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

Main Parts of a Cell

Overview

The cell is the basic unit of life, composed of several key parts, each with distinct functions that contribute to cellular activity and homeostasis.

• Plasma Membrane: Serves as a selective barrier, regulating the entry and exit of substances.

• Cytoplasm: The internal fluid of the cell containing organelles and the cytosol, where many metabolic reactions occur.

• Nucleus: Contains genetic material (DNA) and controls cellular activities through gene expression.

Structure-Function Relationship

The structure of each cell part is closely related to its function. For example, the lipid bilayer of the plasma membrane allows selective permeability, while the nuclear envelope protects genetic material.

Cell Membrane Structure and Selective Permeability

Selective Permeability

The cell membrane is selectively permeable, allowing certain substances to pass while restricting others.

• What can pass through the membrane? Small, nonpolar molecules (e.g., oxygen, carbon dioxide) and some small polar molecules (e.g., water) can pass easily.

• What passes quickly vs. slowly? Lipid-soluble substances pass quickly; ions and large polar molecules pass slowly or require transport proteins.

• Types of chemicals that cannot pass: Large molecules and charged ions typically cannot pass without assistance.

• Example: Glucose requires a transporter to cross the membrane.

Lipids in the Cell Membrane

Types and Functions of Membrane Lipids

The cell membrane contains three major types of lipids: phospholipids, cholesterol, and glycolipids.

• Phospholipids: Form the bilayer structure and act as a barrier to most water-soluble substances.

• Cholesterol: Stabilizes membrane fluidity and integrity, especially at varying temperatures.

• Glycolipids: Contribute to cell recognition and communication.

Structure-Function Relationship: The amphipathic nature of phospholipids (hydrophilic head, hydrophobic tail) is essential for bilayer formation. Cholesterol intercalates between phospholipids, modulating fluidity.

Carbohydrates in the Cell Membrane

Role and Function

Carbohydrates are present on the extracellular surface of the cell membrane, often attached to proteins (glycoproteins) or lipids (glycolipids).

• Function: Cell recognition, signaling, and protection.

• Glycocalyx: The carbohydrate-rich area on the cell surface that aids in cell-cell interactions.

Proteins in the Cell Membrane

Types and Functions

Membrane proteins are crucial for transport, signaling, and structural support.

• Integral Proteins: Span the membrane and function as channels, carriers, or receptors.

• Peripheral Proteins: Attached to the membrane surface, involved in signaling and maintaining cell shape.

• Transport Proteins: Facilitate movement of substances across the membrane.

Membrane Gradients

Concentration and Electrical Gradients

Gradients across the membrane drive the movement of substances.

• Concentration Gradient: Difference in solute concentration across the membrane.

• Electrochemical Gradient: Combined effect of concentration and electrical gradients.

Membrane Transport Mechanisms

Passive and Active Transport

Transport across the membrane can be passive (no energy required) or active (energy required).

• Passive Transport: Includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires ATP to move substances against their gradient.

Factors Influencing Diffusion: Temperature, molecule size, concentration gradient, and membrane permeability.

Types of Passive Transport

• Simple Diffusion: Movement of small, nonpolar molecules directly through the membrane.

• Facilitated Diffusion: Movement of larger or polar molecules via transport proteins.

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

Types of Active Transport

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

• Secondary Active Transport: Uses energy from the movement of another substance down its gradient.

Osmosis and Tonicity

Effects of Solutions on Cells

Tonicity describes how a solution affects cell volume.

• Isotonic: No net movement of water; cell volume remains unchanged.

• Hypertonic: Water moves out; cell shrinks.

• Hypotonic: Water moves in; cell swells.

Membrane Pumps and Transporters

Sodium-Potassium Pump

The sodium-potassium pump is a primary active transporter that maintains cellular ion gradients.

• Function: Pumps 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

• Equation:

$3\text{Na}^+_{\text{(in)}} + 2\text{K}^+_{\text{(out)}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{(out)}} + 2\text{K}^+_{\text{(in)}} + \text{ADP} + \text{P}_i$

Vesicular Transport

Vesicular transport moves large particles or fluids via membrane-bound vesicles.

• Endocytosis: Uptake of substances into the cell (includes phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Exocytosis: Release of substances from the cell.

• Transcytosis: Movement of substances across the cell via vesicles.

Cell Organelles

Major Organelles and Their Functions

Organelles are specialized structures within cells that perform distinct functions.

• Nucleus: Stores genetic material and coordinates cell activities.

• Mitochondria: Site of ATP production via cellular respiration.

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

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

• Lysosomes: Digest cellular waste and foreign material.

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

Cell Cycle and Mitosis

Phases of the Cell Cycle

The cell cycle consists of interphase (G1, S, G2) and mitosis (prophase, metaphase, anaphase, telophase).

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

• Mitosis: Prophase, metaphase, anaphase, telophase; results in two identical daughter cells.

• Purpose: Growth, repair, and maintenance of tissues.

DNA and RNA: Structure and Function

Differences and Roles

DNA and RNA are nucleic acids with distinct structures and functions.

• DNA: Double-stranded, stores genetic information.

• RNA: Single-stranded, involved in protein synthesis (mRNA, tRNA, rRNA).

Transcription and Translation

• Transcription: DNA is used to synthesize mRNA.

• Translation: mRNA is decoded to build proteins.

Key Steps: Initiation, elongation, termination.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT
Osmosis	No	Down gradient	Water
Primary Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Secondary Active Transport	Indirect (uses gradient)	Against gradient	Sodium-glucose symporter
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, exocytosis