a. Cell : unit of function

ISC 11> UNIT 4-UNIT OF LIFE> 4. CELL THEORY

SCOPE OF SYLLABUS

Historical aspects, cell theory, size and shape of cells; general structure of prokaryotic cell; differences between gram +ve and gram –ve bacteria.
General structure of eukaryotic cell, ultrastructure and function of cell wall, cell membrane (description of fluid mosaic model; functions of the plasma membrane: active and passive transport, brief explanation of facilitated diffusion (uniport, symport and antiport) with one example.
Mitochondria, nucleus (structure and types of chromosomes on the basis of the position of centromere, satellite), types of plastids, endomembrane system (endoplasmic reticulum, Golgi complex, lysosomes and vacuoles), ribosomes, microbodies, cytoskeleton, cilia, flagella and centrioles; difference between prokaryotic cell and eukaryotic cell, plant and animal cell, microfilaments and microtubules, flagella and cilia.

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CELL ORGANELLES
NUCLEUS AND CELL INCLUSIONS

Historical aspects

Observe the given picture
He is the person who gave the word "cell".
Identify the person.

What was his contibution to the scientific world?

What is the name of the book he published and what does it contain?

Web Link 1

rediscovering-robert-hooke-video

Observe the given picture.
He is considered as father of microscopy.

Identify the person.

What were his contributions?

Web Link 2

CELL THEORY

The first generalization about cells was given by a botanist Matthais Schleiden. In 1883 he reported that plant tissues are composed of cells and that an embryonic plant arose from a single cell. He declared that the cell is the basic building block of all plant matter.
The second generalization concerning cells and the most important in the development of biology was given by zoologist Theodar Schwann. In 1839, this German biologist, reached the same conclusion as Schleiden about animal tissue being composed of cells, ending speculations that plants and animals were fundamentally different in structure.
Schwann described cellular structures in animal cartilage. In 1855, Rudolph Virchow stated that new cells could only arise from the division of pre existing cells.These observations were put into cell theory-

All living things are made up of cells.
New cells are formed by the division of pre existing cells.
Each cell is made up of protoplasm with a nucleus that contains genetic material passed on to daughter cells.
All cells are basically alike in their structure and metabolic activities.

Exception to cell Theory

Viruses are biologists’ puzzle. They are an exception to cell theory. They lack protoplasm, the essential part of the cell.
Bacteria and cyanobacteria (Blue Green algae) lack well organized nucleus.
Some of the protozons are acellular.
The coenocytic hyphae of some fungi eg. Rhizopus have undivided massof protoplasm, in which many nuclei remain scattered.
Red Blood Corpuscles (RBC) and mature sieve tubes are without nuclei.

Let us recap something about the basic unit of an organism- CELL
Web.Link 3

PROKARYOTIC AND EUKARYOTIC CELLS

Structure of Prokaryotic cell

Prokaryotic cells have no true nucleus as the DNA is not contained within a membrane or separated from the rest of the cell, but is coiled up in a region of the cytoplasm called the nucleoid. The following structures can be found in prokaryotic cells:

Capsule - Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces and nutrients.
Cell Wall - Outer covering of most cells that protects the bacterial cell and gives it shape.
Cytoplasm - A gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.
Cell Membrane or Plasma Membrane - Surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.
Mesosomes -Invaginations of the cell membrane, has enzymes for cellular respiration.
Pili - Hair-like structures on the surface of the cell that attach to other bacterial cells. Shorter pili called fimbriae help bacteria attach to surfaces.
Flagella - Long, whip-like protrusion that aids in cellular locomotion.
Ribosomes - Cell organelles responsible for protein production.
Plasmids - Gene carrying, circular DNA structures that are not involved in reproduction.
Nucleiod Region - Area of the cytoplasm that contains the single bacterial DNA molecule.

Diagram of a prokaryotic cell

Draw and label the diagram

Structure of Eukaryotic cell

Draw and label the diagram

Plant cell and animal cell (under electron microscope)

Structure of Eukaryotic cell

The cell components can be classified as below-

Cell boundaries include cell membrane and cell wall.

Cell wall

Thick, rigid protective envelope.
Present in plants, bacteria, fungi and algae.
Bacteria and fungi – proteins and non-cellulosic carbohydrates.
Algae and plant cells– cellulose.
Cell wall formation – starts as thin layer of pectin.
Cellulose is added inside (secreted by cytoplasm) This forms the Primary cell wall- cellulose microfibrils.
Further layers of cellulose make up -Secondary cell wall
Hemicellulose is a substance that strengthens the cellulose fibers.
Middle lamellae is the fused pectate walls of two adjacent cells. This forms as a demarcation between two cells.

Fine pores in the cell wall are called plasmodesmata.
Electron microscopic studies show that it contains endoplasmic reticulum that is continuous from cell to cell.
This facilitate movement of substances

Functions of cell wall

Provides mechanical support
Protects plasma membrane
Determines cell shape
Protects against osmotic pressure
Translocation of water & other substances
Plays a role in cell expansion
Prevents water loss (cutin and suberin in the cell wall)
Protects against pathogens

Cell membrane

Double layered membrane made up of phospholipids.
This topic is covered as separate chapter.

CELL ORGANELES>