ANIMAL CELLS

As with all of Earth’s organisms, animals are built from microscopic structures called cells. Cells are the basic unit of life and these microscopic structures work together and perform all the necessary functions to keep an animal alive. There is an enormous range of animal cells. Each is adapted to a perform specific functions, such as carrying oxygen, contracting muscles, secreting mucus, or protecting organs.

The cells of animals are advanced and complex. Along with plants and fungi, the cells of animals are eukaryotic. Eukaryotic cells are relatively large cells with a nucleus and specialized structures called organelles.

Although animal cells can vary considerably depending on their purpose, there are some general characteristics that are common to all cells. These include structures such as the plasma membrane, cytoplasm, nucleus, mitochondria, and ribosomes.

GENERAL STRUCTURE OF AN ANIMAL CELL

Animal cells have a number of organelles and structures that perform specific functions for the cell. The huge variety of cells that have evolved to fulfill different purposes do not always have all the same organelles or structures, but in general terms, these are some of the structures you can expect to find in animal cells:

PLASMA MEMBRANE

The plasma membrane is a porous membrane that surrounds an animal cell. It is responsible for regulating what moves in and out of a cell. The plasma membrane is made from a double layer of lipids. Extra compounds such as proteins and carbohydrates are embedded into the lipid membrane and perform roles such as receiving cellular signals and creating channels through the membrane.

NUCLEUS

The cells of animals and plants almost always have a ‘true’ nucleus. A nucleus consists of a nuclear envelope, chromatin, and a nucleolus.

The nuclear envelope is made from two membranes and encapsulates the contents of the nucleus. The double membrane has numerous pores to allow substances to move in and out of the nucleus.

Inside the nuclear envelope, the majority of the nucleus is filled with chromatin. Chromatin contains the majority of a cell’s DNA and condenses down to chromosomes as a cell divides. The nucleolus is the center core of the nucleus and produces organelles called ribosomes.

CYTOPLASM

The cytoplasm is the internal area of an animal cell that isn’t occupied by an organelle or nucleus. It consists of a jelly-like substance called ‘cytosol’ and allows organelles and cellular substances to move around the cell as needed.

ENDOPLASMIC RETICULUM (ER)

The endoplasmic reticulum is a network of membranes found within almost all eukaryotic cells. The membranes are connected to the membrane of the cell’s nucleus and are important for many cellular processes such as protein production and the metabolism of lipids and carbohydrates.

The endoplasmic reticulum includes both the smooth ER and the rough ER. The smooth ER is a smooth membrane and has no ribosomes, whereas the rough ER has ribosomes that are used to produce proteins.

MITOCHONDRIA

Mitochondria are one of the most important of all organelles. They are the site of cellular respiration – the process that breaks down sugars and other compounds into cellular energy. It is in the mitochondria where oxygen is used and CO₂ is produced as a byproduct of respiration.

GOLGI APPARATUS

The golgi apparatus (or golgi body) is another set of membranes found within the cell but is not attached to the nucleus of the cell. It serves many important functions including modifying proteins and lipids and transporting cellular substances out of the cell.

RIBOSOMES

Ribosomes are involved in the process of creating proteins. They can be either attached to the endoplasmic reticulum or floating freely in the cell’s cytoplasm.

PEROXISOMES

These small organelles perform a number of functions regarding the digestion of compounds such as fats, amino acids, and sugars. They also produce hydrogen peroxide and convert it to water.

LYSOSOMES

A lysosome is the waste disposal unit of the cell. They are another small organelle and contain a range of enzymes that allow them to digest molecules such as lipids, carbohydrates, and proteins.

CENTROSOMES

Centrosomes are involved in cell division and the production of flagella and cilia. They consist of two centrioles that are the main hub for a cell’s microtubules. As the nuclear envelope breaks down during cell division, microtubules interact with the cell’s chromosomes and prepares them for cellular division.

VILLI

Villi are needle-like growths that extend from the plasma membrane of a cell. For some cells, such as the cells along the wall of intestines, it is important to be able to rapidly exchange substances with their surrounding environment. Villi increase the rate of exchange of materials between cells and their environment by increasing the surface area of the plasma membrane. This increases the space available for material to move in and out of the cell.

FLAGELLA

Movement is particularly important for certain animal cells. Sperm cells, for example, live for the sole purpose of traveling to an egg and fertilizing it. Flagella (plural of flagellum) provide the mechanical ability for cells to move under their own power. A flagellum is a long, thin extension of the plasma membrane and is driven by a cellular engine made from proteins.

DIFFERENT TYPES OF ANIMAL CELLS

There are heaps of different types of animal cells and these are just a few from common tissues like skin, muscle, and blood.

SKIN CELLS

The skin cells of animals mostly consist of keratinocytes and melanocytes – ‘cyte’ meaning cell. Keratinocytes make up around 90% of all skin cells and produce a protein called ‘keratin’. The keratin in skin cells helps to make skin an effective layer of protection for the body. Keratin also makes hair and nails.

Melanocytes are the second main type of skin cell. They produce a compound called ‘melanin’ which gives skin its color. Melanocytes sit underneath keratinocytes in a lower layer of skin cells and the melanin they produce is transported up to the surface layers of cells. The more melanocytes you have in your skin, the darker your skin is.

MUSCLE CELLS

Myocytes, muscle fibers or muscle cells are long tubular cells responsible for moving an organism’s limbs and organs. Muscle cells can be either skeletal muscle cells, cardiac muscle cells or smooth muscle cells

Skeletal muscle cells are the most common type of muscle cells and are responsible for making general, conscious movements of the body. Cardiac muscle cells control contractions of heart by generating electrical impulses and smooth muscle cells control subconscious movements of tissues such as blood vessels, the uterus, and the stomach.

BLOOD CELLS

Blood cells can be split into red and white blood cells. Red blood cells make up around 99.9% of all blood cells and are responsible for delivering oxygen from the lungs to the rest of the body. Red blood cells are the only animal cells that do not have a nucleus. White blood cells are a vital part of an animal’s immune system and help to battle infections by killing off damaging bacteria and other compounds.

NERVE CELLS

Nerve cells, also called neurons, are the main cells of the nervous system. The human brain alone has around 100 billion nerve cells. They are the message carriers of animal cells and deliver and receive signals using dendrites and axons. Dendrites and axons are extensions from the cell that receive and export signals to and from the cell, respectively.

FAT CELLS

Fat cells, also known as adipocytes or lipocytes, are used to store fats and other lipids as energy reserves. There are two common types of fat cells in animals – white fat cells and brown fat cells. The main difference between the two cell types is the way they store lipids. White fat cells have one large lipid drop whereas in brown fat cells there are multiple, smaller lipid droplets spread through the cell.

DIFFERENCES BETWEEN PLANT, FUNGAL AND ANIMAL CELLS

Animal cells have slight differences to the eukaryotic cells of plants and fungi. The clear differences are the lack of cell walls, chloroplasts and vacuoles and the presence of flagella, lysosomes and centrosomes in animal cells.

Plant and fungal cells have cell walls. A cell wall is an external structure that surrounds the plasma membrane and provides protection and structural support. Plant cells also have chloroplasts and vacuoles. Chloroplasts are the site of photosynthesis and vacuoles are large sac-like organelles used to store substances.

Plant cells lack flagella, lysosomes and centrosomes. Fungal cells typically have lysosomes and centrosomes but very few species have flagella. The main difference between fungal and animal cells is the presence of a cell wall in fungal cells.

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SUMMARY

• Animal cells are typically large, specialized eukaryotic cells – they contain a nucleus and numerous organelles
• The plasma membrane surrounds an animal cell
• Almost all of a cell’s DNA is kept inside its nucleus
• Endoplasmic reticulum (ER) is a network of membranes connected to the nucleus – it includes the smooth ER and the rough ER
• Cellular respiration occurs in the mitochondria
• Ribosomes produce proteins – they can be found in the endoplasmic reticulum or freely floating
• Animal cells have lysosomes for digestion, centrosomes to help with cell division and sometimes flagella to help with movement – none of these three organelles are found in plant cells
• The cells of animals lack cell walls, chloroplasts and vacuoles which are all found in plant cells
• Different types of specialized cells are found in different tissues and have features relative to their function e.g. nerve cells have axons and dendrites to send and receive messages.