Campbell and Reece 8th edition Cell Biology

[Stephen1993]

hello everyone
hopefully i make use of this forum

"We now have an explanation of why carbohydrates is always found on the outside surface of the plasma membrane. It is because the carbohydrate was originally added by enzymes in the lumen of the ER, and membrane asymmetry is preserved when budding or fusing occurs. If carbohydrate is on the inside of the ER membrane, it will also be on the inside of the ER membrane, it will also be on the inside of the vesicle and Golgi membrane, but it will be on the outside of the plasma membrane after exocytosis occurs.
Could you please explain what this means??

thank you

 

[atyy]

When a vesicle in the cell fuses to the plasma membrane, say in exocytosis, the membrane surface that faces the inside of the vesicle will now face the outside of the plasma membrane. Similarly, in exocytosis, the contents of the vesicle are released to the outside of the cell.

http://www.ncbi.nlm.nih.gov/books/NBK26892/figure/A2417/
http://www.ncbi.nlm.nih.gov/books/NBK21471/figure/A4740/?report=objectonly (Check out the movie!)

 

[Stephen1993]

thank you for that atyy

i want to know what the "membrane asymmetry" mean?

thank you

 

[atyy]

"membrane asymmetry" just means that the two sides of the membrane are distinguished by having eg. different proteins in them.

 

[Stephen1993]

thank you for that

Membrane proteins and lipids destined for the cell surface are also transported to the plasma membrane in this way. The membrane lipids are originally made in ER (where the enzymes for phospholipid synthesis are located) and move through the Golgi. The plasma membrane does not grow indefinitely because some of its material is internalised by endocytosis. The two processes are usually balanced.
Could you please explain what this means??

thank you

 

[Stephen1993]

sorry for this late post but i am wondering if you have the Campbell and Reece 8th edition biology textbook because this is where i am studying the cell biology from

thank you again

 

[atyy]

thank you for that

Membrane proteins and lipids destined for the cell surface are also transported to the plasma membrane in this way. The membrane lipids are originally made in ER (where the enzymes for phospholipid synthesis are located) and move through the Golgi. The plasma membrane does not grow indefinitely because some of its material is internalised by endocytosis. The two processes are usually balanced.
Could you please explain what this means??

thank you

In exocytosis, the membrane of the vesicle gets added to the plasma membrane, so the area of the plasma membrane increases. If there is continual exocytosis, without endocytosis, the area of the plasma membrane will keep increasing. This usually doesn't happen, presumably because on average, there are as much endocytosis as there is exocytosis ("balanced").

sorry for this late post but i am wondering if you have the Campbell and Reece 8th edition biology textbook because this is where i am studying the cell biology from

thank you again

I don't have Campbell and Reece, am simply translating the statements you quote into what I hope is plain English.

 

[Stephen1993]

wow
thank you for that

just about that question
"The plasma membrane does not grow indefinitely because some of its material is internalised by endocytosis."
what does "some of its material is internalised by endocytosis" exactly mean?

this is about the endomembrane system

And because the ER membrane is continuous witht the nuclear envelope, the space between the two membrane of the envelope is continuous with the lumen of the ER.

what does the "continuous" mean?

another one
"The vacuole has a major role in the growth of plant cells, which enlarge as their vacuoles absorb water, enabling the cell to become larger with a minimal investment in new cytoplasm."

what does "minimal investment in new cytoplasm." mean?

thank you

 

[Stephen1993]

hi
is there anyone here to help?

thank you

 

[mishrashubham]

just about that question
"The plasma membrane does not grow indefinitely because some of its material is internalised by endocytosis."
what does "some of its material is internalised by endocytosis" exactly mean?

During endocytosis, a part of the plasma membrane itself curves inwards and encloses the extracellular material that needs to be taken inside the cell. This "taking in" or "engulfing" of material is referred to as "being internalised".

And because the ER membrane is continuous witht the nuclear envelope, the space between the two membrane of the envelope is continuous with the lumen of the ER.

what does the "continuous" mean?

Continuous means that the membrane of the endoplasmic reticulum is the same as that of the outer membrane of the nuclear envelope. It is just like an extension. A picture would help.

Notice how the outer nuclear membrane (shown in purple) extends out to form the endoplasmic reticulum (shown in light blue). Therefore the lumen (the space inside the Endoplasmic reticulum) is continuous with the perinuclear space (the space between the inner and outer nuclear membranes, i.e. there is no physical barrier between the two.

another one
"The vacuole has a major role in the growth of plant cells, which enlarge as their vacuoles absorb water, enabling the cell to become larger with a minimal investment in new cytoplasm."

what does "minimal investment in new cytoplasm." mean?

The cytosol along with water is made of many organic molecules and inorganic salts. Therefore it takes energy to make and maintain this cytoplasm. However if you simply want to growth of a cell you don't need all the extra cytoplasm. So what vacuoles do absorb water and thus increase in volume thus also increasing the size of the entire cell, without wasting energy and materials in making more cytoplasm.

 

[Stephen1993]

thank you for that

Just quickly in Pg 85 of the textbook
Figure 5.23 …….
“If the denatured protein remains dissolved, it can often renature when the chemical and physical aspects of its environment are restored to normal.”
What does is mean when it says: “If the denatured protein remains dissolved’?

Pg 155 of the textbook
“For example, if the active site has amino acids with acidic R groups, the active site site may be a pocket of low pH in an otherwise neutral cell.”
What does this mean?

thank you

 

[mishrashubham]

thank you for that

Just quickly in Pg 85 of the textbook
Figure 5.23 …….
“If the denatured protein remains dissolved, it can often renature when the chemical and physical aspects of its environment are restored to normal.”
What does is mean when it says: “If the denatured protein remains dissolved’?

You denature proteins by applying some denaturing agents. So supposee while a denatured protein is dissolved in, may be, an acid (denaturing agent), you can renature the protein by increasing the pH of the solution i.e restoring normal conditions.

Pg 155 of the textbook
“For example, if the active site has amino acids with acidic R groups, the active site site may be a pocket of low pH in an otherwise neutral cell.”
What does this mean?

thank you

The active site of the enzyme has amino acids with acidic R groups (the side chains) i.e. the R group contains carboxyl groups. So naturally the region develops a low pH.

Actually I don't have access to the textbook and therefore it is a bit difficult to give a proper answer without any references to the context of the statement. So if you have any further questions could you also just include the paragraph which contains the sentence that you did not understand? Thanks

 

[Stephen1993]

thank you for that

about the second question there is something with it that i forgot to write

Third, the active site may also provide a microenvironment that is more conductive to a particular type of reaction than the solution itself would be without the enzyme. For example, if the active site has amino acids with acidic R groups, the active site site may be a pocket of low pH in an otherwise neutral cell. In such cases, an acidic amino acid may facilitate H+ transfer to the substrate as a key step in catalysing the reaction.

i kinda get what the last sentence but i don't know what the rest means

thank you so much

 

[mishrashubham]

thank you for that

about the second question there is something with it that i forgot to write

Third, the active site may also provide a microenvironment that is more conductive to a particular type of reaction than the solution itself would be without the enzyme. For example, if the active site has amino acids with acidic R groups, the active site site may be a pocket of low pH in an otherwise neutral cell. In such cases, an acidic amino acid may facilitate H+ transfer to the substrate as a key step in catalysing the reaction.

i kinda get what the last sentence but i don't know what the rest means

Well that's better. Certain reactions require an acidic medium and would only occur in a low pH environment. Such conditions are not present inside a cell. Enzymes are then useful for catalyzing such reactions. As said in the paragraph, the active site contains acidic R groups which create a local region of low pH where the reaction is then possible.

 

[Stephen1993]

thank you for that mishrashubham

 

[mishrashubham]

thank you for that mishrashubham

You are welcome.

 

[Stephen1993]

hi again

Pg 87
The roles of nucleic acids
“The molecular hardware of the cell- the tools for biological functions- consists mostly of proteins. For example, the oxygen carrier in red blood cells is protein haemoglobin, not the DNA that specifies its structure.”
So the last part of the sentence means that the DNA is not the haemoglobin that carries oxygen- it codes for a protein that carries oxygen?
Just want to know what do they mean by the molecular hardware of the cell?

thank you

 

[mishrashubham]

hi again

Pg 87
The roles of nucleic acids
“The molecular hardware of the cell- the tools for biological functions- consists mostly of proteins. For example, the oxygen carrier in red blood cells is protein haemoglobin, not the DNA that specifies its structure.”
So the last part of the sentence means that the DNA is not the haemoglobin that carries oxygen- it codes for a protein that carries oxygen?
Just want to know what do they mean by the molecular hardware of the cell?

thank you

I hope you are aware of what DNA is and how it works.

Here the have used the analogy of software and hardware. A computer as two parts, the hardware and the software. Hardware is that part which physically exists and which you can touch and feel; while software is just the information and set of instructions. So the software actually tells what work needs to be done and the hardware is the part that actually does the work. Let us say you need to calculate what is 2+2. You give that input to the computer, and the software inside sends instructions how to calculate. The processor, which is the microchip and hence forms the hardware, is the thing which does all the work of calculating and sends back the result to be viewed on screen.

Something similar also happens in a cell. The work of your red blood cells is to carry oxygen to different parts of the body. In order to do this they need some sort of machinery. The protein haemoglobin has the ability to hold oxygen. So the DNA inside the cell codes for the protein haemoglobin which then actually does the job. In that sense DNA becomes the software and haemoglobin the hardware. Just a small question; Are you studying the textbook all by yourself?

Also check this for more information
http://en.wikipedia.org/wiki/DNA

 

[Stephen1993]

thank you for that

well i am studying cell biology at university but i prefer to ask them here because i can record the questions and the answers
is that a problem?

pg 308
"For example, 30.3% of human DNA nucleotides have the base A, whereas DNA from bacterium E. coli has only 26.0% A. This evidence of molecular diversity among species, which have been presumed absent from DNA, made DNA a more credible candidate for the genetic material."

what does the last part mean?

 

[mishrashubham]

thank you for that

well i am studying cell biology at university but i prefer to ask them here because i can record the questions and the answers
is that a problem?

pg 308
"For example, 30.3% of human DNA nucleotides have the base A, whereas DNA from bacterium E. coli has only 26.0% A. This evidence of molecular diversity among species, which have been presumed absent from DNA, made DNA a more credible candidate for the genetic material."

what does the last part mean?

As I already said, could you also include the paragraph from which the sentence was taken; because it would be really difficult to give a satisfactory answer based only on a single sentence.

 

[Stephen1993]

ok

pg 308
Additional evidence that DNA is the genetic material
"Chargaff analysed the base composition of DNA from a number of different organisms. In 1950, he reported that the base composition of DNA varies from one species to another. For example, 30.3% of human DNA nucleotides have the base A, whereas DNA from bacterium E. coli has only 26.0% A. This evidence of molecular diversity among species, which have been presumed absent from DNA, made DNA a more credible candidate for the genetic material."

what does the last part mean?

is that better?

 

[Stephen1993]

hello is there anyone to help please?

thank you

 

[mishrashubham]

ok

pg 308
Additional evidence that DNA is the genetic material
"Chargaff analysed the base composition of DNA from a number of different organisms. In 1950, he reported that the base composition of DNA varies from one species to another. For example, 30.3% of human DNA nucleotides have the base A, whereas DNA from bacterium E. coli has only 26.0% A. This evidence of molecular diversity among species, which have been presumed absent from DNA, made DNA a more credible candidate for the genetic material."

what does the last part mean?

is that better?

That was a time when it was not known what exactly carried heritable characteristics from parent to offspring. They knew that different organisms were behaviorally and morphologically different. Therefore it was concluded that the material which carried the information needed to make those bodies, also had to be different. And that was what Chargaff observed in DNA. Hence it became a good candidate for the genetic material.

 

[Stephen1993]

thank you for that

You should already know that coding sequences always begin with the same start codon, AUG, which encodes the amino acid methionine. But there is a problem because AUG is the only codon that specifies methionine. How are the start codon AUG and internal methionine AUG codons distinguished. In eukaryotes, the initiator AUG is simply the first one in from the 5’ end of the mRNA, described in Campbell. In prokaryotes, the initiator AUG is identified by a ribosome binding sequence (a specific sequence of nucleotides called a ‘Shine Dalgarno’ sequence) that precedes it.

thank you

 

[mishrashubham]

thank you for that

You should already know that coding sequences always begin with the same start codon, AUG, which encodes the amino acid methionine. But there is a problem because AUG is the only codon that specifies methionine. How are the start codon AUG and internal methionine AUG codons distinguished. In eukaryotes, the initiator AUG is simply the first one in from the 5’ end of the mRNA, described in Campbell. In prokaryotes, the initiator AUG is identified by a ribosome binding sequence (a specific sequence of nucleotides called a ‘Shine Dalgarno’ sequence) that precedes it.

thank you

AUG is required as a start codon during translation (CTG is used very rarely). However during transcription it is the promoter that tells RNAP where to start.

As you said, the binding site for mRNA is the 5' cap so it would simply start translation from there until it reaches a stop codon.

BTW, there is also something called the Kozak sequence in eukaryotes similar to the SD sequence in prokaryotes. While not ultra-essential, it greatly affects translation.

 

[Stephen1993]

thank you for that

but what i was really after was "How are the start codon AUG and internal methionine AUG codons distinguished."
what does it exactly mean?
does it mean that methionine is an amino acid in the chain? or something else?

thank you

 

[mishrashubham]

You should already know that coding sequences always begin with the same start codon, AUG, which encodes the amino acid methionine. But there is a problem because AUG is the only codon that specifies methionine. How are the start codon AUG and internal methionine AUG codons distinguished. In eukaryotes, the initiator AUG is simply the first one in from the 5’ end of the mRNA, described in Campbell. In prokaryotes, the initiator AUG is identified by a ribosome binding sequence (a specific sequence of nucleotides called a ‘Shine Dalgarno’ sequence) that precedes it.

Is this entire paragraph a quote from your textbook? Do you want a simplification of the text or the answers to the questions posed above?

 

[Stephen1993]

sorry if i haven't made this clear

this is the whole paragraph from a particular textbook
"But there is a problem because AUG is the only codon that specifies methionine. How are the start codon AUG and internal methionine AUG codons distinguished? In eukaryotes, the initiator AUG is simply the first one in from the 5’ end of the mRNA, described in Campbell. In prokaryotes, the initiator AUG is identified by a ribosome binding sequence (a specific sequence of nucleotides called a ‘Shine Dalgarno’ sequence) that precedes it."

in that paragapgh i just don't understand what this part means:
"How are the start codon AUG and internal methionine AUG codons distinguished?"

thank you

 

[mishrashubham]

sorry if i haven't made this clear

this is the whole paragraph from a particular textbook
"But there is a problem because AUG is the only codon that specifies methionine. How are the start codon AUG and internal methionine AUG codons distinguished? In eukaryotes, the initiator AUG is simply the first one in from the 5’ end of the mRNA, described in Campbell. In prokaryotes, the initiator AUG is identified by a ribosome binding sequence (a specific sequence of nucleotides called a ‘Shine Dalgarno’ sequence) that precedes it."

in that paragapgh i just don't understand what this part means:
"How are the start codon AUG and internal methionine AUG codons distinguished?"

thank you

AUG codes for the amino acid methionine. It is also used as a start codon in translation. May be the text is trying to ask how the aug used as a start codon is differentiated from the aug codons used somewhere in between the mRNA sequence.

 

[Stephen1993]

i thought so
well thank you for that

next question:
"During anaphase, the region of overlap is reduced as motor proteins attached to the mircotubules walk them away from one another, using energy from ATP."

i really have trouble with this as i am not sure what they originally mean by the "walking" bit?

thank you

 

[Stephen1993]

hello

sorry but i got an urgent desperate question i want to ask because this is very important!

About the regulation of gene expression in bacteria
What does it mean that in a repressible operons that the default state is ON?
and the opposite for inducible operons
what does default state mean? i tried looking that up in the dictionary

thank you

 

[mishrashubham]

hello

sorry but i got an urgent desperate question i want to ask because this is very important!

About the regulation of gene expression in bacteria
What does it mean that in a repressible operons that the default state is ON?
and the opposite for inducible operons
what does default state mean? i tried looking that up in the dictionary

thank you

An operon is a group of genes that is under the control of a single promoter. Their expression is regulated by proteins called repressors which bind to the DNA and prevent transcription which is decided based upon environmental factors. In repressible operons, the repressors are normally not binded to the DNA in normal conditions; however and inducer can interact with the repressor and cause it to bind with the DNA and hence stop exression of the genes.

It is just the opposite in inducible operons where under natural conditions the repressor prevents expression and an inducer is need to remove it.

Thus default state is the condition in which it is present under normal conditions.

 

[Stephen1993]

thank you for that

so it this something to do with the repressor protein made constitutively?

thank you

 

[mishrashubham]

thank you for that

so it this something to do with the repressor protein made constitutively?

thank you

Sorry I don't get you. What do you mean by "made constitutively"?

 

[Stephen1993]

sorry about that

the repressor protein is made constitutively means that it is produced ALL of the time.