Cell: The Basic Unit of Life
Objective
At the end of the lesson, you should be able to explain how cells carry out functions required for life.
Have you ever wondered what the inside of a cell looks like and how its parts perform
functions required for life?
Learn about it!
The Basic Unit of Life
All organisms are made up of cells. The cell is the basic structural unit found in every living
organism that performs several functions throughout life. The zoo animals such as elephants and
snakes, the plants in the garden, and even yourself, are all living things composed of cells. These
cells can only be seen through the use of a microscope.
Eukaryotic and Prokaryotic Cells
There are two types of cells based on the presence or absence of a nucleus. Cells can be eukaryotic
or prokaryotic.
Eukaryotic cells have a nucleus which contains the genetic material or DNA. They also have
several membrane-bound organelles such as ribosomes and mitochondria. They include animal and
plant cells.
Prokaryotic cells differ in eukaryotic cells because their DNA is found in a region called
the nucleoid rather than a nucleus. They also lack most membrane-bound organelles present in
eukaryotes. However, prokaryotes have cytoplasm where organelles are suspended, flagella that
aids in motility, cell wall made of peptidogycan, cell membrane that serves as a selective barrier,
and ribosomes that make proteins.
Eukaryotic Cells
There are two types of eukaryotic cells: animal and plant cells.
Animal Cells
Aside from the nucleus, the typical animal cell also have other membrane-bound organelles such as
mitochondria, lysosomes, Golgi apparatus, endoplasmic reticulum, nucleus, microtubules, plasma
membrane, cytoplasm, and ribosomes.
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The mitochondria is the powerhouse of the cell because this is where most energy (ATP) is
produced.
The lysosomes break down large molecules into smaller pieces and digest worn out
organelles.
The Golgi apparatus sorts and packages proteins and lipids produced by the smooth and
rough endoplasmic reticulum.
o The cis face receives the materials for processing in transport vesicles.
o The trans face is the discharging end where molecules are released through the
secretory vesicles.
The endoplasmic reticulum has two types: rough ER and smooth ER.
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Rough endoplasmic reticulum is bounded with ribosomes. It is where most protein
synthesis occurs.
o Smooth endoplasmic reticulum has no ribosomes attached. Its function is mainly
for lipid synthesis.
The nucleus is the largest organelle that serves as the control center of the cell. It contains
the hereditary material known as DNA.
The microtubules are components of the cytoskeleton and important in a number of cellular
processes.
The plasma membrane is a semi-permeable membrane that encloses the cell separating its
contents from the surroundings.
The cytoplasm is a semifluid matrix where organelles are suspended.
The ribosomes consist of the large and small subunits. Their main role is to synthesize
proteins needed by the cell.
Plant Cells
Plant and animal cells share the same structures except for the chloroplast, cell wall, and
amyloplast, which are only found in plants. A large vacuole is found in both animal and plant cells
but is a typical and distinct structure in a plant cell.
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The cell wall provides support and protection for the cell. Special openings
called plasmodesmata are used to communicate and transport materials between plant
cells.
The chloroplasts convert light energy to sugars through photosynthesis.
The vacuole is responsible for storing food, water, and metabolic and toxic wastes.
The amyloplast is responsible for the production and storage of starch and the conversion
of starch back to sugar as needed by the plant for energy.
How do cells carry out functions required for life?
Cells have different types which are specialized to perform specific functions. For example, cardiac
muscle cells have numerous mitochondrion because they need a lot of energy. Nerve cells are long
for them to be able to transmit signals from the brain to the rest of the body. Cell membrane of cells
in the intestine is extended to have more surface area to absorb food. Mammalian red blood cells
don’t have nucleus to make more room for hemoglobin, a protein that carries respiratory gases.
Explore!
Cancer cells are basically cells that have gone wrong. Typically, these cells no longer respond to
signals, their growth is uncontrollable, and they even evade apoptosis (programmed cell death).
Since the cells keep on dividing, they eventually form a lump (tumor) that grows in size. How do
normal cells become cancer cells?
What do you think?
Human red blood cells lose their nucleus when they mature. What properties of mature red blood
cells can you infer from this information?
Key Points
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The cell is the basic unit of life.
Prokaryotic cells do not have nucleus. Their genetic material is found in a region called the
nucleoid.
Eukaryotic cells have nucleus which contains the genetic material. They can be classified
as animal or plant cells.
Animal cells have a nucleus and other membrane-bound organelles such as mitochondria,
lysosomes, Golgi apparatus, endoplasmic reticulum, microtubules, plasma membrane,
cytoplasm, and ribosomes.
Plant cells have the same structures as animal cells except for chloroplast, cell wall, and
amyloplast which are only found in plants.
Cells have different types which are specialized to perform specific functions.
Photosynthesis
Objective
At the end of this lesson, you should be able to explain how photosynthetic organisms use light
energy to combine carbon dioxide and water to form energy-rich compounds.
Take a look at this picture.
What can you say about this picture?
Learn about it!
The sun is the primary source of energy needed by all organisms to survive. However, this energy
cannot be consumed by most organisms unless converted into other forms of energy. Think of it this
way. When you go to another country, you need to exchange your money for their currency so that
you can use it. Just like in this case. Only plants and photosynthetic organisms such as algae
convert light energy into forms usable by other organisms.
Photosynthesis is the process where photosynthetic organisms convert light energy to
form sugar. In plants, photosynthesis specifically occurs in the leaves.
The thylakoid membrane contains chlorophyll which is responsible for capturing light energy.
Photosynthesis has two phases: light-dependent reaction and light-independent reaction.
Light-Dependent Reaction
The reaction occurs in the thylakoids. It converts light energy to ATP and NADPH. It can be
summarized into four steps:
1. Light absorption and splitting of water. Light strikes the chlorophyll and an enzyme splits
water (H2O) into protons (H+ions), electrons, and oxygen (O2).
2. Production of ATP. ATP synthase accepts energy from H+ions to produce adenosine
triphosphate (ATP).
3. Hydrogen pump powered by electron acceptors. Electrons pass through proton pump
and the energy from the electrons pumps H+ions back to the thylakoids.
4. Production of NADPH by re-energizing electrons. Light strikes again, the chlorophyll and
electrons get re-energized. The last electron acceptor moves electrons to nicotinamide
adenine dinucleotide phosphate (NADP) and H+ions to produce NADPH.
Light-Independent Reaction
This is also known as the Calvin cycle. It takes place in the stroma and uses ATP and NADPH from
the light-dependent reaction. It reduces CO2 to form sugar. The reaction is summarized below.
1. Fixation of CO2. Ribulose bisphosphate carboxylase (Rubisco) catalyzes CO2 to ribulose 1,
5-bisphosphate (RuBP). A carbon atom sticks to RuBP and results to unstable 6-C molecule
and splits into 3-phosphoglycerate.
2. Reduction of 3-phosphoglycerate. The energy from ATP and hydrogens from NADPH are
added to the 3-C molecule to form 3-phosphoglyceraldehyde (G3P).
3. Regeneration of RuBP from G3P. Most of the G3P are used to generate back the RuBP
and prepares again for CO2fixation.
Photosynthesis is composed of two reactions. Below is the summary:
Explore!
A group of boys placed a bunch of leaves in a sealed plastic bag and left it under the sun. After two
days, they saw drops of water inside the plastic bag. What does this infer?
Try it!
Get a plant and cover some of the leaves with aluminum foil. Place it under the sun for three days.
Examine the leaves and compare it to the other leaves after three days. What do you observe?
What do you think?
How does deforestation affect photosynthesis?
Key Points
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Photosynthesis is the process where plants and other photosynthetic organisms convert
light energy into chemical energy to form sugar.
Photosynthesis has two phases: light-dependent reaction and Calvin Cycle.
The light-dependent reaction converts energy to ATP and NADPH. It has four steps: (1)
Light absorption and splitting of water, (2) production of ATP, (3) hydrogen pump powered by
the movement of electron acceptors and, (4) production of NADPH by re-energizing
electrons.
Calvin cycle uses the ATP and NADPH from the previous reaction. It has three steps: (1)
Fixation of CO2, (2) reduction of 3-phosphoglycerate and, (3) regeneration of RuBP from
G3P.