Cell Structure and Function

• The cell is the fundamental structural and functional unit of life
• All living organisms are composed of cells
• Cells carry out various life processes
• The cell theory states that all living organisms are composed of cells, cells are the basic units of structure and function in living organisms, and all cells arise from pre-existing cells

Cell Structure

• A typical cell consists of the cell membrane, cytoplasm, and nucleus
• The cell membrane is the outer boundary of the cell, separating it from the external environment
• The cytoplasm is the jelly-like substance within the cell, containing various organelles
• The nucleus is the control center of the cell, containing the genetic material (DNA)

Cell Membrane and Transport

• The cell membrane is a selectively permeable membrane that regulates the movement of substances in and out of the cell
• It is composed of a lipid bilayer with embedded proteins
• Transport across the cell membrane can occur through passive or active mechanisms
• Passive transport does not require energy and includes diffusion, osmosis, and facilitated diffusion
  • Diffusion is the movement of molecules from an area of high concentration to an area of low concentration
  • Osmosis is the movement of water molecules from an area of high water concentration to an area of low water concentration across a semipermeable membrane
  • Facilitated diffusion is the movement of molecules across the cell membrane with the help of transport proteins
• Active transport requires energy and involves the movement of molecules against their concentration gradient
  • Examples of active transport include the sodium-potassium pump and endocytosis/exocytosis
• Endocytosis is the process by which cells engulf substances from their surroundings
  • Phagocytosis (cell eating) and pinocytosis (cell drinking) are types of endocytosis
• Exocytosis is the process by which cells release substances to their surroundings

Cell Organelles

• Cell organelles are specialized structures within the cell that perform specific functions
• Key cell organelles include:
  • Nucleus: Contains the cell's DNA and controls cell activities
    • The nucleus is surrounded by a nuclear membrane
    • It contains chromatin, which condenses into chromosomes during cell division
    • The nucleolus is responsible for ribosome synthesis
  • Endoplasmic Reticulum (ER): A network of membranes involved in protein and lipid synthesis
    • Rough ER has ribosomes attached and is involved in protein synthesis
    • Smooth ER lacks ribosomes and is involved in lipid synthesis
  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids
    • It consists of flattened sacs called cisternae
  • Lysosomes: Contain enzymes for intracellular digestion
    • They break down waste materials and cellular debris
  • Mitochondria: Generate energy (ATP) through cellular respiration
    • They have a double membrane structure with inner folds called cristae
  • Ribosomes: Synthesize proteins
    • They can be found free in the cytoplasm or attached to the ER
  • Vacuoles: Store water, nutrients, and waste products
    • Plant cells typically have a large central vacuole
  • Plastids: Found in plant cells and are involved in photosynthesis and storage
    • Chloroplasts contain chlorophyll and are the site of photosynthesis
    • Chromoplasts contain pigments and give color to fruits and flowers
    • Leucoplasts store starch, lipids, or proteins
  • Centrosome: Organizes microtubules and is involved in cell division
    • It contains centrioles in animal cells
  • Cilia and Flagella: Hair-like structures involved in movement
    • Cilia are short and numerous
    • Flagella are long and few in number

Cellular Processes

• Cellular processes are the activities that occur within a cell to maintain life
• Key cellular processes include:
  • Cell division: The process by which a cell divides into two or more daughter cells
    • Mitosis: Cell division that results in two identical daughter cells
      • Involved in growth, repair, and asexual reproduction
    • Meiosis: Cell division that results in four genetically different daughter cells with half the number of chromosomes
      • Involved in sexual reproduction
  • Cellular respiration: The process by which cells break down glucose to produce energy (ATP)
    • Occurs in the mitochondria
  • Photosynthesis: The process by which plant cells convert light energy into chemical energy (glucose)
    • Occurs in the chloroplasts
  • Protein synthesis: The process by which cells produce proteins
    • Involves transcription (DNA to RNA) and translation (RNA to protein)
  • Osmoregulation: The maintenance of water balance in the cell
  • Homeostasis: Maintaining stable internal conditions

Differences between Plant and Animal Cells

• Plant cells have a cell wall, chloroplasts, and a large central vacuole, which are absent in animal cells
• Animal cells have centrioles, which are absent in plant cells
• Plant cells have a fixed shape, while animal cells have a more flexible shape

Cell Size and Shape

• Cells vary in size and shape depending on their function
• Nerve cells are long and slender for transmitting signals
• Red blood cells are small and biconcave for efficient oxygen transport
• Muscle cells are elongated for contraction

Tissues, Organs, and Organ Systems

• Cells are organized into tissues, which are groups of similar cells performing a specific function
• Different tissues are organized into organs, which perform more complex functions
• Organs are organized into organ systems, which work together to carry out major bodily functions