Cell Structure and Function: Study Notes for General Biology

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. They carry out essential processes that sustain life, including metabolism, growth, and reproduction.

• Cell Theory: All living things are composed of cells; the cell is the basic unit of life; all cells arise from pre-existing cells.

• Types of Cells: There are two main types: prokaryotic and eukaryotic cells.

• Microscopy: The development of microscopes enabled the discovery and study of cells.

• Example: Bacteria are prokaryotic cells, while plants and animals are made of eukaryotic cells.

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on the presence or absence of a nucleus and membrane-bound organelles.

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. DNA is located in a region called the nucleoid.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess various membrane-bound organelles.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Cell Organelles and Their Functions

Eukaryotic cells contain specialized structures called organelles, each with distinct functions necessary for cell survival.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Mitochondria: Site of cellular respiration; produces ATP, the cell's energy currency.

• Ribosomes: Synthesize proteins; found free in cytoplasm or attached to the endoplasmic reticulum.

• Endoplasmic Reticulum (ER): Rough ER has ribosomes and synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.

• Chloroplasts: (In plant cells) Site of photosynthesis, converting solar energy into chemical energy.

• Cell Membrane: Semi-permeable barrier that controls the movement of substances in and out of the cell.

• Cell Wall: (In plants, fungi, and some prokaryotes) Provides structural support and protection.

Plasma Membrane Structure and Function

The plasma membrane is a dynamic structure that regulates the internal environment of the cell.

• Phospholipid Bilayer: Composed of two layers of phospholipids with hydrophilic heads and hydrophobic tails.

• Proteins: Embedded in the membrane; function as channels, carriers, receptors, and enzymes.

• Fluid Mosaic Model: Describes the membrane as a flexible, dynamic structure with proteins floating in or on the fluid lipid bilayer.

• Selective Permeability: Allows certain molecules to pass while restricting others.

Transport Across Cell Membranes

Cells must exchange materials with their environment to maintain homeostasis. Transport can be passive or active.

• Passive Transport: Movement of substances down their concentration gradient without energy input. Includes diffusion, osmosis, and facilitated diffusion.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Example: Sodium-potassium pump is an example of active transport in animal cells.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	Oxygen, Carbon Dioxide
Facilitated Diffusion	No	High to Low	Glucose via carrier proteins
Osmosis	No	High to Low (water)	Water movement
Active Transport	Yes (ATP)	Low to High	Sodium-potassium pump

Additional info:

• Some details, such as the full list of organelles and the summary tables, were inferred and expanded for completeness and clarity.

• Definitions and examples were added to ensure the notes are self-contained and suitable for exam preparation.