Characteristics of Living Organisms
All living organisms share defining characteristics that distinguish them from non-living matter. Understanding these characteristics is fundamental to biological classification and study.
MRS GREN - Seven Characteristics of Life
Movement
- All living organisms exhibit some form of movement, either externally visible or internal.
- Plants show growth movements (tropisms) such as phototropism (toward light) and gravitropism (root down, shoot up).
- Animals display locomotion through muscles, cilia, or flagella.
Respiration
- All living systems require energy release through respiration.
- Aerobic respiration: Glucose + Oxygen → Carbon dioxide + Water + Energy
- Anaerobic respiration: Glucose → Lactic acid/Alcohol + Energy
- Occurs continuously in all living organisms at cellular level.
Sensitivity
- Organisms can detect stimuli from their environment and respond accordingly.
- Examples include the pupil reflex, shivering response to cold, and plant responses to light.
- Nerves relay electrical impulses for animals; hormones act as chemical messengers.
Growth
- Permanent increase in dry mass through cell division.
- Not simply getting bigger or accumulating food - must be irreversible growth of cells.
- Involves mitosis to produce more cells.
Reproduction
- Production of new individuals, either sexually or asexually.
- Asexual reproduction: genetically identical offspring from one parent (bacteria binary fission, plants vegetative propagation).
- Sexual reproduction: fusion of nuclei from male and female gametes to form a zygote; offspring genetically different.
- Species defined as a group of organisms that can reproduce to produce fertile offspring.
Excretion
- Elimination of harmful waste products from the body.
- In humans: kidneys excrete urea, excess water and ions through urine; lungs excrete carbon dioxide and water vapour.
- Important: removal of toxic nitrogenous compounds (e.g., urea from protein metabolism).
Nutrition
- Uptake and processing of raw materials to build organic molecules for energy and cell structures.
- Autotrophs make own food via photosynthesis (plants, algae).
- Heterotrophs ingest or absorb nutrients from other organisms (animals, fungi).
- Essential for growth, repair, and maintenance of life processes.
Classification of Living Organisms
The binomial naming system provides a standard method to classify species using Latin/Scientific names: Genus species (e.g., Homo sapiens, Quercus robur).
Construction of Dichotomous Keys
Dichotomous keys are used for identification by making a series of choices between two contrasting features.
Example dichotomous key:
Five Kingdom Classification (Supplement)
Organisms classified into five kingdoms based on shared characteristics and evolutionary relationships:
| Kingdom | Key Features |
|---|---|
| Animalia (Animals) | Multicellular, heterotrophic, no cell walls, mobile at some stage |
| Plantae (Plants) | Multicellular, autotrophic, cell wall present, photosynthetic |
| Fungi | Eukaryotic, cell walls of chitin, absorb nutrients, non-motile |
| Prokaryota (Bacteria) | No nucleus, single circular DNA, prokaryotic cells |
| Protoctista (Protoctists) | Mostly unicellular, euglena, algae, amoeba |
Binary Fission in Single-Cell Organisms (Supplement)
Single-cell organisms reproduce by binary fission:
- The cell replicates its DNA and grows, creating a new membrane within the cell wall.
- Division creates two genetically identical daughter organisms (clones).
Cell Structure
Cells are the basic units of life. Understanding cell structure is essential for understanding how organisms function.
Comparison: Plant vs Animal Cells
Both contain organelles like nucleus, cytoplasm, mitochondria, and ribosomes, but have key differences.
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (cellulose) | Absent |
| Chloroplasts | Present | Absent |
| Large Permanent Vacuole | Large central vacuole | Smaller or none |
| Shape | Usually rectangular | Variable/irregular |
Bacterial Cell Structure
Bacteria are prokaryotes (lack a membrane-bound nucleus). Key structures include:
| Structure | Description | Function |
|---|---|---|
| Cell Wall | Rigid outer layer made of peptidoglycan | Protection, shape maintenance |
| Plasma Membrane | Semi-permeable membrane | Controllles substance passage into/out of cell |
| Cytoplasm | Jelly-like matrix containing water, enzymes and dissolved substances | Site of metabolic reactions |
| Ribosomes | Small granules without membranes | Protein synthesis |
| Circular DNA | Single circular chromosome located in nucleoid region (no nuclear membrane) | Contains genetic information |
| Plasmids | Small rings of extra DNA not found in bacteria | May contain antibiotic resistance genes |
Organelle Structures and Functions
Nucleus
- Control centre of the cell.
- Contains chromosomes made of DNA wrapped around histone proteins.
- Controls cellular activities including growth, metabolism, and reproduction.
Cell Membrane (Plasma Membrane)
- Semi-permeable barrier controlling what enters or leaves the cell.
- Composed mainly of phospholipids with embedded proteins.
- Maintains internal environment homeostasis through selective permeability.
Cytoplasm
- Fluid matrix where organelles are suspended.
- Contains water, ions, small organic molecules and enzymes.
- Site for glycolysis and many metabolic reactions.
Cell Wall (Plant Cells Only)
- Outer layer of cellulose surrounding plasma membrane.
- Provides structural support and protection.
- Rigid structure helps maintain cell shape.
Chloroplasts
- Site of photosynthesis in plant cells and algac.
- Contain:
| Component | Description | Function |
|---|---|---|
| Chlorophyll | Green pigment | Absorbs light energy for photosynthesis |
| Stroma | Fluid surrounding thylakoids | Site of dark/reaction phase of photosynthesis |
| Grans (Thylakoids) | Discs within stroma | Site of light-dependent reactions; contain chlorophyll and electron carriers |
Mitochondria
- Site of aerobic respiration.
- Powerhouse of the cell - produces ATP for energy-requiring activities.
- Adapted structure: folded cristae increase surface area; double membranes allow ion gradients.
Ribosomes
- Site of protein synthesis from mRNA templates.
- Smaller in size when found freely in cytoplasm compared to large ones attached to rough endoplasmic reticulum.
- Translation occurs here where amino acids assemble into polypeptides following base sequence information delivered by mRNA.
Vacuoles
- Membrane-bound organelles that store cell fluids such as sugar, water, pigments and mineral salts.
- In plant cells: large central vacuole provides turgor pressure for structural support.
- In animal cells: smaller lysosomes may be present which perform waste digestion or contain digestive enzymes needed by the cell.
Specialised Cells
Cells differentiate into specialised types to perform specific functions efficiently. Understanding these adaptations helps explain how organisms function at different levels.
Ciliated Epithelium Cells
Found lining surfaces where material movement is required, such as bronchial tubes and fallopian tubes.
| Feature | Description | Function |
|---|---|---|
| Cilia | Hair-like projections on free surface of cell beating synchronously | Move foreign particles or mucus along respiratory tract toward throat for mucociliary clearance; moves egg through oviducts to uterus |
| Cell Body | Basal surface contacts underlying tissue; microvilli and mitochondria present | Provides energy (ATP) from respiration needed to power cilia beating against ciliary beat frequency rate |
Root Hair Cells
Found in root systems of plants extending into soil for water and mineral ion absorption.
| Adaptation | Description | Function |
|---|---|---|
| Long Projections (Root Hairs) | Elongated extensions from epidermis cells into surrounding soil | Increase surface area 50–200 times compared to non-specialised epidermal cells for faster uptake of water/minerals through diffusion and active transport |
| Thin Cell Membranes | Single cell membrane with minimal wall barrier | Facilitates quicker diffusion rates (shorter distance) between soil solution entering cytoplasmic fluid via facilitated or active transport mechanisms |
| Large Vacuole | Filled with concentrated solute mixture including mineral salts | Creates strong water potential gradient drawing water in through osmosis mechanism |
| Mitochondria | Abundant organelles present for energy supply needs | Provides ATP required for active uptake of essential minerals such as nitrates, potassium and magnesium ions from soil solutions via active transport against concentration gradients (Supplement) |
Palisade Mesophyll Cells
Located in upper layers of leaf tissue directly beneath epidermis where light intensity is high.
| Feature | Description | Function |
|---|---|---|
| Elongated Columnar Shape | Tightly packed vertically aligned cylindrical cells forming parallel strands called “palisade” layer arrangement | Maximised photon capture; efficient packing maximises absorption of sunlight available at leaf surface for photosynthesis light reaction phase |
| High Density Chloroplasts | Numerous chloroplasts concentrated in these cells compared to spongy mesophyll below with fewer chloroplasts | Optimal photosynthetic machinery placement: more pigment captures greater energy from incident solar radiation per unit area |
Neurones (Nerve Cells)
Specialised for rapid electrical signal transmission throughout nervous system.
| Adaptation | Description | Function |
|---|---|---|
| Long Axon | Extended process up to 1 metre in size (in some motor neurones connecting spinal cord to foot) | Rapid transmission distances between central processing regions (neuronal relay stations) and effector organs/muscles |
| Myelin Sheath | Fatty insulating layer wrapped repeatedly around axon by Schwann cells (PNS) or oligodendrocytes (CNS) producing multiple segments separated by gaps known as Nodes of Ranvier | Saltatory conduction allows impulses to jump across nodes with faster transmission speeds up to 120 m/s compared to unmyelinated fibres conducting 1–2 m/s; also reduces energy consumption during prolonged use |
| Cell Body (Perikaryon) | Contains nucleus, mitochondria and ribosomes needed for synthesis | Maintains metabolic functions required for sustained firing capacity |
Sizes of Specimens
Magnification Formula
The relationship between actual size, image size and magnification is given by:
Magnification = Image Size / Actual Size
This formula can be rearranged to solve for either image size or actual size:
- Image Size = Magnification × Actual Size
- Actual Size = Image Size / Magnification
Measurement Conversions and Units
Units Used in Biology
| Unit | Symbol | Equivalent | Used For |
|---|---|---|---|
| Metre | m | 1 | Large-scale measurements (room dimensions, animal heights) |
| Millimetre | mm | 1/1000 of a metre (0.001 m) | Measurements up to few millimetres like leaf veins diameter |
| Micrometre | μm | 1/1000 of a millimetre (1×10^(-6) m = 0.000001 m) | Microscopic structures like cells, bacteria |
| Nanometre | nm | 1/1000 of a micrometre (1×10^(-9) m) | Molecular scales: viruses, proteins |
Converting Between Units
Micrometres to Millimetres:
- Divide by 1000: 5 μm = 0.005 mm
- Multiply by 10^(-3): 1000 μm = 1 mm = 0.001 m
Millimetres to Micrometres:
- Multiply by 1000: 2.5 mm = 2500 μm
- Multiply by 10^(6): 0.5 mm = 500 μm
Example calculation from syllabus (Core): Calculate actual size of bacterium given image measure at microscope: If a bacterium appears as 5mm long under a microscope magnified ×5000, what is its actual length?
- Image Size = 5 mm = 5000 μm (convert to same unit)
- Magnification = 5000×
- Actual Size = 5000μ / 5000 = 1 μm (1×10^(-3) mm)
Typical Sizes in Biology
| Organism/Structure | Approximate Size Range |
|---|---|
| Viruses | 20-300 nm (0.02-0.3 μm) |
| Bacteria | 1000–5000 μm (1-5 mm diameter, typical rod-shaped bacteria like E.coli ~1μm × 2-4μm length) |
| Unicellular organisms | 10–500 μm depending on species (yeast ~7μm, amoeba ~300-600μm) |
| Plant Epidermal Cells | 10-50 μm wide |
| Animal Epithelial Cells | 20-50 μm |
| Red Blood Cells | ~7.5 μm diameter |
| Sperm Cell (HUMAN) | Tail length ~60 μm, head ~5 μm |
| Human Hair Diameter | 50–100 μm depending on individual |
Practice Problems
Question 1: A cell is viewed under an electron microscope at a magnification of ×50,000. The image length measures 2.5 cm. Calculate actual cell size in micrometres.
- Convert image to μm: 2.5 cm = 25 mm = 25,000 μm
- Apply formula: Actual Size = Image Size / Magnification
- Actual Size = 25,000 μm / 50,000 = 0.5 μm
Question 2: If a bacteria is 2 μm long and photographed at ×1000 magnification, what would appear on the photograph in mm?
- Image Size = Magnification × Actual Size
- Image Size = 1000 × 2 μm = 2000 μm
- Convert to mm: 2000 μm = 2 mm
Glossary
| Term | Definition | Related Syllabus Section |
|---|---|---|
| Autotroph | Organism that makes its own food via photosynthesis | Chapter 6 Plant Nutrition |
| Heterotroph | Organism that obtains organic molecules by eating other organisms | Chapter 1 Classification |
| Prokaryote | Cell without membrane-bound nucleus | Chapter 2 Bacterial Structure |
| Eukaryote | Cell with nucleus containing DNA in nuclear envelope | Chapter 2 Plant/Animal Cells |
| Plasmid | Small circular DNA molecule separate from chromosomal DNA in prokaryotes | Chapter 2 Bacteria |
| Mitosis | Nuclear division producing identical daughter cells for growth or replacement | Chapter 14 Inheritance |
| Meiosis | Cell division reducing chromosome number (diploid → haploid) to form gametes | Chapter 16 Reproduction |
| Binary Fission | Asexual reproduction in single-celled organisms by simple splitting | Chapter 1 Protoctists/Bacteria |
| Magnification | Ratio of image size to actual specimen dimension | Chapter 2 Sizes of Specimens |