The Biology AS-level Thread

Hey I need help with this question in my bio text book!!
explain using yiur knowlefge of genes (dna), how the amylase enzymes in the fly larva and the adult fly can have a differnet protein structure

Hi, 3 main mechanisms of gene structure and function can explain this:-

1. The DNA that codes for amylase in this organism might code for a specific sequence of amino acids akin to one type of amylase in the larva, then by the time the adult fly hatches from the pupa [chrysalis] at a later stage in its life cycle, pos-transcriptional modification could have occurred so that a slightly different sequence of nitrogen bases will be present in the mRNA of the adult fly, therefore a slightly different sequence of amino acids will be coded for, leading to a different amylase.

2. Some genes can sometimes be either expressed or suppressed based on environmental circumstances - hence, the gene for a particular type of amylase might result in its phenotype being expressed in the larva, and a different phenotype is expressed in the adult.

3. A mutation in the amylase gene could have occurred whereby there has been a substitution OR deletion OR insertion of a base during the part of the life cycle between larva to pupa to adult, resulting a in different sequence of amino acids being produced, and therefore a different amylase.

Hope this helps.
M (specialist biology tutor)

Ok let us look at it in a simple way!

The protein concentration (mainly that of albumin) in the blood determines the osmotic pressure of the blood i.e. a high protein concentration makes for a lower water potential [higher osmotic pressure], so that net movement of water tends to be towards the inside of the blood vessels, as long as these parameters also apply:
- that the osmotic pressure of the tissue fluid (interstitial space) is lower (water potential higher)[check also print in italics below]:

If the opposite of this prevails, i.e. if the osmotic pressure of the plasma is lower than that of the tissue fluid (water potential of plasma is higher than that of tissue fluid), then water will tend to move out of blood vessels into the interstitial space by osmosis = oedema.

I use the words "tends to" because the hydrostatic pressure of the blood due to the pressure created by cardiac contraction is the other variable that plays a part, so it is the net effect of hydrostatic pressure tending to move fluid out of blood vessels and osmotic pressure tending to move water in that will determine the overall direction of fluid flow, and hence whether oedema will occur or not.

Hope this makes matters easier!
M (former medical student)

I understood your explanation.

The book says 'if protein concentration and rate of loss from plasma is not in balance with the protein conc and rate of loss of tissue fluid, oedema may occur'.

Protein conc isnt in balance in plasma with that in the tissue fluid as more proteins move out in the tissue fluid due to high bp.

What do they mean by rate of loss of plasma is not in balance with rate of loss of tissue fluid? I think it means, due to high protein conc in tissue fluid (due to leakage from the arterial end of the capillary bed because of high bp) there is only rate of loss (normally theres gain of fluid by the venous end of the capillary bed as normally proteins dont move out into tissue fluid and hence solute conc is lower of tissue fluid) from capillaries from even the venous capillary end and hence rate of loss is embalanced? As theres fluid loss from both the arterial and venous ends into tissue fluid, that way theres an imbalance. Whereas in normal conditions fluid moves out from arterial end into tissue fluid and some again moves back into the venous end from tissue fluid so rate of loss is equal.

Do I understand correctly?

Thanks.

I need help understanding something related to the transport system of plants.

The book states 'if sucrose was to be translocated through xylem vessels, it would leak out of them as they have no living membranes-membranes can control entry or of solutes'.

My question is, then why doesnt water leak out of xylem vessels? Is lignin impermeable to water but permeable to solutes?

Can anyone explain what exactly is the difference between stem cells and normally dividing cells in our body?

Stem cells can develop into any type of specialized cell - an example of pluripotent cells are embryonic stem cells, which can divide into all of the cell types that make up the body; multipotent cells are more limited than pluripotent cells; adult stem cells are multipotent.

Normal dividing cells are already specialized for performing (a) specific function(s), so when they divide, THEY PRODUCE DAUGHTER CELLS OF THE SAME (ONE) TYPE ONLY.

M

Cells normally dividing by mitosis produce two daughter cells of the same type. So a liver cell undergoing mitosis will produce two daughter liver cells. Such cells are termed 'fully differentiated' - they've transformed into a specific cell type and can't become anything else. These type of cells represent the vast majority of somatic cells.

However, there is another special type of cell loosely called 'stem cells' which are not fully differentiated, but which can go on to form a variety of different cell types. This is called potency, and the amount of change varies with the type of potency the stem cell has - an oligopotent cell can form a couple of different cell types (like a vascular stem cell) whereas a totipotent cell can form any cell type within the organism. Note too that cells can be induced back into various degrees of potency by some very fancy new techniques!

Alright,I get it. Thank you.

Also, can a stem cell divide to again give more stem cells and not differentiate at all? Also, does that happen at all? If yes, then by acting that way stem cells are actually behaving the same way as normal dividing cells.