Animal Cells: Structure and Function
Key Biological Processes
- Nutrition: The process by which living matter absorbs and incorporates energy from the internal and external environment that surrounds it.
- Value: The process by which living beings receive information from the internal and external environment around them and adjust their operation to survive in that environment.
- Reproduction: The process by which living things give rise to other bodies that are similar or equal to them.
Eukaryotic Cell
Eukaryotic cells have their fundamental hereditary material (their genetic makeup) enclosed within a double membrane, the nuclear envelope, that defines a cell nucleus. These cells are microscopic in size but large and varied compared with other cells.
Prokaryotic Cell
Prokaryotic cells lack a distinct cell nucleus; their DNA is dispersed in the cytoplasm. Cells that do have a nucleus with DNA locked behind a membrane are called eukaryotes. Eukaryotes are the more familiar and complex life forms, which form their own empire.
Animal Cell
An animal cell is a type of eukaryotic cell that makes up many tissues in animals.
Features
The animal cell differs in many ways from plant cells. It lacks a cell wall, chloroplasts, and vacuoles. In the absence of a rigid cell wall, animal cells can adopt a variety of forms, and a phagocyte (phagocytic) cell can even store and engulf other structures.
Cell Membrane
The outer limit of the cell, formed by phospholipids. Its function is to delineate the cell and control what enters and exits.
Mitochondria
Tiny dual-membrane cell structures responsible for converting nutrients into the energy-rich compound adenosine triphosphate (ATP), which acts as a fuel cell. For this role, called respiration, it is said that mitochondria are the engine of the cell.
Chromatin
A macromolecular complex formed by the association of deoxyribonucleic acid (DNA) and basic proteins, histones, found in the nucleus of eukaryotic cells.
Lysosome
A sac delimited by a membrane that is found in cells with a nucleus (eukaryotic) and contains digestive enzymes that degrade complex molecules. Lysosomes are abundant in cells that fight disease, such as leukocytes, which destroy harmful invaders and cellular debris.
Golgi Apparatus
A distinct part of the cell membrane system, found in both animal and plant cells.
Cytoplasm
The cytoplasm covers the entire cell volume except for the nucleus. It encompasses many specialized structures and organelles.
Endoplasmic Reticulum
An extensive network of tubes that produce and transport materials within cells with nuclei (eukaryotic cells). The endoplasmic reticulum (ER) consists of limited branching membrane tubules and flattened sacs extending throughout the cytoplasm (cell contents outside the nucleus) and connected with the double membrane surrounding the nucleus. There are two types of ER: smooth and rough.
Rough Endoplasmic Reticulum
The outer surface of rough ER is covered with tiny structures called ribosomes, where protein synthesis occurs. It transports proteins made by ribosomes to cellular regions where they are necessary or to the Golgi apparatus, where they can be exported abroad.
Smooth Endoplasmic Reticulum
The smooth ER serves several functions. It is involved in the synthesis of almost all the lipids that form cell membranes and other membranes that surround other cell structures such as mitochondria. Specialized cells in lipid metabolism, including liver cells, tend to have more smooth ER. The smooth ER is also involved in calcium absorption and release to mediate some types of cellular activity. In skeletal muscle cells, for example, the release of calcium by the ER activates muscle contraction.
Nucleoplasm
The nucleus of eukaryotic cells is a discrete structure that contains the chromosomes, recipients of the genetic makeup of the cell. It is separated from the rest of the cell by a double-layered nuclear membrane and contains a material called nucleoplasm. The nuclear membrane is perforated by pores that allow the exchange of cellular material between the cytoplasm and nucleoplasm.
Nucleus
The most conspicuous structure in almost all animal and plant cells. It is characteristically surrounded by a membrane, is spherical, and is about 5 μm in diameter. Within the nucleus, DNA molecules and proteins are organized into chromosomes that are arranged in pairs and appear identical. The chromosomes are very twisted and tangled, making them difficult to identify separately.
Nucleolus
A structure located within the cell nucleus involved in the formation of ribosomes (cellular organelles responsible for protein synthesis). The cell nucleus typically contains one or more nucleoli, which appear as dense areas of fibers and granules of irregular shape. They are separate from the rest of the nucleus by membrane structures.
Centrioles
Each of the two cylindrical structures that are in the center of a eukaryotic cell organelle called the centrosome. The pair of centrioles are known as Diplosoma; they are arranged perpendicular to each other.
Ribosomes
The animal cell resembles the plant cell in some aspects, such as containing ribonucleic acid (RNA) to bind specific sequences of amino acids and thus form proteins. Ribosomes are found in all cells and also within two cellular structures called mitochondria and chloroplasts. Almost all float freely in the cytoplasm (cell contents outside the nucleus), but many are linked to networks of tubular membranes involved in occupying the whole cell mass and are called the endoplasmic reticulum.
Plasma Membrane
The plasma membrane of eukaryotic cells is a dynamic structure composed of two layers of phospholipid molecules that imbibe cholesterol and protein. Phospholipids have a hydrophilic head and two hydrophobic tails. The two layers of phospholipids are placed with their heads out and tails facing toward the inside. That is, the hydrophilic groups are directed toward the aqueous phase, the outer layer of the membrane into the extracellular fluid, and the inner layer into the cytoplasm.