Cell Structure and Function: General Biology Study Notes

Cell Structure and Function

Sizes of Biological Structures

Cells and their components vary greatly in size, which determines the type of microscopy required for observation. Understanding these sizes is essential for studying cell biology.

• Frog egg: ~1 cm

• Human egg: ~100 μm

• Most plant and animal cells: 10–100 μm

• Nucleus, most bacteria, mitochondrion: 1–10 μm

• Smallest bacteria: ~0.1 μm

• Viruses: 10–100 nm

• Ribosomes: ~25 nm

• Proteins, lipids, small molecules, atoms: 0.1–10 nm

Microscopy:

• Light microscopy: Used for cells and larger organelles.

• Electron microscopy: Required for viruses, ribosomes, and smaller molecules.

Types of Cells

Cells are classified into two main types based on their structure and evolutionary lineage.

• Eukaryotic cells: Characterized by membrane-bound organelles, including a nucleus. Found in domain Eukarya (plants, animals, fungi, protists). Size: 10–100 μm.

• Prokaryotic cells: Lack membrane-bound organelles and nucleus. Found in domains Bacteria and Archaea. Size: 0.1–5 μm.

Examples: Animal cells, plant cells (eukaryotic); bacterial cells (prokaryotic).

Cell Membrane

The cell membrane is a fundamental structure present in all cells, providing a boundary and regulating interactions with the environment.

• Composition: Primarily made of a phospholipid bilayer with embedded proteins.

• Selective permeability: Controls entry and exit of substances.

• Functions: Communication with adjacent cells, signaling, and maintaining homeostasis.

Example: The plasma membrane of animal cells separates the internal cytoplasm from the external environment.

Surface Area to Volume Ratio

Cell size is limited by the surface area to volume ratio, which affects the efficiency of material exchange.

• Formula: $\text{Surface Area} = 6 \times (\text{side length})^2$ for a cube

• Formula: $\text{Volume} = (\text{side length})^3$ for a cube

• Importance: As cells grow, volume increases faster than surface area, limiting the rate of exchange of materials.

Example: Most cells are microscopic to maintain a high surface area to volume ratio for efficient nutrient and waste exchange.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in their internal organization and complexity.

• Prokaryotic cells:

  • No nucleus; DNA located in nucleoid region

  • Cell membrane and cell wall present

  • Cytoplasm contains ribosomes for protein synthesis

• Eukaryotic cells:

  • Compartmentalized into organelles (nucleus, mitochondria, etc.)

  • Each organelle has a specific function and membrane

  • Allows for specialized environments and simultaneous processes

Additional info: Plant cells have extra components such as chloroplasts, central vacuole, cell wall, and plasmodesmata. Animal cells have lysosomes, centrosomes, and flagella.

Nucleus

The nucleus is the control center of eukaryotic cells, housing genetic material and coordinating cellular activities.

• Structure: Enclosed by a double membrane (nuclear envelope) with nuclear pores for transport.

• Contents: Chromosomes (DNA and proteins), nucleolus (site of ribosome synthesis), chromatin (uncondensed DNA).

• Function: Stores genetic information, regulates gene expression, and coordinates cell division.

Example: In plant and animal cells, DNA is found in the nucleus; in bacteria, DNA is in the nucleoid.

Ribosomes

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

• Composition: Made of ribosomal RNA (rRNA) and proteins.

• Function: Translate messenger RNA (mRNA) into polypeptides (proteins).

• Locations: Free in cytoplasm (make proteins for cell use) or bound to endoplasmic reticulum (make proteins for export or membranes).

Endomembrane System

The endomembrane system is a network of membranes and organelles in eukaryotic cells that work together in transport, metabolism, and synthesis.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, plasma membrane.

• Functions: Protein and lipid synthesis, detoxification, transport of materials.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes secretory proteins and membrane phospholipids.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, carbohydrate metabolism, calcium storage, and detoxification.

Golgi Apparatus

• Structure: Stacks of flattened membrane sacs (cisternae).

• Function: Modifies, sorts, and ships proteins and lipids from ER; manufactures some macromolecules.

• Polarity: Has a cis (receiving) and trans (shipping) face.

Lysosomes

• Function: Intracellular digestion, recycling of organic material, destruction of cell components.

• Processes: Phagocytosis (engulfing particles), autophagy (recycling damaged organelles).

• Example: Tay-Sachs disease results from missing lysosomal enzymes, leading to lipid accumulation.

Vacuoles

• Types: Food vacuoles (formed by phagocytosis), contractile vacuoles (pump excess water), central vacuole (in plants, stores water and nutrients).

• Function: Storage, waste disposal, maintaining cell shape and turgor in plants.

Mitochondria and Chloroplasts

These organelles are responsible for energy transformation in eukaryotic cells.

• Mitochondria: Site of cellular respiration; generates ATP from sugars, fats, and oxygen. Contains its own DNA and ribosomes.

• Chloroplasts: Found in plants; site of photosynthesis, producing sugars and oxygen. Contains chlorophyll, DNA, and ribosomes.

• Endosymbiont theory: Suggests mitochondria and chloroplasts originated from engulfed prokaryotes.

Peroxisomes

Peroxisomes are small organelles that carry out oxidation reactions, breaking down fatty acids and detoxifying harmful substances.

• Function: Breakdown of fatty acids, detoxification of alcohol and other compounds.

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, motility, and organization within the cell.

• Microtubules: Hollow tubes made of tubulin; maintain cell shape, serve as tracks for organelle movement, and are involved in cell division.

• Microfilaments (Actin filaments): Solid rods made of actin; support cell shape, involved in muscle contraction and cell movement.

• Intermediate filaments: Fibrous proteins; provide mechanical strength and maintain cell integrity.

Specialized Structures

• Centrosomes and centrioles: Microtubule organizing centers important for cell division.

• Cilia and flagella: Motile structures composed of microtubules; propel cells or move fluids across cell surfaces.

Extracellular Matrix (ECM) and Cell Wall

Cells interact with their environment through the ECM (in animals) or cell wall (in plants).

• ECM: Network of glycoproteins and carbohydrates outside animal cells; provides structural support and regulates cell behavior.

• Cell wall: Rigid structure outside plant cells; composed mainly of cellulose, provides protection and support.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present
Organelles	Absent	Present (membrane-bound)
Cell Size	0.1–5 μm	10–100 μm
Domains	Bacteria, Archaea	Eukarya
Examples	Bacteria, Archaea	Plants, animals, fungi, protists

Additional info: These notes expand on brief points from the original slides, providing definitions, examples, and context for key cell biology concepts.