Cell Biology: Topic 1.2 Ultrastructure of cells Flashcards
Define prokaryotes
Prokaryotes are organisms whose cells lack a nucleus
Prokaryotes belong to the kingdom Monera and have been further classified into two distinct domains:
Archaebacteria – found in extreme environments like high temperatures, salt concentrations or pH (i.e. extremophiles)
Eubacteria – traditional bacteria including most known pathogenic forms (e.g. E. coli, S. aureus, etc.)
Prokaryotic cells will typically contain the following cellular components (9 features)
Cytoplasm – internal fluid component of the cell
Nucleoid – region of the cytoplasm where the DNA is located (DNA strand is circular and called a genophore)
Plasmids – autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer)
Ribosomes – complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryote ribosome = 70S)
Cell membrane – Semi-permeable and selective barrier surrounding the cell
Cell wall – rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis)
Slime capsule – a thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis
Flagella – Long, slender projections containing a motor protein that enables movement (singular: flagellum)
Pili – Hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugation (sex pili)
Define eukaryotes (3 points)
Eukaryotes are organisms whose cells contain a nucleus
They have a more complex structure and are believed to have evolved from prokaryotic cells (via endosymbiosis)
Eukaryotic cells are compartmentalised by membrane-bound structures (organelles) that perform specific roles which can have single or double membranes
Eukaryotes can be divided into four distinct kingdoms:
Protista – unicellular organisms; or multicellular organisms without specialised tissue
Fungi – have a cell wall made of chitin and obtain nutrition via heterotrophic absorption
Plantae – have a cell wall made of cellulose and obtain nutrition autotrophically (via photosynthesis)
Animalia – no cell wall and obtain nutrition via heterotrophic ingestion
What are the advantages of a compartmentalised structure in eukaryotes?
- Enzymes and substrates for a particular process can be much more concentrated than if they were spread throughout the cytoplasm.
- Substances that could cause damage to the cell can be kept inside the membrane of an organelle. For example, the digestive enzymes of a lysosome could digest and kill a cell, if they were not safely stored
inside the lysosome membrane. - Conditions such as pH can be maintained at an ideal level for a particular process, which may be different to the levels needed for other processes in a cell.
- Organelles with their contents can be moved around within the cell
State the structure and function of the nucleus
The nuclear membrane is double and has pores through it. The nucleus contains the chromosomes consisting of DNA associated with histone proteins. Uncoiled chromosomes are spread through the nucleus and are called chromatin. There are often densely staining areas of chromatin around the edge of the nucleus. The nucleus is where DNA is replicated and transcribed to form mRNA, which is exported via the nuclear pores to the cytoplasm.
State the structure and function of the Rough Endoplasmic Reticulum
The rER consists of fattened membrane sacs, called cisternae. Attached to the outside of these cisternae are ribosomes. They are larger than in prokaryotes and are classified as 80S. The main function of the rER is to
synthesize protein for secretion from the cell. Protein synthesized by the ribosomes of the rER passes into
its cisternae and is then carried by vesicles, which bud off and are moved to the Golgi apparatus.
State the structure and function of the Golgi apparatus
This organelle consists of fattened membrane sacs called cisternae, like rER. However the cisternae are not as long, are often curved, do not have attached ribosomes, found near the plasma membrane and have many vesicles nearby. The Golgi apparatus is Involved in the sorting, storing, modification and export (packed in vesicles) of secretory products.
State the structure and function of lysosomes
These are approximately spherical with a single membrane. They are formed from Golgi vesicles. They
contain high concentrations of protein, which makes them densely staining in electron micrographs. They contain digestive enzymes, which can be used to break down ingested food in vesicles or break down organelles in the cell or even the whole cell.
State the structure and function of mitochondrion
A double membrane surrounds mitochondria, with the inner of these membranes invaginated to form structures called cristae. The fluid inside is called the matrix. The shape of mitochondria is variable but is usually spherical or ovoid. They produce ATP for the cell by aerobic cell respiration. Fat is digested here if it is being used as an energy source in the cell.
State the structure and function of free ribosomes
These appear as dark granules in the cytoplasm and are not surrounded by a membrane. Ribosomes are two subunits made of RNA and protein. They are 80s ribosomes in eukaryotic cells and 70s ribosomes in prokaryotic cells. They are the site of polypeptide synthesis (this process is called translation). Nucleolus is the site of ribosome assembly
State the structure and function of vesicles
Very small in size with and has a single membrane with fluid inside. Used to transport materials inside of the cell.
State the structure and function of vacuoles
Single membrane with fluids inside. In plant cells, vacuoles are large and permanent, often occupying majority of the cell’s volume with the function of maintaining the cell’s hydrostatic pressure. In animal cells, vacuoles are small and temporary and used for various reasons.
State the structure and function of microtubules
In the cytoplasm of the cells there are small cylindrical fibres called microtubules that have a variety of roles,
including moving chromosomes during cell division.