Cell Structure. Cellular Metabolism. Cell Growth and Division
1) Cell structure
1.1. The cell is the basic element of life, as we understand it. All cells are built up essentially similarly. The cell must have a cell membrane that borders this element of life from the environment. The inner cell world is composed of the cytoplasm, which is the medium for other structures, and organelles. These may vary among different cell types greatly. Organelles of animal cells are different from plant cells. Bacteria also have specific formations. It must be noted that no matter which cell type is studied, genetic material must be in stock. Genetic material, presented by the DNA or RNA bands, carries all basic information about how the cell must be organized (cell structure), what the cell must do (cell metabolism), and how it should proceed (cell growth and division). In the prokaryotic cells, like bacteria, the genetic material is not organized into a distinct organelle. In the eukaryotic cells, the genetic matter is compacted into the cell nucleus. Plants and animals belong to the eukaryotes and thus considered evolutionary superior over the bacteria.
The key features of a living cell are the following:
- movement. Living creatures are able to move
- respiration. This is a chemical reaction, which enables the cells to release energy
- growth. All live organisms grow
- sensitivity. Live creatures detect changes of the environment and react
- reproduction. Live cells reproduce and give their genetic information to the offspring
- nutrition. Cell uptake nutrients to gain energy from the food by respiration. These two features associate into metabolism
1.2 Table. 1. The key differences between viruses, prokaryotic and eukaryotic cells.
1.3. Human cell.
Membrane. Plasma membrane acts as a barrier protecting the inner structure of the cell from the outer environment (other cells, solutions, water, etc.). The plasma membrane is composed of two lipid layers. The basic element in the lipid layers is the phospholipid. The phospholipid is a molecule that contains a head (hydrophilic because of the phosphorous and nitrogen – they carry electrical charge, marked red) and a tail (hydrophobic because of the lipid part – the hydrocarbon chains are not charged).
Fig.1. Phospholipid. Hydrophilic head marked, hydrophobic tail at the right top.
Phospholipids are organized into a layer, the outer surface of which is hydrophilic and the inner surface is hydrophobic. It is thought that phospholipids arrange spontaneously into a lipid membrane (Jeuken, 2014).
Fig. 2. Phospholipid monolayer with a hydrophilic and a hydrophobic surface (Jeuken, 2014).
Two phospholipid monolayers meet with their hydrophobic parts to make a stable bilayer (Jeuken, 2014).
Fig. 3. Phospholipid bilayer with both surfaces hydrophilic (Jeuken, 2014).
The goal of the phospholipid bilayer is to give the cell a stable watery inner environment for other organelles (Ratoia et al., 2014).
Fig. 4. Simplest model of a cell is possible with a phospholipid bilayer model only (Ratoia et al., 2014).
The entire structure of the cell membrane is complex. The phospholipid bilayer is the matrix that has numerous molecules incorporated. These include various channels for transportation of glucose, ions, water; receptors that receive signals and message information inside the cell; receptors for cell-cell recognition; enzymes to support metabolism; molecules that adhere the cell to other cells and complexes that support the cytoskeleton (Jeuken, 2014).
Fig. 5. Cell membrane actual complex structure (Jeuken, 2014).
Cytoplasm. This is the watery environment of the cell. All organelles are concentrated in the cytoplasm within the cell. Thus, most chemical reactions take place here (BBCa, 2014).
Nucleus. All eukaryotes have genetic material stored in the nucleus. This is the largest organell, it controls the cell’s activity by genetic material. The genetic material is presented by the genes, districts of the DNA band that may be activated. As soon as a gene is activated, a certain protein is synthesized and it will modulate the cell function. The nucleus is composed of a membrane made of a phospholipid bilayer perforated by numerous pores (Smith, 2010).
Fig. 6. Animated model of a nucleus with pores erased (Smith, 2010).
The cell nucleus in the interphase contains chromatin, which is the extended form of the chromosomes, and the nucleolus, the structure that produces the RNA.
Mitochondria. Mitochondria are the place where energy is most effectively produced. Certain energy-rich chemicals are burned here. During this process, Oxygen is consumed and water with Carbon Dioxide given off. Mitochondria are composed of a bilayer membrane, the inner layer folds forming cristae. A cell may have one to one thousand of mitochondria. This organelle is rather big, its dimensions are comparable to an average bacterium (Smith, 2010).
Fig. 7. Animated model of a cell with nucleus erased (Smith, 2010).
Endoplasmic Reticulum. This is a system of phospholipid membranes, which is a continuous network within the cell. Usually, steroids and lipids are synthesized here. However, when ribosomes attach to the endoplasmic reticulum, it appears ‘rough’ under the microscope and acts as a transport system for proteins synthesized by the ribosomes.
Centrioles. These are paired cylinders in the cytoplasm occurring around the nucleus. Centrioles have numerous tubules attached to them to align the chromosomes.
Ribosomes. These are very small organelles that synthesize proteins. Ribosomes convert the genetic information (in the form of an RNA band) into a polypeptide (Epicenter, 2014).
Fig. 8. Ribosome synthesizing a polypeptide (Epicenter, 2014).
Lysosome. A spherical sac composed of a phospholipid layer that contains aggressive enzymes inside. Lysosomes break down old organelles and molecules.
Golgi. Also known as the Golgi apparatus, is similar to the endoplasmic reticulum, but more dense. In the Golgi apparatus, proteins, lipids, enzymes become mature and transported within the cell.
1.4 Stem cells.
A stem cell is a predecessor cell. Stem cells are cells that do not experience certain structural pattern yet. Stem cells have their genetic material on the initial level of differentiation. Thus, organelles and biochemical pathways in these cells may undergo further specialization. Specialized cells, on the contrary, have their genetic material switched into a certain pathway already. In these cells, organelles and aezyme cascades follow their functional load. For example, a stem cell may differentiate into a red blood cell or a neuron. The red blood cell is s specialized cell that contains large amounts of haemoglobin within the dumped cell membrane. A neuron is a cell that has long and short tails (axons and dendrites) that carry electrical signals. Red blood cells carry Oxygen in the blood. Neurons transmit information in the neural system. A red blood cell cannot loose haemoglobin to become a neuron, and the latter will not loose its tails to float in the blood. Both are differentiated highly specialized types of cells. The idea of a stem cell is to have a unique source to restore red blood cells, neurons, skin cells, cardiac cells, or any other type if needed. The stem cell may differentiate into any.
2) Cellular metabolism
2.1 Some small molecules can pass through the cell membrane by simple diffusion, but generally the cell membrane is semi-permeable. It means that some chemical pass through the membrane, while other do not. The nature of the membrane is hydrophobic with hydrophilic surface. To overcome this discrepancy, numerous transport systems exist. These include the channels and transporters.
The channels are proteins organized into tubules that create pores perforating the membrane (Brown, 1996).
Fig 9. Membrane channel model (Brown, 1996).
Ions can move through the channels down the concentration gradient. The channels are selective: the diameter of the pore would permit only certain type of ions, according to the size, charge, type, etc. Channels can be activated bye certain signals, such as chemicals or electrical charge (potential).
Transporters bind specific ions or molecules and transport them through the membrane consuming energy. Each transporter can carry only one type of a molecule, capture at one side and give off at the opposite. Transporters act much slower than channels (Brown, 1996).
Fig. 10. Membrane transporter schematic model (Brown, 1996).
Transportation may be active and passive. Active transport means that a cell uses energy to switch on transporters and pump in/out an ion against the concentration gradient. Passive transportation usually occurs in the channels when the ions flow downhill without energy losses.
Larger particles are transported differently. Macromolecules are excreted by exocytosis, and absorbed by endocytosis. In both, vesicles with certain material fuse with the plasma membrane either to excrete waste products or accept molecules (Brown, 1996).
Fig. 11.Endo- and exocytosis mechanism (Brown, 1996).
For example, glucose, the most known nutrient, enters the cell in human by exocytosis. Insulin activates vesicles with glucose to invaginate into the cell and thus uptake sugar from the blood. Waste products are eliminated from the cell by exocytosis.
2.2. In the cell, chemical reactions do not occur in a random mode. On the contrary, all biochemical cascades follow certain logics and strict regulations. Three basic nutrients exist – sugars (chemically known as carbohydrates), fats (lipids) and amino acids. Sugars (of them, glucose is known best) are the most mobile, while fats are most rich in energy, Amino acids under normal conditions rarely serve as the source of energy. However, no matter from which typ of molecule energy is taken, the general scheme is the same.
Under anaerobic conditions glucose is first converted into pyruvate. Pyruvate is an intermediate product in anaerobic glycolysis. Degrading of glucose into pyruvate is accompanied by energy production. This step includes numerous enzymes and the result is the following:
C6H12O6 → 2 CH3COCOOH + 2 H2O + energy
The pyruvate molecules can be stored in the form of lactic acid if no Oxygen available or to proceed and undergo aerobic glycolisys. If the cell stops glucose catabolism at this point, pyruvate is converted into a more stable lactate due to enzyme lactate dehydrohenase:
CH3COCOOH + NADH + 2H+ → CH3CHOHCOOH
Otherwise, called pyruvate decarboxylase will flake carbon dioxide from pyruvate and produce acetalyl-CoA:
CH3COCOOH → CH3CO-CoA + CO2
Acetyl-CoA is the intermediate product that enters the Crebs cycle. This is a series of reactions that occur in the mitochondria. The Crebs cycle consumes Acetyl-Coa and Oxygen to produce energy in the form of ATP and give off Carbon Dioxide:
CH3CO-CoA + ADP + Pi + 2 H2O → CoA-SH + ATP + 2 CO2
Overall, during anaerobic glycolysis, 2 ATPs are gained, while aerobic glycolysis adds more 35 ATPs per glucose molecule.
Other nutrients follow the same scheme – first degradation to Acetyl-Coa without Oxygen in the cytoplasm and then enter the Crebs cycle breathing in the mitochondria.
2.3 The proteins are polypeptides. This fact helps to understand how proteins are made. First, the DNA band is copied. This must be the gene responsible for the given protein. The DNA will not leave the nucleus, thus it sis copied onto and RNA. The RNA band (which is complementary to the maternal DNA) may leave the nucleus through the pore. The messengener RNA now leaves the nucleus and enters the ribosome just between its subunits. The transporter RNA must find the complementary triplet on the mRNA band and thus make an according polypeptide chain.
Fig. 12. Schematic drawing of protein synthesis involving mRNA travelling from the nucleus.
DNA double helix is the basic molecule that carries information. DNA stores sequences that can further be interpreted as polypeptides and thus become enzymes, receptors, hormones, organelles, etc. RNA may serve various goals: mRNA is a copy of maternal DNA to transport information from the nucleus to the ribosomes, tRNA transports amino acids and recognizes which must follow according to the mRNA sequence, ribosomal RNA is a part of the ribosomes to assist mRNA in protein synthesis. The tRNA and rRNA are found in the cytoplasm mostly, while mRNA is also located in the nucleus. Nucleic acids in the nucleus serve the material for genetic material synthesis, while in cytoplasm they assists in protein synthesis and amino acid transportation.
3) Cell growth and division
3.1 Embryonic stem cells have the potential to enter mitosis. They dived and produce new stem cells. However, they can progress into differentiated cells. If a stem cell would differentiate only into a muscle cell or a neuron, it is called unipotent. Some stem cells can give birth to different branches, these are polypotent. The most basic stem cells are thought to differentiate into any kind of cell – totipotent cells. Cells become specialized because some genes are switched off, only genes for certain functions remain switched on (BBCc, 2014). For example, muscle cells will have genes encoding actin and myosin working, while genes form synaptic receptors untouched. On the contrary, the neurons would have genes encoding neural receptors switched on, while myoglobin genes inactive.
3.2 The cell knows when to start division by analysing the information received by its receptors, like from other tissues or surrounding cells. For example, growth factors are released into the blood stream to stimulate receptors on the cell membrane. As soon as the growth receptor is activated, protein kinase act and stimulate the nucleus to start division.
Interphase is the period of the cell’s life span, it cover over 90% of its time. This is the time of the cell activity. During interphase the cell ‘lives’ performing its functions. Interphase starts after the cytokinesis when daughter cells spread and ends with the beginning of mitosis. During interphase the cell functions (muscle cells contract, hepatocytes perform metabolic pathways, stem cells grow), captures nutrients to live, grows, reads its DNA data to synthesize the necessary enzymes, etc. In the eukaryotes, the majority of cells spend their life times in the interphase.
The interphase is divided into Gap1, Synthesis and Gap2. In G1 the cell functions in its usual mode. The cell grows in G1 giving the largest amounts of proteins and functional activity. In S, DNA is replicated. In G2, the cell resumes growth to prepare for mitosis. Some cells may fall asleep and become inactive and thus enter the G0 phase.
3.3 The mother cell in the S phase duplicates its genetic material. The DNA bands double to create duplicated chromosomes. The arms in these chromosomes are identical. This is so because duplication is not a random process but a well organized nucleotide copying. Next, the chromosomes are condensed and aligned. The centromers divide and sister chromosomes move to the opposite poles. Finally, the identical copies appear it the opposite poles. This is why the daughter cells receive identical information (University of Leicester, 2014).
Fog. 13. Cell cycle scheme (University of Leicester, 2014).
3.4 Normally, cells enter the G1 phase and when needed start preparation for division. Most time will be spent for function. Cancer cells are unable to remain in their G1 for a long period, they divide again and again. Cancer cells divide, then consume a lot of energy and nutrients to prepare for the next division, divide again. As soon as division is over, the cancer cells consume energy and enter the next division endless number of times. The metabolic needs of cancer cells are very high, as the Oxygen demands to produce energy. The normal cells do not prepare for their mitosis all their life span, so the energy needs are lower.