Eukaryotic & Prokaryotic Cells


Cells are the basis of all life forms on the Earth today. They are considered to be the basic building blocks of life.

Cell Theory

Specification Reference

"Understand that cell theory is a unifying concept that states that cells are a fundamental unit of structure, function and organisation in all living organisms."
Cells are the basic building blocks of life. They are the fundamental units of structure, function and organisation in living organisms. Cell theory is a unifying concept which links all life forms on earth together.
  • Structure
  • Function
  • Organisation

Cell Organisation

Specification Reference

"Understand that in complex organisms, cells are organised into tissues, organs, and organ systems."
In multicellular organisms specialised cells are organised into the following structures:

Tissues
  • Group of similar cells
  • Working together
  • To perform a particular function
Organs
  • Groups of different tissues
  • Structured so they work together
  • To perform a particular function
Systems
  • Group of different organs
  • Working together to carry out large scale functions
  • Examples include the digestive system and the nervous system

Prokaryotic Cell Structure

Specification Reference

"Know the ultrastructure of prokaryotic cells and the structure of organelles, including: nucleoid, plasmids, 70S ribosomes and cell wall."
Prokaryotic simply means before the nucleus. Prokaryotic organisms are always single celled.

Basic Properties
  • Single celled organisms
  • DNA suspended freely in the cytoplasm
  • It does not have a nucleus
  • Do not have membrane bound organelles
  • Smaller than eukaryotic cells
Nucleoid
  • Genetic material consists of a single strand of DNA
  • This is often circular
  • The DNA is folded and coiled
  • Area becomes very dense with DNA
  • This area is known as the nucleoid
Plasmids
  • Smaller circles of DNA
  • Code for a particular aspect of the cell such as a toxin
  • Is able to reproduce independently of the nucleoid
70s Ribosomes
  • Not membrane bound
  • Involved in protein synthesis
  • Made up of two smaller sub-units
  • Larger 50s unit and a smaller 30s unit
Cell Walls
  • All bacterial cells have a cell wall
  • Contents of the cell are usually hypertonic so water moves into the cell by osmosis
  • The wall prevents swelling and bursting
  • All have some sort of layer of peptidoglycan
Slime Capsule
  • Surrounds the cell walls
  • Covers cell markers
  • Makes the cell hard to identify
  • Therefore protecting the bacterium from phagocytosis
  • This feature is present in all prokaryotic cells and enables the cell to more easily become pathogenic

Gram Positive & Gram negative Bacteria

Specification Reference

"Be able to distinguish between Gram positive and Gram negative bacterial cell walls and understand why each type reacts differently to some antibiotics."
Bacterial cells can either be Gram positive or Gram negative. Which one they are depends on the type of cell wall it has. This is shown below:

Gram Positive Bacteria
  • Have a thick layer of peptidoglycan
  • Contain chemicals such as teichoic acid
  • Thick layer resists staining
  • Therefore leaving a purple/blue colour after staining
Gram Negative Bacteria
  • Thin layer of peptidoglycan
  • No teichoic acid
  • Thin layer of peptidoglycan is easily broken down
  • Therefore leaving the cell red in colour after staining

Gram Positive & Gram negative Bacteria

Antibiotics can be used to treat most pathogenic bacteria. They kill these bacterial cells by targeting features specific to bacterial cells, preventing them from reproducing. Which antibiotic is used depends upon which type of bacteria is present. If the wrong antibiotic is prescribed the person will see no benefit.

  • Some inhibit the formation of peptidoglycan
  • This makes them effective against Gram positive bacteria
  • Whilst not damaging human cells
  • Polypeptide antibiotics can be used to treat Gram negative bacteria
  • However, these are rarely used because they do interact with human cells
Other antibiotics target common features of bacterial cells such as the 70s ribosomes.

Eukaryotic Cell Structure

Specification Reference

"Know the ultrastructure of eukaryotic cells and the functions of organelles, including: nucleus, nucleolus, 80S ribosomes, rough and smooth endoplasmic reticulum, mitochondria, centrioles, lysosomes, Golgi apparatus, cell wall,chloroplasts, vacuole and tonoplast."
Eukaryotic cells have membrane bound organelles and are the foundations of multi-cellular life.

Nucleus
  • Contains the cells genetic material in chromosomes
  • DNA is coiled around the protein histone
  • Surrounded by a double nuclear membrane
Nucleoulus
  • This is the site of ribosome synthesis and is located within the nucleus
80s Ribosomes
  • Only found in eukaryotic cells
  • Are the site of protein synthesis
Rough Endoplasmic Reticulum
  • A series of folded sacs surrounding the nucleus
  • Lots of ribosomes on its surface
  • Here proteins are synthesised and packaged into vesicles
Smooth Endoplasmic Reticulum
  • Site of lipid synthesis and storage
Mitochondria
  • Bound in a double membrane
  • Contain 70s ribosomes (evidence for the endosymbiosis theory)
  • Produce energy in the form of ATP
Centrioles
  • Found as pairs
  • Each contain 9 tubes
  • Form spindle fibres from microtubules
  • These move the chromosomes in cell division
Lysosomes
  • Single membrane
  • Release hydrolytic enzymes
  • Break down materials for the cell to use
  • Can self destruct a cell as a defence mechanism (apoptosis)
Golgi Apparatus
  • Made up of stacks of flattened membrane sacs
  • Proteins are carried from the RER are altered and modified
  • Then released in a vesicle
  • Have a single membrane
Cell Wall
  • Not found in animal cells
  • Provides strength and support for a plant cell
  • Can be strengthened through lignin
Chloroplasts
  • Found in plant cells
  • Are the site of photosynthesis
  • Are double membrane bound
Vacuole
  • Contains cell sap which acts as a nutrient store and maintains turgor pressure
  • Surrounded by the tonoplast which controls movements into and out of the vacuole
  • Has a single membrane

Microscopy

Specification Reference

"Know how magnification and resolution can be achieved using light and electron microscopy."
Magnification is a measure of how much larger an image of an object is than the actual object itself. Resolutions is a measure of the ability to distinguish between two very close objects. Magnification can be found using the following formula:

  • magnification = size of image / size of object

Staining

Specification Reference

"Understand the importance of staining specimens in microscopy."
  • The cytoplasm of cells is colourless
  • This means that very little can be seen
  • To overcome this, a stain can be used
  • The stain used will react with a specific chemical, allowing you to see a specific organelle
  • Without stains it would be impossible to see most organelles down any microscope