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Introduction to cells

All life on earth requires water to survive.

All living cells contain over 70% water!

Organisms use water in a number of different ways:

  • Temperature control. Water slows down changes in temperature. This helps maintain an optimal temperature for enzymes.
  • Solvent. Most reactions in the cell require the reactants to be in a water-based solution.
  • Transport medium. Both animals and plants use water-based solutions to transport substances around the organism.
  • Blood is roughly 83% water. Blood transports ions, hormones and digested food around the body.

    In plants, water transports mineral salts through xylem vessels.

Plants have additional uses for water. Water is a reactant in photosynthesis reactions. It is also used to maintain the turgidity (stiffness) of cells that keeps plants upright.

Both plants and animals need water to survive.
Both plants and animals need water to survive.

All organisms carry out life processes:

  • Reproduction : living things can produce offspring (copies of the same type).
  • Growth and movement : living things grow and move. For example, plants move in the sense that they grow towards the sun.
  • Reaction to external stimuli: all living things can react to external stimuli. For example, many flowers close at night.
  • Need for food : all living things require nutrition; plants create the food from sun light, while animals need to eat plants or other animals.
  • Respiration for energy: all living things use food to obtain energy. For example, the energy for moving muscles comes from the energy gained by converting sugars to water and carbon dioxide.
  • Excretion of waste: all living things excrete waste products. For example, animals excrete the carbon dioxide obtained when getting energy from sugar via the lungs.

Individual cells in an organism also carry out the life processes. This is why cells are called the basic unit of life. Cells in an organism are also living things.

Cells in a tree may reproduce leading to the growth of the tree. The tree itself may reproduce leading to the appearance of new trees.

Some plants can move rapidly!
Some plants can move rapidly!

Unicellular organisms only have one cell. Bacteria are unicellular.

Multicellular organisms are made from many cells. Animals and plants are multicellular.

A human being has approximately 100 trillion cells!

Important! Viruses are not considered living things and are not made of cells.

There are two types of cells: prokaryotic and eukaryotic. The main difference is that eukaryotic cells have a nucleus.

Characteristics Eukaryotic cells Prokaryotic cells
Nucleus Present Absent
Organelles Present Absent
Structure of DNA Linear Circular
Cell size $${10-100\mu m}$$ $${0.2-2\mu m}$$
Organisms containing these cells Eukaryote Prokaryote
Examples Plant and animal cells Bacteria, archaea

All cells have genetic material, a membrane and a cytoskeleton. Cells in the same organism contain identical genetic material.

Cells within multicellular organisms are specialised for particular functions.

Cells that are specialised are able to perform their functions much more effectively than undifferentiated cells.

The principle is the same as society. A group of people can get more done when each specialises on certain functions.

It would be a waste of resources if all cells in a plant, even root cells and cells inside the stem not exposed to light, developed the structures needed for photosynthesis.

Below are three examples of specialised cells:

Cell type Function Features
Root hair cell
Absorbing water from the soil
  • Finger-like projection increases surface area
  • Selective absorption of minerals and ions
  • Many mitochondria to produce energy for active uptake of ions
Xylem vessel
Transporting water around the plant
  • Strong lignin walls to resist high pressures
  • No cytoplasm increases water flow
  • No cell walls within vessels increases water flow
Red blood cell
Transporting oxygen in the blood
  • Contains haemoglobin that binds oxygen
  • No nucleus allows more space for haemoglobin
  • Biconcave shape increases surface area

Cells in a multicellular organism are specialised for specific functions. Specialised cells are used to form larger structures:

  • Tissue : A group of specialised cells of the same type that organised together so that they function as a unit.
  • The outer layer of your skin (epithelial layer) is an examples of tissue.

  • Organ : A group of different tissue types that work together to perform a common function.
  • The lungs are a pair of organs. They are used for exchanging gases between the blood and the environment. They contain blood vessels and tissue specialised for gas exchange.

  • Organ system : A group of organs that are linked (sometimes in sequence) to perform a higher-level function
  • The digestive system is an organ system that performs digestion of food. The respiratory system exchanges gases between blood and the air.

An organism has several specialised organ systems.

Cells of a type are joined together to form a tissue. Different tissues make an organ and different organs form an organ system.
Cells of a type are joined together to form a tissue. Different tissues make an organ and different organs form an organ system.