Cell structure

• Plasma membrane: formed from the phospholipid bi-layer, it controls the exchange of materials between the internal cell environment and external environment, 10nm
• Cell wall: found in plant cells, outside the plasma membrane and offer structural support, polysaccharide cellulose (plants) and peptidoglycan (bacterial), plasmodesmata connect the cytoplasms of neighbouring cells.
• Nucleus: contains chromatin (a complex of DNA and histone proteins which make up the genetic material of a cell), separated from the cytoplasm by a double membrane (the nuclear envelope), Nuclear pores (are important channels to allow mRNA and ribosome to leave the nucleolus)
• Mitochondria: site of aerobic respiration, surrounded by a double membrane and inner membrane folded to form cristae, this matrix contained enzymes needed for respiration which produce ATP, smaller circular pieces of DNA and ribosomes are also found in the matrix.
• Chloroplasts: found in plant cells, larger than mitochondria, double-membrane, thylakoids containing chlorophyll are stacked to form grana, grana are joined by lamellae, site of photosynthesis (light-dependent stage is in the thylakoids, light-independent takes place in the stroma), also contains circular DNA and ribosmes.
• Ribosomes: found in all cells, freely in the cytoplasm or on the RER, each ribosome is formed by ribosomal RNA and proteins, 80S ribosomes are found in eukaryotic cells, 70s are found in prokaryotes, mitochondria and chloroplasts, site of translation.
• RER: found in plants and animal cells, covered in ribosomes, formed from folds of membrane continuous with the nuclear envelope, processes proteins made by the ribosomes.
• SER: found in plants and animal cells, does not have ribosomes on the surface, involved in production, processing and storage of lipids, carbohydrates and steroids.
• Golgi apparatus: found in plants and animal cells, flattened sacs of membrane, modifies and packages proteins and lipids, vesicle are prepared for exocytosis
• Large permanent vacuoles: a sac in plant cells surrounded by the tonoplast, sequesters water and other substances.
• Lysosomes: specialist forms of vesicles containing hydrolytic enzymes, breaks down waste materials, used by cells in the immune system and in apoptosis.
• Centrioles: hollow fibres made of microtubules, two centrioles are right angles form a centrosome, which organises the spindle fibres during cell division.
• Microtubules: found in eukaryotic cells, makes up the cytoskeleton of the cell about 25nm in diameter, made of alpha and beta tubulin combined to form dimers, the dimers are then joined into protofilaments, 13 protofilaments in a cylinder make a microtubule. forms the cytoskeleton which provides support and movement of the cell.
• Microvilli: Small projections from the surface of the cell that increase the surface area of the cell.
• cilia: hair-like projections made from microtubules, allow the movement of substances over the cell surface.
• Flagella: found in specialised cells, similar to cilia, made of longer microtubules and contract to provide cell movement.
• Structures found only in animal cells: centrioles and microvilli, structures found only in plant cell: the cellulose cell wall, large permanent vacuoles, chloroplasts.
• 1. The DNA from the nucleus is copied into a molecule of mRNA via a process known as transcription

  • The mRNA strand leaves the nucleus through a nuclear pore and attaches to a ribosome on the rough endoplasmic reticulum
  • The ribosome 'reads' the genetic instructions contained within the mRNA and uses this code to synthesise a protein via a process known as translation
• 2)

  • This protein then passes into the lumen (the inside space) of the rough endoplasmic reticulum to be folded and processed
  • Cells that produce a large number of proteins, e.g. enzyme- or hormone-producing cells have an extensive rough endoplasmic reticulum
  • The processed proteins are then transported to the Golgi apparatus (also known as the Golgi body or Golgi complex) in vesicles which fuse with the Golgi apparatus, releasing the proteins
• 3)

  • The Golgi apparatus modifies the proteins, preparing them for secretion
  • Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (e.g. hydrolytic enzymes) or delivered to other organelles
  • The modified proteins then leave the Golgi apparatus in vesicles
  • Finally, these vesicles (containing the final proteins) fuse with the cell surface membrane, releasing the proteins by the process of exocytosis
• • The cytoskeleton is made up of two main types of protein fibres: microfilaments and microtubules
  • Microfilaments are solid strands that are mostly made of the protein actin. These fibres can cause some cell movement and the movement of some organelles within cells by moving against each other
  • Microtubules are tubular (hollow) strands that are mostly made of the protein tubulin. Organelles and other cell contents are moved along these fibres using ATP to drive this movement
  • Intermediate filaments (a third type of fibre) are also found within the cytoskeleton
• function of the cytoskeleton: strengthening and support, supports the organelles keeping them in position, intracellular movement (forms 'tracks' along the cell for the organelles to move along), cellular movement (cilia and flagella both are hair-like extensions which contain microtubules that are responsible for cell movement)
• Comparisons of prokaryotic and eukaryotic cells: animal and plant are E, bacterial are P, P are smaller(0.5-5 micrometres) than E (100 micrometres), P has no membrane-bound in its cytoplasm, P 70s ribosomes, E 80S ribosomes, P no nucleus (only a singular circular DNA molecule), P has a cell wall that contains murein, cell wall made of peptidoglycan, E cell wall plants cellulose/lignin, fungi-chitin . P also have plasmids (small loops of DNA separate from the main DNA), capsules (bacteria have an extra layer called a slime capsule), flagellum.