1.1.1 Eukaryotes and Prokaryotes

In this lesson, we will compare eukaryotic and prokaryotic cells, exploring the structure of plant, animal, and bacterial cells, their differences in size, and how their genetic materialDNA that carries the instructions for cell structure and function. is organised. Additionally, we will develop an understanding of cell scale and perform order of magnitudeA comparison of size using powers of ten. calculations using standard formA way of writing very large or very small numbers as a number between 1 and 10 multiplied by a power of ten..

Eukaryotic Cells

Eukaryotic cells (plant and animal cells) make up the bodies of plants, animals, fungi and protists. They are generally larger and more complex than prokaryotic (bacterial) cells which we will explore later in this lesson. A key feature of eukaryotic cells is that their genetic material (DNA) is enclosed inside a nucleusA membrane-bound organelle in eukaryotic cells that contains DNA.. They also contain a cell membraneA thin, partially permeable barrier surrounding the cell that controls movement of substances in and out., cytoplasmA jelly-like substance in cells where most chemical reactions occur. and a range of specialised sub-cellular structures called organelles, each carrying out specific functions within the cell.

---

Animal Cells

Animal cells are eukaryotic cells that contain membrane-bound organelles. They have several key structures that allow them to carry out essential life processes.

The nucleus contains chromosomes made of DNA and controls the activities of the cell. Mitochondria are the site of aerobic respirationA chemical process that releases energy from glucose using oxygen, occurring in mitochondria., where energy stored in glucose is released. Ribosomes are the site of protein synthesis. The cytoplasm is where most chemical reactions occur, controlled by enzymes. The cell membrane surrounds the cell and regulates the movement of substances in and out.

---

Plant Cells

Plant cells are also eukaryotic and contain the same basic structures as animal cells (nucleus, cytoplasm, cell membrane, mitochondria and ribosomes). However, they have additional structures that allow them to carry out photosynthesisThe process by which plants use light energy to produce glucose. and maintain structural support.

Plant cells have a cellulose cell wallA rigid outer layer that strengthens and supports the cell; made of cellulose in plants and peptidoglycan in bacteria. outside the cell membrane, which strengthens the cell. They often contain chloroplasts, which are the site of photosynthesis and contain chlorophyllA green pigment found in chloroplasts that absorbs light energy for photosynthesis. to absorb light. They also contain a large permanent vacuoleA fluid-filled space in plant cells that maintains internal pressure. filled with cell sapA weak solution of sugars and salts found in the vacuole of plant cells. (a solution of sugars and salts), which helps maintain internal pressure and keep the cell firm (turgid).

---

As we have now seen, the cell membrane, cytoplasm, nucleus, mitochondria, and ribosomes are features of both animal and plant cells. However, the cell wall, chloroplasts, and vacuole are features of plant cells only. This is detailed in Table 1 below.

Feature	Explanation
Cell Membrane	A thin, flexible layer that surrounds the cell. It controls what substances enter and leave the cell.
Cytoplasm	A jelly-like substance where most chemical reactions happen. It contains enzymes that control these reactions and ribosomes for making proteins.
Nucleus	Contains the cell’s genetic material (DNA) arranged in chromosomes. It controls the activities of the cell, including growth and protein production.
Mitochondria	The site of aerobic respiration where energy is released. They release energy from glucose, which the cell uses for activities such as growth and repair.
Ribosomes	Tiny structures where proteins are made. Proteins are needed for growth, repair, and carrying out chemical reactions.
Cell Wall (Plant Cells Only)	A rigid outer layer made of cellulose. It strengthens the cell, supports its shape, and prevents it from bursting.
Chloroplasts (Plant Cells Only)	The site of photosynthesis. They contain chlorophyll, which absorbs light energy to make glucose.
Vacuole (Plant Cells Only)	A large space filled with cell sap (a weak solution of sugars and salts). It helps keep the cell firm by maintaining internal pressure.

This compartmentalisation in eukaryotic cells allows different processes to occur efficiently at the same time, which is important for complex and multicellular organisms.

Prokaryotic Cells

Prokaryotic cells are bacterial cells. They are much smaller than eukaryotic cells and are always single-celled organisms. Unlike eukaryotic cells, prokaryotes do not have a nucleus and do not have membrane-bound organelles. Their genetic material is not enclosed within a nuclear membrane. Instead, the chromosomal DNA forms a single circular loop that lies free in the cytoplasm. Many bacteriaA single-celled prokaryotic microorganism. also contain plasmids which are small, circular DNAA single loop of DNA found free in the cytoplasm of prokaryotic cells. molecules separate from the main chromosomeA structure made of DNA that carries genetic information..

Ribosomes are present in prokaryotic cells and are the site of protein synthesis, but they are smaller than those found in eukaryotic cells. Important processes such as respiration occur in the cytoplasm and at the cell membrane rather than inside mitochondria.  

Feature	Explanation
Cell Membrane	A thin layer that surrounds the cell. It controls the movement of substances into and out of the cell.
Cytoplasm	A jelly-like substance where chemical reactions take place. It contains enzymes and small ribosomes for making proteins.
Cell Wall	A strong outer layer that supports and protects the cell. In bacteria, it is made of a substance called peptidoglycanA substance forming the cell wall of bacterial cells., not cellulose.
Circular DNA	A single loop of chromosomal DNA found free in the cytoplasm. It controls the cell’s activities but is not enclosed in a nucleus.
Plasmids	Small circular rings of extra DNA found in the cytoplasm. They often contain genes that may give advantages, such as antibiotic resistance.
Ribosomes	Small structures where proteins are made. Prokaryotic ribosomes are smaller than those in eukaryotic cells.

Although prokaryotes are structurally simpler, they are highly successful organisms that reproduce rapidly and can survive in a wide range of environments.

Scale and Size of Cells

Bacterial cells are significantly smaller than eukaryotic cells. Eukaryotic cells typically range from 10 to 100 micrometres (μm) in diameter. Prokaryotic cells, such as bacteria, usually range from 1 to 5 micrometres (μm).

Because cells are so small, we use metric units with prefixes:

• A centimetre (cm) is equal to one hundredth of a metre (\(1 \times 10^{-2} \text{m}\)).
• A millimetre (mm) is equal to one thousandth of a metre (\(1 \times 10^{-3} \text{m}\)).
• A micrometre (μm) is equal to one millionth of a metre (\(1 \times 10^{-6} \text{m}\)).
• A nanometer (nm) is equal to one billionth of a metre (\(1 \times 10^{-9} \text{m}\)).
• A picometer (pm) is equal to one trillionth of a metre (\(1 \times 10^{-12} \text{m}\)).

Orders of magnitude describe differences in size using powers of 10. Moving up one order of magnitude means multiplying by 10. Moving down one order of magnitude means dividing by 10. Eukaryotic cells are approximately 10 times larger than prokaryotic cells, which means approximately one order of magnitude.

Standard form is a way to write very large or very small numbers in the form: \(a \times 10^b\), where \(a\) is a number between 1 and 10, and \(b\) is the power of 10.