Dogs and cats, butterflies and slugs, fish and birds, poison ivy and rosebushes and strawberry plants are all organisms. Organisms are composed of organ systems (digestive, circulatory or immune systems, to cite three examples). These systems are in turn composed of organs, which are composed of tissues which are composed of cells. The cell is where the action is!
The definition of life must include the criterion that whatever lives is capable of viable reproduction, which makes it subject to evolution. This definition does not mention cells, but since all life is composed of cells, they are almost a definition of life, too.
There are two kinds of cells, which have already been mentioned in the geology (!) chapter:
- Eukaryotic contain structures called organelles, many of which are not found in prokaryotic cells. One organelle is the nucleus, which houses the DNA.
- Prokaryotic cells are usually smaller. They do not contain nuclei, so their DNA is floating around in the cytoplasm. They do contain ribosomes to carry out protein synthesis.
Cells come in a huge variety of shapes and functions. But cells are the basis of all living beings and they all have their basic character and components in common, a certain indication of the relatedness of all life. Whether they are nerve cells, muscle cells, skin cells, bone cells or any other kind of cells; whether they come from humans or cats or grasshoppers or oak trees or bacteria, they all are based on common features like a membrane filled with cytoplasm, DNA and RNA for reproduction and the fabrication and use of ATP for energy.
For the moment at least, let us look at the kind of cells we are made of – animal eukaryotic cells.
As we saw in the discussion of water, lipid bilayers, in this case, phospholipid bilayers, can form membranes. All cells are surrounded and protected by such membranes, also called plasma membranes. The membrane contains various proteins which serve to pass substances across the membrane, to identify it or to fulfill other functions. This model of the cell membrane, as a mosaic of elements, is called the fluid mosaic model.
Inside the cell is the cytoplasm, including the cytosol – the jelly-like fluid in which other structures float – and the organelles, such as the nucleus or the mitochondria.
The membrane is capable of forming pockets called vesicles. They can form on the inside around material and then break loose from the cell in order to take away the material stored in them. In the other direction, they may form around external material and carry it into the cell.
The cell nucleus
The nucleus holds the DNA, normally wrapped with other proteins into strings of chromatin. Chromosomes are only constituted when needed to facilitate reproduction. The nucleus is surrounded by its own double membrane. The nucleolus within the nucleus serves to synthesize ribosomes, which in turn are sites for synthesis of proteins. The greater the number of proteins produced by a cell, the larger its nucleolus.
Mitochondria are where the final products of digestion, mainly glucose, are converted into ATP, the body’s “energy currency”, which transports energy to whatever metabolic process may need it. A mitochondrion has both an external and an internal membrane. ATP is produced by processes taking place along the inner membrane. The interior of the mitochondrion is called the matrix. Cellular respiration (energy production) will be discussed in detail in the chapter on anatomy and physiology.
Mitochondria originally were bacteria, prokaryotic cells, which moved into other cells and formed a symbiotic relation with them. So mitochondria (and choroplasts) are prokaryotic bacteria living within eukaryotic cells. As a result, mitochondria have their own DNA, independent of that in the cell nucleus, although they have abandoned or given up much of their original DNA to the nucleus.
Ribosomes are where proteins are built following instructions contained in DNA; the instructions are transmitted to the ribosome in the form of messenger RNA, mRNA, about which more later. Ribosomes consist of two subunits, large and small, both of which are constructed in the nucleolus from ribosomal RNA (~75%) and proteins. More on ribosomes in the section on protein synthesis.
The endomembrane system has been referred to as the “post office” of the cell, as its components produce, package and export certain cell products, such as proteins or lipids. It has several components.
- Endoplasmic reticulum — This is actually an extension of the membrane of the nucleus outwards to form folds. The rough ER is studded with ribosomes, to make proteins. The smooth ER is not and produces lipids.
- Golgi apparatus – This serves to dispatch the various products to their final destinations.
- Lysosomes – These “garbage-collection” modules contain enzymes which can break down molecules and even cells. In the process of phagocytosis, for example, macrophages, a type of white blood cell and part of the immune system, gobble up pathogens and then deliver them to lysosomes for destruction. They do not exist in plant cells.
- Peroxisomes – These also contain enzymes for decomposing various molecules such as alcohol.
“Form follows function”, in biology. Cells which produce large amounts of proteins have voluminous endoplasmic reticulums and those which secrete much have large amounts of Golgi bodies. Those, like muscles, which need great amounts of energy contain many mitochondria.
Proteins of the cytoskeleton support the cell and give it shape. They are of three sorts:
- Microtubules are made of the protein tubulum and provide structure, like scaffolding, for the cell; they resist compression. They also form something like a railroad track for RNA to flow along during protein synthesis; it does not just float loose in the cytoplasm. A family of motor proteins, kinesins, pull themselves along the microtubules similar to the way myosin crawls along actin in muscle cells. (See the section on muscles in the next chapter.)
- Microtubules are arranged around a centrosome, consisting of two mutually perpendicular centrioles, each of which is a cylinder of nine triplets of microtubules.
- The stability of microtubules is modified by the interaction of tubulin with tau proteins, especially in the distal portion of CNS neuron axons, including the brain. Defective tau proteins, which no longer stabilize microtubules properly, are associated with pathologies like Alzheimer’s disease (AD) and Parkinson’s disease.
- Microfilaments are thinner and are made of actin and form chains responsible for muscle contraction with the cooperation of myosins. (See the section on muscles in the next chapter.)
- Intermediate filaments are made of keratin and serve, like microtubules, for maintaining cell shape, but, contrary to microtubules, they resist tension which tries to pull apart the cells.
Kinesins and myosins are called motor proteins, since they use actins and microtubules to facilitate movement within the cell and, in the case of muscles, movement of the organism they compose. Since they move themselves along the support structure of the cell, they are probably evolved forms of the cytoskeleton of bacteria.1)This was only observed in the mid-1990s. Lane (2009), 167. Bacteria can be motile in their way too, since they change by adding on elements at one end and leaving them off at the other, effectively moving and generating force. The evolution of motility was an essential step in the spread of plants and – especially – animals some 250-or-more million years ago.
Plant cells differ from animal cells in that they have cell walls outside the cell membrane, and contain a central vacuole and chloroplasts.
The cell wall is a stiff outer layer which supports and protects the plant. The large central vacuole contains liquid which exerts pressure to maintain the plant’s standing position, just like air in a balloon makes the balloon stiff. The liquid also stores proteins.
The chloroplasts are where photosynthesis takes place. Like mitochondria, they contain their own simple form of DNA as well as ribosomes, because, like mitochondria, they originated as bacteria which moved into another cell, felt at home and stayed.
The structure of chloroplasts and their use of energy from sunlight to convert CO2 and H2O into sugar will be presented in the next chapter.
Viruses are not cells because they do not contain all the good things described above. A virus consists of a protective protein coat called a capsid, which contains the viral DNA or RNA (but not both). It may also be surrounded by an envelope which closely resembles a cell membrane – because the virus has stolen it from a cell and adapted it to its own nefarious ends, a virus in cell’s clothing. Viruses have no ribosomes or other organelles to synthesize proteins, so they are dependent for reproduction on normal cells which they invade, substituting their own genetic material for that of the cell and then letting the cell do the job of protein synthesis – but using the virus’s recipe. Real Trojan horses!
Electrochemical considerations – the action potential
Cells are surrounded by a membrane composed of a phospholipid bilayer such that the membrane surfaces are hydrophilic and the interior hydrophobic. So no charged ion, hydrophilic molecule or very large molecule can traverse the membrane. But the cell needs to receive nutriments, expel waste, receive hormonal messages, and so on, so the membrane must not be completely impermeable.
In order to allow the necessary passage of chemicals, the cell membrane is studded with proteins which control the passage of such objects.
- Ion channels are passageways through the cell wall which allow ions to pass naturally in a direction tending to equalize their electrochemical or concentration gradients inside and outside the cell. Such channels may be ion-specific (for instance, allowing only K+ ions to pass), charge-specific (allowing either negative or positive ions, but not both) or size-specific. Some ion channels are leakage channels, opening randomly.
- Gated ion channels allow ions through the channel on occurrence of some event. This might be the arrival, for instance, of an electric potential (voltage-gated ion channel). Voltage-gated channels are essential to the formation of action potentials.
- Ion pumps, especially the sodium-potassium (Na-K) or ATPase pump and the calcium pump, which run all the time in most animal cells, use energy from ATP to pump ions across the membrane against their concentration or electrostatic gradient.
The concentration of chemical substances inside and outside the cell are often not the same and this is crucial to their functioning.
It all starts with a pump, the sodium-potassium (Na-K) pump. This object is powered by ATP to flip back and forth between two states. In one state, it pumps Na+ ions out of the cell; in the other, it pumps K+ ions into the cell. Soon, a concentration gradient is established for each ion, tending to pull the Na+ back into the cell and to push the K+ out.
The pump does more than that, because for every two K+ ion it pulls into the cell, it pushes three Na+ out. The result is the establishment an electrostatic potential across the cell membrane. Because the negative charges inside and the positive ones outside are attracted to each other, they tend to be concentrated close to the membrane surfaces, making the membrane a kind of capacitance.
So the result of the pump is triple:
- a Na+ concentration gradient tending to pull Na+ back into the cell;
- a K+ concentration gradient tending to push K+ out;
- an electric potential across the membrane due to the greater number of positive ions outside the cell.
However, ion channels exist for K+ and these tend to leak K+. The K+ wants to follow its concentration gradient out of the cell, but it is attracted by the relative negative potential of the interior. Eventually, an equilibrium state is reached where the outward pressure due to the concentration gradient balances the input pressure due to the electrostatic gradient and the interior has a potential of -80 mv (relative to the exterior). The cell is said to be polarized. In this state, opening an ion channel would allow positive ions to flow into the cell, depolarizing it.
If we start over and do the same thing, but open a Na+ channel, Na+ rushes into the cell, bringing up the cell’s potential until equilibrium is reached, this time at a potential of +62 mV. So each ion has an equilibrium potential which is a function of the concentration difference across the cell membrane. In general, the cell’s membrane potential is a function of the concentration differences of all the ions inside and outside the cell. If there is no difference in the concentrations, no migration happens and no potential difference arises. Likewise, the higher the concentration difference, the higher the membrane potential. It takes only a very small change in ionic concentration to bring about a much larger change in membrane potential.
Since both K+ and Na+ leakage channels exist, but the Na+ channels leak much more slowly than the K+, the result is to increase somewhat the Na+ concentration within the cell and hence raise the potential a little to around -70 mv.2)Some authors say -65 mv.
Enter the third type of channel, the gated ion channel. This may be of various types depending on the ion allowed to pass and on the circumstances which gate it, which initiates its functioning. The latter property may be of different tiypes:
- mechanically-gated, subject to pressure, for instance, on the surface of the skin or on auditory cilla in the ear;
- voltage-gated, i.e., to charged ions;
- sensitive to molecules (taste, smell, neurons), called ligand-gated;
- photosensitive (retina).
The action potential
Consider a nerve or muscle cell at rest, with a membrane potential of around -70 mV, so the cell is polarized. Suppose something happens which causes the membrane potential to increase (i.e., to become less negative). This could be due to entry of ions through a ligand-gated channel (as in neurons) or a mechanically-gated channel (as in a somatosensory channel) or simply due to K+ leakage (as in a pacemaker cell of the heart). If the increase is small, nothing happens. But if it is sufficient to bring the potential up to the threshold value of -55 mV, then a voltage-gated Na+ channel opens, allowing Na+ to come rushing into the cell, quickly raising the potential to a positive value and depolarizing the cell. When the potential reaches 30 mV, the voltage-gated Na+ channels close but voltage-gated K+ channels open and K+ starts rushing out of the cell, bringing the potential back down (positive charges are leaving the cell) and repolarizing it.
In muscles, release of the neurotransmitter acetylcholine (ACh) by motor nerves opens ligand-gated channels which allow the cell to depolarize to the threshold for voltage-gated sodium channels, which in turn cause an action potential which eventually opens another channel which allows Ca++ into the cell. The Ca++ binds to troponin in the muscle fiber and unlocks it, so that myosin heads can “walk” along the thin filament and contract the muscle.
The heart is special in that it initiates its own action potentials in a repeated, periodic way. The cycle is initiated not by a ligand-gated channel, but is due mostly to leakage channels which allow some Na+ to enter the heart, gradually raising the membrane potential from about -60 mV to -40 mV. This is the threshold value for a gated channel to open, allowing depolarization of the cell. However, because of the leakage, only slow Na+–Ca++ channels are opened, which explains the relatively slow rise of the potential. After about 100-150 milliseconds, the Na+–Ca++ channels close and K+ channels open. As K+ rushes out of the cell, the potential redescends to a bit under its initial resting value. Then the K+ channels also close and the cycle starts over. So the heart beats all by itself due to the membrane potential of the cardiac muscle cells, the slow leakage of Na+ and the voltage-gated Na+–Ca++ and K+ channels.
Continue reading about DNA expression and protein synthesis.
Notes [ + ]
|1.||↑||This was only observed in the mid-1990s. Lane (2009), 167.|
|2.||↑||Some authors say -65 mv.|