B

The cell theory is one of the fundamental principles of biology.
Robert Hooke was an English scientist that was the first one to see a cell under a microscope. In 1655, he explored a cork, which consisted out of the walls of the dead cells.
Anton Van Leeuwenhoek has improved a microscope and was the first to describe bacteria and yeast. He was also the first one to explore the drop of water and capillary blood under a microscope.
In 1838, Matthias Schleiden noticed that the plants are made of cells, while
In 1839, Theodor Schwann concluded that the structural unit of all animals is also the cell.
In 1855, Rudolph Virchow presumed that all cells originate from pre-existing cells.

c. cell

The cell theory states that all the living things are composed of cells, which are their basic functional and structural units, and that all cells originate from the ones that already exist. The cell theory is one of the fundamental principles of biology.

The light microscope uses light waves that pass through the magnifying lenses to the specimen. Since the light waves diffract, we can observe thin specimens that are magnified up to 1000 times. These specimens must be very thin and they are usually transparent, so sometimes chemical colors must bind to the examined structure, so it would be visible.

A microscope is a very important device in biology because it allows us to see the magnified specimens and to understand them better. Robert Hooke was an English scientist that was the first one to see a cell under a microscope, which was a significant discovery in biology.

There are two types of an electron microscope – transmission (TEM) and scanning electron microscopes (SEM). We can observe extremely thin slices of the specimen under a TEM, which gives us a black and white picture. In order to make certain structures visible, colors are added by the computer, which is known as “false coloring”. They give us a two-dimensional image. However, an electron beam of the SEM passes over the surface of the observed specimen, so we get a three-dimensional image, as it is shown in the picture.

Since some of the amoebas can be visible by the human eye, we could see its structure under a light microscope. The light microscope uses light waves that pass through the magnifying lenses to the specimen. Since the light waves diffract, we can observe thin specimens that are magnified up to 1000 times. These specimens must be very thin and they are usually transparent, so sometimes chemical colors must bind to the examined structure, so it would be visible.

Every eukaryotic cell has two major parts – nucleus and the cytoplasm with its organelles. The nucleus contains the genetic material of an organism. However, prokaryotes are simpler organisms than eukaryotes. They don’t have the nucleus, so their DNA is located in the cytoplasm.

Every cell has two major parts – nucleus and the cytoplasm with its organelles.

b. cytoplasm

Proteins are synthesized in a translation process that occurs in the ribosomes. Ribosomes that are attached on the endoplasmatic reticulum makes that part of this organelle granulated. Protein enters this organelle, where its structure may be altered. Ribosomes of the endoplasmatic reticulum make proteins for the cell membrane or the ones that develop functions outside of the cell. Ribosomes that are free in the cytoplasm make proteins that remain inside the cell.

a. ribosomes

Plant cells contain chloroplasts which receive solar energy and changes it into food. These molecules next enter the mitochondria in which they are transformed into chemical energy, that can be easily used in all the cellular processes.

b. chloroplast

Mitochondria convert the energy that is stored in food into chemical energy, that can be easily used in all the cellular processes.

a. mitochondria

Cells are able to move through the environment because of flagella and cilia. Cells can have one, long flagella, and many short cilia on its surface. Centrioles are included in cell division. Lysosomes are organelles that are responsible for digestion.

b. flagella and cilia

The endoplasmic reticulum is an organelle that is included in making proteins. It has two parts – granulated and smooth. Granulated endoplasmic reticulum has ribosomes attached on its surface. A protein that is made by the ribosome enters this organelle, where its structure may be altered. Ribosomes of the endoplasmic reticulum make proteins for the cell membrane or the ones that develop functions outside of the cell. Protein molecules come out of endoplasmic reticulum in a vesicle and go to the Golgi apparatus. They are altered, stored or prepared to be released from the cell in this very organelle.
Smooth endoplasmic reticulum doesn’t have ribosomes on its surface. It contains various enzymes that have a role in the synthesis of fat and carbohydrate molecules.

Vacuoles are larger membrane-enclosed organelles than vesicles. In vacuoles are stored water molecules, proteins, carbohydrates and salts.
Lysosomes are organelles which contain enzymes and are included in the processes of decomposing lipids, carbohydrates, and proteins so their particles could be used by the cell. They also remove the waste from the cell.

Photosynthesis is a process where the light energy is converted to chemical energy. This process occurs in organisms that are known as photoautotrophs, such as most plants, many algae, and cyanobacteria. They all contain a green pigment that is called chlorophyll which absorbs light, lose an electron after which a chain reaction starts. In the final process, a chemical reaction between phosphorus molecule, light, ADP, NADP, and water molecule result in NADPH, ATP, hydrogen ions and molecules of oxygen. The chlorophyll is found in chloroplasts of plants and in walls of cyanobacteria.

Protein molecules are synthesized in endoplasmic reticulum from which they are being transported in a vesicle and to the Golgi apparatus. In this organelle, they are being altered and stored. The function of the Golgi complex is to transport these molecules to their final destination in the cell or out of it, via process of exocytosis. It is also included in lysosomal production and transportation of fat molecules.

The cytoskeleton contains protein fibers that make the form of the eukaryotic cells and they help in their motion. The most significant proteins of the cytoskeleton are microfilaments and microtubules. Microfilaments are made of protein known as actin and their function is to maintain the cells structure but they can also induce ameboid movements of a cell. Microtubules are built from a hollow protein called tubulin. They also maintain the form of a cell. However, cilia and flagella are also made from tubulin and they are responsible for the fast movements of the cell in the liquid environment. Microtubules have an important role in the cell division because they build the mitotic spindle. Centromeres are involved in the cellular division. They are found close to the nucleus, and they are not present in the plant cells.

The cell membrane is composed of the lipid bilayer with protein molecules in it, while carbohydrates are attached to its surface. This structure is also called the mosaic because of the many protein molecules that float in the lipid bilayer. Phospholipids have hydrophobic tails that are pointed to the tails of the other lipid layer, while their hydrophilic heads are directed to water.
The cell membrane separates the cell from the outer environment. It has a function in protection and passing molecules in and out of the cell. The cell membrane is semipermeable, which means that some molecules can move through it, while others can’t. Molecules that can’t cross the cell membrane are usually large or strongly charged.

Every eukaryotic cell has two major parts – nucleus and the cytoplasm with its organelles. The nucleus contains the genetic material of an organism. However, procaryotic cells contain DNA as well as cytoplasm, but they don’t have the nucleus.
The cell membrane separates the cell from the outer environment. It has a function in protection and passing molecules in and out of the cell. This is a characteristic of both prokaryotic and eukaryotic organisms.
Ribosomes are included in the synthesis of protein molecules. Both prokaryotic and eukaryotic organisms have ribosomes. Proteins are altered, stored or prepared to be released from the cell in the Golgi apparatus, which is a characteristic of eukaryotes.
Mitochondria are involved in converting food into chemical energy, that can be easily used in all the cellular processes. They are found in the eukaryotic cells. These processes also occur in the cytoplasm of prokaryotes, but they don’t contain specialized organelles, such as mitochondria.

A homeostasis is the ability of an organism to preserve the ideal conditions in its internal environment. For example, all of our organ systems work in a way to maintain homeostasis. Our cells use oxygen which they metabolize into carbon dioxide. The respiratory system then eliminates carbon dioxide and provides more oxygen for new metabolic processes. Thus, the balance is maintained.

c. homeostasis

In passive transport, molecules doesn’t need ATP to pass through the cell membrane. In diffusion, molecules move from the place of their higher to the place of their lower concentration.

c. diffusion

In passive transport, molecules doesn’t need ATP to pass through the cell membrane. Some molecules, such as glucose, need protein channels to easily pass the membrane. This process is called facilitated diffusion.

b. facilitated diffusion

the cells contain more salt than the freshwater.

active transport uses atp

We can differ passive and active transport of molecules through the cell membrane. In active transport, molecules need ATP to pass through the cell membrane, from the place of lower to the place of their higher concentration. Two main types of active transport are molecular and bulk transport. In molecular transport, protein pumps use the energy to transfer small molecules over the cell membrane. In bulk transport, big molecules are transferred by vesicles or vacuoles in a process of endocytosis or exocytosis. In endocytosis, large molecules that need to enter the cell causes the cell membrane to infold and surrounds these molecules which cause the formation of vacuola or vesicle, depending on its size. In exocytosis, a vesicle merges with the cell membrane which leads to discharging its material out of the cell.

Homeostasis is the ability of an organism to preserve the ideal conditions in its internal environment.
In order to maintain homeostasis, unicellular organisms (prokaryotes and eukaryotes) develop, they react to environmental conditions, convert energy and breed.
Multicellular organisms (like animals and plants) are made of cells, where each one of them is specialized for certain task. They are also able to communicate with each other. Therefore, cell specialization and communication enables the organism to preserve homeostasis.

d multicellular eukaryotes

In order to maintain homeostasis, unicellular organisms (prokaryotes and eukaryotes) develop, they react to environmental conditions, convert energy and reproduce.
Unicellular eukaryotic organisms have the nucleus, while prokaryotes don’t.

c reproduce

Homeostasis is the ability of an organism to preserve the ideal conditions in its internal environment.
In order to maintain homeostasis, unicellular organisms develop, they react to environmental conditions, convert energy and breed.
Multicellular organisms are much more complex than unicellular organisms. They are made of cells, where each one of them is specialized for completing a certain task. They are also able to communicate with each other. In this way, every specialized cell contributes to the overall homeostasis of the organism.
For example, after a meal, the glucose blood level in our organism rises, which stimulates the cells of the endocrine pancreas to secrete insulin, which facilitates the transport of glucose into the cells. That brings glucose level in the blood back to its normal values.

Multicellular organisms are made of cells, where each one of them is specialized for certain task. They are also able to communicate with each other. Therefore, cell specialization and communication enables the organism to preserve homeostasis.
Cells communicate through the signal molecules that bind with the receptors on their surfaces or inside them, in the cytoplasm.

The cell is often compared to a factory in order to understand better its function and structure. The nucleus is like the CEO, which controls processes in the cell by determining which protein molecules will be produced. The cytoplasm is like a floor, where all the employees work. The cell membrane regulates what enters and what molecules leave the cell which is like the shipping/receiving department in the factory model. The cytoskeleton is like walls, floors, and ceilings in the factory because it maintains the shape of the cell. Ribosomes, endoplasmatic reticulum, and the Golgi apparatus are included in the process of making proteins. They act like employes in the assembly line and the ones in the sorting department. We can compare lysosomes with the maintenance workers. Mitochondria and chloroplasts are power plants because they produce energy for the cellular processes.

The cell theory states that all the living things are composed of cells, which are their basic functional and structural units, and that all cells originate from the ones that already exist.
Because all the cells of the multicellular organism originate from the zygote, they all have the same DNA molecule in their nuclei. However, in the various cells, different genes are expressed which leads to the cell specialization.

The osmotic pressure depends on the number of dissolved particles in the fluid. If we have a partially permeable membrane and a concentrated starch solution on its left side, while a diluted starch solution is on the right, the water will flow from the place of the lower osmotic pressure to a place of higher, or, from the right to the left, until the equalization of pressures.

The cell wall maintains the shape of the plant cells but it has pores that allow gases, water molecules, and some other substances to pass through. It is composed of cellulose fibers. Therefore, in the plant cell model, a cell wall could be represented by a porous sheet of paper.

Multicellular organisms are made of cells, where each one of them is specialized for certain task. They are also able to communicate with each other.
All cells of the multicellular organism need to intercommunicate in order to maintain growth, improve their function and to adapt in the environment.
Cells communicate through the signal molecules that bind with the receptors on their surfaces or inside them, in the cytoplasm.
Therefore, cell specialization and communication enables the organism to preserve homeostasis.

If the pacemaker is needed, that tells us that the heart muscle cells or the cells of the cardiac conduction system have difficulties in receiving or sending signals.

For this experiment, we’ll need three beakers, water, and food coloring. The bakers contain water at the various temperatures. The first one is filled with water at room temperature, the second beaker contains ice water, and the third beaker contains hot water. If we drop the equal amount of food coloring into each beaker, we would see that the hot water diffuse the food coloring faster, while the ice water diffuse the food coloring slower than the water at the room temperature. The cause lies in the speed of the molecules, that is higher as the temperature rises.

In the world of science, there is a certain path from the observation of natural processes to posting a theory. The first step is an observation which leads scientists to ask questions they need answers on. The inference is a logical explanation from the known facts, while a hypothesis is a potential explanation of an observed process. In the next step, a scientist must design a controlled experiment, collect and analyze data, based on which the hypothesis will be proven or rejected.
The cell theory states that all the living things are composed of cells, which are their basic functional and structural units, and that all cells originate from the ones that already exist.
Several scientists had an impact on the cell theory, but Matthias Schleiden, Theodor Schwann, and Rudolph Virchow were the ones who formulated it. The cell theory is one of the fundamental principles of biology.
Robert Hooke was an English scientist that was the first one to see a cell under a microscope. In 1655, he explored a cork, which consisted out of the walls of the dead cells.
Anton Van Leeuwenhoek has improved a microscope and was the first to describe bacteria and yeast. He was also the first one to explore the drop of water and capillary blood under a microscope.
In 1838, Matthias Schleiden noticed that the plants are made of cells, while
In 1839, Theodor Schwann concluded that the structural unit of all animals is also the cell.
In 1855, Rudolph Virchow presumed that all cells originate from pre-existing cells.

The nucleus is included in the regulation of the synthesis of proteins, cell division, and cell metabolism. This organelle is compared to the control center of a factory because it contains almost all DNA of the cell (a small amount is in mitochondria). However, the limitation of this model lies in the gene expression. The cell membrane receives external signals which induce transcription of certain proteins in the nucleus. Therefore, the nucleus contains the genome of an organism, but it is not in charge of choosing what genes will be expressed at a certain time.

The plant cells have cell walls which makes them firm and enables their upright growth. Since the plants are stationary organisms, their cell wall must protect them from injuries. If this structure is present in the animal cells, they would be unable to move. Animals have other structures, such as skeleton, which provides protection to their tissues and organs.

Prokaryotes are Escherichia coli and Streptococcus pneumoniae. Eukaryotic cells are human erythrocyte, human ovum, and Saccharomyces cerevisiae.

The width of the human hair is 17 micrometers, the red blood cell is 6 to 8 micrometers and the E. coli is 0.5 to 0.8 micrometers. Therefore we can conclude that 2,1 to 2.8 human erythrocytes or 21 to 34 E. coli could fit across the width of the human hair.

Usually, prokaryotes are smaller than eukaryotic organisms.
If we observe its size, we can conclude that Chlamydomonas reinhardtii is probably a eukaryote.

The size of the glucose molecule is about 190, while carbon dioxide is about 45 daltons.
190 : 45 = 100 : x
x = 23,7 %
This means that the glucose molecule is about 75% larger (100-23,7=76,3) than the molecule of carbon dioxide.

Based on the size of the molecule, we can conclude that carbon dioxide passes the cell membrane faster than glucose because it is about four times smaller.

c. carbon dioxide crosses the cell membrane faster than glucose

The cell theory states that all the living things are composed of cells, which are their basic functional and structural units, and that all cells originate from the ones that already exist.
Cells are the smallest structural and functional units of the human organism. Our whole body develops from one cell, the zygote, which origins from two haploid cells, one from mother (ovum) and the other from a father (sperm). Since our organism is multicellular, all cells with the same structure and function compose a tissue. There are four types of tissues in our body – muscular, connective, nerve and epithelial. Several types of tissue connect into an organ. More organs make an organ system.

The genome contains information about the synthesis of proteins. After the process of transcription, proteins are made in a translation process that occurs in the ribosomes. Eukaryotes have endoplasmic reticulum and the Golgi apparatus. Ribosomes that are attached on the endoplasmic reticulum makes that part of this organelle granulated. Protein enters this organelle, where its structure may be altered. Ribosomes of the endoplasmic reticulum make proteins for the cell membrane or the ones that develop functions outside of the cell. Ribosomes that are free in the cytoplasm make proteins that remain inside the cell. Protein molecules come out of endoplasmic reticulum in a vesicle and go to the Golgi apparatus. They are altered, stored or prepared to be released from the cell in this very organelle.

If we look at figure 8-16, we see three different diagrams of a cell. One represents procaryotic, the other eukaryotic animal, while the last one is an example for the eukaryotic plant cell. These pictures perfectly illustrate the differences in the structure of these types of cell. As it is explained in the text, the procaryotic cell doesn’t have a nucleus, however, it contains cytoplasm with ribosomes, vacuoles, and vesicles, as well as some structures included in maintaining its shape and providing energy. It also has the cell membrane and the cell wall. Every eukaryotic cell has two major parts – nucleus and the cytoplasm with its organelles. The nucleus contains the genetic material of an organism. While the cytoplasm is the part of a cell outside of nucleus which contains:
1. organelles included in making proteins – ribosomes, Golgi apparatus, and endoplasmic reticulum
2. organelles included in storage, cleaning, and support – lysosomes (rare in plant cells), the cytoskeleton (only animal cells contain centromeres), vacuoles, and vesicles
3. organelles that receive and unload the energy – chloroplast (only found in plants), and mitochondria. The animal cells have a cell membrane, while the plant cells also have a cell wall.