Living World, Cell and Tissues

Biology · 4 min read

Characteristics of living things

Living things grow, respire, need nutrition, respond to stimuli (irritability), reproduce, excrete, move and die. Metabolism is the sum of all chemical reactions in the body. Biology was named by Lamarck and Treviranus; Aristotle is the Father of Biology and Father of Zoology; Theophrastus is the Father of Botany.

Cell: the basic unit of life

Robert Hooke (1665) saw the cell in a slice of cork and named it. Anton van Leeuwenhoek first saw living cells (bacteria, protozoa) with a simple microscope.
Cell theory (Schleiden and Schwann, 1838 to 1839; extended by Virchow, 1855): all living things are made of cells; the cell is the basic unit; new cells arise from existing cells.
Robert Brown (1831) discovered the nucleus. Purkinje named protoplasm.
Smallest cell: Mycoplasma (PPLO). Largest cell: ostrich egg. Longest cell: nerve cell (neuron). Largest human cell: ovum; smallest: sperm; red blood cell is among the smallest and lacks a nucleus.
Unicellular organisms: amoeba, paramecium, bacteria, yeast. Multicellular: humans, plants.

Prokaryotic and eukaryotic cells

Feature	Prokaryotic	Eukaryotic
Nucleus	No true nucleus (nucleoid, no membrane)	True nucleus with a membrane
Organelles	No membrane-bound organelles	Present
Size	Small	Larger
Examples	Bacteria, blue-green algae (cyanobacteria)	Plants, animals, fungi, protozoa

Parts of the cell

Part	Function
Cell membrane (plasma membrane)	Selectively permeable; controls entry and exit. In both plant and animal cells
Cell wall	Only in plants, fungi, bacteria; made of cellulose in plants; gives shape and protection
Nucleus	Control centre; contains chromosomes (DNA); nucleolus makes ribosomes
Cytoplasm	Jelly-like fluid where organelles lie
Mitochondria	Powerhouse of the cell; produce ATP by respiration; have own DNA
Ribosomes	Make proteins (the "protein factories")
Endoplasmic reticulum (ER)	Rough ER (with ribosomes) makes proteins; smooth ER makes lipids and detoxifies
Golgi apparatus (Camillo Golgi)	Packages, modifies and dispatches products; "post office"
Lysosomes	"Suicide bags"; digest waste and old cell parts
Plastids	Plant cells only. Chloroplasts (green, photosynthesis, contain chlorophyll), chromoplasts (colour), leucoplasts (store starch, oil, protein)
Vacuole	Large in plants (stores water, sap, maintains turgor); small in animals
Centrosome (centrioles)	Animal cells; help in cell division

Differences: plant cells have a cell wall, plastids, a large central vacuole; animal cells have centrioles and lysosomes (rare in plants).
Diffusion: movement from higher to lower concentration. Osmosis: movement of water through a semi-permeable membrane from lower solute to higher solute concentration. Raisins swell in water.
Plasmolysis: a plant cell shrinks in a strong salt solution; hypotonic (swells), isotonic (no change), hypertonic (shrinks).
Endocytosis and exocytosis carry large particles in and out.
Chromosomes (DNA with proteins) are seen during division. Genes are the units of heredity.
Cell division: mitosis (growth and repair; two identical cells; chromosome number is the same) and meiosis (formation of gametes; four cells with half the chromosomes). Humans have 46 chromosomes (23 pairs); gametes have 23.

Plant tissues

Tissue	Type	Function
Meristematic	Apical (tips), lateral (girth), intercalary (base of leaves)	Cells keep dividing, causing growth
Permanent: simple	Parenchyma (storage), collenchyma (flexibility), sclerenchyma (hard, husk of coconut, lignin)	Support and storage
Complex permanent	Xylem (water and minerals upward; vessels, tracheids), phloem (food downward; sieve tubes, companion cells)	Conduction (vascular tissue)
Protective	Epidermis with stomata, cork (dead, suberin)	Protection, gas exchange

Stomata open and close through guard cells; they are for gas exchange and transpiration.
Xylem and sclerenchyma are dead tissues (mostly).

Animal tissues

Tissue	Types and examples	Function
Epithelial	Squamous (skin, lining of mouth and blood vessels), cuboidal (kidney tubules), columnar (intestine), ciliated (windpipe)	Covering and protection
Connective	Blood, bone, cartilage, tendon (muscle to bone), ligament (bone to bone), adipose (fat), areolar	Support, binding, transport
Muscular	Striated (skeletal, voluntary), smooth (involuntary: gut, blood vessels), cardiac (heart, involuntary, striated)	Movement
Nervous	Neurons	Receive and conduct impulses

Blood is a fluid connective tissue. Cartilage is in the nose, ear and joints. Bone is the hardest tissue (calcium phosphate).
Cardiac muscle never tires; smooth muscles are involuntary.

← PreviousEveryday Chemistry: Water, Air, Fuels and Materials Next →Classification and Diversity of Life

Keep practising

Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.

Free mock tests Join Telegram

Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.