SCIENCE

• Cell


  The basic unit of life
• Matthias Schleiden


  • All plants are made of cells
• Theodor Schwann


  • All animals are made of cells
• All cells come from pre-existing cells
• All organisms are made up of different cells
• Eukaryotic cells


  Cells with a membrane bound nucleus
• Both animal and plant cells are eukaryotic cells
• Organelles

  The different parts of a cell
• Cell Membrane

  A thin semipermeable membrane layer of lipids and proteins surrounding the cell. Its primary role is to protect the cell from its surroundings. Also, it controls the entry and exit of nutrients and other microscopic entities into the cell. For this reason, cell membranes are known as semi-permeable or selectively permeable membranes.
• Nucleus

  It is an organelle that contains several other sub-organelles such as nucleolus, nucleosomes and chromatins. It also contains DNA and other genetic materials.
• Nuclear Membrane


  It is a double-membrane structure that surrounds the nucleus. It is also referred to as the nuclear envelope.
• Centrosome


  It is a small organelle found near the nucleus, which has a thick center with radiating tubules. The centrosomes are where microtubules are produced.
• Lysosome


  They are round organelles surrounded by a membrane and comprising digestive enzymes which help in digestion, excretion and in the cell renewal process.
• Cytoplasm


  A jelly-like material which contains all the cell organelles, enclosed within the cell membrane. The substance found within the cell nucleus, contained by the nuclear membrane is called the nucleoplasm.
• Golgi Apparatus


  A flat, smooth layered, sac-like organelle which is located near the nucleus and involved in manufacturing, storing, packing and transporting the particles throughout the cell.
• Mitochondrion


  They are spherical or rod-shaped organelles with a double membrane. They are the powerhouse of a cell as they play an important role in releasing energy.
• Ribosome


  They are small organelles made up of RNA-rich cytoplasmic granules, and they are the sites of protein synthesis.
• Endoplasmic Reticulum (ER)


  This cellular organelle is composed of a thin, winding network of membranous sacs originating from the nucleus. They form a cisterna.
• Vacuole


  A membrane-bound organelle present inside a cell involved in maintaining shape and storing water, food, wastes, etc.
• Nucleopore


  They are tiny holes present in the nuclear membrane which are involved in the movement of nucleic acids and proteins across the nuclear membrane.
• Cell Wall


  It is a rigid layer which is composed of polysaccharides cellulose, pectin and hemicellulose. It is located outside the cell membrane. It also comprises glycoproteins and polymers such as lignin, cutin, or suberin. The primary function of the cell wall is to protect and provide structural support to the cell. The plant cell wall is also involved in protecting the cell against mechanical stress and providing form and structure to the cell. It also filters the molecules passing in and out of it. The formation of the cell wall is guided by microtubules. It consists of three layers, namely, primary, secondary and the middle lamella. The primary cell wall is formed by cellulose laid down by enzymes.
• Cell membrane


  It is the semi-permeable membrane that is present within the cell wall. It is composed of a thin layer of protein and fat. The cell membrane plays an important role in regulating the entry and exit of specific substances within the cell. For instance, cell membrane keeps toxins from entering inside, while nutrients and essential minerals are transported across.
• Nucleus


  The nucleus is a membrane-bound structure that is present only in eukaryotic cells. The vital function of a nucleus is to store DNA or hereditary information required for cell division, metabolism and growth. Nucleolus: It manufactures cells' protein-producing structures and ribosomes. Nucleopore: Nuclear membrane is perforated with holes called nucleopore that allow proteins and nucleic acids to pass through.
• Plastids


  They are membrane-bound organelles that have their own DNA. They are necessary to store starch and to carry out the process of photosynthesis. It is also used in the synthesis of many molecules, which form the building blocks of the cell. Leucoplasts: They are found in the non-photosynthetic tissue of plants. They are used for the storage of protein, lipids and starch. Chloroplasts: It is an elongated organelle enclosed by phospholipid membrane. The chloroplast is shaped like a disc and the stroma is the fluid within the chloroplast that comprises a circular DNA. Each chloroplast contains a green coloured pigment called chlorophyll required for the process of photosynthesis. The chlorophyll absorbs light energy from the sun and uses it to transform carbon dioxide and water into glucose. Chromoplasts: They are heterogeneous, coloured plastid which is responsible for pigment synthesis and for storage in photosynthetic eukaryotic organisms. Chromoplasts have red, orange and yellow coloured pigments which provide color to all ripe fruits and flowers.
• Central Vacuole


  It occupies around 30% of the cell's volume in a mature plant cell. Tonoplast is a membrane that surrounds the central vacuole. The vital function of the central vacuole apart from storage is to sustain turgor pressure against the cell wall. The central vacuole consists of cell sap. It is a mixture of salts, enzymes and other substances.
• Golgi Apparatus


  They are found in all eukaryotic cells, which are involved in distributing synthesized macromolecules to various parts of the cell.
• Ribosomes


  They are the smallest membrane-bound organelles which comprise RNA and protein. They are the sites for protein synthesis, hence, also referred to as the protein factories of the cell.
• Mitochondria


  They are the double-membraned organelles found in the cytoplasm of all eukaryotic cells. They provide energy by breaking down carbohydrate and sugar molecules, hence they are also referred to as the "Powerhouse of the cell."
• Lysosome


  Lysosomes are called suicidal bags as they hold digestive enzymes in an enclosed membrane. They perform the function of cellular waste disposal by digesting worn-out organelles, food particles and foreign bodies in the cell. In plants, the role of lysosomes is undertaken by the vacuoles.
• Plant Cell Types


  • Collenchyma Cells
  • Sclerenchyma Cells
  • Parenchyma Cells
  • Xylem Cells
  • Phloem Cells

  
• Collenchyma Cells


  • They are hard or rigid cells, which play a primary role in providing support to the plants when there is restraining growth in a plant due to lack of hardening agent in primary walls.
• Sclerenchyma Cells


  • These cells are more rigid compared to collenchyma cells and this is because of the presence of a hardening agent. These cells are usually found in all plant roots and mainly involved in providing support to the plants.
• Parenchyma Cells


  • Parenchyma cells play a significant role in all plants. They are the living cells of plants, which are involved in the production of leaves. They are also involved in the exchange of gasses, production of food, storage of organic products and cell metabolism. These cells are typically more flexible than others because they are thinner.
• Xylem Cells


  • Xylem cells are the transport cells in vascular plants. They help in the transport of water and minerals from the roots to the leaves and other parts of the plants.
• Phloem Cells


  • Phloem cells are other transport cells in vascular plants. They transport food prepared by the leaves to different parts of the plants.
• Plant Cell Functions


  • Photosynthesis is the major function performed by plant cells. Photosynthesis occurs in the chloroplasts of the plant cell. It is the process of preparing food by the plants, by utilizing sunlight, carbon dioxide and water. Energy is produced in the form of ATP in the process. A few plant cells help in the transport of water and nutrients from the roots and leaves to different parts of the plants.
• Cell Cycle


  Cell cycle refers to the series of events that take place in a cell, resulting in the duplication of DNA and division of cytoplasm and organelles to produce two daughter cells.

  
• Interphase


  Also known as the resting phase of the cell cycle; interphase is the time during which the cell prepares for division by undergoing both cell growth and DNA replication. It occupies around 95% time of the overall cycle. The interphase is divided into three phases: G1 phase (Gap 1) - G1 phase is the phase of the cell between mitosis and initiation of replication of the genetic material of the cell. During this phase, the cell is metabolically active and continues to grow without replicating its DNA. S phase (Synthesis) - DNA replication takes place during this phase. If the initial quantity of DNA in the cell is denoted as 2N, then after replication it becomes 4N. However the number of chromosomes does not vary, viz., if the number of chromosomes during G1 phase was 2n, it will remain 2n at the end of S phase. The centriole also divides into two centriole pairs in the cells which contain centriole. G2 phase (Gap 2) - During this phase, the RNA, proteins, other macromolecules required for multiplication of cell organelles, spindle formation, and cell growth are produced as the cell prepares to go into the mitotic phase.

  
• M phase


  This is the mitotic phase or the phase of the equational division as the cell undergoes a complete reorganization to give birth to a progeny that has the same number of chromosomes as the parent cell. The other organelles are also divided equally by the process of cytokinesis which is preceded by mitotic nuclear division. The mitotic phase is divided into four overlapping stages: Prophase, Metaphase, Anaphase, and Telophase.

  
• Mitosis


  The process by which a eukaryotic cell separates the nuclear DNA and chromosomes