Biology Study Guide: Proteins, Lipids, Cell Structure, Metabolism, and Cellular Respiration

Protein Structure and Function

Amino Acid Structure

Amino acids are the building blocks of proteins, each consisting of a central carbon atom (the alpha carbon) bonded to four groups: an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a unique R group (side chain).

• Amino Group: Acts as a base, accepting protons.

• Carboxyl Group: Acts as an acid, donating protons.

• R Group: Determines the chemical properties and identity of the amino acid.

• Peptide Bond: Formed by dehydration synthesis between the carboxyl group of one amino acid and the amino group of another.

Example: Glycine has a hydrogen as its R group, making it the simplest amino acid.

Levels of Protein Structure

Proteins have four levels of structural organization, each contributing to their function.

• Primary Structure: Linear sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding into alpha helices and beta sheets, stabilized by hydrogen bonds.

• Tertiary Structure: Three-dimensional folding due to interactions among R groups (hydrophobic, ionic, hydrogen bonds, disulfide bridges).

• Quaternary Structure: Association of multiple polypeptide chains into a functional protein complex.

Example: Hemoglobin is a quaternary structure protein composed of four polypeptide subunits.

Diverse Roles of Proteins

Proteins perform a wide range of functions in cells:

• Enzymes: Catalyze biochemical reactions.

• Structural Proteins: Provide support (e.g., collagen, keratin).

• Transport Proteins: Move substances across membranes (e.g., hemoglobin, channel proteins).

• Signaling Proteins: Relay messages within and between cells (e.g., hormones).

• Defensive Proteins: Protect against disease (e.g., antibodies).

Additional info: Protein function depends on its shape, which is determined by its structure.

Lipids, Membranes, and the First Cells

Types of Lipids

Lipids are hydrophobic molecules essential for cell structure and energy storage. The three main types are:

• Fats (Triglycerides): Composed of glycerol and three fatty acids; used for energy storage.

• Phospholipids: Contain a glycerol backbone, two fatty acids, and a phosphate group; major component of cell membranes.

• Steroids: Characterized by a four-ring structure; include cholesterol and hormones.

Membrane Structure

Cell membranes are composed of a phospholipid bilayer with embedded proteins.

• Phospholipid Bilayer: Hydrophilic heads face outward, hydrophobic tails face inward.

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

• Peripheral Membrane Proteins: Attach to the membrane surface, often involved in cell signaling or structure.

Example: Channel proteins allow ions to pass through the membrane.

Osmosis and Tonicity

Osmosis is the movement of water across a semipermeable membrane. Tonicity describes the relative concentration of solutes:

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell.

• Isotonic Solution: Equal solute concentration; no net water movement.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell.

Direction of Water Movement: Water moves from areas of low solute concentration to high solute concentration.

Additional info: Osmosis is crucial for maintaining cell volume and function.

Inside the Cell

Plant and Animal Cell Structure

Both plant and animal cells share common organelles but have distinct features.

• Plant Cell: Contains cell wall, chloroplasts, central vacuole, nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and plasma membrane.

• Animal Cell: Contains nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and plasma membrane; lacks cell wall and chloroplasts.

Example: Plant cells have chloroplasts for photosynthesis; animal cells have lysosomes for digestion.

Labelled Cell Diagrams

Key organelles to label:

• Nucleus: Contains genetic material.

• Mitochondria: Site of cellular respiration.

• Chloroplast (plant only): Site of photosynthesis.

• Endoplasmic Reticulum: Protein and lipid synthesis.

• Golgi Apparatus: Protein modification and sorting.

• Cell Wall (plant only): Provides structural support.

• Central Vacuole (plant only): Stores water and nutrients.

• Lysosome (animal only): Digests cellular waste.

Additional info: Both cell types have plasma membranes that regulate entry and exit of substances.

Energy and Enzymes: An Introduction to Metabolism

Exergonic and Endergonic Reactions

Chemical reactions are classified based on energy changes:

• Exergonic Reaction: Releases energy; spontaneous.

• Endergonic Reaction: Requires energy input; non-spontaneous.

Equation:

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

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

Role of ATP

ATP (adenosine triphosphate) is the primary energy carrier in cells.

• Hydrolysis of ATP: Releases energy for cellular processes.

• ATP Cycle: ATP is regenerated from ADP and inorganic phosphate.

Equation:

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

Enzyme Function and Regulation

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

• Active Site: Region where substrate binds.

• Factors Affecting Enzyme Function: Temperature, pH, substrate concentration, inhibitors.

• Regulation: Allosteric regulation, feedback inhibition.

Example: Pepsin is an enzyme active in acidic conditions in the stomach.

Additional info: Enzyme specificity is determined by the shape of the active site.

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose to produce ATP. It consists of several stages:

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate.

• Pyruvate Processing: Converts pyruvate to acetyl-CoA in the mitochondria.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose; produces NADH and FADH2.

• Electron Transport Chain: Uses NADH and FADH2 to generate ATP via oxidative phosphorylation.

Fermentation: Occurs when oxygen is absent; regenerates NAD+ and produces lactic acid or ethanol.

Bird's Eye View of Pathways

Cellular respiration is a multi-step process that maximizes ATP production from glucose.

• Inputs: Glucose, oxygen.

• Outputs: ATP, carbon dioxide, water.

• Fermentation: Produces less ATP than aerobic respiration.

Additional info: The electron transport chain is located in the inner mitochondrial membrane.

Summary Table: Cellular Respiration vs. Fermentation

Process	Oxygen Required?	ATP Yield	End Products
Cellular Respiration	Yes	~30-32 ATP	CO2, H2O
Fermentation	No	2 ATP	Lactic acid or ethanol

Additional info: Fig. 9.2 and the chapter review provide a visual summary of these processes.