Animal and plant cells, cellular organelles

The two important types of eukaryotic cells are animal and plant cells.
Vascular plants are built up of organs, such as leaves. Organs consist of tissues, and tissues, in turn, consist of cells with similar structures and functions. This animation shows a mesophyll cell. These cells are typically found in leaves and carry out intensive photosynthesis.

Eumetazoans are animals that have organs and tissues. This group includes humans. This animation shows a human liver cell.

Typical plant and animal cells exhibit a number of similar features and a number of divergent ones. Both types of cells have nuclei, endoplasmic reticula, that is, a complex network of membrane vesicles, and Golgi apparatuses. They contain a cytoskeleton and cytoplasm. The cells are enclosed by a cell membrane.

In plant cells, the cell membrane is enclosed in a cell wall, made of cellulose. Plant cells commonly contain both vacuoles, filled with cell sap, and photosynthesising chloroplasts.

The nucleus is enclosed by the nuclear membrane. It contains the nucleolus, where ribosomes are produced. The content of the nucleus is made up mostly of chromatin.

Chromatin consists of DNA coiled onto histone proteins. DNA encodes proteins. Thus, it is responsible for the properties of cells and the entire organism.

The nuclear membrane is a double lipid membrane. It contains pores, which are made up of pore-forming membrane proteins through which materials are transported between the cytoplasm and the nucleus.

The endoplasmic reticulum is continuous with the outer layer of the nuclear membrane. The rough endoplasmic reticulum is nearer to the nucleus. Its surface contains ribosomes, which synthesise protein molecules in the order specified by the DNA, which is copied into messenger RNA molecules in the nucleus. The smooth endoplasmic reticulum plays an important role in lipid synthesis and detoxification: it breaks down certain harmful materials.

The proteins produced by the ribosomes in the rough endoplasmic reticulum are processed and packed in the Golgi apparatus. It also plays an important role in determining the final destination of the protein.

The cell membrane is made up of phosphatide molecules. The head of a phosphatide molecule is polar, thus hydrophilic, while the tail is non-polar, thus hydrophobic. The hydrophobic parts turn towards each other and push out the water, forming a double layer. Within a layer, the phosphatide molecules can freely move, but they rarely leave the layer. Therefore, the lipid membrane is a two-dimensional liquid. Membrane proteins in the cell membrane may have several functions: some of them serve as channels, while others function as receptors, which bind signal molecules.

Chloroplasts are important organelles in plant cells; they carry out photosynthesis. They have a double membrane; the folds of the inner membrane form thylakoid discs. These discs are often stacked, forming grana.

Both plant and animal cells contain mitochondria. These are often considered the cells' power stations: they produce a large amount of ATP by breaking down organic molecules. ATP is the molecule that carries out energy transfer in cells.

Living organisms are characterised by cellular organisation. Cells are the smallest unit of life that is classified as living things and are often called the building blocks of life. Cells engage in biological processes, but cell organelles themselves are not living things.