Part 1 - General VCE Biology Tips and BiomacromoleculesPart 2 - Membranes and Cell OrganellesThe cell is the basic unit of structure in an organism.
A cell membrane, cytosol and ribosomes are features constant in all cells.
All cells carry out the process of cellular respiration.
Tissues and OrgansTissues are groupings of cells that carry out a specific function.
Organs are functional groupings of tissues that carry out a specific function.
The diagram below illustrates this section of biological organisation:
In fully formed tissues, cell reproduction and programmed cell death are balanced.
Programmed cell death is known as
apoptosisApoptosisApoptosis is the programmed self-destruction of a cell. It is vital for the survival of an organism as it:
- Prevents the formation of tumors
- Eliminates damaged cells
- Eliminates cells that have fulfilled their functions
- Eliminates irrelevant cells during embryonic development (i.e. webbing between fingers and toes)
It is important for an organism to regulate the level of apoptosis that occurs. Too little apoptosis leads to uncontrolled cell growth and replication (mitosis), and the potential development of a tumor, which can lead to cancer. Too much apoptosis leads to atrophy (wasting away), which in turn can lead to conditions such as Alzheimer's disease.
Cells will carry out apoptosis if signalled to do so by:
- Outside the cell (receptor pathway)
- Inside the cell (mitochondrial pathway)
Mitochondrial Pathway of ApoptosisInitially, serious damage occurs within a cell (such as damage to the DNA sequence or a malfunction of an oxidative enzyme). The following steps are then taken by the cell to self-destruct, thereby limiting the damage the cell can cause:
- Proteins on the surface of the mitochondria activate; the mitochondrial membrane breaks
- Activation of caspase enzymes, which break DNA into small pieces
- Regular apoptotic breakdown occurs (blebbing, breakdown of plasma membrane, rupturing of lysosomes, engulfing and digestion of the cell by a macrophage)
NecrosisOften, there is confusion between apoptosis and cell necrosis. Necrosis is the
non-programmed death of a cell.
Necrosis is usually caused by direct environmental influences, when there is not enough blood flowing to the tissue. This can occur due to the effect of:
Necrosis is irreversible.
The following diagram compares the processes of apoptosis and necrosis:
The Cell CycleThe image below demonstrates the phases of the cell cycle in a eukaryotic cell:
Phase | Function |
G1 - Growth 1/Gap 1 | - New organelles synthesised
- Protein synthesis
This is the main growth stage of many cells |
S - Synthesis | - Replication of DNA
- Detection and repair of DNA damage
|
G2 - Growth 2/Gap 2 | - Rapid cell growth
- Protein synthesis
|
M - Mitosis | - Division of parent cells into daughter cells
- Consists of PMAT phases (Prophase, Metaphase, Anaphase, Telophase)
|
C - Cytokinesis | - Cytoplasm of two daughter cells divide following telophase
|
G
0 is the 'resting' phase.
EukaryotesThe zygote is the first cell of life. It is known as 'the mother of all cells' as it is
totipotent. Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism.
Other, less differentiable cells are known as
pluripotent.
Totipotent - Able to regenerate whole new cells and differentiate into any type of cell.
Pluripotent - Able to develop into many types of mature cell.
Totipotent and pluripotent cells are known collectively as stem cells. (There are other types of cell potencies, being multipotency, oligopotency and unipotency - but I don't recall these being relevant to the VCE course. Feel free to Wikipedia them if you're interested).
The diagram below shows the possibilities of differentiation of a totipotent stem cell:
All of the cells that these stem cells differentiate into are eukaryotic.
This means that they have membrane-bound organelles.Four specific examples of eukaryotes are:
- Animals
- Plants
- Fungi
- Protists
ProkaryotesProkaryotes are another group of organisms who are characterised by:
- Having no membrane-bound nucleus
- Having no membrane-bound organelles
Below is a typical prokaryotic cell:
Note the presence of ribosomes: Even prokaryotes need to synthesise their own proteins!
Also, note the structure of the prokaryotic cell - they have a plasma membrane, a cell wall and a capsule.
Many prokaryotic cells also have a flagellum. This allows motility, or movement.
Prokaryotic cells store their genetic information in different ways. Eukaryotic cells have a membrane-bound nucleus, but prokaryotes do not. Instead, they have a
nucleoid (a highly coiled piece of circular DNA) and free plasmids in their cytoplasm. Generally, plasmids are circular, double-stranded DNA molecules.