Cell Structure and Function: Study Notes for General Biology

6.0 Cell Size - Viewing

Viewing Cells

Understanding how cells are visualized is fundamental in biology. The ability to observe cells depends on several key properties of microscopes and the nature of the specimen.

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

• Resolution: The clarity of an image, defined as the ability to distinguish two closely spaced objects as separate entities.

• Contrast: The difference in brightness between light and dark areas in an image, which helps distinguish structures.

Microscopy Types

• Electron Microscopy:

  • Scanning Electron Microscopy (SEM): Provides detailed images of cell surfaces.

  • Transmission Electron Microscopy (TEM): Reveals internal cell structures.

• Light Microscopy:

  • Brightfield: Can be used with stained or unstained specimens.

  • Fluorescence: Uses fluorescent dyes to label specific cell components.

6.1 Features of Cells

Common Features of All Cells

All cells, whether prokaryotic or eukaryotic, share four fundamental features:

• Plasma Membrane: A selectively permeable barrier that encloses the cell.

• Cytosol: The fluid in which subcellular components are suspended.

• Genetic Material: DNA contained in chromosomes.

• Ribosomes: Specialized structures that facilitate gene expression by synthesizing proteins.

High Surface Area:Volume Ratio

• As cell size increases, the surface area to volume (SA:V) ratio decreases.

• Lower SA:V ratio makes nutrient and waste exchange less efficient.

• Cells cannot grow indefinitely; larger cells require more surface area to support their volume.

• Cells may divide or develop structures (e.g., microvilli) to increase surface area.

Size Determined by Limits

• Lower limit: Cells must be large enough to contain DNA and all necessary macromolecules.

• Upper limit: Limited by the rate at which substances can move across the membrane; too large a cell would not be able to exchange materials efficiently.

6.2 Fundamental Classes of Cells

Two Classes: Prokaryotes and Eukaryotes

Cells are classified based on their size and internal organization.

• Prokaryotes: Bacteria and Archaea; generally smaller and lack membrane-bound organelles.

• Eukaryotes: Eukarya; larger, with internal membrane-bound organelles.

Features	Prokaryotes	Eukaryotes
Domains/Kingdoms of Life	Bacteria, Archaea	Eukarya
Size	1-5 micrometers	10-100 micrometers
Cellular Level	Unicellular	Unicellular or multicellular
Plasma Membrane	Encloses the cell	Encloses parts of the cell
DNA	Circular DNA in nucleoid (not membrane-bound); plasmids present	Linear DNA in nucleus (membrane-bound); complexed with histone proteins
Organelles	Lack membrane-bound organelles	Contain both non-membranous and membranous organelles
Cytosol	Part of cytoplasm not held by organelles	Part of cytoplasm not held by organelles
Ribosomes	Present in cytoplasm; protein synthesis via translation	Produce ribosomal RNA (rRNA); assemble ribosomes using proteins imported from cytoplasm; subunits leave via nuclear pore and are assembled in cytosol; remain free or associate with RER
Cytoskeleton	N/A	Reinforces cell shape, structure, and movement
Cell Walls	Composed of peptidoglycan	Plants: cellulose; Fungi: chitin; Animal: n/a

Prokaryotes

• DNA centralized in the nucleoid: Not membrane-bound.

• Cell wall: Outside the plasma membrane, composed of peptidoglycan (bacteria) or other materials (archaea).

• Glycocalyx: Protective coating of glycoproteins and polysaccharides.

• Fimbriae: Surface appendages for attachment.

• Ribosomes: Present in cytoplasm for protein synthesis.

• Plasmids: Small, non-essential DNA molecules that can confer advantages (e.g., antibiotic resistance).

• Nucleoid: Region where DNA is located (not membrane-bound).

Eukaryotes

• DNA centralized in the nucleus: Membrane-bound by nuclear envelope.

• Some contain cell walls (plants: cellulose; fungi: chitin).

• Cytoplasm: Material within the living cell, excluding the nucleus, for structure and support.

• Cytosol: Part of cytoplasm not held by organelles.

• Ribosomes: Suspended in cytosol or attached to rough ER.

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

• Size: 10-100 micrometers.

Eukaryotic Evolution

• Eukaryotes evolved from prokaryotes about 1.6-2 billion years ago.

• Endosymbiotic theory: Eukaryotes arose from a symbiotic relationship between an archaeal host and a bacterium.

• Endomembrane theory: The plasma membrane of a prokaryotic ancestor invaginated to form internal membranes, including the nuclear envelope and endomembrane system.

Subdivision of Eukaryotic Cells

Feature	Animal Cell	Plant Cell
Domain	Eukarya	Eukarya
Kingdom	Animalia	Plantae
Cellular Level	Unicellular and multicellular	Unicellular and multicellular
Cell Wall	N/A	Made of cellulose, other polysaccharides, and proteins
Organelles (specific)	Centrosome, flagellum	Chloroplasts, plasmodesmata, central vacuole (turgor pressure)
Vacuoles	Lysosome (digestive vacuole)	Central vacuole (turgor pressure)
Ribosomes	Free floating or attached to RER	Free floating or attached to RER
Cytoskeleton	Microtubules, microfilaments, intermediate filaments	Microtubules, microfilaments

Characteristics of Fungal Cells

• Cell wall made of chitin.

• Other features similar to eukaryotic cells.

• Additional info: Fungi are heterotrophic and absorb nutrients from their environment.

6.3 Informational Centre: Nucleus

Structure and Function

The nucleus is the largest organelle in eukaryotic cells and serves as the control center, housing genetic material and coordinating activities such as growth, metabolism, and reproduction.

• Contains DNA: Each cell has a copy of DNA for that organism; organisms have a characteristic number of chromosomes.

• Chromatin:

  • Euchromatin: Partially unwound DNA, accessible for replication and transcription (active genes).

  • Heterochromatin: Fully wound DNA, inaccessible (inactive genes).

• Surrounded by nuclear envelope: Double membrane with nuclear pores for regulated exchange of substances.

• Nuclear lamina: Protein filaments that maintain nuclear shape.

• Nucleolus: Dense region where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

Contents and Functions

1. Storage of DNA: Genetic information for the organism.

2. Transcription of genes: DNA is transcribed to RNA for protein synthesis.

3. Ribosome subunit assembly: rRNA and ribosomal proteins are assembled in the nucleolus; subunits exit via nuclear pores and assemble in the cytosol.

Regulation of Gene Transcription

• Chromatin state (euchromatin vs. heterochromatin).

• Presence of transcription factors (proteins that switch on transcription of specific genes).

• Other regulatory mechanisms (e.g., epigenetic modifications, signaling pathways).

6.4 Endomembrane System

Driving Cell Processes

The endomembrane system is a network of membrane-bound organelles that coordinate the synthesis, transport, and processing of proteins and lipids.

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

• Functions:

  • Synthesis of proteins and lipids

  • Transport of proteins into membranes and organelles or out of the cell

  • Metabolism and movement of lipids

  • Detoxification of poisons

Protein Targeting

• Amino acids in proteins have built-in codes (signal sequences) that direct them to their correct cellular locations.

• Proteins destined for the endomembrane system or secretion are synthesized by ribosomes on the rough ER and transported via vesicles.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes glycoproteins and phospholipids; continuous with nuclear envelope.

• Smooth ER: Lacks ribosomes; synthesizes lipids (including steroids), metabolizes carbohydrates, and detoxifies drugs and poisons.

Additional info: The endomembrane system is essential for compartmentalization in eukaryotic cells, allowing for specialized functions within different organelles.