Cell Structure, Types, and Functions: Foundations of Anatomy & Physiology

Cell Cycle

Overview of the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It consists of interphase, where the cell prepares for division, and the mitotic (M) phase, where division occurs.

• Interphase (~23 hours): The cell grows, replicates its DNA, and prepares for division.

• G1 phase: Cell growth and preparation for DNA replication.

• S phase: DNA replication occurs.

• G2 phase: Further growth and preparation for division.

• Mitotic (M) phase (~1 hour): Division of the nucleus (mitosis) and cytoplasm (cytokinesis) to produce two daughter cells.

Stages of Mitosis:

1. Prophase: Chromosomes condense, spindle fibers form.

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

3. Anaphase: Sister chromatids separate to opposite poles.

4. Telophase: Nuclear envelopes reform around chromosomes.

5. Cytokinesis: Cytoplasm divides, forming two daughter cells.

Example: Skin cells undergo the cell cycle to replace dead or damaged cells. Additional info: The cell cycle is tightly regulated by checkpoints to prevent uncontrolled cell division (cancer).

Types of Cells

Major Cell Types in the Human Body

Cells in the human body are classified based on their structure and function. Each type plays a specific role in maintaining homeostasis.

• Somatic cells: All body cells except reproductive cells; e.g., muscle, nerve, skin cells.

• Gametes: Reproductive cells (ovum and sperm) responsible for sexual reproduction.

• Blood Cells:

  • Erythrocytes (red blood cells): Transport oxygen.

  • Leukocytes (white blood cells): Immune defense.

  • Thrombocytes (platelets): Blood clotting.

• Stem Cells: Undifferentiated cells capable of giving rise to various cell types.

Example: Hematopoietic stem cells in bone marrow produce all types of blood cells.

Cell Shapes and Specialization

Variation in Cell Morphology

Cells exhibit diverse shapes that reflect their specialized functions within tissues and organs.

• Irregular-shaped: Nerve cells for signal transmission.

• Biconcave disc: Red blood cells for efficient gas exchange.

• Cube-shaped: Kidney tubule cells for absorption and secretion.

• Column-shaped: Intestinal lining cells for nutrient absorption.

• Spherical: Cartilage cells for cushioning joints.

• Cylindrical: Skeletal muscle cells for contraction and movement.

Example: The biconcave shape of erythrocytes increases surface area for oxygen transport.

Common Features and General Functions of Cells

Essential Cellular Functions

All cells share fundamental features and perform basic functions necessary for life.

• Maintain integrity and shape:

  • Dependent on plasma membrane and internal contents.

• Obtain nutrients and form chemical building blocks:

  • Cells harvest energy for survival and growth.

• Dispose of wastes:

  • Prevents accumulation of toxic substances that could disrupt cellular activities.

• Cell division:

  • Produces more cells of the same type.

  • Maintains tissue by providing cells for new growth and replacing dead cells.

Example: Liver cells metabolize nutrients and detoxify wastes to maintain homeostasis.

Cell Structure

Major Components of a Cell

Cells are composed of distinct structures that perform specialized functions.

• Plasma membrane: Outer barrier separating internal contents from the external environment; includes extensions such as cilia and microvilli.

• Nucleus: Largest structure, enclosed by a nuclear envelope; contains genetic material (DNA).

• Cytoplasm: Cellular contents between plasma membrane and nucleus; includes cytosol and organelles.

• Cytosol: Viscous fluid with high water content, dissolved macromolecules, and ions.

• Organelles: Specialized structures with unique shapes and functions.

  • Endoplasmic reticulum (ER):

    • Rough ER: Protein synthesis (ribosomes attached).

    • Smooth ER: Detoxification and steroid hormone production.

  • Golgi apparatus: Modifies, packages, and ships proteins via vesicles.

  • Lysosomes: Contain hydrolytic enzymes for breaking down biomolecules and cell debris.

  • Peroxisomes: Contain oxidative enzymes for catabolizing fatty acids and alcohol.

  • Mitochondria: ATP production (cellular energy).

  • Ribosomes: Protein synthesis.

  • Cytoskeleton: Maintains cell shape, assists with cell division and transport.

  • Centrosome: Builds mitotic spindle during cell division.

  • Proteasomes: Degrade damaged proteins.

Additional info: The plasma membrane's selective permeability is crucial for maintaining cellular homeostasis.

Plasma Membrane Structure and Function

Composition and Roles of the Plasma Membrane

The plasma membrane is a dynamic structure that regulates the movement of substances and facilitates communication between the cell and its environment.

• Fluid mixture of lipids and proteins (approximately equal by weight).

• Regulates movement of most substances in and out of the cell.

• Small and nonpolar substances can penetrate without assistance.

• Lipid components:

  • Phospholipids: Modified triglycerides with two fatty acid chains and a phosphate group; form the bilayer.

  • Cholesterol: Scattered within the bilayer; strengthens membrane.

  • Glycolipids: Lipids with attached carbohydrate groups; located on the outer region only.

Example: Cholesterol maintains membrane fluidity at varying temperatures.

Membrane Transport

Mechanisms of Substance Movement Across the Plasma Membrane

Cells use various processes to obtain and eliminate substances across the plasma membrane, maintaining internal balance.

• Passive processes: Do not require energy; substances move down their concentration gradient.

• Active processes: Require energy (ATP); substances move against their concentration gradient.

Functions:

• Physical barrier between cell and interstitial fluid.

• Regulates movement into and out of the cell.

• Establishes and maintains electrochemical gradients.

• Facilitates cell communication.

Membrane Proteins

Types and Functions of Membrane Proteins

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

• Integral proteins: Embedded within and extend across the phospholipid bilayer.

• Peripheral proteins: Loosely attached to external or interior surfaces of the membrane; not embedded in the bilayer.

Functional categories:

• Transport proteins: Regulate movement of substances (channels, carriers, pumps).

• Cell surface receptors: Bind ligands (e.g., neurotransmitters) to initiate cellular responses.

• Identity markers: Communicate to other cells that they belong to the body; distinguish healthy cells from those to be destroyed.

Example: Sodium-potassium pumps maintain electrochemical gradients in nerve cells.

Passive Membrane Transport

Diffusion and Facilitated Diffusion

Passive transport allows substances to move across the membrane without energy input, driven by concentration gradients.

• Simple diffusion: Nonpolar molecules (e.g., O2, CO2, ethanol, urea) pass directly through the phospholipid bilayer.

• Facilitated diffusion: Charged or polar solutes require assistance from membrane proteins.

  • Channel-mediated diffusion: Movement of small ions through water-filled protein channels; channels may be leak (always open) or gated (open in response to stimuli).

  • Carrier-mediated diffusion: Movement of larger or polar molecules via carrier proteins.

Example: Glucose enters cells via carrier-mediated facilitated diffusion. Additional info: Passive transport is essential for processes such as gas exchange in the lungs and nutrient absorption in the intestines.

Table: Comparison of Cell Types

Cell Type	Main Function	Example
Somatic cells	Form tissues and organs	Muscle, nerve, skin cells
Gametes	Reproduction	Ovum, sperm
Blood cells	Transport, immunity, clotting	Erythrocytes, leukocytes, platelets
Stem cells	Differentiate into other cell types	Hematopoietic stem cells

Table: Stages of the Cell Cycle

Stage	Main Event	Duration
G1 phase	Cell growth, preparation for DNA replication	Part of interphase (~23 hours total)
S phase	DNA replication	Part of interphase
G2 phase	Preparation for division	Part of interphase
M phase	Mitosis and cytokinesis	~1 hour

Key Equations

Diffusion Rate

The rate of diffusion across a membrane can be described by Fick's Law:

$J = -D \frac{dC}{dx}$

• J: Diffusion flux

• D: Diffusion coefficient

• \frac{dC}{dx}: Concentration gradient

ATP Production (Cellular Respiration)

Overall reaction for aerobic respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$