cell structure

• role of nucleus - contains cell’s hereditary material + controls cell’s activities . usually between 10 and 20 micrometers in diameter
• nuclear membrane:

  • double membrane surrounding nucleus.
  • outer membrane is continuous with endoplasmic reticulum and often has ribosomes on its surface
  • controls the entry and exit of materials in and out of the nucleus
  • contains any reactions that take place within it
• nuclear pores:

  • allow the passage of large molecules, such as mRNA
  • typically around 3000 pores in each nucleus, each 40-100nm in diameter
• nucleoplasm:

  • granular, jelly-like material that makes up the bulk of the nucleus
• chromosomes:

  • consist of protein-bound, linear DNA, associated to histones
• nucleolus:

  • manufactures ribosomal DNA
  • assembles ribosomes
• mitochondria:

  • site of aerobic respiration
  • responsible for the production of ATP
• mitochondria double membrane:

  • controls entry + exit of material
  • inner membrane folds to form cristae
• cristae:

  • provide large surface area for the attachment of enzymes and other proteins involved in respiration
• matrix:

  • contains proteins, lipids, ribosomes, and DNA that allows the mitochondria to control the production of some of their own proteins such as enzymes
• chloroplasts:

  • organelles that carry out photosynthesis
  • contain chlorophyll made of magnesium
• chloroplast envelope:

  • double plasma membrane
  • highly selective in what enters and leaves the chloroplast
• grana:

  • stacks of up to 100 disk like structures called thylakoids (which contain chlorophyll)
  • first stage of photosynthesis takes place here
• stroma:

  • fluid-filled matrix
  • second stage of photosynthesis
• granal membrane:

  • provides large surface area for the attachment of chlorophyll, electron carriers and enzymes
• fluid of stroma:

  • possesses enzymes needed to make sugars
• chloroplasts contain DNA + ribosomes so that they can quickly manufacture proteins needed for photosynthesis
• Rough Endoplasmic Reticulum:

  • ribosomes present on the outer surface of the membranes
  • tubules + flattened sacs called cisternae
• RER:

  provides large surface area for the synthesis of proteins and glycoproteins

  provides a pathway for the transport of materials, especially proteins throughour the cell
• smooth endoplasmic reticulum :

  • lacks ribosomes, often more tubular in appearance
• SER:

  • synthesise, store and transport lipids + carbohydrates
• Liver and secretory cells (eg epithelial cells) have a lot of endoplasmic reticulum
• golgi apparatus:

  • stack of membranes that make up cisternae
  • vesicles - round hollow structures for transport
• Golgi apparatus:

  • modifies proteins by adding non-protein components, such as carbohydrates
  • modified proteins + lipids are transported in golgi vesicles
  • form lysosomes
  • produce secretory enzymes
  • transport, modify + store lipids
• lysosomes:

  • hydrolyse material ingested by phagocytic cells
  • release enzymes to the outside of the cell to destroy materia around the cell
  • digest worn out organelles so that the useful chemicals they are made of can be reused
  • completely break down cells after they have died
• ribosomes:

  • may be found in cytoplasm or associated to rough endoplasmic reticulum
  • 80S - found in eukaryotic cells, around 25nm
  • 70S - found in prokaryotic cells, mitochondria and chloroplasts, slightly smaller
  • two subunits, large and small, each containing ribosomal RNA and protein
  • site of protein synthesis
• cell wall properties :

  • consist of microfibrils of cellulose, embedded in a matrix
  • have considerable strength
  • middle lamella , marks boundaries between adjacent cells and cements adjacent cells together
  • in algae, cellulose or glycoproteins ore both
  • in fungi, mixture of chitin and glycan and glycoproteins
• cell wall function:

  • provide mechanical strength to prevent cell from burst under osmotic pressure
  • mechanical strength to plant as a whole |
  • allow water to pass along it, contributes to movement though the plant
• vacuoles :

  • single membrane called tonoplast
  • contains solution of mineral salts, sugars, amino acids, wastes and sometimes pigments
  • support herbaceous plants by making cell turgid
  • sugars and amino acids act as temporary food store
  • pigments may colour petals to attract pollinating insects
• tissue - a collection of similar cells that perform a specific function
• all bacteria have a cell wall, made of murein
• some bacteria secrete a capsule of mucilaginous slime around the cell wall
• bacteria have 70s ribosomes
• bacteria have circular DNA and plasmids
• bacteria have no membrane-bound organelles
• viruses:

  • acellular, non-living
  • contain nucleus acids such as DNA or RNA but need a host to replicate
  • nucleic acid enclosed within a protein coat called capsid
  • have attachment proteins
  • HIV surrounded by lipid envelope
• Binary Fission in bacteria:

  • circular DNA replicates + both copies attach to cell membrane
  • plasmids replicate
  • cell membrane grows between two DNA molecules and begins to pinch inwards, separating the cytoplasm
  • new cell wall forms between two molecules for DNA, dividing into two identical daughter cells
• replication of viruses:

  • attach to host cell with attachment protein
  • inject nucleic acid into host cell
  • host cell’s metabolic processed start producing viral components, nucleic acid, enzymes and structural proteins
• treatment of cancer often:

  • prevents DNA from replicating
  • inhibits metaphase by interfering with spindle formation
• drugs against cancer are more effective against rapidly dividing cells, which is why normal body cells are not affected bur hair producing cells are.