Bio Chapter 5 First two slides: Carbohydrates: Structure, Function, and Classification

Preferred Terminology for Carbohydrates

• The generic term "carbohydrate" is not the most precise descriptor.

• More accurate terms include:

  • Monosaccharide: A single, simple sugar unit.

  • Oligosaccharide: A short chain of monosaccharides, typically between $2$ and about $20$ units.

  • Polysaccharide: A long chain of many monosaccharides.

Major Functions of Carbohydrates in Living Organisms

Carbohydrates (monosaccharides, oligosaccharides, polysaccharides, etc.) serve three primary roles in living organisms:

1. Structural Components

• Cell Walls: Carbohydrates are crucial for forming cell walls, providing shape and protection.

  • Plants: Cellulose, a polysaccharide, forms the rigid cell walls of plant cells.

  • Fungi: Fungal cell walls are made of chitin, another polysaccharide.

  • Bacteria: Peptidoglycan, a complex polymer involving monosaccharides, constitutes bacterial cell walls.

  • In all these cases, cell walls are composed of monosaccharides joined together in long chains.

2. Cell Identity and Communication

• Surface Markers: Small groups of monosaccharides are covalently attached to proteins and lipids on the plasma membranes of cells.

• These small groups are called oligosaccharides.

• Glycoproteins: Proteins with one or more oligosaccharides attached.

• Glycolipids: Lipids with one or more oligosaccharides attached.

• Function: Glycoproteins and glycolipids on the cell surface, exposed to the outside, act as crucial cell identity markers.

  • They enable cells to communicate and recognize each other.

  • Immune System Example: Immune cells navigating tissues recognize specific oligosaccharides displayed on cell surfaces. For instance, immune cells can identify liver cells (hepatocytes) by the unique oligosaccharides presented on their plasma membranes, informing the immune system, "I'm in the liver." This allows for tissue-specific recognition and function.

3. Energy Storage

• Carbohydrates are a primary form of stored energy for organisms.

• Plants:

  • Plants synthesize starch through photosynthesis, capturing energy from light.

  • Starch is stored in granules within plant cells (e.g., potato cells, which appear as pinky starch granules in micrographs).

  • This stored energy is for the plant's later use, not for human consumption, though animals (including humans) consume plants to access this stored energy.

• Animals:

  • Animals, when they ingest excess monosaccharides (e.g., from starches), store these glucoses.

  • Glycogen: The primary energy storage polysaccharide in animals. It is stored in the liver and skeletal muscle.

  • "Carbo-loading" Example: Endurance athletes (like cross-country runners) consume high amounts of carbohydrates (e.g., pasta, bread) before events. This process, known as carbo-loading, maximizes glycogen stores in their liver and muscles.

  • During endurance activities, athletes depend on these stored energy reserves, as they typically do not consume food during the race.

The Term "Carbohydrate" and Its Limitations

• The term "carbohydrate" literally means "carbon water" and originates from the general chemical structure of many monosaccharides.

• Idealized Chemical Formula: Most, but not all, monosaccharides fit the formula $(CH<em>2O)</em>n$

  • For every carbon atom, there are typically two hydrogen atoms and one oxygen atom.

• Example: Ribose

  • Ribose has an $n = 5$

  • Its chemical formula is $C<em>5H</em>{10}O_5$

  • This perfectly matches the $(CH<em>2O)</em>n$ formula.

• Limitation: Deoxyribose

  • Deoxyribose, an important monosaccharide found in DNA, has the formula $C<em>5H</em>{10}O_4$

  • It does not fit the $(CH<em>2O)</em>n$ formula because it lacks one oxygen atom relative to its carbon count.

  • This highlights why "carbohydrate" is not always the best descriptor, as not all monosaccharides perfectly adhere to its simplified chemical definition.

Rules for Being a Monosaccharide

To be classified as a monosaccharide, a molecule must meet three simple chemical criteria:

1. Minimum Carbons: Must have at least three carbon atoms.

2. Carbonyl Group: Must possess at least one carbonyl group ($C=O$
   ). It can have more, but one is the minimum requirement.

3. Hydroxyl Groups: Must have at least two hydroxyl groups ($-OH$
   ). It can have more, but two is the minimum requirement.

• If a molecule satisfies these three rules (at least $3$ carbons, at least $1$ carbonyl, at least $2$ hydroxyls), it is a monosaccharide.

Classification of Monosaccharides by Carbonyl Position

Monosaccharides are categorized into two main types based on the location of their carbonyl group:

1. Aldoses: If the carbonyl group ($C=O$ ) is located at the end of the carbon chain.

   • The ending "-ose" is used to denote that it is a monosaccharide (e.g., glucose is an aldose).

2. Ketoses: If the carbonyl group ($C=O$ ) is located internal to the carbon chain.

   • These are also named with the "-ose" suffix (e.g., fructose is a ketose).