The Cell

The Cell – basic unit of life

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

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

Cell Organelles

• Cell organelles are enclosed by membranes.
• The significance of membranes can be illustrated with the example of viruses.
• Viruses lack any membranes & hence do not show characteristics of life until they enter a living body & use its cell machinery to multiply.

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.

1. Endocytosis through Plasma Membrane

• The plasma membrane is flexible & is made up of organic molecules called lipids & proteins.
• The flexibility of the cell membrane also enables the cell to engulf in food & 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.

2. Diffusion through Plasma Membrane

• The plasma membrane is a selectively permeable membrane.
• It is porous & allows the movement of substances or materials both inward & outward.
• Some substances like CO₂ or oxygen can move across the cell membrane by a process called diffusion.
• Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell & its external environment.

Additional Reading: Diffusion, Osmosis & Reverse Osmosis Solution: In chemistry, a solution is homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. For example, in salt water, salt is the solute and water is the solvent. Diffusion: Diffusion is a spontaneous movement of a substance from an area of high concentration to an area of low concentration.           Diffusion through a permeable membrane moves a substance (solute) from a region of high solute concentration (hypertonic solution) to a region where its concentration is low (hypotonic solution).     In living organisms, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment. Exchange of O2 & CO2 in alveoli (millions of tiny, balloon-shaped air sacs in lungs) is an example of diffusion. In a person suffering from pneumonia, the air sacs may fill with fluid or pus. This prevents diffusion of O2 & CO2 in alveoli (breathing issues in Coronavirus patients). Osmosis Water obeys the law of diffusion.  Osmosis is the passage of water (diffusion of molecules of a solvent) from a region of low solute concentration (hypotonic) through a semi-permeable membrane to a region of high solute concentration (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 .     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. 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 so that pure water flows from a region of high solute concentration (hypertonic) through a semi-permeable membrane to a region of low solute concentration (hypotonic).

Cell Wall

• The 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 & provides structural strength to plants.

1. 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, & not dead cells, can absorb water by osmosis. 

2. Plants are at an advantage because of the Cell Wall

• Cell walls permit the cells of plants, fungi & 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.
• The 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 & 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.

Nucleus

• It is an important component of the living cell.
• It is generally spherical & located in the centre of the cell.
• It can be stained & seen easily with the help of a microscope.
• The nucleus is separated from the cytoplasm by a double-layered membrane called the nuclear membrane.
• The nucleus of the bacterial cell is not well organized like the cells of multicellular organisms. There is no nuclear membrane.
• The nuclear membrane is also porous & allows the movement of materials between the cytoplasm & 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.
• The nucleus acts as the control centre of the activities of the cell.
• The nucleus plays a central role in cellular reproduction, the process by which a single cell divides & forms two new cells.
• It also plays a crucial part, along with the environment, in determining the way the cell will develop & what form it will exhibit at maturity, by directing the chemical activities of the cell.

Protoplasm

• Protoplasm includes the cytoplasm & the nucleus. 
• Protoplasm is called the living substance of the cell.

Chromosomes

• Nucleus contains thread-like structures called chromosomes. 
• 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 & Protein.
• DNA molecules contain the information necessary for constructing & organizing cells. 
• Functional segments of DNA are called genes.
• Gene is a unit of inheritance in living organisms. 
• It controls the transfer of a hereditary characteristic from parents to offspring.
• The chromosomes can be seen only when the cell divides.

Chromatin material

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

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 nucleoid.

Prokaryotic Cells vs. Eukaryotic Cells 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.

	Prokaryotes	Eukaryotes
Organisms	Monera: Eubacteria and Archebacteria	Protists, Fungi, Plants & Animals
Meaning of name	Pro = beforeKaryon = nucleus	Eu = afterKaryon = 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 & plants (cellulose & chitin): none in animals
Organelles	usually none	many different ones with specialized functions
Metabolism	anaerobic & aerobic: diverse	mostly aerobic
Genetic material	anaerobic & aerobic: Diverse	single circular double-stranded DNA
Location of genetic information	Nucleoid region	Nucleus
Mode of division	binary fission mostly; budding	mitosis & meiosis using a spindle: followed by cytokinesis

Vacuoles

• Empty structure in the cytoplasm is called vacuole. 
• It could be single & 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 & provide turgidity (swollen due to fluids) & 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 & some proteins.
• In single-celled organisms like amoeba, the vacuole contains the food items that the amoeba has consumed.
• In some unicellular organisms, specialized vacuoles also play important roles in expelling excess water & some wastes from the cell.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is a large network of membrane-bound tubes & 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) & smooth endoplasmic reticulum (SER).

1. 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.

1. Smooth Endoplasmic Reticulum SER

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

2. Functions of Endoplasmic Reticulum (ER)

• Some of these proteins & lipids help in building the cell membrane. 
• This process is known as membrane biogenesis.
• Some other proteins & lipids function as enzymes & 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 & 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 & 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 & therefore constitute another portion of a complex cellular membrane system.
• The material synthesized near the ER is packaged & dispatched to various targets inside & outside the cell through the golgi apparatus.
• Its functions include the storage, modification & 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 & 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.
• 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 & 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 & ribosomes. 
• Therefore, mitochondria are able to make some of their own proteins (ribosomes prepare proteins).

1. Metabolic pathways & ATP

• Metabolic pathways can lead to a more complex structure from a simpler structure (for example, acetic acid becomes cholesterol) or lead to a simpler structure from a complex structure (for example, glucose becomes lactic acid in our skeletal muscle).
• The former cases are called biosynthetic pathways or anabolic pathways. 
• The latter constitute degradation & hence are called catabolic pathways.
• Anabolic pathways, as expected, consume energy. 
• Assembly of a protein from amino acids requires energy input.
• On the other hand, catabolic pathways lead to the release of energy. 
• For example, when glucose is degraded to lactic acid in our skeletal muscle, energy is liberated.
• This metabolic pathway from glucose to lactic acid which occurs in 10 metabolic steps is called glycolysis.
• Living organisms have learnt to trap this energy liberated during degradation & store it in the form of chemical bonds.
• As & when needed, this bond energy is utilized for biosynthetic, osmotic & mechanical work that we perform.
• The most important form of energy currency in living systems is the bond energy in a chemical called adenosine triphosphate (ATP).

Plastids

• You might have noticed several small coloured 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:

1. chromoplasts (coloured plastids) & 
2. leucoplasts (white or colourless plastids).

• Leucoplasts are primarily organelles in which materials such as starch, oils & 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 & ribosomes.

Summary

• Each cell acquires its structure & ability to function because of the organization of its membrane & organelles in specific ways. The cell thus has a basic structural organization. 
• This helps the cells to perform functions like respiration, obtaining nutrition, & 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 & proteins.
• The presence of the cell wall enables the cells of plants, fungi & bacteria to exist in hypotonic media without bursting.
• The ER functions both as a passageway for intracellular transport & as a manufacturing surface.
• The golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification & packaging of substances manufactured in the cell.
• Most plant cells have large membranous organelles called plastids, which are of two types – chromoplasts & leucoplasts.
• Chromoplasts that contain chlorophyll are called chloroplasts & they perform photosynthesis. 
• Leucoplasts help in the storage of oils, starch & protein granules.
• Most mature plant cells have a large central vacuole that helps to maintain the turgidity of the cell & stores important substances including wastes.
• Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, & 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 to 4 centimetres is among the largest unicellular species.

Plant Cell vs. Animal Cell

	Animal Cell	Plant 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 & 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.

Questions

• Can you name the two organelles we have studied that contain their own genetic material?
• What would happen to the life of a cell if there was no Golgi apparatus?
• Where do the lipids & proteins constituting the cell membrane get synthesised?
• What is osmosis?
• Why are lysosomes known as suicide bags?
• Where are proteins synthesized inside the cell?