The parts of a plant and animal cells.

The Nucleus
The nucleus is the control center for every eukaryote cell. Within it, all of the work done by the cell as a whole is regulated. Much of this regulation is done by a substance known as DNA (deoxyribonucleic acid). DNA is a component of chromatin, strands of protein that float within the nucleus.
Part of the chromatin in a cell concentrates itself into an area known as the nucleolus. This area has the task of producing ribosomes, a type of organelle that will be discussed later.
The nucleus regulates the flow of material through its membrane by using a series of pores. These pores connect the two parts of the nuclear envelope. This envelope is made up of a pair of membranes that surround the nucleus. The inner half is wrapped tightly around the nucleus, while the outer half floats loosely around it. In the places where the two membranes are attached, a pore is formed.

Cytoplasm

Many of the parts of a cell that lie between the nuclear envelope and the plasma membrane are suspended in a material that exists between the aforementioned parts of a cell. This material, a jelly-like substance, is called cytoplasm. It provides support for the organelles, and is made up of protein and other macromolecules.

Organelles

Organelles are the parts of the cell that do most of the work necessary to keep the cell functioning. There are many different organelles, each with a different job. The duties of each organelle will be discussed later in this document.

The Plasma Membrane

A component of all cells, the plasma membrane regulates the passage of molecules in and out of the cell, and keeps the cytoplasm and organelles in place. The cytoplasm is made up of a pair of phospholipid walls, according to the most widely accepted theory.
A phospholipid is made up of a phosphate and a lipid, chemically bonded. Since lipids are hydrophobic, the exterior walls of the plasma membrane are made up of the phosphates, and between the walls are the lipids.
The membrane is not stationary; the molecules that make it up can move around to fill up rears in the membrane, and the proteins that fill the gaps in the groups may be replaced by new proteins, as well.

The Cell Wall

Cell walls are not present on animal cells, rather they only exist on the cells of plants, fungi, and some single-celled organisms. The cell wall(s) of a plant are largely composed of cellulose, a polysaccharide. In plants with soft walls, such as grass, only one cell wall, the primary cell wall, exists. In plants such as trees and bushes, as secondary cell wall is build up inside the first one after the cell has grown to its full size.
Plant cells do not touch one another, rather they are separated by the middle lamella, which is made up of pectin. Pectin, a jellylike polysaccharide, is the agent that causes jelly and jam to thicken, and the peels of some citrus fruits are made up of up to 50% pectin.
Unlike other parts of the cell, cell walls do not usually decompose when the cell dies. The xylem cells that make up the bulk of a tree are mainly dead cells, with only a small portion alive and engaged in their duties.

The function of each organelle.

Ribosomes

These organelles are responsible for protein synthesis. They either float in the cytoplasm or are attached to the endoplasmic reticulum. They are made up of proteins and assorted other macromolecules.

Endoplasmic reticulum

This organelle takes the form of a long, folded membrane. It is often continuous with the outer nuclear envelope. When it is covered with ribosomes, it is called rough endoplasmic reticulum. When it is bare, then it is called smooth endoplasmic reticulum. The two types are often continuous with one another. Proteins that were produced by the ribosomes on the rough ER move along the membrane until they reach their destination, the smooth ER. When they reach the smooth ER, a portion of the ER forms a bubble around the protein. Once the bubble separates from the ER, it is called a vesicle. At this time, the vesicle either drops off the protein at the cell membrane, where it is secreted, or transports it to the Golgi apparatus, where it is processed further.

Golgi apparatus

This organelle is used to prepare proteins for secretion. When a membrane-bound vesicle touches the Golgi apparatus, their membranes fuse. The proteins are the sucked into the Golgi apparatus, where each molecule of protein undergoes chemical changes. These changes could include the addition of carbohydrates or the removal of water. The protein molecule is then enclosed in a new vesicle, and carried off to the plasma membrane for secretion.

Mitochondria

These organelles supply energy for the cell. They release enzymes that can liberate the energy stored in food. Each one is surrounded by two membranes. The first separates the mitochondria from the cytoplasm, the second is a tightly packed folded membrane that fills the space inside the outer membrane. The folding of the inner membrane provides extra surface area within a small space. The amount of folding depends on the amount of work that the cell must do; more folding denotes a busy cell. The inner membrane is covered with enzymes, which are used for the chemical reactions that release energy from food.

Plastids

These organelles are unique to plants and some unicellular autotrophs. Examples of plastids include chloroplasts, chromoplasts, and leucoplasts. Chloroplasts contain chlorophyll, a green pigment that captures energy from the sun and uses it to produce carbohydrates. Chromoplasts contain the pigments that give some plants or parts of plants their coloration. Leucoplasts are colorless plastids, in which starch molecules are made from sugar molecules.

Vacuoles

Common in plant cells, these organelles function as storage rooms. They are usually filled with water, which in plants may contain food, salt, or pigments.

Lysosomes

These organelles contain enzymes that are used to reprocess damaged parts of the cell and make them available for reuse. Their membranes are immune to the chemicals of digestion that are trapped inside. Cells often contain many types of lysosomes, each containing a collection of chemicals that can dissolve only a certain type of molecule. Lysosomes are far less common in plant cells than they are in plant cells.

Cytoskeleton

The cytoskeleton is an organelle whose purpose is to provide support and an efficient system of transportation for the organelles within a cell. It is made up of microtubules and other small protein structures. These elements give shape to a cell. The transport of organelles is facilitated with a protein called kinesin, which binds organelles to microtubules, then drags the organelle around the labyrinth of microtubules.

Centrioles

These organelles contain microtubules, and are important in the reproduction of cells. They usually lie near the nucleus, and are present in most animal cells and the cells of some simple plants.

The differences between a plant and an animal cell.

With the variations in organelles aside, the most noticeable difference between plant and animal cells is probably the cell wall. In plants, a single, or occasionally double cell wall exists, while animals lack this feature.
Perhaps plants have a greater need for cell walls, as trees could not possibly maintain their fixed position without the secondary cell wall that provides the rigid support for each cell. Animals, on the other hand, are motile and need flexibility, in the form of flexible cell coverings.
In the realm of organelles, the largest difference between plant and animal cells is probably the utter absence of plastids in animal cells. Animals have no chloroplasts, chromoplasts, or leucoplasts, perhaps because they have no need for them.
Another differences is the fact that few plant cells have lysosomes, the "recycling centers" that are very common in animal cells. And, as a final difference in organelles, I offer the example of centrioles, which appear in almost every animal cell, but only appear in very simple plants.
In the end, we must see that although there are some differences between plant and animal cells, both of them have the same basic structure.

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