Biology – Concept – Important Points For Exam

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Biology

Cell

1- Robert Brown -Discovered Cell Nucleus in 1831

2-Schleiden and Schwann presented the cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life.

3-Robert Hooke – Discovered and coined the term cell in 1665

4- It was possible to observe and understand the complex structure of the cell and its various organelles after the discovery of the electron microscope in 1940.

Plasma Membrane or Cell Membrane

1-Plasma membrane is the outermost covering of the cell.

2-Cell membrane is also called the plasma membrane

3-It can be observed only through an electron microscope.

4-The plasma membrane is made up of organic molecules called lipids and proteins.

* Due to the flexibility nature of the cell membrane, it enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis (endo → internal; cyto → of a cell).

* Amoeba acquires its food through endocytosis process.

* Diffusion is a spontaneous movement of a substance from a region of high concentration (hypertonic solution) to a region where its concentration is low (hypotonic solution)

* Diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.

*Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.

*The plasma membrane is porous and allows the movement of substances or materials both inward and outward.

*Plasma membrane is a selectively permeable membrane

Osmosis

*The movement of water molecules through a selectively permeable membrane is called osmosis.

* Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

* Absorption of water by plant roots is also an example of osmosis

* Unicellular freshwater organisms and most plant cells tend to gain water through osmosis

* Osmosis is a special case of diffusion through a selectively permeable membrane.

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