  1. A round organelle surrounded by a membrane, with no clear internal structure
2. Function
  1. Lysosomes are formed when the vesicles produced by the Golgi apperatus contain enzymes such as proteases and lipases. Lysosomes isolate potentially harmful enzymes from the rest of the cell before releasing them, either to the outside or into a phagocytic vesicle within the cell.
    1. Phagocytic vesicle: Cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells.
    2. Break down material ingested by phagocytic cells
    3. Release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell
      1. Exocytosis: The durable, energy-consuming process by which a cell directs the contents of secretory vesicles out of the cell membrane and into the extracellular space.
    4. Digest worn out organelles so to reuse useful chemicals
    5. Completely break down cells after they have died (autolysis).
      1. Autolysis: The destruction of a cell through the action of its own enzymes. It may also refer to the digestion of an enzyme by another molecule of the same enzyme.
Ribosomes
1. Structure
  1. A very small organelle that floats free in the cytoplasm of a cell or is attached to the rough ER. Small cytoplasmic granules.
    1. 2 subunits: One large and one small.
      1. Each contains ribosomal RNA and protein. They can occur in such vast numbers that they can account for up to 25% of the dry mass of a cell.
    2. 2 types depending on where they are found
      1. 80S Type: Found in eukaryotic cells, around 25nm in diameter
      2. 70S Type: Found in prokaryotic cells, slightly smaller
2. Function
  1. The site where proteins are made
Microvilli
1. Structure
  1. Small finger-like projections of the epithelial cell, folds in the plasma membreane that increase the surface area to allow more efficient absorption.
2. Function
  1. Found on cells incolced in absorption, such as epithelial cells in the small intestine.
    1. Increase surface area for diffusion
    2. Thin walled, decreasing diffusion pathway
    3. Move to help maintain a concentration gradient
    4. Well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain concentration gradient
Plasma Membrane
1. Structure
  1. The membrane is found on the surface of animal cells and just inside the cell wall of plant cells and prokaryotic cells. It's made mainly of lipids and protein.
    1. Phospholipid bilayer
      1. Hydrophilic head
        Interacts with water, not with fat
      2. Hydrophobic tail
        Interacts with fat, not with water
      3. One layer of phospholipids has heads pointing inwards (interacting with water in the cytoplasm)
      4. The other layer has its hydrophilic heads pointing outwards (interacting with water surrounding the cells)
      5. Hydrophobic tails of both layers point into the centre of the membrane
    2. Proteins
      1. Extriinsic proteins
        Occur either on the surface of the biilayer or partially embedded in it, but never extend completely across it. Give mechanical support and act as cell receptors for molecules such as hormones
      2. Intrinsic proteins
        Completely span the phospholipid bilayer. Some act as carriers to transport water-soluable material across the membrane whilst others are enzymes
2. Function
  1. Phospholipids
    1. Allow lipid-soluable substances to enter and leave the cell
    2. Prevent water soluable substances entering and leaving the cell
    3. Make the membrane flexible
  2. Proteins
    1. Provide structural support
    2. Act as carriers
    3. Allow active transport
    4. Form recognition sites by identifying cells
    5. Help cells adhere together
    6. Act as receptors (eg. for hormones)
      1. Hormone receptors: A molecule that can bind to a specific hormone. Receptors for peptide hormones tend to be found on the plasma membrane of cells, whereas receptors for lipid-soluble hormones are usually found within the cytoplasm. Upon hormone binding, the receptor can initiate multiple signaling pathways which ultimately lead to changes in the behaviour of the target cells.
Cytoplasm
1. Function
  1. For storage
  2. Gives shape
2. Structure
  1. Contains gelly like mix of chemicals
  2. Colloid
  3. Contains all organelles
Flagellum
1. Function
  1. For movement in prokaryotes
2. Structure
  1. Fibrous
Centrioles
1. Funtion
  1. Used during cell division
    1. Migrate to opposite poles, produce microtubules of the spinders that pull chromosomes apart
2. Structure
  1. Formed of centrosomes
    1. 2 hollow cylinders

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	Created by Beth Ritchie over 10 years ago