Cell Structure and Function

Surface Area to Volume Ratio

The surface area to volume ratio is a key concept in cell biology, influencing cell size and efficiency of nutrient exchange.

• Definition: The surface area to volume ratio compares the amount of surface area a cell has relative to its volume.

• Importance: As a cell grows, its volume increases faster than its surface area, limiting the rate of material exchange.

• Formula: For a sphere, , .

• Application: Smaller cells have a higher surface area to volume ratio, allowing for more efficient exchange of materials.

Prokaryotic vs. Eukaryotic Cells

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

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. Example: Bacteria.

• Eukaryotic Cells: Have a nucleus and membrane-bound organelles. Example: Animal and Plant cells.

• Similarities: Both have a plasma membrane, cytoplasm, ribosomes, and genetic material.

• Differences: Eukaryotes are generally larger, more complex, and have compartmentalized functions.

Cell Organelles and Their Functions

Organelles are specialized structures within cells that perform distinct functions.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Lysosomes: Contain digestive enzymes to break down waste.

• Vacuoles: Storage organelles; large central vacuole in plant cells stores water and nutrients.

• Mitochondria: Site of cellular respiration; produces ATP.

• Plastids: Found in plant cells; include chloroplasts for photosynthesis.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Provides structural support and facilitates cell movement.

Protein Secretion Pathway

Proteins destined for secretion follow a specific pathway through the cell.

• Steps:

  1. Protein synthesis at ribosomes (often on rough ER).

  2. Transported into the ER lumen.

  3. Packaged into vesicles and sent to the Golgi apparatus.

  4. Modified and sorted in the Golgi.

  5. Packaged into secretory vesicles.

  6. Vesicles fuse with the plasma membrane, releasing proteins outside the cell.

• Example: Insulin secretion by pancreatic cells.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Definition: Mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Both organelles have their own DNA, double membranes, and reproduce independently within the cell.

• Significance: Explains the presence of prokaryotic features in these organelles.

Structure of Mitochondria and Chloroplasts

Mitochondria and chloroplasts are double-membraned organelles with specialized internal structures.

• Mitochondria:

  • Outer membrane

  • Inner membrane (folded into cristae)

  • Matrix (internal fluid)

• Chloroplasts:

  • Outer and inner membranes

  • Stroma (internal fluid)

  • Thylakoids (stacked into grana)

• Function: Mitochondria produce ATP; chloroplasts conduct photosynthesis.

Cytoskeleton Components

The cytoskeleton is a network of protein filaments that provides structural support and facilitates movement.

• Microtubules: Composed of tubulin; form centrioles, cilia, and flagella.

• Microfilaments: Composed of actin; involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical strength.

• Associated Structures: Centrosomes (microtubule organizing centers), basal bodies (anchor cilia/flagella).

• Motor Proteins: Dynein, myosin, and kinesin move along cytoskeletal filaments.

Cilia and Flagella

Cilia and flagella are hair-like structures that enable cell movement.

• Structure: Both have a "9+2" arrangement of microtubules.

• Function: Cilia move substances across cell surfaces; flagella propel cells.

• Bending Mechanism: Dynein arms slide microtubules against each other, causing bending.

• Example: Sperm cells use flagella for movement; respiratory tract cells use cilia to move mucus.

Plant vs. Animal Cells

Plant and animal cells share many features but also have distinct differences.

Cell Junctions and Coordination in Multicellular Organisms

Cell junctions are specialized structures that connect cells and facilitate communication and coordination.

• Tight Junctions: Seal cells together, preventing leakage of materials.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between cells via channels.

• Plasmodesmata (plants): Channels between plant cells for transport and communication.

• Role: Coordinate cellular activities in tissues and organs.