Cell TheoryThe CELL THEORY, or cell doctrine, states that all organisms are composed of similar units of organization, called cells. The concept was formally articulated in 1839 by Schleiden & Schwann and has remained as the foundation of modern biology.

The modern tenets of the Cell Theory include: 

1. all known living things are made up of cells.    2. the cell is structural & functional unit of all living things.   3. all cells come from pre-existing cells by division.             (Spontaneous Generation does not occur).

 4. cells contains hereditary information which is passed from cell to cell during cell division.    5. All cells are basically the same in chemical composition.    6. all energy flow (metabolism & biochemistry) of life occurs               within cells.       

Robert HookeIn 1663 an English scientist, Robert Hooke, discovered cells in a piece of cork, which he examined under his primitive microscope.Hooke only observed cell walls because cork cells are dead and without cytoplasmic contents. Hooke drew the cells he saw and also coined the word CELL.

The word cell is derived from the Latin word 'cellula' which means small compartment.

Anton van LeeuwenhoekTen years later Anton van Leeuwenhoek (1632-1723), a Dutch businessman and a contemporary of Hooke used his own (single lens) monocular microscopes and was the first person to observe bacteria and protozoa

Leeuwenhoek's skill at grinding lenses, together with his naturally acute eyesight and great care in adjusting the lighting where he worked, enabled him to build microscopes that magnified over 200 times, with clearer and brighter images than any of his colleagues at that time.

Two Major Classes of Cells

One kind—a prokaryotic cell (pro KAR ee oh tik)— lacks a nucleus and most other organelles.

Bacteria and another group of organisms called the archaea are prokaryotic cells. Prokaryotic organisms appear earliest in Earth's fossil record.

A bacterium is an example of a prokaryotic cell (pro means "earlier than").

Without a true nucleus and the organelles of eukaryotic cells, prokaryotic cells are much simpler in structure

The DNA in a prokaryotic cell is concentrated in an area called the nucleoid region:

which is not separated from the rest of the cell by a membrane, as is the case in a eukaryotic cell

Eukaryotic cell (yoo KAR ee oh tik) has a nucleus surrounded by its own membrane, and has other internal organelles bounded by membranes.

Protists, fungi, plants, and animals consist of eukaryotic cells. Organisms with eukaryotic cells appeared later in Earth's history.

Membrane Structure• Membranes help keep the functions

of a eukaryotic cell organized. As partitions, the membranes isolate teams of enzymes within a cell's compartments.

• Membranes, unlike walls, regulate the transport of substances across the boundary, allowing only certain substances to pass.

plasma membrane

• The plasma membrane and other membranes of a cell are composed mostly of proteins and a type of lipid called phospholipids

A phospholipid molecule is structured much like the fat molecules but has only two fatty acids instead of three.

The two fatty acids at one end (the tail) of the phospholipid are hydrophobic.

The other end (the head) of the molecule includes a phosphate group, which is negatively charged and hydrophilic.

So the tail end of a phospholipid is pushed away by water, while the head is attracted to water

The structure of phospholipids allows them to form boundaries, or membranes, between two watery environments.

The phospholipids form a two-layer "sandwich" of molecules, called a phospholipid bilayer, that surrounds the organelle or cell.

Many types of proteins are embedded in the membrane's phospholipid bilayer.

Other molecules, such as carbohydrates, may be attached to the membrane as well, but the proteins perform most of the membrane's specific functions.

Another function of membrane proteins is to help cells—especially cells that are part of a multicellular organism—communicate and recognize each other.

Example, chemical signals released by one cell may be "picked up" by the proteins embedded in the membrane of another cell.

Still other membrane proteins, called transport proteins, help move certain substances such as water and sugars across the membrane

DiffusionMolecules in a fluid are constantly in motion, colliding and bouncing as they spread out into the available space.

One result of this motion is diffusion, the net movement of the particles of a substance from area of high (more) concentrated to where they are low (less) concentrated

Passive Transport

Cellular membranes are barriers to the diffusion of some substances. A selectively permeable membrane allows some substances to cross the membrane more easily than others and blocks the passage of some substances altogether.

Diffusion across a membrane is called passive transport because no energy is expended by the cell in the process.

Only the random motion of the molecules is required to move them across the membrane.

Osmosis

The passive transport of water across a selectively permeable membrane is called osmosis (ahs MOH sis).