Revision: Organisation of Life Science SSLC (English Medium) Class 8 Tamil Nadu Board of Secondary Education

A group of similar cells, along with intercellular substances which perform a specific function, is called a tissue.

• Cells that perform a particular function always live in a group. This group of cells is called a tissue.
• For example, blood, phloem, muscle, etc. are examples of tissues.

A group of similar cells which perform a specific function.
example: Muscular tissue in animals.

A tissue is a group of cells that are similar in structure and are organized together to perform a specific task.

The tissue is a group of cells of similar structure and function.

Vascular tissue is the complex plant tissue in higher plants that are composed of xylem and phloem and is concerned with conducting water, minerals, and organic food throughout the plant body.

A tissue, in biology, is defined as a group of cells that have a similar structure and perform a specific function. The word tissue originates from French, which means "to weave."

Diffusion is the free movement of molecules of a substance (solute or solvent, gas, or liquid) from the region of their higher concentration to the region of their lower concentration when the two are in direct contact.

Example: Perfume fills a whole room.

Diffusion is the free movement of molecules of a substance (solute or solvent, gas, liquid) from the region of their higher concentration to the region of their lower concentration when the two are in a direct contact.

Osmosis is the movement of water molecules from their region of higher concentration (dilute solution or with a lower solute concentration) to their region of lower concentration (concentrated solution or with a higher solute concentration) through a semi-permeable membrane.

Osmosis is the movement of water molecules from their region of higher concentration (dilute solution or with a lower solute concentration) to their region of lower concentration (concentrated solution or with a higher solute concentration) through a semi-permeable membrane.

The function of the kidney that involves regulating the composition of blood, including the percentage of water and salts, while removing wastes like urea, is called osmoregulation.

• Anatomy = Study of Internal Structure - Plant anatomy is the study of the internal structure of plants, which includes the organisation and structure of tissues.
• Basic Unit = Cell - The basic unit of plants is the cell. Cells are organised into Tissues → Organs (organisational hierarchy).
• Tissue Definition - A tissue is a group of similar cells having a common origin that perform a specific function together.
• Monocots vs. Dicots - Anatomical (internal structural) differences exist between monocots and dicots, so it's important to know them separately.
• Internal Structure Adapts - The internal structures of plants adapt according to their environment (e.g., water availability, climate), and structural similarities exist in both external and internal morphology of organisms.

• Diffusion - movement of molecules from higher to lower concentration due to kinetic energy; continues till equilibrium is reached.
• Diffusion Pressure (DP) is proportional to the number of diffusing particles. Pure water always has more DP than a solution.
• DPD (Diffusion Pressure Deficit) = DP of pure solvent − DP of solvent in solution. Coined by B.S. Meyer (1938); now called water potential. It is the "thirst" of a cell to absorb water.
• Diffusion is important for absorption of water & minerals, gas exchange, water conduction, and food transport in plants.
• Facilitated diffusion — passive movement of hydrophilic solutes via carrier proteins (aquaporins & ion channels); needs a concentration gradient but no energy.

• Meaning: Osmosis is the movement of water (solvent) through a semipermeable membrane from a dilute solution to a concentrated solution.
• Direction of Movement: Water moves from a hypotonic (low solute, high water) to a hypertonic (high solute, low water) solution.
• Types of Osmosis: Endosmosis: Water enters the cell and makes it turgid.
  Exosmosis: Water leaves the cell and makes it flaccid.
• Types of Solutions:
  Hypotonic: Cell swells.
  Hypertonic: Cell shrinks.
  Isotonic: No net movement of water.
• Cell Pressure: In a fully turgid cell, turgor pressure equals wall pressure, and DPD becomes zero.

1. Kidneys help maintain the balance of water and salts in the blood, a process known as osmoregulation.
2. In summer, more water is lost through sweat, so urine becomes thicker and less frequent due to increased water reabsorption.
3. In winter, water loss through sweat is less, so more urine is passed, and it is more diluted.
4. In cholera, severe water loss through vomiting and diarrhoea reduces water absorption into the blood.
5. Kidneys reabsorb almost all available water, but urea may also accumulate, leading to uremia (urea poisoning).
6. Immediate treatment includes oral rehydration solution (ORS) or glucose-saline drips to restore fluid balance.

• Metabolism is the sum of all biochemical reactions in a cell, including breakdown (catabolism) and synthesis (anabolism) of biomolecules.
• Catabolism releases energy by degrading complex molecules, while anabolism uses energy to build complex molecules from simpler ones.
• Primary metabolites are compounds essential for normal growth and physiology, commonly found in animal tissues (e.g., amino acids, sugars, organic acids).
• Secondary metabolites are mainly found in plant, fungal and bacterial cells and are not directly required for basic survival.
• Important groups of secondary metabolites include terpenes (e.g., carotenoids, rubber), phenolics (e.g., flavonoids, tannins) and nitrogen‑containing compounds (e.g., alkaloids, cyanogenic glycosides).