General Biology Study Notes: Carbohydrates, Lipids, Membranes, and Cells

Chapter 5: Carbohydrates: Structure and Function

Introduction to Carbohydrates

Carbohydrates are essential biomolecules that play critical roles in cell structure, cell identity, and energy storage. They are classified based on the number of sugar units they contain.

• Monosaccharide: Single sugar molecule (e.g., glucose, fructose, galactose).

• Oligosaccharide: Short chains of sugar molecules (few-sugars).

• Polysaccharide: Long chains of sugar molecules (many-sugars), such as starch, glycogen, and cellulose.

Key Point: The function of carbohydrates in living organisms is determined by how their monomers are linked together.

Molecular Structure of Carbohydrates

Carbohydrates generally have the molecular formula (CH2O)n, where "n" is the number of carbon-hydrate groups and can range from 3 to over a thousand. Their structure includes:

• Carbonyl group (C=O)

• Hydroxyl groups (–OH)

• Carbon-hydrogen bonds (C–H)

Not all compounds with the formula are carbohydrates (e.g., formaldehyde is not a carbohydrate).

Sugars as Monomers

Monosaccharides, or simple sugars, serve as the building blocks for larger carbohydrates and provide chemical energy in cells. They also play a significant role in chemical evolution, such as ribose in nucleotide formation.

• Example: Ribose is required for the formation of nucleotides, the building blocks of RNA.

Structural Diversity of Monosaccharides

Monosaccharides vary in several structural aspects, which affect their function:

• Location of the carbonyl group: At the end of the molecule: aldose; in the middle: ketose.

• Number of carbon atoms: Three: triose; Five: pentose; Six: hexose.

• Spatial arrangement of atoms: Different arrangement of hydroxyl groups can result in different sugars (e.g., glucose vs. galactose).

• Linear and ring forms: Sugars often form ring structures in aqueous solutions, and these forms can have different properties.

Key Point: Each monosaccharide has a unique structure and function due to these variations.

Polysaccharides: Structure and Linkages

Formation and Types of Polysaccharides

Polysaccharides are polymers made from monosaccharide monomers. Two sugars linked together form a disaccharide. The linkage occurs via a condensation reaction between two hydroxyl groups, forming a covalent bond called a glycosidic linkage. These linkages can be broken by hydrolysis reactions.

• Glycosidic linkages can form between any two hydroxyl groups. Common types:

  • α-1,4-glycosidic linkage

  • β-1,4-glycosidic linkage

  Both linkages are between the C-1 and C-4 carbons but differ in geometry.

Major Polysaccharides and Their Functions

Polysaccharide	Monomer	Linkage Type	Function
Starch	α-glucose	α-1,4 and α-1,6 (branched)	Energy storage in plants
Glycogen	α-glucose	Highly branched α-1,4 and α-1,6	Energy storage in animals
Cellulose	β-glucose	β-1,4	Structural support in plant cell walls
Chitin	N-acetylglucosamine (NAG)	β-1,4	Structural support in fungi and exoskeletons of insects/crustaceans
Peptidoglycan	Alternating monosaccharides (NAG & NAM)	β-1,4 with peptide cross-links	Structural support in bacterial cell walls

Additional info: In cellulose and chitin, every other monomer is flipped, allowing for linear strands and hydrogen bonding between adjacent strands, which increases structural strength.

Functions of Carbohydrates in Cells

Diverse Cellular Roles

• Precursors for other molecules (e.g., nucleotides, amino acids)

• Provide fibrous structural materials (e.g., cellulose, chitin, peptidoglycan)

• Indicate cell identity (e.g., glycoproteins, glycolipids)

Carbohydrates as Structural Support

• Structural polysaccharides such as cellulose, chitin, and peptidoglycan form long strands with bonds between adjacent strands, organized into fibers or sheets. This provides strength and elasticity to cells and organisms.

• β-1,4-glycosidic linkages are resistant to hydrolysis, making these fibers difficult to break down.

• These fibers exclude water, further hindering hydrolysis.

• Dietary fiber from carbohydrates is important for digestive health.

Role in Cell Identity

• Glycoproteins: Proteins with attached carbohydrates

• Glycolipids: Lipids with attached carbohydrates

• These molecules are crucial for distinguishing "self" cells and for communication between cells.

Carbohydrates and Energy Storage

Energy Storage and Release

• Carbohydrates store and provide chemical energy. In photosynthesis, plants convert sunlight into chemical energy stored in carbohydrate bonds.

• Carbohydrates have more energy than lipids because electrons in C–H bonds have higher potential energy than those in bonds with oxygen.

Enzymatic Hydrolysis of Polysaccharides

• Glycogen is hydrolyzed by the enzyme phosphorylase in animal cells to release glucose.

• Starch is hydrolyzed by amylase enzymes, which are important in digestion.

ATP Production from Glucose

• When a cell needs energy, it breaks down glucose, and the captured energy is used to make ATP.

• ATP is the universal energy currency in cells, driving processes such as polymerization and muscle movement.

Chapter 6: Lipids, Membranes, and the First Cells

Introduction

The plasma membrane, also known as the cell membrane, is a fundamental feature distinguishing living cells from nonliving matter. It is primarily composed of lipids and proteins, forming a selective barrier that regulates the internal environment of the cell.

• Selective barrier: Allows entry of essential materials and prevents entry of harmful substances.

• Facilitates chemical reactions: By sequestering specific chemicals, the membrane enables necessary biochemical reactions for life.

Lipid Structure and Function

Definition and Properties of Lipids

Lipids are carbon-containing compounds that are insoluble in water due to their high proportion of nonpolar carbon–carbon (C–C) and carbon–hydrogen (C–H) bonds.

• Hydrocarbons: Molecules consisting only of carbon and hydrogen; they are nonpolar and hydrophobic.

• Electrons are shared equally in C–H bonds, contributing to their insolubility in water.

Types of Lipids

• Isoprenoid: Hydrocarbon chains that function as pigments, scents, vitamins, and hormone precursors. They serve as building blocks for more complex lipids.

• Fatty acid: A hydrocarbon chain bonded to a carboxyl (–COOH) functional group. Fatty acids typically contain 14–20 carbon atoms and can be either saturated or unsaturated.

Bond Saturation and Hydrocarbon Structure

The degree of saturation in hydrocarbon chains affects the physical and chemical properties of lipids.

• Saturated fatty acids: Contain only single bonds between carbon atoms, resulting in the maximum number of hydrogen atoms. These are typically solid at room temperature.

• Unsaturated fatty acids: Contain one or more double bonds, which introduce kinks in the chain and reduce the number of hydrogen atoms. These are usually liquid at room temperature.

• Polyunsaturated fatty acids: Contain multiple double bonds.

Example: Butter (solid) contains more saturated fats, while olive oil (liquid) contains more unsaturated fats.

Hydrocarbon Structure: Visual Summary

Hydrocarbon chains can be visualized as either straight (saturated) or kinked (unsaturated), affecting how tightly they pack together and thus their physical state at room temperature.

Major Types of Lipids in Cells

Steroids

Steroids are a family of lipids distinguished by a bulky, four-ring structure. They differ by the functional groups attached to the rings.

• Examples: Hormones such as estrogen and testosterone; cholesterol, a key component of plasma membranes.

Fats (Triacylglycerols or Triglycerides)

• Fats are composed of three fatty acids linked to a glycerol molecule. Their primary role is energy storage, as they contain many high-energy bonds.

• Formed by dehydration reactions between the hydroxyl group of glycerol and the carboxyl group of fatty acids, resulting in an ester linkage.

• Fats are not polymers because fatty acids are not linked into chains.

Phospholipids

Phospholipids consist of a glycerol backbone linked to a phosphate group and two hydrocarbon chains. They are the primary component of cell membranes.

• Fatty acid tails are found in Bacteria and Eukarya; isoprenoid tails are found in Archaea.

• Phospholipids are amphipathic, containing both hydrophilic (head) and hydrophobic (tail) regions.

Membrane Lipids and Water

Amphipathic Nature and Membrane Formation

Phospholipids spontaneously form structures in water due to their amphipathic nature:

• Micelles: Spherical aggregates with hydrophilic heads facing outward and hydrophobic tails inward.

• Lipid bilayers: Double-layered sheets with hydrophobic tails facing inward and hydrophilic heads facing outward, forming the basis of biological membranes.

These structures form spontaneously, requiring no input of energy, although the organization decreases entropy at the level of the lipids themselves.

Artificial Membranes

Liposomes are artificial, membrane-bound vesicles formed from phospholipids in the laboratory, used to study membrane properties.

Selective Permeability of Lipid Bilayers

Permeability Characteristics

Phospholipid bilayers exhibit selective permeability:

• Small, nonpolar molecules (e.g., O2) cross quickly.

• Large or charged molecules (e.g., glucose, ions) cross slowly or not at all.

Factors Affecting Membrane Permeability

• Hydrocarbon tail length: Longer tails decrease permeability.

• Cholesterol: Increases membrane density and decreases permeability by packing phospholipids more tightly.

• Temperature: Lower temperatures decrease fluidity and permeability as molecules move more slowly and pack more tightly.

Movement Across Membranes: Diffusion and Osmosis

Diffusion

Diffusion is the spontaneous movement of molecules from regions of high concentration to regions of low concentration, driven by thermal energy.

• Occurs until equilibrium is reached, where molecules are evenly distributed.

• Passive transport: Diffusion across a membrane without energy input.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane from regions of low solute concentration to regions of high solute concentration.

• Hypertonic solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic solution: Lower solute concentration outside; water moves in, cell swells.

• Isotonic solution: Equal solute concentrations; no net water movement.

Proteins in Membranes: Structure and Function

Membrane Proteins

Membranes contain both lipids and proteins. Proteins can be amphipathic, allowing them to insert into the membrane and form passageways for substances.

• Integral (transmembrane) proteins: Span the membrane, with segments facing both interior and exterior.

• Peripheral proteins: Bind to membrane surfaces without passing through.

Fluid-Mosaic Model

The fluid-mosaic model describes the membrane as a dynamic mosaic of phospholipids and proteins, with proteins able to move laterally within the bilayer.

Transport Across Membranes

Channel Proteins

Channel proteins form pores in the membrane, allowing specific ions or molecules to diffuse down their electrochemical gradients.

• Channels are selective, permitting only certain substances to pass.

• Aquaporins: Channel proteins that facilitate water transport.

• Many channels are gated, opening or closing in response to signals.

Carrier Proteins

Carrier proteins facilitate diffusion by binding to specific molecules and undergoing conformational changes to transport them across the membrane.

• GLUT-1: A carrier protein that increases glucose permeability.

Active Transport and Pumps

Active transport moves substances against their concentration gradients, requiring energy input, often from ATP.

• Pumps: Membrane proteins that use energy to transport molecules (e.g., sodium-potassium pump).

• Secondary active transport (co-transport): Uses electrochemical gradients established by pumps to move other substances against their gradients without direct ATP use.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction Relative to Gradient	Example
Simple Diffusion	No	Down gradient	O2 across membrane
Facilitated Diffusion (Channel/Carrier)	No	Down gradient	Glucose via GLUT-1
Active Transport (Pump)	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Against gradient	Co-transport of glucose with Na+