Photosynthesis and Cellular Signaling Mechanisms

Posted on Jul 19, 2025 in Biology

Autotrophic and Heterotrophic Nutrition

Autotrophic Nutrition: Autotrophs sustain themselves without consuming other organisms. They utilize energy from sunlight or the oxidation of inorganic substances to synthesize organic molecules. Plants are photoautotrophs.

Heterotrophic Nutrition: Heterotrophs obtain organic material from other organisms. They are consumers in the biosphere. Humans, for example, depend on food derived from photoautotrophs.

Photosynthetic Autotrophs vs. Chemosynthetic Autotrophs

Photosynthetic autotrophs depend on light (sunlight plus organic compounds) for their food source, while chemosynthetic autotrophs use chemicals (inorganic compounds) as food.

Chloroplast Structure and Function

Location and Structure of the Chloroplast

Chloroplasts are found in the mesophyll interior tissue of leaves, which are the primary sites of photosynthesis. Each mesophyll cell can contain 30-40 chloroplasts. The chloroplast contains membranes and compartments that are crucial for its function.

Summary Equation for Photosynthesis

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

Stages of Photosynthesis

Photosynthesis consists of the Light Reactions and the Calvin Cycle.

Light Reactions

The light reactions split H2O, release O2, reduce NADP+ to NADPH, and generate ATP from ADP through photophosphorylation.

Calvin Cycle

The Calvin cycle involves the fixation of atmospheric CO2 and the reduction of the fixed carbon into carbohydrates.

Photosynthesis as a Redox Process

  1. Reverses the direction of electron flow compared to respiration.
  2. Water (H2O) is oxidized, and carbon dioxide (CO2) is reduced.
  3. It is an endergonic process, with energy provided by light.

Relationship Between Cellular Respiration and Photosynthesis

The summary equations for cellular respiration and photosynthesis are reversed chemical reactions; the products of one process are the reactants of the other.

Absorption and Action Spectra

Absorption Spectrum: The range of wavelengths a pigment can absorb.
Action Spectrum: The relative effectiveness of different wavelengths of radiation in driving a process. For chlorophyll a, the absorption spectrum indicates that violet-blue and red light are most effective for photosynthesis. The action spectrum shows that chlorophyll a is not 100% efficient.

Effective Wavelengths for Photosynthesis

Red and violet-blue light are most effective for photosynthesis.

Role of Carotenoids

Accessory pigments like carotenoids broaden the absorption spectrum for photosynthesis by absorbing excess light that could damage chlorophyll.

Photon Absorption by Chlorophyll

When chlorophyll a in solution absorbs photons, it can fluoresce. In an intact chloroplast, the absorbed light energy excites electrons, which are then transferred to a reaction center. If there is no reaction center, the electron returns to its ground state, releasing energy.

Photosystems and Electron Transport

Photosystem I and Photosystem II

  1. Photon Absorption: A photon strikes a pigment molecule.
  2. Energy Transfer: Energy is passed among pigment molecules until it excites P680 (in PSII) or P700 (in PSI).
  3. Electron Transfer: The excited electron from P680 is transferred to the primary electron acceptor, forming P680+.
  4. Water Splitting: P680+, a strong oxidizing agent, splits H2O. Electrons from hydrogen atoms reduce P680+ back to P680. O2 is released as a byproduct.
  5. Electron Transport Chain (ETC): Electrons move down the ETC from the primary electron acceptor of PSII to PSI. The energy released drives proton gradient formation across the thylakoid membrane.
  6. ATP Synthesis: Diffusion of H+ across the membrane drives ATP synthesis (photophosphorylation).
  7. PSI Excitation: Light energy excites P700 in PSI, which loses an electron to an electron acceptor. P700+ accepts an electron from the ETC of PSII.
  8. NADPH Formation: Electrons from PSI fall down another ETC to ferredoxin, then are transferred to NADP+, reducing it to NADPH.
  9. Calvin Cycle Input: The electrons in NADPH are available for the Calvin cycle. This process also removes H+ from the stroma.

Cyclic Electron Flow

  1. Uses PSI only.
  2. Produces ATP but not NADPH.
  3. No oxygen is released.
  4. Generates surplus ATP, satisfying the higher demand in the Calvin cycle.

Non-Cyclic Electron Flow

  1. Begins in PSII.
  2. Electrons are derived from water splitting.
  3. Electrons are donated to the ETC, leading to H+ pumping and ATP synthesis.
  4. Electrons are passed to PSI.
  5. PSI is excited by light absorption.
  6. Electrons are used to reduce NADP+ to NADPH.

Chemiosmosis in Mitochondria and Chloroplasts

Similarities and Differences

Oxidative phosphorylation occurs in mitochondria, while photophosphorylation occurs in chloroplasts. Both are highly efficient energy-conserving processes that utilize chemiosmosis. Oxidative phosphorylation involves the reduction of O2 to H2O, using electrons from NADH and FADH2, and can occur in light or darkness. Photophosphorylation involves the oxidation of H2O to O2, with NADP+ as the electron acceptor, and is absolutely dependent on light.

Summary of Light Reactions and Location

Equation: 12H2O + 12NADP+ + 18ADP + 18Pi → 6CO2 + 12NADPH + 18ATP
Location: Occurs on the thylakoid membranes within the chloroplast.

Calvin Cycle

Carbon Fixation and Sugar Production

  1. Carbon enters the cycle as CO2 and leaves as glyceraldehyde 3-phosphate (G3P), a three-carbon sugar.
  2. For the net synthesis of one G3P molecule, the cycle must turn three times, fixing three molecules of CO2.

Phases of the Calvin Cycle

  1. Carbon Fixation
  2. Reduction
  3. Regeneration of the CO2 acceptor (RuBP)

Roles of ATP and NADPH

The Calvin cycle uses ATP and the reducing power of NADPH to build sugar from smaller molecules.

Rubisco and Photorespiration

When oxygen concentration is much higher than carbon dioxide concentration, Rubisco can add O2 instead of CO2 to RuBP, initiating photorespiration. This process consumes O2 and organic fuel, releasing CO2 without producing ATP or sugar. It may be an evolutionary relic from a time when the atmosphere had less O2 and more CO2.

Photosynthetic Adaptations

C4 Plants

  1. Minimize photorespiration by initially incorporating CO2 into four-carbon compounds in mesophyll cells, using the enzyme PEP carboxylase.
  2. PEP carboxylase has a higher affinity for CO2 than Rubisco, allowing it to fix CO2 even at low concentrations.

Fate of Photosynthetic Products

Glucose produced during photosynthesis can be converted into starch for storage or combined with fructose to form sucrose for transport.

Cell Signaling

Basic Signal-Transduction Pathway

A signal transduction pathway converts a signal on a cell’s surface into a specific cellular response. These pathways suggest that ancestral signaling molecules evolved in prokaryotes and were later modified in eukaryotes.

Types of Signaling

Paracrine Signaling

A form of cell signaling where the target cell is near the signal-releasing cell. Examples include responses to allergens, tissue repair, and scar tissue formation.

Local Regulation

Messenger molecules that travel only short distances. Hormones are not local regulators as they travel via the bloodstream.

Hormone Travel

In humans and animals, hormones are produced by endocrine glands and released directly into the bloodstream, which carries them to target cells.

Stages of Cell Signaling

  1. Reception: A signal molecule (ligand) binds specifically to a receptor protein, often causing a shape change. Most signal receptors are plasma membrane proteins.
  2. Transduction: A cascade of molecular interactions relays the signal from the receptor to target molecules. This often involves multiple steps, amplifying the signal and providing opportunities for regulation.
  3. Response: The cell’s response to the signal, which can involve regulating transcription or cytoplasmic activities.

Ligands and Receptors

A ligand is a substance that binds specifically to a cell receptor. This binding causes a conformational change in the receptor, initiating a specific reaction. Signal transduction amplifies the original signal by allowing the activated receptor to bind to multiple relay molecules.

Signal Receptor Location

Signal receptors can be located in the plasma membrane or within the cell (intracellular receptors).

Types of Receptors

  • G-Protein-linked Receptor: A plasma membrane receptor that activates a G protein upon ligand binding.
  • Tyrosine-Kinase Receptor: Receptor proteins with enzymatic activity that can trigger multiple pathways simultaneously.
  • Ligand-gated Ion Channel: A protein channel that opens or closes in response to a ligand, allowing or blocking ion flow.

Signal Amplification and Cellular Response

Multistep pathways amplify a small number of extracellular signaling molecules into a large cellular response. The original signal molecule does not need to enter the cell; its binding to a surface receptor initiates a cascade of events within the cell, leading to the response.

Signal Transduction in Cytoplasm and Nucleus

Signal information is transduced through a pathway of reactions where one protein activates another, ultimately reaching a target molecule in the cytoplasm or nucleus to elicit a cellular response.

Signal Amplification Mechanisms

Signal amplification occurs when a receptor binds to multiple proteins, each of which continues down a metabolic pathway, magnifying the initial signal.

Differential Cell Response

Different cell types can respond differently to the same signal molecule due to differential gene expression.

Apoptosis

Role in Development and Disease

Apoptosis, or programmed cell death, is carried out by caspases. It can be triggered by various factors, including death-signaling ligands, DNA damage, or protein misfolding. Apoptosis is essential for animal development and maintenance. Dysregulation of apoptosis may contribute to diseases like Parkinson’s, Alzheimer’s, and cancer.

Apoptosis Process

Apoptosis prevents cellular contents from damaging neighboring cells by packaging the dying cell into vesicles for digestion by scavenger cells.

Cell Division

Reproduction (Meiosis)

Meiosis is a special type of cell division that produces genetically non-identical daughter cells (gametes) with half the number of chromosomes as the parent cell. This ensures haploid cells for sexual reproduction and introduces genetic variation.

Growth and Repair (Mitosis)

Mitosis produces genetically identical daughter cells, providing more cells for growth, tissue repair, or asexual reproduction.

Eukaryotic Genomes and Cell Division

Prokaryotic vs. Eukaryotic Genomes

Prokaryotic genomes are typically single, circular DNA molecules. Eukaryotic genomes are packaged into linear chromosomes, consisting of DNA and protein (chromatin), which condense during cell division. Each eukaryotic species has a characteristic number of chromosomes.

Chromosome Structure

Duplicated chromosomes consist of two sister chromatids attached at the centromere. DNA replication occurs during the S phase of interphase.

Eukaryotic Cell Division

Consists of Mitosis (nuclear division) and Cytokinesis (cytoplasmic division). Meiosis is a variation producing haploid gametes.

Cell Cycle

Chromosome Number Changes

Human somatic cells are diploid (2n = 46), containing two sets of chromosomes. Gametes are haploid (n = 23), containing one set. Fertilization restores the diploid number.

Phases of the Cell Cycle

The cell cycle includes Interphase (G1, S, G2) and Cell Division (Mitosis/Meiosis and Cytokinesis). DNA replication occurs during the S phase. Control of cell division involves surface-volume relationships, chemical signaling, and contact inhibition.

Mitotic Spindle Apparatus

Components

  • Centrosomes: Microtubule organizing centers.
  • Kinetochore Microtubules: Attach to kinetochores on chromosomes.
  • Nonkinetochore Microtubules: Overlap and push poles apart.
  • Asters: Radiate from centrosomes.
  • Centrioles: Located within centrosomes (in animal cells).

Spindle Changes During Mitosis

During mitosis, the spindle apparatus assembles, attaches to chromosomes, and moves them to the metaphase plate. Sister chromatids separate during anaphase and move to opposite poles. Nonkinetochore microtubules elongate the cell.

Chromosome Movement Mechanisms

Motor proteins associated with kinetochore microtubules move chromosomes poleward as microtubules depolymerize. Motor proteins at the spindle poles may also”reel i” chromosomes.

Cytokinesis and Binary Fission

Cytokinesis in Animals and Plants

Cytokinesis differs between animal and plant cells, involving different mechanisms for dividing the cytoplasm.

Binary Fission in Bacteria

In bacteria, binary fission involves replication of the circular chromosome and cell division. Eukaryotic mitosis may have evolved from these mechanisms.

Cell Cycle Control System

Checkpoints, Cyclins, and CDKs

The cell cycle is regulated by checkpoints that ensure proper progression. Cyclins and cyclin-dependent kinases (CDKs) are key regulatory molecules. MPF (Maturation-Promoting Factor) is a critical cyclin-CDK complex.

Internal and External Factors

Internal signals (e.g., chromosome attachment to spindle) and external signals (e.g., growth factors like PDGF) influence cell cycle progression through checkpoints.

Cancer and Cell Division

Abnormal Cell Division

Cancer cells often escape normal cell cycle controls, exhibiting uncontrolled proliferation, loss of density-dependent inhibition, and reduced dependence on growth factors.

Tumor Types

  • Benign Tumors: Grow locally and do not invade or metastasize.
  • Malignant Tumors: Can invade surrounding tissues and metastasize to distant sites.

Inheritance of Traits

Transmission of Traits

Traits are transmitted from parents to offspring through genes, segments of DNA located on chromosomes. Genes are passed via gametes.

Genes and Heredity

Genes are the units of heredity. They determine an organism’s traits. Variation arises from differences in gene nucleotide sequences.

Reproduction Modes

  • Asexual Reproduction: One parent passes genes to offspring without gamete fusion.
  • Sexual Reproduction: Two parents contribute genes, leading to unique combinations in offspring.

Genetic Terminology

  • Somatic Cells: Diploid (2n), containing 23 pairs of chromosomes.
  • Gametes: Haploid (n), containing 23 chromosomes.
  • Karyotype: An ordered display of chromosome pairs.
  • Homologous Chromosomes: Pairs of chromosomes with the same genes at the same loci.

Chromosome Number and Life Cycles

Diploid and Haploid Cells

Diploid cells (2n) have two sets of chromosomes; haploid cells (n) have one set. Human gametes are haploid (n=23).

Meiosis and Fertilization

Meiosis and fertilization alternate in sexual life cycles to maintain a constant chromosome number across generations. Meiosis produces haploid gametes, and fertilization restores the diploid state.

Eukaryotic Life-Cycle Patterns

  • Haploid Life Cycle: Organism is mostly haploid; only the zygote is diploid (e.g., Spirogyra).
  • Diploid Life Cycle: Organism is mostly diploid; gametes are haploid (e.g., humans).
  • Haplo-diploid Life Cycle: Alternation between haploid and diploid phases (common in plants and fungi).

Meiosis I and Meiosis II

Meiosis I

  • Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis) forming tetrads, and crossing over occurs at chiasmata.
  • Metaphase I: Tetrads align at the metaphase plate; homologous chromosomes are attached to spindle fibers from opposite poles.
  • Anaphase I: Homologous chromosomes separate and move to opposite poles; sister chromatids remain attached.
  • Telophase I & Cytokinesis: Two haploid daughter cells are formed, each with chromosomes still composed of two sister chromatids.

Meiosis II

  • Prophase II: Spindle apparatus forms; chromosomes move toward the metaphase plate.
  • Metaphase II: Sister chromatids align at the metaphase plate.
  • Anaphase II: Sister chromatids separate and move to opposite poles.
  • Telophase II & Cytokinesis: Nuclei reform, and daughter cells complete division, resulting in four haploid cells.

Synapsis and Crossing Over

During Prophase I, synapsis allows homologous chromosomes to pair, facilitating crossing over, the exchange of genetic material between non-sister chromatids.

Sources of Genetic Variation

  • Independent Assortment: Random alignment of homologous chromosome pairs during Metaphase I.
  • Crossing Over: Exchange of genetic material between non-sister chromatids during Prophase I.
  • Random Fertilization: Any sperm can fertilize any egg.

Mendelian Genetics

Mendel’s Particulate Mechanism

Mendel’s work proposed that traits are inherited as discrete units (genes), differing from the blending theory of inheritance.

Key Genetic Terms

  • True Breeding: Plants that produce identical offspring when self-pollinated.
  • Hybridization: Mating of two true-breeding varieties.
  • P Generation: Parental generation.
  • F1 Generation: First filial generation (offspring of P).
  • F2 Generation: Second filial generation (offspring of F1).
  • Genotype: Genetic makeup.
  • Phenotype: Observable traits.
  • Alleles: Alternative versions of a gene.
  • Homozygous: Having two identical alleles for a trait.
  • Heterozygous: Having two different alleles for a trait.
  • Dominant Allele: Masks the effect of a recessive allele.
  • Recessive Allele: Masked by a dominant allele.
  • Monohybrid Cross: Cross involving one trait.
  • Dihybrid Cross: Cross involving two traits.

Law of Segregation

Allele pairs segregate during gamete formation, so each gamete receives only one allele from each pair. This results in a 3:1 phenotypic ratio in the F2 generation of a monohybrid cross.

Test-Cross

A test-cross with a homozygous recessive individual determines if an individual with a dominant phenotype is homozygous or heterozygous.

Probability in Genetics

The multiplication rule calculates the probability of independent events occurring together. The addition rule calculates the probability of mutually exclusive events occurring.

Law of Independent Assortment

Genes for different traits segregate independently during gamete formation. This is explained by the random orientation of homologous chromosome pairs during Metaphase I of meiosis.

Dihybrid Cross Predictions

Punnett squares can predict the genotypic and phenotypic ratios of offspring from dihybrid crosses.

Importance of Sample Size

Large sample sizes are crucial in genetic studies to ensure that observed results align with probabilistic predictions.

Genetics Vocabulary

  • Genetics: The study of heredity and variation.
  • Genes: Segments of DNA that specify proteins and control inherited traits.
  • Heredity: The transmission of genes from parents to offspring.
  • Variation: Differences in gene nucleotide sequences among individuals.
  • Chromosome: A linear DNA molecule in eukaryotes, containing genes, centromeres, and telomeres.
  • Locus: The physical location of a gene on a chromosome.
  • Karyotype: An ordered display of chromosomes.
  • Homologous Chromosomes: Chromosome pairs with the same genes at the same loci.
  • Haploid: Cell with one set of chromosomes (n).
  • Diploid: Cell with two sets of chromosomes (2n).
  • Autosome: Non-sex chromosomes.
  • Sex Chromosomes: Determine sex (X and Y in humans).