Lesson 1, Topic 1
In Progress

01-Overview of Cells

March 28, 2021

Human cells are microscopic in size, but vary considerably in shape.

Cells contain cytoplasm, a substance found only in cells. Organelles are specialized structures within the cytoplasm, which is surrounded by a plasma membrane.

The three main parts of a cell are the plasma membrane, the cytoplasm, and the nucleus.

The plasma membrane forms the outer boundary of the cell and is composed of a thin, two-layered membrane of phospholipids containing proteins. Proteins and other molecules embedded in the membrane can function in transport, signaling, self-identification, anchoring of fibers, and chemical processing.

The cytoplasm is the internal fluid of the cell and contains numerous organelles.

Ribosomes are made of two tiny subunits of mostly ribosomal RNA. They may attach to rough endoplasmic reticulum or lie free in the cytoplasm. Ribosomes manufacture enzymes and other protein compounds and are known as “protein factories.”

The endoplasmic reticulum, abbreviated as ER, is a network of connecting sacs and canals that carry substances through the fluid cytoplasm. There are two types: rough and smooth. Rough endoplasmic reticulum collects, folds, and transports proteins made by ribosomes, whereas smooth endoplasmic reticulum synthesizes chemicals and makes new membrane.

The Golgi apparatus is a group of flattened sacs near the nucleus that collects chemicals into vesicles and moves them from the smooth endoplasmic reticulum outward to the plasma membrane. The Golgi apparatus is called the chemical processing and packaging center of the cell.

Mitochondria are composed of inner and outer membranous sacs and are involved with energy-releasing chemical reactions. A mitochondrion contains one DNA molecule and is often called the power plant of the cell.

Lysosomes are membranous-walled organelles that contain digestive enzymes. They serve a protective function in that they engulf and destroy microbes.

Centrioles are paired organelles that lie at right angles to each other near the nucleus. They function in cell reproduction.

Microvilli are small, fingerlike extensions of the plasma membrane. They increase the absorptive surface area of the cell.

Cilia are fine, hair-like extensions found on free or exposed surfaces of some cells. They are capable of moving in unison in what can be described as a wave-like fashion.

Flagella are single projections extending from cell surfaces and are much larger than cilia. The “tails” of sperm cells are the only example of flagella in humans.

The third vital part of the cell is the nucleus. It controls the cell and houses DNA, which is the genetic code that contains the instructions for making proteins. The proteins determine the structures and functions of the cells. The structures of the nucleus include the nuclear envelope, nucleoplasm, nucleolus, and chromatin granules. DNA molecules become tightly coiled chromosomes during cell division. Each cell has 46 chromosomes in the nucleus.

Every human cell has a designated function. Some functions simply help maintain the cell, whereas others regulate life processes of the body itself. Specialized functions of a cell differ depending on the number and type of organelles within the cell.