Biology - The Fundamental Unit of Life

Cell

Basic structural and functional unit of living organisms

Cell biology

Branch of biology that studies cells

Robert Hooke (1665)

Observed dead cells in cork, called them 'cell'

Anton Van Leeuwenhoek

First to discover free living cells in pond water

Robert Brown (1831)

Discovered the nucleus in the cell

Dujardin

Discovered living fluid substance of cell and named Sarcode

J. E. Purkinje

Coined the term ‘protoplasm’ for the fluid substances of the cell

Huxley

Called protoplasm "The physical basis of life"

Rudolf Virchow (1855)

Established that all cells arise from pre-existing cells (“omnis cellula e cellula”)

Cell

A little room; structural and functional unit of living organisms

Compound microscope

Optical instrument with objective lens and eyepiece for magnified images

Electron microscope

Enabled observation of complex cell structure and organelles

Cell Theory

All plants and animals are composed of cells, the basic unit of life

Virchow's expansion of Cell Theory

All cells arise from pre-existing cells

Cell Theory

All living organisms are composed of cells and products of cells; all cells arise from pre-existing cells

Normal size of human cell

20 µm to 30 µm in diameter (1 µm = 1 × 10–6 m)

Largest isolated single cell in animals

Ostrich egg

Longest cell in animals

Nerve cell

Smallest cell

PPLO (Pleuro Pneumonia Like Organism/Mycoplasma, 0.3 µm)

Bacteria size

3-5 m

Viruses

Smaller than bacteria, lack cellular structure

Longest living cell in human body

Nerve cells (neurons)

Human red blood cells size

7 µm in diameter

Shape of cell

Mainly depends upon the specific function it performs

Elongated & branched cell

Nerve cell

Discoidal/saucer shaped cell

RBC

Spindle shaped cell

Muscle cell

Spherical shaped cell

Eggs

Branched shaped cell

Pigment cell of the skin

Slipper shaped cell

Paramecium

Cuboidal shaped cell

Germ cells of gonads

Polygonal shaped cell

Liver cells

Dynamic shapes

Some cells can change shape (e.g., Amoeba, WBC)

Unicellular organisms

Made of a single cell (e.g., Bacteria, Amoeba, Paramecium, Chlamydomonas)

Multicellular organisms

Made of a number of cells (e.g., most fungi, plants, animals)

Cell organelles

Sub-cellular structures with characteristic form, structure, and function

Size of organism

Depends on cell number, not cell size

Cell inclusions

Non-living material in cytoplasm (e.g., pigment granules, starch granules, fat droplets)

Division of labour

Cells are specialised to perform different functions

Cell

All life functions reside in cells, may be specialised

Basic structure of the cell

Cell membrane, Protoplasm (Cytoplasm + Nucleus)

Cytoplasm

Cytosol + Cell organelles

Nucleus

Nuclear membrane, Nucleoplasm, Nucleolus, Chromatin threads

Single membranous organelles

Endoplasmic Reticulum, Golgi body, Lysosomes, Vacuole

Double membranous organelles

Plastid, Mitochondria, Nucleus

Non-membranous organelles

Ribosomes, Centrosomes

Plant Cell

Plasma Membrane, Cytoplasm, Mitochondria, Cell Wall, Ribosome, Nucleus, Endoplasmic Reticulum, Golgi Apparatus, Chloroplast, Vacuole

Animal Cell

Plasma membrane, Cytoplasm, Mitochondria, Ribosomes, Nucleus, Endoplasmic Reticulum, Lysosomes, Golgi Apparatus, Centriole

Onion peel cells

Regularly arranged linear or rectangular compartments with rigid cell walls, nucleus pushed to periphery (central vacuole)

Plant cells

Definite cell wall, plastids, well-developed vacuole; animal cells lack cell wall and plastids, have centrosome

Human cheek cells

Flat and polygonal, distinct rounded nucleus in the middle, thin cell membrane, lightly stained cytoplasm

Plasma membrane

Flexible outer boundary of living cells, made of lipids, proteins, carbohydrates

Endocytosis

Flexibility of plasma membrane enables engulfing food and other materials

Selectively permeable barrier

Allows specific molecules to pass, regulates entry/exit

Diffusion

Movement of small molecules (O2, CO2) from high to low concentration

Active transport

Transport against concentration gradient with energy expenditure

Functions of plasma membrane

Protects internal components, provides shape, allows material entry/exit via pores

Fluid mosaic model

Plasma membrane is a bilayer of phospholipids and proteins

Membrane carbohydrates

Involved in cell recognition by interacting with surface molecules of other cells

Plasmolysis

Shrinking of protoplasm from cell wall due to water loss in hypertonic solution

Crenation

Shrinking of protoplasm due to water loss in hypertonic solution

Robert Hooke's discovery of cell

Observed cork under primitive microscope, described box-like structures as cells in 'Micrographia'

Cell as structural unit

Living organisms made of one or more cells

Cell as functional unit

All life functions reside in cells, may be specialised

Passive transport

Movement of substances along concentration gradient, no energy needed

Diffusion

Movement of substances from high to low concentration to spread uniformly

Osmosis

Net diffusion of water across a selectively permeable membrane towards higher solute concentration

Examples of osmosis

Absorption of water by plant roots, unicellular fresh water organisms

Endosmosis

Osmotic entry of water into a cell or system

Exosmosis

Osmotic withdrawal of water from a cell or system

Hypotonic solution

External solution dilute compared to cell, net water flow into cell

Isotonic solution

External solution same concentration as cell, no net water movement, cell size remains same

Hypertonic solution

External solution more concentrated than cell, net water flow out of cell

Endocytosis

Taking in bulk materials from external environment

Phagocytosis

Cell eating

Pinocytosis

Cell drinking

Diffusion across cell membrane

Move out/in along concentration gradient during respiration

Osmosis

Net diffusion of water across selectively permeable membrane

Hypotonic solution effect

Swelling, may burst (no cell wall)

Hypotonic solution effect

Swelling, turgid (cell wall prevents bursting)

Isotonic solution effect

No net water movement, cell size unchanged

Hypertonic solution effect

Water moves out, cell shrinks

Cell wall

Outermost rigid, freely permeable layer in plant cells, bacteria, fungi, some protists

Cell wall

Absent

Function of cell wall

Withstand hypotonic media without bursting, provides shape and support

Cellulose

Fibrous polysaccharide in plant cell walls

Peptidoglycan

Component of cell walls in bacteria and blue-green algae

Chitin

Component of cell walls in most fungi

Plasmolysis

Plant cell in hypertonic solution loses water, protoplasm shrinks from cell wall

Nucleus

Double membrane bound dense protoplasmic body, controls cell metabolism, contains genetic information

Nucleus

Regulates all cellular activities

Structure of nucleus

Nuclear membrane, Nucleoplasm, Nucleolus, Chromatin threads

Nuclear membrane

Double membrane surrounding nucleus with nucleopores

Nucleopores

Minute pores in nuclear membrane for exchange between nucleoplasm and cytoplasm

Nucleoplasm

Protoplasm within nuclear membrane, contains chromatin and nucleolus

Nucleolus

Usually one per nucleus, storehouse of RNA

Chromatin threads

Darkly stained network of long, fine threads in nucleoplasm, forms chromosomes during cell division

Functions of nucleus

Controls metabolic activities, directs protein synthesis for growth, forms ribosomes, regulates cell cycle, contains hereditary information

Prokaryotes

Organisms lacking a nuclear membrane

Nucleoid

Region in prokaryotes containing genetic material (DNA, RNA)