Eukaryotes


Structure of a Eukaryote

The term eukaryote means 'good or true nucleus' and describes all cells other than bacteria (prostista, fungi, plant cells and animal cells)

  • These cells all possess a nucleus where the DNA is stored
  • They also possess membrane-bound organelles - such as endoplasmic reticulum, mitochondria, lysosomes, golgi apparatus, etc.


Eukaryotic cells are generally more complex than prokaryotes - the presence of membrane-bound organelles allows for compartmenalisation of functions

Eukaryotic cells are believed to have evolved from prokaryotic ancestors, through a process called endosymbiosis


Prokaryotes versus Eukaryotes

Similarities:

  • Both possess genetic material (DNA and RNA)
  • Both contain a cell membrane 
  • Both contain ribosomes


Differences:


Animal Cells

  • Animals are multicellular organisms made up of many eukaryotic cells cooperating to form a functional organism
  • Because of this, individual cells may differ greatly in structure and function, however all share some basic characteristics


Structure of a Generalised Animal Cell

                                      2D Representation                                                                                                                 3D Representation


Plant Cells

  • Plant cells are different from animal cells in a number of key areas:
    • They may contain chloroplast and other plastids for photosynthesis (not including cells that do not undergo photosynthesis - such as root cells)
    • They contain a cell wall (made of cellulose) and therefore have a rigid, rectangular shape
    • They contain a large, central vacuole
    • They store excess glucose as starch (animals store excess glucose as glycogen)
    • They do not have centrioles and do not have cholesterol in their cell membrane

 

Structure of a Generalised Plant Cell


Organelles

Cell Membrane:  Semi-permeable barrier that controls the entry and exit of substances

Cytosol:  The fluid portion of the cytoplasm (does not include the organelles or other insoluble materials)

Nucleus:  Contains hereditary material (DNA) and thus controls cell activities (via transcription) and mitosis (via DNA replication)

Nucleolus:  Site of the production and assembly of ribosome components

Ribosome:  Complexes of RNA and protein that are responsible for polypeptide synthesis (eukaryotic ribosomes are larger than prokaryotes - 80S)

Mitochondria:  Site of aerobic respiration, which produces large quantities of chemical energy (ATP) from organic compounds

Golgi Apparatus:  An assembly of vesicles and folded membranes involved in the sorting, storing and modification of secretory products

Lysosome:  Site of hydrolysis / digestion / breakdown of macromolecules

Peroxisome:  Catalyses breakdwon of toxic substances like hydrogen peroxide and other metabolites

Chloroplasts:  Contain photosynthetic pigment (chlorophyll) and is responsible for photosynthesis (not in animal cells)

Leucoplasts: Responsible for storage of materials (e.g. starch in amyloplasts) in non-photosynthetic plant cells (not in animal cells)

Centrioles:  Microtubule-organising centres involved in cell division (not in plant cells)

Endoplasmic Reticulum:  A system of membranes involved in the transport of materials between organelles

  • Rough ER:  Studded with ribosomes and involved in the synthesis and transport of proteins destined for secretion
  • Smooth ER:  Involved in the synthesis and transport of lipids and steroids, as well as metabolism of carbohydrates


Examples of Organelles as seen in an Electron Micrograph (Liver Cell)


Multicellular Organisms

  • Multicellular organisms, such as most eukaryotes, are capable of performing functions that individual cells could not undertake
  • These new functions are called emergent properties and arise from the communication and interrelationship between distinct cell types
  • Multicellular organisms may be organised as follows
    • Cells may group together to form tissues
    • Organs are then formed from the functional grouping of multiple tissues
    • Organs that interact may form organ systems capable of carrying out specific body functions
    • Organ systems carry out the life functions required by an organism


Levels of Anatomical Organisation


Cell Differentiation

  • All cells of an individual organisms share an identical genome - each cell contains the entire set of  genetic instructions for that organism
  • The activation of different instructions (genes) within a given cell by chemical signals will cause it to differentiate from other cells like it
  • Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature
  • This results in different cells having different structures and functions, such as:
    • Muscle cells having a large number of mitochondria to meet the energy requirements of muscular contraction
    • White blood cells having large numbers of lysosomes in order to digest foreign materials
    • Intestinal cells having a large surface area (microvilli) to maximise absorption of nutrients
    • Pancreatic cells having many golgi bodies to enable large-scale secretion of hormones (insulin or glucagon)
    • Leaf cells having a large number of chloroplasts to carry out photosynthesis


Differential Gene Expression Leading to Specialisation of Cell Structure and Function