General Biology: Cell Structure, Membrane Function, Enzymes, Cellular Respiration, and Photosynthesis Study Guide

Cell Structure and Function

Major Eukaryotic Cell Organelles

Eukaryotic cells contain specialized structures called organelles, each performing distinct functions necessary for cellular life.

• Nucleus: Contains genetic material (DNA) and is the site of ribosomal RNA synthesis.

• Mitochondria: Site of ATP production via cellular respiration.

• Rough Endoplasmic Reticulum (RER): Synthesizes proteins, especially those destined for secretion or membrane insertion.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids and steroid hormones.

• Golgi Apparatus: Sorts and packages proteins for extracellular secretion.

Example: The insulin receptor, a transmembrane glycoprotein, is synthesized in the RER and processed in the Golgi apparatus before being transported to the cell membrane.

Cell Membrane Structure and Function

Fluid Mosaic Model

The cell membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Integral Membrane Proteins: Span the membrane and are involved in transport and signaling.

• Peripheral Membrane Proteins: Attached to the membrane surface, often involved in signaling or structural support.

• Glycoproteins and Glycolipids: Play roles in cell recognition and signaling.

• Cholesterol: Modulates membrane fluidity and stability.

• Diacylglycerol: A lipid component of the membrane.

Key Points:

• Unsaturated fatty acids increase membrane fluidity, especially at lower temperatures, by introducing kinks that prevent tight packing of lipids.

• Phospholipids can move laterally within the membrane, contributing to its fluid nature.

• Membrane proteins may be integral (spanning the membrane) or peripheral (attached to the surface).

Membrane Transport and Cell Processes

Types of Membrane Transport

Cells regulate the movement of molecules across membranes to maintain homeostasis.

• Facilitated Diffusion: Movement of molecules down their concentration gradient via transport proteins.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

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

Example: The sodium-potassium pump uses ATP to move Na+ and K+ ions against their gradients.

Enzymes and Metabolic Reactions

Enzyme Structure and Function

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

• Active Site: The region of the enzyme where substrate binding and catalysis occur.

• Competitive Inhibitors: Bind to the active site, blocking substrate access.

• Non-Competitive Inhibitors: Bind to an allosteric site, changing enzyme shape and reducing activity.

Enzyme Reaction Energy Profile:

• Enzymes lower the activation energy ($E_a$) required for reactions.

• They do not alter the overall free energy change ($riangle G$) of the reaction.

Equation:

• $E_a$ (with enzyme) < $E_a$ (without enzyme)

• $riangle G = G_{products} - G_{reactants}$

Example: ATP hydrolysis releases energy that can be coupled to drive other cellular reactions.

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose to produce ATP.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate, producing ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix; oxidizes acetyl-CoA to produce NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Occurs in the inner mitochondrial membrane; uses NADH and FADH2 to generate a proton gradient for ATP synthesis.

Key Enzymes in Glycolysis:

• Phosphofructokinase (PFK): Uses ATP to create fructose-1,6-bisphosphate.

• Phosphorylase Kinase (PK): Uses ATPs to break glucose into two G3P (glycerol-3-phosphate).

• Pyruvate Kinase (PK): Generates 2 ATPs per glucose by de-phosphorylating phosphoenolpyruvate (PEP).

ATP Yield:

• Approximately 32 ATP molecules are produced from the complete oxidation of one glucose molecule.

• Substrate-level phosphorylation accounts for about 12% of ATP formed during glycolysis.

Equation:

• $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

Example: In oxygen deprivation, muscle cells convert pyruvate to lactate, regenerating NAD+ for glycolysis.

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

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

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

Key Locations:

• Stroma: Site of the Calvin cycle.

• Thylakoid Membrane: Site of light reactions and proton gradient formation.

• Thylakoid Space: Accumulates protons during light reactions.

Equation:

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

Example: The proton gradient in the thylakoid space drives ATP synthesis during the light reactions.

Tables

Cell Membrane Components

Component	Letter
Integral Membrane Protein	B
Peripheral Membrane Protein	F
Glycoprotein	C
Glycolipid	A
Cholesterol	E
Diacylglycerol	D

Cellular Respiration: ATP Production

Process	ATP Yield
Glycolysis	2 ATP
Krebs Cycle (per glucose)	2 ATP
Electron Transport Chain	~28 ATP
Total	~32 ATP

Photosynthesis: Locations of Processes

Process	Location
Calvin Cycle	Stroma
Light Reactions	Thylakoid Membrane
Proton Gradient for ATP Synthesis	Thylakoid Space

Additional info: Some explanations and context have been expanded for clarity and completeness, including definitions and equations relevant to the topics.