12 Aug

Cell

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Content

• Why is cell the fundamental unit of life?
• Shapes of Cells
• Two broad categories of cells – Eukaryotes and Prokaryotes
• Structural Organizations of Cell
• Some things to note
• Important Scientists

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CELL: FUNDAMENTAL UNIT OF LIFE

1) WHY IS CELL THE FUNDAMENTAL UNIT OF LIFE?

• Cell is the fundamental structural and functional unit of living organisms i.e. it is the smallest living unit of an organism. Thus, it is also the basic fundamental unit of life.
• Every cell is capable of doing some basic things like respiration, obtaining nutrition, and clearing the waste material, or forming new proteins.

2) SHAPES OF CELLS 

• With the discovery of electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles.
• The shapes and sizes of the cells are related to the specific function they perform. Some cells like Amoeba have changing shapes. Cells shape can be very peculiar. For example, nerve cells have a typical shape.
• Some organism can have cells of different types
  • For example, humans have different types of cells

3) TWO BROAD CATEGORIES OF CELLS – EUKARYOTES AND PROKARYOTES

• The difference between the structures of Prokaryotes and Eukaryotes is so great that it is considered to be the most important distinction among group of organisms.
  1. Eukaryotic
     • They have membrane bound organelles such as nucleus.
     • They are advanced cells found in plants and animals
     • They are usually found in multi-cellular animals. But there are a lot of unicellular Eukaryotes too.
  2. Prokaryotic
     • They don’t have nucleus or other well-defined organelles. They do have genetic material, but it is not contained within a nucleus.
     • They are found in primitive cells like that of bacteria and Archaea.
     • Prokaryotic cells are always unicellular such as bacteria. But there is some evidence that some bacterial species can aggregate together and divide labor so that the “colony” is working more efficiently. This is the characteristic of a multi-cellular organisms, but there is still a lot of resistance to the idea of calling these prokaryotes multi-cellular.
     • Prokaryotes are usually much smaller than Eukaryotes.

4) STRUCTURAL ORGANIZATIONS OF CELL

• Every living cellhasthe capacitytoperform certain basic functions that are characteristics of all living forms. There is a division of labour seen within a single cell. The cell components called the cell organelles have specialized functions.
• These functions include making new material in the cell, clearing up the waste material from the cell and so on.
• These organelles together constitute the basic unit called the cell. It is interesting to note that all cells are found to have the same organelles, no matter what their function is or what organism they are found in.
• Three Broad components of the cell
  • Plasma Membrane or Cell Membrane
  • Nucleus
  • Cytoplasm

A) PLASMA MEMBRANE OR CELL MEMBRANE

• Cell membrane/Plasma membrane is the outermost covering ofthe cell that separates the contents of the cell from its external environment.
• It allows or permits the entry or exit of some materials in and out of the cells. It also prevents movement of some other materials. The cell membrane is therefore called selectively permeable membrane.
• Cell membrane is not a solid structure. Cell membranes are also described as lipid bilayers.
• There are two layers of phospholipids with protein embedded in the layers.
• How does diffusion of substance take place into the cell?
  • Diffusion
  • Continuous movement of a substance from a region of high concentration to a region where its concentration is low.
  • E.g.: O2 enter the cell by the process of diffusion when the level of concentration of O2 inside the cell decreases.
  • CO2 moves out of the cell when the level of concentration of CO2 inside the cell increases.
  • Water also obeys the law of diffusion: Osmosis – the movement of water molecules through such selectively permeable membrane. It is basically movement of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
• What happens when we put an animal cell or a plant cell into a solution of sugar or salt in water?
  • Hypotonic Solution: If the medium has higher water concentration than the cell, meaning, outside cell is very dilute, the cell will gain water.
  • Isotonic Solution: Same concentration, no movement.
  • Hypertonic Solution: The medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such solution is called a hypertonic solution.
• Thus, osmosis is a special case of diffusion through a selectively permeable membrane.
  • Unicellular freshwater organisms and most cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.
  • Only living cells, and not dead cells, are able to absorb water by osmosis.
  • The flexibility of cell membrane also enables the cell to engulf in food and other material from its external environment. Such process is known as Endocytosis. Amoeba acquires its food through such processes.
• Cell Wall
  • Plant cells in addition to plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.
  • Plasmolysis: When a living plant cell loses water through osmosis there is a shrinkage or contraction of the contents of the cell away from the cell wall. The phenomenon is known as plasmolysis.
  • Plant cells can withstand much greater changes in the surrounding medium than animal cells. Why?Animal cells never have a cell wall.

B) NUCLEUS

• Nucleus acts as brain/control centre of the cell. It stores DNA, the genetic information that tells a cell how to live its life. It controls basic activities like eating, movement, reproduction, basic characteristics etc.
• Sometimes there are more than one nucleus in certain cells. These are called multi-nucleated cells.
• Almost all human cells have one nucleus with identical DNA. Some human cells have no nuclei at all e.g. the Red Blood Cells. Some cells like liver cells and some muscle cells, are multinucleated, meaning they have multiple nuclei.
  • Multinucleated cells are more efficient as they have two control centres. For instance – Liver cells – Hepatocytes do a lot of jobs. They make protein for digestion, help remove harmful stuff from your blood, produce enzymes to digest fats and carbohydrates and store carbohydrate energy for the body. Having two nuclei is like having two sets of blueprints, so the cells can build two proteins at the same time.
• Nucleoid: In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called nucleoid.
• Structure
  • Nucleus has a double layered covering called the nuclear membrane.
  • The nuclear membrane has pores which allow the transfer of material (such as RNA and protein) from inside the nucleus to its outside, i.e. to the cytoplasm.
• The nucleus contains
  • Chromosomes
    • Chromosomes are composed of DNA and proteins.
    • It contains information for inheritance of features from parents to next generation in the form of DNA (Deoxyribonucleic Acid) molecules.
      • DNA molecules contain the information necessary for constructing and organizing cells.
      • Functional segments of DNA are called genes.
      • Chromatin Material
        1. When the cell is in a resting state (not dividing) there is something called chromatin in the nucleus. It is made up of DNA, RNA, and nuclear protein.
        2. Chromatin material is visible as entangled mass    of    thread   like structures.
        3. Whenever the cell is about to divide the chromatin material gets organized into chromosome (the rod shape structure)
• Nucleolus
  • It looks like nucleus inside a nucleus. It is made up of RNA and protein.
  • It is the structure where ribosomes are made.
  • After ribosomes leave the nucleus, they will have the important job of synthesizing proteins.

C) CYTOPLASM

• The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialized cell organelles. Each of these cell organelles perform a specified function for the cell.

A. Cell organelles are enclosed by membranes to keep its own content separate from external environment.

B. In Prokaryotes, besides the absence of a defined nuclear region, the membrane-bound cell organelles are also absent. On the other hand, the eukaryotic cells have nuclear membrane as well as membrane-enclosed organelles.

C. Significance of membranes

• Significance of membranes can be illustrated with the example of viruses.
• Viruses lack any membranes and hence don’t show characteristics of life until they enter a living body and use its machinery to multiply.
• Important Cell Organelles

A. Endoplasmic Reticulum (ER)

• • ER functions both as a passageway for intercellular transport and as a manufacturing surface.
  • It is a large network of membrane bound tubes and sheets to transport material. It looks like long tubules or round and oblong bags (vesicles). The ER membrane is similar in structure to plasma membrane.
  • Some cells like Prokaryotes or RBCs don’t have ER of any kind.
  • Cells that synthesize and release a lot of proteins would need a large amount of ER. Cells from Pancreas or liver will have large number of ER structures.
  • Two types of ER
    • Rough Endoplasmic Reticulum (RER)
      • RER looks rough under a microscope because it has particles called ribosomes attached to its surface. Ribosomes which are present in all active cells, are the sites of protein manufacture.
      • RER looks like sheets or disks of bumpy membranes while smooth ER looks more like tubes.
      • The manufactured proteins are then sent to various places in the cell depending on need, using the ER.
      • The RER are also attached to nuclear envelope that surrounds the nucleus. This attachment allows for movement of molecules through membranes.
    • Smooth Endoplasmic Reticulum (SER)
      • SER help in manufacturing of fat molecules, or lipids, important for cell function.
      • They are mostly shaped like tubes.
      • Some of these lipids help in building of cell membrane. This process of known as membrane biogenesis.
      • Some other proteins and lipids function as enzymes and hormones.
  • Although the ER varies greatly in appearance in different cells, it always forms a network system. Thus, one function of ER is to serve as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
  • The ER also functions as a cytoplasmic framework providing a surface for some of the bio-chemical activities of the cell.
    • e.g.: In liver cells of vertebrates, SER plays a crucial role in detoxifying poisons and drugs

B. Ribosomes (not organelles)

• Note: Ribosomes are not organelles. They are not membrane-enclosed, instead they are macro-molecules made of both RNA and proteins.
• They are the protein factories of the cell. Composed of two subunits, they can be found floating freely in cell’s cytoplasm or embedded within the endoplasmic reticulum.
• Every cell needs Ribosomes, so they are found in both prokaryotes and Eukaryotes.
• Using the templates and instructions provided by two different types of RNA, ribosomes synthesize a variety of proteins that are essential to the survival of the cell.
• There are two sub-units to every ribosome.
• The Process of protein synthesis
  • Protein synthesis starts when mRNA moves from nucleus to a ribosome on the surface of RER.
  • The two sub-units of ribosomes come together and combine with mRNA. They lock onto the mRNA and start the protein synthesis.
  • Ribosome builds the amino acid chain. The process is simple. First, you need an amino acid. Another nucleic acid that lives in the cell is transfer RNA. It is bonded to amino acids floating around the cell. With mRNA offering instructions, the ribosome connects to a tRNA and pulls of one amino acid. The tRNA is then released back into the cell and attached to another amino acid.
  • When the protein is complete RER pinches off a vesicle. That vesicle, a small membrane bubble, can move to the cell membrane or the Golgi apparatus. Some of the protein will be used in the cell and some will be sent out into  intercellular-space.

C. Golgi Apparatus (pronounced ‘GOL-JI)

• Structure: The Golgi apparatus, first described by Camillo Golgi, consists of system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns. These membranes often have connection to membrane of ER and therefore constitute another portion of a complex cellular membrane system.
• Functions of GA include storage, modification and packaging of products in vesicles.
  • Complex sugar can be made out of simple sugar
  • Turning protein into usable form by folding them into different shapes or adding other materials to protein such as lipids or Carbohydrates.
  • It is also involved in formation of lysosomes.
• After making these big molecules, Golgi apparatus packages them into vesicles, and either stores them for later use or sends them out of the cell.

D. Lysosomes

• Structure: Lysosomes are membrane bound sacs filled with powerful digestive enzymes capable of breaking down organic material. These enzymes are made at RER. The membrane ensures that the internal enzymes don’t digest the cell itself.
• Functions
  • It is a kind of waste disposal system of the cell. Lysosome help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles.
• Suicide bags of cell
  • During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst, and enzymes digest their own cell. Therefore, lysosomes are also known as suicide bags of a cell.
• They are not commonly found in plant cells. The tough cell walls keep out the foreign substance.

E. Mitochondria

• Structure:
  • Mitochondria has two membranes covering instead of one.
  • The outer membrane is very porous while the inner membrane is deeply folded. These folds create large surface areas for ATP generating chemical reactions.
  • Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their proteins.
• Functions
  • Mitochondria are known as powerhouse of the cell.
  • The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine Triphosphate) molecules during a process called cellular respiration.
  • ATP is known as the energy currency of the cell. It provides energy for all the cellular activities.
  • Cells which need more energy have more mitochondria. (For e.g. muscle cells)
  • The body uses energy stored in ATP for making new chemical compounds and for mechanical work.
  • Mitochondria are also involved in controlling the concentration of Calcium (Ca2+) ions within cell.

F. PLASTIDS (not found in animal cells)

• Structures:
  • The internal structure of plastids consists of numerous membrane layers embedded in a material called the Stroma.
  • Plastids are similar to mitochondria in external structure.
  • Like mitochondria, plastids also have their own DNA and ribosomes.
• Two types of plastids
  • Chromoplasts (coloured plastids)
  • Leucoplasts (white or colourless)-> these are organelles in which material such as starch, oil and protein granules are stored. Thus, primary purpose of leucoplast is storage.
• Chloroplasts: Plastids containing the green pigment chlorophyll are known as chloroplasts.
  • Chloroplasts are important for photosynthesis in plants and thus are food producers of the cell. They convert light energy of sun into sugars that can be used by cells. The entire process is called photosynthesis and it all depends on little green chlorophyll molecule in each chloroplast. In the process of photosynthesis, plants create sugar and release oxygen.
  • Two membranes (named outer and inner membrane) surrounds the stroma and the grana (stacks of thylakoid). One thylakoid stack is called granum.        The    stacks of thylakoid sacs are connected by    stroma    lamella.     The lamella act like skeleton of the chloroplast, keeping all the sacs a safe distance from each other and maximizing the       efficiency       of       the organelle.
  • Chlorophyll molecules sit on surface of each thylakoid and capture light energy from sun. As energy-rich molecules are created by the light-dependent reactions, they move to the stroma where carbon (C) can be fixed and sugars can by synthesized.They also contain various yellow and orange pigments in addition to chlorophyll.

G. Vacuoles

• Structure:
  • Vacuoles are of small size in animal cells while plant cells have very large vacuoles.
  • The central vacuole of some plant cells may occupy 50-90% of the cell volume.
• Functions
  • Vacuoles are storage sacs for solid and liquid contents. Many substances of importance in life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids, some proteins and waste products.
  • In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
  • In single celled organisms like Amoeba, the food vacuole contains the food items that the amoeba has consumed.
  • In some other unicellular organisms, specialized vacuoles also play important roles in expelling excess water and some wastes from the cell.

H. Cytoskeleton (Not organelles)

• It is the microscopic network of protein and tubules in the cytoplasm of many living cells, giving them shape and coherence.
• It is complex network of interlinking filament and tubules that extend throughout cytoplasm, from the nucleus to plasma membrane.

I. Some unique structures which only some cells have

1. Cilia: In humans,the respiratory tract is lined with cells that have cilia. These are microscopic hair like projections that can move in waves. This feature help in trapping inhaled particles in the air and expels when you cough.
2. Flagella: Some bacteria have flagella. A Flagellum is like a little tail that can help a cell move or propel itself. The only human cell that have a flagellum is sperm cell.

5) SOME THINGS TO NOTE:

• Even every multi-cellular organism come from a single cell.
• While observing a cell under micro-scope, we use iodine solution, safranin solution or methylene blue solution to stain the cells, so that different organelles are clearly visible.
• All cells have a cell membrane, cytoplasm, and genetic material.

6) IMPORTANT SCIENTISTS

• Discovery of Cell (1665)
  • Robert Hooke (father of cytology – the branch of science which studies cell) while examining a thin slice of cork saw that the cork resembled the structure of a honey comb (hexagonal compartments). He in 1665 made the chance observation through a self-designed microscope. He called these boxes cells.
  • This was the very first time that someone had observed that living things appear to consist of separate units.
• Discovery of a living cell (1674)
  • Anton Von Leeuwenhock (father of bacteriology). He studied bacterial, protozoan cells etc.
• Discovery of nucleus (1831)
  • Robert Brown
• Term Protoplasm was coined by Purkinje in 1839 for the fluid substance of the cell.