Cell Structure, Metabolism, Cellular Respiration, and Photosynthesis: Study Guide

Inside the Cell

Prokaryotic Cell Structure and Function

Prokaryotic cells, such as bacteria and archaea, lack a membrane-bound nucleus and organelles. Their internal structure is simpler compared to eukaryotic cells.

• Cell Wall: Provides structural support and protection; composed of peptidoglycan in bacteria.

• Plasma Membrane: Controls the movement of substances in and out of the cell.

• Cytoplasm: Gel-like substance where cellular processes occur.

• Ribosomes: Sites of protein synthesis; smaller than eukaryotic ribosomes.

• Nucleoid: Region containing the cell's DNA; not enclosed by a membrane.

• Flagella: Used for movement.

• Pili: Used for attachment and sometimes for DNA transfer.

Example: Escherichia coli uses flagella for motility and pili for conjugation.

Eukaryotic Cell Structure and Function

Eukaryotic cells have compartmentalized organelles, each with specialized functions, allowing for greater complexity and efficiency.

• Nucleus: Contains genetic material; site of transcription.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Mitochondria: Site of cellular respiration and ATP production.

• Lysosomes: Digestive organelles; recycle cellular material.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Chloroplasts: (in plants) Site of photosynthesis.

• Cytoskeleton: Provides structural support, facilitates movement, and organizes cell contents.

Benefits of Compartmentalization: Increases efficiency by separating incompatible reactions and concentrating substrates and enzymes.

Integration of Organelle Functions

Cellular activities depend on the coordinated function of organelles. For example, protein synthesis involves the nucleus, ER, Golgi, and ribosomes.

• Dominant Organelles: Cells with high energy demands (e.g., muscle cells) have abundant mitochondria; cells involved in secretion (e.g., pancreatic cells) have extensive ER and Golgi.

Lysosomal Recycling Pathways

Lysosomes recycle material via autophagy (self-digestion), phagocytosis (engulfing external particles), and endocytosis (internalizing substances).

Eukaryotic Cytoskeleton Components

The cytoskeleton consists of three major components:

• Microfilaments (Actin Filaments): Support cell shape and enable movement.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Facilitate intracellular transport and cell division.

Metabolism

Energy Transformation in Chemical Reactions

Energy is transformed during chemical reactions, either released (exergonic) or absorbed (endergonic).

• Kinetic Energy: Released when bonds are broken.

• Exergonic Reaction: Releases energy; spontaneous.

• Endergonic Reaction: Requires energy input; nonspontaneous.

Equation:

$\Delta G = \Delta H - T\Delta S$

Where $\Delta G$ is Gibbs free energy, $\Delta H$ is enthalpy, $T$ is temperature, and $\Delta S$ is entropy.

Conditions Affecting Reaction Rate

• Temperature

• Concentration of reactants

• Presence of catalysts (enzymes)

Energetic Coupling and Redox Reactions

Energetic coupling uses exergonic reactions to drive endergonic ones, often via ATP or redox reactions.

• Redox Reactions: Transfer electrons; couple energy release and absorption.

Equation:

$\text{Oxidation:} \quad \text{A} \rightarrow \text{A}^+ + e^-$

$\text{Reduction:} \quad \text{B} + e^- \rightarrow \text{B}^-$

ATP and Energetic Coupling

ATP hydrolysis releases energy to drive endergonic reactions.

Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$

Enzyme Structure and Function

Enzymes are biological catalysts; their structure determines substrate specificity and catalytic efficiency.

• Active Site: Region where substrate binds.

• Cofactors: Inorganic ions required for activity.

• Coenzymes: Organic molecules assisting enzymes.

Regulation of Enzyme Activity

• Allosteric Regulation: Binding of molecules at sites other than the active site.

• Competitive Inhibition: Inhibitor binds to active site.

• Environmental Factors: pH, temperature, and ionic strength affect enzyme activity.

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes that convert glucose into ATP. It consists of four interconnected stages:

• Glycolysis: Glucose is converted to pyruvate in the cytoplasm.

• Pyruvate Oxidation: Pyruvate is converted to acetyl CoA in mitochondria.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA is oxidized, producing NADH and FADH2.

• Electron Transport Chain and Oxidative Phosphorylation: Electrons are transferred to oxygen, generating ATP.

Inputs and Outputs

Process	Inputs	Outputs
Glycolysis	Glucose, 2 ATP, 2 NAD+	2 Pyruvate, 4 ATP (net 2), 2 NADH
Pyruvate Oxidation	2 Pyruvate, 2 NAD+, 2 CoA	2 Acetyl CoA, 2 NADH, 2 CO2
Citric Acid Cycle	2 Acetyl CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP
ETC & Oxidative Phosphorylation	NADH, FADH2, O2, ADP	ATP, H2O, NAD+, FAD

Regulation of Cellular Respiration

• Feedback inhibition regulates glycolysis and citric acid cycle.

• ATP and NADH act as inhibitors; ADP and NAD+ as activators.

Aerobic vs. Anaerobic Respiration

• Aerobic: Uses oxygen; produces more ATP.

• Anaerobic: Does not use oxygen; less ATP produced.

Fermentation

Fermentation allows ATP production without oxygen, yielding less ATP than cellular respiration.

Process	Inputs	Outputs	ATP Yield
Cellular Respiration	Glucose, O2	CO2, H2O, ATP	~30-32 ATP
Fermentation	Glucose	Lactate or Ethanol, ATP	2 ATP

Photosynthesis

Overview of Photosynthesis

Photosynthesis converts light energy into chemical energy in plants, algae, and some bacteria. It consists of light-capturing reactions and the Calvin cycle.

• Light Reactions: Occur in thylakoid membranes; produce ATP and NADPH.

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

Pigments and Light Energy

• Pigments: Molecules like chlorophyll absorb light energy.

• Antenna Pigments: Transfer energy to reaction center.

Photosystem II and ATP Production

Photosystem II uses light energy to split water, release oxygen, and generate ATP via electron transport.

Photosystem I and NADPH Production

Photosystem I uses light energy to produce NADPH.

Z-Scheme and Electron Flow

The Z-scheme describes the interaction between photosystem I and II, linking electron flow and energy production.

• Noncyclic Electron Flow: Produces both ATP and NADPH.

• Cyclic Electron Flow: Produces only ATP.

Carbon Fixation Pathways

• C3 Pathway: Most common; susceptible to photorespiration.

• C4 Pathway: Reduces photorespiration; spatial separation of steps.

• CAM Pathway: Temporal separation; adaptation to arid environments.

Calvin Cycle Phases

• Carbon fixation

• Reduction

• Regeneration of RuBP

Regulation of Photosynthesis

• Light intensity, CO2 concentration, and temperature affect photosynthetic rate.

Comparison Table: C3, C4, and CAM Pathways

Pathway	CO2 Fixation	Photorespiration	Adaptation
C3	Direct via Rubisco	High	Temperate climates
C4	PEP carboxylase, then Rubisco	Low	Hot, sunny climates
CAM	PEP carboxylase at night	Low	Arid climates

Example: Zea mays (corn) uses the C4 pathway; Opuntia (cactus) uses CAM.