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Cell Structure and Organisation

Subject: Biology
Topic: 1
Cambridge Code: 0610 / 0970 / 5090

Organisms and Cells​

Cell - Basic unit of all living organisms

Key Features of Living Organisms​

  • Made of cells
  • Obtain and use energy
  • Grow and reproduce
  • Respond to environment
  • Maintain homeostasis
  • Contain genetic material

Prokaryotic Cells​

Prokaryotic - Organisms without a nucleus (bacteria and archaea)

Features​

  • No membrane-bound nucleus
  • Smaller (typically 1-10 μm)
  • No mitochondria, ER, Golgi apparatus
  • DNA in nucleoid region (not bounded by membrane)
  • Cell wall present (but different from eukaryotic)
  • Ribosomes smaller (70S)

Bacterial Cell Structure​

  1. Cell wall - Rigid layer outside cell membrane (peptidoglycan)
  2. Cell membrane - Semi-permeable, controls entry/exit
  3. Cytoplasm - Contains ribosomes and genetic material
  4. Nucleoid - Region containing DNA (not membrane-bound)
  5. Ribosomes - Site of protein synthesis (70S)
  6. Flagella - Tail-like structures for movement
  7. Pili - Hair-like attachments

Eukaryotic Cells​

Eukaryotic - Organisms with a nucleus (animals, plants, fungi, protists)

Key Features​

  • Membrane-bound nucleus contains DNA
  • Larger (typically 10-100 μm)
  • Many membrane-bound organelles
  • Ribosomes larger (80S)
  • More complex organization

Organelles in Eukaryotic Cells​

Nucleus​

  • Contains chromosomes (DNA + proteins)
  • Controls cell activities
  • Contains nucleolus (ribosome production)
  • Membrane: nuclear envelope with pores

Mitochondrion (plural: Mitochondria)​

  • Function: Aerobic respiration (ATP production)
  • Contains own DNA
  • Double membrane
  • Has cristae (increase surface area)
  • More in active cells

Endoplasmic Reticulum (ER)​

Rough ER:

  • Has ribosomes attached
  • Protein synthesis and transport

Smooth ER:

  • No ribosomes
  • Lipid synthesis, detoxification

Golgi Apparatus (Golgi Body)​

  • Network of flattened sacs
  • Modifies and packages proteins
  • Forms vesicles
  • Transport function

Ribosomes​

  • Site of protein synthesis
  • 80S in eukaryotes, 70S in prokaryotes
  • Found free or on rough ER
  • Not surrounded by membrane

Lysosomes​

  • Contain digestive enzymes
  • Break down worn-out organelles
  • Remove pathogens
  • "Suicide sacs" - can lyse the cell

Centrosome (Centrioles)​

  • Pair of cylindrical structures
  • Involved in cell division
  • Produce spindle fibers
  • Plant cells lack centrosomes

Chloroplast (Plants only)​

  • Photosynthesis
  • Double membrane
  • Contains thylakoids and stroma
  • Own DNA
  • More in leaf cells

Cell Wall (Plants and Fungi)​

  • Outside cell membrane
  • Cellulose in plants, chitin in fungi
  • Rigid, provides support
  • Prevents excessive water loss

Vacuole​

Plant cell:

  • Large central vacuole
  • Turgor pressure (support)
  • Stores water, ions, food

Animal cell:

  • Small vacuoles
  • Temporary storage

Structure of Cell Membrane​

Fluid Mosaic Model - Cell membrane is flexible with embedded proteins

Components​

  1. Phospholipid Bilayer - Forms basic structure

    • Hydrophilic heads face outside
    • Hydrophobic tails face inside

  2. Proteins - Various functions

    • Channel proteins (transport)
    • Receptor proteins (signal)
    • Carrier proteins (active transport)

  3. Cholesterol - Maintains fluidity

  4. Carbohydrates - As glycoproteins and glycolipids

    • Cell recognition
    • Immune response

Differences Between Animal and Plant Cells​

FeatureAnimalPlant
Cell wallNoYes (cellulose)
Large vacuoleNoYes
CentriolesYesNo
ChloroplastsNoYes
ShapeRound/irregularFixed, rectangular
PlasmodesmataNoYes

Key Points​

  1. Prokaryotic cells: no nucleus, bacteria
  2. Eukaryotic cells: nucleus, complex organelles
  3. Every organelle has specific function
  4. Cell membrane: selectively permeable
  5. Plant cells have cell wall and chloroplasts
  6. Mitochondria: site of energy production

Practice Questions​

  1. List 3 differences between prokaryotic and eukaryotic cells
  2. Name 5 organelles and their functions
  3. Describe the fluid mosaic model
  4. Compare animal and plant cells
  5. Why do mitochondria have double membranes?

Revision Tips​

  • Learn each organelle's function
  • Draw and label cells
  • Understand prokaryotic vs. eukaryotic
  • Know structure of cell membrane
  • Practice cell comparison diagrams