Eukaryotic Cells

A eukaryotic cell is a compartmentalised cell that contains organelles such as a nucleus and mitochondria.

There are four main types of eukaryotic cells: plant, animal, fungi and protist cells. In this article, we will mainly cover animal and plant cells. Unlike prokaryotes which do not have a nucleus, all eukaryotes have a nucleus.

Differences between prokaryotic and eukaryotic cells

As mentioned, the main differences between eukaryotic cells and prokaryotic cells are that eukaryotes have a nucleus. Instead of a nucleus, prokaryotes have loose chromosomes that contain DNA information. Bacteria and other cells can also contain plasmids - small, circular DNA. Interestingly, these are separate from the main chromosome and will replicate independently. Almost like a mind of its own! Plasmids often provide a genetic advantage - this is where antibiotic resistance can occur. In addition, cells can exchange these plasmids via bacterial conjugation. Prokaryotes are "smart" with their adaptations.

Below you will find a table showing the differences between eukaryotic and prokaryotic cells, also known as the ultrastructure or the composition of eukaryotic cells.

Table 1. Summary of differences between prokaryotic and eukaryotic cells.

Eukaryotic cell

As you already know, eukaryotic cells are "defined" by the presence of a nucleus. A nuclear membrane surrounds the nucleus. The nucleus contains the cell's genetic information - which is contained within the chromosomes; however, mitochondria and chloroplasts have their own DNA.

Typical eukaryotic cell size

The size of eukaryotic cells varies quite a bit. Eukaryotic cells are usually bigger than prokaryotic cells ranging from 10–100 µm, making them up to 1000 times bigger than prokaryotic cells. When referencing cell size, we are referring to the diameter. Animal cells are usually up to 30µm, while plant cells can reach 100µm.

Plant and animal cells

Below you will find a diagram of two eukaryotic cells - one is a plant, and the other is an animal cell. Note the difference in structures of these two cells.

Example differences between plant and animal cells:

• Size: Animal cells tend to be smaller than plant cells.
• Shape: Due to a cell wall in plant cells, they tend to be a cube or a rectangle in shape. Animal cells do not have cell walls and thus appear in irregular shapes.
• Cilia: Cilia, microtubules that help with cell movement, are found in animal cells but not in plant cells.
• Plastids: Plant cells have chloroplasts which are essential for photosynthesis.
• Lysosomes: A membrane-bound organelle containing digestive enzymes are usually only present in animal cells. Lysosomes break down excess or old/worn down cell parts. Vacuole in plant cells takes care of this.
• Vacuole: The plant contains a large vacuole that takes up most of the space in the cell. It has many functions, but the primary function is to maintain the hydrostatic pressure of the cell. Animal cells can also contain vacuoles; however, they are much smaller and mainly help isolate waste from the cell.

Figure 2. An animal cell. Source: StudySmarter Originals.

Figure 3, A plant cell. Source: StudySmarter Originals.

Shape and size of a cell

The animal cell is depicted as round. However, we know that the membrane around animal cells is fluid and mostly made out of phospholipids, meaning that the shape of the animal cell is irregular. A plant cell has a more restricted shape similar to a cube/rectangle due to the presence of a cell wall.

Figure 4. Phospholipid bilayer of the membrane. Source: Public Domain, via Wikimedia Commons.

Plant organelles

Let's take a look at what A level biology can teach us about plant organelles.

Chloroplasts

Chloroplasts represent another notable distinction between a plant and animal cell. Similar to mitochondria, they have their own DNA and ribosomes. Both mitochondria and chloroplasts are found in plant cells.

Chloroplasts are sites where photosynthesis occurs. The chloroplast has the following structural features:

• Chloroplast envelope: Like the nuclear envelope, chloroplasts have a double membrane.

• Grana: stacks of disc-like structures found inside the chloroplasts. These discs are called thylakoids. This is where chlorophyll is found, which is the pigment that gives plants their green colour. It is also the photosynthetic pigment of plant cells.

• Stroma: is the place where the second part of photosynthesis takes place. In this fluid-filled space, there are a lot of enzymes. These are needed to synthesise sugars.

Vacuoles

While some animal cells can have multiple small vacuoles, the single vacuole in plant cells is much larger and can take up as much as 90% of a plant cell.

The vacuole in animal cells can store and digest substances.

In plants, the vacuole has multiple functions:

• Water storage: A plant's ability to store water is crucial to surviving times of drought and makes it susceptible to forces such as osmosis, which takes water from the cell when in a salty environment and lets it into the cell when in pure water (distilled water).

• pH level regulation: Plant enzymes have a very broad pH range, ranging from 3.0 to 10.0. This range is achieved by the vacuole, which pumps protons (H+ or H3O+) into the cytoplasm, which lowers the pH in the plant cell, making it acidic.

• Turgor maintenance: As mentioned before, the size of the vacuole is controlled by osmosis. When the plant cell has reached its maximum water storage capacity, it is called turgid.

Cell wall

Plant cells have cell walls. The cell wall of plants consists of cellulose. The cell wall is vital for tensile strength and protection against osmotic stress, i.e., it does not allow the cell to burst.

Additionally, the cell wall is responsible for water movement throughout the plant through plasmodesmata (membrane-lined pores that connect adjacent cells). Water can move through symplast and apoplast pathways. Symplast pathways occur through the cell wall, while apoplast through the cytoplasm.

Types of specialised eukaryotic cells

As previously mentioned, plant cells and animal cells are both eukaryotic cells. While they seem like very different organisms at first glance, they have some similarities. The most profound is the presence of specialised cells that have similar characteristics. For example, the skin cells of an animal and epidermal cells of a plant are, in both cases, the outermost layer that protects the underlying structures.

All stem cells can develop into any specialised cell. An early embryo contains these types of cells, which differentiate into different cells in the body. When the embryo is fully developed, stem cells can still be found in the human body in small numbers.

Stem cells in a plant can be found in meristems of the plant, which are located in roots and shoots.

Here is a list of plant and animal cell types for you to compare. We will first look at types of plant cells and then cat types of animal cells in direct comparison.

Examples of specialised animal cells

• Skin cells: Outer layer for protection.

• Blood cells: Oxygen transport and essential for the immune system.

• Muscle cells: For strength and movement.

• Fat cells: For energy storage and source.

• Nerve cells: Present in the whole body to send and receive signals from and to the brain.

• Reproductive cells: Sperm and ovum (egg cells) are needed to produce offspring.

Cells can often differentiate further. For example, above mentioned blood cells can differentiate into white blood cells of various types and red blood cells.

Examples of specialised plant cells

• Parenchyma cells: For synthesis and storage of materials.

• Collenchyma cells: Shock absorbing also have a support function in young plants.

• Sclerenchyma cells: Structural support.

• Xylem cells: Cells for structural support and transportation of water within the plant. When the water in the mesophyll cells evaporates (plant leaves), it is replaced by water transported by the xylem cells.

• Phloem cells: Transportation of nutrients.

• Reproductive cells: For the production of offspring (plants often have both male and female produce cells that together produce seeds).

• Epidermal cell: Tightly packed cells that make up the outer layer of a plant, similar to our skin making up our body's outer, protective layer. Root hair cells are a type of epidermal cell in the root region. They exchange material with the outside world and absorb water and minerals.

Specialisation and organisation of cells

Eukaryotes are mostly multicellular organisms (with some exceptions, including phytoplankton, zooplankton, yeast and others).

The first step in understanding how multicellular structures work is to understand single cells and their organelles. As each organelle performs a specific function, a cell's type can be determined by the number and the size of the organelles found inside. An example would be cells with many mitochondria, which metabolise a lot of ATP for energy - like muscle cells.

Multicellular organisms require cells to perform specific tasks. Cells are tailored to achieve this by having a specific structure and set of organelles. Despite the fact that all cells have the same DNA sequence at the start, only specific genes are expressed in specific cells. For example, muscle cells have a different gene expression than skin cells.

Some cells lose their ability to multiply after differentiating. Others, however, keep that ability and are able to regenerate tissue if there was a lesion, for example. As in the more general statement "all cells come from another cell" that applies to unicellular organisms, all cells within a multicellular organism come from another cell.

Cells can be grouped into tissues where they perform a similar function, such as muscular tissue. Organs are constructed from groups of tissues. For example, the heart is composed of various tissues, such as muscle and epithelial tissue. Organ systems are made up of organs that work together, such as the cardiovascular system, which is made up of the heart and blood vessels.