Cell Structure and Function: General Biology Study Notes

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, possessing an internal environment distinct from the external surroundings. This compartmentalization is essential for maintaining homeostasis and supporting life processes.

• Cell: The smallest living unit, capable of independent existence.

• Plasma Membrane: A selectively permeable bilayer of phospholipids with embedded proteins, separating the cell from its environment.

The plasma membrane has three main functions:

• Isolates the cell's internal contents from the external environment

• Regulates the flow of materials into and out of the cell

• Allows communication with other cells

Cytoplasm and Cytosol

The cytoplasm is the entire region inside the cell membrane and outside the nucleus in eukaryotic cells. It contains the cytosol (fluid component) and various cellular structures.

• Cytosol: The fluid matrix within the cytoplasm.

• Cytoskeleton: A network of protein fibers and filaments that provides structural support and facilitates movement within the cell.

Prokaryotic versus Eukaryotic Cells

Key Differences

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

Prokaryotes	Eukaryotes
No nucleus or nuclear envelope	Nucleus with a nuclear envelope
DNA in a nucleoid region	DNA within the nucleus

• Nucleoid: Dense region of DNA in prokaryotic cells.

• Plasmids: Small, circular DNA molecules found in prokaryotes, separate from the main chromosome.

• Nucleus: Membrane-bound organelle in eukaryotes containing most of the cell's genetic material.

• Nuclear Envelope: Double membrane enclosing the nucleus in eukaryotic cells.

Additional Prokaryotic Features

• Cell Wall: Stiff structure providing shape and protection; permeable to small molecules.

• Pili: Short, hair-like protein projections for attachment to surfaces or other cells.

• Flagella: Rotating structures used for cell propulsion in liquid environments.

Compartmentalization in Eukaryotic Cells

Organelles

Eukaryotic cells contain specialized compartments called organelles, each performing distinct functions.

• Nucleus

• Mitochondrion

• Chloroplast (in plant cells)

• Endoplasmic Reticulum (ER)

• Golgi Apparatus

• Vesicles

Compartmentalization allows for:

• Localized conditions necessary for specific metabolic processes

• Sequestration of reactions to prevent interference between incompatible processes

Nucleus and Genetic Material

The nucleus stores the cell's genetic library. Genetic material is organized as chromatin (DNA and histone proteins), which condenses into chromosomes during cell division.

• Nucleolus: Spherical region within the nucleus where ribosomes are assembled.

• Humans have 46 chromosomes.

Ribosomes

Ribosomes are complexes of rRNA and proteins responsible for protein synthesis. They are found in both prokaryotic and eukaryotic cells.

• Composed of two subunits

• In prokaryotes: free in cytoplasm

• In eukaryotes: free or bound to membranes (e.g., rough ER)

Energy-Transforming Organelles

Mitochondria

Mitochondria are organelles that extract energy from organic molecules to produce ATP, the cell's energy currency.

• Size: 1–10 μm

• Double membrane structure

• Inner membrane highly folded into cristae to increase surface area

• Space between membranes: intermembrane compartment

• Space inside inner membrane: matrix

• Contain their own DNA and ribosomes

Chloroplasts

Chloroplasts are organelles found in plant cells, containing chlorophyll and responsible for photosynthesis.

• Size: 2–5 μm

• Double membrane structure

• Internal fluid: stroma

• Third membrane system: thylakoids (flattened sacs)

• Stacks of thylakoids: grana

• Contain their own DNA and ribosomes

Endomembrane System

Components and Functions

The endomembrane system in eukaryotic cells consists of interconnected organelles involved in synthesis, transport, and degradation of cellular materials.

• Nuclear envelope

• Endoplasmic reticulum (ER: rough and smooth)

• Golgi apparatus

• Lysosomes

• Vacuoles

Components are connected directly (physical contact) or indirectly (via vesicles).

• Vesicles: Membrane-enclosed sacs that transport materials between organelles.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is an extensive network of tubules and sacs, and is the largest part of the cell's membrane system.

• Smooth ER: Lacks ribosomes; synthesizes lipids (phospholipids, steroids).

• Rough ER: Studded with ribosomes; manufactures membranes and synthesizes secretory proteins.

Golgi Apparatus

The Golgi apparatus consists of stacked, flattened membranous sacs that modify, store, and route products from the ER.

• Cis-face: Receiving side for transport vesicles from the ER.

• Trans-face: Shipping side for vesicles leaving the Golgi.

Pathway of a secretory protein:

1. Protein synthesized by a bound ribosome

2. Protein moves through the ER membrane into the ER

3. Vesicle pinched off from ER carries protein to Golgi apparatus

4. Vesicle carries protein to plasma membrane for export

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes for breaking down macromolecules.

• Intracellular digestion: Breakdown of food particles (e.g., food vacuoles formed by phagocytosis)

• Recycling: Degradation and reuse of cellular materials

• Programmed cell destruction: Apoptosis and removal of damaged cells

Summary Table: Major Cell Structures and Functions

Structure	Main Function	Present in
Plasma Membrane	Selective barrier, communication	All cells
Nucleus	Genetic information storage	Eukaryotes
Cytoskeleton	Structural support, movement	All cells
Mitochondria	ATP production	Eukaryotes
Chloroplasts	Photosynthesis	Plant cells
Ribosomes	Protein synthesis	All cells
Golgi Apparatus	Modification and transport of proteins	Eukaryotes
Lysosomes	Digestion and recycling	Eukaryotes

Key Equations

• ATP Production (Cellular Respiration):

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

• Photosynthesis (in 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: These notes expand on the original content by providing definitions, examples, and context for each cell structure and function, as well as summarizing key differences between prokaryotic and eukaryotic cells and including relevant equations for energy transformation processes.