Chapt_3_Cell_part-2.pdf

Cell Structure and Function Recall: The cell is the basic unit of life. It is the smallest structural and functional unit of all living organisms. Cells are microscopic in size and vary in shape and size depending on their function. Most cells are large, like red blood cells (R...

Cell Structure and Function Recall: The cell is the basic unit of life. It is the smallest structural and functional unit of all living organisms. Cells are microscopic in size and vary in shape and size depending on their function. Most cells are large, like red blood cells (RBCs), while some are very small, like nerve cells, ovum (egg cell), etc. Mycoplasmas, the smallest cells, are only 0.3 µm in length. Bacteria could be 3 to 5 µm. The largest isolated single cell is the egg of an ostrich. Among multicellular organisms, human red blood cells are about 7.0 µm in diameter. Nerve cells are some of the longest cells  Shape and Size of the Cell Shape Size Examples Cells vary in shape and size. The size of a cell is also For example, red blood cells are They can be spherical, oval, determined by its function. The small and round because they rectangular, spindle-shaped, or longest living cell in the body is need to be able to move easily irregular. The shape of a cell is the nerve cell (or neuron), which through blood vessels. Nerve determined by its function. can live for an entire lifetime. cells are long and thin because they need to be able to transmit signals over long distances. preencoded.png Shape and Size of the Cell Can you guess the cell from its shape? Nerve cell preencoded.png  Components and Organelles of the Cell The cell membrane, cytoplasm, and nucleus are the three main parts of a cell Plasma Membrane Cytoplasm Nucleus The outermost covering of the cell, The jelly-like substance inside the cell The control center of the cell, regulating the entry and exit of membrane, containing various containing genetic information and substances. It is composed of lipids and organelles and cell inclusions. regulating cellular activities. It is proteins. enclosed by a double membrane.  Plasma Membrane or Cell Membrane The outermost covering of the cell separates the contents of the cell from the external environment. It regulates the entry and exit of substances through the cell (selectively permeable membrane). It is made up of lipids and proteins. Fluid Mosaic Model The fluid mosaic model was proposed by S.J. Singer and Garth L. Nicolson. This model explains the structure of the plasma membrane of animal cells as a mosaic of components such as phospholipids, proteins, cholesterol, and carbohydrates. Why is it called as Fluid-Mosaic Model? The model is called mosaic as it suggests that the membrane is made up of many different constituents like proteins, carbohydrates and lipids which resemble the tiles of an ordinary mosaic. preencoded.png  Plasma Membrane or Cell Membrane Description of Fluid Mosaic Model Phospholipid Bilayer A phospholipid bilayer comprises two layers of phospholipid. Each phospholipid is composed of a hydrophilic head and a hydrophobic tail. The hydrophilic head is attracted to the water in the cytoplasm of the cell as well as to the water surrounding the cell. The hydrophobic tail is repelled by water and attracted towards each other. preencoded.png  Transport Through the Plasma Membrane The movement of substances through the cell membrane takes place by three modes called passive transport, active transport, and bulk transport. Passive Transport Active Transport Bulk Transport Passive transport is a Active transport is the Bulk transport involves mode of transport that movement of ions and the movement of takes place without electrolytes, which micronutrients such the expenditure of takes place across the as proteins and energy. membrane by using polysaccharides which energy produced by the can pass only through It takes place either by cell. a ruptured plasma diffusion or osmosis. To get molecules to membrane. move against the concentration gradient, i.e., from lower concentration to higher concentration, work must be done, hence energy is required. preencoded.png  Transport Through the Plasma Membrane The movement of substances through the cell membrane takes place by three modes called passive transport, active transport, and bulk transport. preencoded.png  Passive Transport Passive transport is a mode of transport that takes place without the expenditure of energy. It takes place either by diffusion or osmosis. Diffusion is the process of movement of substances (liquid & gas) from the region of its higher concentration to the region of its lower concentration to spread uniformly in the given space. CO2 and O2 diffuse into and out of cells. Carbon dioxide is produced by cell respiration. As its concentration increases, CO2, cellular waste diffuses from the cell into the external medium. Similarly, the concentration of oxygen is always higher in the external medium as compared to the cell where it is being consumed in respiration. Therefore, oxygen diffuses from the outside to the inside of the cell. Osmosis is the movement of a water molecule or solvent across a semi-permeable membrane from the region of higher concentration to the region of lower concentration. Osmosis is a selective process since the membrane does not allow all molecules to pass through it. Water is usually the only free-flowing molecule across this membrane. preencoded.png diffusion vs osmosis https://www.youtube.com/watch?v=aDfWK5Q6WZM diffusion vs osmosis https://www.youtube.com/watch?v=aDfWK5Q6WZM preencoded.png Cell Placed In Solution When cells are placed in different solutions, their behavior is influenced by the tonicity of the solution: Hypotonic Solution: Contains a lower concentration of solute compared to the cell's interior, causing water to flow into the cell, leading to swelling or bursting (lysis). Isotonic Solution: Has the same solute concentration as the cell, resulting in no net movement of water, so the cell maintains its normal shape and size. Hypertonic Solution: Contains a higher concentration of solute, causing water to exit the cell, which results in cell shrinkage (crenation). Tonicity is the capability of a solution to modify the volume of cells by altering their water content preencoded.png  Bulk Transport: Exocytosis and Endocytosis Endocytosis The process of taking a particle or substance from outside of the cell and transferring it inside the cell using a vesicle. This is useful in releasing secretory and excretory materials from the cell Exocytosis The process of taking a substance or particle from inside of the cell and transferring it to outside the cell using a vesicle. preencoded.png  Endocytosis Endocytosis is an umbrella term that describes two processes: Phagocytosis and Pinocytosis. Phagocytosis means 'cell eating’. Pinocytosis means 'to drink’. In this process, large particles, such as In this process, the bulk transport cells or macromolecules, are taken into of fluid matter takes place across the cell. the cell membrane by forming For example, when microorganisms small vesicles from outside to invade the human body, neutrophils inside the cell. (types of WBCs) will engulf and destroy them. preencoded.png  Cell Wall Structure The cell wall is the most prominent part of the plant's cell structure. It is made up of cellulose, hemicellulose, and pectin. The cell wall is present exclusively in plant cells. It protects the plasma membrane and other cellular components. Layers The cell wall is multilayered with usually three layers: middle lamella, primary cell wall, and secondary cell wall. All plant cells invariably possess middle lamella, while the primary cell wall and secondary cell wall may or may not be present. Functions A cell wall provides a framework and definite shape to the cell. It gives rigidity and support to the tissues in plants. The cell wall gives mechanical strength to the cell to facilitate the plant to support its long and tall height. preencoded.png  Nucleus Control Center The nucleus is considered the controller or director of the cell. It is the largest cell organelle in plants and animals. Human RBCs lack a nucleus. Some plant cells also lack a nucleus. Chromatin Material The nucleus contains fine thread-like structures called chromatin fibers. Chromatin fibers contain heredity material. Chromosomes are thread-like structures in which DNA is tightly packaged within the nucleus. Structure The nucleus is a double membrane-bound dense protoplasmic body. It is surrounded by a nuclear membrane, which is double- layered and porous. The nuclear membrane separates the nucleus from the cytoplasm. Nucleolus The nucleolus is a spherical, non-membranous structure that varies in number. It is rich in proteins and RNA molecules. The nucleolus is the site for the formation of ribosomes. preencoded.png