Cellular Functions

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sarah_hpj
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152468
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Cellular Functions
Updated:
2012-05-06 07:57:53
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JC biology cellular functions cell structure function
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Cell structure and function
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  1. Eukaryotic cell vs. Prokaryotic cell
  2. Cell surface membrane
    • 7-10nm in width
    • Amphipathic - allows membrane to reseal itself when bilayer is disrupted
    • Dynamic - components (proteins, cholesterol) constantly in motion ("fluid mosaic model")

    • Functions:
    • 1) Serves as a boundary to separate cell contents from external environment
    • 2) Controls exchange of substances across membrane
    • 3) Site for chemical reactions
    • 4) Acts as receptor sites for recognising external stimuli
  3. Nucleus
    • Found in all eukaryotic cells except in mature red blood cells in mammals and sieve tubes in plants
    • Spherical/ ovoid
    • 10 - 20µm in diameter
    • Double membrane
    • Contains genetic material (DNA), controls all cellular activities including cell division and protein synthesis
  4. Nucleolus
    Large, densely-stained

    • Functions:
    • 1) Transcription of rRNA
    • 2) Partial assembly of ribosomal proteins with rRNA
  5. Nuclear envelope
    • Double membrane: outer membrane - continuous with RER
    • Perforated by nuclear pores (each formed by a protein pore complex) to regulate entry/exit of most proteins and RNAs.
  6. Nucleoplasm
    • Semi-fluid
    • Contains chromatin, nucleolus, proteins (enzymes), nucleotides and ions
  7. Chromatin/ chromosomes
    A chromatin thread consists of a DNA molecule coiled around histones protein.

    • In resting/ non-dividing cells - chromatin not visible
    • During nuclear division, chromatin threads condense to become chromosomes - visible under light microscope

    • Types of chromatin:
    • 1) Euchromatin - lightly-stained; genes are transcriptionally active
    • 2) Heterochromatin - densely-stained; genes are transcriptionally inactive
  8. Rough Endoplasmic Reticulum (RER)
    • Flattened, interconnected cisternae - continuous with outer membrane of nuclear envelope
    • Bears ribosomes on surface for protein synthesis

    • Functions:
    • 1) Ribosomes on RER are sites for protein synthesis
    • - Proteins enter RER lumen and undergo modification (e.g. glycosylation)
    • - Forms ER vesicles which transport proteins to Golgi body for further modifications

    • 2) RER membrane contributes to the replenishment and increase in surface area of the cell surface membrane.
    • 3) Provides large surface area of cisternae which serves as a cytoplasmic framework where enzymes of metabolic pathways and other biochemical reactions are bound.
  9. Smooth Endoplasmic Reticulum (SER)
    • Lacks ribosomes
    • Consists of tubular cisternae
    • Has different set of membrane-bound proteins

    • Functions:
    • 1) Synthesis and transport of lipids (e.g. oils, phospholipids, steroids)
    • 2) Detoxification
    • 3) Storage and release of Ca2+ in muscles
    • 4) Contributes phospholipids to replenish cell surface membrane
  10. Golgi Apparatus
    • Single membrane-bound
    • Consists of flattened membranous stacks of cisternae
    • - seen as a single large stack in animal cells; separate stacks in plant cells

    At cis face (outer convex face), new cisternae are constantly formed by fusion of vesicles from rough and smooth ER.

    At trans face (inner concave face), Golgi vesicles bud off from the Golgi apparatus to form vesicles such as lysosomes, and secretory vesicles which contain products destined for secretion out of the cell.

    • Functions:
    • 1) Synthesis of certain macromolecules (e.g. pectin found in cell wall of plant cells)
    • 2) Formation of cell plate in plant cells at final stage of cell division - Golgi-derived vesicles containing pectin and hemicelluloses move to middle of cell where they fuse to form the cell plate.
    • 3) Formation of lysosomes in some animal cells involved in phagocytosis
    • 4) Receives products (proteins and lipids) from ER and chemically modifies them
    • - At trans face, modified proteins are sorted, packaged and transported to various cellular destinations
    • 5) Replenishment of cell surface membrane
  11. Lysosomes
    • 0.2 - 0.5µm in diameter
    • Bound by a single membrane
    • Contains acid hydrolases - contents are acidic; optimal pH of about 5
    • Serves as a storage vesicle to keep enzymes apart from rest of cell, preventing it from destroying the cell (compartmentalisation)

    • Types of lysosomes:
    • 1) Primary lysosomes - bud off from ER or Golgi apparatus
    • 2) Secondary lysosomes - fusion of primary lysosomes with phagocytic/food/pinocytic vacuoles

    • Functions:
    • 1) Digestion of materials
    • - Food material taken in via endocytosis (phagocytosis/pinocytosis)
    • - Primary lysosomes fuse with phagocytotic/food/pinocytotic vacuoles for digestive enzymes to mix with contents and carry out digestion
    • - Useful products released into cytosol; residues discharged by exocytosis

    • 2) Autolysis
    • - Self-destruction of cell by release of lysosomal contents within cell

    • 3) Autophagy
    • - Primary lysosome fuses with worn-out organelles to form secondary lysosome
    • - Digested products returned to cytosol for re-use

    • 4) Enzymes from primary lysosomes involved in remodeling matrix of cell
    • - Tay-Sachs disease - caused by mutated lipid-digesting enzyme in lysosomes which results in impairment of brain and nerve cells due to accumulation of excess lipids.

    - Pompe's disease - caused by absence of a carbohydrase that breaks down lysosomal glycogen, resulting in liver damage.
  12. Mitochondria
    • Double membrane-bound
    • - Outer membrane is smooth and permeable to small molecules
    • - Inner membrane is selectively permeable and extensively folded into shelf-like cristae to increase surface area for enzyme attachment needed for the electron transport chain and oxidative phosphorylation during cellular respiration.

    • - Outer and inner membranes enclose the intermembrane space, which holds high concentration of H+ for the establishment of a proton gradient
    • Moves by cytoplasmic streaming to areas where more ATP is required
    • 1.5 - 10µm in length, 0.25 - 1.00µm in width
    • Mitochondrial matrix - contains proteins, lipids, circular mitochondrial DNA, 70S ribosomes, RNA and enzymes that control the Kreb's cycle and fatty acid oxidation.
    • Mitochondrial DNA - encodes enzymes and proteins responsible for cellular respiration

    • Functions:
    • 1) Site of synthesis of ATP
    • 2) Lipid metabolism - fatty acids broken down to acetyl CoA in matrix and on inner membrane
    • 3) Lipid synthesis
    • 4) Apoptosis - release of certain mitochondrial proteins into cytosol to initiate programmed cell death
  13. Chloroplast
    • Found in photosynthetic organisms
    • Double membrane-bound
    • 3 membranes: outer membrane, inner membrane, thylakoid membrane (flattened sacs)
    • - "Stacks" - Grana
    • - Fluid surrounding thylakoid membrane - Stroma - contains circular chloroplast DNA, 70S ribosomes, enzymes, starch grains

    • Function:
    • 1) Site of photosynthesis
    • Light-dependent reactions - thylakoid membrane
    • Light-independent reactions - stroma
  14. Ribosomes
    • Complex of rRNA and proteins, not bound by membranes
    • Made up of a large subunit and a small subunit
    • Free ribosomes
    • - Float freely in cytosol
    • - Synthesise cytosolic proteins

    • Bound ribosomes
    • - Attached to RER
    • - Synthesise membrane-bound proteins, proteins destined for secretion or for further modification by other organelles such as the Golgi apparatus

    • Types of ribosomes:
    • 1) 70S ribosomes - found in prokaryotes, mitochondria, chloroplasts
    • 2) 80S ribosomes - found in eukaryotes
  15. Centrioles
    • 0.2µm in diameter, 0.3 - 0.5nm in length
    • 9 triplets of microtubules fuse together to give a rod-like structure
    • Found in pairs - positioned at 90° to each other
    • Found in centrosome (near nucleus)

    • Functions:
    • 1) Organisation of spindle fibres during cell division
    • 2) Anchorage for cilia and flagella

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