Cells: Structure, Function, and Membrane Transport (Chapter 3 Study Notes)

Cells

Introduction to Cells

Cells are the fundamental structural and functional units of all living organisms. The study of cells is known as cytology. All living things, including animals, plants, fungi, and microorganisms, are composed of cells. Cells are considered the smallest unit of life capable of carrying out all vital biological processes.

• Definition: A cell is the basic unit of structure and function in living organisms.

• Universality: Cells are found everywhere in living organisms.

• Types: There are many different types of cells in the human body, each specialized for specific functions.

• Examples: Neurons (nerve cells), muscle cells, epithelial cells, blood cells, sperm cells, etc.

Importance of Cells

The human body is made up of approximately trillions of cells. Each cell type has a unique structure and function, contributing to the overall physiology of the organism.

• Diversity: There are over 200 different types of cells in the human body.

• Specialization: Each cell type is specialized to perform specific tasks, such as transmitting signals, contracting, or producing substances.

Cell Functions

General Functions of Cells

Cells perform a variety of essential functions necessary for life. The specific function depends on the cell type.

• Transmit nerve impulses: Neurons are specialized for communication via electrical and chemical signals.

• Transport substances: Blood cells transport oxygen, nutrients, and waste products.

• Provide support: Bone and cartilage cells provide structural support to the body.

• Metabolism: All cells perform metabolism to produce ATP, the energy currency of the cell.

• Synthesize molecules: Cells produce proteins, lipids, carbohydrates, and nucleic acids.

• Store genetic material: DNA and RNA are stored and processed within the cell nucleus.

Cell Membrane (Plasma Membrane)

Structure and Function

The cell membrane (also called the plasma membrane) is a critical component of all human cells. It separates the internal environment of the cell from the external environment and regulates the movement of substances in and out of the cell.

• Extracellular (EC) material: Material outside the cell.

• Intracellular (IC) material: Material inside the cell.

• Functions:

  • Acts as a selective barrier.

  • Facilitates communication and interaction with other cells.

  • Maintains homeostasis by controlling the passage of substances.

Composition of the Cell Membrane

The cell membrane is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid bilayer: Consists of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, creating a semi-permeable barrier.

• Membrane proteins: Serve various functions such as transport, signaling, and structural support.

• Cholesterol: Stabilizes membrane fluidity.

• Carbohydrates: Involved in cell recognition and signaling.

Types of Membrane Proteins

Membrane proteins are essential for the diverse functions of the cell membrane.

• Channel proteins: Form pores that allow specific ions or molecules to pass through the membrane.

• Carrier proteins: Bind to substances and change shape to shuttle them across the membrane.

• Receptor proteins: Bind to chemical messengers (e.g., hormones) and initiate cellular responses.

• Enzymatic proteins: Catalyze chemical reactions at the membrane surface.

• Cell recognition proteins: Allow cells to identify each other (important for immune response).

Membrane Transport

Overview

The cell membrane is selectively permeable, meaning only certain molecules can cross it. Transport across the membrane can be passive (not requiring energy) or active (requiring energy).

Passive Transport

• Simple diffusion: Movement of molecules from an area of high concentration to low concentration without energy input. Examples: oxygen, carbon dioxide.

• Facilitated diffusion: Movement of molecules down their concentration gradient via membrane proteins (channels or carriers). No energy required.

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

Key concept: In passive transport, substances move down their concentration gradient.

Active Transport

• Primary active transport: Uses ATP to move substances against their concentration gradient. Example: sodium-potassium pump.

• Secondary active transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient.

Key concept: In active transport, substances move against their concentration gradient, requiring energy (usually ATP).

Bulk Transport

• Endocytosis: Process by which cells engulf large particles or fluids. Types include:

  • Phagocytosis: "Cell eating"; ingestion of large particles.

  • Pinocytosis: "Cell drinking"; ingestion of fluids and dissolved substances.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

• Exocytosis: Process by which cells expel materials in vesicles that fuse with the plasma membrane.

Osmosis and Tonicity

Osmosis is the movement of water across a membrane. Tonicity describes the ability of a solution to affect cell volume:

Solution Type	Effect on Cell
Isotonic	No net movement of water; cell remains the same size.
Hypertonic	Water moves out; cell shrinks (crenation).
Hypotonic	Water moves in; cell swells and may burst (lysis).

Cellular Organelles

Major Organelles and Their Functions

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

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

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

• Mitochondria: Site of ATP (energy) production; known as the "powerhouse of the cell." $\text{Glucose} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{ATP}$

• Cytoplasm: Jelly-like fluid that holds organelles in place.

• Secretory vesicles: Membrane-bound sacs that transport substances within or out of the cell.

• Flagella: Long, whip-like structures for cell movement (e.g., sperm cells).

• Microvilli: Small projections that increase surface area for absorption (e.g., in the small intestine).

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	Oxygen, CO2
Facilitated Diffusion	No	High to Low	Glucose, ions
Osmosis	No	High to Low (water)	Water
Active Transport	Yes (ATP)	Low to High	Sodium-potassium pump
Endocytosis/Exocytosis	Yes (ATP)	Bulk transport	Phagocytosis, secretion

Additional info: Some details, such as the exact number of cells in the human body (estimated at 37 trillion), and the full list of organelles, have been expanded for academic completeness.