CIE AS/A Level Biology -1.2 Cells as the basic units of living organisms- Study Notes- New Syllabus
CIE AS/A Level Biology -1.2 Cells as the basic units of living organisms- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -1.2 Cells as the basic units of living organisms- Study Notes- New Syllabus
Key Concepts:
- recognise organelles and other cell structures found in eukaryotic cells and outline their structures and functions, limited to:
• cell surface membrane
• nucleus, nuclear envelope and nucleolus
• rough endoplasmic reticulum
• smooth endoplasmic reticulum
• Golgi body (Golgi apparatus or Golgi complex)
• mitochondria (including the presence of small circular DNA)
• ribosomes (80S in the cytoplasm and 70S in chloroplasts
and mitochondria)
• lysosomes
• centrioles and microtubules
• cilia
• microvilli
• chloroplasts (including the presence of small circular DNA)
• cell wall
• plasmodesmata
• large permanent vacuole and tonoplast of plant cells - describe and interpret photomicrographs, electron micrographs and drawings of typical plant and animal cells
- compare the structure of typical plant and animal cells
- state that cells use ATP from respiration for energy-requiring processes
- outline key structural features of a prokaryotic cell as found in a typical bacterium, including:
• unicellular
• generally 1–5 μm diameter
• peptidoglycan cell walls
• circular DNA
• 70S ribosomes
• absence of organelles surrounded by double membranes - compare the structure of a prokaryotic cell as found in a typical bacterium with the structures of typical eukaryotic cells in plants and animals
- state that all viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some viruses have an outer envelope made of phospholipids
Recognise Organelles and Other Cell Structures in Eukaryotic Cells
🌿 Cell Surface Membrane
Structure: Thin phospholipid bilayer with embedded proteins.
Function: Controls entry and exit of substances (selectively permeable barrier).
🧠 Nucleus, Nuclear Envelope & Nucleolus
Nucleus: Spherical organelle containing DNA, controls cellular activities.
Nuclear Envelope: Double membrane with pores; allows material exchange with cytoplasm.
Nucleolus: Dense region inside nucleus; synthesises ribosomal RNA and assembles ribosomes.
🔬 Rough Endoplasmic Reticulum (RER)
Structure: Flattened sacs with ribosomes attached.
Function: Synthesises and transports proteins.
🔬 Smooth Endoplasmic Reticulum (SER)
Structure: Tubular network without ribosomes.
Function: Synthesises lipids and steroids; detoxifies substances.
📦 Golgi Body (Golgi Apparatus or Golgi Complex)
Structure: Stack of flattened membranous sacs.
Function: Modifies, sorts, packages, and transports proteins and lipids.
⚡ Mitochondria
Structure: Double membrane; inner folds called cristae; contains small circular DNA.
Function: Site of aerobic respiration; produces ATP (energy).
🧪 Ribosomes
Structure: Composed of RNA and protein; 80S in cytoplasm, 70S in chloroplasts and mitochondria.
Function: Synthesise proteins.
🗑️ Lysosomes
Structure: Membrane-bound sacs with digestive enzymes.
Function: Break down waste materials, old organelles, and pathogens.
⚙️ Centrioles and Microtubules
Centrioles: Cylindrical structures made of microtubules, involved in cell division (mainly animal cells).
Microtubules: Tubular proteins maintaining cell shape and aiding intracellular transport.
🌬️ Cilia
Structure: Hair-like projections with microtubule arrangement.
Function: Move fluids or the cell itself.
🔍 Microvilli
Structure: Finger-like extensions of the cell membrane.
Function: Increase surface area for absorption.
🌞 Chloroplasts
Structure: Double membrane with internal thylakoid stacks; contains chlorophyll and small circular DNA.
Function: Conduct photosynthesis.
🧱 Cell Wall
Structure: Rigid outer layer composed mainly of cellulose in plants.
Function: Provides structural support and protection.
🔗 Plasmodesmata
Structure: Channels through cell walls connecting adjacent plant cells.
Function: Facilitate communication and transport between cells.
💧 Large Permanent Vacuole and Tonoplast (Plant Cells)
Large Permanent Vacuole: Fluid-filled sac storing water, nutrients, and waste.
Tonoplast: Membrane surrounding the vacuole.
Function: Maintains turgor pressure; stores substances; isolates harmful materials.
📊 Summary Table
| Organelle/Structure | Structure Highlights | Function |
|---|---|---|
| Cell surface membrane | Phospholipid bilayer with proteins | Regulates substance movement |
| Nucleus + nuclear envelope | Double membrane with pores | Controls cell activities |
| Nucleolus | Dense body inside nucleus | Produces ribosomal RNA |
| Rough ER | Flattened sacs with ribosomes | Protein synthesis and transport |
| Smooth ER | Tubular without ribosomes | Lipid synthesis, detoxification |
| Golgi Body | Stacked membranous sacs | Modifies and packages proteins |
| Mitochondria | Double membrane, cristae, circular DNA | ATP production (respiration) |
| Ribosomes | RNA and protein; 80S & 70S types | Protein synthesis |
| Lysosomes | Membrane sacs with enzymes | Digestion and waste removal |
| Centrioles & microtubules | Cylindrical & tubular structures | Cell division, structural support |
| Cilia | Microtubule-based hair-like projections | Movement of cell or fluids |
| Microvilli | Membrane projections | Increase absorption surface area |
| Chloroplasts | Double membrane, thylakoids, DNA | Photosynthesis |
| Cell wall | Rigid cellulose layer | Support and protection |
| Plasmodesmata | Channels through cell walls | Cell-to-cell communication |
| Large vacuole + tonoplast | Membrane sac | Storage and turgor maintenance |
Organelles have specific structures tailored for their functions.
Some organelles like mitochondria and chloroplasts contain their own DNA.
Plant cells have unique features like cell wall, chloroplasts, plasmodesmata, and large vacuole.
Describe and Interpret Photomicrographs, Electron Micrographs, and Drawings of Typical Plant and Animal Cells
🌱 Types of Images
- Photomicrographs: Photographs taken through a light microscope; magnifications up to ×1500; show overall cell shapes and large organelles but limited detail.
- Electron Micrographs: Images from electron microscopes (TEM or SEM); magnifications up to ×1,000,000; reveal ultrastructure like membranes, ribosomes, and detailed organelles.
- Drawings: Hand-drawn or digital illustrations based on observations; highlight key features clearly with labels and simplify complex details for clarity.
🔍 Describing Typical Plant Cells
- Photomicrographs & Drawings:
- Cell wall (rigid outline)
- Large central vacuole (clear area)
- Chloroplasts (green structures in live samples)
- Nucleus, sometimes visible near the edge
- Cytoplasm surrounding organelles
- Electron Micrographs:
- Detailed double membranes of nucleus and chloroplasts
- Thylakoid membranes inside chloroplasts
- Mitochondria with cristae
- Ribosomes as small dots
- Tonoplast membrane around vacuole
🔍 Describing Typical Animal Cells
- Photomicrographs & Drawings:
- Irregular shape without a cell wall
- Nucleus often centrally located
- Cytoplasm filling cell interior
- Visible organelles like mitochondria (if stained)
- May show cilia or microvilli on surface
- Electron Micrographs:
- Clear nuclear envelope with pores
- Detailed mitochondria structure
- Centrioles near the nucleus
- Lysosomes as membrane-bound vesicles
- Microtubules forming cytoskeleton
🧩 Interpreting Images: Key Points
- Identify cell type (plant vs animal) by presence or absence of cell wall, chloroplasts, and vacuole.
- Look for organelles like nucleus, mitochondria, ER, Golgi, lysosomes.
- Note shape and size differences: plant cells are more regular and rectangular; animal cells are irregular.
- Use scale bars and magnification info to estimate sizes.
- Recognise staining patterns in photomicrographs (e.g., darker nucleus).
- Understand grayscale images in electron micrographs showing fine details, membranes, and internal structures.
📊 Summary Table: Features in Different Image Types
| Feature/Organelle | Photomicrograph | Electron Micrograph | Drawing |
|---|---|---|---|
| Cell wall | Visible in plant cells as rigid boundary | Clear double-layered structure | Clear outline |
| Nucleus | Visible, sometimes stained dark | Detailed envelope and nucleolus | Clearly labelled spherical |
| Chloroplasts | Green spots in plant cells | Internal thylakoid membranes | Oval with internal stacks |
| Mitochondria | Occasionally visible | Detailed cristae and membranes | Small oval with folds |
| Vacuole | Large clear area in plant cells | Tonoplast membrane visible | Large central sac |
| Cilia/Microvilli | Visible on cell surface (animal) | Microtubule arrangement visible | Short projections |
🧠 Key Tips for Describing and Interpreting
- Start with overall cell shape and size.
- Identify unique plant or animal features.
- Mention visible organelles and their appearance.
- Use scientific terms and correct labels.
- Refer to the scale bar to estimate size where possible.
- Describe staining or contrast effects in photomicrographs.
Comparing the Structure of Typical Plant and Animal Cells
🌿 Overview
- Both plant and animal cells are eukaryotic, with a nucleus and membrane-bound organelles.
- They have key structural differences adapted to their specific functions.
📊 Comparison Table: Plant Cells vs Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present; rigid cellulose wall for support and protection | Absent; only a flexible cell membrane |
| Shape | Usually regular, rectangular or box-like | Irregular, rounded or varied shapes |
| Large Permanent Vacuole | Present; large central vacuole maintaining turgor pressure | Absent or small temporary vacuoles |
| Chloroplasts | Present; site of photosynthesis with chlorophyll | Absent; no photosynthesis |
| Centrioles | Usually absent | Present; involved in cell division |
| Lysosomes | Rare or absent | Present; contain digestive enzymes |
| Plasmodesmata | Present; channels through cell walls for communication | Absent |
| Cilia and Flagella | Rare; usually absent | Often present; aid movement |
| Energy Storage | Stores starch | Stores glycogen |
| Cell Surface Membrane | Inside cell wall; controls substance entry/exit | Outer boundary; controls entry/exit |
🧠 Summary
| Characteristic | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Yes (cellulose) | No |
| Shape | Fixed, rectangular | Flexible, irregular |
| Vacuole | Large, central | Small or none |
| Chloroplasts | Present | Absent |
| Centrioles | Absent | Present |
| Lysosomes | Rare | Present |
| Plasmodesmata | Present | Absent |
| Energy storage | Starch | Glycogen |
Plant cells are specialised for support, photosynthesis, and storage, while animal cells are more varied in shape and specialised for movement and complex interactions.
Cells Use ATP from Respiration for Energy-Requiring Processes
🌱 What is ATP?
- ATP (Adenosine Triphosphate) is the energy currency of the cell.
- It stores and transports energy within cells.
🔥 How is ATP Produced?
- ATP is produced during cellular respiration in mitochondria.
- Respiration breaks down glucose (or other molecules) to release energy, which is stored in ATP.
⚙️ Role of ATP in Cells
- Provides energy needed for energy-requiring processes such as:
- Active transport: Moving substances against concentration gradients across membranes.
- Protein synthesis: Building proteins from amino acids.
- Cell division: Processes like mitosis and cytokinesis.
- Muscle contraction: Allowing movement in muscle cells.
- Metabolic reactions: Various chemical reactions that require energy input.
- Movement of organelles: Transporting materials inside the cell.
ATP generated by respiration is essential for powering all cellular activities that require energy.
Without ATP, cells cannot perform vital functions necessary for life.
Key Structural Features of a Prokaryotic Cell (Typical Bacterium)
🌱 Basic Characteristics
- Unicellular: Prokaryotes exist as single, independent cells.
- Size: Generally small, about 1–5 µm in diameter.
🧱 Cell Wall
- Made of peptidoglycan, a strong, mesh-like polymer that provides shape and protection.
- Unlike plant cells, no cellulose; distinct bacterial structure.
🧬 Genetic Material
- Contains a single, circular DNA molecule located in the nucleoid region (no true nucleus).
- DNA is not enclosed in a membrane-bound nucleus.
🧪 Ribosomes
- Have 70S ribosomes, smaller than eukaryotic 80S ribosomes.
- Ribosomes are the site of protein synthesis.
🚫 Absence of Membrane-Bound Organelles
- No mitochondria, chloroplasts, endoplasmic reticulum, or Golgi apparatus.
- Cellular processes occur in the cytoplasm or at the cell membrane.
| Feature | Description |
|---|---|
| Unicellular | Single-celled organism |
| Size | 1–5 µm diameter |
| Cell Wall | Peptidoglycan layer |
| DNA | Circular, free in cytoplasm |
| Ribosomes | 70S type |
| Membrane-bound organelles | Absent |
Prokaryotic cells are simpler than eukaryotic cells, lacking nucleus and membrane-bound organelles, but have unique features like peptidoglycan walls and 70S ribosomes essential for their survival.
Comparing Prokaryotic Cells (Typical Bacterium) with Eukaryotic Plant and Animal Cells
🌟 Overview
- Prokaryotic cells are simpler and smaller than eukaryotic cells.
- Eukaryotic plant and animal cells are more complex with membrane-bound organelles and distinct structures.
📊 Comparison Table: Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic Cell (Bacterium) | Eukaryotic Plant Cell | Eukaryotic Animal Cell |
|---|---|---|---|
| Cell Type | Unicellular | Unicellular or multicellular | Unicellular or multicellular |
| Size | Small (1–5 µm) | Larger (10–100 µm) | Larger (10–30 µm) |
| Nucleus | No true nucleus; circular DNA free in cytoplasm | True nucleus with double membrane | True nucleus with double membrane |
| DNA | Circular, naked DNA | Linear DNA wrapped around histones | Linear DNA wrapped around histones |
| Cell Wall | Present; made of peptidoglycan | Present; made of cellulose | Absent |
| Membrane-bound Organelles | Absent | Present (mitochondria, chloroplasts, ER, Golgi) | Present (mitochondria, ER, Golgi, lysosomes) |
| Ribosomes | 70S (smaller) | 80S in cytoplasm, 70S in mitochondria/chloroplasts | 80S |
| Chloroplasts | Absent | Present (photosynthesis) | Absent |
| Vacuoles | Rare, small | Large permanent vacuole | Small or absent |
| Flagella | Simple structure, made of flagellin | Absent or rare | Complex structure (9+2 microtubule arrangement) |
| Cell Division | Binary fission | Mitosis and meiosis | Mitosis and meiosis |
🔍 Key Differences Explained
- Nucleus: Prokaryotes lack a membrane-bound nucleus, their DNA floats freely. Eukaryotes have a defined nucleus.
- Organelles: Eukaryotic cells have many specialized organelles surrounded by membranes, prokaryotes do not.
- Cell Wall Composition: Prokaryotic cell walls contain peptidoglycan, plant cell walls contain cellulose, animal cells have no cell wall.
- Size and Complexity: Prokaryotes are smaller and structurally simpler, eukaryotic cells are larger and more complex.
- Ribosomes: Smaller ribosomes (70S) in prokaryotes vs larger (80S) in eukaryotes, mitochondria and chloroplasts in eukaryotes contain 70S ribosomes.
| Feature | Prokaryotic Cell | Eukaryotic Plant Cell | Eukaryotic Animal Cell |
|---|---|---|---|
| Size | Small (1–5 µm) | Larger (10–100 µm) | Larger (10–30 µm) |
| Nucleus | No | Yes | Yes |
| DNA | Circular, naked | Linear, in nucleus | Linear, in nucleus |
| Cell Wall | Peptidoglycan | Cellulose | None |
| Membrane-bound Organelles | None | Present | Present |
| Ribosomes | 70S | 80S cytoplasm + 70S organelles | 80S |
Prokaryotic cells are small, simple, and lack membrane-bound organelles and a true nucleus.
Eukaryotic cells (plant and animal) are larger and more complex, with a true nucleus and specialized organelles.
Plant cells differ from animal cells mainly by having a cell wall, chloroplasts, and large vacuoles.
Viruses: Structure and Characteristics
🌟 Key Features of Viruses
- Viruses are non-cellular structures – they do not have a cellular organization.
- Each virus contains a nucleic acid core, which can be either:
- DNA (deoxyribonucleic acid) or
- RNA (ribonucleic acid), but never both.
- The nucleic acid core is enclosed within a protective protein coat called a capsid.
- Some viruses possess an additional outer envelope made of phospholipids, usually derived from the host cell membrane.
| Component | Description |
|---|---|
| Non-cellular | Viruses are not made of cells |
| Nucleic Acid Core | Contains either DNA or RNA (genetic material) |
| Capsid | Protein coat protecting the nucleic acid |
| Outer Envelope | Phospholipid layer present in some viruses |
Because they lack cellular structures and metabolism, viruses cannot reproduce independently and must infect host cells to multiply.