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captainGrape

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All of the following statements about endosperm in angiosperms are correct EXCEPT:
A. It has a triploid chromosome number.
B. It forms from a portion of the embryo sac that contains the polar nuclei.
C. It provides nutrients for the growing embryo.
D. It forms entirely from maternal tissue.
E. It originates from a cell that is fertilized by a sperm nucleus from a pollen grain.

D is the answer.
I only understand that D is the right answer because everything else is correct; however, I want to know why D is correct. I thought that the endosperm is formed entirely from the maternal tissue because the only thing that the sperm provides is two nuclei. Comparing this to sexual reproduction in humans; the female ova provides most of the cytoplasm and mitochondria for the developing embryo; so I thought the same applied here in plant.

Thanks in advance!
 

8_man

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If you look at answer choice E, it kind of tells you why. The endosperm "originates from a cell that is fertilized by a sperm nucleus from a pollen grain." Hence, it is a product of both parents. I don't know the specifics, though. :whistle:
 

live.dream.dds

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Hey! Got this question wrong and I don't understand why ... the question comes from cliffnotes AP 3rd edition, animal form and function

Which of these would be activated first in response to a body cell that has been invaded by a virus
a. cytotoxic T cells
b. natural killer cells
c. antibodies
d. macrophages
e. neutrophils

I said B, as natural killer cells are leukocytes in the second line of dense... however, the answer is A. The explanation given is "Cytotoxic T cells would attack the abnormal body cell first. Once the abnormal cell is lysed, nonspecific attacks by macrophages and other phagocytes would occur "
I understand their answer, it is true and it does make sense but I do not understand why B is wrong and A is right.... if that makes any sense
 

tyjacobs

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This is beyond DAT scope but should explain the answer.

what activates cytotoxic t cells?
- t-cell receptor of cytotoxic T-cell binds to antigen presented by target cell MHC I.

what activates natural killer cells? they have 3 excitatory receptors:
- NKG2D: recognizes cell stress markers (MICA, MICB)
- Fc (gamma) receptor III: binds to coated IgG on the target cell (the IgG can be against a viral protein)
- Toll like receptor 3,7: detects viral RNA
But with natural killer cells, they also have inhibitory receptors on their membrane. The excitatory signals must outweigh the inhibitory signals or else the NK cell is not activated.

So you are both right in the fact that both can detect viruses. But they ask which is detected first. Lets go through what would happen in a viral infection chronologically..

Viruses attack cells. Cells that are infected take viral antigens and display them on their MHC 1. Based on what I said above, what activates MHC1? It's going to be cytotoxic T cells. Natural killer cells will come in later, probably after the body creates IgG directed against the viral antigen.

In general though,
know that cytotoxic T cells are going to be activated with MHC1 antigen presentation, and with natural killer cells, its a lot more complicated.
 
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DAT Destroyer

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Hey! Got this question wrong and I don't understand why ... the question comes from cliffnotes AP 3rd edition, animal form and function

Which of these would be activated first in response to a body cell that has been invaded by a virus
a. cytotoxic T cells
b. natural killer cells
c. antibodies
d. macrophages
e. neutrophils

I said B, as natural killer cells are leukocytes in the second line of dense... however, the answer is A. The explanation given is "Cytotoxic T cells would attack the abnormal body cell first. Once the abnormal cell is lysed, nonspecific attacks by macrophages and other phagocytes would occur "
I understand their answer, it is true and it does make sense but I do not understand why B is wrong and A is right.... if that makes any sense

Agreed Answer should be B.

Natural killer cells do not need an antigen to attack a cell. They can recognize a cell that is "Stressed" and attack it. While cytotoxic cells must recognize an antigen to be activated.
So NK cells should react faster.
Hope this helps.
 
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