cell structure

• nucleus -

  controls what enters and leaves the cell, contains genes, coordinates protein synthesis and cell division
• chromatin - a complex of DNA and protein found in eukaryotic cells - important factor in gene expression
• nucleolus - a small dense spherical structure in the nucleus of a cell, it is where ribosomes are made, acts as the brain of the nucleus
• golgi apparatus - a series of membrane-bound flattened sacs that are involved in the modification and packaging of proteins, and in the transport of lipids

  transports, modifies and packages proteins into vesicles for delivery to targeted destinations, receives proteins from endoplasmic reticulum and modifies them, tags vesicles with proteins and sugar molecules for proper delivery
• mitochondria - site of aerobic respiration, where most of the energy is released
• rough endoplasmic reticulum - synthesises and transports lipids, synthesising, folding and modifying proteins, especially those destined for other organelles or secretion, rapidly signaling the nucleus when problems with protein synthesis occur, producing proteins through ribosomes attached to it's surface
• plasma membrane - protect cell from its surroundings
• vacuole - storage, waste management, maintaining water balance and internal pressure, breaking down molecules
• plasmodesma (plant only): a narrow thread of cytoplasm that passes through the cell walls of adjacent plant cells and allows communication between them.
• cell wall - made of cellulose and provides support and structure to the cell, prevents pathogens and viruses from entering
• chloroplasts - p/s
• cytoplasm - where chemical reactions take place, where the cell's energy is stored, supports and suspends organelles and cellular molecules
• vesicle - helps transport materials that an organism needs to survive and suspends organelles and cellular molecules
• ribosome - converts genetic codes into protein molecules, translates encoded information from cell nucleus provided by mRNA
• cytoplasm - cytosol + all organelles (not including nucleus), jelly-like liquid containing the cytoskeleton
• cell-surface membrane = plasma membrane a boundary between the cell and its environment

  it is partially permeable and only allows certain molecules to enter or exit the cell

  controls transport into and out of the cell

  site for chemical reactions and cell-cell signalling

  it is a dynamic structure

  SA increased by microvilli
• microvilli - present in kidney tubules, liver hepatocytes - increase the rate of absorption and secretion
• nuclear envelope is a double membrane that surrounds the nucleus of a cell, protects from cytoplasmic enzymes
• nuclear pores: gaps in nuclear envelope

  entry point for regulatory proteins, nucleotides, steroid hormones

  exit point for mRNA (DNA cant leave as it is too big)
• nucleolus - site of ribosome production (/)
• chromatin: DNA with associated proteins (histones)

  two types:

  heterochromatin (darker patches): (gene expression is switched off) (yr13)

  euchromatin (lighter patched): (gene expression switched on (yr13)
• ultrastructure = the structures within cells that are only visible at resolutions higher than those achieved using a standard light microscope
• prokaryotic cells = bacteria

  • do not have a nucleus - DNA is in the cytoplasm
  • evolved before eukaryotes
• 4.5 bil --> origin of earth

  3.5 bil --> prokaryote bacteria dominate

  2.5 bil --> oxygen accumulates

  1.5 bil --> eukaryotes - first nucleated cell

  0.5 bil --> cambrian explosion of multicellular organisms
• prokaryotes are all unicellular and do not have membrane-bound organelles
• (Prokaryote) a bacterium:

  cell surface membrane

  mesosome: infolding cell membrane, increase S.A., may be an artefact

  capsule: slimy layer

  cell wall: made of peptidoglycan, avoids osmotic swell

  (naked) circular chromosomal DNA in cytoplasm

  ribosomes: free in cytoplasm, smaller than Eukaryotic ribosomes

  bacterial flagellum: allows movement

  pili: allows adhesion
• eukaryotic cells have a nucleus, following kingdoms made up of eukaryotic cells: animal, plant, fungi and protoctist
• eukaryotic organisms can be multicellular or single cellular, have membrane bound organelles
• (eukaryotic) organelles =

  membrane-bound compartments (except for the nucleus) or other structures (not membrane bound) each with a specific function which make up the ultrastructure of cells.

  advantages of compartmentalisation:

  increasing efficiency of reactions by concentrating substrates and enzymes to a specific space, increased likelihood of collisions

  also provides optimal conditions (pH) for specific reactions

  the membranes of membrane-bound organelles are made of the same material as the plasma membrane/ cell surface membrane
• before the cell divides, DNA condenses into chromosomes. the rest of the time DNA appears as 'grainy' chromatin.
• Ribosomes (EU):

  • site of protein synthesis
  • very large macromolecule (29nm, can be seen using electron microscopy => classified as an organelle) made up of protein and ribosomal RNA
  • ribosomes are: 80s in eukaryotes -> large & 70s in prokaryotes -> small
  • ribosomes can be found free in the cytoplasm or on the surface of RER (ribosomes are not surrounded by a membrane)
• Endoplasmic reticulum:

  network of flattened sacs or cisternae originating from outer membrane of nucleus/ continuous with nuclear envelope

  the interior of the cisternae is called the lumen

  the membranes of the cisternae bud off to form vesicles: a small sac formed by a membrane (containing chemical from the lumen of the reticulum)
• There are two types of ER:

  • rough endoplasmic reticulum (RER): site of protein synthesis (on the ribosomes) & protein transport
  • rough as they have ribosomes on the surface
  • newly-formed polypeptides from ribosomes enter lumen of RER and are packaged into vesicles to be taken to golgi apparatus
  • smooth endoplasmic reticulum (SER): site of lipid and carbohydrate synthesis
  • does not have a covering of ribosomes => smooth appearance
• golgi apparatus:

  • site of (protein) modification and secretion
  • where proteins from RER and lipids from SER enter via vesicles and are modified

  The modifications are carried out inside the lumen of the golgi and include:

  • addition of a carbohydrate chain to make glycoproteins and to lipids to make glycolipids
  • prosthetic groups of proteins added e.g. haemoglobin

  the final products are pinched off the end of the cisternae inside a secretory vesicle (transport out of cells using vesicles = secretion)
• secretion: vesicles transport the products to their destination, inside or out of cells
• lysosomes - a vesicle with a specific function:

  vesicles formed by golgi apparatus that remain inside the cell contain hydrolytic (destructive) enzymes called lysozyme which degrades unwanted substances in the cell
• Cytoskeleton: present throughout the cytoplasm of all eukaryotic cells. network of fibres necessary for the shape and stability of a cell. organelles are held in place by the cytoskeleton and it controls cell movement and the movement of organelles within cells
• (cytoskeleton) microfilaments -

  • contractile fibres formed from the protein actin
  • responsible for cell movement and cell contraction during cytokinesis (the process in which the cytoplasm of a single cell divides into two daughter cells)
  • movement of plasma membrane e.g. phagocytosis, exocytosis
• (cytoskeleton) microtubules -

  • globular tubulin proteins polymerise to form tubes that are used to form a scaffold-like structure that determines the shape of a cell
  • they also act as tracks for the movement of organelles, including vesicles, around the cell
  • spindle fibres which have a role in the physical segregation of chromosomes in cell division, are composed of microtubules
• (cytoskeleton) microtubules (part 2):

  • transport mRNA from nucleus to ribosomes
  • transport of vesicles within lumen of RER
  • movement of transport vesicles from RER to Golgi Apparatus
  • movement of secretory vesicles from Golgi to plasma membrane
  • centrioles to allow attachment points for the spindle fibres at the poles
  • spindle fibres to allow movement of chromosomes to opposite poles
  • flagellum to propel cell forward
  • cilia which produce wafting motion to move substances outside the cell
  • help resist compression forces