What Is a Cell?

• A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes.
• A cell can replicate itself independently. Hence, they are known as the building blocks of life. 
• Cells provide structure and support to the body of an organism.
• The cell interior is organized into different individual organelles  surrounded by a separate membrane.
• The nucleus(major organelle) holds genetic information necessary for reproduction and cell growth.

History

• In 1665, Robert Hooke for the first time saw little compartments in the cork, and he called these compartments as cells.
• Hence, dead cells were first saw by Robert Hooke.
• Living cells were first saw by Anton Von Leeuwenhoek.
• Nucleus was first time discovered by Robert brown.
• In 1839, Purkinje coined the term “PROTOPLASM”.
• Cell theory was composed by :

1. Schwann (1839)
2. Schleiden (1838)

• Cell theory was further expanded by Rudolf Virchow(1855).
• Unicellular organisms : organisms those are made up of only single cells.
• Eg- amoeba, paramecium, Chlamydomonas, bacteria.
• Multicellular organisms : organisms those are made of multiple cells.
• Eg – fungi, animals, plants.
• An organelle is a specialized subunit, usually within a cell, that has a specific function.
• Three structures which are common in every cell are :

1. Plasma membrane
2. Nucleus Cytoplasm

Picture of a compound microscope

Membranes

• Plasma membrane

1. This is the outermost covering of the cell that separates the contents of the cell from its external environment.
2. The cell membrane is selectively permeable.

Scientists

Robert Hooke

Robert Brown

Anton Von Leuwenhoek

Purkinje

Diffusion

• Gases like carbon dioxide and oxygen move across the cell membrane by a process known as diffusion.
• Water also move across by the process of diffusion.
• Water movement across such a selectively permeable membrane is accompanied under the process of osmosis.
• Generally unicellular organisms carry out their physiological processes by this process only.

Osmosis

• Absorption of water by roots is done by osmosis.
• The plasma membrane is flexible and is made up of lipoproteins (ie. lipids and proteins).
• Endocytosis and exocytosis process occurs due to the flexibility of the plasma membrane.

Solutions

Tonicity

Cell Wall

• Outside the cell membrane, there is another membrane known as cell wall in case of plants, fungi and some bacteria.
• Unlike cell membrane, cell wall is rigid in nature.
• Cell wall is mainly composed of cellulose.
• Cell wall provides structural strength to the cell.
• 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.

Nucleus

• Solutions used for staining the cells:

1. Iodine solution
2. Methylene blue solution
3. Safranin solution

• The nucleus has a double layered covering known as nuclear membrane.
• The nuclear membrane has pores which allows passage for contents to move across the nucleus and outside the nucleus.
• The nucleus contains chromosomes, a rod – like structures only visible when the cell is about to divide.
• Chromosomes contain the hereditary material known as DNA (deoxyribonucleic acid).
• Chromosomes are composed of DNA and proteins.
• Functional segments of DNA is known as genes.
• Inside nucleus a nucleolus is also present.

Cytoplasm

• Cytoplasm is the fluid contained inside the plasma membrane.
• It contains specialized cell organelles inside it, which carries the metabolic activities of the cell.
• Note : 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.

Types of Cells

• Eukaryotic  Cells

Cells, those have a proper membrane bounded nucleus and its genetic material is organized inside its nucleus.

• Prokaryotic  Cells

Cells, those do not have a proper membrane bounded nucleus and its genetic material is scattered in the cytoplasm.

Cell Organelles

Endoplasmic Reticulum

• It has large network of tubes and sheets.
• It looks like round bags termed as vesicles.
• Two types of endoplasmic reticulum are:

• Rough endoplasmic reticulum (RER) :
  Ribosomes are attached to it.
  Helps in protein synthesis.

• Smooth endoplasmic reticulum (SER) :
  Ribosomes are not attached to it.
  Helps in lipid, fat synthesis.

• Functions of endoplasmic reticulum:

1. Transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
2. Functions as cytoplasmic framework providing a surface for some of the biochemical activities of the cell.
3. SER plays a crucial role in detoxifying of many drugs and poisons.

Golgi Apparatus

• It was first discovered by Camillo Golgi.
• It consists of membrane bound flattened sacs, arranged in stacks parallel to each other called as cisternae.
• These membranes often have connections with the membranes of E.R (endoplasmic reticulum).
• The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.
• Functions of Golgi apparatus :

1. Storage
2. Modification
3. Packaging of products into vesicles
4. Complex sugars may be made from simple sugars.
5. Helps in the formation of lysosomes.

Golgi Apparatus

Lysosomes

• Lysosomes are membrane-bound sacs filled with digestive enzymes.
• These enzymes are made up of RER.
• Foreign bodies like bacteria, food and old organelles end up in the lysosomes, and hence it breaks complex substances into simpler ones.
• When the cell gets damaged, lysosomes bursts, releasing the digestive enzymes and the enzymes digest their own cell. Therefore, lysosomes are also known as “suicidal bags of the cell”.

Mitochondria

• These are known as the power houses of the cell.
• They are double-membraned.
• The inner membrane is covered into folds known as cristae.
• These folds increase the surface area for ATP production. ATP – Adenosine tri-phosphate, it is the energy currency of the cell.
• Mitochondria have their own DNA and ribosomes.
• Since they have their own DNA and ribosomes they can make their own proteins.

Vacuoles

• These are store sacks for solid or liquid contents.
• Plant cells have vacuoles, animals have negligible or very small vacuoles.
• Vacuoles are centrally placed, and occupy 50-90% of the cell volume.
• Vacuoles provide turgidity and rigidity to the cell.
• Vacuoles includes amino acids, sugars, organic acids, some proteins.
• In amoeba, the food vacuoles contain the food contents.
• Vacuoles also play role in expelling excess of water or waste water.
• The covering of vacuoles is known as Tonoplast.

Plastids

• These are also exclusively present in plant cells.
• Types of plastids :

1. Chloroplasts: containing green pigment, help in photosynthesis.
2. Chromoplasts: coloured plastids, containing pigments
   such as “Xanthophylls” & “Carotenoids”.
3. Leucoplasts:  contains starch, oils and protein granules are stored.
4. Aleuroplasts: contains proteins.

• Chloroplasts consists of a matrix/ ground known as stroma.
• Inside the stroma there is stacks of grana known as thylakoid.
• Like mitochondria, plastids have their own DNA and ribosomes.

Cell Division

• New cells are formed in order to grow and replace old cells.
• There are two types of cell division:

1. Mitosis
2. Meiosis

• Mitosis

1. Occurs in somatic/body cells.
2. Results in two diploid daughter cells.
3. This is required for general body growth, and repair of cells.
4. It occurs only in one step.

• Meiosis

1. Occurs in reproductive/gamete cells.
2. Results in four haploid daughter cells.
3. This is required for reproduction process.
4. It occurs in 2 steps (ie. Meiosis I and Meiosis II).

Stages of Mitosis

Stages of Meiosis