Download Functions of a Cell

Functions of a Cell (7.12EF)
Student Expectation
The student is expected to compare the functions of a cell to the functions of organisms
such as waste removal AND recognize that according to cell theory all organisms are
composed of cells and cells carry on similar functions such as extracting energy from
food to sustain life.
Key Concepts





Key Concept 1: Each organelle has a different and specific job that the others
rely on for the total survival of the cell, like the waste removal role of the
lysosome.
Key Concept 2: The function of each organelle’s essential task can be compared
with the functions of organisms to obtain energy, such as the “power factories” of
mitochondria and the “storage facilities” of endoplasmic reticulum.
Key Concept 3: Cells make up all living organisms, unicellular or multicellular,
and have similarities in structure cells and all cells need genetic and
environmental information in order to function. The cell theory states that new
cells come from old survive.
Key Concept 4: Cells use a series of chemical reactions to break down nutrients
in food to create energy and produce waste through a process called metabolism.
Key Concept 5: Cellular respiration is the process of breaking down
carbohydrates, fats and proteins to release energy that can be delivered to each
cell of the organism. Plant cells use photosynthesis to convert sunlight into
oxygen and energy for consumption by other organisms.
Teacher Background
Objective(s):
7.12E Compare the functions of a cell to the functions of organisms such as waste
removal.
7.12F Recognize that according to cell theory all organisms are composed of cells and
cells carry on similar functions such as extracting energy from food to sustain life.
Foundation:
Cells are the structural and functional units common to all living organisms. A cell is the
smallest unit of life that is classified as a living thing. Some organisms are unicellular,
meaning they consist of only a single cell. Most bacteria are unicellular. Other organisms,
including humans, are multicellular, consisting of many cells. For example, humans have
about 100 trillion cells.
There are two distinct types of cells: prokaryotic cells (e.g. bacterial cells) and eukaryotic
cells (e.g. plant or animal cells). The main difference between the two is a well-defined
nucleus surrounded by a membranous nuclear envelope that is present in only eukaryotic
cells. Both types of cells share many common features. The genetic information is stored
in genes. Proteins serve as the main structural material. Ribosomes are used to synthesize
proteins. And a cell membrane controls what substances enter and leave the cell. The
primary difference between the two types is that prokaryotes lack a true nucleus.
Multicellular organisms, such as plants and animals, have various levels of organization
within them. Although individual cells can perform specific functions, they become
dependent on one another and can work together for the good of the entire organism. The
first level, cells, is the simplest level as cells are the basic structural and functional units
in living things. Examples include blood cells, bone cells, root cells, etc. The next level,
tissues, are made up of cells that are similar in structure and function and which work
together to perform a specific activity. Humans, for example, have four basic tissues:
connective, epithelial, muscle, and nerve. Organs are made up of tissues that work
together to perform a specific activity. Examples of this level are the heart, brain, skin,
etc. The fourth level is the organ systems. These are groups of two or more organs that
work together to perform a specific function for the organism. Examples in the human
body include the circulatory, nervous, skeletal, muscular, integumentary, endocrine,
digestive, immune, reproductive, excretory, and respiratory systems. The final level of
organization is the organism. Organisms are entire living things that carry out all basic
life processes. Organisms take in materials, release energy from food, release wastes,
grow, respond to the environment, and reproduce. Although an organism can be made up
of only one cell such as bacteria, most organisms, including plants and animals, are
usually made up of organ systems performing these functions. The levels of organization
from smallest to largest are:
cells → tissues → organs → organ systems → organisms
The cell membrane is an important structure present in all cells. It has many functions
including regulating the entry and exit of molecules in and out of the cell, maintaining the
boundaries of the cell, supporting its contents, and maintaining proper cell to cell contact.
The structure of the cell membrane includes a phospholipid bilayer. The hydrophilic parts
are on the outside and the hydrophobic parts point inwards towards each other. This
prevents the entry of polar solutes and is one of the main factors responsible for
regulating what enters and exits the cell. Integral membrane proteins present in the cell
membrane structure have many important functions. They serve as receptors for the cell
and can act as carriers for active transport of substances in and out of the cell.
The cell wall is a structure that is present only in plant cells which surrounds the cell
membrane. This is a special characteristic that helps distinguish plant from animal cells.
The cell wall is composed of polysaccharides including cellulose, lignin, protein, certain
lipids, and water for example. The cell wall has many functions for a plant cell. It
determines the shape of the cell, helping monitor the rate and direction of growth of the
cell. Since it is rigid in nature, the cell wall provides strength and support to plant cells.
Cell walls maintain turgor pressure, giving the plant rigidity. It is semi-permeable,
allowing exchange of substances in and out of the cell. The cell wall also provides
protection for the cell, and is the first line of defense for the cell from pathogens or
harmful microorganisms.
The nucleus is a highly specialized spherical structure that is the control center of a
eukaryotic cell. It is one of the largest structures inside a cell, accounting for about 10%
of the cell’s volume. The structure of the nucleus consists of a surrounding nuclear
membrane, a nucleolus, nucleoplasm, and chromosomes (chromatin). The nucleus
controls the hereditary characteristics of an organism and is responsible for protein
synthesis, cell division, growth, and differentiation. It is the site for transcription in which
mRNA are produced for protein synthesis. Proteins and RNA are stored in the nucleolus.
Ribosomes are produced in the nucleolus as well. As the control center of the cell, the
nucleus stores all the chromosomal DNA of an organism.
The region between the cell membrane and the nucleus of a cell is the cytoplasm. It is a
gel-like substance which is part of the cell and holds important organelles. The cytoplasm
is the site for many biochemical reactions necessary for maintaining life. It is the place
where cell expansion and growth take place. Cell reproduction, protein synthesis,
glycolysis, and cytokinesis are some of the vital functions that are carried out in the
cytoplasm.
The cell has many organelles present in the cytoplasm that have specific vital functions.
The mitochondria are rod-shaped organelles made up of a double layered membrane and
an inner part which is made up of folds called cristae. The primary function of
mitochondria is the production of energy in the form of ATP and the regulation of
cellular metabolism. The mitochondria also help perform other functions such as
monitoring of cell differentiation and growth, managing apoptosis, synthesis of steroids
and formation of hormones, and building, breakdown, and recycling of products that are
needed for the cell to function normally.
Chloroplasts are organelles found only in plant cells that carry out the process of
photosynthesis in which light energy is converted into chemical energy. Chloroplasts
contain chlorophyll, giving plants a green color. They are present in each and every part
of the plant including stems and even fruits. They also play an important role in storage
of energy and synthesis of metabolic substances.
Vacuoles are large, round, water-filled sacs in the cytoplasm that are the storage areas of
the cell. Plant cells usually have one large vacuole taking up a large amount of space. At
times, it can occupy more than 90% of the plant cell space. The structure of the vacuole
includes a membrane-structure called a tonoplast and a central, hollow region containing
a sap-like fluid. This substance contains various secretory and excretory compounds and
enzymes. The tonoplast helps separate the contents of the vacuole from those floating in
the cytoplasm. It does not allow harmful substances present in the vacuole from entering
and harming the rest of the cell. It is selectively permeable and helps maintain the pH and
ionic concentration of the cell by regulating what travels in and out of the vacuole.
Vacuoles store food and other materials needed by the cell. Most of the water in plant
cells is stored in vacuoles.
A single cell is a system, made up of many individual parts that work together, much the
same way that an entire organism is a system made up of many structures with specific
functions. A cell’s organelles and other structures work to carry out functions necessary
to sustain life. A cell requires various substances and energy for growth, as does any
living organism.
The cell uses many different chemical reactions to survive. These processes create a large
amount of products that the cell needs to remove. This waste includes byproducts of
reactions, toxic substances, and many of the parts of the cell that break down over time
and need to be eliminated. Lysosomes are organelles that help break down part of the cell
that are worn out and help get rid of waste products that are created by different parts of
the cell. They contain a number of different enzymes that break down waste, similar to
enzymes in the digestive systems of organisms. The cell has specialized pumps that use
energy to move the smaller amounts of waste product out of the cell. Exocytosis through
the plasma membrane allows the cell to get rid of large quantities of waste products all at
once. The process in which a cell removes waste is similar to the processes of the
excretory systems of many organisms.
The cell theory is an explanation of the relationship between cells and living organisms.
It states that all living organisms are composed of cells, cells are the basic unit of
structure and function in living things, and that cells arise from pre-existing cells. This
theory holds true for all living things, unicellular or multicellular. Modern variations of
the theory include the ideas that energy flow occurs within cells, hereditary information
(DNA) is passed on from cell to cell, and all cells have the same basic chemical
composition.
To carry out their day to day functions, cells require energy. The ultimate source of this
energy is the sun. Some organisms can trap energy directly from the sun, storing it away
in the bonds of organic molecules such as glucose through photosynthesis, and organisms
which are capable of photosynthesis are called autotrophs. Organisms which are not
capable of photosynthesis are called heterotrophs, and must acquire their energycontaining organic molecules through their diet instead. To convert the energy stored in
organic molecules into a form that is usable, both autotrophs and heterotrophs must take
large molecules and break them down, and then recapture the energy released in the
process and store it the bonds of smaller, easier to use molecules. Cellular respiration is
one of the key ways a cell gains useful energy to fuel cellular processes by producing
ATP. It occurs in both prokaryotic and eukaryotic cells and has three main stages. In
glycolysis, glucose is split into two molecules of a three carbon sugar. Carbohydrates,
fats, and proteins can all be used for fuel as well. Two molecules of ATP, two molecules
of pyruvic acid, and two high energy electron carrying molecules of NADH are
produced. The Krebs cycle produces several compounds capable of storing high energy
electrons along with two ATP molecules. The electron transport chain is a series of
electron carriers in the membrane of mitochondria. As the high energy electrons are
passed to oxygen, a gradient is formed and ATP is produced. Prokaryotic cells can yield a
maximum of 38 ATP molecules while eukaryotic cells yield a maximum of 36 (the
NADH molecules produced in glycolysis pass through the mitochondria membrane,
which “costs” 2 ATP molecules).