book- chapter 2

• Cell theory states that all organisms are made of cells, which are the basic unit of life and arise from pre-existing cells
• We use a microscope to view cells because they are very small, too small to be seen clearly with the naked eye
• Cells are made up of various parts, including:

  • Cell membrane: outer boundary of the cell
  • Cytoplasm: parts of the cell within the cell membrane, except for the nucleus
  • Organelles: structures suspended in the cytoplasm that carry out specific functions
  • Cytosol: liquid part of the cytoplasm
  • Cytoskeleton: internal scaffolding of protein fibers within the cytoplasm
  • Inclusions: chemical substances occurring as granules or liquid droplets in the cytoplasm
• All cells have a similar basic structure despite variations in size and shape
• Nucleus is the largest organelle in the cell, contains DNA, and controls the structure and function of the cell
• Ribosomes are responsible for joining amino acids to make proteins
• Endoplasmic reticulum (ER) provides a surface for chemical reactions and storage or transport of materials; can be rough (with ribosomes) or smooth (without ribosomes)
• Golgi body modifies proteins and packages them for secretion from the cell
• Lysosomes contain digestive enzymes that break down large molecules and worn-out organelles
• Mitochondria release energy for the cell through cellular respiration and are known as the 'powerhouses' of the cell
• Cytoskeleton consists of microtubules and microfilaments that give the cell its shape and assist in cell movement
• Cilia and flagella are fine projections on some cells that help in cell or substance movement; cilia are short and numerous, while flagella are longer and fewer in number
• Inclusions are chemical substances found in the cytoplasm of the cell but are not part of the cell structure
• Cells need to take in substances from tissue fluid and produce materials that must be removed from the cell
• During cellular respiration, glucose and oxygen are used to produce carbon dioxide, water, and energy
• All cells require oxygen and glucose, while carbon dioxide and water are removed
• Cell membranes separate the internal and external environment of cells
• The cell membrane is composed of phospholipid molecules arranged in a bilayer
• The fluid mosaic model is the currently accepted model for cell membrane structure
• Cholesterol and protein molecules are embedded in the phospholipid bilayer of the membrane
• Proteins in the cell membrane include receptor proteins, channel proteins, carrier proteins, and cell-identity markers
• Cell membranes act as a physical barrier, regulate the passage of materials, are sensitive to changes, and help support the cell
• Cell membranes are differentially permeable, allowing certain ions and molecules to pass through while restricting others
• Transport across cell membranes can occur through simple diffusion, facilitated transport, or vesicular transport
• Diffusion is the spreading out of particles so that they are evenly distributed over the available space
• Osmosis is the diffusion of water across a differentially permeable membrane to balance the concentration of solutes
• Facilitated transport involves proteins in the cell membrane allowing molecules to be transported across
• Protein channels and carrier proteins are involved in facilitated transport
• Carrier proteins are only open on one side of the membrane at a time
• When a specific substance binds to the binding site within the protein, the protein changes shape and opens to the other side
• Carrier proteins are specific and will only bind to a particular molecule
• Carriers can become saturated, meaning any increase in the concentration of molecules to be transported cannot increase the rate of movement
• Carrier activity is regulated by substances such as hormones
• There are two main types of carrier-mediated transport: facilitated diffusion and active transport
• Facilitated diffusion occurs along the concentration gradient and does not require energy input
• Active transport requires energy in the form of ATP and moves substances against the concentration gradient
• Vesicular transport is the movement of substances across the cell membrane in membranous sacs called vesicles
• Endocytosis is taking liquids or solids into the cell by vesicular transport
• Exocytosis is when the contents of a vesicle inside the cell are passed to the outside
• Molecules and ions move within the cell mostly by diffusion