Cell Structure and Function
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Prokaryotic vs. Eukaryotic Cells
Prokaryotic Cells
Definition: Cells that lack a membrane-bound nucleus and organelles.
Key Characteristics:
- Genetic material (DNA) freely floating in the cytoplasm
- No membrane-bound organelles
- Smaller in size (typically 1-5 μm)
- Found in bacteria and archaea
- Single circular chromosome
- Cell wall present (in most bacteria)
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Examples: Bacteria such as E. coli, Streptococcus.
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Eukaryotic Cells
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Definition: Cells that contain a membrane-bound nucleus and specialized organelles.
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Key Characteristics:
- DNA enclosed within a nucleus
- Membrane-bound organelles present
- Larger in size (typically 10-100 μm)
- Found in animals, plants, fungi, and protists
- Multiple linear chromosomes
- No cell wall in animal cells
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Examples: Animal cells (muscle cells, nerve cells, blood cells)
Key Differences Summary
Prokaryotic, Simple. Nucleus absent, DNA in cytoplasm, no organelles, 1-5 μm
Eukaryotic, Complex. Nucleus present, DNA in nucleus, organelles, 10-100 μm
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Major Organelles and Their Functions
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Nucleus
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Structure:
- Surrounded by nuclear envelope (double membrane)
- Contains nuclear pores for transport
- Houses nucleolus (ribosome assembly site)
- Contains chromatin (DNA + proteins)
Functions:
​- Controls cell activities (cell's "control center")
- Stores genetic information (DNA)
- Regulates gene expression
- Site of DNA replication and transcription
Analogy: Think of it as the cell's headquarters or library containing all the instructions.
Mitochondria
Structure:
- Double membrane organelle
- Outer membrane smooth, inner membrane folded (cristae)
- Contains mitochondrial DNA
- Matrix contains enzymes for cellular respiration
Functions:
- Cellular respiration (aerobic respiration)
- ATP production ("energy currency")
- Heat generation
- Calcium storage
- Involved in cell death (apoptosis)
Analogy: The cell's powerhouse or battery, converting fuel into usable energy.
Key Process: Glucose + Oxygen → ATP + Carbon dioxide + Water
Ribosomes
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Structure:
- Small organelles made of RNA and proteins
- Two subunits: large and small
- Found free in the cytoplasm or attached to the ER
- No membrane boundary
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Functions:
- Protein synthesis (translation)
- Read mRNA and assemble amino acids into proteins
- Free ribosomes make proteins for the cytoplasm
- Bound ribosomes make proteins for secretion
Analogy: Protein factories or assembly lines of the cell.
Endoplasmic Reticulum (ER)
Rough ER (RER)
Structure:
- Network of membranes with ribosomes attached
- Connected to nuclear envelope
- Extensive folded membrane system
Functions:
- Protein synthesis and modification
- Quality control of proteins
- Transport of proteins to Golgi apparatus
- Membrane production
Smooth ER (SER)
Structure:
- Network of membranes without ribosomes
- More tubular than rough ER
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Functions:
- Lipid synthesis (including steroids)
- Carbohydrate metabolism
- Detoxification of harmful substances
- Calcium storage
Analogy: Think of ER as the cell's highway system - rough ER is like a highway with factories (ribosomes) alongside, smooth ER is like a highway for transport and processing.
Golgi Apparatus
Structure:
- Stack of flattened membrane sacs (cisternae)
- Has cis face (receiving) and trans face (shipping)
- Associated with vesicles
Functions:
- Modifies proteins from the rough ER
- Packages proteins and lipids
- Adds carbohydrate groups to proteins (glycosylation)
- Ships products to final destinations
Analogy: The cell's post office receives, processes, packages, and ships items.
Lysosomes
Structure:
- Membrane-bound vesicles
- Contain digestive enzymes
- Acidic interior (pH ~4.5)
Functions:
- Digest worn-out organelles (autophagy)
- Break down harmful substances
- Digest materials from outside the cell
- Cell death processes
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Analogy: The cell's recycling center and garbage disposal system.
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Other Important Organelles
Centrosomes and Centrioles
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- Function: Organize microtubules, important in cell division
- Structure: Two centrioles arranged at right angles
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Cytoskeleton
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- Components: Microfilaments, intermediate filaments, microtubules
- Functions: Cell shape, organelle movement, cell division
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Specialised Animal Cells
Muscle Cells (Myocytes)
Skeletal Muscle Cells
Structure:
- Long, cylindrical, multinucleated
- Contains myofibrils with actin and myosin filaments
- Striated appearance
- Many mitochondria for energy
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Function:
- Voluntary movement
- Posture maintenance
- Heat generation
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Adaptations:
- Specialized proteins (actin, myosin) for contraction
- Multiple nuclei for protein synthesis
- Abundant mitochondria for ATP production
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Cardiac Muscle Cells
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Structure:
- Shorter than skeletal muscle
- Single nucleus
- Intercalated discs connect cells
- Striated appearance
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Function:
- Involuntary heart contractions
- Continuous rhythmic pumping
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Adaptations:
- Intercalated discs for synchronized contraction
- Rich blood supply
- Resistant to fatigue
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Smooth Muscle Cells
Structure:
- Spindle-shaped
- Single nucleus
- No striations
- Shorter than skeletal muscle
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Function:
- Involuntary movements in organs
- Control of blood vessel diameter
- Movement in the digestive system
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Adaptations:
- Can maintain prolonged contractions
- Less energy is required than skeletal muscle
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Epithelial Cells
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General Characteristics:
- Form continuous sheets
- Rest on basement membrane
- Closely packed with tight junctions
- Avascular (no blood vessels)
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Types and Functions:
Simple Squamous Epithelium:
- *Structure*: Single layer of flat cells
- *Location*: Alveoli, blood vessels
- *Function*: Diffusion and filtration
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Simple Cuboidal Epithelium:
- *Structure*: Single layer of cube-shaped cells
- *Location*: Kidney tubules, glands
- *Function*: Secretion and absorption
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Simple Columnar Epithelium:
- *Structure*: Single layer of tall cells
- *Location*: Intestines, stomach
- *Function*: Absorption and secretion
- *Special features*: May have cilia or microvilli
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Stratified Squamous Epithelium:
- *Structure*: Multiple layers, top layer flattened
- *Location*: Skin, mouth, esophagus
- *Function*: Protection
Adaptations:
- Tight junctions prevent leakage
- Microvilli increase surface area
- Cilia for movement of materials
- Rapid cell division for replacement
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Nerve Cells (Neurons)
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Structure:
- Cell body (soma) contains nucleus and organelles
- Dendrites: receive signals
- Axon: transmits signals
- Synapses: connections with other neurons
- Myelin sheath: insulation (in some neurons)
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Function:
- Receive, process, and transmit information
- Generate and conduct electrical impulses
- Communication throughout body
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Types:
1. Sensory neurons: Carry information from receptors to CNS
2. Motor neurons: Carry signals from CNS to muscles/glands
3. Interneurons: Connect neurons within CNS
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Adaptations:
- Long axons for long-distance communication
- Myelin sheath increases conduction speed
- Multiple dendrites for receiving many signals
- Specialized neurotransmitters for chemical communication
- High metabolic activity - many mitochondria
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Blood Cells
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Red Blood Cells (Erythrocytes)
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Structure:
- Biconcave disc shape
- No nucleus (in mammals)
- Packed with hemoglobin
- Flexible membrane
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Function:
- Oxygen transport
- Some carbon dioxide transport
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Adaptations:
- Biconcave shape increases surface area
- No organelles = more space for hemoglobin
- Flexible for passage through capillaries
- Hemoglobin binds oxygen efficiently
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White Blood Cells (Leukocytes)
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Types and Functions:
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Neutrophils:
- *Structure*: Multi-lobed nucleus, granular cytoplasm
- *Function*: First responders to infection, phagocytosis
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Lymphocytes:
- *Structure*: Large nucleus, little cytoplasm
- *Function*: Immune response (B cells make antibodies, T cells kill infected cells)
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Monocytes:
- *Structure*: Large cells, kidney-shaped nucleus
- *Function*: Differentiate into macrophages, phagocytosis
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Eosinophils:
- *Structure*: Bi-lobed nucleus, red granules
- *Function*: Combat parasites, allergic reactions
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Basophils:
- *Structure*: Bi-lobed nucleus, blue granules
- *Function*: Release histamine in allergic reactions
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Platelets (Thrombocytes)
Structure:
- Cell fragments (not complete cells)
- No nucleus
- Contain clotting factors
Function:
- Blood clotting
- Wound healing
Adaptations:
- Small size allows rapid movement to injury sites
- Contain clotting proteins
- Sticky surface for clot formation
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Structure-Function Relationships
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Key Principles:
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1. Form follows function: Cell structure is adapted to its specific role
2. Surface area to volume ratio: Affects the efficiency of transport
3. Specialization: Cells become specialized for specific functions
4. Organelle cooperation: Different organelles work together
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Examples:
- Muscle cells: Long shape and contractile proteins enable movement
- Red blood cells: Biconcave shape maximizes oxygen-carrying capacity
- Nerve cells: Long projections enable long-distance communication
- Epithelial cells: Tight connections create effective barriers
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