Prokaryotes and eukaryotes (Edexcel GCSE Biology Combined Science): Revision Notes

Eukaryotes & Prokaryotes

All living organisms are made up of cells.

Cells can be classified into two main types: Eukaryotic and Prokaryotic cells.

Eukaryotic Cells:

• Eukaryotes include all plants, animals, fungi, and protists.
• These cells are more complex and usually larger than prokaryotic cells.

Key features:

• Nucleus: Contains the cell's genetic material (DNA) enclosed within a nuclear membrane.
• Cell membrane: Controls the movement of substances in and out of the cell.
• Cytoplasm: Gel-like substance where most chemical reactions happen.
• Mitochondria: Where aerobic respiration occurs, providing energy for the cell.
• Ribosomes: Sites of protein synthesis.
• Chloroplasts (in plant cells): Site of photosynthesis, contains chlorophyll.
• Cell wall (in plant cells): Made of cellulose, strengthens the cell.

Prokaryotic Cells:

• Prokaryotes include bacteria and archaea.
• These cells are simpler and generally smaller than eukaryotic cells.

Key features:

• No true nucleus: Genetic material (DNA) is not enclosed within a membrane; instead, it is a single loop of DNA found in the cytoplasm.
• Plasmids: Small rings of DNA that can replicate independently of chromosomal DNA.
• Cell membrane: Similar function to eukaryotes, controlling the movement of substances.
• Cytoplasm: Contains ribosomes, but they are smaller than those in eukaryotes.
• Cell wall: Provides structure and protection, but it is not made of cellulose.
• Flagella (in some bacteria): Tail-like structures that help the cell move.

Differences Between Eukaryotic and Prokaryotic Cells:

• Size: Eukaryotic cells are generally larger (10-100 micrometres) compared to prokaryotic cells (0.1-5 micrometres).
• Nucleus: Eukaryotes have a nucleus; prokaryotes do not.
• Organelles: Eukaryotes have membrane-bound organelles (e.g., mitochondria, chloroplasts); prokaryotes do not.
• DNA Structure: Eukaryotes have linear DNA associated with proteins (histones); prokaryotes have circular DNA without histones.
• Reproduction: Eukaryotic cells divide by mitosis or meiosis; prokaryotic cells divide by binary fission.

Animal Cells (Eukaryotic Cells)

Nucleus:

• The nucleus controls the activities of the cell and contains the cell's genetic material (DNA). This DNA carries instructions that tell the cell how to function and grow. Cell Membrane:

• The cell membrane controls what enters and leaves the cell. It's like a barrier, protecting the cell, while allowing essential nutrients to pass through and waste to exit. Mitochondria:

• These are known as the "powerhouses" of the cell. Mitochondria carry out respiration, which is the process of converting glucose and oxygen into energy that the cell can use. Ribosomes:

• Ribosomes are responsible for making proteins. These proteins are essential for repairing damage and directing chemical processes in the cell.

Plant Cells (Eukaryotic Cells)

Nucleus:

• Like in animal cells, the nucleus in plant cells controls the cell's activities and contains the DNA. Cell Membrane:

• The cell membrane works similarly to that in animal cells, controlling what substances enter and leave the cell. Mitochondria:

• Mitochondria provide energy for the plant by converting glucose into energy through respiration, just like in animal cells. Ribosomes:

• Ribosomes make proteins, which are needed for the cell's structure and processes.

Differences between Plant and Animal

Cell Wall:

• The cell wall is an additional layer outside the cell membrane, giving the plant cell strength and structure. It's made of a tough substance called cellulose. Chloroplasts:

• Chloroplasts are where photosynthesis happens. They contain a green pigment called chlorophyll, which absorbs sunlight to produce food for the plant (glucose) using water and carbon dioxide. Vacuole:

• The vacuole is a large, fluid-filled space in plant cells that stores nutrients and waste products. It also helps the cell maintain its shape.

Bacteria Cells (Prokaryotic Cells)

Chromosomal DNA:

• Unlike eukaryotic cells, bacteria don't have a nucleus. Instead, their genetic material is found in a single circular strand of DNA located in the cytoplasm. Plasmid DNA:

• In addition to chromosomal DNA, bacteria also have small rings of extra DNA called plasmids. These can carry extra genes, such as those for antibiotic resistance. Cell Membrane:

• The cell membrane in bacteria controls what substances move in and out of the cell, similar to eukaryotic cells. Ribosomes:

• Just like in eukaryotic cells, ribosomes in bacteria make proteins that are essential for the cell's survival and functions. Flagella:

• Flagella are tail-like structures that help bacteria move. They act like little motors, allowing the bacteria to swim towards food or away from harmful substances.

Key Differences Between Cell Types:

• Eukaryotic cells (Animal and Plant cells): These have a nucleus and other membrane-bound organelles.
• Prokaryotic cells (Bacteria): These do not have a nucleus or other membrane-bound organelles. Instead, their genetic material floats freely in the cell. These sub-cellular structures are vital for the specific functions that each cell type performs.