Photosynthesis and Cellular Reproduction: Study Guide (Chapters 7 & 8)

Photosynthesis: Using Light to Make Food (Chapter 7)

7.1 Photosynthesis Powers Most Life on Earth

Photosynthesis is the process by which light energy is converted into chemical energy, sustaining most life on Earth. Organisms are classified based on how they obtain energy and carbon.

• Chemotrophs: Organisms that obtain energy by oxidizing inorganic or organic chemicals rather than from light.

• Autotrophs: Organisms that produce their own food from inorganic substances (e.g., CO2), typically using light or chemical energy.

• Photoautotrophs: Autotrophs that use light energy to synthesize organic compounds from CO2 and water. Example: Plants, algae, and some bacteria.

• Heterotrophs: Organisms that obtain organic food molecules by eating other organisms or their by-products.

7.2 Photosynthesis Occurs in Chloroplasts in Plant Cells

Photosynthesis takes place in specialized organelles called chloroplasts, primarily within the mesophyll cells of leaves.

• Chloroplasts: Double-membraned organelles in plant cells where photosynthesis occurs.

• Stroma: The thick fluid inside the chloroplast where the Calvin cycle (sugar synthesis) takes place.

• Mesophyll: Leaf cells specialized for photosynthesis; chloroplasts are concentrated here.

• Thylakoids: Membranous sacs within the chloroplast, containing chlorophyll; arranged in stacks called grana (singular: granum).

• Thylakoid Membrane: Site of the light reactions of photosynthesis.

• Stomata: Tiny pores on the leaf surface through which CO2 enters and O2 exits.

Example: In a typical plant cell, the chloroplast contains an outer membrane, an inner membrane, stroma, thylakoid stacks (grana), and thylakoid membranes where light reactions occur.

7.4 Photosynthesis is a Redox Process

Photosynthesis involves redox (reduction-oxidation) reactions, where electrons are transferred between molecules.

• Redox Reaction: A chemical reaction involving the transfer of electrons from one substance to another.

• In photosynthesis, water is oxidized (loses electrons) and carbon dioxide is reduced (gains electrons).

Equation:

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

7.5 Photosynthesis Occurs in Two Stages, Linked by ATP and NADPH

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle (dark reactions).

• Light Reactions: Occur in the thylakoid membranes; convert solar energy to chemical energy, producing ATP and NADPH, and releasing O2.

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to synthesize sugars from CO2.

• Products of Light Reactions: ATP, NADPH, O2

7.6 Visible Radiation Absorbed by Pigments Drives the Light Reactions

Light energy is absorbed by pigments, initiating the light reactions.

• Electromagnetic Spectrum: The entire range of electromagnetic radiation, from gamma rays to radio waves; visible light is the portion plants use for photosynthesis.

7.7 Photosystems Capture Solar Energy

Photosystems are complexes of proteins and pigments that capture light energy.

• Reaction Center: The core of a photosystem, containing a special pair of chlorophyll a molecules that initiate electron transfer.

7.9 The Light Reactions Take Place Within the Thylakoid Membranes

The light reactions involve electron transport and ATP synthesis.

• ATP Synthase: An enzyme embedded in the thylakoid membrane that synthesizes ATP from ADP and inorganic phosphate as protons flow through it.

• Electrons lost from the reaction center of Photosystem II are replaced by electrons from water, producing O2 as a by-product.

Equation for ATP Synthesis:

$\mathrm{ADP} + \mathrm{P}_i \xrightarrow{\text{ATP synthase}} \mathrm{ATP}$

The Cellular Basis of Reproduction and Inheritance (Chapter 8)

8.1 Cell Division

Cell division is essential for growth, repair, and reproduction. Organisms reproduce either asexually or sexually.

• Asexual Reproduction: A single parent produces genetically identical offspring; only one individual is required. Example: Binary fission in bacteria.

• Sexual Reproduction: Two parents contribute genetic material to produce genetically unique offspring.

• Except for identical twins, siblings from the same parents are similar but not identical due to genetic recombination.

8.3 Eukaryotic vs. Prokaryotic Cells

Eukaryotic cells differ from prokaryotic cells in several key ways.

• Eukaryotic Cells: Larger, more complex, contain many more genes, and have a membrane-enclosed nucleus.

• Sister Chromatids: Identical copies of a chromosome, joined together at a region called the centromere.

8.4 The Eukaryotic Cell Cycle

Eukaryotic cells spend most of their life in interphase, during which they grow and replicate DNA.

• Interphase: The phase where the cell grows, performs normal functions, and duplicates its DNA in preparation for division.

8.5 Mitosis: Key Stages

Mitosis is the process of nuclear division in eukaryotic cells, consisting of several stages.

• Prophase: The mitotic spindle begins to form.

• Metaphase: Chromosomes align at the cell's equator, attached to spindle fibers.

• Telophase: The nuclear envelope re-forms around the separated chromosomes.

8.6 Plant vs. Animal Cell Division

Plant and animal cells divide differently due to structural differences.

• Plant Cells: Form a cell plate during cytokinesis, which develops into a new cell wall.

• Animal Cells: Lack cell walls; cytokinesis occurs via cleavage furrow formation.

8.7 Regulation of Cell Division

Cell division is regulated by physical and chemical factors.

• Density-Dependent Inhibition: The phenomenon where crowded cells stop dividing; a key factor is the availability of growth factors and space.

8.11 Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes carrying genes for the same traits.

• Homologous Chromosomes: Two chromosomes in a nucleus that carry genes controlling the same inherited characteristics.

8.18 Nondisjunction

Nondisjunction is an error in cell division that leads to abnormal chromosome numbers.

• Nondisjunction: The failure of chromosome pairs (sister chromatids or homologous chromosomes) to separate properly during anaphase of meiosis or mitosis.

• This can result in gametes or cells with extra or missing chromosomes, leading to genetic disorders.

Table: Comparison of Asexual and Sexual Reproduction

Feature	Asexual Reproduction	Sexual Reproduction
Number of Parents	One	Two
Genetic Variation	Offspring identical to parent	Offspring genetically unique
Examples	Bacteria (binary fission), some plants	Animals, flowering plants

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard biology textbooks.