4. Eukaryotic cells

Talking book

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
is protected
be found

Weblink 3: Inside a cell

4.1. Organelles

Talking book

Organelles are 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

Talking book

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

Talking book

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.

Mitochondria

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.

**A mitochondrion seen through a scanning electron.**

Chloroplasts

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.

**A chloroplast seen through a transmission electron microscope.**

4.2. Movement structures

Talking book

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

Talking book

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**

Weblink 5: Flagella and cilia movement

4.2.2. Changes in the viscosity of cytoplasm

Talking book

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

Talking book

**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

Talking book

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

Talking book

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.

The number of chromosomes in gametes (sex cells) varies from somatic cell. The haploid number (n) is the number of chromosomes in a gamete. The diploid number (2n) is the number of chromosomes in somatic cells. There are two sets of haploid cells in a somatic cell, one from each parent.

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.

Apply

Use the diagram below to locate the different structures of the mitochondrion in the photograph taken with a scanning electron.

Create

Find information and draw a diagram that relates the endoplasmatic reticulum to the Golgi apparatus.

Once you have finished your diagram, scan it and e-mail it to your teacher. Make sure you include your name, class year and activity number in the e-mail.

Create

Write a small report that outlines the similarities and differences between mitochondria and chloroplasts.

Evaluate

Is the following statement true?

'Plant cells have chloroplasts instead of mitochondria.'

True

False
Correct answer

Wrong answer

Explain your answer.

Understand

Relate pseudopodia and phagocytosis with their definitions.

Pseudopodia
Phagocytosis

Process in which a cell absorbs a particle from the external environment by producing pseudopodia to engulf the particle.
They are extensions of the cytoplasm produced as a result of viscosity changes.

Understand

Complete the table below showing the differences between cilia and flagella according to their shape and movement.

	Cilia	Flagella
Shape
Movement

Analyse

What is the role of the centriole during the following stages?

Apply

Which sturcutres of the nucleus can you see in image B?

B. Animal cell seen through an electronic microscope.

Understand

Relate the terms chromatid and centromere with their corresponding definition.

Chromatid
Centromere

Half of one chromosome that contains all the genetic information.
The place where chromatids meet.

Understand

Discuss the following statement: ' A cell with chromosomes does not have chromatin.'

Is it correct?

Yes

No
Correct answer

Wrong answer

Understand

A human ovum has 23 chromosomes. How many chromosomes does a human epithelial cell have?

A human epithelial cell has chromosomes.

Apply

Look at the images below and decide if they are from animal or plant cells. Explain your answers.

A. . We can see .

B. . We can see .

C. . We can see .

Create

In your notebook, create your own table that displays the main differences between animal and plant cells. Include a diagram.

Once you have finished your table and diagram, scan them and e-mail them to your teacher. Make sure you include your name, class year and activity number in the e-mail.

,
You have completed the lesson!

Below is the time you have spent on the activity and the score you obtained.

Time spent

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4. Eukaryotic cells

4.1. Organelles

4.1.1. Organelles that process nutrients

4.1.2. Organelles that produce energy

4.2. Movement structures

4.2.1. Cilia and flagella

4.2.2. Changes in the viscosity of cytoplasm

4.3. The nucleus

4.3.1. Interphase nucleus

4.3.2. Nuclear division

4.4. Animal cells and plant cells

Activity 13

Activity 14

Activity 15

Activity 16

Activity 17

Activity 18

Activity 19

Activity 20

Activity 21

Activity 22

Activity 23

Activity 24

Activity 25