Cell Structure, Function, and Tissue Types: Study Notes for Anatomy & Physiology

Cell Structure and Function

Parts of a Cell

The cell is the basic unit of life, composed of various structures that perform specialized functions. Understanding the main components of a cell is essential for studying anatomy and physiology.

• Plasma Membrane: The outer covering of the cell, acting as a selective barrier that regulates the movement of substances in and out of the cell.

• Cytoplasm: Gel-like interior containing the nucleus, organelles, and chemicals necessary for cellular processes.

• Nucleus: The control center of the cell, containing DNA and surrounded by a double membrane. It directs cellular activities and houses genetic material.

• Organelles: Specialized structures within the cytoplasm that carry out distinct functions (see organelle table below).

• Cytoskeleton: A network of protein filaments (microfilaments, microtubules) that provides structural support and facilitates movement.

• Plasma Membrane Extensions: Structures such as cilia, flagella, and villi that aid in movement or increase surface area.

Plasma Membrane Structure

The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. This structure allows selective permeability and communication with the environment.

• Fluid Mosaic Model: Describes the membrane as a dynamic arrangement of phospholipids and proteins.

• Phospholipid Bilayer: Hydrophilic (water-loving) heads face outward, while hydrophobic (water-fearing) tails are sandwiched in the middle.

• Cholesterol: Stabilizes membrane fluidity.

• Proteins: Serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins or lipids, involved in cell recognition.

Plasma Membrane Transport Functions

The plasma membrane controls the movement of substances into and out of the cell, maintaining homeostasis.

• Passive Transport: Movement of materials from high to low concentration without energy input. Includes diffusion, osmosis, and facilitated diffusion.

  • Diffusion: Movement of molecules down a concentration gradient.

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

  • Facilitated Diffusion: Uses protein channels or carriers for transport.

• Active Transport: Movement of materials from low to high concentration, requiring ATP. Includes pumps (e.g., Na+/K+ pump) and bulk transport (endocytosis, exocytosis).

  • Endocytosis: Uptake of materials via vesicles.

  • Exocytosis: Release of materials to the outside.

Other Plasma Membrane Functions

Beyond transport, the plasma membrane plays roles in cell communication, adhesion, and protection.

• Cell Junctions: Structures such as tight junctions, desmosomes, and gap junctions connect neighboring cells and regulate permeability.

• Barrier Functions: Examples include the blood-brain barrier.

Cell Organelles

Major Organelles and Their Functions

Organelles are specialized structures within the cell that perform distinct tasks necessary for cell survival and function.

Name	Description	Function
Mitochondrion	Double membrane; inner membrane forms cristae	Powerhouse of the cell; produces ATP via cellular respiration
Ribosome	Made of two subunits	Site of protein synthesis
Rough Endoplasmic Reticulum (RER)	System of canals with ribosomes attached	Transports and modifies proteins
Smooth Endoplasmic Reticulum (SER)	No ribosomes attached	Lipid synthesis, detoxification, calcium storage
Golgi Apparatus	Stack of flattened sacs	Modifies, packages, and sorts proteins
Lysosome	Membrane-bound vesicle with digestive enzymes	Breakdown and recycling of cellular waste
Peroxisome	Contains peroxidase enzymes	Detoxifies harmful substances
Centrioles	Cylindrical structures, visible during cell division	Organize spindle fibers in mitosis
Cilia	Hair-like projections	Move materials across cell surfaces
Flagella	Whip-like tail	Propels sperm cells
Villi	Fold-like projections	Increase surface area for absorption

Nucleus and Genetic Material

Nucleus Structure and Function

The nucleus is the control center of the cell, housing genetic material and coordinating cellular activities.

• Nuclear Envelope: Double membrane that surrounds the nucleus.

• Chromatin: Loosely organized DNA and protein; condenses into chromosomes during cell division.

• Chromosome: Rod-shaped structure of condensed DNA; humans have 46 chromosomes (23 pairs).

• Nucleolus: Site of ribosome synthesis.

• Inclusions: Droplets of fat, stored sugar (glycogen), pigment.

Cell Cycle and Division

Life Cycle of a Cell: Interphase and Mitosis

The cell cycle includes periods of growth (interphase) and division (mitosis), ensuring proper replication and distribution of genetic material.

• Interphase: Cell grows, performs normal functions, and replicates DNA.

• DNA Replication: DNA duplicates to ensure each new cell receives a complete set of genetic material.

  • DNA double helix unwinds; complementary base pairing occurs: A-T, G-C.

  • Enzymes synthesize new strands, resulting in two identical DNA molecules.

• Mitosis: Division of the nucleus into two genetically identical daughter cells. Four phases:

  1. Prophase: Chromatin condenses, spindle fibers form.

  2. Metaphase: Chromosomes align at the cell's equator.

  3. Anaphase: Sister chromatids separate and move to opposite poles.

  4. Telophase: Nuclear envelopes reform, chromosomes decondense.

Meiosis

Meiosis is a specialized cell division process that produces gametes (egg and sperm) with half the normal amount of DNA. It is essential for sexual reproduction.

• Reduces chromosome number by half.

• Ensures genetic diversity in offspring.

• Relevant in reproductive system studies.

Genetic Information and Protein Synthesis

Gene

A gene is a segment of DNA that contains instructions for building a specific polypeptide or protein.

Transcription

Transcription is the process by which information in DNA is copied into messenger RNA (mRNA) in the nucleus.

• DNA unwinds; RNA polymerase synthesizes mRNA using DNA as a template.

• Each group of three bases (codon) in mRNA codes for a specific amino acid.

• Example: If DNA strand is TAC GGC GAC TGC TGC GGC ATT, mRNA after transcription would be AUG CCG CUG ACG ACG CCG UAA.

Translation

Translation is the process by which ribosomes use mRNA to assemble a polypeptide chain (protein).

• Ribosomes move along mRNA, reading codons.

• Transfer RNA (tRNA) brings amino acids to the ribosome.

• Peptide bonds form between amino acids, creating a polypeptide chain.

Cell Diversity

Variation in Cell Structure and Function

Cells vary widely in structure and function, reflecting their specialized roles in the body.

• Different cell types have unique shapes, sizes, and organelles.

• Examples: Red blood cells (biconcave discs), neurons (long extensions), macrophages (phagocytic cells).

• Some cells have cilia or flagella for movement; muscle cells contain filaments for contraction.

Tissue Types

The Four Main Tissue Types

Tissues are groups of similar cells that perform specific functions. The four basic tissue types are:

• Epithelial Tissue: Forms sheets or layers; lines, covers, or forms glands.

• Connective Tissue: Supports, connects, and protects; includes bone, blood, fat, cartilage.

• Muscle Tissue: Responsible for movement; includes skeletal, cardiac, and smooth muscle.

• Nervous Tissue: Receives stimuli and conducts electrical impulses; includes neurons and supporting cells.

Naming Rules for Epithelial Tissue

Epithelial tissues are named according to the number of layers and cell shape.

• Simple: Single layer of cells.

• Stratified: Multiple layers of cells.

• Cell Shapes: Squamous (flat), cuboidal (cube-shaped), columnar (tall and narrow).

Specific Tissue Types (BIO 163 Lab)

Examples of specific tissues studied in lab:

• Epithelial Tissues:

  • Simple squamous epithelium

  • Simple cuboidal epithelium

  • Simple columnar epithelium

  • Stratified squamous epithelium

  • Pseudostratified ciliated columnar epithelium

  • Transitional epithelium

• Connective Tissues:

  • Areolar (loose packaging for organs)

  • Adipose (fat storage)

  • Reticular (lymphatic system)

  • Bone (support, protection)

  • Hyaline cartilage (airway, ribs)

  • Blood (transport, immune function)

• Muscle Tissues:

  • Skeletal muscle

  • Cardiac muscle

  • Smooth muscle

• Nervous Tissue:

  • Neurons (main cells for signal transmission)

Additional info:

• Equations for cellular respiration in mitochondria: $\text{Food} + O_2 \rightarrow \text{ATP} + \text{Heat} + CO_2 + H_2O$

• DNA base pairing: $A \leftrightarrow T$, $G \leftrightarrow C$

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$