Cell Structure and Function

Overview of Cell Structures Necessary for Life Functions

Cells are the fundamental units of life, and their internal organization allows them to perform essential functions. Eukaryotic cells contain specialized structures called organelles, each with distinct roles in maintaining cellular processes.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.

• Lysosomes: Contain digestive enzymes to break down waste.

• Chloroplasts: (in plants) Site of photosynthesis.

• Plasma Membrane: Selectively permeable barrier controlling entry and exit of substances.

• Cytoskeleton: Provides structural support, shape, and aids in movement.

Example: Antibiotics can target bacterial cell structures (e.g., cell wall, ribosomes) that differ from those in human cells, making them effective against bacteria but not human cells.

Microscopy and Cell Size

Types of Microscopes and Their Uses

Microscopes are essential tools for visualizing cells and their components, which are often too small to be seen with the naked eye. Different types of microscopes provide varying levels of magnification and resolution.

• Light Microscope: Uses visible light to observe living cells and tissues; suitable for general cell structure.

• Fluorescence Microscope: Uses fluorescent dyes to label specific cell components; allows visualization of specific proteins or structures.

• Electron Microscope: Uses electron beams for much higher resolution; includes two main types:

  • Scanning Electron Microscope (SEM): Provides detailed 3D images of cell surfaces.

  • Transmission Electron Microscope (TEM): Reveals internal cell structures in high detail.

Example: Ribosomes and viruses are typically visualized with electron microscopes due to their small size, while whole cells can be seen with light microscopes.

Relative Sizes of Biological Structures

Biological structures vary greatly in size, from atoms to whole cells. Understanding their scale is important for choosing the appropriate microscopy technique.

• Atoms: ~0.1 nm

• Ribosomes: ~20-30 nm

• Small Bacteria: ~0.5-1 μm

• Mitochondria: ~1-2 μm

• Animal Cells: ~10-30 μm

• Chicken Egg: ~1-2 cm

Example: The plasma membrane is best visualized with an electron microscope, while the overall shape of a white blood cell can be seen with a light microscope.

Surface Area to Volume Ratio and Cell Size

The surface area to volume (SA:V) ratio is a key factor limiting cell size. As a cell grows, its volume increases faster than its surface area, reducing the efficiency of nutrient uptake and waste removal.

• Formula for Surface Area of a Cube:

• Formula for Volume of a Cube:

• Surface Area to Volume Ratio:

Example: Smaller cells have a higher SA:V ratio, allowing for more efficient exchange with their environment. This is why most cells are small.

Additional info: Table values inferred from standard cube calculations.

Prokaryotic and Eukaryotic Cells

Prokaryotic Cell Structure

Prokaryotes (Bacteria and Archaea) are simpler than eukaryotes and lack membrane-bound organelles. Their DNA is located in a region called the nucleoid.

• Size: Typically 1/10th the size of eukaryotic cells.

• Cell Wall: Provides structure and rigidity; contains peptidoglycan in bacteria.

• Ribosomes: Smaller than those in eukaryotes; site of protein synthesis.

• Plasma Membrane: Controls entry and exit of substances.

• Flagella: Used for movement.

• Chromosome: Single, circular DNA molecule.

Example: Antibiotics like penicillin target the bacterial cell wall, which is absent in human cells.

Differences in Cell Wall Structure: Gram-Positive vs. Gram-Negative Bacteria

Additional info: Table content inferred from standard microbiology knowledge.

Eukaryotic Cell Structure

Eukaryotic cells are more complex and contain membrane-bound organelles. They can be plant or animal cells, with some differences between the two.

• Plant Cells: Have cell walls, chloroplasts, and large central vacuoles.

• Animal Cells: Lack cell walls and chloroplasts, have smaller vacuoles.

Example: Only plant cells can perform photosynthesis due to the presence of chloroplasts.

Cell Organelles and Their Functions

Major Organelles and Their Roles

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement. It also plays a role in intracellular transport and cell division.

• Microfilaments: Composed of actin; involved in cell movement and shape changes.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Hollow tubes; guide organelle movement and chromosome separation during cell division.

Example: The cytoskeleton helps transport vesicles from the nucleus to the plasma membrane.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cells