1.1.1Key Concepts in Biology

Eukaryotic Organisms

5 min read 9 parts Lesson 2 of 17 1 quiz

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Section 1 of 9Cells: the basic unit of life

A basic fungal cell (cell wall of chitin)

All living organisms are made of cells.

A cell is the smallest unit that can carry out the processes of life.

Some organisms, such as bacteria and amoeba, consist of a single cell.

Multicellular

Others, such as humans and oak trees, are multicellular and are made of millions of cells that work together.

Eukaryotic

Cells are classified into two main types: eukaryotic and prokaryotic.

This lesson looks at eukaryotic cells.

Section 2 of 9What makes a cell eukaryotic?

Eukaryotic and Prokaryotic Gene Expression

A eukaryotic cell has a nucleus that contains the genetic material, enclosed by a nuclear membrane.

Organelles

The cell also has other membrane-bound compartments called organelles, each with a particular job.

Because the organelles are separated by membranes, different chemical reactions can take place at the same time without interfering with each other.

Cytoplasm

The cell is surrounded by a cell membrane, and the inside, apart from the nucleus, is the cytoplasm, a watery solution where many chemical reactions occur.

Eukaryotic cells are generally larger than prokaryotic cells, typically 10 to 100 micrometres across.

The separate compartments make eukaryotic cells efficient.

What makes a cell eukaryotic?

For example, enzymes for respiration are held together inside mitochondria where conditions suit them, and the DNA is protected in the nucleus so that it is not damaged by the many reactions that happen in the cytoplasm.

Most eukaryotic cells also have a transport network and a cytoskeleton of protein fibres that holds organelles in place and helps the cell keep its shape, although these are not examined in detail at this level.

Section 3 of 9The main groups of eukaryotes

protist groups archaezoa euglenozoa alveolata

Main Uses of Glucose in Plants — 2D Simulation

Organisms with eukaryotic cells include:

The main groups of eukaryotes

Animals

Animals multicellular, feed on other organisms, cells have no cell wall (for example insects, fish, mammals).

Plants

Plants multicellular, make food by photosynthesis, have cell walls made of cellulose and chloroplasts (for example grass, ferns, trees).

Fungi

Fungi such as mushrooms, moulds and yeast. Their cell walls are made of chitin, and they feed by secreting enzymes onto dead or living matter and absorbing the products. Yeast is single-celled; most others are made of threads called hyphae.

Protists

Protists mostly single-celled organisms such as Amoeba, Euglena and Chlorella. Some, like Chlorella, have chloroplasts; some, like Plasmodium, are parasites.

Fungi reproduce by spores, and many are saprotrophs: they secrete digestive enzymes onto dead material and absorb the soluble products, which makes them important decomposers.

The main groups of eukaryotes

Protists are a varied group. Amoeba engulfs food, Euglena photosynthesises but can also feed, and the protist that causes malaria is carried by mosquitoes.

Animals and plants have the most specialised cell types because they are multicellular.

Section 4 of 9Organelles common to eukaryotic cells

DNA → Chromosome → Nucleus | Realistic Cell — All Organelles Inside

Structure	Function
Nucleus	Contains chromosomes of DNA; controls the activities of the cell
Cytoplasm	Where most chemical reactions take place
Cell membrane	Controls the passage of substances into and out of the cell
Mitochondria	Site of aerobic respiration, releasing energy
Ribosomes	Make proteins by translating genetic information

The nucleus holds DNA in the form of chromosomes. The DNA carries genes, which are the instructions for making proteins. Mitochondria are very small and are found in large numbers in cells that need a lot of energy, such as muscle cells. Ribosomes are tiny structures in the cytoplasm that join amino acids in the right order to make proteins.

Section 5 of 9Sizes and comparison with other cells

Eukaryotic organisms: protists, fungi, plants, animals

Comparison table of prokaryotes and eukaryotes

Comparison of electron and light microscopes

The diameter of a typical animal cell is around 20 micrometres. This is far too small to see with the naked eye, so we use microscopes. Differences between eukaryotes and prokaryotes are summarised here and developed in the lesson on prokaryotic organisms.

Feature	Eukaryote	Prokaryote
Nucleus	Present	Absent; DNA free in cytoplasm
Membrane-bound organelles	Present	Absent
Size	Usually 10-100 micrometres	Usually 1-5 micrometres
Examples	Animals, plants, fungi, protists	Bacteria

The differences between plant, animal and other eukaryotic cells reflect how each obtains energy and how it is supported.

Sizes and comparison with other cells

For example, plant cells need a rigid wall to support the plant body, and chloroplasts to make food, whereas animal cells need flexibility to change shape.

Yeast cells, as fungi, have a cell wall but no chloroplasts, so they cannot photosynthesise and must absorb nutrients from their surroundings.

Section 6 of 9Cells, tissues and organs

Levels of organisation: organelles, cells, tissues, organs, organ systems

Inside a leaf: levels of organisation question

Leaf and internal structure: levels of organisation question

In multicellular eukaryotes, cells with similar structure and function form tissues, tissues combine to form organs, and organs working together form organ systems. This organisation lets cells specialise, so the whole organism is more efficient. The specialisation of cells is covered in a later lesson.

Section 7 of 9Exam-style questions

Animal cell labelled (nucleus, mitochondria, ribosomes)

Preparing a Cheek-Cell Slide — Real Laser + Universal Wheel V11

State two structures found in both a human cheek cell and a yeast cell. [2]2 marks
Explain why muscle cells contain a large number of mitochondria. [3]3 marks
Name the cell structure that controls the entry and exit of substances in an animal cell, and one that contains the genetic material. [2]2 marks

Section 8 of 9Key terms

Gas exchange by diffusion in a single-celled protist

Term	Meaning
Eukaryote	An organism whose cells have a nucleus and other membrane-bound organelles.
Organelle	A specialised structure inside a cell with a particular function.
Nucleus	The organelle that contains the genetic material.
Mitochondrion	The organelle where aerobic respiration takes place.
Ribosome	A tiny structure where proteins are made.
Cytoplasm	The jelly-like substance in which cell reactions occur.
Protist	A mostly single-celled eukaryote that is not an animal, plant or fungus.

Section 9 of 9Exam technique

Saying that the nucleus is the "brain" of the cell. It contains the DNA with instructions; it controls activities by directing protein synthesis.

Thinking mitochondria make energy. They release energy from glucose in respiration; energy is not created.

Writing that fungi are plants. They have chitin walls and no chloroplasts.

Believing all cells have a nucleus. Bacterial cells do not.