VCE Biology Question Thread

[heids]

i forgot to press enter

2. The major factor limiting cell size is the
a. concentration of water in the cytoplasm.
b. need for energy.
c. presence of membrane-enclosed organelles.
d. ratio of surface area to volume.
e. composition of the plasma membrane.

D.  I would approach this by considering each one in order:
a - what does water concentration have to do with it? irrelevant
b - so if a cell has unlimited energy, will it grow to the size of a baseball? (obviously, no)
c - irrelevant, how could this impact cell size?
d - think about what you know about SA to volume: necessary molecules diffuse through the membrane (SA) and thus for enough to filter in so the cell can support all the organelles etc.  Hence a big cell has low SA to volume and wouldn't survive.
e - this is more reasonable, but assume that the membrane is both strong and fluid and can support larger cells

[yang_dong]

thank you bangla_lok,

when they say volume, do they mean the volume of the cytoplasm?

Can you please explain it again, I'm so sorry I didn't quite get it ~

[wobblywobbly]

Thanks for the encouragement! If you don't mind answering, how did you go in unit 4 sacs and the exam?

Ah. "A+ average" on Unit 4, then A+ on the exam.

Tailoring to your question on getting a 40+ SS, an A+ on the exam is essential in getting that, without a doubt. Luckily, it's only 93-95 marks out of 110 (a near-perfect mark in the MC will get you very close), which is definitely achievable if you work at it. Just do really well in knowing the content, applying it, and knowing how to answer the questions.

[sushibun]

Why does the nervous system have a localized effect whereas the endocrine system has a more widespread effect?
Can't the endocrine system have a localized effect?

Is it because the endocrine system isn't restrictive enough and just releases hormones into the bloodstream, expecting it to reach the target cells?

[Rishi97]

Why does the nervous system have a localized effect whereas the endocrine system has a more widespread effect?
Can't the endocrine system have a localized effect?

Is it because the endocrine system isn't restrictive enough and just releases hormones into the bloodstream, expecting it to reach the target cells?

Yes basically and also some cells may have the same receptors

[cosine]

Why does the nervous system have a localized effect whereas the endocrine system has a more widespread effect?
Can't the endocrine system have a localized effect?

Is it because the endocrine system isn't restrictive enough and just releases hormones into the bloodstream, expecting it to reach the target cells?

Correct. The endocrine system releases hormones which may take relatively long periods of time to take their effect. Whereas the nervous system sends out action potentials to the effector, causing an almost immediate reaction.

[sushibun]

Yes basically and also some cells may have the same receptors

Correct. The endocrine system releases hormones which may take relatively long periods of time to take their effect. Whereas the nervous system sends out action potentials to the effector, causing an almost immediate reaction.

Thanks! 

[vox nihili]

Why does the nervous system have a localized effect whereas the endocrine system has a more widespread effect?
Can't the endocrine system have a localized effect?

Is it because the endocrine system isn't restrictive enough and just releases hormones into the bloodstream, expecting it to reach the target cells?

Paracrines have a localised effect, endocrines are always distributed in the blood.

[tiff_tiff]

heyy,

can someone confirm this with me: animal cells don't have walls?? I thought they did??? so they just have a membrane???
are plant cells the only things that have a cell wall???

[cosine]

heyy,

can someone confirm this with me: animal cells don't have walls?? I thought they did??? so they just have a membrane???
are plant cells the only things that have a cell wall???

Animal cells do not have cell walls. They have an outer membrane (boundary) which is known as the phospholipid bilayer. Plant cells have a phospholipid bilayer, as well as a cell wall on top of it. The cell wall is not another phospholipid bilayer, though. It is composed of cellulose and it a rigid boundary which further protects and shapes plant cells. No eukaryotic cells have cell walls besides plant cells, some fungal and protists, but there are also some prokaryotic cells that also have them such as some bacteria.

Hope it clarified things!

[mahler004]

Animal cells do not have cell walls. They have an outer membrane (boundary) which is known as the phospholipid bilayer. Plant cells have a phospholipid bilayer, as well as a cell wall on top of it. The cell wall is not another phospholipid bilayer, though. It is composed of cellulose and it a rigid boundary which further protects and shapes plant cells. No eukaryotic cells have cell walls besides plant cells, but there are some prokaryotic cells that also have them such as some bacteria.

Hope it clarified things!

Fungal cells and some protists have cell walls.

[heids]

when they say volume, do they mean the volume of the cytoplasm?

Can you please explain it again, I'm so sorry I didn't quite get it ~

Volume = the contents of the cell, i.e. cytosol, organelles and nucleus.  The organelles all need certain molecules to function, like carbon, oxygen or potassium.  The greater the area of the membrane, the more of these necessary molecules can diffuse through to the cytoplasm.

Think of a 1x1x1 cm object - the surface area is 6cm2 to 1cm3 volume (6:1).  But double each dimension - 2x2x2 - and surface area is 24cm2 to 8cm3 (3:1).  So this means that as a cell gets bigger, the ratio of surface area (and thus how much can diffuse into/out of the cell) gets LOWER compared to volume.  So when a cell gets too big, not enough nutrients will be able to diffuse across for the large number of organelles/cytoplasm to function.

Sorry, can't explain very clearly

[cosine]

In the condensation reaction of two glucose monomers, where is the energy supplied to form sucrose (or any disaccharide)? I know H2O is released, but this is an anabolic reaction, which requires energy. Thanks

[mahler004]

In the condensation reaction of two glucose monomers, where is the energy supplied to form sucrose (or any disaccharide)? I know H2O is released, but this is an anabolic reaction, which requires energy. Thanks

The energy comes from 'activated' phosphorylated glucose molecules.

Sucrose is not a disaccharide of glucose and glucose, it is a disaccharide of glucose and fructose. The disaccharide of glucose and glucose is maltose.

[cosine]

The energy comes from 'activated' phosphorylated glucose molecules.

Sucrose is not a disaccharide of glucose and glucose, it is a disaccharide of glucose and fructose. The disaccharide of glucose and glucose is maltose.

It comes from activated phosphorylated glucose molecules. Does this mean that an inorganic phosphate group is attached to glucose before it can bind with another glucose monomer to form maltose?