Cell Structure and Function: General Biology Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Cell Structure and Function

Introduction to Cells

Cells are the fundamental unit of life, forming the basis of all living organisms. They can exist as single-celled entities or as part of complex multicellular organisms.

Single-celled organisms: Examples include Bacteria and Protists.
Multicellular organisms: Composed of many cells (e.g., humans have 60–100 trillion cells and over 200 different cell types).

Cell Theory

The cell theory is a foundational concept in biology, describing the properties and functions of cells.

Definition: The cell is the smallest unit of matter that can be considered alive.
Key contributors:
- Matthias Schleiden (Botanist) and Theodor Schwann (Zoologist): All living material is made up of cells or the products of cells (1839).
- Rudolf Virchow (1821–1902): Cells arise from the division of other cells.
Cell sizes:
- Most cells are small (micrometers in diameter).
- Examples: Bacteria (1–10 μm), Eukaryotic cells (10–100 μm).

Classification of Cells

Cell Classes

Cells are classified into two major types based on their internal structure:

Prokaryotic cells
Eukaryotic cells

Prokaryotic Cells

Prokaryotic cells are simpler and lack a membrane-bound nucleus. They are found in organisms of the kingdom Monera (bacteria and archaea).

Origin of term: Greek 'pro' (before) + 'karyon' (nucleus).
Characteristics:
- No true nucleus; genetic material is in a nucleoid region.
- Lack most organelles found in eukaryotes.
- Cell wall present in most species.
- Single, circular DNA molecule.
Examples: Escherichia coli, Streptococcus

Eukaryotic Cells

Eukaryotic cells are more complex and possess a true, membrane-bound nucleus. They are found in animals, plants, fungi, and protists.

Origin of term: Greek 'eu' (true) + 'karyon' (nucleus).
Characteristics:
- Distinct membrane-enclosed nucleus.
- Variety of organelles (e.g., mitochondria, endoplasmic reticulum).
- Specialized internal compartments for chemical reactions.

Cell Structure and Organelles

Plasma Membrane

The plasma membrane defines the cell boundary and regulates the movement of substances in and out of the cell.

Structure: Phospholipid bilayer with embedded proteins.
Functions:
- Cell boundary
- Cell compartments

Cytoplasm

The cytoplasm includes all contents of the cell between the nucleus and the plasma membrane.

Contains organelles and the cell matrix.

Nucleus

The nucleus is the control center of the cell, storing hereditary information and directing protein synthesis.

Functions:
- Stores DNA
- Controls cell activities

Chromatin and Chromosomes

Chromatin: Diffuse mass of DNA within the nucleus.
Chromosomes: Condensed, rod-shaped DNA structures formed during cell division.
- After replication, chromatin condenses into chromosomes.
- Each chromosome consists of two sister chromatids joined at a centromere.

Nucleolus

The nucleolus is a specialized region within the nucleus responsible for producing ribosomal RNA (rRNA).

Two or more nucleoli may be present in a nucleus.

Nuclear Envelope

The nuclear envelope is a double membrane that surrounds the nucleus and contains nuclear pores for transport.

Regulates movement of molecules between nucleus and cytoplasm.

Ribosomes

Ribosomes are the sites of protein synthesis in the cell.

Types:
- Free ribosomes (in cytoplasm)
- Bound ribosomes (attached to endoplasmic reticulum)

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubules within the cytoplasm, involved in synthesis and transport.

Types:
- Rough ER (RER): Studded with ribosomes; involved in chemical modification of proteins.
- Smooth ER (SER): Lacks ribosomes; involved in synthesis and modification of lipids and carbohydrates.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins for shipment within or outside the cell.

Acts as the cell's 'address label' system for proteins.

Lysosomes

Lysosomes are membrane-bound organelles containing hydrolytic enzymes for digestion.

Formed by the Golgi apparatus.
Digest macromolecules and combine with food vacuoles for digestion.

Endomembrane System

The endomembrane system is a collection of membranous organelles that manufacture and export cell products.

Proteins: Ribosomes → RER → transport vesicle → Golgi apparatus → secretory vesicle → cell membrane
Carbohydrates and Lipids: SER → transport vesicle → Golgi apparatus → secretory vesicle → cell membrane

Cytoskeleton

The cytoskeleton maintains cell shape, anchors organelles, and facilitates movement.

Composed of microtubules, microfilaments, and intermediate filaments.

Cell Motility Structures

Cells may possess structures for movement, such as flagella and cilia.

Flagella: Long, whip-like structures for locomotion.
Cilia: Short, hair-like structures for movement or fluid transport.

Mitochondria

Mitochondria are the sites of cellular respiration, converting chemical energy in food to ATP for cellular activities.

Double membrane organelle.
Present in most eukaryotic cells.
Equation for cellular respiration:

Chloroplasts

Chloroplasts are organelles found in plants and photosynthesizing protists, responsible for photosynthesis.

Double membrane organelle.
Convert solar energy to chemical energy in sugars.
Equation for photosynthesis:

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No true nucleus	Membrane-bound nucleus
Organelles	Few, non-membranous	Many, membrane-bound
DNA	Single, circular DNA	Multiple, linear chromosomes
Cell Size	1–10 μm	10–100 μm
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Summary

Cells are the basic units of life, classified as prokaryotic or eukaryotic based on their structure. Eukaryotic cells possess a variety of organelles that perform specialized functions, while prokaryotic cells are simpler. Understanding cell structure and function is essential for studying all aspects of biology.