Cell Theory
Cell theory states that the cell is the fundamental unit of life, organisms are made up of one or more cells, and cells arise from preexisting cells
In multicellular organisms, cells are specialised to carry out different functions
A cell is the simplest entity that we can define as living → it has certain features such as the ability to reproduce, respond to the environment, harness energy, evolve
Structure and function of a cell are closely related
All cells have genetic material in the form of DNA
Prokaryotes have no nucleus
Eukaryotes have a nucleus
First forms of life were prokaryotic cells
Their genetic material is organized in one circular chromosome with many loops
This genetic material is concentrated in a region of cell known as the nucleoid (a cell structure with multiple loops formed from supercoils of DNA)
They also have a cell wall surrounding the cell membrane, which helps to maintain their shape
Some bacteria have flagella (singular, flagellum), structures that extend from their surface and allow them to move
Prokaryotic cells are much smaller than eukaryotic cells
The small size of prokaryotic cells means that they have a relatively high ratio of surface area to volume → makes sense for an organism that absorbs nutrients from the environment → a large amount of membrane surface area is available for absorption relative to volume of the cell it serves
In eukaryotes, nucleus houses the vast majority of the cell’s DNA, which takes the form of multiple linear chromosomes in contrast to single, circular chromosomes of prokaryotes
In eukaryotes, transcription and translation are separated by place and time
There are differences in the types of lipids that make up cell membranes of prokaryotes and eukaryotes
Plant cells have intercellular connections called plasmodesmata, channels that allow the passage of large molecules such as mRNA and proteins between neighboring cells
Plant cells have a cell wall outside the cell membrane, vacuoles specialised for water uptake, and chloroplasts that convert energy of sunlight into chemical energy
Membranes of these organelles are either physically connected by membrane “bridges” or transiently connected by vesicles → small membrane-enclosed sacs that transport substances within a cell or from the interior to the exterior of the cell
Vesicles form by budding off an organelle, taking with them a piece of the membrane and internal contents of the organelle from which they derive
They then fuse with another organelle or the cell membrane, re-forming a continuous membrane and unloading their contents
These interconnected membranes make up the endomembrane system
The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, the cell membrane, and the vesicles that move between them
In plants, the endomembrane system is actually continuous between cells through plasmodesmata
Eukaryotes evolved from prokaryotes
Prokaryotes do not have nucleus
DNA of bacteria is located in a region called nucleotide
As there is no membrane surrounding chromosome, prokaryotes are less able to regulate what comes into contact with their DNA
Eukaryotes have a range of membrane bound organelles which carry out specific cell functions - membrane bound organelles allow more efficient and more complicated functions to be carried out in the eukaryotic cell
Mitochondria - where eukaryotes produce ATP - Prokaryotes produce the same energy producing pathway but in their cell membrane
Endoplasmic reticulum - transport system that facilitates the transport of newly produced proteins around the cell
In eukaryotes ribosomes are distributed throughout the cytoplasm
Golgi apparatus - Receives proteins from the er in membrane Bound vesicles that fuse with cis face of golgi apparatus, depositing the proteins into the lumen - These protein can be modified by enzymes - Two of the main Post translational modifications of proteins are the addition of sugars (glycosylation) and the addition of phosphate groups (Phosphorylation) - Processed proteins are packaged into membrane bound vesicles for distribution around the cell or for secretion from the cell
Prokaryotes are able to carry out post translational modifications including phosphorylation and glycosylation, Although these are much less abundant in the cell - transport around the cell is regulated but many purses are diffusion limited - The reduce capacity of prokaryotes for Specialized functions and transport around the cell is one of the reasons why prokaryotes are much smaller than eukaryotes
Both prokaryotes and eukaryotes have DNA, ribosomes, cytoplasm and plasma membrane
Prokaryotes are the oldest type of cell - they are small and generally simple - they have no membrane bound organelles - they are most often single-celled and their DNA is organised into a single circular chromosome
Who carriers evolved from prokaryotes - they are much larger than prokaryotes - they are consistently more complex - they can be single-celled or multi cellular and they have multiple linear chromosomes
Evolution of eukaryotes from prokaryotes
Eukaryotes evolved from one type of archaea
Archaeal ancestor of the eukaryotes can be referred to as the first eukaryotic common ancestor
Some billion years ago, this earliest ancestor engulfed a bacterium from the class Alphaproteobacteria
A symbiotic relationship with established whereby the bacterial cell contributed ATP to the archaeal and in return the archaeal cell provided nutrients for the bacterial cell - endosymbiosis
The mutual benefit provided by this relationship led to the evolution of codependence
Many genes that were in the bacteria were transferred to the nucleus of the host cell
The mitochondria has also evolved cristae which are folded membranes that increase the amount of surface area in which ATP production can occur
How do we know that mitochondria evolved from a prokaryotic cell?
The first evidence comes from the two membrane layers of the mitochondria
The outer membrane arose due to the process of the prokaryotic ancestor of the mitochondria being engulfed by the eukaryotic host
Analysis of this outer membrane revealed that it is comprised of phospholipids that are characteristic of eukaryotic cells
The phospholipids and proteins that comprise the inner membrane of the mitochondria are more like a prokaryotic cell membrane
The second line of evidence comes from DNA as the mitochondria still has its own genome - however, it is not linear like eukaryotes but circular
Mitochondrial genes are more similar to prokaryotic genes to eukaryotic genes
The endosymbiosis of photosynthetic bacteria known as cyanobacteria into the ancestor of plant cells led to the evolution of chloroplasts