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August 21, 2019, 04:59:53 pm

Author Topic: A Guide to Success in VCE Biology - Part 2 (Membranes and Cell Organelles)  (Read 12426 times)  Share 

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alondouek

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Part 1 - General VCE Biology Tips and Biomacromolecules

Part 2 - Membranes and Cell Organelles

The 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 Organs

Tissues 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 apoptosis

Apoptosis

Apoptosis 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 Apoptosis

Initially, 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)


Necrosis

Often, 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:

  • Injury
  • Radiation
  • Chemicals

Necrosis is irreversible.

The following diagram compares the processes of apoptosis and necrosis:



The Cell Cycle

The image below demonstrates the phases of the cell cycle in a eukaryotic cell:


PhaseFunction
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

G0 is the 'resting' phase.


Eukaryotes

The 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


Prokaryotes

Prokaryotes 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.
« Last Edit: September 20, 2014, 09:53:08 pm by alondouek »
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shoomer010

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+1
wow this is really helpful thanks :)

Yacoubb

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Thank you so much :)

Just quick question. Is this answer okay?

what is the difference between apoptosis and necrosis?
Apoptosis is the natural, programmed death of cells, whereas necrosis is the non-programmed death of cells caused by physical trauma.

Scooby

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Thank you so much :)

Just quick question. Is this answer okay?

what is the difference between apoptosis and necrosis?
Apoptosis is the natural, programmed death of cells, whereas necrosis is the non-programmed death of cells caused by, for example, physical trauma.

That's a bit better  :P

Also - necrotic cells release cytotoxic substances, such as glutamate, into the extracellular environment, which may in turn cause surrounding cells to become necrotic. These cytotoxic substances aren't released by apoptotic cells
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Yacoubb

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That's a bit better  :P

Also - necrotic cells release cytotoxic substances, such as glutamate, into the extracellular environment, which may in turn cause surrounding cells to become necrotic. These cytotoxic substances aren't released by apoptotic cells

Thank you :) and so I probably might leave out the physical trauma bit, because its best to be to the point and state the main difference regarding programming of cell death :)

Scooby

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Thank you :) and so I probably might leave out the physical trauma bit, because its best to be to the point and state the main difference regarding programming of cell death :)

Yeah, that's alright  :)

Necrosis can happen for a wide range of reasons - infection, ischemia, physical trauma, exposure to cytotoxins, etc
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Yacoubb

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Yeah, that's alright  :)

Necrosis can happen for a wide range of reasons - infection, ischemia, physical trauma, exposure to cytotoxins, etc

When interferon is secreted and coats neighbouring cells with this interferon, does this prevent pathogenic agents from infecting the neighbouring cells? Does this then prevent necrosis?

Scooby

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Interferon is a substance released by a virally-infected cell that triggers neighbouring cells to begin producing enzymes that will enable them to resist infection by the virus. That doesn't mean the cell won't be infected by the virus; it's just more prepared to combat it

I'd be wary of describing cell death following viral infection as necrosis
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Yacoubb

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Okay so I'll rephrase that:

Interferon is a protein-based substance released by virus-infected cells that triggers neighbouring cells to release enzymes that enable the cell to combat the virus if it happens to enter the cell. This is a part of the immune system's second line of defence.

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DJA

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Re: A Guide to Success in VCE Biology - Part 2 (Membranes and Cell Organelles)
« Reply #10 on: December 26, 2013, 01:50:50 pm »
0
Is Cell Apoptosis a part of the VCE course that we need to know?
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alondouek

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Re: A Guide to Success in VCE Biology - Part 2 (Membranes and Cell Organelles)
« Reply #11 on: December 26, 2013, 02:02:19 pm »
+1
You should understand what it is, why it is needed and how it differs from necrosis (unregulated cell death). You probably won't need to know the ways apoptosis occurs, such as the caspase pathway (though NoB should have a small section on this if you're interested).
« Last Edit: December 26, 2013, 02:21:45 pm by alondouek »
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DJA

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Re: A Guide to Success in VCE Biology - Part 2 (Membranes and Cell Organelles)
« Reply #12 on: December 26, 2013, 02:07:46 pm »
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You should understand what it is, why it is needed and how it differs from necrosis (unregulated cell death). You probably won't need to know the ways apoptosis occurs, such as the cascade pathway (though NoB should have a small section on this if you're interested).

Thanks for the quick reply!  :)

Just ensuring that I'm not learning extraneous things which won't be tested.
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DJA

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Re: A Guide to Success in VCE Biology - Part 2 (Membranes and Cell Organelles)
« Reply #13 on: December 26, 2013, 02:12:20 pm »
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Just a further question.

Difference between Cytosol and Cytoplasm?

Is the cytoplasm basically everything within the plasma membrane-including all organelles. Would the plasma membrane be classed as an organelle-and therefore part of the cytoplasm?

Thanks in advance! :)
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alondouek

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Re: A Guide to Success in VCE Biology - Part 2 (Membranes and Cell Organelles)
« Reply #14 on: December 26, 2013, 02:20:18 pm »
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The cytosine is the cytosol + organelles + 'cell inclusions'.  The cell membrane is not considered part of the cytoplasm :)

Edit: Here is a better place for specific biology questions: Biology Question Thread [3/4]
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