Cellular Respiration and Photosynthesis

Overview of Energy Conversion in Cells

Cells obtain energy from food through cellular respiration, while photosynthesis allows plants and some organisms to convert light energy into chemical energy. These processes are fundamental for life, providing ATP and organic molecules necessary for cellular functions.

• Cellular Respiration: The process by which cells break down glucose and other molecules to produce ATP, the cell's energy currency.

• Photosynthesis: The process by which plants, algae, and some bacteria convert light energy, water, and carbon dioxide into glucose and oxygen.

Equations:

• Cellular Respiration:

• Photosynthesis:

Autotrophs and Heterotrophs

Organisms are classified based on how they obtain energy and carbon.

• Autotrophs: Organisms that produce their own food from inorganic sources (e.g., plants, algae). They are considered producers.

• Heterotrophs: Organisms that obtain energy by consuming other organisms. They are considered consumers.

• Example of Autotroph: Arabidopsis thaliana (a model plant)

• Example of Heterotroph: Homo sapiens (humans)

Cellular Respiration: Aerobic vs. Anaerobic

Cellular respiration can occur with or without oxygen.

• Aerobic Respiration: Requires oxygen and produces more ATP.

• Anaerobic Respiration: Occurs without oxygen, producing less ATP and often resulting in fermentation.

Stages of Cellular Respiration

1. Glycolysis

Occurs in the cytoplasm; breaks down glucose into pyruvate.

• 1 molecule of glucose is split into 2 molecules of pyruvate.

• NAD+ picks up high-energy electrons to become NADH.

• ATP is produced: net gain of 2 ATP per glucose.

2. Citric Acid Cycle (Krebs Cycle)

Occurs in the mitochondrial matrix; completes the breakdown of glucose.

• Acetyl-CoA enters the cycle, producing CO2 as waste.

• NAD+ and FAD pick up electrons to become NADH and FADH2.

• ATP is produced: 2 ATP per glucose.

3. Electron Transport Chain (ETC)

Occurs in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to create a proton gradient that drives ATP synthesis.

• Oxygen is the final electron acceptor, forming water.

• ATP produced: about 32-34 ATP per glucose.

Summary Table: Cellular Respiration Stages

Photosynthesis

Chloroplasts and Light Energy

Photosynthesis occurs in the chloroplasts of plant cells, using light energy to synthesize glucose.

• Chloroplasts: Organelles containing chlorophyll, the pigment responsible for capturing light energy.

• Chlorophyll: Green pigment found in the thylakoid membranes; absorbs light, mainly in the blue and red wavelengths.

• Photosystem: Protein complexes in the thylakoid membrane that play a key role in the light reactions.

Light Energy and Absorption

• Light travels in waves; the distance between crests is called wavelength.

• The full range of wavelengths is the electromagnetic spectrum.

• Shorter wavelengths have more energy; longer wavelengths have less energy.

• Chlorophyll absorbs mainly blue and red light, reflecting green.

Stages of Photosynthesis

1. Light Reactions ("Photo")

Occur in the thylakoid membrane; convert light energy into chemical energy (ATP and NADPH).

• Light excites electrons in chlorophyll (photosystem II).

• Water is split, releasing electrons, protons, and oxygen.

• Electrons move through the electron transport chain, pumping protons to create a gradient.

• ATP synthase uses the gradient to produce ATP.

• Electrons are transferred to NADP+, forming NADPH.

2. Calvin Cycle ("Synthesis")

Occurs in the stroma; uses ATP and NADPH to convert CO2 into glucose.

• CO2 is fixed into a 3-carbon sugar (G3P).

• ATP and NADPH from light reactions provide energy and electrons.

• Glucose is synthesized from G3P.

Summary Table: Photosynthesis Stages

Nucleic Acids: DNA and RNA

Structure of Nucleotides

Nucleic acids are polymers of nucleotides, which are the building blocks of DNA and RNA.

• Each nucleotide consists of:

  • A phosphate group

  • A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

  • A nitrogenous base (A, T, C, G in DNA; A, U, C, G in RNA)

• DNA is double-stranded; RNA is usually single-stranded.

Comparison Table: DNA vs. RNA

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division.

• Helicase: Unwinds the DNA double helix.

• DNA Polymerase: Synthesizes new DNA strands by adding nucleotides to the template strand.

• Replication is semiconservative: each new DNA molecule has one old and one new strand.

Transcription and mRNA Processing

Transcription is the synthesis of RNA from a DNA template, occurring in the nucleus.

• RNA Polymerase: Enzyme that synthesizes RNA from DNA.

• Promoter: DNA sequence where RNA polymerase binds to start transcription.

• Terminator: DNA sequence signaling the end of transcription.

• mRNA: Messenger RNA; carries genetic information from DNA to ribosomes for protein synthesis.

mRNA Processing

• Three main modifications:

  1. 5' cap addition

  2. Poly-A tail addition

  3. Splicing out introns

• Introns: Non-coding regions removed from mRNA.

• Exons: Coding regions retained in mature mRNA.

Base Pairing Rules

• In DNA: A pairs with T, C pairs with G.

• In RNA: A pairs with U, C pairs with G.

Key Terms and Definitions

• Gene: A segment of DNA that codes for a protein or functional RNA.

• Polymer: A large molecule made of repeating subunits (monomers).

• Sugar-phosphate backbone: The structural framework of nucleic acids, formed by sugar and phosphate groups.

Example: The process of transcription in eukaryotic cells occurs in the nucleus, where DNA is used as a template to synthesize mRNA, which then exits to the cytoplasm for translation.

Additional info: Some details, such as the specific number of ATP produced in each stage, were inferred from standard biology knowledge to complete the study guide.