General Biology Study Notes: The Working Cell, Cellular Respiration, and Photosynthesis

The Working Cell

Energy Concepts

Cells require energy to perform work, maintain structure, and support life processes. Energy exists in various forms and is governed by physical laws.

• Energy: The capacity to do work or cause change.

• Kinetic Energy: Energy of motion (e.g., moving objects).

• Potential Energy: Stored energy due to position or structure (e.g., chemical bonds).

• Conservation of Energy: Energy cannot be created or destroyed, only transformed.

• Entropy: Measure of disorder; energy transformations increase entropy.

Example: A car and a human both use fuel (gasoline or food) to perform work, converting chemical energy into kinetic energy and heat.

ATP and Cellular Work

ATP (adenosine triphosphate) is the primary energy currency of the cell, enabling cellular work by transferring energy.

• ATP Structure: Adenosine + 3 phosphate groups.

• ATP Function: Stores and releases energy for cellular processes.

• ATP Cycle: ATP is regenerated from ADP + Pi via cellular respiration.

• Energy Transfer: ATP transfers phosphate groups to molecules, driving cellular work (e.g., muscle contraction, synthesis).

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

Example: ATP powers active transport, muscle contraction, and biosynthesis.

Enzymes

Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed.

• Activation Energy (EA): Energy needed to start a reaction by breaking reactant bonds.

• Specificity: Enzymes act on specific substrates due to their 3D shape.

• Catalysis: Substrate binds to active site, enzyme changes shape, reaction occurs.

• Inhibition: Molecules can block or alter enzyme activity (e.g., drugs, toxins).

• Regulation: Enzyme activity is regulated to control metabolic pathways.

Example: Lactase breaks down lactose in milk; penicillin inhibits bacterial enzymes.

Membrane Function

Cell membranes regulate the movement of materials into and out of cells, maintaining homeostasis.

• Selective Permeability: Only certain molecules can pass based on size, charge, and properties.

• Passive Transport: Movement down concentration gradient; no energy required (e.g., diffusion, osmosis).

• Facilitated Diffusion: Proteins assist movement of molecules across membrane.

• Active Transport: Uses ATP to move molecules against gradient (e.g., sodium-potassium pump).

• Large Molecule Transport: Endocytosis (import), exocytosis (export), phagocytosis (engulfing particles).

Transport Type	Energy Required?	Direction	Example
Diffusion	No	High to low	O2 diffuses into cells
Osmosis	No	Water moves across membrane	Plant cells in hypotonic solution
Facilitated Diffusion	No	High to low	Glucose transport via protein
Active Transport	Yes (ATP)	Low to high	Sodium-potassium pump
Endocytosis/Exocytosis	Yes	Import/Export	Phagocytosis, neurotransmitter release

Cellular Respiration

Aerobic Harvest of Food Energy

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, using oxygen.

• Overall Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP)}$

• Three Main Stages:

  1. Glycolysis: Occurs in cytoplasm; breaks glucose into pyruvate; produces ATP and NADH; does not require oxygen.

  2. Citric Acid Cycle (Krebs Cycle): Occurs in mitochondria; completes breakdown of glucose; produces CO2, ATP, NADH, FADH2.

  3. Electron Transport Chain: Occurs in inner mitochondrial membrane; uses NADH and FADH2 to produce most ATP; oxygen is final electron acceptor.

Stage	Location	Main Inputs	Main Outputs
Glycolysis	Cytoplasm	Glucose, 2 ATP	2 Pyruvate, 2 NADH, 4 ATP (net 2 ATP)
Citric Acid Cycle	Mitochondria	Acetyl-CoA, NAD+, FAD	CO2, ATP, NADH, FADH2
Electron Transport Chain	Inner mitochondrial membrane	NADH, FADH2, O2	ATP, H2O

Fermentation: Anaerobic process (without oxygen) that allows glycolysis to continue; produces lactic acid or ethanol.

Photosynthesis

Using Light to Make Food

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

• Overall Equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Sites of Photosynthesis: Occurs in chloroplasts; chlorophyll absorbs light energy.

• Chloroplast Structure: Inner membrane encloses stroma; thylakoids are stacked into grana.

• Light Reactions: Occur in thylakoid membranes; convert light energy to ATP and NADPH; release O2.

• Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into glucose.

Process	Location	Main Inputs	Main Outputs
Light Reactions	Thylakoid membrane	Light, H2O	ATP, NADPH, O2
Calvin Cycle	Stroma	CO2, ATP, NADPH	Glucose, ADP, NADP+

Example: Photosynthesis in plants removes carbon dioxide from the air and produces oxygen, supporting life on Earth.

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