THE CELL
Scientific investigations go hand in hand with technological advances because they provide new opportunities to scientists in their investigations. The single most important technological advance was the invention of the microscope which enabled minute details of the cells to be examined.
The ancient microscope, forerunner of the modern microscope, was first discovered by Anton Von Leeuwenhook in about the year 1600. He used it to observe microorganisms. A new microscope was developed by Robert Hooke in 1665. He formed slides from cork and investigated them under microscope. He observed that each section of cork was made up of 'small identical empty rooms'. He first used the term cell to describe these structures.
In the following years, informations about cell biology was improved by the investigation of scientists ranging from Mirbel-1802, Oken-1805, Lamarch-1809, Dutrochet-1824 to Turpin-1826. The exact form of cell theory was determined by Schlieden and Schwann in 1838.
The cell theory states two basic terms:
1- All organisms are composed of cells
There may be structural and functional differences from cell to cell. However, basically they are all made up similar components such as the possession of a nucleus, mitochondria, etc. Focusing on the differences between plant and animal cells, plant cells contain chloroplast to carry out photosynthesis but animal cells do not. A procaryotic bacterium has a mesosome, but eucaryotic cells, lack this structure. On the other hand, viruses can not be structurally considered as having a cell form.
2-All cells originate from identical parent cells by means of cells division.
A parent cell divides to form to new daughter cells. These cells mature and divide again into new daughter cells. This means that genetic continuity perpetuates from cell to cell by means of cell division.
The structure of all cells is similar but there are some differences from kingdom to kingdom. Every cell has
- A plasma membrane to protect and limit the cytoplasm
- A cytoplasm to carry out metabolic activities
- Hereditary material to direct metabolic activity and to provide genetic continuity.
Cells are categorized basically into two groups as procaryotic and eucaryotic.
A. PROCARYOTIC CELL
Procaryotic cells have no true nucleus and the hereditary material is free in the cytoplasm. They lack any membranous organelle. Only the Kingdom Monera is made up of procaryotic organisms. Bacteria are typical examples of this kind of cell.
B. EUCARYOTIC CELL
Eucaryotic cells confine their hereditary material within a membrane in the cytoplasm and possess a true nucleus. Most organisms except Monerans, have eucaryotic cells. The eucaryotic cell has the following compartments when it is observed under the light microscope.
1. Plasma membrane
2. Cytoplasm
3. Nucleus
A cell does not have a fixed shape. It differs according to its function and the tissue, which includes it. In general, animal cells are oval and plant cells are rectangular because of their cell wall. Cells are microscopic therefore they cannot usually be observed by naked eye. The exceptions to this are some algae and unfertilized frog and bird eggs as they are macroscopic and therefore can be seen by the naked eye.
The pigment carried inside it determines the color of the cell. For instance, red blood cells are red in color because of their hemoglobin. Leaf cells are green because of their chlorophyll and fat cells are yellow in color.
1. The Plasma Membrane
The plasma membrane is a thin layer that surrounds the entire cell and encloses the cell organelles located within the cytoplasm. In addition, the membrane controls the passage of material from the exterior to the interior and vice versa. It is selectively permeable, namely it allows the exchange of some materials while excluding others.
The function of the plasma membrane can be summarized as follows:
- It regulates material exchange.
-It detects chemical messengers arriving at the cell surface.
-It links cells together by junctions.
-It determines the shape of cells.
-It protects the cells to some extent against external conditions.
Structure of the Plasma Membrane
The first investigations in to the cell membrane were made by Gorter, Grandel and Ghost. The first model, known as the unit membrane model was proposed in 1935 by British scientists Danielli and Davson. According to this model, the plasma membrane is a lipid bilayer composed of two lipid layers and one of protein. The model was however advanced by Singer and Nicolson and they proposed the fluid mosaic model.
The fluid mosaic model differs from the unit membrane model in which description of the active transport whereas the unit membrane model is stable.
The fluid mosaic model is the most famous and valid model put forward for plasma membrane. This model claims that the membrane is a lipid bilayer composed of the two lipid layers. In constrast to the other models, claims that proteins are distributed along the membrane and embedded in it. Additionally, some proteins move freely within the plasma membrane because the lipid bilayer is fluid.
The thickness of the plasma membrane is 75-100 A (angstrom). Proteins are abundant (60%), lipid molecules constitute 40% of the plasma membrane whereas sugar molecules make up only a small percentage of the membrane.
The hydrophobic, hydrophilic nature of lipid molecules makes them ideal for their function in the plasma membrane. The hydrophilic part is nonpolar and forms the tail of lipid. The heads have close connections between them and extend outwards away from the centre of the plasma membrane. There are tiny holes called pores which are inside the protein molecules and these open into the cytoplasm. Lipids and proteins are attached to glucose chains to form glycolipids and glycoproteins. These glycolipids and glycoproteins are structurally similar and serve as receptor molecules for hormones.
Transport Across The Plasma Membrane
a. Diffusion
The movement of molecules from high concentrations to lower concentrations is called diffusion. It is a passive form of transport because there is no energy required for movement. Molecules move due to the gradient difference in one area to another area. Diffusion ceases when the concentrations of molecules is equal in all areas.
Diffusion in cells is possible for only water soluble molecules in solid, gas or liquid state.
The rate of diffusion is affected by the number of pores in the plasma membrane. A high number of pores provides rapid diffusion for molecules through the plasma membrane. Without so many pores, the diffusion rate slows down. Another factor which affects the diffusion rate is the size of molecules. Small molecules can easily diffuse through the plasma membrane where as big molecules can not. Any molecule bigger than a glucose molecule can not diffuse through the plasma membrane. The temperature of the environment also affects the movement of the molecules. It increases the kinetic energy of molecules. Fast moving molecules of suitable size easily diffuse in or out of cells. An additional factor which helps the rate of the diffusion is pressure. For instance, high blood pressure in the capillaries forces food molecules to diffuse into the tissues.
The sides of the pores are electrically charged. One side of the pore is positively charged and other side is negatively charged. When an ion, either (+) or (-) charged enters a pore, it is attracted by the ions within it. Neutral molecules diffuse more rapidly than the charged ions as they are not attracted to either side of the pore.
Some molecules such as ethers do not use pores to enter cells, they are lipid soluble molecules and dissolve directly in the plasma membrane and than in to the cell.
b. Osmosis
In this process, water molecules move from an environment where the water concentration is high, to a viscous environment where the concentration of water is low. As the density of the solution decreases, the water concentration increases. Therefore high amount of water is present and less material. Osmosis is the diffusion of water across a semi permeable membrane. The human body always needs water to maintain homeostasis. In fact, 75% of the human body is made of water.
There are two extreme situations in osmosis; plasmolysis and deplasmolysis.
Plasmolysis: this occurs when a cell is put in a very viscous environment. There is more water in the cytoplasm and solution is less concentrated than that of the external environment. Water moves from the cytoplasm across the plasma membrane to the outside. The volume of cell is reduced. For example, if a cell is put in sea water it will shrink because sea water is saltier and is more concentrated than the cytoplasm of cell. A solution in which a cell plasmolysis is termed hypertonic.
Deplasmolysis: a cell takes in water when it is put in a hypotonic solution. The volume of the cell increases and exerts a pressure on the plasma membrane. In animal cells the membrane ruptures but in plant cells, the plasma membrane is supported by the cell wall and does not burst. The pressure of the cell contents on the plasma membrane in plant cells is called turgor pressure.
The advantages of turgor pressure for plants are as follows;
- It gives shape to the plant. It provides support for the meristematic tissues.
- Stomata are opened and closed by the effects of turgor pressure.
- Irritation of some plants is provided by turgor pressure.
c. Active Transport
The transportation of certain ions is achieved by a special system called active transport. In this system, the direction of the flow is the reverse of that of diffusion. Molecules are transported from an area of low concentration to a high concentration. This process requires energy and carrier enzymes. Therefore there can be no equilibrium in this process.
In active transport, a carrier enzyme binds to the molecule. It changes the shape, rotates to the other side and releases the molecule on the other side of the membrane. During these activities energy is consumed.
The Sodium-Potassium pump (Na+-K+) is a typical example of active transport. Sodium (Na) ions are forced across the membrane in to the cell where the concentration of sodium is already very high. Simultaneously, potassium ions are pumped out of the cell by ATPase enzyme.
d. Phagocytosis
Phagocytosis is a process where by large molecules which can not pass through the plasma membrane are transport into the cell. Large molecules are transported into the cell by following mechanism. The cell first senses the presence of a molecule outside using receptor molecules. Then a pseudopod and a gulf are formed on the plasma membrane. A large food molecule is engulfed and a food vacuole is formed in the cytoplasm. This food vacuole combines with lysosomes secreted from golgi bodies. The vacuole contents are hydrolyzed by the digestive enzymes of the lysosomes. The useful materials diffuse into the cytoplasm and the wastes are removed from the cell by exocytosis.
e. Exocytosis
This process is similar to phagocytosis. The synthesized molecules accumulate in golgi bodies and are packaged into a sac. This sac is then pushed through the cytoplasm. When it reaches the plasma membrane it fuses with the structure of the membrane and releases its contents to the exterior.
2. Cytoplasm
Cell Organelles
In unicellular organisms, metabolic activities such as reproduction and digestion are performed by the organelles. A unicellular organism is well organized and self-sufficient. Multicellular, complex organisms have essentially the same system but the organ or tissue performs a particular function and the component cells use their organelles for more specific purposes than those of unicellular organisms.
a. Mitochondria
The main function of mitochondria is to provide energy for the cell. Ingested foodstuffs are broken into their components to release energy that the cell can use. The foodstuffs contain energy in the form of chemical bond energy that may be converted into ATP energy by oxidative phosphorylation in mitochondria. Both plant and animal cells contain mitochondria in their cytoplasm. In cells where energy consumption is high, mitochondria are abundant. For example, a liver cell contains about 1200 mitochondria. They are also abundant in muscle and nerve cells where energy consumption is high. A mitochondrion possess its own hereditary material and is capable of division.
C6H12O6+6O2 --> 6CO2+6H2O
Each mitochondrion is surrounded by two membranes; an inner and an outer and it also has two compartments; the matrix and cristae. The matrix is the fluid part of the mitochondrion in which the hereditary material (DNA and RNA) and ribosomes are found. The inner membrane is folded into the matrix and forms cristae that carries ETS (electron transport system) enzymes.
b. Ribosomes
Ribosomes are the most essential organelles in cells. They play an important role in protein synthesis by translating the information on into protein. If a cell has hereditary material and ribosomes, it can synthesize cell components itself.
Ribosomes are the smallest, nonmembraneous cell organelles. They are made up of ribosomalRNA (rRNA) and proteins. Ribosomes may be located on endoplasmic reticulum, free in the cytoplasm, in the outer membrane of the nucleus, in the nucleus, in mitochondria and in chloroplasts.
There are structurally two types of ribosomes. Eucaryotic ribosomes are bigger than prokaryotic ribosomes found in prokaryotic cells, mitochondria and chloroplasts. In prokaryotic cells, the ribosomes come together and form groups called polysomes. They are different in structure but their function is the same in every cell and they synthesize proteins as do their eucaryotic counterparts.
c. Endoplasmic Reticulum (ER)
Endoplasmic Reticulum is formed from the furrowing or folding of the plasma membrane into the cytoplasm in young cells. It is highly circulate materials within the cell. It constitutes most of the cytoplasm and serves as the skeleton of cell. There are two types of Endoplasmic Reticulum and a simple classification system is used to distinguish them. If the ER has ribosomes attached to it, it is termed Rough endoplasmic reticulum. If there are no ribosomes present it is as Smooth endoplasmic reticulum. sER is more tubular in shape. But rER is made up of flattened sacs of membranes. The smooth ER forms golgi bodies and synthesizes lipid molecules.
ER has many functions. It serves as the circulatory system of the cell for example, in which material is transported between the cytoplasm and the intercellular matrix by diffusion and active transport.
Rough ER has ribosomes on it to synthesize exportable proteins, after which they are exported from the cell.
During cell division the ER is degraded so as to provide free movement for the chromosomes in the cytoplasm.
d. Golgi complex
Golgi bodies package the material which is secreted from the cell. Secretory cells such as goblet cells in the trachea contain many golgi bodies because they synthesize carbohydrates in golgi cisternae and combine them with the proteins to form glycoproteins. These are then secreted from the cell.
In structure, they are flattened sacs coming from endoplasmic reticulum. Initially, an empty vesicle breaks out from ER. It flattens into the golgi complex and forms a layer of it. The chemical substances such as hormones or enzymes formed on ER begin to accumulate at the golgi bodies and a secretory vesicla is broken away from the golgi complex and is secreted from the cell.
In plant cells, the cell plate is formed by the secretions of golgi bodies during cytokinesis, otherwise known as cytoplasmic division.
e. Lysosomes
Lysosomes are the vesicles formed by golgi bodies. Digestive enzymes accumulate at a golgi body which packages them into a vesicle, so forming a lysosome. Their digestive enzymes can destroy foreign substances. For example, bacteria engulfed by human white blood cells (WBC). This process is called phagocytosis and lysosomes are abundant in these cells. When a bacterium enters the cell, it is engulfed into a sac, forming a food vacuole, and the bacterial contents are digested by lysosomal enzymes.
Lysosomes are also responsible for the degregation of old and damaged cells. Lysosomes are commonly known as suicide capsules for the reason. This action is called autolysis. Furthermore, the fast decomposition of dead organisms is achieved by the activities of lysosomes.
f. Vacuole
Vacuoles are organelles of both plant and animal cells. They are sacs filled with fluid materials. In plant cells, they contain salt, organic molecules and metabolic wastes and serve as a storage tank of cells. At the same time, the accumulation of anthicyanin in the vacuoles gives color to the cells. When the plant cell is in turgor, it absorbs the water from the cytoplasm to keep homeostasis in the cell. In young plant cells, they are greater in number but are smaller in size. Conversely in old plant cells, their size is but there are fewer of them. This is accounted for by the accumulation of metabolic wates and the resulting enlargement of the vacuoles which then fuse together. Big vacuoles are thus formed. In old plant cell, vacuoles may constitute most of the cytoplasm so confining cytoplasmic contents to a small area. The vacuoles of animal cells are always small because they excrete waste instead of accumulating it.
In protests, contractile vacuoles accumulate metabolic wastes and excess water until the vacuole can hold no more. The contents are then excreted from cell by means of contraction and relaxation.
g. Plastids
Plastids are present only in plant cells and give colour to them. The most well known plastids are chloroplasts, chloroplasts and leucoplasts.
The Chloroplast is a prominent plastid in plants. It gives green colour to the plant and has an important function in photosynthesis. Its structure resembles that of a mitochondrion. It is surrounded by a double membrane and has DNA in its matrix. It converts radiant energy from sunlight into chemical bond energy.
6CO2+6H2O->C6H12O6+6O2
The fluid part of the chloroplast is called the stroma where the enzymatic reactions of photosynthesis take place. The grana is made up of a pile granums with chlorophylls located between them. The chlorophylls are a source of electrons (e). they absorb sunlight and convert it to chemical bond energy. In the cytoplasm of prokaryotes, chlorophylls are found in the free form. The green pigment in plants is produced by Mg ions in the chlorophylls.
Chloroplasts as in the case with mitochondria behave as prokaryotic cells do. They can divide into two by binary fission and therefore have their own hereditary information.
Chromoplasts give orange color to fruits and flowers. For instance, a carrot contains carotene, tomato contains leucopine, etc. a chromoplast does not carry out photosynthesis, it transmits absorbed light to the chlorophylls.
Leucoplasts are colourless plastids. They do not impart color to plants. Their function is the storage of the products of photosynthesis
It is the most prominent part of the cell as it performs the metabolic activity of it. There is at least one nucleus in almost all cells. A notable exception is mature red blood cells which do not have any nucleus. In contrast, striated muscle cells are multinucleated. The nucleus has basically two main functions:
The synthesis of molecules in cells. All the all the genetic information for the regulation of metabolic activity is located in DNA in the nucleus.
-It stores the hereditary material and transmits characteristics to the next generation during cell division so maintaining genetic continuity.
The nucleus is generally located in the middle of cell and is isolated from the cytoplasm by a double layered nuclear membrane which is the blind end of Smooth Ers. Some of the ribosomes are located on the nuclear membrane. Nuclear pores are distributed along it. These pores are larger then the pores on the plasma membrane so that RNA can pass through. The nucleoplasm is the fluid part of the nucleus and is filled with Chromatins, nucleotides and related enzymes.
There is least one area of condensed granular RNA in the nucleus called Nucleus where manufacture of ribosomes takes place.
DNA is a sequence of nucleic acids and stays in the nucleus in the form of chromatin. More information about DNA can be found in chapter 3 Organic Molecules.
DNA combines with histone proteins to form chromatins which are slightly folded linear sequences of DNA. The m-RNA is synthesized from DNA which is in the form of chromatin. It coils into short, thick chromosomes during cell division.
Normal cells contain two sets of chromosomes and are therefore called diploid cells. The somatic cells of animals are diploid but the reproductive cells, namely male or female gametes, otherwise referred to as the egg and sperm have on set of chromosomes and are haploid or monoploid.
The diploid number of chromosomes is fixed for all members of the species whereas it differs from one species to another. However, there may be different species that have the same chromosomes number.
tomato cell 16 chromosomes
Drosophila 8 chromosomes
wheat cell 42 chromosomes
Ligustrum vulgare 46 chromosomes
moly fish cell 46 chromosomes
human cell 46 chromosomes
For example, a human cell, a moly fish cell and a Ligustrum cell have 46 chromosomes, two of them is an animal and the other is a plant. The number of chromosomes is not related to the complexity of organisms. The important point is the information located on the chromosomes.
C. CELL CYCLE
At beginning of its life, every multicellular organism is comprised of a only one cell called a zygote. Cells divide many times during the development of an organism. When a tissue is damaged it is repaired by cell division. The life cycle of a cell is as follows: it begins with the division of the mother cell into new immature and divide again into other cells by the process of mitosis. The rate of cell division varies according to the life cycle of an organism.
Why does a cell divide?
The structure of a cell is spherical with a finite size and volume if the cell becomes too large, it must divide. During of the development of the cell, its volume increases three fold. The plasma membrane can not supply enough nutrients to the cell if it becomes too large. Thus, cell has to divide to survive. Nerve cells however, do not divide rapidly because their volume of cytoplasm is less so the surface area of the membrane is sufficient.
When the DNA of a developed cell replicates, two nuclei are formed. This process is called caryokinesis. The cytoplasm is then divided into two. This process is called cytokines.
The life cycle of a cell has the following stages:
1-INTERPHASE
1- G1 phase: This stage occurs between the end
of mitosis and the start of the S phase. Most of the metabolic
activity occurs in this stage and towards the S phase, the synthesis
activity of DNA replication enzymes increases. This stage of mitosis
takes the longest period of time.
2- S phase: The major significance of this stage is
DNA replication. The DNA is replicated by DNA polymerase
enzymes. As a result two sets of chromatin are present in the cell.
3- G2 phase: This marks is the final preparation
stage before cell division. The rate of protein synthesis increases
in the G2 phase. Centrioles replicate and spindle fibers begin to form.
2-MITOSIS
4- PROPHASE: This is the first
visible phase of mitosis. In animal cells the first
visible event is the division of the centrioles
(centrosomes). After dividing, each pair moves
to opposite poles of the cell and forms the spindle
fibers on which the chromosomes move.
The replicated chromatin materials
shorten and thicken, thus forming the visible
chromosomes. A chromosome is composed of
two CHROMATIDS and a CENTROMERE which
holds the chromatids in place. The nuclear membrane
and nucleoli disappear in the cytoplasm at this point.
5- METAPHASE: In this stage the
chromosomes are arranged on the equatorial plane
and the centromeres split. Now the chromosomes are
ready to separate.
6- ANAPHASE: Right after the
separation of the centromeres, the sister chromatids
repel each other towards opposite poles. Possibly
these chromatids are pulled by the shortening of
the spindle fibers. Anaphase ends with the arrival
of the chromosomes at the poles. The chromosomes
then start to uncoil to form long strands of chromatin.
7- TELOPHASE: The final stage
of mitosis is telophase and is recognized by the
reformation of daughter nuclei and the pinching
of membrane. The chromosomes completely
unwind and form the chromatin material. The
spindle fibers disappear, then the nuclear membrane
and nucleoli are reformed.
At the same time cytokinesis begins
and after cytokines two identical cells are formed.
In plant cells the cell wall is too
hard and strong to bend and to form a furrow. For this reason, in plant cells, the cell plate formed by the order of golgi bodies in the equator of cell.
The two new daughter cells are structurally identical because both of the same DNA structure as the sister chromatids were segregated during the anaphase stage. The cytoplasm in plant cells is unequally distributed between the daughter cells.
Mitotic cell division provides the following advantages for organisms:
1. It allows asexual reproduction of organisms. It therefore stabilizes the characteristics of organisms reproduced by asexual reproduction.
2. In multicellular organisms, it provides development and growth.
3. It enables the damaged structures of organisms to be reformed.
4. It provides the information of gametes in some organisms e.g; male honey bee.