NCERT Summary: Summary of Biology - 6

Plant Reproduction

Flowering plants (Angiosperms)

Flowering plants, or angiosperms, are the most recently evolved group of seed plants. They first appeared during the Mesozoic era, over 100 million years ago. All angiosperms produce flowers; when sexual reproduction occurs, fertilisation produces a diploid zygote and a triploid nutritive tissue (endosperm) to support the developing embryo.

Flowers - structure and function

A flower is a compact reproductive shoot in which nodes are very close together and the floral parts are arranged in whorls. Flowers have both sterile and reproductive parts. The sterile parts are the sepals (collectively the calyx) and petals (collectively the corolla). When sepals and petals are similar in form they are called tepals. The reproductive parts are the stamens (male; the androecium) and one or more carpels (female; a single carpel is often called a pistil and the set of carpels is the gynoecium).

Pollen

Pollen grains carry the male gametophyte (microgametophyte) stage of the life cycle. Pollen arises by meiosis of microspore mother cells within the anther microsporangia. Each pollen grain has an outer wall called the exine, composed mainly of the durable biopolymer sporopollenin. Inside the pollen grain are typically two haploid cells: the tube cell (or tube nucleus), which will direct formation of the pollen tube, and the generative cell that divides to form two sperm cells. The generative cell may divide before or after pollen is released (before or after pollination).

Pollination

Pollination is the transfer of pollen from an anther to a stigma. Pollination may be:

• by insects (entomophily),
• by wind (anemophily),
• by birds or bats,
• by water, or
• assisted by humans (artificial pollination).

Some flowers are self-pollinating (autogamous), while many species have mechanisms promoting cross-pollination (allogamy). Flower colour, scent and nectar often indicate the likely pollinator: for example, red and tubular flowers tend to attract birds, yellow and blue hues attract bees, and pale nocturnal flowers may attract moths. Wind-pollinated flowers typically have reduced or absent petals (for example, many grasses and oaks) and produce large quantities of light pollen.

Gynoecium: stigma, style and ovary

The gynoecium consists of one or more carpels. Each carpel usually has a stigma (a receptive surface where pollen lands), a style (a stalk or channel that positions the stigma and provides a route for the pollen tube), and an ovary (the basal chamber that contains one or more ovules). Pistils formed from fused carpels may be compound, and the degree of fusion is an important characteristic in plant classification.

The stigma provides a surface and biochemical environment for pollen adhesion, hydration and germination. The style places the stigma at a species-specific distance from the ovary and often guides the pollen tube towards the ovules. In maize (corn), the long hairs of the silk are part stigma and style.

Ovules and the female gametophyte

Each ovule within the ovary contains the female gametophyte (the embryo sac in angiosperms). The embryo sac develops from a megaspore mother cell by meiosis and subsequent mitoses to produce a typical seven-celled, eight-nucleate structure (in most angiosperms): one egg cell, two synergids, three antipodal cells and a central cell containing two polar nuclei. The micropyle is the opening through which the pollen tube usually enters the ovule.

Double fertilisation

After pollination the pollen germinates on the stigma and the pollen tube grows down the style toward the ovule. The generative cell divides to form two sperm cells, which travel down the pollen tube. Upon reaching the embryo sac, the pollen tube usually enters via a synergid. One sperm cell fuses with the egg cell to form a diploid zygote that develops into the embryo. The second sperm fuses with the two polar nuclei in the central cell to form the typically triploid endosperm, a nutritive tissue that supports embryo growth. This coordinated fusion of two separate fertilisation events is called double fertilisation and is a defining feature of angiosperms.

Fruit and seed development

Following fertilisation the ovary wall (and sometimes other floral parts) develops into a fruit that encloses and helps disperse the seeds. Fruits may be fleshy (e.g. mango, tomato), dry (e.g. legumes), multiple (derived from several flowers) or aggregate (derived from many carpels of one flower). Each seed develops from a fertilised ovule and contains the embryo and a food reserve, usually endosperm or stored cotyledon tissue. Seeds germinate under suitable conditions to produce the next-generation sporophyte.

Note: View the Seeds of Life site for illustrations and information about fruits and seeds.

The Digestive System

Digestive systems in different organisms

Single-celled organisms absorb nutrients directly from their environment across the cell membrane. Multicellular animals, whose cells are generally not in direct contact with the external medium, have developed specialised organs and compartments for ingestion, digestion and absorption.

Animals obtain nutrients by two broad processes: feeding (taking in food) and digestion (breaking food down into absorbable units).

Modes of feeding

• Ingestive feeders: most animals that take food into a mouth and digest it in an internal tract.
• Absorptive feeders: organisms such as tapeworms that absorb nutrients across their body surface from a host.
• Filter feeders: animals such as oysters and mussels that strain small particles from water.
• Substrate feeders: organisms such as earthworms and termites that feed within the material they live in (soil, wood).
• Fluid feeders: animals such as aphids that suck plant or animal fluids.

Stages in the digestive process

Food consists largely of macromolecules (starch, proteins, fats) that must be reduced to small molecules before absorption. The digestive process involves several coordinated activities:

• movement: transport and mechanical processing of food through the digestive tract;
• secretion: release of digestive juices, enzymes and mucus in response to chemical or neural signals;
• digestion: chemical and mechanical breakdown of macromolecules into their monomeric units;
• absorption: transport of digested molecules across the intestinal epithelium into the blood or lymph;
• elimination: removal of undigested residues and wastes.

In common usage, digestion refers to the breakdown of food, absorption to uptake of the digested products into the circulation, and assimilation to incorporation of nutrients into body tissues.

The human digestive system - overview

The human alimentary canal is a muscular, coiled tube about 6-9 metres long when fully extended, extending from the mouth to the anus. Major regions include the mouth, pharynx, oesophagus, stomach, small intestine and large intestine. Accessory digestive organs connected by ducts include the salivary glands, pancreas, liver and gall bladder.

The mouth and pharynx

Mechanical breakdown of food begins in the mouth by chewing with the teeth and manipulation by the tongue. Chemical digestion starts with salivary amylase, secreted by salivary glands, which begins hydrolysis of starch to maltose and other oligosaccharides. Mucus moistens and lubricates the bolus of food; bicarbonate in saliva helps neutralise acids. Swallowing moves the food from the mouth into the pharynx and then the oesophagus, where coordinated muscular contractions (peristalsis) carry food to the stomach.

The tongue also contains taste buds that detect sweet, salty, sour, bitter and umami sensations and assist in food selection and swallowing. Most vertebrates possess specialised teeth adapted for diet; birds have a beak and typically lack teeth.

The stomach

The stomach stores food and performs mechanical and chemical digestion. An empty human stomach has a volume of about 50-100 millilitres and during a meal it typically fills to about 1 litre; it can distend to larger volumes under discomfort.

The stomach lining secretes about 2 litres of gastric juice per day. Gastric juice contains:

• hydrochloric acid (HCl), which creates a highly acidic environment (pH 1.5-2.5) that helps denature proteins, kill many microbes and activate zymogens;
• pepsinogen, an inactive precursor (zymogen) secreted by chief cells and activated to pepsin in acidic conditions; pepsin begins protein hydrolysis to peptides;
• mucus, secreted by surface epithelial cells to protect the stomach lining from acid and proteolytic damage.

Gastric secretions are regulated by neural and hormonal signals (for example, sight or smell of food stimulates secretion). The stomach mechanically churns food into an acidic semi-fluid called chyme, which is gradually released into the duodenum through the pyloric sphincter. Alcohol and aspirin can be absorbed directly across the stomach lining into the bloodstream.

Ulcers

Peptic ulcers arise when the protective mechanisms of the stomach lining fail, allowing acid and pepsin to damage tissue. Severe ulcers can bleed and, if they perforate the stomach wall, create a life-threatening condition. Most peptic ulcers (at least 90%) are associated with infection by the bacterium Helicobacter pylori; other contributing factors include non-steroidal anti-inflammatory drugs (NSAIDs) like aspirin and stress. Ulcer treatment targets eradication of H. pylori when present and reduction of acid with medications.

The small intestine - digestion and absorption

The small intestine is the principal site of final digestion and nutrient absorption. It is a highly folded tube whose length and internal specialisations provide an enormous surface area for absorption. The small intestine is anatomically divided into the duodenum (the most active in digestion), the jejunum and the ileum.

Epithelial cells of the small intestine form finger-like projections called villi, and each epithelial cell bears microscopic microvilli on its apical surface; together these structures multiply the absorptive surface dramatically (estimates of the total absorptive surface vary, but it is many square metres). Intestinal epithelial cells also produce enzymes in their brush border that complete digestion of disaccharides and peptides.

The pancreas delivers alkaline pancreatic juice containing digestive enzymes (amylases, proteases, lipases) and bicarbonate to neutralise stomach acid. The liver produces bile (stored and concentrated in the gall bladder), which contains bile salts that emulsify fats, aiding enzymatic action by lipases.

Digestion and absorption pathways:

• Carbohydrates are broken down to monosaccharides (glucose, fructose, galactose) by pancreatic amylase and brush-border enzymes; monosaccharides are absorbed into enterocytes and then into blood capillaries.
• Proteins are broken into peptides and amino acids by proteases (pepsin in the stomach, pancreatic proteases in the small intestine) and brush-border peptidases; amino acids are absorbed by active transport into enterocytes and then into capillaries.
• Fats are emulsified by bile salts into small droplets, digested by lipases to fatty acids and monoglycerides, which form micelles with bile salts to reach the epithelial surface; fatty acids and monoglycerides enter enterocytes, are re-esterified to triglycerides, packaged into chylomicrons and transported into the lymphatic system (lacteals) rather than directly into the blood.

Absorption is largely an active, energy-requiring process for many nutrients. The small intestine also reabsorbs most of the water and electrolytes from the digestive contents. The ileum reclaims bile salts for recycling by the liver.

Enzyme deficiencies can impair digestion and absorption: for example, lactose intolerance results from reduced production of the brush-border enzyme lactase, causing inability to hydrolyse lactose into glucose and galactose for absorption. Coeliac disease (gluten enteropathy) is an immune-mediated disorder in which gluten ingestion damages the intestinal lining, impairing nutrient absorption.

The liver

The liver is a large, metabolically active gland located in the upper right abdomen. It performs multiple functions related to digestion and general metabolism:

• production of bile, the bile salts of which emulsify dietary fats in the small intestine;
• detoxification of substances in the blood;
• synthesis of important blood proteins, including clotting factors and albumin;
• destruction of old erythrocytes and conversion of haemoglobin breakdown products into bile pigments (bilirubin);
• storage of glucose as glycogen and release of glucose when blood levels fall;
• deamination of amino acids and conversion of resultant ammonia into urea for excretion by the kidneys;
• metabolism of cholesterol and many drugs and toxins.

The gall bladder

The gall bladder stores and concentrates bile produced by the liver and releases it into the duodenum in response to a fatty meal. The gall bladder can be surgically removed (cholecystectomy) with many individuals continuing to digest fats adequately, although they may need to moderate fat intake after removal.

Glycogen, hormones and blood sugar regulation

Glycogen is the animal storage form of glucose, a branched polymer of glucose residues. When blood glucose is low, the hormone glucagon stimulates the liver to break down glycogen to glucose and release it into the blood. When glucose is abundant, insulin promotes uptake of glucose into cells and its conversion to glycogen. In prolonged low-glucose states (starvation), the liver can synthesise glucose from non-carbohydrate precursors (gluconeogenesis) such as amino acids, following deamination.

Liver diseases

Jaundice is a yellowing of the skin and sclera due to elevated bilirubin in the blood, often reflecting impaired liver function or obstruction of bile flow. Major liver diseases include hepatitis and cirrhosis.

Hepatitis A, B and C

• Hepatitis A is usually an acute, self-limiting infection transmitted primarily by the faecal-oral route (contaminated food or water).
• Hepatitis B is transmitted by blood and bodily fluids and can become chronic; chronic infection increases the risk of primary liver cancer. Safe sexual practices and vaccination reduce risk.
• Hepatitis C is mainly blood-borne; many infections become chronic and can progress to cirrhosis and hepatocellular carcinoma. Transmission via sexual contact is less common but possible.

Cirrhosis

Cirrhosis is a chronic liver disease in which normal hepatic architecture is replaced by regenerative nodules and fibrous tissue. Common causes include chronic alcohol abuse, long-standing viral hepatitis and some parasitic infections. Cirrhosis impairs metabolic, synthetic and detoxification functions of the liver and can lead to portal hypertension and other systemic complications.

The pancreas

The pancreas is both an exocrine gland (producing pancreatic juice containing digestive enzymes and bicarbonate delivered to the duodenum) and an endocrine gland (islets of Langerhans secrete hormones such as insulin and glucagon to regulate blood glucose). Pancreatic bicarbonate neutralises acid chyme entering the duodenum, allowing pancreatic enzymes to function. Impaired pancreatic function or insulin sensitivity contributes to disorders such as diabetes mellitus. Prediabetes is a condition of reduced insulin sensitivity that can progress to Type II diabetes; lifestyle measures (dietary modification, increased physical activity) and medications may delay or prevent progression.

The large intestine

The large intestine comprises the caecum (with the appendix), colon, rectum and anal canal. It receives mostly indigestible residues and liquid from the small intestine. Primary functions of the large intestine are:

• recovery of water and electrolytes,
• formation and storage of faeces, and
• microbial fermentation of indigestible materials.

The large intestine hosts a diverse microbial community that produces enzymes to break down some otherwise indigestible molecules and synthesises vitamins such as vitamin K, which can be absorbed. Secretions are primarily alkaline mucus that protect the epithelium and buffer bacterial metabolites.

Nutrition - nutrients and their roles

Nutrition concerns the composition of food, its energy content and the supply of essential organic and inorganic nutrients needed for growth, repair and metabolism. Organisms that obtain energy from inorganic chemical reactions are called chemotrophs; those that convert light to chemical energy are phototrophs; and organisms that rely on consuming organic matter are heterotrophs. In human nutrition:

• Macronutrients include carbohydrates, lipids and proteins; these are required in relatively large amounts and provide energy and structural components.
• Water is essential for life; proper water balance is vital.
• Carbohydrates: a minimum intake of carbohydrate is recommended to meet basal metabolic needs and spare proteins from being used as energy; sources include grains, fruits and vegetables.
• Proteins: polymers of amino acids needed for growth and repair. Humans require about twenty amino acids in proteins; eleven can be synthesised by the body and nine are essential amino acids that must be provided in the diet. In prolonged starvation the body may catabolise muscle proteins for energy.
• Lipids and fats: high-energy molecules stored in adipose tissue. Some fatty acids (e.g. linoleic acid) are essential and must be obtained from the diet. Dietary fats aid absorption of fat-soluble vitamins (A, D, E and K).
• Vitamins: organic micronutrients required for enzymatic reactions and metabolism; they are needed in trace amounts and are either water-soluble or fat-soluble.
• Minerals: inorganic elements required for structural roles, co-factors and physiological functions (examples: iron for haemoglobin, iodine for thyroxine, calcium for bone and nerve function, sodium for nerve conduction and fluid balance).

Proper balance of nutrients is essential for health; nutritional imbalances contribute to conditions such as cardiovascular disease, hypertension and some cancers.

Digestion in animals - selected facts (NCERT)

• Starfish: a starfish feeds on shelled animals by prying open the shell and everting its stomach through the mouth to digest the soft tissues; the stomach is later retracted into the body and digestion continues internally.
• Saliva: contains the enzyme that begins starch breakdown (salivary amylase).
• Liver location: the liver lies in the upper right part of the abdomen and is the largest gland in the body.
• Macromolecule breakdown: carbohydrates are digested to simple sugars (e.g. glucose), fats to fatty acids and glycerol, and proteins to amino acids.
• Ruminants: grass-eating ruminant mammals (e.g. cows) swallow grass quickly into a specialised stomach compartment (rumen) where microbial fermentation partially digests cellulose; the cud is regurgitated and rechewed in a process called rumination.
• Cellulose digestion: cellulose is abundant in plant material but humans cannot digest cellulose because they lack the enzyme cellulase.
• Amoeba: when an amoeba encounters food it engulfs the particle by extending pseudopodia to form a food vacuole where digestion occurs intracellularly.