Cell Biology Introduction Flashcards

Fundamental Principles of Cell Biology

• The Cell as the Unit of Life: Cells are defined as the smallest units of matter considering living.
• Hierarchical Composition:
  • Cells contain molecules.
  • Molecules are composed of atoms.
• Life Classifications:
  • Unicellular Life: Organisms consisting of a single cell.
  • Multicellular Life: Organisms like animals composed of many cells.
• Cellular Continuity: A fundamental statement in biology is that all cells come from pre-existing cells. Currently, no cells are being created "from scratch"; they always arise from cells that already exist.
• Universality of Function: While different types of cells (such as plant and animal cells) look different, the basic structures and functions of internal cell parts are largely universal across life forms.

Classification of Life: Prokaryotes vs. Eukaryotes

• Broad Categorization: Life on Earth is fundamentally divided into two primary categories: prokaryotes and eukaryotes.
• Prokaryotic Organisms (Prokaryotes):
  • Etymology: The prefix "pro" refers to "earliest" or "before."
  • Domains: Prokaryotes encompass two domains of life: Domain Bacteria and Domain Archaea.
  • Nucleus Status: They do not possess a membrane-bound nucleus. Instead, their genetic material is located in a central area called the nucleoid region.
  • Size: Generally, prokaryotic cells are about $10 \times$ smaller than eukaryotic cells.
  • Internal Structure: They exhibit very little compartmentalization and lack membrane-bound organelles.
  • Evolutionary Context: Prokaryotes are believed to represent the earliest forms of life on Earth.
• Eukaryotic Organisms (Eukaryotes):
  • Etymology: "Eukaryotic" translates to "true nucleus."
  • Defining Feature: Presence of a membrane-bound nucleus that holds the DNA/genetic material.
  • Organelles: These cells contain many internal compartments called organelles, which segregate different functions.
  • Diversity: Includes animal cells and plant cells.

Structural Components of Prokaryotic Cells

• Standard Structures (Found in All Prokaryotes):
  • Plasma Membrane: Also called the cell membrane. It surrounds the cell, keeping the "outsides out and the insides in."
  • Cytoplasm: A watery matrix (sometimes described as jelly-like) that allows enzymes, chemicals, and internal structures to move and interact.
  • Ribosomes: Described as "granular bodies" or little dots; they are the sites of protein synthesis.
  • DNA: Found in a central region (nucleoid). Prokaryotic DNA is typically organized into one long, circular loop. Some smaller extra loops of DNA may also exist in the cytoplasm.
• Additional and Extracellular Structures:
  • Cell Wall: A structural material outside the plasma membrane made of various sugars that provides support and protection.
  • Pili: Hair-like structures that help the bacteria adhere to surfaces.
  • Flagella: Long, whip-like tails used for propulsion and swimming.
  • Capsule: A structure outside the cell wall that serves as a protective layer, helping the bacteria resist drying out or enter a dormant state.

Cellular Dimensions and the Surface Area-to-Volume Constraint

• Relative Scale of Cells:
  • Largest Cells: Certain eggs, such as chicken and ostrich eggs, are unusually large cells visible to the naked eye. Frog eggs are also on the larger side, followed by slightly smaller human eggs.
  • Microscopic Range: Most typical animal and plant cells require a microscope to see.
  • Sub-cellular Scales: Mitochondria and bacteria are similar in size (both smaller than eukaryotic cells). Proteins and lipids are smaller still, followed by the smallest unit, the atom.
• The Size Limit Principle:
  • Cells do not grow to be as large as humans because of the surface area-to-volume ($SA/V$) ratio.
  • Constraint: A cell must have enough surface area (membrane) relative to its internal volume to facilitate the efficient transfer of materials in and out.
  • The Critical Size: If a cell becomes too large, it can no longer move materials in and out in the time needed to sustain life.

Eukaryotic Architecture: Organelles of the Animal Cell

• Organelles: The term refers to small, membrane-bound structures inside the cell.
• The Nucleus:
  • Nuclear Envelope: The double membrane surrounding the nucleus.
  • Chromatin: The genetic material (DNA) plus associated proteins located inside the nucleus.
• Waste Management Organelles:
  • Peroxisome: A spherical compartment that metabolizes or breaks down chemical waste products produced by the cell.
  • Lysosome: Described as the "garbage disposal" of the cell. It uses digestive enzymes to break down worn-out structures (like mitochondria) and wastes. It maintains an acidic environment with approximately $1000 \times$ more hydrogen ions ($H^+$) than the surrounding cytoplasm.
• Endoplasmic Reticulum (ER):
  • Rough ER: Located closest to the nucleus, it is studded with ribosomes and serves as the site for protein synthesis. These membranes often form channels for material transport.
  • Smooth ER: A series of membranes without ribosomes. Its primary function is making more membranes, such as phospholipids.
• Golgi Apparatus: Named after Emilio Golgi. It consists of a series of folded membranes separate from the ER. It modifies, processes, and packages proteins for use or export (e.g., antibodies moved into the bloodstream).
• Mitochondria:
  • Function: Known as the "powerhouse of the cell" and the site of energy production.
  • Energy Currency: Produces $ATP$ (Adenosine Triphosphate).
  • Structure: Has a "bag-in-a-bag" structure consisting of an outer membrane and a highly folded inner membrane. The intermembrane space is functional and critical for $ATP$ production.
• Cytoskeleton: A network of proteins that provides structural support and maintains the shape of the cell (cyto- meaning cell; skeleton meaning support).
• Vacuoles and Vesicles: Membrane spheres used for storage. Vesicles are generally smaller and geared for movement around the cell, while vacuoles are larger and relatively stationary.

Distinctive Features of Plant Cells

• Shared Structures: Plant cells share many features with animal cells, including the nucleus, ER, Golgi, ribosomes, mitochondria, peroxisomes, and cytoskeleton.
• Unique Structures:
  • Cell Wall: A rigid outer layer for structure and protection, located outside the plasma membrane.
  • Central Vacuole: A very large, stationary compartment filled with "cell sap" (watery storage for minerals and dissolved materials).
  • Chloroplasts: The site of photosynthesis. These organelles capture solar energy and transform it into chemical energy in the form of glucose ($C_6H_{12}O_6$), which is then passed through the food chain.

Specialized Movement Structures

• Cilia: Thin, hair-like structures found on animal cells (and some other types) that bend back and forth to help a cell move or to sweep materials across a surface (e.g., the lining of human respiratory surfaces).
• Flagella: Longer, whip-like structures used specifically for cell movement.

The Endosymbiotic Theory and Evolutionary Origins

• Origins of Mitochondria and Chloroplasts: Scientific evidence suggests that mitochondria and chloroplasts were once free-living prokaryotic organisms (bacteria) billions of years ago.
• The Mechanism: Large cells engulfed these smaller bacteria but did not digest them. Instead, they were maintained as symbionts. Over time, this symbiosis became essential, and they can no longer live independently.
• Supporting Evidence for Endosymbiosis:
  • Double Membrane: Both organelles have an outer and an inner membrane.
  • Independent Division: They divide within the cell through a process called binary fission, similar to how bacteria divide.
  • Genetic Material: They possess their own DNA in a circular loop, identical to bacterial DNA.
  • Ribosomes: They have their own internal ribosomes, distinct from the cell's main ribosomes.
  • Size: They are roughly the same size as modern bacteria.