Uni Bio 3

Chapter 7: Cell Structure and Function

Concept 7.1: Biologists Use Microscopes and Biochemistry to Study Cells

  • Cells are typically too small for the naked eye, yet they can be quite complex.

Microscopy

  • Microscopes are essential for visualizing cells.

  • Light Microscopes (LM):

    • Use visible light to illuminate the specimen.

    • Light is refracted by glass lenses, creating magnified images.

Important Parameters of Microscopy

  • Magnification:

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

  • Resolution:

    • The clarity of the image; the smallest distance between two points that can be resolved.

  • Contrast:

    • Visible differences in parts of a sample.

Light Microscope Capabilities

  • Magnification up to 1,000x.

  • Techniques available to enhance contrast and color various cell components.

Limitations of Light Microscopy

  • Unable to resolve details finer than 0.2 µm (200 nm); thus, subcellular structures usually remain unseen.

Electron Microscopy

  • Utilizes beams of electrons rather than light to achieve higher resolution.

  • Two Basic Types of Electron Microscopes:

    • Scanning Electron Microscope (SEM):

      • Scans sample surfaces coated with a thin layer of gold.

      • Produces three-dimensional images by detecting electrons emitted from the surface.

    • Transmission Electron Microscope (TEM):

      • Passes electrons through thin sections of stained specimens.

      • Primarily used for studying internal cell structures, relying on heavy metals for contrast.

Usage of EM

  • Both SEM and TEM utilize electromagnets instead of glass lenses for focusing.

  • Electron microscopy significantly enhances the understanding of subcellular structures; however, sample preparation often kills the cells.

Cell Fractionation

  • A methodology for breaking down cells and separating organelles based on size and density.

  • Enables bulk preparation of specific cell components for functional study.

Eukaryotic versus Prokaryotic Cells

  • Eukaryotic cells contain membrane-bound organelles, unlike prokaryotic cells, which lack a nucleus and bound organelles.

  • Common features of all cells include plasma membranes, cytosol, chromosomes, and ribosomes.

Features of Prokaryotic Cells

  • No nucleus; DNA present in a nucleoid.

  • No organelles; cytoplasm is bound by plasma membrane.

Features of Eukaryotic Cells

  • DNA is housed within a nucleus surrounded by a nuclear envelope.

  • Contain membrane-bound organelles; larger in size compared to prokaryotic cells.

Plasma Membrane

  • Acts as a selective barrier for the passage of oxygen, nutrients, and waste.

  • The structure of the plasma membrane contains a lipid bilayer with embedded proteins, which plays vital roles in cell function and integrity.

Surface Area to Volume Ratio

  • The ratio limits cell size; as cells grow, volume increases faster than surface area.

  • Smaller cells possess higher ratios, facilitating the exchange of materials efficiently with their environment.

Eukaryotic Cell Structure Overview

  • Internal membranes partition the cell into organelles, each providing specific environments to carry out various metabolic processes.

  • Organelles common to both plant and animal cells include the nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, and others.

Nucleus: Information Hub

  • Contains most of the cell's DNA and is typically the most prominent organelle.

  • Enclosed by a double-membrane nuclear envelope comprising lipid bilayers with proteins.

Nuclear Organization

  • Chromatin: DNA complexed with proteins, condensing into chromosomes during division.

  • Nucleolus: Location of ribosomal RNA (rRNA) synthesis and ribosomal subunit assembly.

Ribosomes: Protein Synthesis

  • Consist of ribosomal RNA and proteins; involved in synthesizing proteins in the cytosol (free ribosomes) and on the rough ER (bound ribosomes).

Endomembrane System

  • Composed of various organelles involved in the trafficking and processing of proteins, including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.

Endoplasmic Reticulum (ER)

  • Forms a membranous network consisting of two types: smooth ER (synthesis of lipids) and rough ER (synthesis and processing of proteins).

  • Functions of Smooth ER:

    • Lipid synthesis, carbohydrate metabolism, detoxification of drugs, calcium ion storage.

Functions of Rough ER

  • Secretory protein synthesis, distribution of transport vesicles, and membrane production.

Golgi Apparatus

  • Composed of flattened sacs (cisternae) involved in modifying, sorting, and shipping products from the ER.

  • Serves as a quality control center where modifications occur as materials transit from the cis face to the trans face.

Lysosomes: Digestive Organelles

  • Membranous sacs containing hydrolytic enzymes for digestion of macromolecules, functioning best in acidic conditions.

Vacuoles: Various Functions

  • Large vesicles serving various roles, including storage, maintenance of ion concentrations, and digestion.

  • Central Vacuole: Serves as a storage area in plant cells; stores cell sap consisting of inorganic ions.

Mitochondria and Chloroplasts

  • Mitochondria: Site of cellular respiration, generating ATP from energy substrates.

  • Chloroplasts: Organelle for photosynthesis in plants, converting solar energy into chemical energy.

Conclusion

  • Integration of cellular processes exemplified by the macrophage's ability to ingest bacteria illustrates how cells function as cohesive units.

  • Overall, cellular functions arise from the organization and interaction of various cellular components.