Calcium in biology

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The calcium ion (Ca ² ) plays an important role in the physiology and biochemistry of organisms and cells. They play an important role in the signal transduction pathway, where they act as the second messenger, in the neurotransmitter release of neurons, the contraction of all types of muscle cells, and in conception. Many enzymes require calcium ions as cofactors, those who use a blood coagulation cascade become an important example. Extracellular calcium is also important to maintain potential differences across cell membranes, as well as proper bone formation.

Calcium levels in mammals are strictly regulated, with bone functioning as a primary mineral reservoir. The calcium ion, Ca ² , is released from the bone into the bloodstream under controlled conditions. Calcium is transported through the bloodstream as dissolved ions or bound to proteins such as serum albumin. The parathyroid hormone secreted by the parathyroid glands regulates the resorption of Ca ² from bone, reabsorption in the kidney back into circulation, and increased vitamin D activation ₃ to calcitriol. Calcitriol, the active form of vitamin D ₃ , increases calcium absorption from the intestine and mobilization of calcium ions from the bone matrix. The calcitonin secreted from parafollicular cells of the thyroid gland also affects calcium levels by fighting parathyroid hormone; However, physiological significance in humans is dubious.

Calcium storage is an intracellular organelle, which constantly accumulates Ca ² ions and releases them during certain cellular events. Intracellular Ca Storage ² includes mitochondria and endoplasmic reticulum. The concentration of calcium characteristics in model organisms is: in E. coli 3mM (bound), 100nM (free), in 2mM (tied) yeast buds, in mammalian cells 10-100nM (free) and in blood plasma 2mM.

Video Calcium in biology

Animal

Vertebrata

In vertebrates, calcium ions, like many other ions, are essential for many physiological processes whose concentration is maintained within certain limits to ensure adequate homeostasis. This is evidenced by human plasma calcium, which is one of the most strictly physiologic variables set in the human body. Normal plasma levels vary between 1 and 2% for a certain time. Approximately half of all ionized calcium circulates in an unbound form, with the other half complexed with plasma proteins such as albumin, as well as anions including bicarbonate, citrate, phosphate, and sulfate.

Different tissues contain calcium in different concentrations. For example, Ca ² (mostly calcium phosphate and some calcium sulfate) is the bone element (most importantly) and the calcification cartilage. In humans, the total body content of calcium is mostly present in the form of bone mineral (about 99%). Under these circumstances, most are not available for exchange/bioavailability. The way to overcome this is through the process of bone resorption, in which calcium is released into the bloodstream through the action of bone osteoclasts. The remaining calcium is present in extracellular and intracellular fluids.

In a typical cell, the intracellular concentration of ionized calcium is about 100 nM, but can increase 10 to 100-fold over various cellular functions. Intracellular calcium levels are maintained relatively low with respect to extracellular fluids, with an estimated magnitude of 12,000 fold. This gradient is maintained through various pumps of plasma membrane calcium utilizing ATP for energy, as well as considerable storage in the intracellular compartment. In excitable electric cells, such as skeletal muscle and the heart and neurons, membrane depolarization leads to Ca ² transient with a concentration of Ca 2 cytosolic up to 400 nM upwards. Mitochondria are able to sequester and store some Ca ² . It is thought that the mitochondrial matrix-free calcium concentration rises to dozens of in situ micromolar levels during neuronal activity.

Effects

The effects of calcium on human cells are specific, meaning that different types of cells respond in different ways. However, in certain circumstances, the action may be more common. Ca ² ion is one of the two most widely used messengers in signal transduction. They get them into the cytoplasm either from outside the cell through a cell membrane through a calcium channel (such as a calcium-binding protein or a voltage-gated calcium channel), or from some internal calcium deposits such as the endoplasmic reticulum and mitochondria. The intracellular calcium levels are governed by the transport proteins that secrete them from the cell. For example, the sodium-calcium exchanger uses energy from the sodium electrochemical gradient by combining the entry of sodium into the cell (and down its concentration gradient) by transporting calcium out of the cell. In addition, the plasma membrane Ca ² ATPase (PMCA) obtains energy to pump calcium out of cells by hydrolyzing adenosine triphosphate (ATP). In neurons, the calcium-selective ion channel depends on the critical stresses for synaptic transmission through the release of neurotransmitters into the synaptic cleft by the fusion of synaptic vesicle vesicles.

The function of calcium in muscle contraction was discovered as early as 1882 by Ringer. His subsequent investigation was to reveal his role as a messenger about a century later. Because the action is interconnected with cAMP, they are referred to as synodistic messengers. Calcium can bind several different calcium-modulated proteins such as troponin-C (first identified) and calmodulin, a protein necessary to increase muscle contraction.

In endothelial cells lining the inside of the blood vessels, Ca ² ions can regulate some signaling pathways that cause smooth muscles to circle around the blood vessels to relax. Some of the paths contracted Ca ² These include eNOS stimulation to produce nitric oxide, as well as channel stimulation K _ca to the end of K and cause hyperpolarization of the cell membrane. Both nitric oxide and hyperpolarization cause smooth muscle to relax to regulate the number of tones in the blood vessels. However, dysfunction in these Ca ² pathways can cause an increase in tone caused by unregulated smooth muscle contractions. This type of dysfunction can be seen in cardiovascular disease, hypertension, and diabetes.

Co-ordinate calcium plays an important role in defining the structure and function of proteins. Examples of proteins with calcium coordination are von Willebrand factor (vWF) which has an important role in the process of forming blood clots. It was discovered using single-molecule optical tweezers measurements - that the vWF-calcium-bound acts as a shear force sensor in the blood. The shear force causes the exposure of the A2 domain of vWF whose refill rate increases dramatically with the presence of calcium.

Adaptation

Ca ² ion flow regulates some secondary messenger systems in nerve adaption for visual, auditory, and olfactory systems. It may often be bound to calmodulin as in the olfactory system to increase or suppress the cation channel. Other times calcium level changes can actually release guanylyl cyclase from inhibition, as in the fotoreception system Ca ² The ions can also determine the speed of adaptation in the nervous system depending on the receptor and the varying proteins. affinity to detect calcium levels to open or close channels at high concentrations and low calcium concentrations in the cell at the time.

Negative effects and pathology

A substantial decrease in extracellular Ca ² concentration can lead to a condition known as tetanic hypocalcemic, characterized by spontaneous motor neuron release. In addition, severe hypocalcemia will begin to affect aspects of blood coagulation and signal transduction.

Ca ² ion may damage cells if they enter excessive amounts (eg, in the case of excitotoxicity, or over-excitation of neural circuits, which can occur in neurodegenerative diseases, or after insults such as brain trauma or stroke). Inclusion of excessive calcium into cells can damage it or even cause it to have apoptosis, or death from necrosis. Calcium also acts as one of the main regulators of osmotic pressure (Osmotic shock). Chronically elevated plasma calcium (hypercalcemia) is associated with cardiac arrhythmias and decreased neuromuscular stimuli. One of the causes of hypercalcemia is a condition known as hyperparathyroidism.

Invertebrates

Some invertebrates use calcium compounds to build exoskeleton (shell and carapace) or endoskeleton (calcareous calcium echinoderms and poriferan spicules).

Maps Calcium in biology

Plants

Stomatal closing

When the cell's abscisic acid signal absorbs, Ca ² ions enter the cytosol both from outside the cell and the internal store, reversing the concentration gradient so that the K ions start out of the cell. The loss of solute makes the cell soft and closes the pores of the stomata.

Mobile sharing

Calcium is the necessary ion in the formation of mitotic spindles. Without spindle mitosis, cell division can not occur. Although younger leaves have higher calcium requirements, older leaves contain higher amounts of calcium because calcium is relatively immobile through plants. It is not transported through the phloem because it can bind with other nutrient ions and precipitates from the liquid solution.

Structural role

Ca ² ion is an important component of plant cell walls and cell membranes, and is used as a cation to balance organic anions in plant vacuoles. The concentration of Ca ² of the vacuole can reach millimolar levels. The most striking use of Ca ² ions as structural elements in algae occurs in marine coccolithophores, which use Ca ² to form calcium carbonate plates, with which they are covered.

Calcium is needed to form pectin in the central lamella of newly formed cells.

Calcium is needed to stabilize the cell membrane permeability. Without calcium, the cell wall can not stabilize and hold its contents. This is very important in developing fruits. Without calcium, the cell walls are weak and can not accommodate the contents of the fruit.

Some plants accumulate Ca in their tissues, making it much firmer. Calcium is stored as crystal-oxalate in plastids.

Cell signaling

Ca ² ion is usually stored at the nanomolar level in the plant cell cytosol, and acts in a number of signal transduction pathways as the second messenger.

src: www.biochemj.org

Man

Dietary recommendations

The US Institute of Medicine (IOM) established the Recommended Dietary Allowances (RDAs) for calcium in 1997 and updated those values ​​in 2011. See table. The European Food Safety Authority (EFSA) uses the term Population Reference Intake (PRIs) as a substitute for RDAs and assigns slightly different figures: ages 4-10 800 mg, age 11-17 1150 mg, age 18-24 1000 mg, and & gt; 25 years 950 mg.

Because of concerns of long-term side effects such as calcification of arteries and kidney stones, IOM and EFSA both established Active Tolerant Intakes (ULs) for dietary combinations and calcium supplements. From IOM, people aged 9-18 years should not exceed 3,000 mg/day; for ages 19-50 not exceeding 2,500 mg/day; for ages 51 and older, not exceeding 2,000 mg/day. EFSA establishes UL at 2,500 mg/day for adults but decides information for children and adolescents is not enough to determine UL.

For the labeling of food and US dietary supplements, the amount in the presentation is expressed as a percent of Daily Value (% DV). For the purpose of labeling 100% calcium from Daily Value is 1000 mg, but on May 27, 2016 it was revised to 1300 mg to make it agree with the RDA. The old and new daily Adult Value table is given in Daily Intake References. The original deadline to meet is July 28, 2018, but on September 29, 2017 the FDA released the proposed rule that extends the deadline to January 1, 2020 for large companies and 1 January 2021 for small companies.

Approved health claims (US)

Although as a general rule, the labeling of dietary supplements and marketing is not permitted to make disease prevention or treatment claims, the FDA has for some food and dietary supplements reviewing science, concluding that there are significant scientific agreements, and issuing specially tailored health claims.. Initial decisions allowing health claims for dietary supplements of calcium and osteoporosis were then changed to include calcium and vitamin D supplements, effective January 1, 2010. Examples of permitted words are shown below. To be eligible for calcium health claims, dietary supplements contain at least 20% of dietary reference intake, which for calcium means at least 260 mg/portion.

• "Enough calcium during life, as part of a balanced diet, can reduce the risk of osteoporosis."
• "Adequate calcium as part of a healthy diet, along with physical activity, can reduce the risk of osteoporosis later in life."
• "Enough calcium and vitamin D throughout life, as part of a balanced diet, can reduce the risk of osteoporosis."
• "Enough calcium and vitamin D as part of a healthy diet, along with physical activity, may reduce the risk of osteoporosis later in life."

Food sources

The United States Department of Agriculture (USDA) website has a very complete list of calcium content (in milligrams) in food, per common size as per 100 grams or per normal portion.

The amount of calcium in the diet, per 100 grams:

• parmesan (cheese) = 1140 mg
• milk powder = 909Ã, mg
• hard goat cheese = 895 mg
• Cheddar Cheese = 720Ã, mg
• tahini paste = 427Ã, mg
• molasses = 273 mg
• almond = 234Ã, mg
• collard greens = 232Ã, mg
• kale = 150Ã, mg
• goat's milk = 134Ã, mg
• sesame seeds (not milled) = 125 mg
• nonfat cow's milk = 122 mg
• yogurt whole-milk tasteless = 121 mg
• hazelnuts = 114Ã, mg
• spinach = 99Ã, mg
• ricotta (skim milk cheese) = 90 mg
• brown sugar = 85 mg
• lentil = 79Ã, mg
• wheat germ = 72Ã, mg
• peas = 62.7 mg
• beans = 53.1 mg
• egg, boiled = 50Ã, mg
• flour = 41Ã, mg
• orange = 40Ã, mg
• ASI = 33Ã, mg
• rice, white, whole grains, half cooked, enriched, cooked = 19 mg
• trout = 19Ã, mg
• beef = 12 mg
• cod = 11 mg

Measures in blood

The amount of calcium in the blood (more specifically, in blood plasma) can be measured as total calcium , which includes calcium-bound protein and free calcium. In contrast, ionized calcium is a measure of free calcium. Abnormally high levels of calcium in the plasma are called hypercalcemia and abnormally low levels are called hypocalcemia, with "abnormal" generally referring to levels outside the reference range.

The main methods for measuring serum calcium are:

• O-Cresolphalein Complexone Method; The disadvantage of this method is that the volatile nature of 2-Amino-2-Methyl-1-Propanol used in this method makes it necessary to calibrate the method every few hours in a clinical laboratory setting.
• Method of Arsenazo III; This method is stronger, but arsenic in the reagent is a health hazard.

The total amount of Ca ² present in the tissue can be measured using atomic absorption spectroscopy, where the tissue is evaporated and burned. To measure Ca ² concentration or spatial distribution in the cell cytoplasm in vivo , various fluorescent journalists may be used. These include calcium-binding fluorescent dyes that can absorb cells like Fura-2 or a genetically engineered variant of the green fluorescence protein (GFP) called Cameleon.

Corrected calories

Since access to ionized calcium is not always available, corrected calcium may be used instead. To calculate the corrected calcium in mmo/l, one takes total calcium in mmol/L and adds it to ((40 minus serum albumin in g/L) multiplied by 0.2).

src: msb.embopress.org

See also

• Bone seeker

src: rsfs.royalsocietypublishing.org

References

src: jeb.biologists.org

External links

• The USDA national nutritional database, the calcium content of selected foods
• Calcium and vitamin D.

Source of the article : Wikipedia