Cell Structure and Function: General Biology Study Notes

Cell Structure and Function

Introduction

Cells are the fundamental units of life, possessing an internal environment distinct from their surroundings. Understanding cell structure and function is essential for grasping the basics of biology, as all living organisms are composed of cells.

Basic Cell Components

Plasma Membrane

The plasma membrane is a selectively permeable barrier composed primarily of phospholipids and proteins. It separates the cell's internal environment from the external environment and regulates the movement of substances in and out of the cell.

• Functions of the plasma membrane:

  • 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 region inside the plasma membrane and outside the nucleus (in eukaryotic cells). It contains the cytosol (the fluid portion) and all cellular structures.

• Cytosol: The liquid matrix of the cytoplasm.

Cytoskeleton

The cytoskeleton is a network of protein fibers and filaments in the cytoplasm that provides structural support, maintains cell shape, and facilitates movement.

• Composed of microfilaments, intermediate filaments, and microtubules.

• Involved in intracellular transport and cell division.

Genetic Material

All cells contain genetic material in the form of deoxyribonucleic acid (DNA), which carries hereditary instructions.

Prokaryotic versus Eukaryotic Cells

Comparison Table

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

Prokaryotic Cell Features

• Nucleoid: Dense region of DNA in prokaryotic cells.

• Plasmids: Small, circular DNA molecules found in the cytoplasm, separate from chromosomal DNA.

• Cell Wall: Rigid structure that provides shape and protection; permeable to small molecules.

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

• Flagella: Long, whip-like structures used for cell movement in liquid environments.

Eukaryotic Cell Features

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

• Nuclear Envelope: Double membrane enclosing the nucleus.

• Organelles: Specialized compartments within the cytoplasm, each with specific functions.

Cell Organelles and Their Functions

Major Organelles

• Nucleus

• Mitochondrion

• Chloroplast (in plants and algae)

• Endoplasmic Reticulum (ER)

• Golgi Apparatus

• Vesicles

Compartmentalization in Eukaryotic Cells

Compartmentalization allows for specialized environments and processes within the cell, preventing interference between incompatible metabolic reactions.

Nucleus

• Contains the cell's genetic library.

• Genetic material is organized as chromatin (DNA + histone proteins).

• Chromosomes: Long threadlike associations of genes, composed of chromatin.

• Nucleolus: Spherical region within the nucleus where ribosomes are assembled; may be two or more per cell.

Ribosomes

• Sites of protein synthesis.

• Present in both prokaryotic and eukaryotic cells.

• Composed of ribosomal RNA (rRNA) and proteins.

• Consist of two subunits.

• In prokaryotes, ribosomes are free in the cytoplasm; in eukaryotes, they may be free or attached to membranes.

Mitochondria

Mitochondria are organelles that extract energy from organic molecules to p

\\roduce ATP, the cell's energy currency.

• Size: 1-10 μm

• Number per cell varies with metabolic activity

• Can move and change shape

• 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

• Semi-autonomous: can grow and reproduce

Chloroplasts

Chloroplasts are organelles found in plants and algae that contain chlorophyll and are the sites of photosynthesis.

• Size: 2.0-5.0 μm

• Number per cell varies with metabolic activity

• Can move and change shape

• Double membrane structure

• Space inside inner membrane: stroma (viscous fluid)

• Third membrane system: thylakoids (flattened sacs)

• Grana: Stacks of thylakoids

Endomembrane System

Components

• Nuclear envelope

• Endoplasmic reticulum (rough and smooth types)

• Golgi apparatus

• Lysosomes

• Vacuoles

Components are connected directly by physical contact or indirectly via vesicles.

Vesicles

• Membrane-enclosed sacs that transport materials between organelles.

• Formed by pinching off portions of membranes.

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. It is found in two forms:

• Smooth ER: Lacks bound ribosomes; synthesizes lipids (especially phospholipids and steroids).

• Rough ER: Has bound ribosomes; manufactures membranes and synthesizes secretory proteins.

Pathway of a Secretory Protein

1. Protein synthesized by a bound ribosome

2. Newly synthesized protein moves through the ER membrane into the ER

3. Vesicle carrying the protein is pinched off the ER and travels to the Golgi apparatus

4. Golgi apparatus finishes, sorts, and ships the protein to the plasma membrane for export

Golgi Apparatus

• Stacked, flattened membranous sacs

• Modifies, stores, sorts, and routes products of the ER

• Has two faces:

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

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

Lysosomes

Lysosomes are membrane-bound organelles that digest all major classes of macromolecules.

• Intracellular digestion

  • Food vacuoles are formed by phagocytosis of food particles

• Recycling of cellular material

• Programmed cell destruction

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Phagocytosis: The process by which a cell engulfs particles to form an internal compartment known as a vacuole.

• Histone: Protein that helps package DNA into chromatin in eukaryotic cells.

Summary Table: Major Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
DNA Location	Nucleoid region	Nucleus
Organelles	Absent	Present (membrane-bound)
Cell Wall	Present (most)	Present (plants, fungi)
Ribosomes	Smaller (70S)	Larger (80S)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Important 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):

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

Conclusion

Understanding the structure and function of cells, including the differences between prokaryotic and eukaryotic cells and the roles of various organelles, is foundational for further study in biology. Mastery of these concepts is essential for exploring more advanced topics in cellular and molecular biology.