Energy, Cellular Respiration, and Photosynthesis: Study Guide

Energy and the Working Cell

Types of Energy and Conservation

Energy is the capacity to do work. In biological systems, energy exists in several forms, and the law of conservation of energy states that energy cannot be created or destroyed, only transformed.

• Kinetic Energy: The energy of motion. Example: A moving muscle or flowing water.

• Potential Energy: Stored energy due to position or structure. Example: Chemical bonds in glucose.

• Chemical Energy: A form of potential energy stored in chemical bonds of molecules.

• Heat (Thermal Energy): Energy associated with the random movement of atoms and molecules.

Law of Conservation of Energy: Energy can be transferred and transformed, but it cannot be created or destroyed.

ATP: The Cell's Energy Currency

ATP (adenosine triphosphate) is the primary energy carrier in cells. It stores energy in its high-energy phosphate bonds.

• Structure: Composed of adenine, ribose, and three phosphate groups.

• Energy Storage: Energy is stored in the bonds between phosphate groups, especially the terminal phosphate bond.

• Cellular Work: Cells use ATP to power mechanical, transport, and chemical work by transferring a phosphate group to other molecules (phosphorylation).

Equation:

$\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \text{energy}$

Enzymes and Enzyme Inhibition

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Function: Enzymes bind specific substrates at their active site, facilitating chemical reactions.

• Enzyme Inhibitors: Molecules that decrease enzyme activity. They can be competitive (bind active site) or noncompetitive (bind elsewhere, changing enzyme shape).

Example: Many drugs and toxins act as enzyme inhibitors.

Cell Membrane Structure and Function

The cell membrane is a selectively permeable barrier composed mainly of a phospholipid bilayer with embedded proteins.

• Structure: Phospholipid bilayer with hydrophilic heads and hydrophobic tails; proteins serve as channels, receptors, or enzymes.

• Function: Regulates entry and exit of substances, communication, and cell recognition.

Transport Across Membranes

• Diffusion: Movement of molecules from high to low concentration. Small, nonpolar molecules (e.g., O2, CO2) cross easily.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Passive transport using membrane proteins for molecules that cannot diffuse directly.

• Active Transport: Movement of substances against their concentration gradient using energy (ATP).

• Endocytosis: Process by which cells engulf large particles or liquids by forming vesicles.

• Exocytosis: Process by which cells expel materials using vesicles that fuse with the membrane.

Cellular Respiration

Autotrophs vs. Heterotrophs

• Autotrophs: Organisms that produce their own food (e.g., plants via photosynthesis).

• Heterotrophs: Organisms that obtain food by consuming other organisms (e.g., animals, fungi).

Overview of Photosynthesis and Cellular Respiration

These two processes are interconnected in the cycling of energy and matter in ecosystems.

• Photosynthesis: Converts light energy, CO2, and H2O into glucose and O2.

• Cellular Respiration: Breaks down glucose and O2 to produce ATP, CO2, and H2O.

Simplified Equations:

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

Cellular Respiration: $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2 \rightarrow 6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{ATP}$

Stages of Cellular Respiration

Cellular respiration occurs in three main stages, each with specific inputs, outputs, and cellular locations.

Stage	Location	Inputs	Outputs
Glycolysis	Cytoplasm	Glucose, 2 ATP, 2 NAD+	2 Pyruvate, 4 ATP (net 2), 2 NADH
Citric Acid Cycle (Krebs Cycle)	Mitochondrial matrix	2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP
Electron Transport Chain (ETC)	Inner mitochondrial membrane	NADH, FADH2, O2	H2O, ~28 ATP

Role of Oxygen

• Oxygen is the final electron acceptor in the electron transport chain, allowing for the production of large amounts of ATP.

• Stages requiring oxygen: Citric Acid Cycle and Electron Transport Chain (aerobic stages).

• Glycolysis: Does not require oxygen (anaerobic).

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue when oxygen is unavailable.

• Occurs in: Cytoplasm

• Products: Lactic acid (in animals) or ethanol and CO2 (in yeast)

• Purpose: Regenerates NAD+ for glycolysis

Photosynthesis

Leaf Structure and Function

Leaves are specialized organs for photosynthesis, containing various cell types with distinct roles.

• Epidermis: Protective outer layer

• Mesophyll: Main site of photosynthesis, rich in chloroplasts

• Stomata: Pores for gas exchange (CO2 in, O2 out)

• Veins: Transport water and nutrients

Chloroplast Structure

• Outer and Inner Membranes: Enclose the organelle

• Stroma: Fluid-filled space where the Calvin Cycle occurs

• Thylakoids: Membranous sacs containing chlorophyll; site of light reactions

• Grana: Stacks of thylakoids

Chlorophylls and Light Absorption

• Chlorophylls: Pigments that absorb light energy, mainly blue and red wavelengths; reflect green light.

• Function: Absorb light energy to drive photosynthesis.

• When chlorophyll absorbs light: Electrons are excited to higher energy states, initiating the light reactions.

Role of Water and Carbon Dioxide

• Water (H2O): Split during the light reactions to provide electrons and protons; releases O2 as a byproduct.

• Carbon Dioxide (CO2): Used during the Calvin Cycle to build glucose.

Stages of Photosynthesis

Stage	Location	Inputs	Outputs
Light Reactions	Thylakoid membrane	Light, H2O, NADP+, ADP	O2, ATP, NADPH
Calvin Cycle (Dark Reactions)	Stroma	CO2, ATP, NADPH	Glucose (G3P), NADP+, ADP

Calvin Cycle Product and Rounds

• Final Product: G3P (glyceraldehyde-3-phosphate), which can be used to form glucose and other organic molecules.

• Number of Rounds: Three turns of the Calvin Cycle are required to produce one G3P; six turns are needed to form one glucose molecule.