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Cells Are Basic Units of Life
The structure and physiology, or function, of living things can be analyzed on a cellular level. The
different types of cells in an organism determine how the organism as a whole appears and
behaves. Although the various cells carry out different tasks, the cells within an organism do not
function independently. They are organized into tissues. Tissues are groups of cells that work
together for a particular purpose. Tissues are organized into larger components—organs and organ
systems. Humans have many different organ systems, including the circulatory, respiratory, and
nervous systems. Together these systems regulate how the body functions and responds to its
environment.
Cells Arise From Pre-Existing Cells
All cells arise from pre-existing cells. For unicellular—or one-celled—organisms, the production of a
new cell results in a new organism. In multicellular organisms, new cells increase the size of tissues
and replace old or damaged cells. The process of making new cells allows organisms to grow and to
recover from sickness and injuries. Studying this process helps doctors and scientists understand
how organisms develop and how to treat damage or disease.
Prokaryotic and Eukaryotic Cells
Cells are categorized as either prokaryotic or eukaryotic. Prokaryotic cells are found in bacteria and
in archaea. All other organisms have eukaryotic cells. The root of these terms, “karyose”, comes
from the Greek word for kernel. In biology, this root refers to the nucleus of a cell, which is the
cellular structure containing genetic material, or DNA. One of the main differences
between prokaryotic cells and eukaryotic cells relates to the nucleus.
A prokaryotic cell lacks a membrane-bound nucleus. In contrast, a eukaryotic cell has a distinct
membrane-bound nucleus. This nucleus contains linear strands of DNA. In contrast, in prokaryotic
cells the DNA is unenclosed and usually organized as a circular chromosome.
Furthermore, prokaryotic cells lack other membrane-bound organelles. Eukaryotic
cells contain organelles that are enclosed in membranes.
What structures make up cells?
Components of Prokaryotic Cells
The prokaryotic cells of Bacteria and Archaea lack membrane-bound organelles, but have distinct
components that carry out important functions within the cell. In prokaryotic cells, DNA is coiled up in
a region called the nucleoid. This DNA usually consists of a single circular chromosome. Unlike
the nucleus of eukaryotic cells, a membrane does not enclose the nucleoid. Prokaryote DNA is in
direct contact with the cytoplasm, a gel-like component that contains subcellular
structures. Prokaryotic cells may also contain plasmids. These are relatively small, circular DNA
molecules. Plasmids contain fewer genes than the nucleoid’s DNA strand, and their genes are not
needed for normal cell functions. However, plasmids confer useful properties, for example antibiotic
resistance in some bacteria. Structures called ribosomes are also found in prokaryotic cells.
Ribosomes are small bodies within the cell. Ribosomes are involved in protein synthesis.
In prokaryotic cells, the ribosomes are suspended in the cytoplasm.
Prokaryotic cells are enclosed by a cell membrane. The cell membrane serves as a barrier to
unwanted chemicals, and allows cells to recognize one another. A key function of the cell
membrane is to control which substances move into and out of the cell. Photosynthetic prokaryotic
cells may also have inner membranes called thylakoids, which facilitate photosynthesis. Outside
the cell membrane, most prokaryotic cells have a cell wall that protects the cell and gives it shape.
Some bacteria have an outer covering called a capsule. The capsule is a hard coat that retains
moisture and prevents the bacteria from being engulfed and destroyed by other cells.
Components of Eukaryotic Cells
Eukaryotic cells are found in plants, animals, fungi, and protists. The membrane-bound subcellular
structures in eukaryotic cells are called organelles. These subcellular structures are part of the
cytoplasm or cytosol. The membrane isolates the organelle, allowing it to carry out specific functions.
A membrane also encloses the entire cell, containing the cytoplasm and separating the cell from its
environment. Some eukaryotic cells also have a tough cell wall surrounding the cell membrane.
Not all eukaryotic cells are the same, but they do share many of the same organelles.
One organelle that is present in all eukaryotic cells is the nucleus. Most of the cell’s genetic
information, in the form of DNA, resides within the nucleus. Like other organelles, a membrane
encloses the nucleus. It serves as the control center of the cell.
Another type of organelle found in most eukaryotic cells is endoplasmic reticulum (ER). There are
two kinds of ER. Rough ER is the site of protein synthesis. Rough ER has ribosomes on its surface
that manufacture proteins. Smooth ER produces lipids, detoxifies chemicals, and transports
materials throughout the cell. Smooth ER lacks ribosomes. The ER works closely with
another organelle called the Golgi apparatus or Golgi body. The Golgi apparatus modifies and
packages proteins from the ER and sends them to other parts of the cell. Proteins processed by the
Golgi apparatus may be transported to the cell membrane to be released from the cell.
Other structures found in eukaryotic cells include mitochondria, lysosomes, vacuoles, and
chloroplasts. Mitochondria are oblong or ovoid organelles that serve as the power packs of the cell.
The mitochondria release the cell’s stored energy. Lysosomes are structures that contain digestive
enzymes. These enzymes break down food and waste material. Some eukaryotic cells store
nutrients and waste products in vacuoles. Vacuoles are fluid-filled structures enclosed within a
membrane. The size and function of vacuoles differ for plant cells and animal cells. Some plant
cells also contain chloroplasts. These organelles produce sugars and starches via the process of
photosynthesis.
Some cells have organelles that extend from the cell. There are two main types, cilia and flagella.
Cilia can be used for moving the cell. Cilia can also move substances outside the cell. Some cilia act
as sensory devices. Those cilia that move cells are usually present in large numbers. Some small
organisms use them to move around. Most of these belong to a group of called the ciliates. Large
eukaryotes use cilia in large numbers. The cilia of adjacent cells often work together. For example,
cells lining the windpipe use cilia to sweep mucus and dirt away from the lungs. Other cilia are
adapted to sense the environment around the cell. Flagella are longer than cilia. They are usually
present in fewer numbers. They have a whip-like movement that can move a cell. They are mostly
found in protists and simple organisms like sponges.
What are the differences between plant and animal cells?
Plant and Animal Cells
Plants and animals are both composed of eukaryotic cells, but their cellular structures differ in
important ways. Plant cells have cell walls, while animal cells do not. The plant cell wall is made
of cellulose. The cellulose is organized in a matrix comprising a tough outer layer that supports the
plant and allows pressure from liquid to build up inside the cell. The cell wall also provides protection
against mechanical damage. Small holes in the cell wall allow materials to enter and leave the cell.
The cell wall makes plant cells rigid, preventing free movement of the cell within plant tissue. In
contrast, many animal cells can move freely within an organism.
Chloroplasts are found in most plant cells and in some protists, but not in animal cells. Chloroplasts
allow plants and certain protists to make their own food through photosynthesis. Via photosynthesis,
chloroplasts use the energy in sunlight to drive a chemical reaction between carbon dioxide and
water. This chemical reaction produces oxygen and sugars. The sugars serve as food for the plant.
Plant and animal cells differ in the size and function of their vacuoles. Plant cells typically have a
single large vacuole, while some protists and animal cells have smaller, often multiple, vacuoles. In
both plant and animal cells, vacuoles store nutrients and waste. Vacuoles in plant cells have the
additional function of maintaining pressure against the cell wall. This pressure helps keep the plant
upright. The amount of water available to the plant affects the degree to which vacuoles maintain
pressure. A shortage of water causes vacuoles to shrink. This in turn causes the plant to wilt. When
more water becomes available, vacuoles expand and the plant stands upright again.
What is the role of mitochondria in eukaryotic cells?
Structure of Mitochondria
Mitochondria are specialized organelles found in eukaryotic cells. The number of mitochondria in
cells varies. Most cells contain hundreds of mitochondria . They have an outer and an inner
membrane. The smooth outer membrane encloses the organelle and separates it from the
surrounding cytoplasm. The inner membrane has a series of deep folds called cristae. This folding
increases the surface area of the membrane. The large surface area facilitates the chemical
reactions that occur on the inner membrane. Mitochondria are filled with a semifluid matrix that also
contains ribosomes and mitochondrial DNA.
Function of Mitochondria
Mitochondria supply energy to the cell. They do this through a process called cellular respiration. In
cellular respiration, oxygen combines with food molecules to release energy in the form of adenosine
triphosphate (ATP). Cells use ATP to provide energy necessary for key cellular processes. These in
turn drive the activities of tissues, such as muscle contraction and nerve impulses.