Cell Structure and Function: Study Notes for ANP College Students

Cells: The Living Units

Basic Parts of Human Cells

Human cells share similar structures and functions, which are essential for life. The three main parts of a cell are the cell membrane, cytoplasm, and nucleus.

• Cell Membrane: The boundary that separates the cell from its environment.

• Cytoplasm: The internal fluid and organelles within the cell.

• Nucleus: The control center containing genetic material.

Cell Membrane Structure and Function

Phospholipid Bilayer

The cell membrane is composed of two adjacent layers of phospholipids, forming a bilayer. Each phospholipid molecule has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails.

• Hydrophilic Head: Contains a phosphate group and faces outward toward water.

• Hydrophobic Tails: Made of fatty acids and face inward, away from water.

Membrane Composition

The cell membrane separates the intracellular fluid (ICF) from the extracellular fluid (ECF). It is primarily composed of phospholipids (70-75%), glycolipids (5%), and cholesterol (20%), which stabilize the membrane.

Membrane Proteins

Membrane proteins are crucial for cell function and are classified as integral or peripheral proteins.

• Integral Proteins: Embedded within the membrane; include channel proteins for selective transport.

• Peripheral Proteins: Attached to the inner or outer surface of the membrane.

• Functions: Transport, cell recognition, receptors, anchors, enzymes, and joining areas.

Transport Across the Cell Membrane

Selective Permeability

The cell membrane allows only certain substances to pass through unaided, mainly small, nonpolar molecules like oxygen and carbon dioxide.

Passive Membrane Transport

Passive transport does not require energy and relies on concentration gradients. Molecules move from areas of higher concentration to lower concentration.

• Diffusion: Movement of particles down their concentration gradient.

• Facilitated Diffusion: Uses channel or carrier proteins for transport of larger or charged molecules.

Osmosis

Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. Water moves from areas of low solute concentration (high water) to high solute concentration (low water).

Tonicity and Cell Volume

Tonicity describes the effect of osmolarity on cell volume:

• Isotonic Solution: Equal osmolarity inside and outside; no net water movement.

• Hypertonic Solution: Higher osmolarity outside; water leaves cell, causing shrinkage.

• Hypotonic Solution: Lower osmolarity outside; water enters cell, causing swelling or bursting.

Active Membrane Transport

Active transport requires ATP to move substances against their concentration gradient, often using protein carriers.

• Active Transport: Uses pumps to move ions or molecules.

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

Exocytosis

Exocytosis is the process of exporting material from the cell using vesicles. The vesicle membrane fuses with the cell membrane, releasing its contents.

Membrane Potential

Ion Gradient and Electrical Charge

The separation of ions across the plasma membrane creates a membrane potential, with a positive charge outside and negative charge inside.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm consists of cytosol and organelles. Cytosol is the jelly-like fluid where biochemical reactions occur. Organelles perform specialized functions.

Types of Organelles

• Membranous Organelles: Separated from cytosol by a membrane (e.g., mitochondria, Golgi apparatus).

• Non-membranous Organelles: Not separated by a membrane (e.g., filaments, centrosome).

Endomembrane System

The endomembrane system includes the endoplasmic reticulum (ER), Golgi apparatus, and vesicles, which work together to produce, package, and export cellular products.

Endoplasmic Reticulum (ER)

The ER is a system of channels continuous with the nuclear envelope. It is involved in protein and lipid synthesis, transport, and storage.

Rough ER (RER)

RER is studded with ribosomes and is involved in protein synthesis and modification.

Smooth ER (SER)

SER lacks ribosomes and is involved in lipid synthesis, calcium storage, carbohydrate metabolism, and detoxification.

Golgi Apparatus

The Golgi apparatus sorts, modifies, and ships products from the ER. It also produces lysosomes.

Lysosomes and Peroxisomes

• Lysosomes: Contain digestive enzymes for breaking down organic compounds.

• Peroxisomes: Specialized for detoxification and lipid metabolism, mainly in liver cells.

Ribosomes

Ribosomes are the site of protein synthesis, composed of ribosomal RNA subunits. They can be free-floating or bound to ER.

Mitochondria

Mitochondria are bean-shaped organelles responsible for energy production. Cells with high energy demands, like muscle and neurons, contain many mitochondria.

Nucleus

Structure and Function

The nucleus is the control center of the cell, storing genetic instructions in DNA. It is surrounded by a nuclear envelope with pores for transport.

Nucleolus

The nucleolus manufactures RNA for ribosome construction.

Chromatin and Chromosomes

Chromatin is composed of DNA and proteins. Chromosomes are the condensed form of chromatin, containing genes.

Summary Table: Cell Membrane Components

Component	Function
Phospholipids	Form bilayer, barrier to water-soluble substances
Glycolipids	Cell recognition
Cholesterol	Membrane stability
Integral Proteins	Transport, receptors
Peripheral Proteins	Support, signaling

Summary Table: Types of Membrane Transport

Type	Energy Required?	Example
Passive (Diffusion)	No	O2, CO2
Facilitated Diffusion	No	Glucose, ions
Osmosis	No	Water
Active Transport	Yes	Na+/K+ pump
Vesicular Transport	Yes	Endocytosis, exocytosis

Key Equations

Osmosis

Osmosis is governed by osmotic pressure:

$\Pi = iMRT$

• \Pi: Osmotic pressure

• i: van 't Hoff factor

• M: Molarity

• R: Gas constant

• T: Temperature (Kelvin)

Membrane Potential

The Nernst equation describes equilibrium potential for ions:

$E = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• E: Equilibrium potential

• R: Gas constant

• T: Temperature

• z: Ion charge

• F: Faraday constant

Conclusion

Understanding cell structure and membrane function is fundamental to anatomy and physiology. The cell membrane regulates transport, maintains homeostasis, and enables communication. Organelles perform specialized tasks essential for cell survival and function.