Where do animal cells get their glucose from?
Erythrocytes, the red blood cells, are not cells! In contrast to other eukaryotic cells, human erythrocytes have neither a nucleus nor mitochondria!
Erythrocyte: no nucleus, no mitochondria.
Despite this “deficiency”, erythrocytes are perfectly adapted and optimally equipped to take over gas exchange in the body. The main actor is the transport protein hemoglobin (also important for the pH!). There are around 4.5–5 million erythrocytes per µl of blood!
Metabolism of the red blood cells
What does the loss of mitochondria mean?
- no citric acid cycle
- no beta oxidation
- no respiratory chain
- the only way to generate energy: glycolysis
In general: reduced ability of the erythrocytes to regenerate: maximum lifespan: 120 days
When you have worked out the processes of cell respiration in the chapter on cell respiration, you can think about this Abitur task idea again!
Energy metabolism of the erythrocytes
Exclusively via anaerobic glycolysis, therefore glucose as the sole source of energy!
Approx. 30 g of glucose are converted into lactate per day.
- ATP for Na+/ K+-ATPase
- to maintain the internal ion composition (for red blood cells: a lot of potassium inside, little sodium)
- ATP is also required for the biosynthesis of glutathione!
- “Glycolysis special” in the erythrocytes
- personal erythrocyte byway via 2,3-bisphosphoglycerate *
(* Small repetition in the margin: How do you know that? How was that again with the regulation? Maybe you look at the cell respiration)
- This special path causes the loss of one ATP per mole of glucose !! Like everything in nature, this has its purpose and reason (read under iron balance).
- Pentose phosphate pathway in the erythrocytes
- oxidative pathway (from glucose-6-phosphate) to ribulose-5-phosphate is important. Here are 2 NADPH + H+
- NADPH + H+ is used to regenerate glutathione!
Help, I need glutathione! But for what?
- It serves as an oxidation protection, it is a reducing agent.
- Glutathione is built up as a tripeptide from the three amino acids glutamate, cysteine and glycine.
- special linkage of glutamate and cysteine via the c-carboxyl group of glutamate (atypical tripeptide)
- Task and mode of operation of glutathione
- Oxidation of 2 x glutathione to glutathione disulfide (glutathione peroxidase)
Why do erythrocytes need this system?
- come into contact with oxygen J
- unwanted oxygen radicals can arise, these are very reactive
- cause e.g. oxidation of Hb (Fe2+) to methemoglobin (Fe3+) and a superoxide radical
- this reacts with 2 H+ and O2 to hydrogen peroxide ...
- Hydrogen peroxide is detoxified again into water via glutathione peroxidase
- Regeneration: via glutathione reductase
- requires NADPH + H+ from the pentose phosphate route
What do radicals do?
- network fatty acids in the cell membrane, which leads to their destruction
- cross-link proteins, which lose their functionality as a result
- Summary: Protection of the erythrocytes:
- Glutathione as protection against oxidation 
- Glutathione reductase and NADPH + H+ as regeneration machinery
- Superoxide dismutase eliminates superoxide radicals
- Glutathione peroxidase and catalase reduce hydrogen peroxide
How can this system be disturbed?
- Deficiency in glucose-6-phosphate dehydrogenase  causes hemolytic anemia
Breakdown of erythrocytes
- Life expectancy of the erythrocyte: 120 days
- The red blood cells go through a kind of apoptosis at the end of life. The calcium content of the cell increases and its shape changes. Phosphatidylserine is shifted to the outside, this serves as a receptor for macrophages.
- The erythrocytes are broken down in the spleen, but also in the liver and bone marrow (sickle cell anemia: because sickle cell erythrocytes do not get that old and are constantly broken down, the spleen is overloaded).
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