THE CELL

CELL ANATOMY

• The vast majority of cells are minuscule; the average diameter of a cell ranges from 20 to 30 micrometers (m), which means that 40 cells lined up would span a period.
• Neurons (nerve cells) and muscle fiber cells (myofibers) are two examples of highly specialized cells that can grow to be more than 30 centimeters (more than 12 inches) in length.
• The cell membrane, also known as the plasma membrane, can be thought of as the "skin" of the cell.
• Organelles are the structures that may be found within a cell; each organelle has its own unique size, shape, and function.
• These organelles do not move around randomly within the cell.
• The interior of the cell is very well organized, with compartments that are connected to one another by sheets and membranes.
• These compartments are held in place by a flexible "skeleton" that is made up of even smaller tubules and filaments.

EMBRYONIC STEM CELL

• Stem cells are "starter" cells that are not specialized but have the potential to develop into specialized cells.
• Stem cells in an embryo have the potential to differentiate into any of the body's more than 200 different types of specialized cells.

CELL TYPES

Epithelial cells

• These cells are responsible for the formation of skin, as well as the covering of most organs and the lining of hollow cavities, such as the digestive tract.

Photoreceptor cell

• In the retina of the eye is a type of cell that is sensitive to both light and color called a cone cell.

Red blood cell

• A red cell, also known as an erythrocyte, is essentially a sack filled with oxygen-carrying hemoglobin molecules.
• These molecules are arranged in a double-dished (biconcave) shape.

Adipose (fat) cell

• Adipocytes, which are the primary kind of adipose cell, are large and packed to the brim with lipid droplets, which are responsible for the storage of energy.

Smooth muscle cell

• The enormous, spindle-shaped cells that make up smooth muscle contract by sliding strands of protein along the interior of the cell.

Nerve cell

• Each cell has what are called dendrites, which are short branches, and an axon, which is a long "wire."
• Dendrites are responsible for receiving nerve impulses.

Sperm cell

• Each sperm has a head that is responsible for transporting the genetic material of the father, and a whip-like tail that is used for movement.

Ovum (egg) cell

• These enormous cells hold the genetic material from the mother as well as the energy supplies necessary for the embryo to begin its first cell divisions.

CELL MEMBRANE

• The membrane is able to accomplish its dual duties of safeguarding the contents of the cell and allowing materials to go into and out of the cell due to the presence of a number of features that make this possible.
• Phospholipid molecules form a double layer that serves as the fundamental structural element of this membrane.
• Each phospholipid consists of a head group that is water-loving (also known as hydrophilic) and two tail groups that are water-hating (also known as hydrophobic).
• The structure of the two layers places the heads on the outside and the inside of the cell membrane, respectively, and the tails in the middle of the space between them.
• The phospholipids are arranged in a pattern that is broken up at regular intervals by protein molecules and chains of carbohydrates.
• This pattern is what allows other cells in the body to recognize the cell.

SURFACE ORGANELLES

• Certain cells in the body have specialized structures that protrude from their surface. These structures are called protrusions.
• The cells that line the small intestine include tiny projections that look like fingers called microvilli.
• These microvilli increase the surface area that is available for the absorption of nutrients.
• Some cells in the female reproductive system have minute, hairlike cilia that wave to transport the ovum along the oviduct.
• Similar ciliated cells in the respiratory tract carry small particles out of the airways.
• Ciliated cells in the digestive tract move food particles out of the digestive tract.
• The human sperm are the only cells in the human body that have a lengthy flagellum that looks like a whip and is employed for propulsion.

SPERM

• The human sperm cell has a slender tail, known as a flagellum, that extends from it.
• This tail acts as a propeller, assisting the sperm as they move up the female genital tract.

CILIATED CELLS

• Some of the cells that line the fallopian tubes have cilia that look like hair and are colored pink in this micrograph.
• These cilia move an egg closer to the uterus as they do so.

MEMBRANES OF ORGANELLES

• Membranes are responsible for the subdivision of the cytoplasm into parts, the regulation of the movement of substances between these sections, the provision of attachment sites and storage spaces, and the formation of channels along which substances travel.

GOLGI COMPLEX

• Processing of proteins that come from the endoplasmic reticulum takes place inside the membrane-bound sacs that make up the Golgi complex.

ENDOPLASMIC RETICULUM (ER)

• In most cases, a single continuous labyrinthine space is enclosed by a succession of ER membranes that are heavily folded and curled.

MITOCHONDRION

• Folds are made in the inner membrane to enhance the surface area available for energy release.

TRANSPORT

• There are three different processes that can be responsible for the movement of materials through the cell membrane.
• Across a process known as diffusion, even relatively insignificant molecules like water, oxygen, and carbon dioxide are able to pass through the membrane.
• Facilitated diffusion is necessary for the passage of molecules that are unable to pass through the phospholipid layer.
• When there is a lower concentration of a substance on the outside of the cell than there is on the interior of the cell, that substance can only be transported into the cell by active transport, which takes energy.
• Examples of such substances include minerals and nutrients.

DIFFUSION

• There is a natural movement of many molecules from a region in which they are present in high concentration to a region in which they are present in lower concentration.

FACILITATED DIFFUSION

• After changing its shape and releasing the molecule into the cell, a carrier protein first connects with a particular molecule that is located outside of the cell.

ACTIVE TRANSPORT

• The binding of molecules to a receptor location on the cell membrane causes a protein to transform into a channel that allows molecules to pass through.

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