What are Living Organisms Made Up of?

• Robert Hooke made the initial discovery of cells in 1665. He used a simple microscope to look at the cells in a slice of cork. With the aid of an improved microscope, Leeuwenhoek (1674) made the initial discovery of free living cells in pond water.

• Robert Brown made the discovery of the cell's nucleus in 1831.

• The term "protoplasm" was first used by Purkinje to describe the fluid component of cells in 1839.

• The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839).

• Virchow (1855) added to the cell theory by proposing that all cells originate from pre-existing cells.

• The complex structure of the cell and its different organelles could be seen and understood due to the invention of the electron microscope.

UNICELLULAR ORGANISMS

It is now understood that a single cell can make up an entire creature, as seen in bacteria, amoebas, chlamydomonas, and paramecium. They are referred to as unicellular creatures.

MULTICELLULAR ORGANISMS

In multicellular creatures (multi = many), such as some fungi, plants, and animals, numerous cells combine together in a single body and take on different functions in it to form various body parts.

CELLS DIVIDE AND RE-DIVIDE

Every organism with several cells originated from a single cell called zygote. How? Cells divide to create more of the same kind of cell. Thus, all cells emerge from pre-existing cells.

• Cells’ distinct functions are correlated with their size and shape.

• Amoeba cells, for example, can change their shape.

• For example, nerve cells have a typical shape that may be more or less fixed and peculiar for that particular type of cell.

Brief introduction to cell organelles

• Cell organelles are distinct internal parts that are unique to each cell.

• Each type of cell organelle carries out a specific function, such as producing new material within the cell or removing waste from the cell, among other things.

• These organelles allow a cell to survive and carry out all of its necessary functions.

• Together, these organelles make up the basic building block known as the cell. It is interesting that, irrespective of their purpose or the organism they are found in, all cells have the same organelles.(All plant cells have same organelles and all animal cells have same organelles but a plant cell and an animal cell does not have same organelles)

Structural Organisation of a Cell

Under a microscope, we would notice three characteristics in almost every cell.

• Plasma membrane

• Nucleus

• Cytoplasm.

Because of these characteristics, all activities within the cell and interactions between the cell and its environment are possible.

Plasma Membrane/Cell membrane

The plasma membrane is the cell's outermost covering that separates its contents from its external environment.

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

• However, we can observe the structure of the plasma membrane only through an electron microscope.

• The plasma membrane allows or allows certain materials to enter and exit the cell.

• It also keeps some other materials from moving.As a result, the cell membrane is referred to as a selectively permeable membrane.

• Diffusion allows certain substances, such as carbon dioxide or oxygen, to move across the cell membrane.

Diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration.

The cell's ability to absorb food and other substances from its surrounding environment is also made possible by the flexibility of the cell membrane. Endocytosis is the term for such processes. Such procedures are used to obtain nourishment for amoebas.

How diffusion works inside the cell?

• Some substances, such as CO2 (which is cellular waste that must be excreted by the cell), accumulate in high concentrations within the cell.

• The concentration of CO2 in the cell's external environment is lower than that inside the cell.

• When the concentration of CO2 inside and outside a cell differs, CO2 diffuses out of the cell, from a region of high concentration to a region of low concentration outside the cell.

• When the level or concentration of O2 inside the cell decreases, O2 enters the cell via diffusion.

• Thus, diffusion is important in gaseous exchange between cells as well as between cells and their surroundings.

• Water follows the diffusion law as well. Osmosis is the movement of water molecules through a selectively permeable membrane.

• The amount of substance dissolved in water also influences the movement of water across the plasma membrane.

Osmosis

Osmosis is defined as the net diffusion of water across a selectively permeable membrane in the direction of a higher solute concentration.

Isotonic Solution

• There won't be any net movement of water across the cell membrane if the medium and the cell both contain exactly the same amount of water. An isotonic solution is one such solution.

• Although water enters and exits the cell membrane in opposite directions, there is no total movement of water because the amounts are equal. The size of the cell will not change.

Hypotonic Solution

• The cell will absorb water through osmosis if the medium around it has a higher water concentration than the cell, indicating that the outside solution is severely diluted. A hypotonic solution is one such solution.

• Although water molecules can freely move in both ways across the cell membrane, more water will enter the cell than will exit it. Water enters the cell as a result, which is the total outcome. The cell is probably going to swell.

Hypertonic Solution

• The cell will osmotically lose water if the medium is a highly concentrated solution with a lower concentration of water than the cell. A hypertonic solution is one such as this.

• Once more, water passes through the cell membrane both ways, but this time, more water exits the cell than enters it. Consequently, the cell will contract.

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 exhange 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.

Cell Wall

• In addition to the plasma membrane, plant cells also feature a cell wall, which is a hard outer layer.

• The cell wall lies outside the plasma membrane

• The plant cell wall is mainly composed of cellulose

• Plants receive their structural strength from cellulose, a complex material present in cell wall

• Osmosis causes the contents of a living plant cell to shrink or contract away from the cell wall when the cell loses water.. This phenomenon is known as plasmolysis

Nucleus

• Nuclear membrane refers to the double-layered membrane that surrounds the nucleus.

• The nuclear membrane has pores that enable material to move from the nucleus' inside to its outside, or to the cytoplasm.

• Chromosomes are found in the nucleus and are only observable as rod-like structures when the cell is about to divide.

• Chromosomes store information in the form of DNA (Deoxyribonucleic Acid) molecules that allows traits to be passed down from parents to offspring. DNA and protein are the components of chromosomes.

• The knowledge needed to construct and organise cells is found in DNA molecules.

• Genes are functional sections of DNA.

• This DNA is a component of the chromatin of a cell that is not dividing. Chromatin is evident as a tangled mass of structures that resemble threads. The material in the chromatin is organised into chromosomes whenever the cell is about to split.

• A single cell divides into two new cells by the process of cellular reproduction, which is mostly controlled by the nucleus.

• Nucleus 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.(FUNCTION)

• Due to the lack of a nuclear membrane, the nuclear area of the cell may be poorly defined in some organisms, such as bacteria. A nucleoid is a nuclear region that is undefined and made up entirely of nucleic acids. These creatures are referred to as prokaryotes because their cells lack a nuclear membrane.

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

Cytoplasm

• The liquid substance that makes up the plasma membrane is called cytoplasm.

• Numerous specialised cell organelles are present in cytoplasm. These organelles each have a particular purpose for the cell.

• Membranes enclose the cell organelles. Prokaryotes lack membrane-bound cell organelles in addition to having an undefined nuclear area. The nuclear membrane and organelles that are enclosed in membranes, however, are both present in eukaryotic cells.

• Since viruses lack membranes, they do not exhibit signs of life until they enter a living thing and use its cell machinery to reproduce.

• The cytoplasm is in charge of holding the elements of the cell and protecting them from damage.(FUNCTION)

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a vast network of tubes and sheets that are membrane-bound.

• It seems to be circular, oblong, or lengthy tubules (vesicles). The ER membrane resembles the plasma membrane in terms of structure.

• Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum(SER) are the two forms of ER

• 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.(FUNCTION)

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

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

• 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

• 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

• The Golgi apparatus, first theorised by Camillo Golgi, is a network of membrane-bound vesicles (flattened sacs) stacked roughly parallel to one another and known as cisterns.

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

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

• The Golgi apparatus is also involved in the formation of lysosomes

Lysosomes

• Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes

• These enzymes are made by RER.(Rough Endoplasmic Reticulum)

• A type of cell waste disposal system is the lysosome. By breaking down any foreign objects and worn-out cell organelles, they aid in keeping the cell clean.

• Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break complex substances into simpler substances

• Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material.

• For instance, when a cell is damaged, lysosomes may burst, releasing enzymes that may then begin to digest the damaged cell. As a result, lysosomes are also referred to as the cell's "suicide bags."

Mitochondria

• Mitochondria are known as the powerhouses of the cell.

• There are two membranes that cover mitochondria.The inner membrane is deeply folded and the outer membrane is porous.

• These folds increase surface area for ATP generating chemical reactions. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules

• 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 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

Plastids

• Plastids are present only in plant cells

• There are three types of plastids – chloroplast(green),chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids).

• Chloroplasts are chlorophyll-containing organelles in plant cells; they play a vital role for life on Earth since photosynthesis takes place in chloroplasts.

• Many flowers and fruits have yellow, orange, and red colouring because of chromoplasts. Some roots, like carrots, and tubers, such sweet potatoes, also contain them.

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

• The internal organisation of the Chloroplast consists of numerous membrane layers embedded in a material called the stroma.

• These are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.

Vacuoles

• Storage sacs for solid or liquid contents of cell are called vacuoles.

• Animal cells have small vacuoles, but plant cells have very large vacuoles.

• 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 and rigidity to the cell.

• Vacuoles are storage spaces for a variety of compounds vital to plant cell survival. These include certain proteins, carbohydrates, different organic acids, and amino acids.

• The food that the single-celled amoeba has consumed is stored in the food vacuole.

• Specialized vacuoles also play key functions in removing excess water and certain wastes from the cell in some unicellular organisms.

As a result, the cell is the basic building block of all living things. It is also the fundamental building block of life.

Cell Division

• In order for an organism to grow, replace old, damaged, or dead cells, and create the gametes needed for reproduction, new cells must be created.

• Cell division refers to the process of creation of new cells.

• There are two main types of cell division: mitosis and meiosis.

Mitosis

• Mitosis is the name of the cell division process that most cells use to divide in order to expand.

• Each mother cell divides into two exactly alike daughter cells throughout this procedure.

• The number of chromosomes in the daughter cells matches that of the mother cell. It aids in tissue growth and repair in living things.

Meiosis

• In both animals and plants, certain cells found in reproductive organs or tissues divide to create gametes, which, when fertilised, produce offspring. Meiosis, a different process by which they split involves two consecutive divisions

• When a cell divides by meiosis it produces four new cells instead of just two

• The new cells only have half the number of chromosomes than that of the mother cells.