Cell Structure, Membrane Function, and Photosynthesis: General Biology Study Notes

Cell Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of the cell membrane as a dynamic combination of lipids, proteins, and carbohydrates. This model explains how membranes are flexible and selectively permeable.

• Phospholipids: Form the bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Transport Proteins: Facilitate movement of substances across the membrane.

• Enzymes: Catalyze reactions at the membrane surface.

• Glycoproteins: Proteins with carbohydrate chains involved in cell recognition.

• Attachment Proteins: Anchor the membrane to the cytoskeleton and extracellular matrix.

• Receptor Proteins: Bind signaling molecules and initiate cellular responses.

Selective Permeability: The cell membrane allows certain substances to pass while restricting others, maintaining homeostasis.

Membrane Transport Mechanisms

Cells use various mechanisms to move substances across membranes, depending on the properties of the molecules and the needs of the cell.

• Diffusion: Movement of molecules from high to low concentration.

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

• Passive Transport: Includes diffusion and facilitated diffusion; does not require energy.

• Active Transport: Movement against a concentration gradient; requires energy (ATP).

• Facilitated Diffusion: Uses channel or carrier proteins to move substances down their concentration gradient.

• Exocytosis and Endocytosis: Bulk transport mechanisms for large molecules or particles.

• Phagocytosis, Pinocytosis, Receptor-Mediated Endocytosis: Types of endocytosis for uptake of solids, liquids, or specific molecules.

Concentration Gradient: The difference in concentration of a substance across a space.

Carrier Proteins, Channel Proteins, Aquaporins: Specialized proteins that facilitate transport of specific molecules.

Effects of Tonicity on Cells

Tonicity refers to the ability of a solution to cause a cell to gain or lose water.

• Isotonic Solution: No net movement of water; cell volume remains stable.

• Hypotonic Solution: Water enters the cell; cell may swell and burst.

• Hypertonic Solution: Water leaves the cell; cell shrinks.

Cell Structure: A Tour of the Cell

Basic Features of All Cells

All cells share certain structural features essential for life.

• Plasma Membrane: Encloses the cell, regulates entry and exit of substances.

• Cytoplasm: Jelly-like substance where cellular processes occur.

• DNA/RNA: Genetic material for storing and transmitting information.

• Ribosomes: Sites of protein synthesis.

Eukaryotic Cell Structures

Eukaryotic cells contain membrane-bound organelles that perform specialized functions.

• Cell Wall: Provides structure and protection (in plants, fungi, some protists).

• Cytoskeleton: Network of microfilaments, intermediate filaments, and microtubules for shape and movement.

• Flagella and Cilia: Structures for movement.

• Nucleus: Contains DNA, chromatin, nucleolus.

• Ribosomes: Free or attached to endoplasmic reticulum.

• Endoplasmic Reticulum (Rough and Smooth): Protein and lipid synthesis.

• Vesicles: Transport materials within the cell.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Digestive organelles.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in structure and complexity.

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Wall	Present (most)	Present (plants, fungi)
DNA Location	Nucleoid region	Nucleus
Ribosomes	Smaller (70S)	Larger (80S)

Cell Junctions

Cells connect and communicate through specialized junctions.

• Anchoring Junctions: Attach cells to each other or to the extracellular matrix.

• Tight Junctions: Prevent leakage of materials between cells.

• Gap Junctions: Allow direct communication between cells.

• Plasmodesmata: Channels between plant cells for transport and communication.

Photosynthesis: Using Light to Make Food

Autotrophs vs. Heterotrophs

Autotrophs (producers) make their own food using light or chemical energy. Heterotrophs (consumers) obtain energy by eating other organisms.

Chloroplast Structure and Function

The chloroplast is the site of photosynthesis in plant cells.

• Stroma: Fluid-filled space where the Calvin cycle occurs.

• Thylakoid: Membranous sacs containing chlorophyll; site of light reactions.

• Granum: Stack of thylakoids.

Photosynthesis Overview

Photosynthesis converts light energy into chemical energy in the form of glucose.

• Light Reactions: Occur in thylakoid membranes; convert light energy to chemical energy (ATP and NADPH).

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

General Equation for Photosynthesis:

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

Chlorophyll and Light Absorption

Chlorophyll is the pigment responsible for capturing light energy.

• Absorbs light primarily in the blue and red wavelengths.

• Reflects green light, giving plants their color.

Light Reactions vs. Calvin Cycle

Process	Location	Main Inputs	Main Outputs
Light Reactions	Thylakoid membrane	Light, H2O	ATP, NADPH, O2
Calvin Cycle	Stroma	CO2, ATP, NADPH	Glucose, ADP, NADP+

Cellular Respiration: Harvesting Chemical Energy

Overview of Cellular Respiration

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

General Equation for Aerobic Cellular Respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$

Major Steps of Cellular Respiration

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

• Pyruvate Oxidation: Pyruvate enters mitochondria and is converted to acetyl-CoA.

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

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

ATP Production and Energy Carriers

• ATP: Main energy currency of the cell.

• NADH and FADH2: Electron carriers that transfer energy to the electron transport chain.

Importance: Cellular respiration provides energy for all cellular activities.

Summary Table: Photosynthesis vs. Cellular Respiration

Process	Location	Main Function	Key Reactants	Key Products
Photosynthesis	Chloroplast	Energy storage	CO2, H2O, light	Glucose, O2
Cellular Respiration	Mitochondria	Energy release	Glucose, O2	CO2, H2O, ATP

Additional info:

• Some details about cell junctions, organelles, and metabolic pathways were expanded for clarity and completeness.

• Equations and tables were formatted for academic clarity.