Chapter One: Part Three - Cell Structure and Function

Overview of Cell Structure and Organelles

• Organelles: Functionally and structurally distinct parts of a cell, typically surrounded by membranes with specific functions.
• Ultrastructure: General detailed structures of a cell, including components not necessarily surrounded by membranes (e.g., cell membrane).

Types of Cells

• Prokaryotes: Bacteria and archaea.
• Eukaryotes: Includes animals, plants, and other organisms.
• Focus on eukaryotic cell structures in this section.

Key Eukaryotic Cell Structures

1. Cell Surface Membrane

• Also called plasma membrane.
• Approximately 7 nm thick; visible only under an electron microscope.
• Appears as a tri-laminar structure due to phospholipid bilayer.
• Function: Controls movement of substances in and out of the cell.

2. Nucleus

• Largest organelle visible under a light microscope.
• Surrounded by a double membrane (nuclear envelope) with nuclear pores for substance movement.
• Contains DNA for protein synthesis (transcribed into mRNA).
• Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly.
• Chromatin: DNA wrapped around proteins (histones).

3. Ribosomes

• Smallest organelle; not membrane-bound (25 nm in size).
• Composed of rRNA and proteins; consists of two subunits (large and small).
• Function: Site of protein synthesis; types include:
  • 80S Ribosomes: Found in cytoplasm and rough ER.
  • 70S Ribosomes: Found in mitochondria and chloroplasts.

4. Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; involved in protein synthesis and modification.
  • Function: Protein folding, glycosylation, and transport to Golgi apparatus.
• Smooth ER: Lacks ribosomes; involved in lipid synthesis (cholesterol, steroid hormones).

5. Golgi Apparatus (Golgi Body)

• Stack of flattened membrane sacs (cisternae) with no direct connection to other membranes.
• Functions:
  • Protein and lipid modification (glycosylation, phosphorylation).
  • Packaging of molecules into vesicles for transport (secretory vesicles and lysosomes).
  • Continuous formation and breakdown of vesicles like microbots.

6. Lysosomes

• Special vesicles containing hydrolytic enzymes for breaking down unwanted structures (damaged organelles, bacteria).
• Enzymes function best in an acidic environment.

7. Mitochondria

• Double membrane organelle known as the site of aerobic respiration.
• Synthesizes ATP, contains 70S ribosomes and circular DNA.
• Cristae: Folded inner membranes to increase surface area for ATP synthesis.
• Hypothesized to have a prokaryotic origin due to similarities with bacteria.

8. Chloroplasts

• Large organelle involved in photosynthesis; visible under light microscope.
• Contains chlorophyll, thylakoids (stacked as grana), and stroma (fluid).
• Also contains 70S ribosomes and circular DNA.
• Function: Site of ATP synthesis during light-dependent reactions and conversion of carbon dioxide into glucose (light-independent reactions).

Important Processes

• Protein Synthesis and Secretion:
  • Ribosomes synthesize proteins, which are transported to the Golgi apparatus for modification.
  • Secretory vesicles bud off from Golgi body, fuse with plasma membrane, and release contents via exocytosis.
• Endocytosis: Process by which cells intake substances, countering the effects of exocytosis.

Summary

• Understanding the structure and function of organelles is essential for grasping cellular processes.
• Review key terms and organelle functions for better comprehension in future chapters.