Cell Structure and Function: Study Notes for General Biology

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Cell Structure and Function

Microscopy and the Tools of Cytochemistry

Microscopy is essential for studying cells, as it allows scientists to visualize structures too small for the naked eye. Different types of microscopes provide varying levels of resolution and magnification.

Resolution: The ability to distinguish two close points as separate entities. Higher resolution allows for clearer images of small structures.
Light Microscopes (LM): Use visible light to magnify specimens up to about 1,000 times. Useful for viewing living cells and tissues.
Electron Microscopes (EM): Use beams of electrons for much higher resolution (up to 2 nm). Useful for viewing ultrastructure of cells.
Scanning Electron Microscope (SEM): Provides detailed 3D images of cell surfaces.
Transmission Electron Microscope (TEM): Used to view internal cell structures at very high resolution.

Example: TEM can reveal the arrangement of organelles within a cell, while SEM can show the surface texture of a cell membrane.

Prokaryotes: Nature of Adaptation

Prokaryotes are single-celled organisms that thrive in a wide range of habitats. They are the earliest known life forms and exhibit remarkable adaptability.

Size: Typically 0.5–5 μm in diameter.
Habitat: Found in diverse environments, including extreme conditions.
Adaptations: Rapid reproduction, genetic diversity, and metabolic versatility.

Example: Escherichia coli is a common prokaryote found in the human gut, adapted to survive in both oxygen-rich and oxygen-poor environments.

Prokaryotic vs. Eukaryotic Cells

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

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Size	0.5–5 μm	10–100 μm
DNA location	Nucleoid region	Nucleus
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Additional info: Eukaryotic cells are generally larger and more complex than prokaryotic cells.

Cell Membranes and Internal Compartments

All cells are surrounded by a plasma membrane that regulates the movement of substances in and out of the cell. Eukaryotic cells contain internal membranes that compartmentalize functions.

Plasma Membrane: Composed of a phospholipid bilayer with embedded proteins. Selectively permeable.
Internal Membranes: Form organelles such as the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplasts.
Compartmentalization: Allows for specialized environments and efficient metabolic processes.

Example: The mitochondrion has a double membrane that creates distinct compartments for energy production.

Genetic Material and Chromosomes

Cells store genetic information in the form of DNA, which is organized into chromosomes.

Prokaryotic Cells: DNA is located in the nucleoid region and usually consists of a single circular chromosome.
Eukaryotic Cells: DNA is contained within the nucleus and organized into multiple linear chromosomes.
Chromatin: DNA and associated proteins that make up chromosomes.

Example: Human cells have 46 chromosomes, while most bacteria have a single chromosome.

Ribosomes: Protein Synthesis

Ribosomes are the molecular machines responsible for protein synthesis in all cells.

Structure: Composed of ribosomal RNA (rRNA) and proteins.
Location: Free in the cytoplasm or attached to the endoplasmic reticulum (in eukaryotes).
Function: Translate messenger RNA (mRNA) into polypeptide chains.

Example: Ribosomes in pancreatic cells synthesize digestive enzymes for secretion.

Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that coordinates the synthesis, modification, and transport of cellular products.

Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.
Functions: Protein and lipid synthesis, detoxification, and transport.

Example: The rough ER is studded with ribosomes and is involved in the synthesis of membrane-bound and secretory proteins.

Endoplasmic Reticulum (ER)

The ER is a continuous membrane system that plays a central role in the synthesis and transport of proteins and lipids.

Rough ER: Has ribosomes attached; synthesizes proteins for secretion or membrane insertion.
Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Example: Liver cells have abundant smooth ER for detoxifying drugs and poisons.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Structure: Stacks of flattened membranous sacs called cisternae.
Function: Receives products from the ER, modifies them, and ships them to their destinations.

Example: The Golgi apparatus adds carbohydrate groups to proteins, forming glycoproteins.

Lysosomes

Lysosomes are membrane-bound organelles containing hydrolytic enzymes for intracellular digestion.

Function: Breakdown of macromolecules, recycling of cellular components, and defense against pathogens.
Autophagy: The process by which lysosomes digest and recycle the cell's own organelles.

Example: White blood cells use lysosomes to destroy bacteria and viruses.

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs involved in transport, storage, and waste disposal.

Vesicles: Small sacs that transport materials within the cell.
Vacuoles: Larger sacs, prominent in plant cells, that store water, nutrients, and waste products.

Example: The central vacuole in plant cells maintains turgor pressure and stores nutrients.

Mitochondria and Chloroplasts: Energy Conversion

Mitochondria and chloroplasts are organelles responsible for energy conversion in eukaryotic cells.

Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP.
Chloroplasts: Found in plants and algae; site of photosynthesis, converting light energy into chemical energy.

Example: Muscle cells contain many mitochondria to meet high energy demands.

Equation for Cellular Respiration:

Equation for Photosynthesis:

Internal Membranes and Compartmentalization

Internal membranes create specialized compartments within eukaryotic cells, allowing for efficient and regulated metabolic processes.

Advantages: Separation of incompatible reactions, increased surface area for reactions, and localized concentration of enzymes.
Examples: Lysosomes for digestion, mitochondria for energy production, and peroxisomes for detoxification.

Additional info: Compartmentalization is a key feature distinguishing eukaryotic cells from prokaryotic cells.