MBBS Biology

Cell Structure and Function

  • Robert Hooke first saw and namedcells. Antony van Leeuwenhoek was the first person to see living cells.
  • Before the development of microscopes, the existence of cellular life was unknown. The development of light microscopes and laterel ectron microscopes helped scientists learn more about the cell. Most of the discoveries about cell structure since the 1950s have been made due to the use of electron microscopes.
  • The cell theory states that all living things are made of one or more cells, that cells are the basic unit of life, and that cells come only from other cells.
  • Cell size is limited by a cell’s surface area to volumeratio. A cell’s shape is determined by its function.
  • Parts common to all cells are the plasma membrane, the cytoplasm, ribosomes, and genetic material.
  • Prokaryotic cells lack a nucleus and other membrane-bound organelles.
  • The plasma membrane is a selectively permeable lipid bilayer that contains mostly lipids and proteins. These lipids and proteins are involved in many cellular processes.
  • The gel-like material within the cell that holds the organelles is called cytoplasm. The cytosol, which is the watery substance that does not contain organelles, is made up of 80% to 90% water.
  • The cytoskeleton has many functions. It helps to maintain cell shape, it holds organelles in place, and for some cells, it enables cell movement. The cytoskeleton also plays important roles in both the intracellular movement of substances and in cell division. Three main kinds of cytoskeleton fibers are microtubules, intermediate filaments, and microfilaments.
  • Cilia are extensions of the cell membrane that contain microtubules. Although both are used for movement, cilia are much shorter than flagella. Cilia cover the surface of some single-celled animals, such as paramecium, but cover only one side of cells in some multicellular organisms.
  • There are three features that plant cells have that animal cells do not have: a cell wall, a large central vacuole, and plastids.
  • Mitochondria use energy from organic compounds to make ATP.
  • Ribosomes are exported from the nucleolus, where they are made, to the cytoplasm.
  • The Golgi apparatus is a large organelle that is usually made up of five to eight cupshaped, membrane-covered discs called cisternae. It modifies, sorts, and packages different substances for secretion out of the cell, or for use within the cell.
  • Individual organisms from a colonial organism or biofilm can, if separated, survive on their own, while cells from a multicellular organism (e.g., liver cells) cannot.
  • A tissue is a group of connected cells that have a similar function within an organism. An organ is a group of tissues that has a specific function or group of functions, and an organ system is a group of organs that act together to perform complex related functions, with each organ focusing on a part of the task.
  • Molecules and ions cross the plasma membrane either by passive transport or active the transport.
  • Passive transport is the movement of molecules across the cell membrane without an input of energy from the cell.
  • Diffusion is the movement of molecules or ions from an area of high concentration to an area of lower concentration. The molecules keep moving down the concentration gradient until equilibrium is reached.
  • Osmosis is the diffusion of water molecules across a semipermeable membrane and down a concentration gradient. They can move into or out of a cell, depending on the concentration of the solute.
  • Active transport moves molecules across a cell membrane from an area of lower concentration to an area of higher concentration. Active transport requires the use of energy.
  • The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins that are found in the membrane.
  • The sodium-potassium pump is an example of a cell membrane pump. It moves three sodium ions out of the cell and two potassium ions in to the cell. The sodium-potassium pump uses ATP.
  • Endocytosis and exocytosis are active transport mechanisms in which large molecules enter and leave the cell inside vesicles.
  • In endocytosis, a substance or particle from outside the cell is engulfed by the cell membrane. The membrane folds over the substance and it becomes completely enclosed by the membrane. There are two main kinds of endocytosis: pinocytosis and phagocytosis.
  • Communication between cells is important for coordinating cell function in an organism. Membrane proteins and vesicles are involved in cellular communication.
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