4. Eukaryotic cells
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Eukaryotic cells originated after prokaryotic cells. They are more complex and have a series of advantages over prokaryotic cells.
Their cytoplasm contains a range of structures that perform different functions. These structures are called organelles .
Genetic material is contained in the nucleus. This way, it is protected and provides better stability for cells.They have a cytoskeleton : a microscopic network of protein filaments and microtubules. Its function is to maintain the cell's shape and internal organisation. It also helps cells move.
Key structure
Passive voice: to be + past participle
is contained
Cytoskeletal components
Weblink 3: Inside a cell
4.1. Organelles
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Organelles are membranous structures found in the cytoplasm. There are different organelles with different functions, so each organelle is made up of the molecules needed to perform its functions.
Ribosomes are an exception, as they are not membranous structures. They are small spherical shaped organelles with no membrane. They can be found scattered throughout the cytoplasm or attached to the rough endoplasmic reticulum. They are responsible for protein synthesis
The other organelles can be subdivided into organelles that process nutrients and organelles that produce energy.
4.1.1. Organelles that process nutrients
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Organelles in this group are in charge of processing nutrients. They are: the endoplasmic reticulum, the Golgi apparatus, lysosomes and vacuoles.
Lysosome (red) digesting substances (pink).
Lysosomes : small vesicles that contain substances capable of digesting molecules captured by cells.
Endoplasmic reticulum : very complex set of tubules and vesicles responsible for manufacturing and transporting different substances, such as lipids and proteins. There are two types: the rough endoplasmic reticulum, which has ribosomes, and the smooth endoplasmic reticulum.
Vacuoles (pale green) in a plant cell
Vacuoles : structures that store different substances; plant cells have more and larger vacuoles than animal cells.
Golgi apparatus : organelle formed by grouped vesicles and flattened sacs; it takes substances from the endoplasmic reticulum, modifies them and introduces them into the vesicles for secretion.
Weblink 4: The endoplasmic reticulum, Golgi apparatus and lysosomes
4.1.2. Organelles that produce energy
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Mitochondria and chloroplasts provide cells with energy so they can perform their vital functions. They both have their own genetic information, so they can function independently.
They are cylindrical organelles made of a double membrane. The external membrane is smooth and the internal one has folds called cristae . Inside is the matrix , which is mainly made up of genetic material, ribosomes and enzymes.
Mitochondria are considered the power plants of eukaryotic cells, as they are in charge of obtaining energy through the process of cell respiration. Mitochondria can be found in all eukaryotic cells.
The inside of a mitochondrion
A mitochondrion seen through a scanning electron.
They are egg-shaped organelles with a double membrane. They have a series of disc-shaped sacs named thylakoids , which contain the pigment that gives them their distinctive green colour.
Photosynthesis happens here. During this process, chloroplasts synthesise organic molecules from inorganic ones using chemical energy obtained from the Sun. Chloroplasts are only present in the cells of photosynthetic organisms, such as plants and some protists.
The inside of a chloroplast
A chloroplast seen through a transmission electron microscope.
4.2. Movement structures
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Certain eukaryotic cells are able to move in two different ways: using their appendixes (cilia and flagella) or by changing the viscosity of their cytoplasm.
Ciliated epithelial cells in the bronchioles
4.2.1. Cilia and flagella
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Cilia and flagella are mobile organelles. They are formed by protein fibres from the cytoskeleton.
Their movement is coordinated by a structure known as centriole . It is formed by protein tubules arranged like cilia and flagella. The centriole is also involved in cellular division.
Cellular arrangement of cilia and flagella
Movement of cilia and flagella
Weblink 5: Flagella and cilia movement
4.2.2. Changes in the viscosity of cytoplasm
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Proteins found in the cytoskeleton are responsible for changes in the viscosity of cytoplasm. They do this by grouping together or separating themselves. They produce pseudopodia , an extension of cytoplasm, and modify the shape of the cell.
Pseudopodia are also used to surround and capture certain materials from the environment (microbes, food particles and so on). This process is referred to as phagocytosis .
4.3. The nucleus
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Animal cell seen through an electronic microscope.
The nucleus, which contains genetic material, is the control centre of the cell. It is normally located in the centre of the cell. In some cases, such as secreting cells and many plant cells, it can be found in peripheral areas.
Its structure varies depending on the moment in life of the cell. It has two different structures: interphase nucleus , when the cell is not dividing, and nuclear division , when the cell is dividing.
Video 5: The cell nucleus
4.3.1. Interphase nucleus
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It has a porous double membrane that surrounds the nucleoplasm , which is similar to cytoplasm. Inside the nucleoplasm there is chromatin , a substance formed by the double helix of DNA joined to histones (proteins), and a nucleolus , a spherical organelle involved in the synthesis of ribosomes.
4.3.2. Nuclear division
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When cell division begins, the nucleus changes completely: chromatin condenses into chromosomes . Chromosomes are X-shaped structures and they are present in different numbers depending on the species.
Each chromosome is made up of two chromatin filaments called chromatids , which are joined by a centromere . Both chromatids are identical, so the genetic information is duplicated. The centromere separates two regions in each chromatid, named arms .
Chromatin condensation
The number of chromosomes in gametes (sex cells) varies from somatic cell haploid number (n) diploid number ( 2n)
4.4. Animal cells and plant cells
Talking book
Most organelles are common to all eukaryotic cells. However, there are some differences between plant and animal cells. The main differences are:
Animal cell Plant cell
Wall No
Yes
Chloroplasts No
Yes
Centrioles Yes
No
Cilia and flagella In some cases
No
Science experiment: Observing cells
Key concepts
Each organelle has a specific function.
The endoplasmic reticulum, the Golgi apparatus, lysosomes and vacuoles are organelles that process nutrients.
Mitochondria and chloroplasts are organelles that produce energy.
The nucleus contains genetic material formed by chromatin. When a cell divides, the chromatin condenses into chromosomes.
Animal and plant cells are eukaryotic cells.
