Cell Structure and Function

Microscopy in Biology

Microscopy is essential for studying cells and their components, as most are too small to be seen with the naked eye. Different types of microscopes use light or electrons to visualize specimens, each with unique advantages.

• Light Microscopy: Uses visible light to illuminate specimens. Can observe living cells and general cell structure.

• Electron Microscopy: Uses beams of electrons for much higher resolution. Two main types:

  • Transmission Electron Microscope (TEM): Electrons pass through thin sections of specimens, revealing internal structures.

  • Scanning Electron Microscope (SEM): Electrons scan the surface, producing detailed 3D images of cell surfaces.

• Key Terms:

  • Magnification: The ratio of an object's image size to its real size.

  • Resolution: The ability to distinguish two close objects as separate.

  • Contrast: The difference in brightness between parts of the sample.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area. A high surface area-to-volume ratio is important for efficient exchange of materials with the environment.

Cellular Structure: Plasma Membrane and Common Features

All cells share certain structural features that are essential for life.

• Plasma Membrane: A selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It regulates the movement of substances in and out of the cell.

• Four Common Features of All Cells:

  • Plasma membrane

  • Cytoplasm (or cytosol)

  • Genetic material (DNA)

  • Ribosomes (for protein synthesis)

Cytosol vs. Cytoplasm

The terms cytosol and cytoplasm are related but distinct.

• Cytosol: The fluid portion of the cell's interior, excluding organelles.

• Cytoplasm: The entire region between the plasma membrane and the nucleus, including the cytosol and organelles.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

Unique Structures in Animal vs. Plant Cells

Some cellular structures are unique to either animal or plant cells.

• Structures Primarily in Animal Cells: Lysosomes, centrosomes with centrioles, and flagella (in some animal cells).

• Structures Primarily in Plant Cells: Cell wall, chloroplasts, central vacuole, and plasmodesmata.

Functional Categories of Eukaryotic Cellular Components

Eukaryotic cells contain specialized organelles grouped by function. Each group includes specific components with distinct structures and roles.

• 1. Genetic Control

  • Nucleus: Contains most of the cell's DNA; surrounded by a double membrane (nuclear envelope) with nuclear pores for transport.

  • Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

  • Ribosomes: Complexes of rRNA and protein; carry out protein synthesis in the cytosol (free ribosomes) or on the rough endoplasmic reticulum (bound ribosomes).

• 2. Endomembrane System

  • Endoplasmic Reticulum (ER): Network of membranes; rough ER has ribosomes and synthesizes proteins, smooth ER synthesizes lipids and detoxifies chemicals.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

  • Lysosomes: Contain digestive enzymes to break down macromolecules and old organelles (mainly in animal cells).

  • Vacuoles: Storage organelles; central vacuole in plants stores water, ions, and nutrients.

  • Vesicles: Small membrane-bound sacs for transport within the cell.

  • Plasma Membrane: Part of the endomembrane system, controls entry and exit of substances.

• 3. Energy Processing

  • Mitochondria: Sites of cellular respiration; convert chemical energy in food to ATP.

  • Chloroplasts: (Plants and algae) Sites of photosynthesis; convert solar energy to chemical energy in sugars.

• 4. Structural Support, Movement, and Communication

  • Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) that provide structural support, cell shape, and movement.

  • Cell Wall: (Plants, fungi, some protists) Rigid outer layer for protection and support.

  • Extracellular Matrix (ECM): (Animals) Network of proteins and carbohydrates outside the cell membrane for support and signaling.

  • Cell Junctions: Structures that connect cells to each other (e.g., tight junctions, desmosomes, gap junctions in animals; plasmodesmata in plants).

  • Flagella and Cilia: Structures for cell movement or moving substances along cell surfaces.

Summary Table: Functional Categories of Eukaryotic Organelles

Endosymbiont Theory

The endosymbiont theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. It proposes that these organelles originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells, forming a symbiotic relationship.

• Supporting Evidence:

  • Both mitochondria and chloroplasts have their own circular DNA, similar to prokaryotes.

  • They have double membranes, consistent with engulfment.

  • They reproduce independently within the cell by a process similar to binary fission.

  • Ribosomes in these organelles resemble those of bacteria.

Cellular Features Supporting Endosymbiont Theory

• Structural: Double membranes, size similarity to bacteria, presence of their own DNA and ribosomes.

• Functional: Independent replication, protein synthesis using their own genetic information.

Example: The mitochondrion in animal cells and the chloroplast in plant cells both originated from ancient symbiotic bacteria, according to the endosymbiont theory.