The Cell: Structure, Function, and Processes (Chapter 3 Study Notes)

The Cell

Basic Processes of Cells

Cells are the fundamental units of life, carrying out essential processes to maintain homeostasis and support the organism. These processes include metabolism, transport, communication, and reproduction.

• Cell Metabolism: The sum of all chemical reactions in a cell, including anabolic (building), catabolic (breaking down), and oxidation-reduction reactions.

• Substance Transport: Movement of compounds into, out of, or within the cell.

• Communication: Cells interact with their environment and other cells via signaling molecules and receptors.

• Cell Reproduction: Most cells divide to produce new cells, essential for growth and repair.

Overview of Cell Structure

Most animal cells share three basic components: the plasma membrane, cytoplasm (including cytosol, organelles, and cytoskeleton), and the nucleus.

• Plasma Membrane: Separates the cell from its environment, provides structural support, mediates communication, regulates transport, and identifies the cell.

• Cytoplasm: Contains cytosol (intracellular fluid), organelles (specialized structures), and the cytoskeleton (protein filaments for support and movement).

• Nucleus: Surrounded by a double membrane (nuclear envelope), contains most of the cell’s DNA, and is the site of RNA production.

Cell Diversity: Cells vary greatly in size and shape, allowing for specialized functions (e.g., red blood cells, nerve cells, epithelial cells, skeletal muscle cells).

The Plasma Membrane

The Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which forms a selective barrier between the extracellular fluid (ECF) and the cytosol.

• Hydrophilic (polar) heads: Face the aqueous environments inside and outside the cell.

• Hydrophobic (nonpolar) tails: Face inward, away from water, creating a barrier to most water-soluble substances.

The Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which highlights its dynamic nature and the presence of various proteins, lipids, and carbohydrates.

• Integral (Transmembrane) Proteins: Span the membrane and are involved in transport and signaling.

• Peripheral Proteins: Attached to one side of the membrane, often involved in signaling or structural support.

• Cholesterol: Stabilizes membrane structure, especially with temperature changes.

• Glycolipids and Glycoproteins: Involved in cell recognition and signaling.

Membrane Proteins: Structure and Function

Membrane proteins are essential for the diverse functions of the plasma membrane, including transport, communication, and structural support.

Function	Structure
Channels: Membrane proteins act as channels through which substances pass to enter or exit the cell.	Protein channel spanning the membrane, allowing specific molecules to pass.
Carriers: Membrane proteins bind and transport substances into or out of the cell.	Carrier protein changes shape to move substances across the membrane.

Transport Across the Plasma Membrane

Passive Transport

Passive transport does not require energy and relies on concentration gradients to move substances across the membrane.

• Diffusion: Movement of solute molecules from higher to lower concentration.

• Simple Diffusion: Nonpolar molecules (e.g., O2, CO2, lipids) pass directly through the bilayer.

• Facilitated Diffusion: Polar or charged molecules (e.g., ions, glucose) cross via channel or carrier proteins.

• Osmosis: Movement of water across a selectively permeable membrane from lower to higher solute concentration, often through aquaporins.

Tonicity

Tonicity describes the ability of a solution to cause a cell to gain or lose water.

• Isotonic: Equal solute concentration; no net water movement.

• Hypertonic: Higher solute concentration outside; cell loses water and shrivels (crenates).

• Hypotonic: Lower solute concentration outside; cell gains water and may swell or lyse.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients, using carrier proteins called pumps.

• Primary Active Transport: Direct use of ATP to transport substances (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses the energy from a concentration gradient established by primary active transport to move another substance.

Electrophysiology

Electrophysiology studies the membrane potential, the electrical potential difference across the plasma membrane due to the distribution of ions.

Cytoplasmic Organelles

Overview of Organelles

Organelles are specialized structures within the cytoplasm that perform distinct functions necessary for cell survival.

Mitochondria

Mitochondria are the cell’s powerhouses, producing most of the ATP via oxidative catabolism. They have a double membrane, with the inner membrane folded into cristae.

Ribosomes

Ribosomes are the sites of protein synthesis, composed of ribosomal RNA and proteins. They can be free in the cytosol or bound to the endoplasmic reticulum.

Endomembrane System

The endomembrane system includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and related vesicles, which work together to synthesize, modify, and transport cellular products.

• Rough ER (RER): Studded with ribosomes; synthesizes and folds proteins.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids, detoxifies substances, and stores calcium ions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules, old organelles, and foreign substances.

The Cytoskeleton

Types of Cytoskeletal Filaments

The cytoskeleton provides structural support, maintains cell shape, and enables movement. It consists of three main types of protein filaments:

• Actin Filaments (Microfilaments): Thin filaments involved in cell movement and shape maintenance.

• Intermediate Filaments: Provide mechanical strength and support for the cell and nucleus.

• Microtubules: Hollow tubes that maintain cell structure, facilitate organelle movement, and form the core of cilia and flagella.

The Nucleus

Structure and Function

The nucleus directs cellular activities and houses DNA. It is surrounded by a double membrane (nuclear envelope) with nuclear pores for transport. The nucleolus within the nucleus assembles ribosomes.

Protein Synthesis

Gene Expression

Protein synthesis involves two main steps: transcription (DNA to mRNA) and translation (mRNA to protein). Genes are segments of DNA that code for proteins, with exons as coding regions and introns as noncoding regions.

• Transcription: RNA polymerase synthesizes mRNA from a DNA template.

• Translation: Ribosomes read mRNA codons and assemble amino acids into a polypeptide chain.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle consists of interphase (G1, S, G2 phases) and the mitotic phase (mitosis and cytokinesis). Mitosis divides the genetic material, while cytokinesis divides the cytoplasm, resulting in two identical daughter cells.

Summary Table: Plasma Membrane Transport

Type of Transport	Definition	Example(s)
Simple Diffusion	Movement of solute with its concentration gradient through the plasma membrane unaided by a transport protein.	Oxygen, carbon dioxide, lipids
Facilitated Diffusion	Movement of solute with its concentration gradient with the help of a carrier or channel protein.	Sodium ions, potassium ions, glucose
Osmosis	Movement of water from lower to higher solute concentration through a selectively permeable membrane.	Water absorption in intestines, kidneys
Primary Active Transport	Movement of solute against its concentration gradient using ATP.	Na+/K+ ATPase pump
Secondary Active Transport	Uses the energy from a concentration gradient to power transport of another substance.	Glucose, chloride ions