Cell Types

Summary: Tour of Eukaryotic Cells

Definition:

• Eukaryotic cells have membrane-bound organelles — internal compartments with specific functions.

Key Membrane-Bound Organelles

1. Nucleus

   • Contains DNA (genetic info)

   • Surrounded by nuclear membrane

   • Contains nuclear pores that allow mRNA to exit

2. Endoplasmic Reticulum (ER)

   • Connected to nucleus

   • Two types:

     • Rough ER (with ribosomes): protein synthesis

     • Smooth ER (no ribosomes): lipid synthesis, detox

   • Ribosomes use mRNA to make proteins (can be attached to ER or free in cytoplasm)

3. Golgi Body (Apparatus)

   • Packages and modifies proteins/lipids

   • Prepares materials for secretion out of the cell

4. Mitochondria

   • "Powerhouse of the cell" – makes ATP (cell energy)

   • Has its own DNA (inherited maternally)

   • Thought to have evolved from independent organisms (endosymbiosis theory)

5. Chloroplasts (plants/algae only)

   • Site of photosynthesis

   • Contain thylakoids (internal membrane stacks)

   • Also have their own DNA

6. Vacuole

   • Large in plants, smaller in animals

   • Stores water, nutrients, and waste

   • Can also break down materials (enzymes)

7. Lysosome

   • Contains digestive enzymes

   • Breaks down waste, foreign substances, or damaged cell parts

   • Mostly in animal cells, but some found in plants

Other Notes

• Cells are 3D and highly dynamic

• Many non-membrane-bound structures also exist (not covered here)

• Cells are far more complex than they appear — like a microscopic city full of activity

Conclusion:
Eukaryotic cells are defined by their internal complexity and compartmentalization through organelles. This allows specialized functions and efficient operation, making them far more advanced than they seem at a glance.

Summary of the Endomembrane System

Purpose:
The endomembrane system is a network of membrane-bound organelles in eukaryotic cells that modifies, packages, and transports lipids and proteins. It's essential for protein secretion (e.g., digestive enzymes from pancreatic cells).

Components of the Endomembrane System

( Excludes mitochondria, chloroplasts, and peroxisomes)

1. Nuclear Envelope

• Surrounds the nucleus.

• Connects directly to the endoplasmic reticulum (ER).

2. Endoplasmic Reticulum (ER)

• Rough ER (RER):

  • Studded with ribosomes.

  • Synthesizes and modifies proteins.

  • Sends proteins in vesicles to the Golgi.

• Smooth ER (SER):

  • No ribosomes.

  • Synthesizes lipids, carbohydrates, steroids.

  • Detoxifies drugs/poisons.

  • Stores Ca²⁺ ions (especially in muscle as sarcoplasmic reticulum).

  • Contains transitional ER regions for vesicle formation.

3. Golgi Apparatus

• Modifies, tags, sorts, and packages proteins/lipids from the ER.

• Cis face receives vesicles; trans face ships them out.

• Sends vesicles to:

  • Plasma membrane (for secretion)

  • Lysosomes or vacuoles (for digestion/storage)

4. Plasma Membrane

• Final destination for secreted proteins or membrane proteins.

• Fuses with transport vesicles to release or embed proteins.

5. Lysosomes (in animal cells)

• Contain digestive enzymes.

• Break down cellular waste and foreign particles.

• Example: Macrophages use lysosomes to destroy pathogens via phagocytosis (fusion of phagosome and lysosome).

6. Vacuoles (in plant cells)

• Large central vacuole functions like a lysosome.

• Stores water, waste, toxins, pigments.

• Helps maintain water balance and cell structure.

Not Part of the Endomembrane System

• Peroxisomes:

  • Break down fatty acids and detoxify substances (e.g., alcohol).

  • Produce hydrogen peroxide (H₂O₂) as a by-product.

  • Not derived from the ER or Golgi.

Core Function:
The endomembrane system manages the flow of proteins and lipids from synthesis to their final destinations (inside or outside the cell).

Summary: Mitochondria & Chloroplasts

Key Roles:

• Mitochondria: Powerhouse of the cell. Converts fuel molecules (like sugars) into ATP via cellular respiration.

• Chloroplasts: Found in plants/algae. Capture light energy to make sugars via photosynthesis.

Structure & Function:

Chloroplasts

• Found only in plants and photosynthetic algae

• Function: Photosynthesis – converts sunlight + CO₂ into sugars

• Structure:

  • Double membrane

  • Internal stacks of thylakoids (contain chlorophyll)

  • Fluid-filled stroma surrounds thylakoids

  • Grana = stacks of thylakoids

Mitochondria

• Found in almost all eukaryotic cells (not in red blood cells)

• Function: Cellular respiration – converts food into ATP

• Structure:

  • Double membrane

  • Inner membrane forms folds called cristae

  • Matrix inside inner membrane holds DNA + ribosomes

  • High-energy cells (e.g., muscles) have more mitochondria

Origin (Endosymbiont Theory):

• Mitochondria and chloroplasts were once free-living bacteria

• They were engulfed by ancestral eukaryotic cells:

  1. First engulfed aerobic bacteria → became mitochondria

  2. Then engulfed photosynthetic bacteria → became chloroplasts

• Evidence:

  • Both have own DNA

  • Both have ribosomes

  • Resemble bacteria in size and structure