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Human cells are eukaryote cells: their genetic material is found inside their nucleus. They are also heterotrophic cells: they feed on organic matter.
Cells are the basic structural and functional units that form our organism.
2.1. Cells: Functional units
Talking book
Cells can have different shapes and sizes, however they all present a basic structure composed of:
Membrane: it is a thin layer that separates the cell from the external environment. It allows entrance and release of substances.
Cytoplasm: this is the cell's interior, where organelles can be found. The majority of vital chemical reactions take place in this area.
Nucleus: it contains genetic material that has the essential information to control the vital functions.
Weblink 2: Animal cell: a tiny society
Video 3: The inner life of a cell
2.2. Exchange with the environment
Talking book
Diffusion through the plasma membrane
Cells have to exchange substances with the environment in order to obtain what they need to perform their functions, like eliminating waste products.
The cell membrane is semipermeable and this means it only allows the passage of certain substances. Substances can go through the membrane in different ways:
Diffusion. This refers to the free passage of small molecules, such as oxygen and carbon dioxide. This movement is always performed from the environment where the concentration of these molecules is greater to the one that has a lower concentration of molecules.
Osmosis. This is a special type of diffusion that consists in the passage of water from the more diluted to the more concentrated medium.
Active transport
Tonicity is a measure of the concentration of a solution. If we compare two solutions, we say that one is hypertonic to the other one when the amount of solute is higher (it is more concentrated); hypotonic, if it is less (it is more diluted) or isotonic, if it is the same(both solutions have the same level of concentration).
Active transport: sometimes the concentration of substances is less in the exterior of the cell and they can't go through the membrane by the process of diffusion. If the substances are essential for the cell, the cell can incorporate them by using an active transport system that uses energy. The same process can occur when the cell needs to eliminate substances into a medium where the concentration of substances is higher.
Endocytosis: sometimes the cell needs to incorporate large particles that cannot go through the plasma membrane. When this happens, the membrane collapses to encompass the particles and forms a small vesicle that is incorporated into the cytoplasm. The opposite process is called exocytosis.
Video 4: Transport across cell membranes
Key concepts
Human cells are formed by a membrane, cytoplasm, nucleus and a range of cell organelles.
Cell organelles perform specific functions and the membrane makes up the surface where the exchange with the environment takes place.
The exchange of substances can be carried out in different ways: diffusion, active transport, endocytosis or exocytosis.
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Activity 5 Why are cells considered to be the functional units of living things?
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Activity 6 What is the relationship between ribosomes, endoplasmic reticulum and the Golgi apparatus? Complete the sentence in order to show your answer.
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Activity 7 Look at the image, read all the information and match each organelle to its function.
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Activity 8 Are the particles entering or exiting the cell?
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Activity 9 Indicate how solutions A, B and C are related to the cell that is submerged in them.
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Activity 10 When we place a teabag in hot water, the water takes a brownish colour. Explain which process has taken place.
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Activity 11 What would happen to a human cell if it were to be placed in distilled water?
