🧬AP Biology Unit 2 – Cell Structure and Function

Key Concepts

• Cells are the fundamental unit of life, serving as the building blocks for all living organisms
• Cell theory states that all living things are composed of cells, cells are the basic units of structure and function in living things, and all cells come from pre-existing cells
• Cells can be classified as either prokaryotic (lacking a nucleus and membrane-bound organelles) or eukaryotic (possessing a nucleus and membrane-bound organelles)
• The cell membrane is a selectively permeable barrier that controls the movement of substances in and out of the cell
  • Consists of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol
• Organelles are specialized structures within a cell that perform specific functions
  • Examples include the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus
• Cells transport materials across their membranes through various mechanisms, including diffusion, osmosis, and active transport
• Energy production in cells occurs primarily through the processes of cellular respiration (in the mitochondria) and photosynthesis (in chloroplasts)
• Cells communicate with each other through various signaling pathways, such as chemical messengers and receptor proteins

Cell Theory and Discovery

• Cell theory was developed in the 19th century by scientists Matthias Schleiden, Theodor Schwann, and Rudolf Virchow
• Schleiden and Schwann independently concluded that cells are the basic units of life and that all living things are composed of cells
• Virchow added to the theory by proposing that all cells come from pre-existing cells through the process of cell division
• The invention of the microscope in the 17th century by Anton van Leeuwenhoek and Robert Hooke was crucial to the discovery and study of cells
  • Hooke coined the term "cell" after observing the boxlike structures in a thin slice of cork
• Advancements in microscopy, such as the electron microscope, have allowed for more detailed observations of cell structure and function
• Cell theory has been refined over time to incorporate new discoveries, such as the existence of organelles and the role of DNA in heredity

Types of Cells

• Cells can be broadly classified into two main categories: prokaryotic and eukaryotic
• Prokaryotic cells are typically smaller and simpler than eukaryotic cells
  • Lack a nucleus and membrane-bound organelles
  • Examples include bacteria and archaea
• Eukaryotic cells are more complex and larger than prokaryotic cells
  • Possess a nucleus and membrane-bound organelles
  • Examples include animal cells, plant cells, and fungi
• Plant cells have unique features that distinguish them from animal cells, such as a cell wall, chloroplasts, and a large central vacuole
• Animal cells lack a cell wall and chloroplasts but contain centrioles, which are involved in cell division
• Specialized cells, such as neurons and muscle cells, have unique structures and functions that enable them to perform specific tasks within an organism

Cell Membrane Structure

• The cell membrane is a selectively permeable barrier that separates the interior of the cell from the external environment
• Composed of a phospholipid bilayer, with the hydrophilic heads facing the aqueous environment and the hydrophobic tails facing each other
• Membrane proteins are embedded within the phospholipid bilayer and serve various functions, such as transport, signaling, and cell recognition
  • Integral proteins span the entire membrane, while peripheral proteins are attached to the surface
• Carbohydrates are attached to some membrane proteins and lipids, forming glycoproteins and glycolipids, which play a role in cell recognition and communication
• Cholesterol is a steroid molecule that helps maintain membrane fluidity and stability
  • More cholesterol results in a less fluid membrane, while less cholesterol increases membrane fluidity
• The fluid mosaic model describes the dynamic nature of the cell membrane, with its components able to move laterally within the plane of the membrane

Organelles and Their Functions

• The nucleus is the control center of the cell, containing the genetic material (DNA) and directing cellular activities
  • Nuclear pores in the nuclear envelope allow for selective transport of molecules between the nucleus and cytoplasm
• Mitochondria are the powerhouses of the cell, generating ATP through the process of cellular respiration
  • Contain their own DNA and ribosomes, suggesting an endosymbiotic origin
• The endoplasmic reticulum (ER) is a network of membranous channels involved in protein and lipid synthesis, as well as transport
  • Rough ER is studded with ribosomes and synthesizes proteins, while smooth ER lacks ribosomes and synthesizes lipids
• The Golgi apparatus is responsible for modifying, packaging, and sorting proteins and lipids for transport to their final destinations
• Lysosomes are membrane-bound organelles containing digestive enzymes that break down cellular waste, foreign particles, and damaged organelles
• Ribosomes are the sites of protein synthesis, translating mRNA into polypeptide chains
  • Can be found free in the cytoplasm or attached to the rough ER
• Chloroplasts are the site of photosynthesis in plant cells, converting light energy into chemical energy (glucose)

Cell Transport Mechanisms

• Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration, driven by a concentration gradient
  • Oxygen and carbon dioxide can diffuse directly through the cell membrane
• Osmosis is the diffusion of water across a selectively permeable membrane from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration)
  • Cells in hypotonic solutions will swell, while cells in hypertonic solutions will shrink
• Facilitated diffusion involves the use of transport proteins to move specific molecules across the membrane down their concentration gradient
  • Examples include glucose transporters (GLUT) and ion channels
• Active transport requires energy (ATP) to move molecules against their concentration gradient, from an area of low concentration to an area of high concentration
  • Examples include the sodium-potassium pump and the proton pump in mitochondria
• Endocytosis is the process by which cells take in large particles or macromolecules by engulfing them with the cell membrane
  • Phagocytosis involves the ingestion of solid particles, while pinocytosis involves the uptake of liquid droplets
• Exocytosis is the process by which cells release large particles or macromolecules by fusing a vesicle with the cell membrane

Energy Production in Cells

• Cellular respiration is the process by which cells break down organic molecules (glucose) to produce ATP, the primary energy currency of the cell
  • Occurs in the mitochondria and consists of four stages: glycolysis, pyruvate oxidation, the Krebs cycle, and the electron transport chain
• Glycolysis is the anaerobic breakdown of glucose into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH
• Pyruvate oxidation converts pyruvate into acetyl-CoA, releasing carbon dioxide and generating NADH
• The Krebs cycle (also known as the citric acid cycle) oxidizes acetyl-CoA, producing carbon dioxide, ATP, NADH, and FADH2
• The electron transport chain is a series of protein complexes in the inner mitochondrial membrane that use the energy from NADH and FADH2 to pump protons into the intermembrane space, creating an electrochemical gradient
  • The flow of protons back into the matrix through ATP synthase drives the synthesis of ATP
• Photosynthesis is the process by which plant cells and some other organisms convert light energy into chemical energy (glucose)
  • Occurs in the chloroplasts and consists of two stages: the light-dependent reactions and the Calvin cycle
• The light-dependent reactions capture light energy and use it to generate ATP and NADPH, while also splitting water molecules to release oxygen
• The Calvin cycle (also known as the light-independent reactions) uses the ATP and NADPH from the light-dependent reactions to fix carbon dioxide into organic compounds (glucose)

Cell Communication

• Cells communicate with each other and their environment through various signaling pathways
• Signaling molecules (ligands) bind to specific receptor proteins on the target cell's surface or inside the cell
  • Examples of signaling molecules include hormones, neurotransmitters, and growth factors
• Receptor-ligand binding initiates a series of intracellular events, known as signal transduction, which ultimately leads to a cellular response
  • Responses can include changes in gene expression, protein activity, or cell behavior
• G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that associate with G proteins to initiate intracellular signaling cascades
  • Examples include receptors for adrenaline, serotonin, and dopamine
• Receptor tyrosine kinases (RTKs) are another class of cell surface receptors that, when activated by ligand binding, phosphorylate specific tyrosine residues on intracellular signaling proteins
  • Examples include receptors for insulin and growth factors
• Intracellular receptors are located inside the cell and bind to signaling molecules that can pass through the cell membrane, such as steroid hormones and thyroid hormones
  • Upon ligand binding, these receptors directly influence gene expression by acting as transcription factors
• Gap junctions are channels that directly connect the cytoplasm of adjacent cells, allowing for the rapid exchange of small molecules and ions
  • Play a crucial role in coordinating the activities of cells in tissues and organs

Applying Your Knowledge

• Understanding cell structure and function is essential for many practical applications, such as drug development, disease treatment, and biotechnology
• Knowledge of cell membrane structure and transport mechanisms can be used to design targeted drug delivery systems
  • Example: Liposomes can be engineered to carry drugs and release them specifically at the site of a tumor
• Insights into cell signaling pathways can help identify potential targets for therapeutic intervention
  • Example: Blocking the activity of overactive signaling proteins (oncogenes) can slow or stop the growth of cancer cells
• Manipulating the genetic material of cells (DNA) through techniques like CRISPR-Cas9 can be used to correct genetic disorders or create genetically modified organisms with desired traits
  • Example: CRISPR-Cas9 has been used to correct mutations in human embryos that cause genetic diseases such as cystic fibrosis
• Understanding the differences between prokaryotic and eukaryotic cells can aid in the development of antibiotics that specifically target bacterial cells while leaving human cells unharmed
• Knowledge of cellular energy production (respiration and photosynthesis) can be applied to create more efficient biofuels or optimize crop yields
  • Example: Genetically engineered algae can be used to produce high levels of biofuels, such as biodiesel
• Studying the structure and function of specialized cells, such as stem cells, can lead to advances in regenerative medicine and tissue engineering
  • Example: Induced pluripotent stem cells (iPSCs) can be generated from a patient's own cells and used to create personalized therapies for diseases like Parkinson's or spinal cord injuries