Cell Organelle A small organ-like structure present inside the cell is called a cell organelle. It has a particular structural makeup and performs a specific function. Depending upon the presence or absence of membrane, cell organelles can be classified into three categories, namely: Without membrane: Some cell organelles like ribosome are not bounded by any membrane. They are present in prokaryotic as well as eukaryotic organisms. Single membrane-bound: Some organelles are bounded by a single membrane. For example, vacuole, lysosome, Golgi Apparatus, Endoplasmic Reticulum etc. They are present only in a eukaryotic cell. Double membrane-bound: Cell organelles like mitochondria and chloroplast are double membrane-bound organelles. They are present only in a eukaryotic cell. Structure and Functions of Cells Structure and functions of different cell inclusions are as follows: Cell Organelle Cell Membrane/ Plasma Membrane Occurrence/ Characteristic & Structure Function Present in both plant cell and animal cell. Selectively Permeable: Allows the materials in and out of the cell according to the requirement of the cell. Encloses the contents of the cell. Provides shape: animal cell. Allows transport: by Diffusion and Osmosis. Made up of bilipid layer and protein (Fluid Mosaic Model) Cell Wall Cytoplasm Nucleus Present only in a plant cell. Hard and rigid. Fully permeable. Made up of Cellulose in plant and peptidoglycan in bacteria. Protection Gives shape and turgidity. Contains 80-90% water and many organic and inorganic compounds. Colloidal, Viscous, Jelly like fluid inside the cell. Contains enzymes responsible for all the metabolic activity taking place inside the cell. Covered by a double membranous nuclear membrane in a Eukaryotic Cell. Controls the activity of the cell. Contains DNA, RNA, Protein, nucleolus, and Chromatin network. Starts cell division. It has the chromosomes or DNA which controls the hereditary characters (Director/ Brain of the Cell) *Double membranous structure. *Autonomous body as contains its own DNA. *Self-duplicates *The main seat of respiration. *Stores energy in the form of ATP molecules. Mitochondria (The Power House of The Cell / Storage Batteries) Discovered by Camillo Golgi in 1898. Originates from RER. Contains Sac like Cisternae and Vesicles. Has two faces – cis face or receiving face and trans face or supplying face. Modification, Packaging, and transport of materials Synthesis of lysosomes, plasma membrane A network of membranes. RER bears ribosomes and appears rough SER does not have ribosomes Forms the skeletal framework of the cell. Transport of materials from one cell to other. Provides a surface for the synthesis of material – Proteins in RER and Lipids in SER. Formation of lysosomes, Golgi bodies and vacuoles Membrane Biogenesis Detoxification of harmful substances in the liver. Arise from ER and GB Surrounded by tonoplast and filled with cell sap Golgi Bodies (Shipping Department of Cell) Endoplasmic Reticulum (Framework of Cell) Vacuole *Store cell sap which may be liquid or solid food, toxic byproduct. *Provide rigidity and turgidity to plant cell Lysosomes (Suicidal bags of Cell, natural Membrane-bound organelles Present in all animal cells and few plant cells Intracellular digestion of food in unicellular organisms. Tiny circular single membrane-bound structures filled with digestive enzymes scavenger, cellular housekeeper) Without a membrane Consist of two subunits – 60S and 40S in eukaryote both made up of RNA Synthesis of Proteins Ribosomes (Protein Factories) *Double membrane-bound Types: 1. 1. Leucoplast –Colourless plastid; 2. 2. Chromoplast –Coloured Plastid – blue, red, yello 3. 3. Chloroplast – Green plastid *Autonomous self-duplicating body Plastids Chloroplast – Perform Photosynthesis – Helps in the release of oxygen Chromoplast – impart colour to flowers which help in pollination Leucoplast – Storage Amyloplast – Store starch Aleuroplast – Store Protein Elaioplast – Store fat The cell cycle and mitosis Key terms Term Meaning Cell cycle The series of growth and development steps a cell undergoes between its formation and reproduction Interphase Phase of the cell cycle where the cell grows and makes a copy of its DNA Mitosis Phase of the cell cycle where the cell separates its DNA into two sets and divides, forming two new cells Cancer A disease of uncontrolled cell growth The cell cycle In eukaryotic cells, the cell cycle is divided into two major phases: interphase and mitosis (or the mitotic (M) phase). Interphase is the longest part of the cell cycle. This is when the cell grows and copies its DNA before moving into mitosis. During mitosis, chromosomes will align, separate, and move into new daughter cells. The prefix inter- means between, so interphase takes place between one mitotic (M) phase and the next. Interphase is composed of G1 phase (cell growth), followed by S phase (DNA synthesis), and followed by G2 phase (cell growth). At the end of interphase comes the mitotic phase, which is made up of mitosis and cytokinesis and leads to the formation of two daughter cells. Mitosis precedes cytokinesis, though the two processes typically overlap somewhat. Interphase Interphase consists of three steps: G_11start subscript, 1, end subscript phase: first gap phase; the cell grows larger and organelles are copied S phase: synthesis phase; the cell synthesizes a complete copy of the DNA in its nucleus G_22start subscript, 2, end subscript phase: second gap phase; the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis Cells that are meant to divide will complete G_22start subscript, 2, end subscript and enter mitosis. Other types of cells that divide slowly or not at all may exit the G_11start subscript, 1, end subscript phase and enter a non-dividing state called G_00start subscript, 0, end subscript. Some cells remain here indefinitely, while others may re-enter division under the right conditions. Mitosis (the M phase) The process of mitosis, or cell division, is also known as the M phase. This is where the cell divides its previously-copied DNA and cytoplasm to make two new, identical daughter cells. Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase. Stages of mitosis Early prophase. The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears. Late prophase (prometaphase). The nuclear envelope breaks down and the chromosomes are fully condensed. Metaphase. Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. Anaphase. The sister chromatids separate from one another and are pulled towards opposite poles of the cell. The microtubules that are not attached to chromosomes push the two poles of the spindle apart, while the kinetochore microtubules pull the chromosomes towards the poles. Telophase. The spindle disappears, a nuclear membrane re-forms around each set of chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes also start to decondense. Cytokinesis in animal and plant cells. Cytokinesis in an animal cell: an acting ring around the middle of the cell pinches inward, creating an indentation called the cleavage furrow. Cytokinesis in a plant cell: the cell plate forms down the middle of the cell, creating a new wall that partitions it in two.