Cells

cell review cell metabolism review Saladin-4th-Edition-Chapter-4-Genetics-and-Cellular-Function/ A and p review

Various Cell Types: (a) Nasal sinus cells (viewed with a light microscope), (b) onion cells (viewed with a light microscope), and (c) Vibrio tasmaniensis bacterial cells (seen through a scanning electron microscope) are from very different organisms, yet all share certain characteristics of basic cell structure.

Components of Plasma Membranes

The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals.

LEARNING OBJECTIVES

Describe the function and components of the plasma membrane

KEY TAKEAWAYS

Key Points

• The principal components of the plasma membrane are lipids ( phospholipids and cholesterol), proteins, and carbohydrates.

• The plasma membrane protects intracellular components from the extracellular environment.

• The plasma membrane mediates cellular processes by regulating the materials that enter and exit the cell.

• The plasma membrane carries markers that allow cells to recognize one another and can transmit signals to other cells via receptors.

Key Terms

• plasma membrane: The semipermeable barrier that surrounds the cytoplasm of a cell.

• receptor: A protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell.

Structure of Plasma Membranes

The plasma membrane (also known as the cell membrane or cytoplasmic membrane) is a biological membrane that separates the interior of a cell from its outside environment.

The primary function of the plasma membrane is to protect the cell from its surroundings. Composed of a phospholipid bilayer with embedded proteins, the plasma membrane is selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells. Plasma membranes must be very flexible in order to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries.

The plasma membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues. The membrane also maintains the cell potential.

In short, if the cell is represented by a castle, the plasma membrane is the wall that provides structure for the buildings inside the wall, regulates which people leave and enter the castle, and conveys messages to and from neighboring castles. Just as a hole in the wall can be a disaster for the castle, a rupture in the plasma membrane causes the cell to lyse and die.

The plasma membrane: The plasma membrane is composed of phospholipids and proteins that provide a barrier between the external environment and the cell, regulate the transportation of molecules across the membrane, and communicate with other cells via protein receptors.

The Plasma Membrane and Cellular Transport

The movement of a substance across the selectively permeable plasma membrane can be either “passive”—i.e., occurring without the input of cellular energy —or “active”—i.e., its transport requires the cell to expend energy.

The cell employs a number of transport mechanisms that involve biological membranes:

1. Passive osmosis and diffusion: transports gases (such as O₂ and CO₂₎ and other small molecules and ions

2. Transmembrane protein channels and transporters: transports small organic molecules such as sugars or amino acids

3. Endocytosis: transports large molecules (or even whole cells) by engulfing them

4. Exocytosis: removes or secretes substances such as hormones or enzymes

The Plasma Membrane and Cellular Signaling

Among the most sophisticated functions of the plasma membrane is its ability to transmit signals via complex proteins. These proteins can be receptors, which work as receivers of extracellular inputs and as activators of intracellular processes, or markers, which allow cells to recognize each other.

Membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, which then trigger intracellular responses. Some viruses, such as Human Immunodeficiency Virus (HIV), can hijack these receptors to gain entry into the cells, causing infections.

Membrane markers allow cells to recognize one another, which is vital for cellular signaling processes that influence tissue and organ formation during early development. This marking function also plays a later role in the “self”-versus-“non-self” distinction of the immune response. Marker proteins on human red blood cells, for example, determine blood type (A, B, AB, or O).

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane structure as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates.

LEARNING OBJECTIVES

Describe the fluid mosaic model of cell membranes

KEY TAKEAWAYS

Key Points

• The main fabric of the membrane is composed of amphiphilic or dual-loving, phospholipid molecules.

• Integral proteins, the second major component of plasma membranes, are integrated completely into the membrane structure with their hydrophobic membrane-spanning regions interacting with the hydrophobic region of the phospholipid bilayer.

• Carbohydrates, the third major component of plasma membranes, are always found on the exterior surface of cells where they are bound either to proteins (forming glycoproteins ) or to lipids (forming glycolipids).

Key Terms

• amphiphilic: Having one surface consisting of hydrophilic amino acids and the opposite surface consisting of hydrophobic (or lipophilic) ones.

• hydrophilic: Having an affinity for water; able to absorb, or be wetted by water,”water-loving.”

• hydrophobic: Lacking an affinity for water; unable to absorb, or be wetted by water,”water-fearing.”

The fluid mosaic model was first proposed by S.J. Singer and Garth L. Nicolson in 1972 to explain the structure of the plasma membrane. The model has evolved somewhat over time, but it still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness. For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or approximately 1,000 times wider than a plasma membrane. The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type. For example, myelin contains 18% protein and 76% lipid. The mitochondrial inner membrane contains 76% protein and 24% lipid.

Components of the Plasma Membrane

Component

Location

Phospholipid

Main fabric of the membrane

Cholesterol

Attached between phospholipids and between the two phospholipid layers

Integral proteins (for example, integrins)

Embedded within the phospholipid layer(s). May or may not penetrate through both layers.

Peripheral proteins

On the inner or outer surface of the phospholipid bilayer; not embedded within the phospholipids

Carbohydrates (components of glycoproteins and glycolipids)

Generally attached to outside of membrane layer

The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and some of the proteins.

The fluid mosaic model of the plasma membrane: The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane.

The main fabric of the membrane is composed of amphiphilic or dual-loving, phospholipid molecules. The hydrophilic or water-loving areas of these molecules are in contact with the aqueous fluid both inside and outside the cell. Hydrophobic, or water-hating molecules, tend to be non- polar. A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the middle of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent lipid bilayer cell membrane that separates fluid within the cell from the fluid outside of the cell.

The structure of a phospholipid molecule: This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.

Proteins make up the second major component of plasma membranes. Integral proteins (some specialized types are called integrins) are, as their name suggests, integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the the phospholipid bilayer. Single-pass integral membrane proteins usually have a hydrophobic transmembrane segment that consists of 20–25 amino acids. Some span only part of the membrane—associating with a single layer—while others stretch from one side of the membrane to the other, and are exposed on either side. Some complex proteins are composed of up to 12 segments of a single protein, which are extensively folded and embedded in the membrane. This type of protein has a hydrophilic region or regions, and one or several mildly hydrophobic regions. This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid.

Structure of integral membrane proteins: Integral membrane proteins may have one or more alpha-helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3).

Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains may consist of 2–60 monosaccharide units and can be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other. This recognition function is very important to cells, as it allows the immune system to differentiate between body cells (called “self”) and foreign cells or tissues (called “non-self”). Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them. These carbohydrates on the exterior surface of the cell—the carbohydrate components of both glycoproteins and glycolipids—are collectively referred to as the glycocalyx (meaning “sugar coating”). The glycocalyx is highly hydrophilic and attracts large amounts of water to the surface of the cell. This aids in the interaction of the cell with its watery environment and in the cell’s ability to obtain substances dissolved in the water.

Membrane Fluidity

The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.

LEARNING OBJECTIVES

Explain the function of membrane fluidity in the structure of cells

KEY TAKEAWAYS

Key Points

• The membrane is fluid but also fairly rigid and can burst if penetrated or if a cell takes in too much water.

• The mosaic nature of the plasma membrane allows a very fine needle to easily penetrate it without causing it to burst and allows it to self-seal when the needle is extracted.

• If saturated fatty acids are compressed by decreasing temperatures, they press in on each other, making a dense and fairly rigid membrane.

• If unsaturated fatty acids are compressed, the “kinks” in their tails push adjacent phospholipid molecules away, which helps maintain fluidity in the membrane.

• The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperatures.

• Cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity.

Key Terms

• phospholipid: Any lipid consisting of a diglyceride combined with a phosphate group and a simple organic molecule such as choline or ethanolamine; they are important constituents of biological membranes

• fluidity: A measure of the extent to which something is fluid. The reciprocal of its viscosity.

Membrane Fluidity

There are multiple factors that lead to membrane fluidity. First, the mosaic characteristic of the membrane helps the plasma membrane remain fluid. The integral proteins and lipids exist in the membrane as separate but loosely-attached molecules. The membrane is not like a balloon that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst; the membrane will flow and self-seal when the needle is extracted.

Membrane Fluidity: The plasma membrane is a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane.

The second factor that leads to fluidity is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms; there are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight. In contrast, unsaturated fatty acids do not contain a maximal number of hydrogen atoms, although they do contain some double bonds between adjacent carbon atoms; a double bond results in a bend of approximately 30 degrees in the string of carbons. Thus, if saturated fatty acids, with their straight tails, are compressed by decreasing temperatures, they press in on each other, making a dense and fairly rigid membrane. If unsaturated fatty acids are compressed, the “kinks” in their tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules. This “elbow room” helps to maintain fluidity in the membrane at temperatures at which membranes with saturated fatty acid tails in their phospholipids would “freeze” or solidify. The relative fluidity of the membrane is particularly important in a cold environment. A cold environment tends to compress membranes composed largely of saturated fatty acids, making them less fluid and more susceptible to rupturing. Many organisms (fish are one example) are capable of adapting to cold environments by changing the proportion of unsaturated fatty acids in their membranes in response to the lowering of the temperature.

In animals, the third factor that keeps the membrane fluid is cholesterol. It lies alongside the phospholipids in the membrane and tends to dampen the effects of temperature on the membrane. Thus, cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity too much. Cholesterol extends in both directions the range of temperature in which the membrane is appropriately fluid and, consequently, functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.

The Plasma Membrane and the Cytoplasm

The plasma membrane is made up of a phospholipid bilayer that regulates the concentration of substances that can permeate a cell.

LEARNING OBJECTIVES

Explain the structure and purpose of the plasma membrane of a cell

KEY TAKEAWAYS

Key Points

• All eukaryotic cells have a surrounding plasma membrane, which is also known as the cell membrane.

• The plasma membrane is made up by a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment.

• Only relatively small, non- polar materials can easily move through the lipid bilayer of the plasma membrane.

• Passive transport is the movement of substances across the membrane that does not require the use of energy while active transport is the movement of substances across the membrane using energy.

• Osmosis is the diffusion of water through a semi- permeable membrane down its concentration gradient; this occurs when there is an imbalance of solutes outside of a cell compared to the inside the cell.

Key Terms

• phospholipid: Any lipid consisting of a diglyceride combined with a phosphate group and a simple organic molecule such as choline or ethanolamine; they are important constituents of biological membranes

• hypertonic: having a greater osmotic pressure than another

• hypotonic: Having a lower osmotic pressure than another; a cell in this environment causes water to enter the cell, causing it to swell.

The Plasma Membrane

Despite differences in structure and function, all living cells in multicellular organisms have a surrounding plasma membrane (also known as the cell membrane). As the outer layer of your skin separates your body from its environment, the plasma membrane separates the inner contents of a cell from its exterior environment. The plasma membrane can be described as a phospholipid bilayer with embedded proteins that controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the membrane.

Eukaryotic Plasma Membrane: The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.

The cell membrane is an extremely pliable structure composed primarily of two adjacent sheets of phospholipids. Cholesterol, also present, contributes to the fluidity of the membrane. A single phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic, and a non-polar lipid “tail,” which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails. The phospholipid bilayer consists of two phospholipids arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads contact the fluid inside and outside of the cell.

Phospholipid Bilayer: The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads contact the fluid inside and outside of the cell.

The plasma membrane’s main function is to regulate the concentration of substances inside the cell. These substances include ions such as Ca⁺⁺, Na⁺, K⁺, and Cl^–; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO₂), which must leave the cell.

The membrane’s lipid bilayer structure provides the cell with access control through permeability. The phospholipids are tightly packed together, while the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the plasma membrane, only relatively small, non-polar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these materials are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—such as glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer.

Transport Across the Membrane

All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive (non-energy requiring) transport is the movement of substances across the membrane without the expenditure of cellular energy. During this type of transport, materials move by simple diffusion or by facilitated diffusion through the membrane, down their concentration gradient. Water passes through the membrane in a diffusion process called osmosis. Osmosis is the diffusion of water through a semi-permeable membrane down its concentration gradient. It occurs when there is an imbalance of solutes outside of a cell versus inside the cell. The solution that has the higher concentration of solutes is said to be hypertonic and the solution that has the lower concentration of solutes is said to be hypotonic. Water molecules will diffuse out of the hypotonic solution and into the hypertonic solution (unless acted upon by hydrostatic forces).

Osmosis: Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic.

In contrast to passive transport, active (energy-requiring) transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP). The energy is expended to assist material movement across the membrane in a direction against their concentration gradient. Active transport may take place with the help of protein pumps or through the use of vesicles. Another form of this type of transport is endocytosis, where a cell envelopes extracellular materials using its cell membrane. The opposite process is known as exocytosis. This is where a cell exports material using vesicular transport.

Cytoplasm

The cell’s plasma membrane also helps contain the cell’s cytoplasm, which provides a gel-like environment for the cell’s organelles. The cytoplasm is the location for most cellular processes, including metabolism, protein folding, and internal transportation.

The Endoplasmic Reticulum

The endoplasmic reticulum is an organelle that is responsible for the synthesis of lipids and the modification of proteins.

LEARNING OBJECTIVES

Describe the structure of the endoplasmic reticulum and its role in synthesis and metabolism

KEY TAKEAWAYS

Key Points

• If the endoplasmic reticulum (ER) has ribosomes attached to it, it is called rough ER; if it does not, then it is called smooth ER.

• The proteins made by the rough endoplasmic reticulum are for use outside of the cell.

• Functions of the smooth endoplasmic reticulum include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions.

Key Terms

• lumen: The cavity or channel within a tube or tubular organ.

• reticulum: A network

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions are performed in separate areas of the ER: the rough ER and the smooth ER. The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

Rough ER

Rough Endoplasmic Reticulum: This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell.

The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope. Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles —or secreted from the cell (such as protein hormones, enzymes ). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane. Since the RER is engaged in modifying proteins (such as enzymes, for example) that will be secreted from the cell, the RER is abundant in cells that secrete proteins. This is the case with cells of the liver, for example.

Smooth ER

The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions. In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells.

The Golgi Apparatus

The Golgi apparatus sorts and packages materials before they leave the cell to ensure they arrive at the proper destination.

LEARNING OBJECTIVES

Describe the structure of the Golgi apparatus and its role in protein modification and secretion

KEY TAKEAWAYS

Key Points

• The Golgi apparatus is a series of flattened sacs that sort and package cellular materials.

• The Golgi apparatus has a cis face on the ER side and a trans face opposite of the ER.

• The trans face secretes the materials into vesicles, which then fuse with the cell membrane for release from the cell.

Key Terms

• vesicle: A membrane-bound compartment found in a cell.

The Golgi Apparatus

We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need to be sorted, packaged, and tagged so that they wind up in the right place. Sorting, tagging, packaging, and distribution of lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes.

The Golgi apparatus sorts and packages cellular products: The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower portion of the image. Several vesicles can be seen near the Golgi apparatus.

The receiving side of the Golgi apparatus is called the cis face. The opposite side is called the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is the addition of short chains of sugar molecules. These newly-modified proteins and lipids are then tagged with phosphate groups or other small molecules so that they can be routed to their proper destinations.

Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the trans face of the Golgi. While some of these vesicles deposit their contents into other parts of the cell where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

In another example of form following function, cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundance of Golgi. In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell.

Lysosomes

Lysosomes are organelles that digest macromolecules, repair cell membranes, and respond to foreign substances entering the cell.

LEARNING OBJECTIVES

Describe how lysosomes function as the cell’s waste disposal system

KEY TAKEAWAYS

Key Points

• Lysosomes breakdown/digest macromolecules (carbohydrates, lipids, proteins, and nucleic acids), repair cell membranes, and respond against foreign substances such as bacteria, viruses and other antigens.

• Lysosomes contain enzymes that break down the macromolecules and foreign invaders.

• Lysosomes are composed of lipids and proteins, with a single membrane covering the internal enzymes to prevent the lysosome from digesting the cell itself.

• Lysosomes are found in all animal cells, but are rarely found within plant cells due to the tough cell wall surrounding a plant cell that keeps out foreign substances.

Key Terms

• enzyme: a globular protein that catalyses a biological chemical reaction

• lysosome: An organelle found in all types of animal cells which contains a large range of digestive enzymes capable of splitting most biological macromolecules.

A lysosome has three main functions: the breakdown/digestion of macromolecules (carbohydrates, lipids, proteins, and nucleic acids), cell membrane repairs, and responses against foreign substances such as bacteria, viruses and other antigens. When food is eaten or absorbed by the cell, the lysosome releases its enzymes to break down complex molecules including sugars and proteins into usable energy needed by the cell to survive. If no food is provided, the lysosome’s enzymes digest other organelles within the cell in order to obtain the necessary nutrients.

In addition to their role as the digestive component and organelle-recycling facility of animal cells, lysosomes are considered to be parts of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body’s immune system. In a process known as phagocytosis or endocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen.

Lysosomes digest foreign substances that might harm the cell: A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium and then fuses with a lysosomes within the cell to destroy the pathogen. Other organelles are present in the cell but for simplicity are not shown.

A lysosome is composed of lipids, which make up the membrane, and proteins, which make up the enzymes within the membrane. Usually, lysosomes are between 0.1 to 1.2μm, but the size varies based on the cell type. The general structure of a lysosome consists of a collection of enzymes surrounded by a single-layer membrane. The membrane is a crucial aspect of its structure because without it the enzymes within the lysosome that are used to breakdown foreign substances would leak out and digest the entire cell, causing it to die.

Lysosomes are found in nearly every animal-like eukaryotic cell. They are so common in animal cells because, when animal cells take in or absorb food, they need the enzymes found in lysosomes in order to digest and use the food for energy. On the other hand, lysosomes are not commonly-found in plant cells. Lysosomes are not needed in plant cells because they have cell walls that are tough enough to keep the large/foreign substances that lysosomes would usually digest out of the cell.

Peroxisomes

Peroxisomes neutralize harmful toxins and carry out lipid metabolism and oxidation reactions that break down fatty acids and amino acids.

LEARNING OBJECTIVES

Name the various functions that peroxisomes perform inside the cell

KEY TAKEAWAYS

Key Points

• Lipid metabolism and chemical detoxification are important functions of peroxisomes.

• Peroxisomes are responsible for oxidation reactions that break down fatty acids and amino acids.

• Peroxisomes oversee reactions that neutralize free radicals, which cause cellular damage and cell death.

• Peroxisomes chemically neutralize poisons through a process that produces large amounts of toxic H₂O₂, which is then converted into water and oxygen.

• The liver is the organ primarily responsible for detoxifying the blood before it travels throughout the body; as a result, liver cells contain large amounts of peroxisomes.

Key Terms

• enzyme: a globular protein that catalyses a biological chemical reaction

• free radical: Any molecule, ion or atom that has one or more unpaired electrons; they are generally highly reactive and often only occur as transient species.

Peroxisomes

A type of organelle found in both animal cells and plant cells, a peroxisome is a membrane-bound cellular organelle that contains mostly enzymes. Peroxisomes perform important functions, including lipid metabolism and chemical detoxification. They also carry out oxidation reactions that break down fatty acids and amino acids.

Peroxisomes: Peroxisomes are membrane-bound organelles that contain an abundance of enzymes for detoxifying harmful substances and lipid metabolism.

In contrast to the digestive enzymes found in lysosomes, the enzymes within peroxisomes serve to transfer hydrogen atoms from various molecules to oxygen, producing hydrogen peroxide (H₂O₂). In this way, peroxisomes neutralize poisons, such as alcohol, that enter the body. In order to appreciate the importance of peroxisomes, it is necessary to understand the concept of reactive oxygen species.

Reactive oxygen species (ROS), such as peroxides and free radicals, are the highly-reactive products of many normal cellular processes, including the mitochondrial reactions that produce ATP and oxygen metabolism. Examples of ROS include the hydroxyl radical OH, H₂O₂, and superoxide (O₋₂). Some ROS are important for certain cellular functions, such as cell signaling processes and immune responses against foreign substances. Many ROS, however, are harmful to the body. Free radicals are reactive because they contain free unpaired electrons; they can easily oxidize other molecules throughout the cell, causing cellular damage and even cell death. Free radicals are thought to play a role in many destructive processes in the body, from cancer to coronary artery disease.

Peroxisomes oversee reactions that neutralize free radicals. They produce large amounts of the toxic H₂O₂ in the process, but contain enzymes that convert H₂O₂ into water and oxygen. These by-products are then safely released into the cytoplasm. Like miniature sewage treatment plants, peroxisomes neutralize harmful toxins so that they do not cause damage in the cells. The liver is the organ primarily responsible for detoxifying the blood before it travels throughout the body; liver cells contain an exceptionally high number of peroxisomes.

Mitochondria

Mitochondria are organelles that are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule.

LEARNING OBJECTIVES

Explain the role of the mitochondria.

KEY TAKEAWAYS

Key Points

• Mitochondria contain their own ribosomes and DNA; combined with their double membrane, these features suggest that they might have once been free-living prokaryotes that were engulfed by a larger cell.

• Mitochondria have an important role in cellular respiration through the production of ATP, using chemical energy found in glucose and other nutrients.

• Mitochondria are also responsible for generating clusters of iron and sulfur, which are important cofactors of many enzymes.

Key Terms

• alpha-proteobacteria: A taxonomic class within the phylum Proteobacteria — the phototropic proteobacteria.

• adenosine triphosphate: a multifunctional nucleoside triphosphate used in cells as a coenzyme, often called the “molecular unit of energy currency” in intracellular energy transfer

• cofactor: an inorganic molecule that is necessary for an enzyme to function

One of the major features distinguishing prokaryotes from eukaryotes is the presence of mitochondria. Mitochondria are double-membraned organelles that contain their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. Eukaryotic cells may contain anywhere from one to several thousand mitochondria, depending on the cell’s level of energy consumption. Each mitochondrion measures 1 to 10 micrometers (or greater) in length and exists in the cell as an organelle that can be ovoid to worm-shaped to intricately branched.

Mitochondria Structure

Most mitochondria are surrounded by two membranes, which would result when one membrane-bound organism was engulfed into a vacuole by another membrane-bound organism. The mitochondrial inner membrane is extensive and involves substantial infoldings called cristae that resemble the textured, outer surface of alpha-proteobacteria. The matrix and inner membrane are rich with the enzymes necessary for aerobic respiration.

Mitochondrial structure: This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane.

Mitochondria have their own (usually) circular DNA chromosome that is stabilized by attachments to the inner membrane and carries genes similar to genes expressed by alpha-proteobacteria. Mitochondria also have special ribosomes and transfer RNAs that resemble these components in prokaryotes. These features all support the hypothesis that mitochondria were once free-living prokaryotes.

Mitochondria Function

Mitochondria are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a by-product.

It is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don’t get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.

In addition to the aerobic generation of ATP, mitochondria have several other metabolic functions. One of these functions is to generate clusters of iron and sulfur that are important cofactors of many enzymes. Such functions are often associated with the reduced mitochondrion-derived organelles of anaerobic eukaryotes.

Origins of Mitochondria

There are two hypotheses about the origin of mitochondria: endosymbiotic and autogenous, but the most accredited theory at present is endosymbiosis. The endosymbiotic hypothesis suggests mitochondria were originally prokaryotic cells, capable of implementing oxidative mechanisms. These prokaryotic cells may have been engulfed by a eukaryote and became endosymbionts living inside the eukaryote.

Intermediate Filaments and Microtubules

Microtubules are part of the cell’s cytoskeleton, helping the cell resist compression, move vesicles, and separate chromosomes at mitosis.

LEARNING OBJECTIVES

Describe the roles of microtubules as part of the cell’s cytoskeleton

KEY TAKEAWAYS

Key Points

• Microtubules help the cell resist compression, provide a track along which vesicles can move throughout the cell, and are the components of cilia and flagella.

• Cilia and flagella are hair-like structures that assist with locomotion in some cells, as well as line various structures to trap particles.

• The structures of cilia and flagella are a “9+2 array,” meaning that a ring of nine microtubules is surrounded by two more microtubules.

• Microtubules attach to replicated chromosomes during cell division and pull them apart to opposite ends of the pole, allowing the cell to divide with a complete set of chromosomes in each daughter cell.

Key Terms

• microtubule: Small tubes made of protein and found in cells; part of the cytoskeleton

• flagellum: a flagellum is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells

• cytoskeleton: A cellular structure like a skeleton, contained within the cytoplasm.

Microtubules

As their name implies, microtubules are small hollow tubes. Microtubules, along with microfilaments and intermediate filaments, come under the class of organelles known as the cytoskeleton. The cytoskeleton is the framework of the cell which forms the structural supporting component. Microtubules are the largest element of the cytoskeleton. The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins. With a diameter of about 25 nm, microtubules are the widest components of the cytoskeleton. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can dissolve and reform quickly.

4. LYSOSOMES - quite variable in size and shape. Contain powerful digesting enzymes - breakdown and destroy foreign particles, microorganisms, damaged or worn out cells and cell parts

5. CENTROSOME (central body) - Usually near the G.A. and nucleus. Composed of 2 "cylinders" called CENTRIOLES (each composed of numerous microtubules), which always lie perpendicular to each other. Active involved in cell reproduction - SPINDLE forms from the centrioles

6. VESICLES - tiny sacs in which substances are transported

7. MICROFILAMENTS & MICROTUBULES - threadlike structures

MICROFILAMENTS - involved in cellular movement, as in muscle cells

MICROTUBULES - larger than filaments, maintain shape ("skeleton" of the cell)

Cell:

A cell consists of three main parts---the ______________, the cell "stuff" called ______________, and the outer _________________________.

2. Cell Membrane: The cell membrane is extremely _________and is __________________ permeable.

function: The cell membrane regulates the_________________________________________,

participates in signal transduction, and helps cells adhere to other cells

structure: The basic framework of the cell membrane consists of a double layer of _________________

_________________ are found in the cell membrane, including some which are transmembrane and some that are peripheral membrane.

3. Cytoplasm: The cytoplasm consists of a clear liquid called ______________________, a supportive ____________________, and networks of membranes and organelles.

4. endoplasmic reticulum: provides a tubular __________________________ system inside the cell.

rough: why does it appear rough? ________________________

What does it function in the synthesis and transport of? _____________________

smooth: Why does it appear smooth? _____________________

What does it function in the transport of? _____________________

5. ribosome: Where are they found? _____________________

What are they composed of? _____________________

What do they help in the production of? _____________________

6. golgi apparatus: is composed of flattened __________and it packages the cells products.

These packages are released in the form of ________________________

7. lysosomes: contain __________________ enzymes to break up old cell components and bacteria.

They are sometimes called the "______________________________" of the cell.

8. microfilaments and microtubules: are thin threadlike structures that serve as the _________________________________of the cell.

Microfilaments, made of the protein ____________________, cause various cellular movements.

Mictotubules, made of the globular protein _________________.

9. centrosome: is a structure made up of two hollow cylinders called ________________________. What is their function?

10. cilia & flagella: are motile extensions from the cell. Which one is shorter? ______________

What is its function in the human body? _____________________

What is the only flagellated cell in the body? _____________________

11. Nucleus: is bounded by a ___________-layered nuclear membrane containing relatively large nuclear ______________________ that allow the passage of certain substances.

12. nucleolus: Where is it found? _____________________

Does it have its own membrane? _____________________

What chemicals is it made of? _____________________

13. chromatin: What chemicals is the chromatin made of? _____________________

14. Movement Through Cell Membrane:

The cell membrane controls what passes through it.

PASSIVE TRANSPORT: Mechanisms of movement across the membrane may be passive, requiring no ______________ from the cell (diffusion, facilitated diffusion, osmosis, and filtration).

diffusion: from area of ___________ concentration to area of low concentration to reach _____________________________.

osmosis: Only substance that is moved by osmosis is ___________________________.

What substances diffuse in the human body? _____________________

15. Facilitated Diffusion: - uses membrane proteins that function as ______________________ to move molecules (such as glucose) across the cell membrane.

16. Filtration: Filtration forces molecules through _______________________ and is commonly used to separate solids from __________________________

17. Active Transport: moves from area of ____________ concentration to area of ____________

concentration. Requires ___________________ proteins: (pumps). Also requires energy in the form of _______________.

18. Endocytosis and Exocytosis: In _______________________ molecules that are too large to be transported by other means are engulfed by an invagination of the cell membrane and carried into the cell surrounded by a vesicle.

_______________________ is a form in which cells engulf liquids.

_______________________ is a form is which the cell takes in larger particles, such as a white blood cell engulfing a bacterium.

In ___________________________molecules are pushed out of the cell

19. Cell Cycle: The series of changes a cell undergoes from the time it is formed until it _________________________ is called the cell cycle.

The cell cycle consists of what four stages? ______________________________________

The cell cycle is highly regulated. Most cells do not divide continually. Cells have a maximum number of times they can divide because of built-in "clocks"called _____________ on the tips of chromosomes.

20. Cell Reproduction: There are two types of cell division, mitosis and meiosis. Meiosis produces ______________ cells.

21. mitosis: How many daughter cells are produced in mitosis? ______ Are they identical to the "mother" cell? _____________

22. interphase: Interphase is a period of great metabolic activity in which the cell grows and 23. synthesizes new molecules and organelles. During the S phase of interphase, the __________ of the cell is replicated in preparation for cell division.

24. prophase: What disappears during this phase? _____________________

What appears or becomes visible during this phase? _____________________

25. metaphase: Why is this phase the easiest to see on a microscope slide? (hint, what are the chromosomes doing?) _____________________

26. anaphase: What characterizes this phase? _____________________

27. telophase: What reappears during this phase? _____________________ What have the chromosomes done? _____________________

28. Cytokinesis begins during anaphase of mitosis and continues as the cell pinches into _________________________________

29. differentiation: The process by which cells develop into different types of cells with specialized ___________________ is called differentiation. What controls this?

30. What is the death of a cell called? ______________________

Labeling Practice