Cell Structure, Membrane Function, and Transport Mechanisms: Study Notes for Anatomy & Physiology

Cell Structure and Diversity

Introduction to Cells

The cell is the fundamental unit of life, forming the basis for all living organisms. All organisms are composed of one or more cells, which serve as the structural and functional building blocks of life.

• Cells arise from other cells primarily through the process of mitosis.

• Most body cells are produced by mitosis.

Cell Diversity and Similarity

Cells vary in structure and function but share common features. They can be classified based on their roles:

• Cells that connect body parts: e.g., fibroblasts, erythrocytes, epithelial cells.

• Cells that move organs and body parts: e.g., smooth muscle cells, skeletal muscle cells.

• Cells that store nutrients: e.g., fat cells.

• Cells that fight disease: e.g., macrophages.

• Cells that gather information and control body functions: e.g., nerve cells.

• Cells of reproduction: e.g., sperm cells.

Similarity: All cells have the same basic parts and some common functions.

Generalized Cell Structure

• Plasma membrane: The outer boundary, acting as a selectively permeable barrier.

• Cytoplasm: Intracellular fluid packed with organelles.

• Nucleus: Organelle that controls cellular activities, usually near the cell's center.

Plasma (Cell) Membrane

Extracellular and Intracellular Materials

Substances contributing to body mass are found inside and outside cells:

• Extracellular fluid: Includes interstitial fluid, blood plasma, and cellular secretions (e.g., saliva, mucus, digestive fluids).

• Extracellular matrix: Nonliving material in connective tissue, consisting of ground substances and fibers.

Most body cells are in contact with a jellylike substance composed of proteins and polysaccharides.

Membrane Structure

The plasma membrane is a flexible structure composed of phospholipids, cholesterol, and proteins. It separates cell contents from the extracellular environment.

• Fluid mosaic model: Describes the membrane as a dynamic, double-layer (bilayer) of lipids with embedded proteins.

Membrane Lipids

• Phospholipid bilayer: The basic fabric of the membrane, with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Stabilizes the membrane.

• Dynamic fluid structure: Lipids and proteins move freely within the membrane.

Membrane Proteins

Membrane proteins are responsible for most specialized functions and communication with other cells.

• Integral proteins: Inserted into the lipid bilayer, often spanning the membrane.

• Peripheral proteins: Loosely attached to integral proteins or anchored to the membrane.

Types of Membrane Proteins

• Transport proteins: Control entry and exit of substances.

• Receptors for signal transduction: Bind chemical messengers and initiate cellular responses.

• Enzymatic activity: Catalyze metabolic pathways.

• Cell-cell recognition: Glycoproteins act as identification tags.

• Attachment to cytoskeleton and extracellular matrix: Maintain cell shape and fix protein location.

• Intercellular joining: Form cell junctions.

Specialized Structures of Membrane

• Tight junctions: Impermeable junctions that prevent molecules from passing between cells.

• Desmosomes: Anchoring junctions that prevent cell separation.

• Gap junctions: Communicating junctions allowing ions and molecules to pass between cells.

• Microvilli: Increase surface area for absorption.

Membrane Potential and Ion Distribution

Difference in K+ and Na+ Distribution

The membrane potential is determined mainly by the concentration gradient of potassium (K+) and the differential permeability of the plasma membrane to K+ and other ions.

• K+ diffuses out of the cell down its concentration gradient, making the inside of the membrane more negative.

• Na+ enters the cell, but the membrane is less permeable to Na+.

• At a membrane voltage of -90 mV, K+ concentration gradient is balanced by the electrical gradient.

Equation:

$\text{Membrane Potential} \approx -90\ \text{mV}$

Cytoplasm and Organelles

Cytoplasm

• Cytosol: Viscous, semitransparent fluid.

• Inclusions: Chemical substances that may or may not be present.

• Organelles: Specialized subcellular structures.

Mitochondria

• Structure: Threadlike, lozenge-shaped organelles.

• Function: Power plants of the cell, providing most ATP.

Ribosomes

• Composition: Single-stranded, sugar ribose, and base uracil (RNA).

• Function: Sites of protein synthesis (transcription and translation).

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; manufactures proteins and phospholipids.

• Smooth ER: Synthesizes lipids, cholesterol, phospholipids, and steroid hormones; detoxifies chemicals; stores calcium.

Golgi Apparatus

• Consists of stacked, flattened cisterns.

• Modifies, concentrates, and packages proteins and lipids.

Vesicles

• Lysosomes: Contain digestive enzymes.

• Peroxisomes: Contain powerful enzymes for detoxification.

Cytoskeleton

• Network of structural proteins supporting cell shape and movement.

Cilia and Flagella

• Cilia: Whiplike, motile extensions on cell surfaces.

• Flagella: Longer projections for movement (e.g., sperm cell).

Nucleus

• Contains genetic material (DNA).

• Controls cell activities.

Plant Cells

Unique Structures

• Central vacuole: Stores water, salts, nutrients, and waste products.

• Cell wall: Rigid outer layer for support and protection.

• Plastids: Sites of photosynthesis (e.g., chloroplasts).

Cell Types

• Eukaryotic cell: True nucleus and membrane-bound organelles.

• Prokaryotic cell: No true nucleus; genetic material in nucleoid.

Transport Across Membrane

Passive Transport

• Diffusion: Movement of particles from high to low concentration.

• Facilitated diffusion: Passive transport via carrier proteins or channels for large/charged molecules.

• Channel-mediated diffusion: Channels allow passage of specific ions or water.

• Osmosis: Movement of water through a selectively permeable membrane.

Factors affecting diffusion rate:

• Concentration gradient

• Molecular size

• Temperature

Active Transport

• Pumps: Use ATP to move substances against their concentration gradients.

• Vesicular transport: Transport of large particles/macromolecules via membrane-bound sacs.

Summary Table: Cell Membrane Components

Component	Structure	Function
Phospholipid Bilayer	Double layer of phospholipids	Barrier, fluidity, selective permeability
Integral Proteins	Embedded in bilayer	Transport, receptors, enzymes
Peripheral Proteins	Attached to membrane surface	Support, signaling
Cholesterol	Interspersed among phospholipids	Stabilizes membrane
Glycoproteins	Proteins with carbohydrate chains	Cell recognition

Key Equations

• Membrane Potential: $\text{Membrane Potential} \approx -90\ \text{mV}$

• Cellular Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Additional info:

• Some context and examples have been expanded for clarity and completeness.

• Scientific terms have been defined and grouped logically for study purposes.