Cells and Organisation
Key Facts
- Cells are the basic building blocks of all living organisms.
- Most human cells are between 10 and 30 micrometres across.
- Plant cells have features that animal cells do not, including a cell wall, chloroplasts and a permanent vacuole.
- Cells become specialised to carry out particular functions.
The Animal Cell
An animal cell is surrounded by a thin cell membrane that controls what enters and leaves the cell. Inside you find the cytoplasm, a jelly-like substance where most chemical reactions take place. The nucleus is a large, roughly spherical structure that contains the cell's DNA and controls its activities. Scattered throughout the cytoplasm are mitochondria — small, sausage-shaped organelles where aerobic respiration occurs, releasing energy the cell needs. Animal cells also contain ribosomes, tiny structures where proteins are assembled.
The Plant Cell
Plant cells contain all the same organelles as animal cells, plus three additional structures. A rigid cell wall made of cellulose surrounds the cell membrane, providing support and preventing the cell from bursting when it absorbs water. Chloroplasts are green, disc-shaped organelles that contain chlorophyll and are the site of photosynthesis — they absorb light energy and convert carbon dioxide and water into glucose and oxygen. A large permanent vacuole filled with cell sap (a dilute solution of sugars and salts) sits in the centre of the cell, helping to keep the cell turgid and maintain its shape.
Worked Example — Comparing Cells
Question: Name two structures found in plant cells but not in animal cells and state the function of each.
Answer 1: Cell wall — made of cellulose, it provides structural support and prevents the cell from bursting.
Answer 2: Chloroplasts — they contain chlorophyll and are the site of photosynthesis, converting light energy into glucose.
Specialised Cells
As organisms develop, their cells differentiate — they become specialised to perform a particular job. A red blood cell has no nucleus to make more room for haemoglobin, which carries oxygen. Its biconcave disc shape gives a large surface area to volume ratio for efficient gas exchange. A nerve cell (neuron) is long and thin with branched endings to make connections with other cells and carry electrical impulses quickly across the body. A root hair cell in a plant has a long, thin extension that increases the surface area for absorbing water and mineral ions from the soil. A sperm cell has a streamlined head, a mid-section packed with mitochondria for energy and a long tail (flagellum) for swimming towards the egg.
Levels of Organisation
In multicellular organisms, cells are organised into a hierarchy. Similar specialised cells group together to form a tissue — for example, muscle tissue is a group of muscle cells working together to contract. Different tissues combine to form an organ. The stomach, for instance, contains muscular tissue, glandular tissue and epithelial tissue. Organs work together in an organ system. The digestive system includes the stomach, small intestine, large intestine, liver and pancreas, all working together to break down and absorb food. Finally, organ systems combine to make a complete organism. The hierarchy runs: cells, tissues, organs, organ systems, organism.
Worked Example — Levels of Organisation
Question: Put these in order from smallest to largest: organ system, cell, tissue, organism, organ.
Answer: Cell → Tissue → Organ → Organ system → Organism.
Practice Questions
- Name three organelles found in both animal and plant cells.
- What is the function of mitochondria?
- Explain how a red blood cell is adapted to its function.
- Describe the difference between a tissue and an organ.
- A palisade mesophyll cell in a leaf is packed with chloroplasts. Explain why.