Cell Membrane Structure, Membrane Transport, and Cellular Respiration Study Guide

Cell Membrane Structure & Function

Phospholipids and the Phospholipid Bilayer

The cell membrane is primarily composed of phospholipids, which are amphipathic molecules containing both hydrophilic and hydrophobic regions. This unique structure allows phospholipids to form a bilayer, creating a selective barrier between the cell and its environment.

• Hydrophilic head: Attracted to water; faces outward toward aqueous environments.

• Hydrophobic tail: Repelled by water; faces inward, away from water, forming the interior of the bilayer.

• Bilayer formation: Occurs naturally due to the amphipathic nature of phospholipids.

• Membrane proteins: Hydrophilic regions interact with water; hydrophobic regions are embedded within the membrane.

• Example: In a diagram, phospholipid heads are on the exterior and interior surfaces, while tails are sandwiched in the middle.

The Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a dynamic structure with a fluid phospholipid bilayer and embedded proteins. This model emphasizes both the movement and diversity of membrane components.

• Fluid: Phospholipids and some proteins move laterally within the membrane.

• Mosaic: Various proteins and other molecules are scattered throughout the membrane.

• Embedded proteins: Amphipathic proteins are integrated within the bilayer, not simply attached to the surface.

• Example: The membrane is best described as a fluid bilayer with embedded amphipathic proteins.

Membrane Fluidity

Membrane fluidity is crucial for cell function and is influenced by the movement of phospholipids and the presence of cholesterol.

• Lateral movement: Phospholipids commonly move side-to-side within the same layer.

• Flip-flop: Rarely occurs; phospholipids seldom switch sides across the bilayer.

• Cholesterol: Located within the bilayer, regulates fluidity:

  • At warm temperatures: Restrains phospholipid movement.

  • At cool temperatures: Prevents phospholipids from packing too tightly.

• Example: Cholesterol is depicted as small molecules between phospholipid tails in diagrams.

Integral, Transmembrane & Peripheral Proteins

Membrane proteins are classified based on their association with the membrane.

• Peripheral proteins: Attached to the membrane surface; do not penetrate the hydrophobic core.

• Integral proteins: Penetrate the hydrophobic interior; often amphipathic.

• Transmembrane proteins: A subset of integral proteins that span the entire membrane.

• Amphipathic nature: Integral proteins have hydrophobic regions interacting with the membrane interior and hydrophilic regions exposed to water.

• Example: Transmembrane proteins are shown crossing the bilayer in diagrams.

Reading a Fluid Mosaic Model Diagram

Understanding membrane diagrams is essential for identifying structural components and their properties.

• Phospholipid heads/tails

• Cholesterol

• Integral, peripheral, and transmembrane proteins

• Extracellular and cytoplasmic sides

• Hydrophilic and hydrophobic regions

• Strategy: Hydrophilic regions touch water; hydrophobic regions are buried inside.

Selective Permeability

The plasma membrane allows some substances to cross more easily than others, a property known as selective permeability.

• Cross easily: Small, hydrophobic/nonpolar molecules (e.g., O2, CO2, hydrocarbons).

• Cross with difficulty: Hydrophilic/polar substances (e.g., sugars, water, ions).

• Example: O2 crosses the bilayer faster than sucrose or Na+.

Transport Proteins

Transport proteins facilitate the movement of hydrophilic substances across the membrane.

• Channel proteins: Provide hydrophilic tunnels for molecules to pass through.

• Carrier proteins: Bind specific molecules and change shape to transport them.

• Specificity: Transport proteins are highly specific; e.g., a glucose carrier will not transport fructose.

Aquaporins

Aquaporins are channel proteins specialized for water transport, significantly increasing the rate at which water crosses the membrane.

• Function: Facilitate rapid water movement.

• Defects: If aquaporins are defective, water transport is greatly reduced.

• Example: Aquaporins are depicted as water channels in membrane diagrams.

Cellular Respiration

Mitochondria & Cellular Respiration

Mitochondria are the organelles where cellular respiration occurs, a process that uses oxygen to generate ATP.

• Structures: Outer membrane, inner membrane, cristae, intermembrane space, mitochondrial matrix.

• Cristae: Folds of the inner membrane that increase surface area for ATP-producing enzymes.

• Matrix: Site of several metabolic steps in cellular respiration.

Oxidation and Reduction

Cellular respiration involves redox reactions, where electrons are transferred between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

• Application: Removal of hydrogen/electrons from glucose is oxidation.

Stages of Cellular Respiration

Cellular respiration occurs in a series of stages, each with distinct products and processes.

• Sequence: Glucose → Glycolysis → Pyruvate Oxidation → Citric Acid Cycle → Oxidative Phosphorylation

• Order: Understanding the correct sequence is essential for tracing energy flow.

Glycolysis

Glycolysis is the first stage of cellular respiration, breaking down glucose into pyruvate and producing ATP and NADH.

• Net products (per glucose): 2 pyruvate, 2 ATP, 2 NADH

• Location: Cytoplasm

Citric Acid Cycle

The citric acid cycle (Krebs cycle) further oxidizes pyruvate, generating ATP, NADH, and FADH2.

• Products (per cycle): ATP, NADH, FADH2

• Note: One glucose yields two pyruvate, so products may need to be doubled.

Electron Transport Chain

The electron transport chain uses electrons from NADH and FADH2 to create a proton gradient across the mitochondrial membrane, driving ATP synthesis.

• NADH and FADH2: Electron carriers

• Electrons: Transferred through protein complexes

• H+ gradient: Energy released pumps protons, creating a gradient used for ATP production

Substrate-Level Phosphorylation

Substrate-level phosphorylation is a direct method of ATP production during glycolysis and the citric acid cycle, distinct from oxidative phosphorylation.

• Occurs in: Glycolysis and citric acid cycle

• Oxidative phosphorylation: ATP produced via the electron transport chain and chemiosmosis

Key Terms and Definitions

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions

• Hydrophilic: Water-attracting

• Hydrophobic: Water-repelling

• Selective permeability: Property allowing some substances to cross the membrane more easily

• Channel protein: Forms a tunnel for molecules

• Carrier protein: Binds and transports specific molecules

• Aquaporin: Water channel protein

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• Substrate-level phosphorylation: Direct ATP formation from a substrate

Summary Table: Membrane Components and Functions

Component	Location	Function
Phospholipid Head	Exterior/interior surfaces	Hydrophilic, interacts with water
Phospholipid Tail	Interior of bilayer	Hydrophobic, avoids water
Cholesterol	Between phospholipid tails	Regulates membrane fluidity
Integral Protein	Embedded in bilayer	Transport, signaling, structure
Peripheral Protein	Surface of membrane	Cell signaling, support
Transmembrane Protein	Spans entire membrane	Transport, communication
Aquaporin	Embedded in membrane	Facilitates water transport

Summary Table: Stages of Cellular Respiration

Stage	Main Location	Key Products
Glycolysis	Cytoplasm	2 ATP, 2 NADH, 2 Pyruvate
Pyruvate Oxidation	Mitochondrial matrix	Acetyl-CoA, NADH, CO2
Citric Acid Cycle	Mitochondrial matrix	ATP, NADH, FADH2, CO2
Oxidative Phosphorylation	Inner mitochondrial membrane	ATP, H2O

Key Equations

• Cellular Respiration Overall Equation:

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

• Oxidation and Reduction:

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

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

Additional info:

• Understanding diagrams is emphasized for exam preparation; students should be able to identify all major membrane and mitochondrial structures.

• Carrier proteins' specificity is a key concept, especially in distinguishing glucose and fructose transport.

• Substrate-level phosphorylation occurs in glycolysis and the citric acid cycle, while oxidative phosphorylation is associated with the electron transport chain.