Sunday, September 13, 2009

Kitap: Power, Sex, Suicide

Hücre-molekül seviyesinde olan bitenin hayvanlar aleminden daha fantastik olduğunu gördükçe (ki hayvanlar alemi de az fantastik değil hani) mikro alem hakkında bilgimi arttırmaya karar verip Power, Sex, Suicide - Mitochondria and the Meaning of Life kitabını amazon'dan aldım.

Kitabın tezi bakterinin ötesine geçip bugün gördüğümüz karmaşık organizmaları mümkün kılanın hücrelerin enerji üretimi için mitokondri teknolojisine geçmeleri. Kitabın adından da anlaşılacağı üzere enerji, cinsiyet ve yaşlanma/ölümün müsebbibi mitokondri. Kitapta bir miktar anlatılan ancak ayrıntılı bir incelemeyi hak eden konu ise hücre enerji üretim mekanizması olan elektron iletim zinciri. Biyolojiden çok kimya/fizik bilmeyi gerektiriyor. Detaylı bakıp anlamaya çalışmalı bir ara.

Hayat nedir, benim burada ne işim var, "ben" neyim diyenlere önemli ipuçları sağlıyor. "Neden" sorusundan çok "nasıl" sorusunun yanıtı için güzel hipotezler sunuyor. Kitabın tabiri ile [sayfa 321]:
If they don't show us the meaning of life, they do at least make some sense of its shape. And what is meaning in this world, if it doesn't make sense?
Daha az felsefi ve günlük hayata yakın olan "antioksidanlar yaşam süresini uzatır mı" benzeri sorulara da yanıtlar var (antioksidanlar yaşam süresini uzatamazmış).

Kitapta altını çizdiğim bölümler:
[p.4] Every mitochondrion contains 5 to 10 copies of its genes. Because there are usually hundreds of mitochondria in every cell, there are many thousands of copies of the same genes in each cell, whereas there are only two copies of genes in the nucleus (the control centre [sic] of the cell).

[p.8] ...if you know how something happened, you begin to see why it happened.

[p.12] All in all, there are said to be 10 million billion mitochondria in an adult human, which together constitute about 10 per cent of our body weight.

[p.17] ...bacteria have been collaborating and competing among themselves for nearly four billion years, and yet only came up with the eukaryotic cell once. The acquisition of mitochondria was the pivotal moment in the history of life.

[p.80] ...oxygen respiration can produce 19 time more ATP per molecule of glucose than does fermentation. ATP is produced at a rate of 9*10^20 molecules per second, which equates to a turnover rate (the rate at which it is produced and consumed) of about 65 kg every day.

[p.97] Other forms of respiration use sulphate or nitrate, or even iron, instead of oxygen - and all of them pump protons across a membrane.

[p.121] Why is it that there are no bacteria with more than 10 million DNA letters, when ... the single-celled eukaryote Amoeba dubia has managed to accumulate 670 billion letters - 67,000 times more letters than the largest bacteria, and for that matter 200 times more than humans.

[p.121] An intact membrane is necessary for energy generation. Eukaryotic cells use the inner mitochondrial membrane to generate ATP, while bacteria, which do not have organelles, must use their external cell membrane.

[p.126] Internalization of energy production therefore enables both the loss of the cell wall and a much greater cell volume.

[p.126] A large energetic cell does not have to spend all its time replicating its DNA, but can instead spend time and energy developing an arsenal of protein weapons. It can behave like a fungal cell, and squirt lethal enzymes onto neighbouring cells to digest them before absorbing their juices. Or it can turn predator and live by engulfing smaller cells whole, digesting then inside itself. Either way, it doesn't need to replicate quickly to stay ahead of competition - it can simply eat competition.

[p.137] As a proportion of genes lost, all species have now lost between 95 and 99.9 per cent of their mitochondrial genes.

[p.159] animals increase in size their metabolic rate slows down by a factor that correspond to the ratio of surface area to mass.

[p.179] On 'average', in the wild, a mammal uses about thirty times more energy to stay alive than an equivalent reptile. In practical terms, this means that a mammal must eat in one day the amount of food that would sustain a reptile for a whole month.

[p.200] Typically, a cell must accumulate eight to ten mutations in rather specific genes before it can transform into a malignant cell, whereupon the transformed cell puts its own interests before those of the body.

[p.258] The female sex specializes to provide the mitochondria (100,000 of them in humans) in the large, immobile egg cells, while the male sex specializes to eliminate mitochondria from tiny motive sperm cells... it often seems to boil down to conflict between genetically different populations of mitochondria. To restrict the opportunity for conflict, mitochondria are usually inherited from only one of the two parents.

[p.260] The requirement for a close match means it is critical for mitochondrial and nuclear genes to co-adapt to each other in synchrony, otherwise respiration cannot work.

[p.272] Most proposed clocks don't keep good time. The telomeres, for example - the 'caps' on the end of chromosomes that wear away over our lives at a steady rate - display such divergent patterns across species that they can't possibly be the primary cause of ageing.

[p.277] To date, calorie restriction is the only mechanism proved to extend the lifespan of mammals like rats and mice.

[p.302] Rather than simply causing damage and destruction, free radicals play a vital role in keeping respiration fine-tuned to needs, and in signalling respiratory deficiencies to the nucleus.

[p.303] Simply attempting to quash free radical generation with massive doses of antioxidants is likely to exacerbate the situation, if it could be made to work at all.


Nart Bedin Atalay said...

Gercekler fantastikmis gercekten.

Mitochondria'nin Power ile iliskisi konusunda bir fikrim olustu ama Sex, ve Suicide konularini iliskilendiremedim.

Samil Korkmaz said...

Cinsiyetle konusunda s.258'deki alıntı biraz fikir veriyor ama mevzu daha derin, bkz. Evolution of sexual reproduction. Detay için kitabı okumak lazım.

Suicide için ise bkz. Mitochondrial Regulation

Tabi tüm bunların hipotez olduğunu unutmamalı.