Edexcel IGCSE Biology: Study of Living Organisms

• Cells are the fundamental building blocks of living organisms.
• Unicellular organisms consist of a single cell, while multicellular organisms are composed of many cells.

Levels of Organization

• In multicellular organisms, cells are specialized to perform specific functions.
• Specialized cells form tissues, tissues form organs, and organs form organ systems.
• Multicellular organisms exhibit multiple levels of organization.

Cell Structures

• Cells are enclosed by a cell membrane, separating them from their surroundings.
• The cytoplasm is located within the cell membrane.
• Eukaryotic cells contain organelles within their cytoplasm.
• Organelles are compartments within the cell where specific processes occur.

Organelles in Eukaryotic Cells

• The nucleus contains genetic material.
• Mitochondria are the sites of aerobic respiration.
• Ribosomes are the sites of protein synthesis.
• Plant cells have additional structures, including a cell wall, chloroplasts, and a permanent vacuole.
• Ribosomes and mitochondria are visible with an electron microscope but too small to see clearly with a light microscope.

Animal Cells

• Key structures include the nucleus, cell membrane, mitochondria, ribosomes, and cytoplasm.

Plant Cells

• In addition to animal cell structures, plant cells have a cell wall made of cellulose, providing shape and protection.
• A permanent vacuole in plant cells is filled with cell sap, maintaining cell turgidity.
• Plant cells in leaves and stems may contain chloroplasts for photosynthesis.

Cell Differentiation and Specialization

• Structural differences in cells enable them to perform specific functions.
• Cell differentiation is the process by which cells become specialized.
• Specialized cells have unique characteristics that allow them to perform specific functions.
• Cell differentiation involves development of specific structures and subcellular components to carry out a particular function.
• Animal cells mostly differentiate early in development, limiting their ability to differentiate later in life.
• Adult stem cells in animals retain the ability to differentiate for repair and replacement of cells.
• Many types of plant cells can fully differentiate throughout the plant's life.

Examples of Specialized Cells

• Ciliated cells in the trachea and bronchi move mucus using hair-like cilia.
• Nerve cells conduct impulses and are long, facilitating communication throughout the body, axons are covered in a fatty sheath that insulates and speeds up nerve transmission.
• Red blood cells transport oxygen efficiently with a biconcave shape and lack a nucleus to maximize space for oxygen transport and contain haemoglobin.
• Root hair cells absorb water and mineral ions from the soil using long extensions for maximum absorption.
• Palisade mesophyll cells perform photosynthesis, are column-shaped to maximize light absorption and are tightly packed beneath the upper epidermis of the leaf to optimize photosynthesis.

Stem Cells

• Stem cells are undifferentiated cells capable of unlimited division and can differentiate into other cell types.
• Embryonic stem cells, being totipotent, are crucial for forming all tissues and organs during development.
• Adult stem cells primarily replace damaged or lost cells and produce new cells for growth.
• Meristem cells in plants are unspecialized cells that can differentiate into cells needed for growth.

Chemical Elements

• Most molecules in living organisms are carbohydrates, proteins, or lipids.
• These organic molecules all contain carbon.