1.2 Ultrastructure of cells Flashcards
Light microscopes are limited in resolution by the wavelengths of
visible light (400–700 nm).
Electrons have a much shorter wavelength (2-12 pm) therefore
electron microscopes have a much higher resolution
Resolution
is defined as the shortest distance between two points that can be distinguished
Ultrastructure
is all the structures of a biological specimen that are at least 0.1nm in their smallest dimension
Light microscopes allow us to see the…
structure of cells
Electron microscopes allow us to see the
ultrastructure of cells, such as these pancreatic exocrine cells
Electron microscopes can see viruses (0.1μm diameter) ,
but light microscopes cannot
First organisms to evolve on Earth..
(pro “before”, karyotic “nucleus”)
Simple cell structure, some with..
pilli and at least one flagellum
prokaryotes have a…
cell membrane, cell wall, 70S ribosomes (smaller than in eukaryotes), and a single circular DNA molecule (nucleoid)
No membrane to separate cellular components
What are the functions of these prokaryotic cell parts?
Movement – flagellum
Protein synthesis – ribosomes
Attachment, DNA transfer – pili
Protective coating – cell wall
Contains enzymes for metabolism – cytoplasm
Region containing closed-loop DNA – nucleoid
Controls entry and exit of substances – plasma membrane
Prokaryotes reproduce asexually using the process of binary fission
Single circular chromosome is replicated semi conservatively [2.7.U1]
Each DNA loops moves to opposite ends of the cell and attach to the membrane
The membrane elongates and pinches off (cytokinesis) forming two separate cells
The two daughter cells are genetically identical (clones)
Binary fission is also used by some organelles in eukaryotes [links to 1.5.U3]
Cytoplasm is divided into compartments by
single or double membranes to form organelles
There are several advantages in being compartmentalized:
Efficiency of metabolism - enzymes and substrates can localized and much more concentrated
Localised conditions - pH and other such factors can be kept at optimal levels. The optimal pH level for one process in one part of the cell
Toxic / damaging substances can be isolated, e.g. digestive enzymes (that could digest the cell itself) are stored in lysosomes
Numbers and locations of organelles can be changed dependent on the cell’s requirements.
Nucleus
Generally spherical with a double membrane
Pores (holes) are present in the membrane
Contains genetic information in the form of chromosomes (DNA and associated histone proteins)
Uncoiled chromosomes are referred to as chromatin – they stain a dark colour and are concentrated at the edges of the nucleus
mRNA is transcribed in the nucleus (prior to use in protein synthesis in the cytoplasm)
mRNA leaves the nucleus via the pores
(DNA is too large to move through the pores)
The Mitochondrion (pl. Mitochondria)
Has a double membrane
A smooth outer membrane and a folded inner membrane
The folds are referred to as cristae
Variable in shape
Site of ATP production by aerobic respiration (if fat is used as a source of energy it is digested here)
Free ribosomes
80S Ribosomes (approx. 20nm diameter) - larger than the ribosomes found in prokaryotes
No membrane
These appear as dark granules in the cytoplasm
Synthesizes proteins to function in the cytoplasm, for use within the cell, e.g. enzymes
The Rough Endoplasmic Reticulum (rER)
The consists of flattened membrane sacs, called cisternae
Often located near to the nucleus
80S Ribosomes are attached to the outside of the cisternae are ribosomes
rER synthesizes proteins which are transported, by vesicles, to the golgi apparatus for modification before secretion outside the cell
The Golgi apparatus
this organelle also consists of flattened membrane sacs called cisternae, like rER.
Different to rER:
No attached ribosomes
Often sited close to the plasma membrane
The cisternae are shorter and more curved that those of the rER
The Golgi apparatus processes (modifies) proteins from from the rER. The proteins are then repackaged in vesicles for secretion outside the cell.
Vesicles
A single membrane with fluid inside
Very small in size
Used to transport materials inside of a cell
Lysosomes
Generally spherical with a single membrane
Formed from Golgi vesicles.
They contain digestive enzymes for breakdown of:
ingested food in vesicles
unwanted/damaged organelles
The cell itself
High concentration of enzymes (a type of protein) cause this organelle to stain heavily and hence appear dark on micrographs
Vacuoles
Single membrane with fluid inside
In Plant cells vacuoles are large and permanent, often occupying the majority of the cell volume
In animals vacuoles are smaller and temporary and used for various reasons, e.g. to absorb food and digest it
Flagellum (Flagella pl.)
Thin projection (usually singular) from the cell surface.
Contain microtubules
Used to move the cell
Cilia
Thin projections from the cell surface.
Contain microtubules
Used to either move the cell or to move the fluids next to the cell
Microtubules
Small cylindrical fibres called microtubules
Have a variety of functions, e.g. part of the structure of flagella and they play a role in cell division
Centrioles
Consist of two groups of nine triple microtubules
Are mainly found in animal cells, not present in vascular plants or fungi.
Chloroplast
Many, but not all, plant cells contain chloroplasts
A double membrane surrounds the chloroplast
Inside are stacks of thylakoids
Each thylakoid is a disc composed of a flattened membrane
The shape of chloroplasts is variable but is usually ovoid
The site of photosynthesis and hence where glucose is produced.
Starch grains maybe present if photosynthesis is happening quickly
Cell wall
an extracellular component not an organelle.
secreted by all plant cells (fungi and some protists also secrete cell walls).
Plant cell walls consist mainly of cellulose which is:
Permeable - does not affect transport in and out of the cell
Strong – gives support to the cell and prevent the plasma membrane bursting when under pressure
Hard to digest –resistant to being broken down, therefore lasts along time without the need for replacement/maintenance
