The Fundamental Unit of Life-Notes
Biology - Notes
Cells
Cells are the basic building blocks of all living things. Cells have many parts, each with a different function. Some of these parts, called organelles, are specialized structures that perform certain tasks within the cell.
Prominent three features in almost every cell; plasma membrane, nucleus and cytoplasm.
Structural Organisation of living organism
PLASMA MEMBRANE
This is the outermost layer of the cell that separates the cell's contents from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents the movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane. Carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.
The plasma membrane is flexible and composed of organic molecules called lipids (Phospholipids) and proteins.
Active and Passive Transport
Active transport is the movement of molecules across a membrane from a region of lower concentration to a region of higher concentration against the concentration gradient, often assisted by enzymes and requires energy
Passive transport is the movement of ions and molecules across the cell membrane without requiring energy.
DIFFUSION
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration down the concentration gradient.
OSMOSIS
Water also obeys the law of diffusion. The movement of water molecules through such a selectively permeable membrane is called osmosis.
The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.
Hypotonic Solution
If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution.
Isotonic Solution
If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution.
Hypertonic Solution
If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.
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.
Endocytosis
Endocytosis is defined as the process of trapping a particle or even a substance from the external environment by the process of engulfing it. The flexibility of the cell membrane helps the cell to engulf the food and also other materials from the external environment.
Amoeba acquires its food through such processes.
CELL WALL
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.
Plasmolysis
Plasmolysis is the process in which cells lose water when they are placed in a hypertonic solution. It causes contraction or shrinking of the plasma membrane away from the cell wall. It is a reversible process and the cell can get back to normal when placed in a hypotonic solution.
NUCLEUS
The nucleus is a double-membraned organelle that contains the genetic material and other instructions required for cellular processes. It is exclusively found in eukaryotic cells and is also one of the largest
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 characters from parents to next generation in the form of DNA (Deoxyribonucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organising cells. Functional segments of DNA are called genes.
Nucleoid
A nucleoid is a region within a prokaryotic cell that contains the genetic material (DNA). Unlike eukaryotic cells, prokaryotic cells do not have a membrane-bound nucleus.
Prokaryotes
Prokaryotes are those organisms which do not contain a nucleus and other membrane-bound organelles. They are unicellular.
Bacteria are an example of prokaryotes.
Eukaryotes
Eukaryotes are those organisms which contain a nucleus and membrane-bound organelles. They are either unicellular or multicellular.
The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag like structures) but not with plastids as in eukaryotic cells.
CYTOPLASM
Cytoplasm is the gelatinous liquid that fills the inside of a cell. It is composed of water, salts, and various organic molecules. Some intracellular organelles, such the nucleus and mitochondria, are enclosed by membranes that separate them from the cytoplasm.
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.
CELL ORGANELLES
The cellular components are called cell organelles. These cell organelles include both membrane and non-membrane-bound organelles, present within the cells and are distinct in their structures and functions. They coordinate and function efficiently for the normal functioning of the cell.
Biogenesis
Biogenesis is the process by which proteins and lipids are used to synthesise the cell membrane. The endoplasmic reticulum is in charge of making these membranes. Membranes produced by the membrane biogenesis process aid in the division of the cell into morphological and chemical compartments.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells. All eukaryotic cells contain an ER. In animal cells, the ER usually constitutes more than half of the membranous content of the cell. The ER membrane is similar in structure to the plasma membrane.
The function of the ER is to serve as channel for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
The ER also functions as a cytoplasmic framework, providing a surface for some of the biochemical activities in the cell. In the liver cells of the group of animals called vertebrates. SER plays a crucial role in detoxifying many poisons and drugs.
GOLGI APPARATUS
The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles (flattened sacs) arranged approximately parallel to each other in stacks called cisterns.
The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its functions include the storage, modification and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus. The Golgi apparatus is also involved in the formation of lysosomes
LYSOSOMES
Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER. Lysosomes are a kind of waste disposal system of the cell. These help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles.
Lysosomes may burst, and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.
MITOCHONDRIA
Mitochondria are known as the powerhouses of the cell. Mitochondria have two membrane coverings. The outer membrane is porous while the inner membrane is deeply folded. These folds increase surface area for ATP- generating chemical reactions. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work.
Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.
PLASTIDS
Plastids are present only in plant cells. There are two types of plastids – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids). Chromoplasts containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.
The internal organisation of the Chloroplast consists of numerous membrane layers embedded in a material called the stroma. These are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.
VACUOLES
Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.
vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins.
In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed.
CELL DIVISION
New cells are formed in organisms in order to grow, to replace old, dead and injured cells, and to form gametes required for reproduction. The process by which new cells are made is called cell division. There are two main types of cell division: mitosis and meiosis.
Mitosis
The process of cell division by which most of the cells divide for growth is called mitosis. In this process, each cell, called a mother cell, divides to form two identical daughter cells. The daughter cells have the same number of chromosomes as the mother cell. It helps in the growth and repair of tissues in organisms.
Meiosis
Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilisation give rise to offspring. They divide by a different process called meiosis which involves two consecutive divisions. When a cell divides by meiosis it produces four new cells instead of just two The new cells only have half the number of chromosomes than that of the mother cells.
