Cell Structure and Function

Microscopy and Biochemical Techniques in Cell Biology

Biologists use various types of microscopes and biochemical methods to study cells, extending human senses to observe structures invisible to the naked eye. Understanding these tools is essential for exploring cell structure and function.

• Microscopy: The development of microscopes allowed scientists like Robert Hooke and Anton van Leeuwenhoek to first view cells and microorganisms.

• Light Microscopes: Use lenses to bend light and magnify images. Key features include:

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

  • Resolution: Measure of image clarity; minimum distance two points can be distinguished.

  • Contrast: Difference in brightness between parts of the image.

• Most cells are between 1 and 100 μm in diameter.

• Electron Microscopes (EM): Overcome the resolution barrier of light microscopes, allowing study of organelles.

  • SEM (Scanning Electron Microscope): Used for detailed study of specimen surface topography. Electrons scan the surface, producing a 3D image.

  • TEM (Transmission Electron Microscope): Used to study internal cell structure. Electrons pass through thin sections of specimens, revealing cellular details.

  • Super-resolution microscopy: Allows researchers to distinguish subcellular structures as small as 10–20 μm.

  • Cryo-electron microscopy: Preserves specimens at extremely low temperatures for viewing in their environment.

• Cell Fractionation: Technique to separate cell components by centrifugation, enabling study of specific organelles and their functions.

Prokaryotic and Eukaryotic Cells

Cells are classified into two major types: prokaryotic and eukaryotic. Both share certain features but differ in complexity and organization.

• Prokaryotic Cells: Include bacteria and archaea.

• Eukaryotic Cells: Include protists, fungi, animals, and plants.

• Common Features: Plasma membrane, cytosol, chromosomes, and ribosomes.

• Key Differences:

  • Location of DNA: Eukaryotic DNA is in the nucleus; prokaryotic DNA is in the nucleoid (not membrane-bound).

  • Organelles: Eukaryotic cells have membrane-bound organelles; prokaryotic cells do not.

  • Cytoplasm: In eukaryotes, the region between the nucleus and plasma membrane contains organelles; in prokaryotes, it is not compartmentalized.

  • Size: Eukaryotic cells are generally larger (10–100 μm) than prokaryotic cells (0.1–5 μm).

• Surface Area to Volume Ratio: As cell size increases, volume grows faster than surface area, limiting the rate of material exchange. Cells may have adaptations (e.g., microvilli) to increase surface area.

• Cell Wall: Plant cells have a strong cell wall made of cellulose; animal cells do not.

Internal Membranes and Compartmentalization

Eukaryotic cells have internal membranes that compartmentalize functions, allowing specialized processes to occur simultaneously.

• Plasma Membrane (PM): A selective barrier regulating passage of oxygen, nutrients, and waste.

• Organelle Membranes: Many enzymes are embedded in membranes, supporting cell metabolism.

• Cytoskeleton: Provides structural framework and supports cell shape.

• Mitochondria: Produce ATP for energy; enzymes are embedded in their membranes.

• Nucleus: Enclosed by a double membrane (nuclear envelope), houses DNA, and is the site of ribosome assembly.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane.

  • Functions: Synthesis and transport of proteins and lipids, metabolism, detoxification, and movement of materials.

  • Membranes are connected by direct contact or by transfer of vesicles.

The Nucleus: Information Center

The nucleus contains most of the cell's genetic material and is responsible for storing and transmitting information for cell function.

• Nuclear Envelope: Double membrane (lipid bilayer) with pores that regulate entry and exit of molecules.

• Nuclear Lamina: Protein network supporting the envelope.

• Chromosomes: DNA organized with proteins (histones) into chromatin; condenses into visible chromosomes during cell division.

• Ribosome Assembly: Nucleolus synthesizes rRNA and assembles ribosomal subunits.

• mRNA Synthesis: DNA is transcribed into mRNA, which exits the nucleus for protein synthesis in the cytoplasm.

• Chromosome Number: Each species has a characteristic number of chromosomes (e.g., humans have 46; sex cells have 23).

Ribosomes: Protein Factories

Ribosomes are the cellular components that carry out protein synthesis, found either free in the cytosol or bound to the ER or nuclear envelope.

• Free Ribosomes: Synthesize proteins that function in the cytosol.

• Bound Ribosomes: Synthesize proteins destined for membranes, organelles, or secretion.

• Structurally identical; location determines protein destination.

The Endomembrane System: Protein Trafficking and Metabolic Functions

The endomembrane system is a network of membranes involved in synthesis, transport, and processing of proteins and lipids.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes and secretes proteins. Proteins enter the ER lumen, fold into shape, and are often glycoproteins (proteins with carbohydrate chains).

  • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons, and stores calcium ions.

• Golgi Apparatus: Consists of flattened membranous sacs (cisternae) with structural and functional directionality (cis face receives, trans face ships). Modifies, sorts, and packages proteins and lipids for delivery.

• Vesicles: Transport materials between ER, Golgi, plasma membrane, and other organelles.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Plant Cell Vacuole: Regulates cytoplasm composition, stores compounds, and maintains turgor pressure.

Key Terms and Concepts

• Selective Permeability: Property of membranes to allow certain substances to pass while blocking others.

• Peripheral Proteins: Proteins attached to the membrane surface.

• Transport Proteins: Facilitate movement of substances across membranes.

• Glycoproteins: Proteins with carbohydrate chains attached.

• Concentration Gradient: Difference in concentration of a substance across a space.

• Passive Transport: Movement of substances without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement of substances against a concentration gradient, requiring energy.

• Tonicity: Ability of a solution to cause a cell to gain or lose water.

• Membrane Potential: Voltage across a membrane due to ion distribution.

Formulas and Equations

• Surface Area of a Sphere:

• Volume of a Sphere:

• Surface Area to Volume Ratio:

Example Table: Comparison of Prokaryotic and Eukaryotic Cells

Example Table: Functions of Endomembrane System Components

Additional info:

• Some details on super-resolution and cryo-electron microscopy were inferred for completeness.

• Tables were constructed to summarize comparisons and functions for clarity.