Introduction to Cells
Biology: AQA A Level, topic 2.1.1
Eukaryotic vs Prokaryotic Cells
All life on Earth exists as cells. A cell is the smallest unit of a living thing. All cells can be grouped into either eukaryotic or prokaryotic cells. These different categories have similarities and differences.
Similarities
- Both prokaryotic and eukaryotic cells contain organelles.
- The cell-surface membrane in eukaryotic and prokaryotic cells is made of a phospholipid bilayer.
- This bilayer is responsible for controlling the passage of substances across exchange surfaces.
Differences
- Prokaryotic cells make up single-celled prokaryotic organisms (e.g. bacteria). In contrast, eukaryotic cells make up complex eukaryotic organisms (e.g. animals, plants, fungi and algae).
- Eukaryotic cells are larger and more complex than prokaryotic cells.
- Cells arise from other cells by binary fission in prokaryotic cells and by mitosis or meiosis in eukaryotic cells.
Size comparison
Types of Eukaryotic Cells
Eukaryotic cells have similar structures across different types of organisms. But there are some key differences that are outlined here.
Animal cell
Animal cell
- Most animal cells have the following organelles:
- Mitochondria.
- Ribosomes.
- Endoplasmic reticulum (ER).
- Golgi.
- Lysosomes.
- Nucleus.
- Animal cells are enclosed by a cell membrane.
Plant cell
Plant cell
- Plant cells contain all the organelles found in animal cells.
- Plant cells also possess:
- Vacuole (a repository of cell sap).
- Chloroplasts (the site of photosynthesis).
- Cell wall (made of cellulose and contains plasmodesmata, through which cells exchange substances with each other).
Algal cells
- Algal cells and plant cells have an identical set of organelles.
Fungal cells
- Fungal cells are similar to plant cells, apart from:
- There are no chloroplasts in fungal cells.
- The cell walls of fungal cells are made from chitin instead of cellulose.
Specialisation and Organisation of Cells
In complex multicellular organisms, cells gain specific features. This is called specialisation. Specialised cells are then organised into groups to perform a function.
Cell specialisation
- A cell specialises because the shape and contents of a cell help it to carry out its function. Examples include:
- Muscle cells are very active so contain lots of mitochondria in order to produce ATP.
- Red blood cells have a biconcave shape and no nucleus to maximise space to carry oxygen. They also have lots of haemoglobin.
- Palisade cells have a long, upright shape and contain chlorophyll to absorb light for photosynthesis.
Cell organisation
- Specialised cells are organised into tissues, tissues into organs and organs into organ systems.
- Tissues (e.g. muscle tissue, xylem tissue).
- Organs (e.g. the animal heart or plant leaf).
- Organ systems (e.g. the female reproductive system, which includes the uterus, ovaries, mammary glands and breasts).
Test yourself on Introduction to Cells
Which of these statements about prokaryotic and eukaryotic cells is true?
Which cells contain organelles?
Which of these organelles are NOT found in animal cells?
Which organelles are present in plant cells but not in animal cells?
Which of these eukaryotes does NOT have a cell wall?