Cell Structure, Function, and Evolution: Study Notes for General Biology

Cell Structure and Function

Overview of the Cell

Cells are the fundamental units of life, and all living organisms are composed of one or more cells. Cells can be broadly classified as prokaryotic or eukaryotic, each with distinct structural features and organelles.

• Prokaryotic cells lack a nucleus and membrane-bound organelles. Bacteria and Archaea are prokaryotes.

• Eukaryotic cells have a nucleus and various membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes.

Carbohydrates: Structure of Fructose

Numbering Carbons in Fructose

Fructose is a monosaccharide (simple sugar) with the molecular formula C6H12O6. Correctly numbering the carbon atoms in fructose is essential for understanding its structure and function in biological systems.

• Linear form: Numbering starts from the end closest to the carbonyl group (C=O).

• Ring form: Numbering continues in the same order as the linear form, with the ring closure typically involving carbons 2 and 5.

• Importance: Correct carbon numbering is crucial for identifying isomers and understanding biochemical reactions involving fructose.

Example: In the linear form, carbon 1 is the carbonyl carbon (ketone group), and carbon 6 is the terminal CH2OH group. In the ring form, the numbering follows the same sequence, with the ring closure between C2 and C5.

Ribosomes: Structure and Differences

Prokaryotic vs. Eukaryotic Ribosomes

Ribosomes are molecular machines responsible for protein synthesis in all living cells. While both prokaryotic and eukaryotic cells have ribosomes, there are key differences in their structure and composition.

• Function: Both types of ribosomes interact with RNA to synthesize proteins.

• Prokaryotic ribosomes: 70S ribosomes, composed of a 50S large subunit and a 30S small subunit.

• Eukaryotic ribosomes: 80S ribosomes, composed of a 60S large subunit and a 40S small subunit.

• rRNA content: Prokaryotic ribosomes contain 16S, 23S, and 5S rRNA; eukaryotic ribosomes contain 18S, 28S, 5.8S, and 5S rRNA.

• Location: Eukaryotic ribosomes can be free in the cytoplasm or bound to the endoplasmic reticulum; prokaryotic ribosomes are only found free in the cytoplasm.

Table: Comparison of Prokaryotic and Eukaryotic Ribosomes

Feature	Prokaryotic Ribosome	Eukaryotic Ribosome
Total Size (Svedberg units)	70S	80S
Large Subunit	50S (23S + 5S rRNA)	60S (28S + 5.8S + 5S rRNA)
Small Subunit	30S (16S rRNA)	40S (18S rRNA)
Location	Cytoplasm	Cytoplasm and rough ER

Mitochondria: Structure and Function

Role in Aerobic Metabolism

Mitochondria are membrane-bound organelles found in most eukaryotic cells. They are often referred to as the "powerhouses" of the cell because they generate ATP, the cell's main energy currency, through aerobic respiration.

• Structure: Mitochondria have a double membrane: an outer membrane and a highly folded inner membrane (forming cristae), which increases surface area for metabolic reactions.

• Compartments: The space between the membranes is the intermembrane space; the innermost area is the matrix.

• Function: When oxygen is available, mitochondria convert glucose (or other molecules) into ATP via cellular respiration.

Equation for Cellular Respiration:

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

Example: Muscle cells have many mitochondria to meet high energy demands during contraction.

Origin of Mitochondria: The Endosymbiont Theory

Evidence and Significance

The endosymbiont theory proposes that mitochondria originated from free-living bacteria that were engulfed by ancestral eukaryotic cells. Over time, these bacteria formed a symbiotic relationship with their host cells, eventually evolving into modern mitochondria.

• Double membrane: Mitochondria are surrounded by two membranes, similar to Gram-negative bacteria.

• Own DNA: Mitochondria contain circular DNA, like bacteria.

• Ribosomes: Mitochondria have their own ribosomes, which are more similar to prokaryotic ribosomes than to eukaryotic cytoplasmic ribosomes.

• Reproduction: Mitochondria replicate independently of the cell by a process similar to binary fission.

Significance: The endosymbiont theory explains the evolutionary origin of mitochondria and highlights the importance of symbiosis in the evolution of complex life.

Additional info: The theory was first formally proposed by Lynn Margulis in 1967 and is now widely accepted due to molecular and genetic evidence.

Chloroplasts and Other Organelles

Chloroplasts: Structure and Function

Chloroplasts are organelles found in plant and algal cells responsible for photosynthesis. Like mitochondria, they have a double membrane and their own DNA.

• Structure: Outer and inner membranes, with internal stacks of thylakoids (grana) surrounded by stroma.

• Function: Capture light energy and convert it into chemical energy (glucose) via photosynthesis.

Equation for Photosynthesis:

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

Nucleus and Endomembrane System

Nucleus

The nucleus is the control center of eukaryotic cells, containing most of the cell's genetic material (DNA). It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for the exchange of materials.

• Chromatin: DNA and associated proteins found within the nucleus.

• Nucleolus: Region where ribosomal RNA is synthesized and ribosome assembly begins.

Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins.

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

• Functions: Protein and lipid synthesis, transport, metabolism, and detoxification.

Endoplasmic Reticulum (ER)

The ER is a major site of synthesis and transport within the cell, and is continuous with the nuclear envelope. There are two types:

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

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies poisons, and stores calcium ions.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for delivery to different destinations inside or outside the cell.

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

• Function: Receives products from the ER, modifies them, and ships them to their final destinations.

Summary Table: Major Eukaryotic Organelles

Organelle	Main Function	Key Features
Nucleus	Stores genetic material, controls cell activities	Double membrane, nuclear pores, nucleolus
Mitochondrion	ATP production via aerobic respiration	Double membrane, cristae, own DNA
Chloroplast	Photosynthesis	Double membrane, thylakoids, own DNA
Ribosome	Protein synthesis	Large and small subunits, rRNA and proteins
Endoplasmic Reticulum	Protein and lipid synthesis, detoxification	Rough (ribosomes), smooth (no ribosomes)
Golgi Apparatus	Modification and transport of proteins/lipids	Stacked cisternae