History of Cells & Cell Biology

• Robert Hooke discovered that cork resembled a honeycomb structure with little compartments by observing a thin slice of cork with a self-designed microscope.
• Hooke called the compartments cells, meaning "little rooms" in Latin.
• Cork comes from tree bark.

Living Organisms

• Onion bulbs of different sizes have similar cells visible under a microscope.
• Cells are the basic building units of the onion bulb.
• All organisms consist of cells.
• Some organisms are single cells acting independently.

History of Cell Biology (Cytology)

• Robert Hooke (1665) is considered the father of cytology (cell biology)
• Antonie van Leeuwenhoek with an improved microscope discovered free-living cells in pond water
• Leeuwenhoek is considered the father of microbiology.
• Robert Brown discovered the nucleus in the cell.
• Purkinje coined the term 'protoplasm' (nucleus + cytoplasm)
• Schleiden (botanist) and Schwann (zoologist) proposed cell theory: plants and animals are composed of cells, and the cell is the basic unit of life.
• Rudolf Virchow added all cells arise from pre-existing cells to the cell theory
• A single cell can form a whole organism like Amoeba, Chlamydomonas, Paramecium, and bacteria.

Unicellular and Multicellular Organisms

• Organisms made of single cells are unicellular (uni = single)
• Multicellular organisms (multi = many) consist of many cells grouped together to perform different functions; examples include fungi (except yeast), plants, and animals.
• Every multicellular organism originates from a single cell.
• Cells divide to produce cells of the same kind.

Shape & Size of Cells

• Cell shape and size correlate with their specific functions.
• Amoeba cells have changing shapes.
• Some cells have fixed and peculiar shapes, like nerve cells.
• Each living cell can perform some basic functions.
• Multicellular organisms, like humans, exhibit a division of labour.
• Division of labour is seen within a cell.
• Cells contain specific components known as cell organelles.
• Each organelle performs a specific function.
• Cell organelles enable a cell to live and perform its functions.
• Cell organelles make up the basic unit called the cell.
• All cells share the same organelles.

Cell Composition

• Every cell contains plasma membrane, nucleus, and cytoplasm.
• All activities and interactions of the cell are possible.

Plasma Membrane

• Plasma membrane (cell membrane) is the outermost cell covering that separates the cell's contents from its external environment.
• Components of the cell bound by the plasma membrane is protoplasm.
• It allows entry and exit of some materials.
• It prevents some materials from moving.
• The cell membrane is selectively permeable.
• Substances like carbon dioxide and oxygen move across the cell membrane by diffusion.
• CO2 diffuses across the plasma membrane because CO2 concentration is high inside the cell and low outside.
• O2 enters the cell when the O2 concentration inside the cell decreases.
• Diffusion plays a role in gaseous exchange between cells.
• Water also obeys the law of diffusion.
• The movement of water through a selectively permeable membrane is osmosis.
• The movement of water across the plasma membrane is affected by substances dissolved in water.
• Osmosis is water passage from a high to low concentration through a selectively permeable membrane until equilibrium.

Solutions and their tonicity

• A hypotonic solution surrounding a cell has a high water concentration, so the cell gains water by endosmosis and swells up.
• An isotonic solution has the same water concentration as the cell, resulting in no net water movement, and the cell maintains its size.
• A hypertonic solution has a lower water concentration than the cell, causing the cell to lose water by exosmosis and shrink.
• Unicellular freshwater organisms and most plant cells gain water through osmosis.
• Absorption of water by plant roots is an example of osmosis.
• Diffusion facilitates exchange of gases and water in a cell.
• The cell obtains nutrition from its environment.
• Different molecules move in and out of cells through active transport (requires energy).
• The plasma membrane is flexible and consists of lipids and proteins.
• The structure of the plasma membrane is visible via electron microscope.
• Cell’s flexibility enables it to engulf food through endocytosis (phagocytosis for Amoeba).

Cell Wall

• Plant cells have a rigid outer covering called the cell wall, located outside the plasma membrane
• The cell wall is non-living.
• Living components inside the cell wall are protoplast.
• Plant cell walls are mainly made of cellulose, which provides the structural strength to plants.
• Plasmolysis is when a plant cell loses water through osmosis causing contents to shrink, away from the cell wall.
• Only living cells can absorb water by osmosis.
• Cell walls allow plant cells, fungi, and bacteria to withstand very dilute (hypotonic) external media without bursting.
• Cells in dilute media take up water, building up pressure against the cell wall.
• Cell walls counteract the pressure from the swollen cell.
• The surrounding medium changes the cell walls of plants more than it does with animal cells.

Nucleus

• Iodine, safranin, or methylene blue solutions stain cells.
• Nucleus is a dark, oval or spherical structure in the center of each cell
• A double-layered nuclear membrane covers the nucleus.
• A nuclear membrane contains pores allowing nucleoplasm to cytoplasm transfer.
• Nucleus contains chromosomes, visible as rod-shaped structures when the cell divides.
• Chromosomes contain inheritable information from parents in the form of DNA molecules.
• Chromosomes consist of DNA and protein.
• DNA molecules contain information for constructing and organizing cells.
• Functional segments of DNA are genes.
• A non-dividing cell contains DNA as chromatin material, which appear as threadlike structures.
• Chromatin material organizes into chromosomes when the cell divides.
• Nucleus plays a role in cellular reproduction.
• The cell depends on the environment and the nucleus to decide to develop and how it will develop as the organism matures
• Bacteria lack a well-defined nuclear region due to the absence of a nuclear membrane.
• A nucleoid is the undefined nuclear region that contains only nucleic acids.

Prokaryotes vs Eukaryotes

• Prokaryotes lack nuclear membrane.
• Eukaryotes have nuclear membrane.
• Prokaryotes also lack most cytoplasmic organelles.
• The functions of prokaryotic organelles are performed by poorly formed parts of the cytoplasm.
• Chlorophyll in photosynthetic prokaryotic bacteria associates with membranous vesicles but not with plastids like in eukaryotic cells.

Cytoplasm

• Cytoplasm is a large region of the cell enclosed by the cell membrane.
• Cytoplasm takes up minimal stains under observation
• Cytoplasm contains fluid and special organelles.
• Each organelle specializes in functions for the cell
• Cell organelles are enclosed by membranes in eukaryotes
• Prokaryotes do not have membrane-bound organelles.
• Eukaryotes have membrane-enclosed organelles as well as a nuclear membrane.
• Viruses do not show characteristics of life until they multiply inside a living host.

Sizes of Cells

• Prokaryotic cells are small with undefined nuclear regions whereas Eukaryotic cells are large and have double layered nuclear membrane
• In a prokaryote, the chromatin condenses to a single chromosome, whereas eukaryotes have multiple.
• No membrane-bound organelles in prokaryotes. Membrane-bound organelles in eukaryotes

Cell Organelles

• Cells use membrane-bound little structures called cell organelles to support various complicated activities
• Some chemicals separate with membrane-bound little structures

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets.
• ER looks like either tubules or vesicles (round or oblong bags).
• The ER membrane structurally resembles the plasma membrane.

Two Types of Endoplasmic Reticulum

• Rough endoplasmic reticulum (RER) appears rough as it is attached to ribosomes, which are the sites of protein manufacture.
• The ribosomes, which produce RER are present in every active cell
• Ribosomes’ manufactured proteins are transported to various places inside the cell via ER
• Smooth endoplasmic reticulum (SER) helps in the manufacture of lipids for cell function.
• Proteins and lipids help in membrane biogenesis and function as enzymes and hormones
• Endoplasmic Reticulum (ER) varies in appearance in different cells, but forms a network system.
• ER transports materials between cytoplasm regions between cytoplasm to nucleus.
• ER serves as a framework for biochemical activities.
• In vertebrates, SER in the liver cells detoxifies poisons and drugs.

Golgi Apparatus

• Camillo Golgi first described the Golgi apparatus.
• Golgi body consists of flattened sacs of membrane-bound vesicles arranged parallel called cisterns
• Golgi membranes connect with the ER membranes.
• ER synthesizes, stores, modifies, packages, and dispatches materials inside and outside the cell with the Golgi apparatuses
• In some cases, complex sugars are produced y the Golgi apparatus from simple sugars.
• Golgi apparatus is responsible for the formation of lysosomes

Lysosomes

• Lysosomes are membrane-bound sacs with digestive enzymes made by RER and modified by Golgi bodies.
• They are the cell’s waste disposal system
• Lysosomes keep cells clean by digesting foreign material and worn out organelles.
• During cell damage lysosomes may burst and digest the cell.
• Lysosomes are known as the 'suicide bags' of the cell.

Mitochondria

• Mitochondria are referred to as "powerhouses of the cell".
• Double membrane coverings.
• The outer membrane is porous whereas the inner membrane is deeply folded.
• ATP is created when folds increase surface area for ATP generating chemical reactions.
• The energy required for life’s activities is released by mitochondria in the form of ATP (Adenosine tri-phosphate) molecules
• ATP functions as the energy currency of the cell.
• The body uses ATP to make new chemical compounds and work mechanically.
• Mitochondria have their own DNA and ribosomes.
• Mitochondria can form their own proteins.

Plastids

• Plastids are found in plant cells
• Plastids contain DNA and ribosomes

Types of Plastids

• Chromoplasts (colored plastids) containing chlorophyll are named chloroplasts, they are important for photosynthesis and yellow/orange pigments.
• Internal organization of Chloroplast consists of membrane layers embedded in stroma
• Leucoplasts (white or colourless plastids) store materials such as starch, oils and protein granules.

Vacuoles

• Vacuoles store solid or liquid contents.
• Vacuoles are small in animal cells and large in plant cells (occupying 50-90% of the cell volume)
• Vacuoles in plants store cell sap in cell sap which provides turgidity and rigidity.
• Vacuoles store substances like amino acids, sugars, various organic acids, and some proteins etc.
• Amoeba has a food vacuole with consumed food items.
• Unicellular organisms have contractile vacuoles for expelling excess water and some wastes.
• Membrane, organelles, structure, and function are due to the organization.
• The structural organization helps to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins.
• The cell is the fundamental structural and functional unit of life.

Cell Division

• Cell division creates new cells to form organisms, replace old cells and create gametes
• The process is known is cell division
• The process is called cell division
• Mitosis: the process of cell division for growth and repair, a mother cell divides and forms two clones of identical cells, each daughter cell has a same clone for chromosomes.
• Meiosis: the cells divide by two consecutive divisions where reproductive organs of animals and plants form gametes by fertilisation to give rise to offspring, 4 new cells created each with half the number of chromosomes of the mother cells.