Life Processes

I. The Fundamental Nature of Life

Visible movement such as growth, running, or breathing is not the only criterion of life. According to biologists, continuous invisible molecular movement is the true requirement for maintaining life.

Why Molecular Movement Is Necessary

Living organisms are highly organised structures made of cells, tissues, and organelles.
These structures naturally tend to break down due to environmental effects and chemical changes.
Organisms must continuously repair and maintain these structures through molecular movements.
Without maintenance, the organised structure of the organism would eventually collapse and life would stop.

Life Processes

Life processes are the basic maintenance activities required to keep an organism alive, even when it is inactive or asleep.

The major life processes include:

Nutrition
Respiration
Transportation
Excretion

Energy and Carbon Basis of Life

Maintenance activities require energy from external sources, generally obtained through food.
Life on Earth is primarily carbon-based.
Most food substances contain carbon-based molecules.
Complex chemical reactions, especially oxidation-reduction reactions, break down food molecules into a common usable energy form.

II. Nutrition

Nutrition is the process by which organisms obtain food and use it for energy, growth, repair, and maintenance.

A. Autotrophic Nutrition

Autotrophic nutrition is the process in which organisms synthesise complex, high-energy organic substances from simple inorganic substances such as carbon dioxide and water using an external energy source.

Photosynthesis Equation

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
In the presence of sunlight and chlorophyll

Three Main Events of Photosynthesis

1. Absorption: Chlorophyll present in chloroplasts captures light energy.

2. Conversion and Splitting: Light energy is converted into chemical energy and water molecules are split into hydrogen and oxygen.

3. Reduction: Carbon dioxide is reduced to form carbohydrates such as glucose.

Temporal Variation in Desert Plants

Desert plants often take in carbon dioxide during the night and prepare an intermediate compound. During the day, this intermediate is acted upon by the chemical energy obtained from sunlight.

Leaf Anatomy

A typical leaf cross-section contains:

Waxy cuticle
Upper epidermis
Lower epidermis
Air spaces
Vascular bundles
Xylem
Phloem

Stomata and Guard Cells

Stomata are tiny pores present on leaves that allow exchange of gases between the plant and the atmosphere.

Guard cells regulate the opening and closing of stomata.
When water enters guard cells, they swell and the stomatal pore opens.
When guard cells lose water, they shrink and the pore closes.
Closing stomata helps plants conserve water.

Storage of Food

Plants

Store unused food energy as starch.

Humans

Store excess food energy as glycogen.

Mineral Requirements

Plants absorb several minerals from the soil, including:

Nitrogen
Phosphorus
Iron
Magnesium

Nitrogen is essential for protein synthesis. Plants absorb nitrogen mainly as inorganic nitrates and nitrites or as organic compounds prepared by bacteria.

B. Heterotrophic Nutrition

Heterotrophic organisms cannot synthesise their own food and depend on other organisms for nutrition.

Mode	Description	Examples
Saprophytic	Food is broken down outside the body and then absorbed.	Fungi, bread moulds, yeast, mushrooms
Holozoic	Whole food material is taken in and digested internally.	Humans and other animals
Parasitic	Nutrition is obtained from a living host without immediately killing it.	Cuscuta, ticks, lice, leeches, tapeworms

Nutrition in Unicellular Organisms

Amoeba

Amoeba uses temporary finger-like projections called pseudopodia.
Pseudopodia surround food and form a food vacuole.
Digestive enzymes break complex substances into simpler substances.
The digested nutrients diffuse into the cytoplasm.

Paramoecium

Paramoecium has a definite body shape.
It uses cilia for movement.
Cilia sweep food towards a specific intake region.

III. Human Digestive System

The human alimentary canal is a continuous tube through which food passes and is digested, absorbed, and finally eliminated.

Mouth

Teeth crush food into smaller particles.
Salivary glands release saliva.
Salivary amylase breaks starch into simpler sugars.
The muscular tongue mixes food with saliva.

Oesophagus

The oesophagus pushes food towards the stomach through rhythmic muscular contractions called peristaltic movements.

Stomach

Gastric glands release:

Hydrochloric Acid (HCl): Creates an acidic medium.
Pepsin: Digests proteins.
Mucus: Protects the stomach lining from acid.

The exit of food from the stomach is controlled by a sphincter muscle.

Small Intestine

The small intestine is the main site of complete digestion and absorption of nutrients.

Herbivores generally have longer small intestines because cellulose requires more time for digestion.
Carnivores generally have shorter small intestines.

Role of Liver

The liver produces bile juice.
Bile makes the food mixture alkaline.
Bile salts emulsify fats into smaller globules.

Role of Pancreas

Trypsin: Digests proteins.
Lipase: Digests emulsified fats.

Intestinal Juice

Intestinal enzymes complete digestion by converting:

Food Component	Final Product
Proteins	Amino acids
Carbohydrates	Glucose and other simple sugars
Fats	Fatty acids and glycerol

Absorption through Villi

The inner wall of the small intestine contains numerous finger-like projections called villi.
Villi greatly increase the surface area for absorption.
Blood vessels inside the villi transport absorbed nutrients.
These nutrients are used for energy, growth, tissue formation, and repair.

Large Intestine

The large intestine absorbs water from unabsorbed food.
Remaining waste is passed out through the anus.
The anal sphincter controls the elimination of faeces.

IV. Respiration

Respiration is the process of breaking down food molecules to release usable energy for life activities.

Common First Step

In the cytoplasm, one six-carbon glucose molecule breaks down into two three-carbon molecules of pyruvate.

Glucose (6C) → 2 Pyruvate molecules (3C each)

Different Pathways of Pyruvate Breakdown

Condition	Location	Products	Energy Output
Anaerobic respiration in yeast	Cytoplasm	Ethanol + CO₂ + Energy	Low
Anaerobic respiration in human muscles	Cytoplasm	Lactic acid + Energy	Low
Aerobic respiration	Mitochondria	CO₂ + H₂O + Energy	High

Fermentation in Yeast

Pyruvate → Ethanol + CO₂ + Energy

Anaerobic Respiration in Human Muscles

Pyruvate → Lactic Acid + Energy

Lactic acid accumulation during oxygen shortage may cause muscle cramps.

Aerobic Respiration

Pyruvate + Oxygen → CO₂ + H₂O + Large Amount of Energy

ATP — Energy Currency of the Cell

ATP stands for Adenosine Triphosphate. It is the primary energy currency used by cells.

ATP is synthesised from ADP and inorganic phosphate.

ADP + Pi + Energy → ATP

Breaking the terminal phosphate bond of ATP releases approximately 30.5 kJ/mol of energy.

Respiratory Surfaces

Plants exchange gases mainly through stomata.
Carbon dioxide elimination is more significant at night, while oxygen release is more significant during the day.
Aquatic organisms breathe faster because water contains less dissolved oxygen than air.
Terrestrial organisms have large respiratory surfaces located inside the body for protection.

Human Respiratory System

Nostrils → Throat → Trachea → Bronchi → Bronchioles → Alveoli

Nostrils contain hairs and mucus that filter dust and germs.
The trachea is supported by rings of cartilage to prevent collapse.
Bronchi divide into smaller bronchioles.
Bronchioles end in tiny air sacs called alveoli.
The alveoli provide a respiratory surface of approximately 80 m².

Transport of Respiratory Gases

Haemoglobin in red blood cells has a high affinity for oxygen.
Carbon dioxide is more soluble in water than oxygen.
Therefore, most carbon dioxide is transported in dissolved form.

Role of Cilia

Cilia are hair-like structures in the upper respiratory tract that remove dust and germs. Smoking damages or destroys these cilia.

V. Transportation Systems

A. Human Circulatory System

Blood Composition

Plasma: Transports food, carbon dioxide, and nitrogenous wastes.
Red Blood Corpuscles: Transport oxygen using haemoglobin.

The Heart

The heart is a muscular pumping organ approximately the size of a fist.

Right Side of the Heart

Receives deoxygenated blood through the vena cava.
Pumps this blood to the lungs through the pulmonary artery.

Left Side of the Heart

Receives oxygenated blood through the pulmonary veins.
Pumps oxygenated blood to the body through the aorta.

Heart Chambers and Valves

The heart has four chambers.
Ventricles have thicker walls than atria.
Thicker ventricular walls help pump blood at higher pressure.
Valves prevent the backward flow of blood.

Double Circulation

In double circulation, blood passes through the heart twice during one complete cycle of circulation.

Four-chambered hearts in birds and mammals allow complete separation of oxygenated and deoxygenated blood, making circulation highly efficient.

Blood Pressure

Normal Blood Pressure = 120/80 mm of Hg

120: Systolic pressure
80: Diastolic pressure
Measured using a sphygmomanometer.

Blood Vessels

Blood Vessel	Structure	Function
Arteries	Thick and elastic walls	Carry blood away from the heart under high pressure
Veins	Thin walls and valves	Carry blood towards the heart
Capillaries	Walls one cell thick	Allow exchange of materials between blood and tissues

Platelets

Platelets help clot blood at injury sites and prevent excessive blood loss.

Lymph

Lymph is a colourless fluid containing relatively little protein.
It drains excess fluid from tissues.
It transports absorbed fats from the intestine.

B. Transportation in Plants

Xylem — Transport of Water and Minerals

Xylem transports water and dissolved minerals from roots to other parts of the plant.

Root Pressure

Root cells actively absorb ions from the soil.
This creates a concentration gradient.
Water enters the roots and is pushed upward.
Root pressure is especially significant at night.

Transpiration Pull

Water evaporates from the stomata of leaves.
Evaporation creates suction or tension in the water column.
This pulls water upward from the roots.
Transpiration pull is the major force during the day.

Phloem — Translocation

Phloem transports sucrose and amino acids from leaves to storage organs such as roots and fruits, as well as to growing regions.

ATP is used to actively load sucrose into phloem tissue.
This increases the osmotic pressure inside the phloem.
Water enters the phloem, creating pressure that pushes food towards lower-pressure regions.

VI. Excretion

Excretion is the process of removing harmful metabolic wastes from the body.

Human Excretory System

The human excretory system consists of:

A pair of kidneys
Ureters
Urinary bladder
Urethra

The Nephron

The nephron is the basic structural and functional unit of the kidney.

It contains a network of capillaries called the glomerulus.
The glomerulus lies inside a cup-shaped structure called Bowman’s capsule.

Filtration and Reabsorption

Approximately 180 litres of filtrate may be formed in the kidneys daily.
Only about 1–2 litres of urine are finally excreted.
Useful substances are selectively reabsorbed into the blood.

Substances reabsorbed include:

Glucose
Amino acids
Required salts
Most of the water

Urinary Bladder

The urinary bladder is a muscular sac that stores urine. Its emptying is controlled by nervous signals.

Artificial Kidney — Haemodialysis

Haemodialysis is a process in which an artificial kidney removes nitrogenous wastes from the blood using semi-permeable tubes and dialysing fluid.

Waste materials move out of the blood by diffusion.
The dialysing fluid helps maintain the concentration gradient.
Unlike natural kidneys, an artificial kidney does not selectively reabsorb useful substances.

Excretion in Plants

Plants release oxygen and excess water through gaseous exchange and transpiration.
Wastes may be stored inside vacuoles.
Wastes may accumulate in ageing leaves, which later fall off.
Some wastes are stored as resins and gums in old xylem.
Certain wastes are released directly into the surrounding soil.

Quick Revision

Life Processes

Nutrition, respiration, transportation, and excretion.

Photosynthesis

Uses CO₂, water, sunlight, and chlorophyll.

Plant Storage

Plants store food as starch.

Human Storage

Humans store excess glucose as glycogen.

Common Respiration Step

Glucose breaks into two pyruvate molecules.

ATP Energy

Approximately 30.5 kJ/mol.

Alveoli Area

Approximately 80 m².

Normal Blood Pressure

120/80 mm of Hg.

Water Transport

Xylem transports water and minerals.

Food Transport

Phloem transports sucrose and amino acids.

Nephron

Basic filtration unit of the kidney.

Daily Filtrate

Approximately 180 L formed, but only 1–2 L urine excreted.