bio

• Electrons are located in energy levels or shells around the nucleus of an atom.
• Unicellular organisms include Chlamydomonas, Paramoecium, and bacteria
• All multicellular organisms have many cells that group together in a single body to form various body parts
• Every multicellular organism originates from a single cell through cell division
• Cells divide to produce cells of their own kind
• All cells come from pre-existing cells
• Cells were first discovered by Robert Hooke in 1665
• Leeuwenhoek discovered free living cells in pond water for the first time
• Robert Brown discovered the nucleus in the cell
• Purkinje coined the term 'protoplasm' for the fluid substance of the cell
• The cell theory, that all plants and animals are composed of cells and that the cell is the basic unit of life, was presented by Schleiden and Schwann
• Virchow suggested that all cells arise from pre-existing cells
• The shape and size of cells are related to the specific function they perform
• Each living cell has the capacity to perform certain basic functions
• Division of labour is seen within a single cell with specific components known as cell organelles
• Cell organelles perform special functions such as making new material in the cell and clearing up waste material
• Plasma membrane, nucleus, and cytoplasm are three features found in almost every cell
• Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment
• Plasma membrane is a selectively permeable membrane that allows the entry and exit of some materials in and out of the cell
• Diffusion is the process by which substances like carbon dioxide or oxygen move across the cell membrane
• Osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration
• In a hypotonic solution, the cell gains water by osmosis and swells up
• In an isotonic solution, there is no net movement of water across the cell membrane
• In a hypertonic solution, the cell loses water by osmosis and shrinks
• Plant cells have a rigid outer covering called the cell wall, mainly composed of cellulose
• Plasmolysis is the shrinkage or contraction of the contents of a plant cell away from the cell wall when the cell loses water through osmosis
• Living cells, not dead cells, are able to absorb water by osmosis
• Cell walls permit the cells of plants, fungi, and bacteria to withstand very dilute (hypotonic) external media without bursting
• In hypotonic media, cells tend to take up water by osmosis, causing the cell to swell and build up pressure against the cell wall
• The cell wall exerts an equal pressure against the swollen cell
• Cells with walls can withstand greater changes in the surrounding medium than animal cells
• Nucleus
• The nucleus has a double-layered covering called the nuclear membrane
• The nuclear membrane has pores that allow the transfer of material from inside the nucleus to the cytoplasm
• Chromosomes are visible as rod-shaped structures when the cell is about to divide
• Chromosomes contain DNA molecules with information for inheritance of characters from parents to the next generation
• DNA molecules contain information necessary for constructing and organizing cells
• Functional segments of DNA are called genes
• In a cell not dividing, DNA is present as part of chromatin material, visible as an entangled mass of thread-like structures
• The nucleus plays a central role in cellular reproduction and in determining the way the cell will develop