Living Cell

The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". The study of cells is called cell biology.

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals). While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion(10¹²) cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres.

Robert Hooke	Discovered and coined the term cell in 1665
Robert Brown	Discovered Cell Nucleus in 1831
Schleiden and Schwann	Presented The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life. Schleiden (1838) and Schwann (1839).

The cell was discovered by Robert Hooke in 1665, who named the biological units for their resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from preexisting cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. Cells emerged on Earth at least 3.5 billion years ago.

With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles.

 

Cell Organelles

Plasma Membrane or Cell Membrane

·         Cell membrane is also called the plasma membrane.

·         It can be observed only through an electron microscope.

·         Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment.

Endocytosis

·         The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.

·         The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis (endo → internal; cyto → of a cell). Amoeba acquires its food through such processes.

Diffusion

·         Plasma membrane is a selectively permeable membrane [The plasma membrane is porous and allows the movement of substances or materials both inward and outward].

·         Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion [spontaneous movement of a substance from a region of high concentration (hypertonic solution) to a region where its concentration is low (hypotonic solution)].

·         Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.

Osmosis

·         Water also obeys the law of diffusion. The movement of water molecules through a selectively permeable membrane is called osmosis.

·         Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Thus, osmosis is a special case of diffusion through a selectively permeable membrane.

·         Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.

·         Thus, diffusion is important in exchange of gases and water in the life of a cell. In additions to this, the cell also obtains nutrition from its environment.

·         Different molecules move in and out of the cell through a type of transport requiring use of energy in the form of ATP.

Reverse Osmosis (RO)

·         Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove larger particles from drinking water.

·         In reverse osmosis, an applied pressure is used to overcome osmotic pressure.

·         Reverse Osmosis is a phenomenon where pure water flows from a dilute solution [hypotonic] through a semi permeable membrane to a higher concentrated solution [hypertonic].

·         Semi permeable means that the membrane will allow small molecules and ions to pass through it but acts as a barrier to larger molecules or dissolved substances.

Cell Wall

·         Cell wall is absent in animals.

·         Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the 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 shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis (plasma → fluid; lysis → disintegration, decomposition).

·         Only living cells, and not dead cells, are able to absorb water by osmosis. Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute [hypotonic] external media without shrinkage.

·         In such media the cells tend to lose water by osmosis. The cell shrinks, building up pressure against the cell wall. The wall exerts an equal pressure against the shrunken cell.

·         Cell wall also prevents the bursting of cells when the cells are surrounded by a hypertonic medium (medium of high concentration).

·         In such media the cells tend to gain water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell.

·         Because of their walls, plant cells can withstand much greater changes in the surrounding medium than animal cells.

Cytoplasm

·         It is the jelly-like substance present between the cell membrane and the nucleus.

·         The cytoplasm is the fluid content inside the plasma membrane.

·         It also contains many specialized cell organelles [mitochondria, golgi bodies, ribosomes, etc].

·         Each of these organelles performs a specific function for the cell.

·         Cell organelles are enclosed by membranes.

·         The significance of membranes can be illustrated with the example of viruses.

·         Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.

Nucleus

·         It is an important component of the living cell.

·         It is generally spherical and located in the center of the cell.

·         It can be stained and seen easily with the help of a microscope.

·         Nucleus is separated from the cytoplasm by a double layered membrane called the nuclear membrane.

·         This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus [diffusion].

·         With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus.

·         In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.

·         Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring. This means that your parents pass some of their characteristics on to you.

·         Nucleus, in addition to its role in inheritance, acts as control center of the activities of the cell.

·         The entire content of a living cell is known as protoplasm [cytoplasm + nucleus]. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.

·         The nucleus of the bacterial cell is not well organized like the cells of multicellular organisms. There is no nuclear membrane.

·         Every cell has a membrane around it to keep its own contents separate from the external environment.

·         Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function.

·         To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or ‘organelles’) within themselves.

Chromosomes

·         The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.

·         Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (deoxyribo nucleic acid)

·         Chromosomes are composed of DNA and Protein.

·         DNA molecules contain the information necessary for constructing and organizing cells. Functional segments of dna are called genes.

·         In a cell which is not dividing, this dna is present as part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.

·         The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells.

·         It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.

Prokaryotic Cells vs. Eukaryotic Cells

Prokaryotic cells are cells without a nucleus. The DNA in prokaryotic cells is in the cytoplasm rather than enclosed within a nuclear membrane. Prokaryotic cells are found in single-celled organisms, such as bacteria, like the one shown in Figure. Organisms with prokaryotic cells are called prokaryotes. They were the first type of organisms to evolve and are still the most common organisms today.

This diagram shows the structure of a typical prokaryotic cell, a bacterium. Like other prokaryotic cells, this bacterial cell lacks a nucleus but has other cell parts, including a plasma membrane, cytoplasm, ribosomes, and DNA. Identify each of these parts in the diagram.

Eukaryotic cells are cells that contain a nucleus. A typical eukaryotic cell is shown in Figure. Eukaryotic cells are usually larger than prokaryotic cells, and they are found mainly in multicellular organisms. Organisms with eukaryotic cells are called eukaryotes, and they range from fungi to people.

Eukaryotic cells also contain other organelles besides the nucleus.

An organelle is a structure within the cytoplasm that performs a specific job in the cell. Organelles called mitochondria, for example, provide energy to the cell, and organelles called vacuoles store substances in the cell. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can. This allows eukaryotic cells to have greater cell specificity than prokaryotic cells. Ribosomes, the organelle where proteins are made, are the only organelles in prokaryotic cells.

In some ways, a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit, the cytoplasm of the cell also contains various structures, such as a nucleus and other organelles. 

Prokaryotic Cells vs. Eukaryotic Cells could also be elaborated as:

·         Organisms whose cells lack a nuclear membrane, are called prokaryotes (pro = primitive or primary; karyote ≈karyon = nucleus).

·         Organisms with cells having a nuclear membrane are called eukaryotes.

·         Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells.

·         Many of the functions of such organelles are also performed by poorly organised parts of the cytoplasm.

·         The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag like structures) but not with plastids as in eukaryotic cells.

Prokaryotes → defined nuclear region, the membrane-bound cell organelles are absent.

Eukaryotic Cells → have nuclear membrane as well as membrane-enclosed organelles.

Characteristics	Prokaryotes	Eukaryotes
Organisms	Monera: Eubacteria and Archebacteria	Protists, Fungi, Plants and Animals
Meaning of name	Pro = before Karyon = nucleus	Eu = after Karyon = nucleus
Evolution	3.5 billion years ago (older type of cell)	1.5 billion years ago
Uni-/multicellular	Unicellular (less complex)	Multicellular (more complex)
Cell wall	almost all have cell walls (murein)	fungi and plants (cellulose and chitin): none in animals
Organelles	usually none	many different ones with specialized functions
Metabolism	anaerobic and aerobic: diverse	mostly aerobic
Genetic material	single circular double stranded DNA	complex chromosomes usually in pairs; each with a single double stranded DNA molecule and associated proteins contained in a nucleus
Location of genetic information	Nucleoid region	Nucleus
Mode of division	binary fission mostly; budding	mitosis and meiosis using a spindle: followed by cytokinesis

Nucleoid

·         In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a

Vacuoles

·         Empty structure in the cytoplasm is called vacuole. It could be single and big as in an onion cell (plant cell). Cheek cells (animal cells) have smaller vacuoles.

·         Large vacuoles are common in plant cells. Vacuoles in animal cells are much smaller.

·         Vacuoles are storage sacs for solid or liquid contents.

·         The central vacuole of some plant cells may occupy 50-90% of the cell volume.

·         In plant cells vacuoles are full of cell sap and provide turgidity [swollen and distended or congested] and rigidity to the cell.

·         Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins.

·         In single-celled organisms like amoeba, the food vacuole contains the food items that the amoeba has consumed.

·         In some unicellular organisms, specialized vacuoles also play important roles in expelling excess water and some wastes from the cell.

The functions of the vacuole include:

·         Isolating materials that might be harmful or a threat to the cell

·         Containing waste products

·         Containing water in plant cells

·         Maintaining internal hydrostatic pressure or turgor within the cell

·         Maintaining an acidic internal pH

·         Containing small molecules

·         Exporting unwanted substances from the cell

·         Allows plants to support structures such as leaves and flowers due to the pressure of the central vacuole

·         By increasing in size, allows the germinating plant or its organs (such as leaves) to grow very quickly and using up mostly just water.^[4]

·         In seeds, stored proteins needed for germination are kept in 'protein bodies', which are modified vacuoles

Endoplasmic Reticulum (ER)

·         The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or long bags (vesicles).

·         The ER membrane is similar in structure to the plasma membrane.

·         There are two types of ER –– rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Rough Endoplasmic Reticulum RER – Ribosomes

·         RER looks rough under a microscope because it has particles called ribosomes attached to its surface.

·         The ribosomes, which are present in all active cells, are the sites of protein manufacture.

·         The manufactured proteins are then sent to various places in the cell depending on need, using the ER.

Smooth Endoplasmic Reticulum SER

·         The SER helps in the manufacture of fat molecules, or lipids, important for cell function.

Functions of Endoplasmic Reticulum (ER)

·         Some of these proteins and lipids help in building the cell membrane. This process is 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 the 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 biochemical activities of the cell.

·         In the liver cells of the group of animals called vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.

Golgi Apparatus or Golgi Complex

·         The golgi apparatus consists of a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns.

·         These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.

·         The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the golgi apparatus.

·         Its functions include the storage, modification and packaging of products in vesicles.

·         In some cases, complex sugars may be made from simple sugars in the golgi apparatus.

·         The golgi apparatus is also involved in the formation of lysosomes.

Lysosomes

·         Lysosomes are a kind of waste disposal system of the cell.

·         Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles.

·         Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material.

·         During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.

·         Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER.

Mitochondria

·         Mitochondria are known as the 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.

Note: If Mitochondria is the Power Plant. ATP is the Electricity.

·         ATP is known as the energy currency of the cell.

·         The body uses energy stored in ATP for making new chemical compounds and for mechanical work.

·         Mitochondria have two membrane coverings instead of just one.

·         The outer membrane is very porous while the inner membrane is deeply folded. These folds create a large surface area 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 own proteins [ribosomes prepare proteins].

Plastids

·         You might have noticed several small colored bodies in the cytoplasm of the cells of Tradescantia leaf. They are scattered in the cytoplasm of the leaf cells. These are called plastids.

·         They are of different colours. Some of them contain green pigment called chlorophyll. Green coloured plastids are called chloroplasts. They provide green colour to the leaves.

·         Chloroplasts are important for photosynthesis in plants.

·         Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.

·         Plastids are present only in plant cells. There are two types of plastids – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids).

·         Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.

·         The internal organization of the plastids consists of numerous membrane layers embedded in a material called the stroma.

·         Plastids are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own dna and ribosomes.

Summary

·         Each cell acquires its structure and ability to function because of the organization of its membrane and organelles in specific ways. The cell thus has a basic structural organization. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins. Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.

·         Cells are enclosed by a plasma membrane composed of lipids and proteins.

·         The presence of the cell wall enables the cells of plants, fungi and bacteria to exist in hypotonic media without bursting.

·         The ER functions both as a passage way for intracellular transport and as a manufacturing surface.

·         The golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification and packaging of substances manufactured in the cell.

·         Most plant cells have large membranous organelles called plastids, which are of two types – chromoplasts and leucoplasts.

·         Chromoplasts that contain chlorophyll are called chloroplasts and they perform photosynthesis. Leucoplasts help in the storage of oils, starch and protein granules.

·         Most mature plant cells have a large central vacuole that helps to maintain the turgidity of the cell and stores important substances including wastes.

·         Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, and they have only very small ribosomes as organelles.

·         A white blood cell (WBC) in human blood is an example of a single cell which can change its shape.

·         Bacterial cell also has a cell wall.

·         In egg white material is albumin which solidifies on boiling. The yellow part is yolk. It is part of the single cell.

·         Valonia ventricosa, a species of algae with a diameter that ranges typically from 1 to4 centimetres is among the largest unicellular species.

Plant Cell vs. Animal Cell

 

 

Nucleus	Present	Present
Cilia	Present	It is very rare
Shape	Round (irregular shape)	Rectangular (fixed shape)
Chloroplast	Animal cells don’t have chloroplasts	Plant cells have chloroplasts because they make their own food
Cytoplasm	Present	Present
Endoplasmic Reticulum (Smooth and Rough)	Present	Present
Ribosomes	Present	Present
Mitochondria	Present	Present
Vacuole	One or more small vacuoles (much smaller than plant cells).	One. large central vacuole taking up 90% of cell volume.