AS Level Biology Cell Structure Explained: Detailed Guide

Understanding cell structure is fundamental to AS Level Biology. This detailed guide explains cell types, organelles and their functions, membrane structure, and how cells work, with clear explanations and visual descriptions.

Cell Types: Prokaryotic vs Eukaryotic

Prokaryotic Cells

• No nucleus (DNA in cytoplasm)
• No membrane-bound organelles
• Smaller (1-5 μm)
• Simpler structure
• Example: bacteria

Key Features:

• Circular DNA (plasmid)
• Ribosomes (smaller, 70S)
• Cell wall (peptidoglycan)
• May have flagella or pili

Eukaryotic Cells

• Has nucleus (DNA in nucleus)
• Membrane-bound organelles
• Larger (10-100 μm)
• More complex structure
• Examples: plant and animal cells

Key Features:

• Linear DNA in chromosomes
• Ribosomes (larger, 80S)
• Many organelles
• More complex functions

Cell Organelles and Functions

Nucleus

• Structure: Surrounded by nuclear envelope, contains DNA and nucleolus
• Function: Controls cell activities, stores genetic information
• Key Points: Largest organelle, contains chromosomes, nucleolus makes ribosomes

Mitochondria

• Structure: Double membrane, inner membrane folded (cristae), contains matrix
• Function: Site of aerobic respiration, produces ATP (energy)
• Key Points: Has own DNA, many in active cells, “powerhouse” of cell

Ribosomes

• Structure: Small structures made of RNA and protein
• Function: Site of protein synthesis
• Key Points: Found on rough ER or free in cytoplasm, very small, essential for all cells

Endoplasmic Reticulum (ER)

• Rough ER: Has ribosomes attached, involved in protein synthesis and transport
• Smooth ER: No ribosomes, involved in lipid synthesis and detoxification
• Key Points: Network of membranes, connected to nucleus, transports materials

Golgi Apparatus

• Structure: Stack of flattened membrane-bound sacs (cisternae)
• Function: Modifies, packages, and distributes proteins and lipids
• Key Points: Receives from ER, forms vesicles, prepares for secretion

Lysosomes

• Structure: Membrane-bound vesicles containing digestive enzymes
• Function: Break down waste materials and cellular debris
• Key Points: Acidic interior, “recycling center” of cell, involved in apoptosis

Chloroplasts (Plant cells only)

• Structure: Double membrane, contains thylakoids and stroma, has chlorophyll
• Function: Site of photosynthesis, converts light energy to chemical energy
• Key Points: Has own DNA, green color from chlorophyll, only in plant cells

Vacuole (Plant cells)

• Structure: Large membrane-bound sac
• Function: Stores water, ions, and waste, maintains turgor pressure
• Key Points: Large in plant cells, small in animal cells, helps support plant

Cell Membrane Structure

Fluid Mosaic Model

• Phospholipid bilayer: two layers of phospholipids
• Proteins embedded: integral and peripheral proteins
• Cholesterol: maintains fluidity
• Glycoproteins and glycolipids: cell recognition

Key Components:

• Phospholipids: Hydrophilic heads, hydrophobic tails, form bilayer
• Proteins: Channel proteins, carrier proteins, receptor proteins
• Cholesterol: Reduces fluidity at high temperatures, increases at low temperatures
• Glycoproteins: Cell recognition and communication

Functions:

• Controls what enters/leaves cell (selectively permeable)
• Cell recognition and communication
• Maintains cell shape
• Separates cell from environment

Transport Across Membranes

Diffusion

• Movement from high to low concentration
• No energy required (passive)
• Through membrane or channel proteins
• Example: oxygen, carbon dioxide

Osmosis

• Movement of water
• Through selectively permeable membrane
• From high to low water potential
• Passive process
• Important for plant turgor

Active Transport

• Movement against concentration gradient
• Requires energy (ATP)
• Uses carrier proteins
• Example: ion pumps, glucose absorption

Endocytosis and Exocytosis

• Endocytosis: taking in large particles (phagocytosis, pinocytosis)
• Exocytosis: releasing large particles
• Requires energy
• Used for large molecules

Cell Specialization

Animal Cell Specialization:

• Muscle cells: contractile proteins, many mitochondria
• Nerve cells: long extensions, transmit signals
• Red blood cells: no nucleus, carry oxygen
• Sperm cells: flagellum for movement

Plant Cell Specialization:

• Root hair cells: large surface area for absorption
• Xylem cells: hollow, transport water
• Phloem cells: transport sugars
• Guard cells: control stomata

Cell Division

Mitosis

• Produces identical cells
• For growth and repair
• Four stages: prophase, metaphase, anaphase, telophase
• Results in two diploid cells

Meiosis

• Produces gametes (sex cells)
• Reduces chromosome number by half
• Two divisions
• Results in four haploid cells

Common Mistakes to Avoid

1. Confusing Organelles

• Know differences between similar structures
• Understand functions clearly
• Practice labeling diagrams

2. Prokaryotic vs Eukaryotic

• Clear understanding of differences
• Know examples of each
• Understand evolutionary significance

3. Membrane Structure

• Understand fluid mosaic model
• Know components and functions
• Link structure to function

Related Resources

• AS Level Biology Past Papers Cell Structure
• AS Level Biology Revision Notes
• AS Level Biology Tutor

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Understanding cell structure is essential for AS Level Biology. Practice regularly and seek help when needed to master this fundamental concept.