organelles.html

ORGANELLES
ANIMAL CELLS

NUCLEUS

Also known as the control center of the cell, the nucleus contains all of the "instructions" to the cell. It is the site were nucleic acids are synthesized and and directs all activities to the cell. It is surrounded by a double membrane known as the nuclear envelope. Substances such as proteins and nucleic acids enter the nucleus through holes in the envelope called nuclear pores. Inside the nuclear envelope is a material called nucleoplasm. This material contains a combination of deoxyribosnucleic acid (DNA) and proteins known as chromatin. When a cell goes through mitosis, a strand of this chromatin coils up and forms a chromosome. Most nuclei contain a dark spot called a nucleolus and is the site of ribosome formation. A ribosome is a small structure found on the rough endoplasmic reticulum and floating freely in the cytoplasm. A nucleolus is shown in the picture below.

MITOCHONDRIA

The mitochondrion is where adenosine triphosphate (ATP) is formed, therefore this organelle is commonly known as the "powerhouse" of the cell. ATP provides the chemical energy that drives the chemical reactions of the cell. Cells such as muscle cells or liver cells have a lot of mitochondria because they need a lot of energy. A mitochondria has two membranes, a smooth outer membrane that serves as the boundary between the mitochondrion and the cytoplasm. The inner membrane has many long folds, called cristae, that enlarge the internal surface area, providing more space for the reaction of cell respiration that take place there. Mitochondria are the sites of aerobic respiration. Many biologists believe that the mitochondrion was a descendant of some bacteria that was endocytosed by a larger cell billions of years ago. Evidence of mitochondrion DNA and the similarity of bacteria ribosomes and the ribosomes found in the mitochondria support this theory. The electron microscope of a mitochondria is found below.

ENDOPLASMIC RETICULUM (ER)

The endoplasmic reticulum is the "highway" to the cell. It is simply a membrane system of folded sacs and tunnels. There are two types of ER, one is the rough endoplasmic reticulum, the other is the smooth endoplasmic reticulum. The difference between them is that rough ER has ribosomes attached to it while the smooth ER does not. A picture of rough endoplasmic reticulum is shown below, notice the ribosomes on the surface (the black dots).

GOLGI APPARATUS

The Golgi apparatus is the processing, package, and secreting organelle of the cell. This organelle simpifies molecules and packages them into small membrane bound sacs called vesicles. It consists of a stack of membranes or sacs filled with fuid and dissolved or suspended substances. The Golgi apparatus works as an assembly line in a factory. After protein is synthesized on a ribosome, the protein passes into the interior of the ER membrane to an area of smooth ER. There the protein is enclosed in a vesicle that buds off from smooth ER. The vesicle then moves and fuses with a Golgi sac. The contents are then modified as they pass through the Golgi apparatus. At the last Golgi sac, the new vesicle moves to the cell membrane and discharges it's contents outside the cell. Below is a picture of the Golgi apparatus.

LYSOSOMES

The lysosome is an organelle that contains digestive enzymes. They are small, spherical organelles that are surrounded by a single membrane. These organelles digest food particles, disease-causing bacteria captured by the white blood cells, and worn-out and broken parts of cells. These organelles are extremely important in early development, on the other hand, lysosomes can destroy important structures in the cell such as DNA or RNA this explains the reason for the nuclear pores and the membrane. Notice in the picture below the dark spots, these are lysosomes.

CHLOROPLASTS

Chloroplasts are the site of photosynthesis. These organelles are usually oval and have two outer membranes. These two layers surround a semi-fluid interior called the stroma. Within the stroma, another system of interconnecting disk-shaped compartments are formed these disks are known as grana. Below is a picture of chloroplast in an onion cell.

VACUOLE

The vacuole in a plant cell takes up around 90% of the cell's volume. This organelle is a place for storage and commonly holds water for the cell. The vacuole must stay full to keep the plant upright. The pressure put on the cell wall by the vacuole is what keeps the cell rigid and standing against gravity. Can you find the vacuole in the picture below?

CELL WALL

The cell wall is the rigid covering of a plant cell. It is primarily made of long chains of cellulose embedded in hardening compounds such as pectin and lignin. Pores in the cell wall allow ions and molecules to pass to and from the cell membrane (this structure will be discussed later but CLICK HERE to go to the page). There are two types of cell walls, primary and secondary. The primary cell wall is formed during cell growth. The secondary wall is formed after the cell has stopped growing. These two wall together help strenghten the cell.

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