Year 10 Edexcel IGCSE Biology Revision Guide

The Characteristics of Living Organisms

All living organisms, regardless of their complexity, share a specific set of characteristics that distinguish them from non-living matter. These characteristics can be remembered using the acronym MRS GREN, which stands for Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.

Movement is an action by an organism or part of an organism causing a change of position or place. While animals often move their entire bodies to find food or escape predators, plants exhibit movement by turning their leaves toward light or extending their roots toward water. Respiration describes the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. Most living things perform aerobic respiration, which requires oxygen, though some can perform anaerobic respiration in the absence of oxygen.

Sensitivity refers to the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses. This includes sensing light, sound, touch, and chemical changes. Growth is a permanent increase in size and dry mass by an increase in cell number or cell size or both. Even single-celled organisms grow before they divide.

Reproduction involves the processes that make more of the same kind of organism, ensuring the survival of the species through sexual or asexual means. Excretion is the removal from organisms of the waste products of metabolism, which are chemical reactions in cells including respiration, as well as the removal of toxic materials and substances in excess of requirements. Finally, Nutrition is the taking in of materials for energy, growth, and development. Plants require light, carbon dioxide, water, and ions to make their own food via photosynthesis, while animals require organic compounds and water and usually ingest their food.

The Variety of Eukaryotic Organisms

Eukaryotic organisms are characterized by having cells that contain a nucleus and other membrane-bound organelles. This group includes plants, animals, fungi, and protoctists, each with distinct structural and functional properties.

Plants are multicellular organisms. Their cells contain chloroplasts and are able to carry out photosynthesis. They possess cellulose cell walls and store carbohydrates as starch or sucrose. Examples of plants include flowering plants, such as a cereal like maize, and non-flowering plants, such as mosses and ferns. Animals are also multicellular organisms, but their cells do not contain chloroplasts and are not able to carry out photosynthesis. They have no cell walls and usually possess nervous coordination, allowing them to move from one place to another. Animals often store carbohydrates as glycogen. Examples include mammals, such as humans, and insects, such as houseflies and mosquitoes.

Fungi are organisms that are not able to carry out photosynthesis. Their body is usually organized into a mycelium made from thread-like structures called hyphae, which contain many nuclei. Some examples are multicellular, such as Mucor, which has a typical fungal hyphal structure. Others are single-celled, such as yeast. Fungal cells have walls made of chitin. They feed by extracellular secretion of digestive enzymes onto food material and absorption of the organic products; this is known as saprotrophic nutrition. Fungi may store carbohydrate as glycogen.

Protoctists are microscopic single-celled organisms. Some, like Amoeba, live in pond water and have features like an animal cell, while others, such as Chlorella, have chloroplasts and are more like plants. A pathogenic example is Plasmodium, which is the organism responsible for causing malaria.

Prokaryotic Organisms: Bacteria

Prokaryotes are organisms whose cells lack a nucleus and membrane-bound organelles. Bacteria are the primary example of prokaryotic organisms. These are microscopic single-celled organisms that possess a cell wall, cell membrane, cytoplasm, and plasmids. They lack a nucleus but contain a circular chromosome of DNA.

While some bacteria can carry out photosynthesis, most feed on other living or dead organisms. Examples include Lactobacillus bulgaricus, a rod-shaped bacterium used in the production of yogurt from milk, and Pneumococcus, a spherical bacterium that acts as the pathogen causing pneumonia.

Pathogens and the Nature of Viruses

Pathogens are organisms that cause disease and can include fungi, bacteria, protoctists, or viruses. Viruses are unique because they are not considered living organisms. They are small particles, substantially smaller than bacteria, and are parasitic in nature. They can only reproduce inside living cells and can infect every type of living organism.

Viruses have a wide variety of shapes and sizes. They have no cellular structure but possess a protein coat and contain one type of nucleic acid, which can be either DNA or RNA. Notable examples include the tobacco mosaic virus, which causes discoloring of the leaves of tobacco plants by preventing the formation of chloroplasts; the influenza virus, which causes the flu; and HIV, which causes AIDS.

Preventing the transmission of diseases caused by pathogens involves several strategies. These include maintaining good hygiene (such as handwashing), using barriers (like masks or condoms), vaccination to provide immunity, and isolating infected individuals to stop the spread of contagious agents.

Structures and Functions in Living Organisms

Organisms are organized into levels of increasing complexity to carry out life processes efficiently. The fundamental unit of life is the cell. Within cells, specialized structures called organelles perform specific functions, such as the nucleus controlling cell activities or mitochondria generating energy.

A group of similar cells with a similar function is called a tissue. Different tissues working together to perform a specific function form an organ. For example, the stomach is an organ made of muscular, nervous, and epithelial tissues. Several organs working together to perform a major body function form an organ system, such as the digestive system or the circulatory system. This hierarchical organization allows multicellular organisms to maintain homeostasis and survive in various environments.