Lesson 11: Cell Structure and Micro-organisms

In the figure : In the figure the levels of organization in living organisms are shown: cells, tissues, organs, organ systems and the organism.

Rule 1: Levels of organization — Cells join to form tissues, tissues form organs, organs form organ systems and organ systems form the organism.

• Easy : Many muscle cells form muscle tissue.
• Medium : The heart is an organ made of tissues; the heart and blood vessels form the circulatory system.
• Exam-type : Order from smallest to largest: cell → tissue → organ → organ system → organism.

In the figure : In the figure Robert Hooke saw the compartments of a thin section of cork under a microscope in 1665 and named them cells. A marks one compartment.

Rule 2: Discovery of the cell — In 1665 Robert Hooke saw hexagonal compartments like a bee-hive in cork and called them 'cells' (Latin 'cella' = small room). In 1673 Anton van Leeuwenhoek made a microscope from lenses and first saw live bacteria and protozoa. In 1838 Schleiden and Schwann gave the cell theory: all living organisms are made of cells. In 1885 Rudolph Virchow said all cells arise from pre-existing cells.

• Easy : Hooke named cells in 1665.
• Medium : Leeuwenhoek's microscope came in 1673, which is 1673 − 1665 = 8 years after Hooke's cork.
• Exam-type : The cell theory (1838) came 1838 − 1665 = 173 years after the naming of cells, and Virchow's statement 1885 − 1838 = 47 years after the theory.

In the figure : In the figure 1 millimetre = 1000 micrometres and 1 micrometre = 1000 nanometres, so each unit is 1000 times smaller than the one above it.

Rule 3: Measuring cells — Cells cannot be seen with the naked eye. They are measured in micrometres (microns) and nanometres. 1 mm = 1000 µm; 1 µm = 1000 nm.

• Easy : 2 mm = 2 × 1000 = 2000 µm.
• Medium : 3 µm = 3 × 1000 = 3000 nm.
• Exam-type : 0.5 mm = 0.5 × 1000 = 500 µm = 500 × 1000 = 500,000 nm.

In the figure : In the figure of a compound microscope: A eyepiece, B objective lens, C stage (the slide is placed here), D mirror or light and E arm.

Rule 4: Compound microscope — A compound microscope is used to see cells. Onion peel is observed with a drop of dilute iodine or eosin, covered with a cover-slip, under the 10X objective.

• Easy : The slide is placed on the stage.
• Medium : Stain: a drop of dilute iodine or eosin makes the cells easier to see.
• Exam-type : The parts in order from your eye to the slide: eyepiece, objective lens, stage.

Spot the difference

In the figure : In the figure of a plant cell: A cell wall, B chloroplast, C large vacuole, D nucleus and E cytoplasm. The cell wall and chloroplasts are not found in animal cells.

Rule 1: Parts of a cell — Four main parts: cell wall (outermost covering, only in plant cells), plasma membrane (thin, delicate, flexible; outermost covering of animal cells), cytoplasm (the liquid between the plasma membrane and the nucleus) and cell organelles.

• Easy : Only plant cells have a cell wall.
• Medium : The cytoplasm is the liquid part between the plasma membrane and the nucleus.
• Exam-type : A plant cell has a definite shape because of its cell wall and one large vacuole.

In the figure : In the figure of an animal cell: A plasma membrane, B nucleus, C mitochondrion (the powerhouse), D cytoplasm and E a small vacuole.

Rule 2: Organelles and their work — Nucleus (double porous membrane) controls all functions. Endoplasmic reticulum changes proteins made by ribosomes and sends them to Golgi bodies. Golgi bodies (flat sacs) distribute proteins. Mitochondria produce energy: the powerhouses of the cell. Chloroplasts carry out photosynthesis. Vacuoles throw out waste; small in animal cells, one large in plant cells. Organelles are studied with the electron microscope, which magnifies up to 2 × 10⁹ (two billion) times.

• Easy : Mitochondria: powerhouse (energy).
• Medium : Chloroplasts make food by photosynthesis, so only green plant cells have them.
• Exam-type : A friend says "animal cells have a cell wall and chloroplasts." Wrong: they have neither.

Spot the difference

In the figure : In the figure the sizes named in this chapter are shown (not drawn to scale): Paramoecium about 100 micrometres, typhoid bacteria 1 to 3 micrometres and polio virus 28 nanometres.

Rule 1: Where they live, and how big — They live in air, water, soil, food, sewage, garbage and in the bodies of plants and animals. Some are solitary (amoeba, paramoecium), some live in colonies, some on dead plants and animals. We cannot see objects smaller than 100 micrometres. If 1000X magnification is not enough, an electron microscope is used.

• Easy : Amoeba and paramoecium live alone.
• Medium : Paramoecium (100 µm) is about 100 ÷ 3 ≈ 33 times larger than a 3 µm typhoid bacterium.
• Exam-type : Polio virus is 28 nm = 28 ÷ 1000 = 0.028 µm, much smaller than bacteria.

In the figure : In the figure a bacterium is a prokaryotic cell: it has a plasma membrane (A), a nucleoid (B) and cytoplasm (C), but no membrane-bound organelles.

Rule 2: Prokaryotic cells — Bread fungus and algae in ponds are multi-cellular. Most bacteria and viruses are unicellular, with a plasma membrane, cytoplasm and nucleoid but no membrane-bound organelles: prokaryotic cells. NCCS, Pune does research in cell science.

• Easy : Most bacteria are unicellular.
• Medium : Prokaryotic cells have no membrane-bound organelles, unlike plant and animal cells.
• Exam-type : Of bread fungus, bacteria and a plant cell, the prokaryotic one is the bacterium.

In the figure : In the figure the five groups of micro-organisms are shown: A algae, B fungi, C protozoa, D bacteria and E viruses.

Rule 3: Surviving and classification — Each microbe needs a specific environment; some need oxygen and some do not. Some survive on the ocean floor, in polar ice and in hot springs. In adverse conditions they form a thick covering and stop life processes until conditions are favourable. They are classified as algae, fungi, protozoa, bacteria and viruses.

• Easy : Five groups: algae, fungi, protozoa, bacteria, viruses.
• Medium : Some microbes survive in polar ice and hot springs.
• Exam-type : In bad conditions a microbe makes a thick covering and waits; life processes restart when conditions are right.

Spot the difference

In the figure : In the figure a few drops of yoghurt in lukewarm milk, kept for 8 to 10 hours, turn the milk into yoghurt.

Rule 1: Everyday uses — Microbes decompose garden waste, dung and fruit peel into manure of the best quality (glass, metal and plastic do not change) and help in sewage disposal. Microbes in the soil and in root nodules of leguminous plants (pea, bean, fenugreek) change atmospheric nitrogen into compounds that increase soil fertility and the protein content of pulses.

• Easy : Fruit peels in a compost pit turn into manure.
• Medium : Glass, metal and plastic do not decompose, so do not put them in a compost pit.
• Exam-type : Peas, beans and fenugreek roots hold microbes that enrich the soil with nitrogen compounds.

In the figure : In the figure the uses of micro-organisms are shown: manure from garden waste (P), curd (Q), bread by fermentation (R) and vaccines (S).

Rule 2: Food, medicine and industry — Butter, buttermilk and cheese are made by microbes. Fermentation is used for yoghurt, alcohol, bread, acetic, citric and lactic acids, vitamins and antibiotics. Antibiotics (e.g. penicillin) destroy pathogens or slow their growth; tuberculosis, typhoid and cholera are now under control. Use them only on a doctor's prescription and complete the course. A vaccine is made in a laboratory with the help of microbes and gives immunity against a disease. Microbes are also used in tanning, making ropes from agave, clearing oil slicks and in biogas plants.

• Easy : Idli batter rises because of fermentation.
• Medium : Penicillin is an antibiotic that destroys or slows pathogens.
• Exam-type : An antibiotic course should be completed and taken only when a doctor prescribes it.

Spot the difference

In the figure : In the figure pathogens spread through dirty water and food (P, typhoid), through the air by sneezing and coughing (Q, tuberculosis) and through the bite of a female mosquito (R, dengue).

Rule 1: How pathogens spread — Contaminated food and water cause amoebiasis, typhoid, cholera, hepatitis and gastro. Sneezing and coughing release pathogens that cause the common cold, diphtheria, pneumonia and tuberculosis. Female mosquitoes breeding in garbage, drains and stagnant water spread malaria, dengue, elephantiasis, yellow fever, chikungunya and Zika fever.

• Easy : Typhoid spreads through dirty water and food.
• Medium : Tuberculosis spreads through the air when an infected person coughs or sneezes.
• Exam-type : Dengue and malaria spread through the bite of a female mosquito.

In the figure : In the figure with a sample reading: the normal body temperature is about 37 °C; when microbes enter, the immune system acts and the temperature rises (here to 39.5 °C).

Rule 2: Food spoilage and precautions — Food spoils when fungi grow on it or when microbes release toxic enterotoxins; eating it causes food poisoning (loose motions and vomiting). In humid air, fungi spoil gunny bags, cotton, paper, rubber and leather. Precautions: eat fresh, covered food; drink boiled water; hold a handkerchief while coughing or sneezing; do not let garbage and water accumulate. Almost 80% of diseases occur due to uncleanliness (Swachha Bharat Abhiyan).

• Easy : Eat fresh, covered food and drink boiled water.
• Medium : A rise of body temperature from 37 °C to 39.5 °C is 39.5 − 37 = 2.5 °C.
• Exam-type : If 80% of diseases come from uncleanliness, then out of 100 diseases, 80 can be prevented by cleanliness.

Spot the difference