chapter 2 part 4 Flashcards
Cellulose cell wall:
- Plant cell walls are made of cellulose, a complex carbohydrate.
- They are freely permeable so substances can pass into and out of the cell through the cellulose wall.
- The cell walls of a plant cell give it shape.
- The contents of the cell press against the cell wall making it rigid.
- This supports both the individual cell and the plant as a whole.
- The cell wall also acts as a defence mechanism, protecting the contents of the cell against invading pathogens.
- All plant cells have cellulose cell walls.
Structures which are unique to plant cells include:
Vacuoles
Chloroplasts
Vacuoles
Vacuoles are membrane lined sacs in the cytoplasm containing cell sap.
Many plant cells have large permanent vacuoles which are very important in the maintenance of turgor, so that the contents of the cell push against the cell wall and maintain a rigid framework for the cell.
The membrane of a vacuole in a plant cell is called the tonoplast.
It is selectively permeable, which means that some small molecules can pass through it but others cannot.
If vacuoles appear in animal cells, they are small and transient (not permanent).
Chloroplasts
Chloroplasts are the organelles responsible for photosynthesis in plant cells.
They are found in the cells in the green parts of plants such as the leaves and the stems but not in the roots.
They have a double membrane structure, similar to mitochondria.
The fluid enclosed in the chloroplast is called the stroma.
They also have an internal network of membranes, which form flattened sacs called thylakoids.
Several thylakoids stacked together are called a granum (plural grana).
The grana are joined by membranes called lamellae.
The grana contains the chlorophyll pigments, where light-dependent reactions occur during photosynthesis.
Starch produced by photosynthesis is present as starch grains.
Like mitochondria, chloroplasts also contain DNA and ribosomes.
Chloroplasts are therefore able to make their own proteins.
The internal membranes provide the large surface area needed for the enzymes, proteins and pigment molecules necessary in the process of photosynthesis
diagram of chloroplast
unicellular organisms can be classed into two evolutionary domai
Archaea and Bacteria, which evolved from an ancient common ancestor.
Prokaryotic cells:
Prokaryotic cells may have been among the earliest forms of life on Earth.
They first appeared around 3.5 billion years ago when the surface of the Earth was a very hostile environment.
Scientists believe that these early cells were adapted to living in extremes of salinity, pH and temperature - These organisms are known as extremophiles and they still exist today.
They can be found in hydrothermal vents and salt lakes - similar environments to those believed to have made up the early Earth.
They are usually of the domain Archaea and more recently they have been found in more hospitable environments such as soil and the human digestive system.
Prokaryotic organisms are always unicellular with a relatively simple structure.
Their DNA is not contained within a nucleus, they have few organelles and the organelles they do have are not membrane-bound.
DNA within prokaryotes:
The structure of the DNA contained within prokaryotes is fundamentally the same as in eukaryotes but it is packaged differently.
Prokaryotes generally only have one molecule of DNA, a chromosome, which is supercoiled to make it more compact.
The genes on the chromosome are often grouped into operons, meaning a number of genes are switched on or off at the same time.
Ribosomes within prokaryotes:
The ribosomes in prokaryotic cells are smaller than those in eukaryotic cells.
Their relative size is determined by the rate at which they settle, or form a sediment, in solution.
The larger eukaryotic ribosomes are designated 80S and the smaller prokaryotic ribosomes, 70S.
They are both necessary for protein synthesis, although the larger 80S ribosomes are involved in the formation of more complex proteins.
Cell wall within prokaryotes:
Prokaryotic cells have a cell wall made from peptidoglycan, also known as murein.
It is a complex polymer formed from amino acids and sugars.
Flagella within prokaryotes:
The flagella of prokaryotes is thinner than the equivalent structure of eukaryotes and does not have the 9+2 arrangement.
The energy to rotate the filament that forms the flagellum is supplied from the process of chemiosmosis, not from ATP as in eukaryotic cells.
The flagellum is attached to the cell membrane of a bacterium by a basal body and rotated by a molecular motor.
The basal body attaches the filament comprising the flagellum to the cell-surface membrane of a bacterium.
A molecular motor causes the hook to rotate giving the filament a whip-like movement, which propels the cell.
prokaryotic flagellum diagram
A comparison with eukaryotic cells:
The first eukaryotic cells appeared about 1.5 billion years ago and are much more complex than prokaryotic cells.
Their DNA is present within a nucleus and exists as multiple chromosomes, which are supercoiled, and each one wraps around a number of proteins called histones, forming a complex for efficient packaging.
This complex is called chromatin and chromatin coils and condenses to form chromosomes.
Eukaryotic genes are generally switched on and off individually.
eukaryotic cells have membrane-bound organelles including mitochondria and chloroplasts
Organisms from the plant, animal, fungi, and protoctista kingdoms are all composed of eukaryotic cells.
Many are multicellular.
prokaryotic cell diagram
eukaryotic cell diagram
The similarities and differences between prokaryotic and eukaryotic cells:
