Cell Structure and Function

Key Terms and Definitions

This section defines essential terms related to cell biology, microscopy, and cellular components. Understanding these terms is foundational for studying cell structure and function.

• Magnification: The process of enlarging the appearance of an object using optical instruments.

• Resolving Power: The ability of a microscope to distinguish two close objects as separate entities.

• Cell Theory: The scientific theory stating that all living things are composed of cells, and all cells arise from pre-existing cells.

• Prokaryotic Cell: A cell lacking a nucleus and membrane-bound organelles; found in Bacteria and Archaea.

• Eukaryotic Cell: A cell with a true nucleus and membrane-bound organelles; found in plants, animals, fungi, and protists.

• Chromosome: A structure composed of DNA and proteins that contains genetic information.

• Organelle: Specialized subunit within a cell that has a specific function (e.g., mitochondria, chloroplasts).

• Capsule: A gelatinous outer layer found in some prokaryotes, providing protection and aiding in adherence.

• Pili: Hair-like appendages on prokaryotic cells used for attachment or DNA transfer.

• Phospholipid: A lipid molecule with a hydrophilic (polar) head and two hydrophobic (nonpolar) tails; major component of cell membranes.

• Cell Junction: Structures that connect adjacent cells and facilitate communication (e.g., tight junctions, gap junctions).

• Nuclear Envelope: Double membrane surrounding the nucleus in eukaryotic cells.

• Nuclear Pores: Openings in the nuclear envelope that allow exchange of materials between the nucleus and cytoplasm.

• Endomembrane System: A group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins (includes ER, Golgi apparatus, lysosomes, etc.).

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis; rough ER has ribosomes, smooth ER does not.

• Transport Vesicle: Small, membrane-bound sac that moves molecules between organelles or to the cell membrane.

• Food Vacuole: Membrane-bound sac containing ingested food particles, found in some eukaryotes.

• Photosynthesis: The process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose).

• Cellular Respiration: The process by which cells extract energy from glucose and other molecules, producing ATP.

• Stroma: The fluid-filled space inside chloroplasts, surrounding the thylakoids; site of the Calvin cycle.

• Grana: Stacks of thylakoids within chloroplasts, where light-dependent reactions of photosynthesis occur.

• Matrix: The innermost compartment of the mitochondrion, containing enzymes for the Krebs cycle.

• Cristae: Folds of the inner mitochondrial membrane, increasing surface area for ATP production.

• Microtubules: Hollow rods made of tubulin proteins; part of the cytoskeleton, involved in cell shape, transport, and division.

• Microfilaments: Thin protein filaments (actin) that support cell shape and are involved in movement.

• Intermediate Filaments: Cytoskeletal components that provide mechanical support for the cell.

Cellular Organization

Unicellular vs. Multicellular Organisms

Organisms can be classified based on the number of cells they possess and their taxonomic domain or kingdom.

• Unicellular: Consist of a single cell (e.g., most Bacteria, Archaea, some Protists).

• Multicellular: Composed of many cells (e.g., animals, plants, most fungi).

• Domains: Bacteria and Archaea are primarily unicellular; Eukarya includes both unicellular and multicellular organisms.

• Kingdoms: Animalia and Plantae are multicellular; Protista and Fungi include both unicellular and multicellular species.

Microscopy

Light Microscopes

Light microscopes use visible light and glass lenses to magnify small objects, allowing observation of living cells.

• How it works: Light passes through the specimen and is bent by lenses to produce a magnified image.

• Key factors:

  • Magnification (how much larger the image appears)

  • Resolving Power (the clarity or ability to distinguish two points as separate)

• Limitations: Maximum useful magnification is about 1000x; limited by the wavelength of light.

Electron Microscopes

Electron microscopes use beams of electrons for much higher resolution imaging than light microscopes.

• How it works: Electrons are focused by electromagnetic lenses; specimens must be in a vacuum.

• Types: Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM).

• Benefits: Much greater resolving power (up to 2 million times); can reveal ultrastructure of cells.

• Disadvantages: Specimens must be dead and specially prepared; cannot observe living cells.

Comparison: SEM vs. TEM

SEM and TEM are two main types of electron microscopes, each suited for different applications.

Cell Types: Prokaryotic vs. Eukaryotic

Common Features

Both prokaryotic and eukaryotic cells share several fundamental structures and functions.

• Plasma membrane

• Cytoplasm

• Ribosomes

• Genetic material (DNA)

Differences

Prokaryotic and eukaryotic cells differ in complexity and organization.

• Prokaryotic Cells:

  • No nucleus; DNA in nucleoid region

  • No membrane-bound organelles

  • Generally smaller (1-10 μm)

  • Examples: Bacteria, Archaea

• Eukaryotic Cells:

  • True nucleus with nuclear envelope

  • Membrane-bound organelles (e.g., mitochondria, ER, Golgi)

  • Larger (10-100 μm)

  • Examples: Animals, plants, fungi, protists

Cell Organelles: Animal vs. Plant Cells

Organelle Distribution

Animal and plant cells share many organelles, but some are unique to each cell type.

• Found in animal cells but not in plant cells: Lysosomes, centrioles

• Found in plant cells but not in animal cells: Chloroplasts, cell wall, central vacuole

Cell Membranes and Molecules

Polarity and Amphipathic Molecules

Cell membranes are composed of molecules with distinct chemical properties that affect their interactions with water.

• Hydrophilic: "Water-loving"; polar molecules that dissolve readily in water (e.g., salts, sugars).

• Hydrophobic: "Water-fearing"; nonpolar molecules that do not dissolve in water (e.g., oils, fats).

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Phospholipids: Form the bilayer of cell membranes, with hydrophilic heads facing outward and hydrophobic tails inward.

Example: The amphipathic nature of phospholipids allows them to form the basic structure of biological membranes, creating a selective barrier for cells.