Cell Structure and the Endomembrane System: Study Guide

Cell Structure and the Endomembrane System

Key Terms and Concepts

This section introduces essential vocabulary and concepts related to cell structure and the endomembrane system, which are foundational for understanding eukaryotic cell organization and function.

• Organelle: Specialized subunit within a cell that has a specific function.

• Nucleus: Membrane-bound organelle containing genetic material (DNA).

• Endomembrane system: Group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Semiautonomous organelles: Organelles like mitochondria and chloroplasts that contain their own DNA and can replicate independently.

• Chromatin: Complex of DNA and proteins found in the nucleus.

• Nuclear envelope: Double membrane surrounding the nucleus.

• Endoplasmic reticulum (ER): Network of membranes involved in protein and lipid synthesis; includes rough ER (with ribosomes) and smooth ER (without ribosomes).

• Golgi apparatus: Organelle responsible for modifying, sorting, and packaging proteins and lipids.

• Lysosomes: Organelles containing digestive enzymes for breaking down cellular waste.

• Peroxisomes: Organelles involved in the breakdown of fatty acids and detoxification.

• Vacuoles: Membrane-bound sacs for storage and transport.

• Mitochondria: Powerhouse of the cell, site of cellular respiration.

• Chloroplasts: Organelles in plant cells responsible for photosynthesis.

Endomembrane System

Definition and Components

The endomembrane system is a network of membranes within eukaryotic cells that work together to produce, modify, and transport proteins and lipids. It includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and vacuoles.

• Nuclear envelope: Separates the nucleus from the cytoplasm and regulates passage of molecules.

• Endoplasmic reticulum (ER): Divided into rough ER (with ribosomes) and smooth ER (without ribosomes).

• Golgi apparatus: Modifies, sorts, and packages macromolecules for secretion or use within the cell.

• Lysosomes: Digestive organelles containing hydrolytic enzymes.

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

• Vacuoles: Storage organelles, especially prominent in plant cells.

Structure of the Nucleus

The nucleus is the control center of the cell, containing most of the cell's genetic material.

• Nuclear envelope: Double membrane with nuclear pores for transport of molecules.

• Nuclear lamina: Network of proteins providing structural support.

• Nucleolus: Site of ribosome assembly.

Ribosomes: Free vs. Bound

Ribosomes are the molecular machines that synthesize proteins. They can be free in the cytosol or bound to the rough ER.

Type of Ribosome	Location	Product
Free ribosomes	Cytosol	Proteins for use within the cell
Bound ribosomes	Attached to rough ER	Proteins for secretion or for use in membranes/organelles

Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in protein and lipid synthesis. It is divided into two types:

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion and membrane insertion.

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

Major functions of the smooth ER:

• Lipid synthesis (e.g., phospholipids, steroids)

• Detoxification of drugs and poisons

• Storage of calcium ions

Golgi Apparatus

The Golgi apparatus is the cell's packaging and distribution center. It consists of flattened membrane sacs called cisternae.

• Cis face: Receiving side, closest to the ER.

• Trans face: Shipping side, facing the plasma membrane.

• Processes include glycosylation (addition of sugars), sorting, and packaging of proteins and lipids.

Pathway of macromolecules: Proteins and lipids are transported from the ER to the Golgi in vesicles, modified in the Golgi, and then sent to their final destinations.

Secretory Pathway

The secretory pathway describes the route by which proteins are synthesized, processed, and exported from the cell.

1. Protein synthesis begins in the rough ER.

2. Proteins are packaged into transport vesicles.

3. Vesicles fuse with the Golgi apparatus for further modification.

4. Modified proteins are sorted and sent to the plasma membrane or other organelles.

Lysosomes, Peroxisomes, and Vacuoles

These organelles are involved in digestion, detoxification, and storage.

• Lysosomes: Contain hydrolytic enzymes; pH inside is acidic (around 4.5-5.0); involved in autophagy (recycling cellular components).

• Peroxisomes: Break down fatty acids and neutralize toxins; compartmentalization prevents damage to other cellular components.

• Vacuoles: Store nutrients, waste products, and help maintain turgor pressure in plant cells.

Semiautonomous Organelles and Endosymbiosis

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are organelles with their own DNA, supporting the endosymbiosis theory of their origin.

• Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP.

• Chloroplasts: Site of photosynthesis; converts light energy into chemical energy (glucose).

Structural comparison:

Feature	Mitochondria	Chloroplasts
Membranes	Double membrane (outer and inner)	Double membrane plus thylakoid membrane
Compartments	Intermembrane space, matrix	Intermembrane space, stroma, thylakoid space
DNA	Present	Present
Ribosomes	Present	Present

Endosymbiosis Theory

The endosymbiosis theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Both organelles have their own circular DNA.

• Both have double membranes.

• Both contain ribosomes similar to those of bacteria.

Functions of Chloroplasts and Peroxisomes

• Chloroplasts: Carry out photosynthesis, producing glucose and oxygen from carbon dioxide and water using light energy.

• Peroxisomes: Detoxify harmful substances and break down fatty acids; compartmentalization allows for safe handling of reactive molecules.

Summary Table: Key Organelles and Their Functions

Organelle	Main Function
Nucleus	Stores genetic material, controls cell activities
Rough ER	Protein synthesis and processing
Smooth ER	Lipid synthesis, detoxification, calcium storage
Golgi apparatus	Modification, sorting, and packaging of macromolecules
Lysosomes	Digestion and recycling of cellular waste
Peroxisomes	Breakdown of fatty acids, detoxification
Vacuoles	Storage and maintenance of cell turgor
Mitochondria	ATP production via cellular respiration
Chloroplasts	Photosynthesis

Key Equations

• Cellular respiration (mitochondria):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Photosynthesis (chloroplasts):

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Additional info:

• Autophagy is a process by which lysosomes recycle cellular components, maintaining cellular health.

• Compartmentalization in cells allows for specialized environments and efficient metabolic processes.