Organisation of the Organism

The One Thing

Cells are the basic structural and functional units of living organisms, and their structures are adapted to perform particular functions. Cells combine hierarchically into tissues, organs, organ systems and organisms, while microscopes enable their study through magnification and resolution.

Reactions, Processes and Experiments

Key Terms

• Cell: The smallest unit of an organism that can carry out all the processes needed for life.
• Cell membrane: A partially permeable boundary that controls the movement of substances into and out of the cell.
• Cytoplasm: A jelly-like substance where many chemical reactions take place.
• Nucleus: A cell structure containing genetic material that controls the activities of a eukaryotic cell.
• Mitochondrion: An organelle where aerobic respiration releases energy for the cell.
• Ribosome: A small structure where proteins are made.
• Cell wall: A strong layer outside the cell membrane that supports and strengthens plant and bacterial cells.
• Chloroplast: An organelle containing chlorophyll, where photosynthesis occurs in plant cells.
• Vacuole: A fluid-filled space containing cell sap; in plant cells it helps maintain internal pressure and support.
• Permanent vacuole: A large, stable vacuole in a plant cell surrounded by a membrane and filled with cell sap.
• Eukaryotic cell: A cell with genetic material enclosed inside a nucleus, such as an animal, plant or fungal cell.
• Prokaryotic cell: A cell without a nucleus; its circular DNA is found in the cytoplasm, as in bacteria.
• Bacterial cell: A prokaryotic cell containing cytoplasm, a cell membrane, a cell wall, ribosomes and circular DNA; some also have plasmids, a capsule or a flagellum.
• Plasmid: A small circular loop of DNA found in many bacteria, separate from the main bacterial chromosome.
• Flagellum: A whip-like structure that helps some bacterial cells move.
• Magnification: How many times larger an image appears compared with the actual specimen.
• Resolution: The ability to distinguish two points as separate; higher resolution produces clearer detail.
• Light microscope: A microscope that uses visible light and lenses to magnify specimens; it can usually show whole cells and some larger organelles.
• Electron microscope: A microscope that uses electrons to produce images with much greater magnification and resolution than a light microscope.
• Magnification formula: Magnification equals image size divided by actual size.
• Tissue: A group of similar cells working together to carry out a particular function.
• Organ: A structure made from different tissues working together to perform a specific function.
• Organ system: A group of organs working together to carry out a major life process.
• Organism: An individual living thing made of one or more cells.
• Specialised cell: A cell adapted in structure to perform a particular function efficiently.
• Cell differentiation: The process by which an unspecialised cell develops features that enable it to carry out a specific function.
• Resolution limit: The smallest distance between two points that a microscope can distinguish as separate.

Easily Confused

• Cell wall and cell membrane: The cell wall provides support and is freely permeable to many substances; the cell membrane controls movement more selectively.
• Magnification and resolution: Magnification increases image size; resolution determines whether additional detail can be distinguished.
• Light microscope and electron microscope: A light microscope generally has lower resolution but can observe living cells; an electron microscope has much greater magnification and resolution but requires prepared specimens.
• Eukaryotic cell and prokaryotic cell: Eukaryotic genetic material is enclosed in a nucleus; prokaryotic circular DNA is found in the cytoplasm.
• Plant cell and bacterial cell: Both have a cell wall, but plant cells have a nucleus, chloroplasts and a large permanent vacuole, whereas bacterial cells have circular DNA not enclosed in a nucleus and may contain plasmids, a capsule or a flagellum.
• Vacuole and permanent vacuole: A vacuole is a fluid-filled space containing cell sap; a permanent vacuole is the large, stable plant-cell vacuole surrounded by a membrane.
• Tissue and organ: A tissue is made from similar cells; an organ is made from different tissues.
• Organ and organ system: An organ performs a specific function using different tissues; an organ system consists of organs working together in a major life process.
• Plasmid and bacterial chromosome: A plasmid is a small circular loop of DNA separate from the main bacterial chromosome.
• Flagellum and cilia: A flagellum helps some bacterial cells move; cilia move substances such as mucus along a tissue surface.

What Gets Asked

• Compare animal, plant and bacterial cells, including the structures present in each. Marks are lost by describing bacterial circular DNA as being enclosed in a nucleus.
• Explain how a specialised cell is adapted to its function, such as a sperm cell, root hair cell, palisade mesophyll cell, red blood cell, nerve cell or ciliated cell. Marks are lost by naming a structure without linking it to its function.
• Use Magnification = image size ÷ actual size, Actual size = image size ÷ magnification or Image size = magnification × actual size. Marks are lost when units are not made the same before dividing.
• Distinguish magnification from resolution. Marks are lost by assuming that a larger image necessarily contains more detail.
• Compare light and electron microscopes. Marks are lost by omitting that light microscopes can observe living cells whereas electron microscopes require prepared specimens and normally cannot be used to observe living cells.
• Arrange or apply the sequence organelle → cell → tissue → organ → organ system → organism. Marks are lost by confusing tissues with organs or organ systems.