Chapter 3: Cells – Structure, Function, and Processes

Cells: The Living Units

Introduction

Cells are the fundamental units of life, forming the basis of structure and function in all living organisms. Understanding cell structure, membrane dynamics, and cellular processes is essential for the study of anatomy and physiology.

Major Regions of a Generalized Cell

• Plasma Membrane: The outer boundary that regulates entry and exit of substances.

• Cytoplasm: The intracellular fluid containing organelles.

• Nucleus: The control center containing genetic material (DNA).

Plasma Membrane: Structure and Function

Phospholipid Bilayer

• Composition: Composed of a double layer of phospholipids with embedded proteins, cholesterol, and carbohydrates.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of the membrane, where proteins float in or on the fluid lipid bilayer.

• Functions: Selective permeability, cell signaling, protection, and cell recognition.

Membrane Transport

Passive Transport (Includes Diffusion and Osmosis)

• Passive Transport: Movement of substances across the membrane without energy input.

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

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

• Facilitated Diffusion: Transport of substances via carrier or channel proteins.

• Tonicity: The ability of a solution to change the shape of cells by altering their internal water volume.

Example: Red blood cells in a hypertonic solution shrink due to water loss.

Active Transport (Directly or Indirectly Uses ATP)

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

• Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

• Secondary Active Transport: Indirect use of ATP via ion gradients.

• Vesicular Transport: Includes endocytosis (into cell) and exocytosis (out of cell).

Equation: $\text{Na}^+/\text{K}^+$ ATPase: 3 Na+ out, 2 K+ in per ATP hydrolyzed.

Cellular Organelles

Membranous Organelles

• Mitochondria: Site of ATP production via aerobic respiration.

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

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

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Lysosomes: Contain digestive enzymes for breakdown of waste.

Non-Membranous Organelles

• Ribosomes: Sites of protein synthesis.

• Cytoskeleton: Provides structural support and facilitates movement.

• Centrioles: Involved in cell division and formation of cilia/flagella.

Extensions of the Cell

• Cilia: Move substances across cell surfaces.

• Flagella: Propel the cell (e.g., sperm cell).

• Microvilli: Increase surface area for absorption.

Nucleus and Associated Structures

• Nucleus: Contains DNA and controls cellular activities.

• Nucleolus: Site of ribosome assembly.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

Organelle Table

Organelle	Structure/Description	Function
Mitochondria	Double membrane, inner folds (cristae)	ATP production
Ribosomes	Small RNA-protein complexes	Protein synthesis
Golgi Apparatus	Stack of flattened membranes	Protein modification and packaging
Lysosomes	Membrane-bound vesicles with enzymes	Digestion of waste
Endoplasmic Reticulum	Network of membranes (rough/smooth)	Protein/lipid synthesis
Peroxisomes	Small vesicles with oxidases	Detoxification

Endomembrane System

• Includes: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, plasma membrane.

• Function: Synthesis, modification, and transport of proteins and lipids.

Cell Cycle and Mitosis

Phases of the Cell Cycle

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for mitosis).

• Mitotic Phase: Mitosis (division of nucleus) and cytokinesis (division of cytoplasm).

Stages of Mitosis

Stage	Description
Prophase	Chromatin condenses, spindle forms
Metaphase	Chromosomes align at cell equator
Anaphase	Sister chromatids separate
Telophase	Nuclear envelopes reform, chromosomes decondense

Cytokinesis

• Division of the cytoplasm, resulting in two daughter cells.

DNA Structure and Function

• DNA Replication: Process by which DNA makes a copy of itself during cell division.

• Base Pairing: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).

• Genetic Code: Sequence of nucleotides that determines protein synthesis.

Protein Synthesis: Central Dogma

• Transcription: DNA is transcribed into mRNA in the nucleus.

• Translation: mRNA is translated into protein at the ribosome.

• Key Players: DNA, mRNA, tRNA, rRNA.

Example: DNA sequence GCT-AGA-TCT is transcribed to mRNA as CGA-UCU-AGA.

Stages of Transcription and Translation

• Transcription: Initiation, elongation, termination.

• Translation: Initiation, elongation, termination.

Function of Genes

• Genes encode instructions for synthesizing proteins, which determine cell structure and function.

Additional info: This guide expands on the review questions by providing definitions, examples, and context for each topic, making it suitable for exam preparation and self-study.