Biology: Cells and Passage of Substances

Basic Units and Organization of Life

• Definition of a Cell: The cell is defined as the basic structural and functional unit of all known living organisms. It is the smallest unit of life and is often referred to as the "building block of life."
• Occurrence of Cells: All living things consist of one or more cells. An organism may be composed of trillions of cells, featuring hundreds of different types and functions. For example, a human body contains approximately $100 \times 10^{12}$ (100 trillion) cells.
• Unicellular Organisms: These are organisms composed of only one cell that performs all functions required for the organism's survival. Bacteria are a primary example.
• Multicellular Organisms: These are organisms composed of many cells working together.
• Organelles: Every cell contains smaller structures known as organelles, which perform specific functions including metabolism, production, transportation, and the secretion of substances.

Biological Organization: From Atoms to Organisms

Biological structures are organized hierarchically from the simplest to the most complex:

1. Atoms: The smallest units.
2. Molecules: Combinations of atoms.
3. Nucleus of a Cell: An organelle within a cell containing genetic material.
4. Cells: The basic unit of life.
5. Tissues: A group of similar cells working together to perform a particular function. Examples include:
   • Ciliated Epithelial Tissue: Located in the trachea and bronchus. It features cilia that exhibit flicking movements to sweep dust, bacteria, and mucus out of air passages.
   • Human Cheek Epithelial Tissue: Lines the mouth. It is made of closely packed cells and secretes mucus to maintain a moist environment.
   • Nervous Tissue: The main component of the nervous system, composed of long neurons that receive and transmit impulses rapidly between the body and the nervous system.
   • Muscle Tissue: A soft tissue composed of long muscle fiber cells containing contractile proteins that allow for movement.
6. Organs: A group of different tissues working together to perform a particular function.
7. Organ Systems: A group of organs performing several closely related functions.
8. Organism: A complete living entity made of organ systems working together.

Human and Plant Organ Systems

Human Organ Systems

The human body contains 13 major organ systems, including:

• Integumentary System: Includes skin, hair, nails, and sweat/oil glands.
• Digestive System: Organs include teeth, tongue, salivary glands, liver, gall bladder, duodenum, caecum, appendix, rectum, anus, oesophagus, stomach, colon, and ileum. It functions to ingest, digest, and absorb food while removing undigested waste.
• Other Systems: Skeletal, Muscular, Lymphatic, Endocrine, Nervous, Respiratory, Excretory (Urinary), Circulatory, Male Reproductive, and Female Reproductive systems.

Vital Organs in Humans

There are 78 total organs in the human body. Five are considered vital:

1. Heart: Part of the circulatory system. Pumps blood through vessels to the body.
2. Brain: Part of the nervous system. Controls body activity and coordinates information from sense organs.
3. Lungs: Part of the respiratory system. Breathe in $O_2$ for the bloodstream and remove $CO_2$.
4. Kidneys: Part of the excretory system. Filter blood to remove excess water and metabolic waste like urea.
5. Liver: Part of the digestive system. Functions include detoxification and bile production. Note: The skin is the largest organ in the human body, designed for protection.

Plant Organ Systems

• Shoot System: Includes leaves and stems.
• Root System: Below-ground structures.
• Reproductive System: In angiosperms, this includes flowers, seeds, and fruits. In conifers, reproductive structures are found in cones.
• Leaf: Part of the shoot system. Converts sunlight into chemical energy for food production.
• Flower: Part of the reproductive system. Attracts pollinators for fertilization to create seeds and fruits.

Specialized Cell Adaptations

Cells are modified to perform specific roles within an organism. Their structures are ideally suited for these functions:

• Root Hair Cell: Responsible for absorbing water and mineral ions from the soil. It has a long extension that significantly increases its surface area for absorption.
• Palisade Cell: The main site of photosynthesis. It contains many chloroplasts filled with chlorophyll to trap the maximum amount of light.
• Xylem Cell: Transports water and minerals from roots to leaves. It is a dead, hollow cell with no end walls, ensuring no obstruction to the flow of water.
• Red Blood Cell: Transports oxygen ($O_2$) to all body cells. It lacks a nucleus and organelles to maximize space for haemoglobin, which carries the oxygen.
• Sperm Cell: Functions to fertilize the egg cell. It possesses a tail (flagellum) to assist in swimming.
• Motor Nerve Cell: Transmits electrical impulses to effectors like muscles. It has a very long axon insulated by a myelin sheath to prevent the leakage of electrical impulses.
• Muscle Cell: Specialized for contraction to produce movement. They contain protein fibers that shorten when energy is available.

Plant vs. Animal Cell Structure

Both cell types are eukaryotic, meaning they contain a membrane-bound nucleus and membrane-bound organelles (mitochondria, vacuoles, etc.).

Comparative Table

Diffusion: Principles and Importance

• Definition: Diffusion is the net movement of molecules, atoms, and ions from a region of their higher concentration to a region of their lower concentration, down a concentration gradient. This continues until an equilibrium is achieved, where particles are evenly distributed.
• Characteristics:
  • It is a passive process requiring no energy.
  • Molecules possess kinetic energy (energy of motion), moving randomly and colliding. Collisions are more frequent in high-concentration areas, resulting in the spreading of molecules.
  • Concentration Gradient: The difference in concentration of a substance between two regions.
• Simple Diffusion in Organisms: This is the primary method for moving substances over short distances. Essential substances like $O_2$, $CO_2$, urea, and nutrients (fatty acids, glucose, amino acids, and vitamins) diffuse across the cell surface membrane.

Importance for Living Organisms

1. Photosynthesis/Gaseous Exchange in Plants: $CO_2$ diffuses from the air into leaves via stomata because the internal concentration is lower (as cells use it up). $O_2$ (waste) diffuses out from high internal concentrations to the air.
2. Pollination: Flowering plants release fragrance molecules that diffuse from petals into the air to attract pollinators like bees.
3. Human Gaseous Exchange: In the alveoli of the lungs:
   • $O_2$ concentration is high in alveoli and low in blood capillaries, so $O_2$ diffuses into the blood.
   • $CO_2$ concentration is high in blood and low in alveoli, so $CO_2$ diffuses into the alveoli to be exhaled.

Practical Investigations of Diffusion

Experiment: Diffusion in Water

1. A colored sugar cube or teabag is placed in distilled water.
2. Initially, there is a high concentration of sugar/dye at the source, creating a steep concentration gradient.
3. The particles dissolve and begin net movement.
4. Over time, they spread from high to low concentration.
5. Result: Molecules become equally distributed (equilibrium).

Experiment: Diffusion of Ammonia Gas ($NH_3$)

1. Cotton wool soaked in ammonia solution is placed at one end of a glass tube filled with red litmus paper spaced at intervals.
2. Ammonia gas diffuses from the high concentration (cotton wool) toward the low concentration (other end).
3. Observation: The litmus paper turns blue sequentially as the gas reaches it.

Osmosis: Concepts and Mechanics

• Syllabus Definition: Osmosis is the passage of water molecules from a region of higher water potential to a region of lower water potential through a partially (selectively) permeable membrane.
• Selectively Permeable Membrane: A membrane that is "picky," allowing some substances to pass through while blocking others.
• Water Potential ($\Psi$):
  • Represented by the Greek symbol $\Psi$.
  • High Water Potential: A dilute solution with many free water molecules and few or no solutes.
  • Low Water Potential: A concentrated solution with many solute molecules and fewer free water molecules.

Effects of Osmosis on Plant Cells

Plant Cells in Hypotonic Solutions

• Environment: Pure (distilled) water, which is weak/dilute and has a high $\Psi$.
• Cell State: The cell sap/vacuole is hypertonic (concentrated) with a low $\Psi$.
• Process: Endosmosis (water moving into the cell).
• Result: The vacuole gains water and swells. The cell becomes turgid. The rigid cellulose cell wall prevents the cell from bursting by expanding only slightly.

Plant Cells in Hypertonic Solutions

• Environment: Concentrated salt solution, which has a low $\Psi$.
• Cell State: The cell sap/cytoplasm is hypotonic (dilute) with a high $\Psi$.
• Process: Exosmosis (water moving out of the cell).
• Result: The cell cytoplasm and vacuole shrink. The cell surface membrane eventually tears away from the cell wall.

Active Transport

• Definition: Active transport is the movement of ions or substances from a region of their lower concentration to a region of their higher concentration, against a concentration gradient. This process is distinct from the passive nature of diffusion and osmosis.

Questions & Discussion

Discussion on Unicellular Organisms (Sperm Cells)

• Millie: Argues a sperm cell is a unicellular organism because it is made of one cell and can live outside the body.
• Oscar: Argues a sperm cell is not a unicellular organism. While it is one cell, it cannot reproduce to produce more sperm cells.
• Madeline: Argues it is a unicellular organism because it carries out life cycle processes and "reproduces" when it fertilizes the egg.

Multiple Choice Question

• What is meant by osmosis?
  • Correct Answer: (d) Osmosis is the movement of water molecules from a dilute to a more concentrated solution through a partially permeable membrane. (Explanation: A dilute solution has high water potential; a concentrated solution has low water potential).