Ch3 The Cell: Structure, Function, and Processes – Study Notes for Anatomy & Physiology

The Cell: Structure, Function, and Processes

Basic Processes of Cells

Cells are the fundamental units of life, carrying out essential processes to maintain homeostasis and support the organism.

• 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 mechanisms.

• 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, and nucleus.

• Plasma Membrane: The outer boundary that separates the cell from its environment.

• Cytoplasm: The region between the plasma membrane and nucleus, containing cytosol, organelles, and the cytoskeleton.

• Nucleus: The control center housing DNA and the site of RNA production.

Functions of the Plasma Membrane

The plasma membrane is a dynamic structure that isolates the cell, provides support, communicates, regulates transport, and identifies the cell.

• Intracellular Space: Contains intracellular fluid (cytosol).

• Extracellular Space: Contains extracellular fluid (ECF).

Components of the Cytoplasm

• Cytosol: Watery gel with proteins, solutes, and RNA; site of many cellular processes.

• Organelles: Specialized structures performing specific functions.

• Cytoskeleton: Protein filaments providing support, shape, and transport within the cell.

Nucleus

The nucleus is surrounded by a double membrane (nuclear envelope), contains most of the cell’s DNA, and is the site for RNA production. DNA and RNA direct cellular functions by coding for proteins.

Cell Size and Diversity

Cells vary greatly in size and shape, allowing for specialized functions. Examples include red blood cells, nerve cells, epithelial cells, and skeletal muscle cells.

The Plasma Membrane: Structure and Function

The Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, forming a barrier between the ECF and cytosol.

• Hydrophilic (polar) heads: Face water on both sides of the membrane.

• Hydrophobic (nonpolar) tails: Face inward, away from water.

The Fluid Mosaic Model

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

• Fluidity: Phospholipids and proteins move laterally, allowing flexibility and function.

• Integral Proteins: Span the membrane; some are transmembrane proteins.

• Peripheral Proteins: Located on one side of the membrane.

Functions of Membrane Proteins

• Channels: Allow passage of substances.

• Carriers: Transport substances across the membrane.

• Receptors: Bind ligands to trigger cellular changes.

• Enzymes: Catalyze reactions.

• Structural Support: Maintain cell shape.

• Linkers: Connect cells together.

Other Membrane Components

• Cholesterol: Stabilizes membrane structure.

• Glycolipids and Glycoproteins: Involved in cell recognition.

Transport Across the Plasma Membrane

Passive Transport

Passive transport does not require energy and includes diffusion and osmosis.

• Diffusion: Movement of solute from high to low concentration, driven by a concentration gradient.

• Simple Diffusion: Nonpolar solutes and gases pass directly through the bilayer.

• Facilitated Diffusion: Polar or charged solutes cross via channel or carrier proteins.

• Osmosis: Movement of water across a selectively permeable membrane from low to high solute concentration.

• Tonicity: Describes the ability of a solution to cause osmosis.

• Isotonic: No net water movement.

• Hypertonic: Cell loses water and shrivels (crenates).

• Hypotonic: Cell gains water, swells, and may lyse.

Active Transport

Active transport requires energy (ATP) to move substances against their concentration gradients.

• Primary Active Transport: Direct use of ATP, e.g., sodium-potassium pump ($3 \text{Na}^+$ out, $2 \text{K}^+$ in).

• Secondary Active Transport: Uses the gradient created by primary active transport to move another substance.

Vesicular Transport

Large particles are transported via vesicles in processes requiring ATP.

• Endocytosis: Bringing substances into the cell (phagocytosis for particles, pinocytosis for fluids).

• Exocytosis: Releasing substances from the cell.

• Transcytosis: Substances move into, across, and out of the cell.

Summary Table: Plasma Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, Ions
Osmosis	No	Water: Low to High Solute	Water
Primary Active Transport	Yes (ATP)	Low to High	Na+/K+ Pump
Secondary Active Transport	Indirect (ATP)	Low to High (coupled)	Glucose/Na+ Cotransport
Vesicular Transport	Yes (ATP)	Varies	Phagocytosis, Exocytosis

Cytoplasmic Organelles

Membrane-Bound Organelles

• Mitochondria: Site of ATP production; contains its own DNA and ribosomes; inner membrane forms cristae.

• Peroxisomes: Use oxygen to detoxify substances, break down fatty acids, and synthesize certain phospholipids.

• Endoplasmic Reticulum (ER):

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

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

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

• Lysosomes: Contain digestive enzymes for breaking down macromolecules and worn-out organelles.

Non-Membrane-Bound Organelles

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins; can be free or bound to ER.

The Cytoskeleton

Types of Cytoskeletal Filaments

• Actin Filaments (Microfilaments): Support cell shape, involved in movement, and muscle contraction.

• Intermediate Filaments: Provide mechanical strength and structural support.

• Microtubules: Hollow tubes that maintain cell shape, move organelles, and form cilia and flagella.

Cellular Extensions

• Microvilli: Increase surface area for absorption; supported by actin filaments.

• Cilia and Flagella: Motile structures for movement; composed of microtubules.

The Nucleus

Structure and Function

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Chromatin: DNA wrapped around histones; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosome assembly.

Protein Synthesis

Gene Expression

• Transcription: DNA code is copied into mRNA in the nucleus.

• Translation: Ribosomes read mRNA and synthesize polypeptides in the cytoplasm.

Genetic Code

• DNA triplets are transcribed into mRNA codons, which are translated into amino acids.

• Mutations in DNA can lead to diseases, including cancer.

The Cell Cycle

Phases of the Cell Cycle

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• M Phase: Mitosis (division of genetic material) and cytokinesis (division of cytoplasm).

Mitosis Stages

• Prophase: Chromatin condenses, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate.

• Telophase and Cytokinesis: Nuclear envelope reforms, cell splits.

Cell Cycle Control and Cancer

• Checkpoints regulate cell division; failure can lead to uncontrolled growth (tumors).

• Benign Tumor: Non-invasive growth.

• Malignant Tumor (Cancer): Invades tissues and can metastasize.

Additional info: This summary covers the structure and function of the cell, plasma membrane, transport mechanisms, organelles, cytoskeleton, nucleus, protein synthesis, and the cell cycle, as outlined in a typical Anatomy & Physiology curriculum.