Cell Membranes, Transport, Metabolism, and Cellular Respiration: General Biology Study Notes

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Phospholipid Bilayer Architecture

Structure and Properties

Biological membranes are primarily composed of a phospholipid bilayer, which provides selective permeability and structural integrity to cells.

Hydrophilic heads: Polar head groups face aqueous environments (inside and outside the cell).
Hydrophobic tails: Fatty acid tails are oriented toward the center, away from water.
Amphipathic structure: Contains both hydrophilic and hydrophobic regions.

The hydrophobic core prevents water-soluble substances from freely crossing, making the bilayer essential for cellular function.

Cholesterol's Role in Membrane Fluidity

Cholesterol acts as a temperature buffer to maintain optimal membrane fluidity.

Temperature	Effect on Membrane	Cholesterol's Action
Increased	More fluid	Prevents excessive fluidity
Decreased	More solid	Prevents excessive rigidity

Membrane Protein Classification

Types of Membrane Proteins

Membrane proteins are essential for transport, signaling, and structural support.

Transmembrane proteins: Span the entire membrane from one side to the other.
Integral proteins: Embedded in the hydrophobic core (includes transmembrane proteins).
Peripheral proteins: Associated with one side of the membrane only.

Additional Membrane Components:

Extracellular matrix: Glycoproteins and proteins on the outer surface.
Actin cortex: Actin filaments beneath the plasma membrane provide structural support.

Permeability Categories

Classification of Molecule Passage

Membrane permeability depends on molecule size, charge, and polarity.

Permeability Level	Examples	Characteristics
High	O2, CO2, N2, ethanol	Small, uncharged molecules
Moderate	H2O	Small but requires aquaporins for efficient transport
Low	Ions, proteins, polysaccharides	Large size or charged nature blocks passage

Transport Protein Types

Channel proteins: Form specific channels through the membrane, highly selective (e.g., aquaporins for water only), facilitate passive transport down concentration gradients.
Carrier proteins: Bind and transport substances across the membrane, can facilitate both passive and active transport.

Passive vs. Active Transport

Transport Mechanisms

Cells use different mechanisms to move substances across membranes.

Passive Transport:
- Movement down concentration gradient
- No energy investment required
- Relies on diffusion
- Channel proteins facilitate passive transport only
Active Transport:
- Movement against concentration gradient
- Energy investment required (ATP)
- Establishes electrochemical gradients

Osmosis & Tonicity

Tonicity Conditions

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the effect of solute concentration on cell volume.

Solution Type	Solute Concentration	Water Movement	Cell Response
Isotonic	Equal inside/outside	Equal movement	Maintains normal shape
Hypotonic	Lower outside	Water moves into cell	Cell swells and may burst
Hypertonic	Higher outside	Water moves out of cell	Cell shrinks

Osmoregulation

Cells maintain water balance to prevent lysis in hypotonic environments or dehydration in hypertonic conditions.

Homeostasis: Regulation of water and solute balance.
Hypotonic environments: Cells swell and may burst.
Hypertonic environments: Cells shrink and may die.
Critical: No net movement allowed; cells must avoid extreme osmotic conditions.

Metabolism Overview

Metabolic Pathways

Metabolism encompasses all chemical reactions occurring in cells at any given moment.

Pathway Type	Direction	Energy Requirement	Spontaneity	Examples
Catabolic	Break down	Releases energy	Spontaneous	Glucose → CO2
Anabolic	Build up	Requires energy	Non-spontaneous	Amino acids → proteins

Energy Forms

Types of Energy

Cells utilize different forms of energy for metabolic processes.

Potential energy: Stored energy in complex molecules.
Kinetic energy: Energy of motion.
Chemical energy: Type of potential energy stored in molecular bonds.

Complex molecules (proteins, polysaccharides) contain more potential energy than simple molecules.

Free Energy Changes

Gibbs Free Energy Formula

Free energy change determines whether a reaction is spontaneous.

ΔG Value	Reaction Type	Energy Flow	Spontaneity
Positive (+)	Endergonic	Requires energy input	Non-spontaneous
Negative (−)	Exergonic	Releases energy	Spontaneous

Exergonic: Glycogen breakdown (more free energy in reactants).
Endergonic: Protein synthesis (more free energy in products).

Enzyme Function

Role of Enzymes

Enzymes are biological catalysts that:

Lower activation energy barriers
Speed up reactions without changing products
Do not alter final free energy of products

Adding enzyme to a reaction with keeps ; only reaction rate increases.

Environmental Factors Affecting Enzymes

Factor	Human Enzyme	Heat-tolerant Bacteria
Temperature	37°C optimum	~80°C optimum
pH	Varies by enzyme (stomach: pH 2, intestinal: pH 8)	—

Enzyme Specificity & Inhibition

Substrate Binding

Active site: Where substrates bind
Specificity: Each enzyme binds only its specific substrate

Inhibition Types

Competitive Inhibition:
- Mechanism: Inhibitor competes with substrate for active site
- Solution: Add more substrate to overcome inhibition
Non-competitive (Allosteric) Inhibition:
- Binds to allosteric site (not active site)
- Causes shape change in protein structure
- Makes active site inaccessible to substrate
- Independent of substrate concentration
Feedback Inhibition:
- Occurs when the end product of a pathway inhibits an early enzyme in that pathway
- Example: Isoleucine pathway—when enough isoleucine accumulates, it acts as a non-competitive inhibitor
- Prevents waste of cellular resources

Redox Reactions

Basic Principles

Oxidation: Loss of electrons
Reduction: Gain of electrons
OIL RIG: Oxidation Is Losing, Reduction Is Gaining (electrons)

Electron Transfer Process

Electron carriers (NAD+) become reduced (gain electrons)
Reduced carriers (NADH) transport electrons to electron transport chain
Carriers become oxidized again (lose electrons)
Electron transport chain components undergo redox reactions

Cellular Respiration Overview

Three Stages of Glucose Breakdown

Stage	Location	Key Products	ATP Yield
Glycolysis	Cytoplasm	2 Pyruvate, 2 NADH	2 net ATP
Citric Acid Cycle	Mitochondrial matrix	CO2, NADH, FADH2	2 ATP
Oxidative Phosphorylation	Inner mitochondrial membrane	H2O, NAD+, FAD	~32 ATP

Glycolysis Details

Process Overview

Glycolysis splits 6-carbon glucose into two 3-carbon pyruvate molecules in the cytoplasm.

Investment vs. Payoff Phase:
- Investment: Uses 2 ATP initially
- Payoff: Generates 4 ATP
- Net gain: 2 ATP per glucose
Additional Products: 2 NADH (electron carriers), 2 Pyruvate molecules

Citric Acid Cycle

Location and Function

Occurs in mitochondrial matrix
Completes glucose oxidation
All carbon atoms from glucose released as CO2
Generates 2 ATP per glucose
Produces additional electron carriers (NADH, FADH2)

Electron Transport Chain

Location and Structure

Located in inner mitochondrial membrane
Membrane is highly folded (cristae) to increase surface area
Separates matrix from intermembrane space

Electron Flow Process

Reduced carriers (NADH, FADH2) deposit electrons
Electrons move down the chain in redox reactions
Energy release powers proton pumping
Electrochemical gradient established across membrane

Proton Gradient Establishment

Protons (H+) pumped from matrix to intermembrane space
Creates electrochemical gradient
Higher concentration in intermembrane space
Gradient energy used for ATP synthesis

Mitochondrial Structure

Key Compartments

Outer membrane: Smooth boundary
Inner membrane: Highly folded with electron transport chains
Intermembrane space: Between outer and inner membranes
Matrix: Innermost compartment where citric acid cycle occurs

Importance of Structure: The compartmentalization and folding of the inner membrane are critical for efficient ATP production.

Cellular Respiration Overview (Aerobic Pathway)

Stages and Key Points

Stage	Location	ATP Yield	Key Products
Glycolysis	Cytoplasm	2 ATP	2 pyruvate, electrons to carriers
Citric Acid Cycle	Matrix	2 ATP	More electrons to carriers
Electron Transport	Inner membrane	~34 ATP	Proton gradient + ATP

Oxygen is required for aerobic respiration. Without oxygen, cells use fermentation or anaerobic respiration, which yield much less ATP.
Key points:
- Pyruvate enters the mitochondria and is converted to acetyl-CoA before entering the citric acid cycle.
- The citric acid cycle occurs in the matrix.

Photosynthesis and Cellular Respiration Connection

Overview

Photosynthesis and cellular respiration are interconnected processes. Photosynthesis stores energy in glucose, while cellular respiration releases energy from glucose to produce ATP.

Light Reactions: Occur in chloroplasts, produce ATP and NADPH.
Calvin Cycle: Uses ATP and NADPH to fix CO2 into glucose.
Cellular Respiration: Breaks down glucose to release energy for ATP synthesis.

Example: The ATP produced by cellular respiration powers cellular activities, while the glucose produced by photosynthesis serves as the primary energy source for most organisms.