General Biology Study Guide: Photosynthesis, Cell Membranes, Cell Signaling, Cell Division, Mitosis & Meiosis, and Cancer

Photosynthesis

Definition and Overview

Photosynthesis is the process by which plants, algae, and certain bacteria convert light energy into chemical energy, producing carbohydrates from carbon dioxide and water. It is essential for life on Earth as it provides the primary energy source for most organisms.

• Light energy is captured by pigments (mainly chlorophyll).

• Water is oxidized, releasing oxygen.

• CO2 is reduced, forming sugars.

Overall equation:

$6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$

Light-Dependent Reactions

These reactions occur in the thylakoid membranes and require light to produce ATP and NADPH.

• Water is split (photolysis), releasing O2.

• Electrons move through the electron transport chain.

• ATP is produced via chemiosmosis (H+ gradient powers ATP synthase).

• NADP+ is reduced to NADPH.

Calvin Cycle (Light-Independent Reactions)

The Calvin Cycle uses ATP and NADPH to fix CO2 into carbohydrates. It occurs in the stroma of the chloroplast.

• Carbon fixation: CO2 is attached to RuBP by the enzyme Rubisco.

• Reduction: ATP and NADPH are used to convert 3-PGA to G3P.

• Regeneration: RuBP is regenerated for the cycle to continue.

To make one glucose: 6 CO2, 18 ATP, 12 NADPH are needed.

Accessory Pigments

Accessory pigments such as chlorophyll b and carotenoids absorb wavelengths of light that chlorophyll a cannot, broadening the spectrum of light used for photosynthesis.

• Chlorophyll a: absorbs blue and red light, reflects green.

• Carotenoids: absorb additional wavelengths, protect against photo-damage.

Cell Membranes

Structure of the Plasma Membrane

The plasma membrane is a fluid mosaic composed of phospholipids, proteins, and cholesterol. It acts as a selectively permeable barrier.

• Phospholipid bilayer: hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: integral (transmembrane) and peripheral.

• Cholesterol: modulates fluidity.

Factors Affecting Membrane Fluidity

• Temperature: Higher temperature increases fluidity; lower temperature decreases fluidity.

• Cholesterol: Prevents freezing at low temperature, prevents excess fluidity at high temperature.

• Saturated vs. unsaturated fatty acids: Unsaturated increase fluidity.

Membrane Proteins

• Transmembrane proteins: span the membrane, involved in transport and signaling.

• Peripheral proteins: attached to the surface, involved in shape and signaling.

Osmosis and Tonicity

Osmosis is the movement of water across a membrane from low solute concentration to high solute concentration.

Solution Type	Effect on Cell
Hypertonic	Water leaves cell; cell shrinks
Hypotonic	Water enters cell; cell swells (may burst in animal cells)
Isotonic	No net movement of water

Active vs. Passive Transport

• Passive transport: Down concentration gradient, no energy required (e.g., diffusion, osmosis, channels).

• Active transport: Against gradient, requires ATP (e.g., pumps).

Bulk Transport

• Phagocytosis: Engulfing solid particles.

• Pinocytosis: Engulfing fluid.

• Endocytosis: General term for uptake into cell.

• Exocytosis: Releasing contents outside cell.

Cell Signaling

Types of Chemical Signaling

• Autocrine: Cell signals itself.

• Paracrine: Signals nearby cells.

• Juxtacrine: Direct contact between cells.

• Endocrine: Hormones travel through bloodstream to distant cells.

What a Ligand Does

• Ligand binding causes receptor shape change, initiating a signal transduction pathway.

• Results in cellular responses (e.g., metabolism, growth).

Major Receptor Types

• Ion channel-linked receptors: Allow ions to flow across membrane.

• G-protein coupled receptors (GPCRs): Activate G-proteins, trigger second messengers.

• Protein kinase receptors: Add phosphate groups to proteins, activating cascades.

Second Messengers

Second messengers amplify signals inside the cell, leading to large cellular responses.

• cAMP

• IP3

• DAG

• Ca2+

MAPK Pathway

The MAPK pathway is a major signaling cascade controlling cell division.

• Growth factor binds receptor.

• Activates Ras protein (GTP-bound).

• Triggers MAPK cascade, leading to cell division.

Cell Division

Requirements for Cell Division

• Reproductive signal

• DNA replication

• Segregation of chromosomes

• Cytokinesis (division of cytoplasm)

Binary Fission (Prokaryotes)

• Single circular chromosome

• Replication starts at origin

• Cell elongates, DNA separates, membrane pinches, two identical cells form

Mitosis vs. Meiosis

Process	Result	Purpose
Mitosis	2 identical diploid cells	Growth, repair
Meiosis	4 genetically unique haploid cells	Sexual reproduction

Interphase

• G1: Cell grows, performs normal functions

• S: DNA replication

• G2: Cell prepares for division

Meiosis Mechanisms

• Synapsis: Homologous chromosomes pair up (Prophase I)

• Crossing Over: Exchange of DNA between homologs (Prophase I)

• Chiasmata: Physical X-shaped points where crossing over occurs

• Independent Assortment: Random alignment of homologs (Metaphase I)

Genetic Variation in Meiosis

• Crossing over

• Independent assortment

• Random fertilization

Chromosome Abnormalities

• Nondisjunction: Failure of homologs or sister chromatids to separate properly (can lead to trisomy or monosomy)

• Trisomy 21: Down syndrome

Cancer

Mutated Proto-Oncogenes

• Proto-oncogenes are "gas pedal" genes that promote cell growth.

• When mutated, they become oncogenes, causing uncontrolled growth.

Mutated Tumor Suppressor Genes

• Normally inhibit cell cycle or promote apoptosis.

• Mutation leads to loss of control, higher chance of forming tumors.

Cell Death: Necrosis vs. Apoptosis

• Necrosis: Accidental cell death, swelling and bursting.

• Apoptosis: Programmed cell death, cell fragments into pieces.

Cancer Treatments (Overview)

• Surgery

• Chemotherapy

• Radiation

• Targeted therapy (e.g., monoclonal antibodies, genetic profiling)

Cancer Summary Table

Gene Type	Normal Function	Effect of Mutation
Proto-oncogene	Promote cell growth	Uncontrolled growth (oncogene)
Tumor suppressor	Inhibit cell cycle, promote apoptosis	Loss of control, higher tumor risk

Additional info: These notes expand on the original outline by providing definitions, mechanisms, and examples for each topic, ensuring a self-contained study guide suitable for exam preparation in General Biology.