Eukaryotes are multicellular organisms whose cells have a membrane-bound nucleus. This nucleus contains membrane-bound organelles, such as mitochondria and the Golgi apparatus, and the genetic material. Examples of eukaryotes include humans and plants.
Eukaryotic organisms can have several types of tissues that consist of many different cell types, a feature that is not possible in prokaryotes. Eukaryotic reproduction occurs sexually through gamete fusion and meiosis as well as asexually through mitosis.
Mitosis involves cell division that gives rise to two identical cells, while meiosis is characterized by DNA replication followed by two rounds of cell division. This division finally produces four daughter cells, each having half the chromosomes from the original parent.
Although eukaryotes represent a small minority of all living things, due to their larger size, they have a collective biomass equal to that of prokaryotes.
Eukaryotic cell structure cut-away - scientifically accurate 3d illustration - Image Copyright: Mopic / Shutterstock
Structure of eukaryotes
The structure of eukaryote cells are somewhat complex and involve many different structural components. These components include:
- Nucleus - a double membrane found in a eukaryotic cell which surrounds the nucleolus and chromosomes.
- Cell membrane - protein and lipids are the primary building blocks of the eukaryotic cell membrane which envelopes eukaryotic cells.
- Cell wall - plants and fungi have a rigid cell wall apart from the cell membrane; animal cells do not have a cell wall.
- Cytoplasm - a gelatinous solution in which the cytoskeleton and organelles float around.
- Cytoskeleton, or the cell's skeleton, gives shape and stability to cells and is made of three kinds of fibers called microfilaments, microtubules, and actin filaments.
- Organelles are small structures inside the cytoplasm that have specialized functions in the body.
Eukaryotic organelles also include various components, including the following:
- Endoplasmic reticulum (ER) - a special membrane that often has ribosomes on its surface, thus assisting in protein synthesis as well as mechanical support and transport.
- Mitochondria - features an inner and outer membrane and functions in the synthesis of adenosine triphosphate.
- Lysosomes - spheres enclosed by membranes and contain strong digestive enzymes; they destroy damaged cells and digest phagocytosed materials.
- Ribosomes - contain ribosomal RNA (rRNA) and protein and are the site of protein synthesis in cells.
- Flagella and cilia - hair-like projections found in some human cells and help the cell in locomotion.
- Centrioles - cylindrical structures found near the nucleus that have key functions in cell division.
- Villi - projections in the cell membrane that increase the cell’s surface area.
- Chloroplasts or plastids - an organelle unique to plant cells that store chlorophyll and are the site of photosynthesis.
- Vacuoles - bags or sacs filled with fluid; animal cells have several small vacuoles while plant cells have one large vacuole that occupies nearly 90% of the cell volume.
The cellular environment
Eukaryotic cells have many components that play critical roles in various essential cellular pathways. These components are briefly discussed below:
The cell of most living things is composed of 60 - 90% water, which is a good solvent and is useful in many key metabolic reactions.
Sodium, calcium, potassium, and other ions are usually found in intracellular and extracellular fluid. Differences in ion concentrations are important in osmoregulation and cell signaling.
Carbohydrates, such as glucose and starch, account for nearly 3% of a cell’s dry mass. They serve as one of the most important energy sources for cells.
Water-insoluble lipids account for nearly 40% of a typical cell’s dry mass. These lipids are mostly composed of carbon and hydrogen and help the cell in long-term energy storage.
Proteins constitute about 60% of a typical cell and have important structural functions in the cell.
DNA is the genetic material stored inside the nucleus of eukaryotes and encodes instructions for different types of proteins essential for life. RNA is also present in different forms such as mRNA, tRNA, and rRNA.