All BUT ONE describes a function of apoptosis. That is A) Cell death that occurs when cells are exposed to toxins or are injured. B) Maintain a balance within the immune system by getting rid of spent immune cells. C) Cells undergo cellular suicide when they receive certain cues indicating that the cell is damaged. D) Occurs during embryological development to make sure that extra body tissues are not present once the embryo has developed.
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But what is your question dear. You have not mentioned it.
Answer:
Apoptosis is a form of programmed cell death, or “cellular suicide.” It is
Necrosis (the messy way)
When cells are damaged by harmful factors (such as injury or toxic chemicals), they usually “spill their guts” as they die. Because the damaged cell’s plasma membrane can no longer control the passage of ions and water, the cell swells up, and its contents leak out through holes in the plasma membrane. This often causes inflammation in the tissue surrounding the dead cell.
Apoptosis (the tidy way)
Cells that undergo apoptosis go through a different and much more orderly process. They shrink and develop bubble-like protrusions (technical name: “blebs”) on their surface. The DNA in the nucleus gets chopped up into small pieces, and some organelles of the cell, such as the endoplasmic reticulum, break down into fragments. In the end, the entire cell splits up into small chunks, each neatly enclosed in a package of membrane.
What happens to the chunks? They release signals that attract debris-eating (phagocytic) immune cells, such as macrophages. Also, the fragments of the dying cell display a lipid molecule called phosphatidylserine on their surface. Phosphatidylserine is usually hidden on the inside of the membrane, and when it is on the outside, it lets the phagocytes bind and "eat" the cell fragments.
Why do cells undergo apoptosis?
Many cells in the human body have the built-in ability to undergo apoptosis (in the same way that they have the built-in ability to copy their DNA or break down fuels). Basically, apoptosis is a general and convenient way to remove cells that should no longer be part of the organism.
Some cells need to be “deleted” during development – for instance, to whittle an intricate structure like a hand out of a larger block of tissue.
Some cells are abnormal and could hurt the rest of the organism if they survive, such as cells with viral infections or DNA damage.
Cells in an adult organism may be eliminated to maintain balance – to make way for new cells or remove cells needed only for temporary tasks.
Apoptosis is part of development
Other examples of apoptosis during normal development include the loss of a tadpole’s tail as it turns into a frog, and the removal of unneeded neurons in as neural circuits in the brain are “wired.”
Apoptosis can eliminate infected or cancerous cells
In some cases, a cell can pose a threat to the rest of the body if it survives. For instance, this may be the case for cells with DNA damage, pre-cancerous cells, and cells infected by viruses. If these cells undergo apoptosis, the threat to the rest of the organism (such as cancer or spread of a viral infection) is removed.
When a cell’s DNA is damaged, it will typically detect the damage and try to repair it. If the damage is beyond repair, the cell will normally send itself into apoptosis, ensuring that it will not pass on its damaged DNA. When cells have DNA damage but fail to undergo apoptosis, they may be on the road to cancer.
Sometimes, pre-cancerous cells that have avoided internal apoptosis cues are detected by immune cells, which try to trigger apoptosis through an external signaling pathway. Successful cancer cells, however, manage to duck both internal and external cues that would normally trigger apoptosis. This allows them to divide out of control and accumulate mutations (changes in their DNA).
Apoptosis is key to immune function
Apoptosis also plays an essential role in the development and maintenance of a healthy immune system. When B and T cells (immune cells that bind specific molecules) are first produced, they’re tested to see if they react against any of the body’s own “self” components. Cells that do are eliminated right away by apoptosis. If this process fails, self-reactive cells may be released into the body, where they can attack tissues and cause autoimmune conditions.
Apoptosis also plays an important role in allowing the immune system to turn off its response to a pathogen. When a pathogen is detected, the immune cells that recognize the pathogen divide extensively, undergoing a huge increase in numbers with the purpose of destroying the pathogen. Once the pathogen is cleared from the body, the large numbers of pathogen-specific immune cells are no longer needed and must be removed by apoptosis to maintain homeostasis (balance) in the immune system.
Summary
Apoptosis is a form of programmed cell death, or “cellular suicide.” It is different from necrosis, in which cells die due to injury. Apoptosis is not the only form of programmed cell death, but it is the form we understand best.
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