Unit 4: Cell Structure, Function and Movement

Preview Key Concepts: Cells are the basic unit of life. Eukaryotic cells share many similarities. The cell membrane is a barrier that separates a cell from the external environment. Materials move across membranes because of concentration differences. Cells use energy to transport materials that cannot diffuse across a membrane.

Review Academic Vocabulary:1. Polymer of amino acids. 2. Biological catalysts. 3. Examples include fats and cholesterol. 4. What an enzyme works on.

Preview Biology Vocabulary: Try to guess the meaning of each boldfaced word from its context.Phrase My Guess

1. The cell membrane, like a tea bag, is characterized by selective permeability

2. The smell of baking bread spreads throughout your home by diffusion.

3. Cells in your immune system gobble up invading bacteria and recycle old cell parts through phagocytosis.

Cell TheoryKey Concept: Cells are the basic unit of life.Early Studies led to the development of the cell theory – Cells are the very smallest parts of life. All living things are made of cells. However, most cells cannot be seen without a microscope. How did scientists find out about cells when they couldn’t see them?Discovery of Cells – Over many years, may scientists observed and studied cells under the microscope. As early scientists improved both microscopes and lenses, they could learn more and more about cells. Some of these finding are listed in the table below.Scientist FindingHooke (1665) Identified and named cells.Leeuwenhoek (1674) Observed living cells, could see greater detail due to better lenses.Schleiden (1838) Noted that plants are made of cells.Schwann (1839) Concluded that all living things are made of cells.Virchow (1855) Proposed that all cells come from other cells.Cell Theory – The discoveries of these early scientists came together into the Cell Theory. Today’s scientists agree with this cell theory. It says three things:

All living things are made of cells. ALL cells come from other living cells. The cell is the most basic unit of life. There is nothing living that is smaller than a cell.

Which part of the cell theory states where cells come from?

Prokaryotic cells lack a nucleus and most internal structures of eukaryotic cells – Cells come in different shapes and carry out different jobs. However, they all share some features. Cells are very small. They are surrounded by a membrane that controls what enters and leaves the cell. They have cytoplasm, a jelly-like material that contains the building blocks needed for life. And they are made of similar molecules. Once of these molecules is DNA, the genetic information.

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Vocabulary Check:1. Name two types of cells.2. In both types of cells, the jelly-like substance is .3. Parts that carry out specific jobs within a cell are .4. List the three parts of the cell theory.

Eukaryotic Cells and Cell Organelles.Key Concept: Eukaryotic cells share many similarities.Cells have an internal structure –

Several organelles are involved in making and processing proteins – Proteins are a very important type of molecule that are used in all life functions. Basic life functions include reproduction, repair and growth of inured cells or body parts. Life functions also include the regulation of circulation and digestive systems. You need proteins to digest the foods you eat or to move your muscles when you ride a bike. Proteins are at work when your heart beats or your eye blinks. Some hormones such as insulin, which controls your blood sugar levels, are also proteins.Proteins are very important, and many organelles work together to make them. These organelles include the nucleus, endoplasmic reticulum, ribosomes, the Golgi apparatus, and vesicles. Ribosomes are found in BOTH eukaryotic and prokaryotic cells. However, the other organelles – those surrounded by a membrane – are found ONLY in eukaryotic cells.

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There are two main types of cells, prokaryotic cells and eukaryotic cells. Prokaryotic Cells are extremely small. Their DNA floats in the cytoplasm, and they have no distinct internal parts. Prokaryotes, such as bacteria, are made of only one cell. Eukaryotic Cells have a nucleus, which is a membrane that separate DNA from the cytoplasm. The nucleus is a type of Organelle, a small part that carries out a specific job in a cell. Eukaryotic cells have many types of organelles. Like the nucleus, most organelles are covered by a membrane. Eukaryotes, such as plants and animals, are made of one cell or many cells.

Your skeleton is made of bones that help keep all your body parts in place. Eukaryotic cells have a skeleton too. It is called the cytoskeleton. The cytoskeleton is a network of protein fibers. They look like a lot of tiny strings that crisscross a cell. The cytoskeleton is the framework of a cell. It gives a cell shape, support, and strength. The cytoskeleton can change as a cell needs to change.What might a cell look like if it had no cytoskeleton?

Nucleus – The Nucleus stores and protects the DNA of the cell. DNA contains the genes that are the instructions for making proteins. The Nucleus is enclosed by the nuclear envelope. This is similar in structure and function to the cell membrane. The nucleus also contains the nucleolus, which produces ribosomes.

Endoplasmic Reticulum – The Endoplasmic Reticulum is a network of thin, folded membranes that helps in the production of proteins and other molecules. The membranes are like a maze; they fold back on themselves and have little spaces inside the folds. There are two type of ER, Smooth and Rough. The Rough ER looks bumpy because it has ribosomes attached to it, the Smooth ER does NOT.Ribosomes – are the tiny organelles that link amino acids together to form proteins. They are found on the surface of the ER and floating freely in the cytoplasm.

Other Organelles have various functions – Some organelles do jobs other than making proteins.

Plant Cells have cell walls and chloroplasts – Plant cells have two parts that animal cells do NOT have. Plant cells have cell walls the chloroplasts.

Vocabulary Check1. Little sacs that carry molecules.2. Bumps that link amino acids.3. Green organelles for photosynthesis.4. The skeleton of the cell.5. Bean-shaped energy supplier.6. Holder and protector of DNA.7. What do the nucleus, ER, ribosomes, Golgi apparatus, and vesicles work together to do?8. How are plant cells different from animal cells?

Cell Membrane3

Golgi Apparatus – is a stack of layers of membranes. In the Golgi apparatus, proteins are changed, put into packages and carried to other places in the cell.Vesicles – are small sacs. They carry different molecules to where they are needed. Vesicles are generally short-lived and are formed and recycled as needed.

Mitochondria – bean-shaped organelles that produce chemical energy that is usable b a cell. They have two membranes. The inner membrane has lots of folds that form compartments. Mitochondria also have their own ribosomes and DNA. At one time, mitochondria may have been independent prokaryotes that were taken in by larger cells (Endosymbiotic Theory).

Vacuoles – sacs of fluids that store materials in a cell. These materials include water, food molecules, ions and enzymes. Many plant cells have a large central vacuole. When filled with fluid, the central vacuole exerts pressure that can help support the plant.Lysosomes – are organelles that contain enzymes. They protect a cell by attacking incoming bacteria or viruses. They also break down old cell

Centrioles – shaped like cylinders. They are made of tiny tubes in a circle. They move when animal cells divide in two. Centrioles help form cilia and flagella, structures that help cells to move or to move liquids past a cell. Centrioles are surrounded by the centrosome. The centrosome is a small region of cytoplasm that organizes proteins into fibers that help the cell divide.

Cell Walls – a strong, rigid layer that protects, supports, and shapes the plant cell. It surrounds the cell membrane. Some cell walls are very thick, such as those in a tree. When joined with others, they can support a very large plant. Some cell Walls are thinner, like those in celery.Chloroplasts – carry out photosynthesis that stores energy from sunlight as chemical energy for the plant. The chloro- part of the name chloroplast means “green”. Chloroplasts contain chlorophyll, the green pigment that helps capture energy from sunlight. Like mitochondria, chloroplasts have two membranes and their own ribosomes and DNA.

Key Concept: The cell membrane is a barrier that separates a cell from the external environment.Cell Membranes are composed of two phospholipid layers.The Cell Membrane is the package that a cell comes in. It is a thick layer that separates the inside of the cell from the outside of the cell. It controls what comes into and goes out of the cell. The cell membrane is made up of a double layer of phospholipids. A phospholipid is made of three parts: a phosphate group, a glycerol, and two fatty acid chains.

Selective Permeability – The cell membrane has selective permeability. This means that it allows some materials, but NOT all, to cross it, or permeate it. In other words, the cell membrane is semipermeable. You might own a semipermeable jacket. The jacket is waterproof; it does not allow water in from the outside. But water vapor form your sweat can exit through the fabric. Just like the jacket, a semipermeable cell membrane lets only some materials through.

Chemical signals are transmitted across the cell membrane.

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The phosphate group and glycerol make up the “head”. The fatty acids make up the “tail”. In the cell membrane, phospholipids are arranged tail-to-tail so that the heads face outward. The phosphate group has an electrical charge, so the heads on the outside are polar. They interact with water. All through the two layers of phosphoplipids are other molecules. Some are proteins; some are cholesterol; some are carbohydrates.The heads are hydrophilic (water loving), the tails are hydrophobic (water fearing).

Fluid Mosaic Model – Scientists have developed the fluid mosaic model to describe the cell membrane. The membrane is fluid because the phospholipids in each layer can move and slide. It acts like a film of oil on the surface of water. The membrane is like a mosaic because of all the different molecules embedded among the phospholipids. Together, they look like a mosaic.

In a cell, small nonpolar molecules can usually cross the membrane by themselves. Small polar molecules can be carried by proteins. Large molecules can be moved in vesicles.Name another semipermeable object. What can pass through it? What can not?

Cells are exposed to lots of signals. These signals may be molecules in a cell’s environment. They may be molecules secreted by other cells. Or they may be molecules on the surface of another cell. How can a cell make sense of all these signals? Cells have receptors. A receptor is a protein that detects a signal and acts because of it. Only certain molecules will bind to a receptor. A cell responds to a signal only if it has a receptor for that signal.When a receptor binds to a signal, it changes shape. As a result, the receptor interacts with other molecules in new ways. The binding between a receptor and its signal can set off a chain of events that will affect certain genes. The signal may cause the genes to make proteins, or it may cause the genes to stop making proteins.Cells have receptors both inside the cell and in the cell membrane. Recall that some molecules can easily cross the cell membrane, but others cannot. Molecules that can cross the membrane bind to receptors in the membrane. When the signal binds to a receptor, the receptor changes its shape. Other molecules inside the cell respond to the changing shape of the receptor. The signal molecule itself never enters the cell.

How does a receptor in the membrane transmit a message to the cell?

Vocabulary Check1. Which material has selective permeability, a sponge or a shovel?2. Which contains the other, cell membrane, or phospholipid?3. Which receives signals, a fluid mosaic model, or a receptor?4. What does the cell membrane do for the cell?5. Describe how the phospholipids are arranged in the cell membrane?6. Do cells respond to every signal? Explain.7. Explain how a tea bag is an example of selective permeability.

Diffusion and OsmosisKey Concept: Materials move across membranes because of concentration differences.Diffusion and osmosis are types of passive transport.Cell are constantly taking in and sending out substances. But cells do NOT have to use energy to move all those molecules. Passive Transport is the movement (transport) of molecules without a cell using energy.

Solutions – If you dissolve salt in water, you have made a solution. The more salt you put in the water, the higher the concentration of slat becomes, and the lower the concentration of water becomes. The salt is the solute, and the water is the solvent.Cells are usually surrounded by fluid. The type of solution that a cell is in can have a big effect on the cell. There are three types of solution: isotonic, hypotonic, and hypertonic. These terms are comparisons. They compare the concentration of one solution to the concentration of another solution.

A solution is Isotonic to a cell if it has the same concentration of solutes that the cell has. Iso- means “equal”. In an isotonic solution, water moves into and out of a cell at equal rates. As a result, cell size remains constant.

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Concentration – the amount of molecules of one type in an area. If there are few molecules, the area has a low concentration. If there are many molecules, the area has a high concentration. Concentration can vary from one region to another. A concentration gradient is the difference in the concentration of a substance form one location to another. Molecules move from one place to another because of this difference in concentration.Diffusion is the movement of molecules from a place of higher concentration to a place of lower concentration. When molecules diffuse, they are described as moving down their concentration gradient. The diffusion of molecules across the cell membrane is a type of passive transport. It happens because of the natural motion of particles. Diffusion does NOT need energy from a cell.The diffusion of water molecules is called Osmosis. The process of osmosis is exactly the same as diffusion but refers ONLY to water molecules. Water molecules diffuse across a membrane form a place of higher water concentration to a place of lower water concentration.

A solution is Hypertonic if it has a higher concentration of solutes than a cell. Hyper- means “more”. This means the cell has a higher concentration of water than the surrounding fluid. As a result, water diffuses out of the cell, and the cell shrivels.

A solution is Hypotonic if it has a lower concentration of solutes than a cell. Hypo- means “less”. This means the cell has a lower concentration of water than the surrounding fluid. As a result, water diffuse into the cell, and the cell grows larger.

Notice how water moves from the area of higher water concentration to the area of lower water concentration in two of the pictures above.Some Molecules diffuse through selective transport proteins.

Vocabulary Check1. Which two words mean nearly the same thing?2. Which word includes “helper”?3. Which word describes a slope?4. How does passive transport benefit a cell?5. Why do cells need facilitated diffusion?6. Why does a cell swell in a hypotonic solution?

Active Transport, Endocytosis, ExocytosisKey Concept: Cells use energy to transport material that cannot diffuse across a membrane.

Proteins can transport materials against a concentration gradient.You have seen that the cell membrane controls the passive transport of material into and out of a cell. However, cells often need large amounts of materials that cannot diffuse across the membrane. Cells can use energy to move molecules from an area of lower concentration to an area of higher concentration.

Endocytosis and exocytosis transport material across the membrane in vesicles.Sometimes a material is too large to cross the membrane. Cells can use energy to transport these materials in vesicles.

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If some molecules can’t diffuse through the cell membrane by themselves, they can get help. Transport proteins give them a ride through the membrane. This process is called facilitated diffusion. The word facilitate means “to make easier”. Different transport proteins make it easier for certain molecules to get through the cell membrane without a cell using energy. Facilitated diffusion is another type of passive transport.In facilitated diffusion, do molecules move down their concentration gradient? Explain.

This process is called Active Transport. As the figure shows, active transport uses energy to drive molecules through transport proteins. This process plays an important role in helping cells to maintain homeostasis.What is the difference between active transport and passive transport?

Vocabulary Check1. Draw the last step of exocyctosis.2. Draw the first step of endocytosis.3. The biggest difference between active transport and passive transport is the need for the cell to use

.4. Draw a Venn diagram to compare and contrast active transport and passive transport.

Review1. What are the three major parts of the cell theory?2. Draw and describe the major organelles in plant and animal cells.3. Which of the following is an example of selective permeability?

a. A magnet b. a window screen c. a mild jug d. a bottle rocket4. The head of a phospholipid interacts with water because it is . 5. Diffusion is an example of

a. Passive transport b. active transport c. membrane receptors d. proteins6. A solution has a higher concentration of solutes than a cell.7. A is a protein that recognizes and responds to a signal.

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A cell uses endocytosis to take in large materials or liquids. The prefix endo- means “in”. In endocytosis, the cell membrane starts to fold in, forming a pocket around a substance. The pocket breaks off inside the cell making a vesicle. The contents of the vesicle are then broken down or released into the cell. Phagocytosis is a type of endocytosis in which the cell membrane grows out to surround large particles. The word literally means “cell eating”. Phagocytosis plays an important role in the immune system when white blood cells “eat” bacteria and other invaders.

Exocytosis is the opposite of endocytosis. The prefix exo- means “out”. It is the process that moves substances out of the cell. In exocytosis, a vesicle surrounds materials that need to be removed. The vesicle then goes to the cell membrane, fuses with it, and lets go of the contents. Exocytosis is the cell’s way of getting rid of wastes or secreting molecules.What functions do endocytosis and exocytosis carry out for the cell?