General Biology: Cell Structure, Function, and Metabolism Study Guide

Cell Structure and Organelles

Protein Processing and Secretion

Cells contain specialized organelles that manage the synthesis, processing, and secretion of proteins. The Golgi apparatus is crucial for the final processing and packaging of proteins destined for secretion.

• Ribosomes: Sites of protein synthesis; translate mRNA into polypeptides.

• Endoplasmic Reticulum (ER): Rough ER is involved in initial protein folding and modification; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins for secretion or delivery to other organelles.

• Nucleus: Contains genetic material; directs protein synthesis via transcription.

• Example: Insulin is synthesized in the rough ER, processed in the Golgi, and secreted from the cell.

Phagocytic Cells and Lysosomes

Neutrophils are white blood cells that perform phagocytosis, engulfing pathogens and debris. They contain abundant lysosomes, which are organelles filled with digestive enzymes.

• Lysosomes: Break down macromolecules, cellular debris, and pathogens.

• Phagocytosis: The process by which cells ingest large particles or cells.

• Example: Neutrophils destroy bacteria by engulfing them and digesting them in lysosomes.

Muscle Filaments: Thick and Thin Components

Muscle contraction is driven by the interaction of thick and thin myofilaments. The principal protein of thick filaments is myosin, while actin is the main component of thin filaments.

• Thick Filaments: Composed mainly of myosin.

• Thin Filaments: Composed mainly of actin, with associated proteins like tropomyosin and troponin.

• Example: Skeletal muscle contraction involves myosin heads binding to actin filaments.

Plant vs. Animal Cell Structures

Plant cells possess unique organelles not found in animal cells, such as chloroplasts and a cell wall.

• Chloroplasts: Organelles for photosynthesis, containing chlorophyll.

• Cell Wall: Rigid outer layer providing structural support; composed mainly of cellulose.

• Example: Chloroplasts enable plants to convert sunlight into chemical energy.

Cell Boundaries

All cells have a plasma membrane that encloses the cytoplasm. In plant cells, a cell wall is found outside the plasma membrane.

• Plasma Membrane: Semi-permeable barrier controlling entry and exit of substances.

• Cell Wall: Provides additional protection and support in plants.

Microscopy and Cell Transport

Microscopy Parameters

Microscopy involves several key parameters:

• Magnification: Ratio of image size to actual size.

• Resolution: Ability to distinguish two close objects as separate.

• Contrast: Difference in light intensity between specimen and background.

Ion Concentrations and Active Transport

Cells maintain specific ion gradients across their membranes. The sodium-potassium pump uses ATP to transport sodium and potassium ions against their concentration gradients.

• Sodium (Na+): Lower concentration inside the cell than outside.

• Potassium (K+): Greater concentration inside the cell than outside.

• ATP: Required to power the sodium-potassium pump during active transport.

• Equation: $3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Types of Membrane Transport

Transport across cell membranes can be passive or active:

• Passive Transport: Molecules move along the concentration gradient; no energy required (e.g., diffusion, osmosis).

• Active Transport: Molecules move against the concentration gradient; requires energy (ATP).

• Example: Glucose uptake in the intestine uses active transport.

Osmosis and Membrane Permeability

Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to higher solute concentration.

• Semipermeable Membrane: Allows selective passage of certain molecules, especially water.

• Example: Water uptake by plant roots via osmosis.

Biological Membrane Structure

The fluid mosaic model describes the cell membrane as a bilayer of phospholipids with proteins embedded within.

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

• Proteins: Serve as channels, receptors, and enzymes.

Cellular Processes and Metabolism

Endocytosis Types

Endocytosis is the process by which cells engulf external substances. Phagocytosis is a type of endocytosis involving the ingestion of large particles, such as dead cells.

• Phagocytosis: "Cell eating"; used by immune cells to remove debris.

• Pinocytosis: "Cell drinking"; uptake of fluids.

• Receptor-mediated Endocytosis: Specific uptake via receptor binding.

Passive vs. Active Transport Characteristics

Passive transport relies on kinetic energy and does not require ATP, while active transport requires energy input.

• Passive Transport: Driven by kinetic energy; does not use ATP.

• Active Transport: Requires ATP; moves substances against gradients.

Membrane Permeability and Ion Passage

Small nonpolar molecules (e.g., oxygen, carbon dioxide) pass easily through the plasma membrane, while ions (e.g., sodium ion) face greater difficulty due to their charge.

• Easy Passage: Oxygen, carbon dioxide, steroid hormones.

• Difficult Passage: Sodium ions, large polar molecules.

ATP Production in Mitochondria

The mitochondrion produces ATP primarily through oxidative phosphorylation, which occurs after glycolysis and the citric acid cycle.

• Oxidative Phosphorylation: Uses electron transport chain and chemiosmosis to generate ATP.

• Equation: $\text{ADP} + \text{P}_i + \text{energy} \rightarrow \text{ATP}$

Anabolism and Catabolism

Anabolism refers to the synthesis of complex molecules from simpler ones, while catabolism is the breakdown of complex molecules into simpler ones.

• Anabolism: Formation of proteins from amino acids.

• Catabolism: Breakdown of glucose during cellular respiration.

Enzyme Activity Factors

Enzyme activity is influenced by several factors:

• pH: Each enzyme has an optimal pH.

• Temperature: High temperatures can denature enzymes.

• Inhibitors: Chemicals that reduce enzyme activity.

Entropy and Spontaneous Reactions

Spontaneous reactions generally increase the entropy (disorder) of a system.

• Entropy ($S$): Measure of disorder; spontaneous processes tend to increase $S$.

Feedback Inhibition in Metabolic Pathways

Feedback inhibition is a regulatory mechanism where the end product of a pathway inhibits an earlier step, preventing overproduction.

• Example: ATP inhibits phosphofructokinase in glycolysis.

Energy in Chemical Bonds

Chemical bonds, such as C-H bonds in glucose, store potential energy that can be released during metabolism.

• Potential Energy: Stored energy due to position or structure.

Enzyme-Catalyzed Reaction Energy Diagram

Enzymes lower the activation energy required for reactions. The energy diagram shows:

Label	Description
A	Transition state
B	Activation energy in the absence of an enzyme
C	Activation energy in the presence of an enzyme

• Activation Energy ($E_a$): Minimum energy required for a reaction to proceed.

• Enzymes: Biological catalysts that lower $E_a$.