- CELL ORGANELLES
- PLASMA MEMBRANE OR CELL MEMBRANE
- CELL WALL
CELL BIOLOGY – 01
|Robert Hooke||Discovered and coined the term cell in 1665|
|Robert Brown||Discovered Cell Nucleus in 1831|
|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. Schleiden (1838) and Schwann (1839).|
- Cell membrane is also called the plasma membrane.
- It can be observed only through an electron microscope.
- Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment.
- The plasma membrane is flexible and is made up of organic molecules called Lipids & Proteins.
- The flexibility of the cell membrane also 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 such processes.
Plasma membrane is a selectively permeable membrane .The plasma membrane is porous and allows the movement of substances or materials both inward and outward.
- Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion [spontaneous movement of a substance from a region of high concentration (hypertonic solution) to a region where its concentration is low (hypotonic solution)].
- Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.
- Water also obeys the law of diffusion. 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. Thus, osmosis is a special case of diffusion through a selectively permeable membrane.
- Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.
- Diffusion is important in exchange of gases and water in the life of a cell. In additions to this, the cell also obtains nutrition from its environment.
- Different molecules move in and out of the cell through a type of transport requiring use of energy in the form of ATP.
Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove larger particles from drinking water.
- In reverse osmosis, an applied pressure is used to overcome osmotic pressure.
- Reverse Osmosis is a phenomenon where pure water flows from a dilute solution [hypotonic] through a semi permeable membrane to a higher concentrated solution [hypertonic].
- Semi Permeable means that the membrane will allow small molecules and ions to pass through it but acts as a barrier to larger molecules or dissolved substances.
- Cell wall is absent in animals.
- Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane.
- The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.
- 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 .
- Only living cells, and not dead cells, are able to absorb water by osmosis. Cell walls permit the cells of plants, Fungi And Bacteria to withstand very dilute external media without shrinkage.
- Cell wall also prevents the bursting of cells when the cells are surrounded by a hypertonic medium (medium of high concentration).
- Because of their walls, plant cells can withstand much greater changes in the surrounding medium than animal cells.
- It is the jelly-like substance present between the cell membrane and the nucleus.
- The cytoplasm is the fluid content inside the plasma membrane.
- It also contains many specialized cell organelles [Mitochondria, Golgi Bodies, Ribosomes, Etc].
- Each of these organelles performs a specific function for the cell.
- Cell organelles are enclosed by membranes.
- The significance of membranes can be illustrated with the example of viruses.
- 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.
- It is an important component of the living cell.
- It is generally spherical and located in the center of the cell.
- It can be stained and seen easily with the help of a microscope.
- Nucleus is separated from the cytoplasm by a double layered membrane called the nuclear membrane.
- This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus [diffusion].
- With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the NUCLEOLUS.
- In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The CHROMOSOMES can be seen only when the cell divides.
- GENE is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring. This means that your parents pass some of their characteristics on to you.
- Nucleus, in addition to its role in inheritance, acts as control center of the activities of the cell.
- The entire content of a living cell is known as PROTOPLASM [cytoplasm + nucleus]. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.
- The nucleus of the bacterial cell is not well organized like the cells of multicellular organisms. There is no nuclear membrane.
- Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function.
- The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
- Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (deoxyribo nucleic acid)
- Chromosomes are composed of DNA and Protein.
- DNA molecules contain the information necessary for constructing and organizing cells. Functional segments of dna are called genes.
- In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
- It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.