Biology Study Guide: Understanding Nucleotides, Enzymes, and Cell Energy

Layla Guthrie

12/2/2025

AP Biology

Biology Study Guide : Nucleotides, Enzymes, and Cell Energy

Subjects

AP Biology

Cell Structure and Function

The Cell Membrane Structure and Function

Membrane

•

Dec 2, 2025

•

5 pages

Biology Study Guide: Understanding Nucleotides, Enzymes, and Cell Energy

Layla Guthrie

@laylaguthrie_aoly

Dive into the fascinating world of cells, the building blocks... Show more

1 / 5

1. What are nucleotides composed of?
Nucleotides have three components: pentose sugar (5-carbon sugar),
phosphate group, and nitrogenous bas

Cellular Basics and Organelles

The cell is the fundamental unit of life, coming in two main types: smaller prokaryotic cells and larger eukaryotic cells. Every cell contains specialized parts that work together to keep it functioning.

Nucleotides, the building blocks of DNA and RNA, consist of three components: a pentose sugar, a phosphate group, and a nitrogenous base. In DNA, the nucleotides include adenine, thymine, cytosine, and guanine, while RNA uses uracil instead of thymine.

The mitochondria earn their nickname as the "powerhouse of the cell" because they extract energy from food through cellular respiration, producing ATP (adenosine triphosphate), the energy currency cells use to power their activities. This process is crucial for all cellular functions!

💡 Quick Tip: Remember the difference between Rough ER and Smooth ER: Rough ER has ribosomes attached (making it "rough") and produces proteins, while Smooth ER (without ribosomes) produces lipids and detoxifies chemicals.

The Central Dogma of Molecular Biology explains the flow of genetic information: DNA → RNA → Protein. This fundamental principle describes how the instructions in your genes become the proteins that do the actual work in your cells.

Cell Membranes and Size Limitations

The cell membrane forms the outer boundary of all cells, maintaining shape and controlling what enters and exits. Made of a phospholipid bilayer, this semi-permeable barrier allows only certain molecules to cross, helping cells get nutrients and remove wastes.

Cell size matters! The surface area-to-volume ratio explains why cells can't grow indefinitely. Smaller cells have a higher ratio, making it easier for materials to diffuse in and out quickly. Larger organisms solve this problem by developing specialized organs that increase surface area and using circulatory systems to move materials.

Between complementary nucleotides $like A-T and G-C$ , hydrogen bonds form the connections. These bonds are weak individually but together create the stable structure of DNA's double helix. Purines (adenine and guanine) have a double-ring structure, while pyrimidines have single rings.

💡 Remember This: Cells maintain homeostasis - the stable internal conditions necessary for survival - through active processes that regulate everything from temperature to pH levels.

Most cells are microscopic - plant and animal cells cannot be seen with the unaided eye. To view detailed cellular structures like ribosomes, scientists must use an electron microscope which provides much higher magnification than light microscopes.

Prokaryotes vs. Eukaryotes

The key difference between prokaryotic and eukaryotic cells lies in their genetic organization. A nucleus is a membrane-bound structure in eukaryotes that houses multiple chromosomes, while a nucleoid is an irregularly shaped region in prokaryotes that contains a single chromosome without a protective membrane.

Ribosomes, composed of ribosomal RNA and about 50 different proteins, are the protein factories in all cells. In prokaryotes, many cells move using flagella, whip-like structures that propel them through their environment. Some bacteria also have a capsule that protects them from being engulfed by predator cells.

The Golgi apparatus acts like the cell's post office, receiving proteins from the rough ER, then modifying, packaging, and shipping them to their final destinations. In plant cells, the Golgi also produces polysaccharides needed for cell wall construction.

💡 Cool Biology Fact: The endosymbiotic theory explains how mitochondria and chloroplasts came to exist inside our cells - they were once free-living bacteria that were engulfed by larger cells and developed a mutually beneficial relationship!

Chloroplasts, found only in plant and algae cells, are the sites of photosynthesis where sunlight energy is converted into sugar. Unlike animal cells, plant cells have rigid cell walls made of cellulose that provide structural support and protection.

Cell Communication and Transport

Cells respond differently depending on their environment's tonicity. In an isotonic solution, cells maintain their volume. In a hypertonic solution, water leaves the cell, causing it to shrink. In a hypotonic solution, water enters the cell, potentially causing it to burst.

Cell junctions are specialized structures that connect adjacent cells. Tight junctions create seals that prevent materials from passing between cells, desmosomes form strong adhesive connections, and gap junctions allow for direct communication between neighboring cells.

Glycoproteins $carbohydrate + protein$ and glycolipids $carbohydrate + lipid$ on cell surfaces act as recognition sites and play key roles in cell-cell identification. Homotypic binding occurs when identical molecules on different cells bind together, like how your skin cells stick to each other.

💡 Transport Simplified: Think of passive transport as going downhill (no energy needed) and active transport as going uphill (requires energy input from the cell).

Water doesn't freely cross cell membranes but moves through special channels called aquaporins. Gases can be transported along with water. Understanding how substances enter and exit cells is crucial for comprehending nearly all cell functions!

Energy and Enzymes

The laws of thermodynamics govern all energy transfers in cells. The First Law states that energy is neither created nor destroyed, only converted from one form to another. The Second Law explains that when energy is converted, some becomes unavailable for work.

Cells store energy in two main forms: potential energy (stored in chemical bonds, concentration gradients, or charge imbalances) and kinetic energy (the energy of movement). Reactions that build molecules consume water, while hydrolysis reactions break down molecules and use water.

Enzymes are protein catalysts that speed up chemical reactions by lowering the activation energy required to start a reaction. These biological helpers can be disabled through denaturation (structural changes that destroy function), often caused by extreme temperatures or pH.

💡 Enzyme Inhibition Simplified: Think of competitive inhibition as someone stealing your parking spot (an impostor molecule blocks the enzyme's active site), while allosteric inhibition is like someone changing the shape of your car key so it doesn't work properly (binding elsewhere changes the enzyme's shape).

Enzyme function can be regulated through different mechanisms. In competitive inhibition, molecules similar to the substrate compete for the enzyme's active site. In allosteric inhibition, molecules bind to a different part of the enzyme, changing its shape and slowing down reactions.

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Subjects

AP Biology

Cell Structure and Function

The Cell Membrane Structure and Function

Membrane

AP Biology

•

Dec 2, 2025

•

5 pages

Biology Study Guide: Understanding Nucleotides, Enzymes, and Cell Energy

Layla Guthrie

@laylaguthrie_aoly

💡 Enzyme Inhibition Simplified: Think of competitive inhibition as someone stealing your parking spot (an impostor molecule blocks the enzyme's active site), while allosteric inhibition is like someone changing the shape of your car key so it doesn't work properly (binding elsewhere changes the enzyme's shape).