Lipids, Cell Structure, and the Endomembrane System: Study Notes for General Biology

Lipids

Overview of Lipids

Lipids are a diverse group of hydrophobic biological molecules, primarily composed of hydrocarbon regions. They play crucial roles in energy storage, membrane structure, and signaling.

• Hydrophobic: Lipids do not mix well with water due to their nonpolar hydrocarbon chains.

• The three most biologically important lipids are fats, phospholipids, and steroids.

Fats and Energy Storage

Fats are the primary energy storage molecules in animals and some plants. They are more energy-dense than carbohydrates like starch.

• Major function: Long-term energy storage in adipose (fat) cells.

• Adipose tissue also cushions vital organs and insulates the body.

• Fats provide about twice as much energy per gram as starch.

• Plants store energy mainly as starch, which is less energy-dense and more rigid than fat.

• Seeds in plants store fats to provide high-density energy reserves for germination.

• Examples of fat-rich plant foods: olive oil, avocados, nuts.

Structure of Fats

Fats are constructed from two types of smaller molecules: glycerol and fatty acids.

• Glycerol: A three-carbon alcohol with a hydroxyl group attached to each carbon.

• Fatty acid: A carboxyl group attached to a long hydrocarbon skeleton.

• In a fat molecule, three fatty acids are joined to glycerol by an ester linkage, forming a triacylglycerol (triglyceride).

• The fatty acids in a fat can be all the same or of two or three different kinds.

Phospholipids and Steroids

Phospholipids and steroids are other important classes of lipids with distinct structures and functions.

• Phospholipid: Two fatty acids and a phosphate group are attached to glycerol.

• The fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head.

• Steroids: Characterized by a four-ring hydrocarbon skeleton. Example: cholesterol, sex hormones.

• Steroid hormones are transported in the bloodstream by special proteins (e.g., sex hormone-binding globulin).

Cell Theory and Cell Structure

Cell Theory

The cell theory is a fundamental concept in biology, describing the properties and organization of all living organisms.

• The cell is the basic unit of structure and organization in organisms.

• All living organisms are made up of one or more cells.

• The cell is the fundamental building block of life (the "atom" of biology).

• All cells arise from pre-existing cells; there is no spontaneous generation.

• Basic features of all cells: plasma membrane, aqueous solution called cytosol or cytoplasm.

Types of Cells: Prokaryotic vs. Eukaryotic

Cells are classified as either prokaryotic or eukaryotic, with key structural differences.

• Eukaryotic cells:

  • Have DNA in a nucleus

  • Contain membrane-bound organelles (e.g., mitochondria)

• Prokaryotic cells:

  • No nucleus

  • No membrane-bound organelles

  • No mitochondria

  • Smallest living cells

• The main difference in cell structure is related to size and evolutionary adaptations to being large.

Scale in Biology

Understanding biological scale is essential for visualizing molecular and cellular biology.

• Biological systems range in size from 0.1 nm (atoms) to thousands of km (ecosystems).

• Common metric prefixes: milli- (10-3), micro- (10-6), nano- (10-9).

• Order of magnitude: Multiples of 10; used to estimate and compare sizes.

• Light microscopes cannot resolve objects smaller than the wavelength of light (~0.5 μm).

Diffusion and Brownian Motion

Diffusion is a key process in cells, but it is limited by cell size and the random nature of molecular movement.

• Diffusion: Random movement of molecules from point A to point B.

• Random movement is called Brownian motion or a "random walk".

• Brownian motion is slow and inefficient for moving molecules over large distances.

• The problem of being big in cells is called diffusion limitation.

Adaptations to Large Cell Size

Key Eukaryotic Cell Structures

Eukaryotic cells have evolved specialized structures to overcome the limitations of large size.

• Endomembrane system

• Mitochondria

• Cytoskeleton (support and transport)

Compartmentalization

Compartmentalization in eukaryotic cells provides local environments for specialized functions and allows incompatible processes to occur simultaneously.

• Shrinks the space available for chemical reactions, increasing efficiency.

• Examples of compartments: endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, plasma membrane.

The Endomembrane System

Functions and Components

The endomembrane system is a network of membranes within eukaryotic cells that plays a central role in gene expression, protein synthesis, and transport.

• Includes: nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, plasma membrane.

• Major function: Provides compartments for gene expression and protein synthesis.

Nucleus and Chromosomes

• The nucleus contains most of the cell's DNA.

• DNA is organized into discrete units called chromosomes.

• Each chromosome contains one DNA molecule associated with proteins, forming chromatin.

• The nuclear envelope encloses the nucleus and is a double membrane with pores for molecular transport.

Endoplasmic Reticulum (ER)

The ER is a major component of the endomembrane system, continuous with the nuclear envelope and accounting for more than half of the total membrane in many eukaryotic cells.

• Two regions: Rough ER (with ribosomes) and Smooth ER (without ribosomes).

Rough ER

• Has bound ribosomes for protein synthesis.

• Provides an environment for protein folding and modification.

• Produces glycoproteins (proteins covalently bonded to carbohydrates).

• Synthesizes cell membranes and membrane proteins.

Smooth ER

• Not involved in protein synthesis.

• Synthesizes lipids.

• Detoxifies drugs and poisons.

• Stores calcium ions.

Ribosomes

Ribosomes are complexes of ribosomal RNA and protein, responsible for protein synthesis.

• "Free" ribosomes are found in the cytoplasm.

• Some proteins are synthesized in the cytoplasm, but this can be inefficient due to crowding, slow mRNA transport, unsuitable chemical environments, or the need for export.

• Many proteins (e.g., insulin, oxytocin) are synthesized on ribosomes bound to the ER.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Size	Smallest living cells	Generally larger
Mitochondria	Absent	Present
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Equations and Concepts

• Order of Magnitude: $10^n$ (where n is an integer)

• Diffusion Limitation: The time for diffusion increases with the square of the distance: $t \propto d^2$

Additional info: Some explanations and examples have been expanded for clarity and completeness, such as the details of the endomembrane system and the comparison table.