Eukaryotic cell structure

• Eukaryotic cells have a distinct nucleus and possess membrane-bound organelles
• the nucleus contains the organism’s hereditary material and controls the cell’s activities
• Nuclear envelope: A double membrane that surrounds the nucleus (controls the entry and exit of materials from the nucleus) (contains the reactions taking place within it) (continues from the endoplasmic reticulum)
• Nuclear pores: allows the passage of large molecules out of the nucleus
• Nucleoplasm: The gel-like substance that fills the nucleus.
• Chromosomes: long strands of linear DNA that carry genes.
• Nucleolus: Site of RNA synthesis and ribosome assembly (in nucleoplasm)
• Nucleus functions:

  • acts as the control centre of the cell through the production of mRNA, tRNA (and hence protein)
  • contain the genetic material of the cell (to form DNA)
  • manufacture ribosomes and ribosomal RNA
• Mitochondrion structure:

  • double membrane:controls the entry/exit of material (inner membrane is folded into cristae)
  • cristae: provide large sa for the attachment of enzymes and proteins involved in respiration
  • the matrix: contains protein, lipids, ribosomes and DNA (allowing control of the production of some of their own proteins, enzymes found here)
• Mitochondria: site of aerobic respiration (ATP production), high number in cells with high metabolic activity (muscle and epithelial cells)
• Chloroplast: site of photosynthesis
• Chloroplast functions:

  • the chloroplast envelope
  • the grana
  • the stroma
• Chloroplast

  • chloroplast envelope: a double plasma membrane that surrounds the organelle, highly selective in what it allows in/out
• Chloroplast

  • the grana: stacks of up to 100 disc-like structures (thylakoids), chlorophyll in thylakoids, some thylakoids have tubular extensions allowing them to connect with thylakoids in adjacent grana, first stage of photosynthesis
• Chloroplasts

  • stroma: a fluid-filled matrix where the second stage of photosynthesis takes place, contains starch grains
• Chloroplast

  • granal membrane allows for a large sa (for attachment of chlorophyll, electron carriers and enzymes)
• Chloroplast

  • fluid of the stroma possesses all the enzymes needed to make sugars (2nd stage)
• Chloroplast

  • contain both DNA and ribosomes so they can manufacture some of the proteins needed for photosynthesis
• Why do chloroplasts contain DNA and ribosomes?

  so chloroplasts can manufacture proteins needed for photosynthesis
• Endoplasmic reticulum: spreads throughout the cytoplasm, continuous with the outer nuclear membrane
• RER: has ribosomes on the outer surface of the membrane
• RER functions:

  • provide large sa for protein and glycoprotein synthesis
  • provide pathway for the transport of proteins throughout the cell
• SER: more tubular in appearance
• SER functions:

  • synthesise, store and transport lipids
  • synthesise, store and transport carbohydrates
• SER:

  • cells that produce / store large quantities pf carbohydrates, lipids and proteins have lots of ER
  • Eg, liver and epithelial cells
• Golgi apparatus:

  • a series of flattened membrane sacs (cisternae) with small hollow structures called vesicles
  • vesicles move to the cell surface and fuse to the membrane, releasing its contents
• Golgi apparatus functions:

  • add carbs to proteins (glycoproteins)
  • produce secretory enzymes
  • secrete carbohydrates
  • transport, modify and store lipids
  • form lysosomes
• Gogli apparatus:

  • especially well developed in secretory cells (epithelial)
• Lysosomes:

  • formed when the vesicles produced by the golgi apparatus contain enzymes such as proteases and lipases
• Lysosomes functions

  • hydrolyse materials ingested by phagocytosis
  • exocytosis (release enzymes) in order to destroy material around the cell
  • digest worn out organelles (re-use chemicals)
  • autolysis (break down dead cells)
• Lysosomes:

  abundant in secretory cells (epithelial and phagocytic)
• Ribosomes:

  small cytoplasmic granules

  found in: cytoplasm or associated with RER
• Ribosomes:

  80S: eukaryotic cells, 25nm

  70S: prokaryotic, mitochondria, chloroplasts, smaller
• Cell wall:

  • all plant cells
  • microfibrils of cellulose (polysaccharide) embedded in a matrix
• Cell wall:

  • consist of many polysaccharides
  • thin layer (middle lamella) which separates adjacent cell walls and cements adjacent cells together
• Cell wall functions:

  • provide mechanical strength (against osmotic pressure)
  • mechanical strength to the plant as a whole
  • allow water to pass along it (contribute to flow of water)
• Vacuoles:

  • a fluid filled sac bound by a single membrane
  • membrane = tonoplast
• Vacuole functions:

  • support herbaceous plants (turgid)
  • temporary food store (sugars and amino acids)
  • pigments attract pollinating insects