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B6. Eukaroytic Cell Membranes - Biology

B6. Eukaroytic Cell Membranes - Biology


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We have studied lipids, proteins, and carbohydrates. Several examples of such attachments include:

  • N-myristoylation (attached myristic acid - 14:0 - through an amide link)
  • S-palmitoylation (attached palmitic acid - 16:0 - through a thioester link with a Cys
  • farnesyl or geranylgeranyl additon to a CAAX carboxy-terminal sequence in a target protein, where C is Cys, A is aliphatic, and X is any amno acid
  • addition of a protein to a glycosyl phophatidylinositol (GPI), through a complex which usually contains a conserved tetrasaccharide core of 3 Man and 1 GlcNAc residues linked to a protein. The GPI can be further modified with extra Gal's and Man, as well as additions to the PI group, which secures the protein in the membrane. GPIs are found in eukaryotic cells, and link many surface antigens, adhesion molecules, and hydrolases to the membrane. GPIs from Plasmoidium falciparum, the malarial parasite which kills about two million people each year, appears to act as a toxin and is the most common CHO modification of the parasite protein. Mice immunized against the GPI sequence, NH2-CH2-CH2-PO4-Man (a1-2) 6Man (a1-2) Man (a1-6) Man (a1-4) GlcNH2 (a1-6) myo-inositol-1,2-cyclic-phosphate, were substantially protected from malarial symptoms and death after they were exposed to the actual parasite.


Figure: Biological Membranes: Simple to Complex


Figure: A cool view of a membrane surface


Section Summary

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleolus within the nucleus is the site for ribosome assembly. Ribosomes are found in the cytoplasm or are attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria perform cellular respiration and produce ATP. Peroxisomes break down fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.

Animal cells also have a centrosome and lysosomes. The centrosome has two bodies, the centrioles, with an unknown role in cell division. Lysosomes are the digestive organelles of animal cells.

Plant cells have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole expands, enlarging the cell without the need to produce more cytoplasm.

The endomembrane system includes the nuclear envelope, the endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport membrane lipids and proteins.

The cytoskeleton has three different types of protein elements. Microfilaments provide rigidity and shape to the cell, and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural elements of centrioles, flagella, and cilia.

Animal cells communicate through their extracellular matrices and are connected to each other by tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other by plasmodesmata.

Additional Self Check Questions

1. What structures does a plant cell have that an animal cell does not have? What structures does an animal cell have that a plant cell does not have?


The Nucleus

Typically, the nucleus is the most prominent organelle in a cell ( [Figure 1]). The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail ( [Figure 4]).

Figure 4: The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.


The exocyst at the interface between cytoskeleton and membranes in eukaryotic cells

Delivery and final fusion of the secretory vesicles with the relevant target membrane are hierarchically organized and reciprocally interconnected multi-step processes involving not only specific protein-protein interactions, but also specific protein-phospholipid interactions. The exocyst was discovered as a tethering complex mediating initial encounter of arriving exocytic vesicles with the plasma membrane. The exocyst complex is regulated by Rab and Rho small GTPases, resulting in docking of exocytic vesicles to the plasma membrane (PM) and finally their fusion mediated by specific SNARE complexes. In model Opisthokont cells, the exocyst was shown to directly interact with both microtubule and microfilament cytoskeleton and related motor proteins as well as with the PM via phosphatidylinositol 4, 5-bisphosphate specific binding, which directly affects cortical cytoskeleton and PM dynamics. Here we summarize the current knowledge on exocyst-cytoskeleton-PM interactions in order to open a perspective for future research in this area in plant cells.

Keywords: Exo70 actin cytoskeleton exocyst microtubule cytoskeleton myosin phospholipids secretion small GTPases.


B6. Eukaroytic Cell Membranes - Biology

Cell Structure and Membrane

Introduction
A cell is a smallest unit of an organism that can live independently. There are two cell types: prokaryotic and eukaryotic cells. Prokaryotic cells include bacteria and a large group of other microorganisms with no nucleus. Eukaryotic cells include plant cell and animal cells, they have distinct nucleus and cell organelles.

How the cell maintains life using organelles?
The cell maintains life by assigning each responsibility to separate specialized machines. These machines are called organelles. An organelle is a compartmentalized structure that performs a specialized function within a cell. An animal cell contains a nucleus, ribosomes, mitochondria, rough endoplasmic reticulum, smooth endoplasmic reticulum, plasma membrane, Golgi apparatus and lysosomes. The nucleus controls the cell function.

Organelle structures and functions
Ribosomes: make proteins for the cell. Each ribosome is made of two protein subunits: the large subunit and the small subunit. The units clasp around a strand of nucleic acid instructions from the nucleus. The ribosome reads the strand instructions to make proteins for the cell to use in its normal activities.
Endoplasmic reticulum: Including rough ER and smooth ER. Rough ER is found attached to the outside of the nucleus. It appears rough because of the ribosomes on its surface. It helps the attached ribosomes in finishing protein synthesis. Smooth ER is NOT attached to the nucleus and DOES NOT have attached ribosomes (thus smooth). Smooth ER synthesizes carbohydrates and lipids.
The Golgi apparatus: made up of flattened, folded sacs, ships packages around the cell.
Mitochondria: converts carbohydrates taken from food into ATP -- produce energy to power the cell.
Lysosome: highly acidic, destroy waste to clean up the cell.

Cell Membrane: composition and function
A cell membrane is a selectively permeable structure that envelops the cell and protects the cell&rsquos internal environment. The cell&rsquos membrane is made of phospholipids, which have carbohydrate heads and lipid tails. Proteins can be embedded or anchored on cell membrane. Cell membranes provides a stable environment for cells, perform communication function among cells via the surface proteins, and selectively exchange material between a cell and its environment.

A cell is a building unit of an organism that can function independently. The cell maintains life by assigning each responsibility to separate specialized machines. These machines are called organelles. An organelle is a compartmentalized structure that performs a specialized function within a cell. An animal cell contains a nucleus, ribosomes, mitochondria, rough endoplasmic reticulum, smooth endoplasmic reticulum, plasma membrane, Golgi apparatus and lysosomes. The nucleus controls the cell function. Other organelles provide energy and building blocks for cells. Cell membrane is a selectively permeable structure that envelops the cell and protects the cell&rsquos internal environment. The cell&rsquos membrane is made of bi-layer of phospholipids and proteins, which can communicate with other cells or environment.

  • Concept maps to explain the topics which are dealt with.
  • Flow-chart type of explanation on each function of a cell.
  • Elegant structure details of a cell and each organelle.
  • Detailed explanation on synthesis, delivery and function of proteins
  • What is a cell
  • Type of cells
  • Requirement for life
  • A cell maintain life by assigning each responsibility to organelles
  • What are organelles
  • Nucleus
  • Ribosomes
  • Endoplasmic Reticulum
  • Golgi Apparatus
  • Protein synthesis and delivery
  • Mitochondria
  • Lysosomes
  • Summary
  • What is a cell membrane?
  • Cell membrane composition
  • Cell membrane synthesis
  • Cell membrane function

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B6. Eukaroytic Cell Membranes - Biology

The first major distinction we can make between living organisms is whether they are composed of prokaryotic or eukaryotic cells. Eukaryotic organisms can be unicellular or multicellular. Whereas eukaryotic cells contain a true nucleus enclosed in a membrane, prokaryotic cells do not contain a nucleus. The major organelles are identified in the eukaryotic cell in Figure 1.1.

Figure 1.1. Eukaryotic Cell Numerous membrane-bound organelles are found in the cytoplasm of a eukaryotic cell.

Each cell has a cell membrane enclosing a semifluid cytosol in which the organelles are suspended. In eukaryotic cells, most organelles are membrane bound, allowing for compartmentalization of functions. Membranes of eukaryotic cells consist of a phospholipid bilayer. This membrane is unique in that its surfaces are hydrophilic, electrostatically interacting with the aqueous environments inside and outside of the cell, while its inner portion is hydrophobic, which helps to provide a highly selective barrier between the interior of the cell and the external environment. The cell membrane is such an important topic on the MCAT that an entire chapter&mdashChapter 8 of MCAT Biochemistry Review&mdashis devoted solely to discussing the structure and physiology of biological membranes. The cytosol allows for the diffusion of molecules throughout the cell. Within thenucleus, genetic material is encoded in deoxyribonucleic acid (DNA), which is organized into chromosomes. Eukaryotic cells reproduce by mitosis, allowing for the formation of two identical daughter cells.

The Nucleus

As the control center of the cell, the nucleus is the most heavily tested organelle on the MCAT. It contains all of the genetic material necessary for replication of the cell. The nucleus is surrounded by the nuclear membrane or envelope, a double membrane that maintains a nuclear environment separate and distinct from the cytoplasm. Nuclear pores in the nuclear membrane allow for selective two-way exchange of material between the cytoplasm and the nucleus.

The nuclear envelope creates two distinct environments within the cell because it separates the nucleus from the cytoplasm. This allows for compartmentalization of transcription (the formation of hnRNA from DNA, which is subsequently processed to form mRNA) and translation (the formation of a peptide from mRNA). These processes are discussed in Chapter 7 of MCAT Biochemistry Review.

The genetic material (DNA) contains coding regions called genes. Linear DNA is wound around organizing proteins known as histones, and is then further wound into linear strands called chromosomes. The location of DNA in the nucleus allows for the compartmentalization of DNA transcription separate from RNA translation. Finally, there is a subsection of the nucleus known as the nucleolus, where the ribosomal RNA (rRNA) is synthesized. The nucleolus actually takes up approximately 25 percent of the volume of the entire nucleus and can often be identified as a darker spot in the nucleus.

Mitochondria

Mitochondria, shown in Figure 1.2, are often called the power plants of the cell, in reference to their important metabolic functions. The mitochondrion contains two layers: the outer and inner membranes. The outer membrane serves as a barrier between the cytosol and the inner environment of the mitochondrion. The inner membrane, which is thrown into numerous infoldings called cristae, contains the molecules and enzymes necessary for the electron transport chain. The cristae are highly convoluted structures that increase the surface area available for electron transport chain enzymes. The space between the inner and outer membranes is called the intermembrane space the space inside the inner membrane is called the mitochondrial matrix. As described in Chapter 10 of MCAT Biochemistry Review, the pumping of protons from the mitochondrial matrix to the intermembrane space establishes the proton-motive force ultimately, these protons flow through ATP synthase to generate ATP during oxidative phosphorylation.

The serial endosymbiosis theory attempts to explain the formation of some of the membrane-bound organelles it posits that these organelles formed by the engulfing of one prokaryote by another and the establishment of a symbiotic relationship between the two. In addition to mitochondria, chloroplasts in plant cells and organelles of motility (such as flagella) are believed to have evolved through this process.

Mitochondria are different from other parts of the cell in that they are semi-autonomous. They contain some of their own genes and replicate independently of the nucleus via binary fission. Mitochondria are thought to have evolved from an anaerobic prokaryote engulfing an aerobic prokaryote and establishing a symbiotic relationship.

Figure 1.2. Mitochondrial Structure

In addition to keeping the cell alive by providing energy, the mitochondria are also capable of killing the cell by release of enzymes from the electron transport chain. This release kick-starts a process known as apoptosis, or programmed cell death.

Lysosomes are membrane-bound structures containing hydrolytic enzymes that are capable of breaking down many different substrates, including substances ingested by endocytosis and cellular waste products. The lysosomal membrane sequesters these enzymes to prevent damage to the cell. However, release of these enzymes can occur in a process known as autolysis. Like mitochondria, when lysosomes release their hydrolytic enzymes, it results in apoptosis. In this case, the released enzymes directly lead to the degradation of cellular components.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a series of interconnected membranes that are actually contiguous with the nuclear envelope. The single membrane of the endoplasmic reticulum is folded into numerous invaginations, creating complex structures with a central lumen. There are two varieties of ER: smooth and rough. The rough ER (RER) is studded with ribosomes, which permit the translation of proteins destined for secretion directly into its lumen. On the other hand, the smooth ER (SER) lacks ribosomes and is utilized primarily for lipid synthesis and the detoxification of certain drugs and poisons. The SER also transports proteins from the RER to the Golgi apparatus.

Golgi Apparatus

The Golgi apparatus consists of stacked membrane-bound sacs. Materials from the ER are transferred to the Golgi apparatus in vesicles. Once in the Golgi apparatus, these cellular products may be modified by the addition of various groups, including carbohydrates, phosphates, and sulfates. The Golgi apparatus may also modify cellular products through the introduction of signal sequences, which direct the delivery of the product to a specific cellular location. After modification and sorting in the Golgi apparatus, cellular products are repackaged in vesicles, which are subsequently transferred to the correct cellular location. If the product is destined for secretion, then the secretory vesicle merges with the cell membrane and its contents are released via exocytosis. The relationships between lysosomes, the ER, and the Golgi apparatus are shown in Figure 1.3.

Figure 1.3. Lysosomes, the Endoplasmic Reticulum, and the Golgi Apparatus

KEY CONCEPT

Not all cells have the same relative distribution of organelles. Form will follow function. Cells that require a lot of energy for locomotion (such as sperm cells) have high concentrations of mitochondria. Cells involved in secretion (such as pancreatic islet cells and other endocrine tissues) have high concentrations of RER and Golgi apparatuses. Other cells, such as red blood cells, which primarily serve a transport function, have no organelles at all.

Peroxisomes

Peroxisomes contain hydrogen peroxide. One of the primary functions of peroxisomes is the breakdown of very long chain fatty acids via &beta-oxidation. Peroxisomes participate in the synthesis of phospholipids and contain some of the enzymes involved in the pentose phosphate pathway, discussed in Chapter 9 of MCAT Biochemistry Review.

The cytoskeleton, shown in Figure 1.4, provides structure to the cell and helps it to maintain its shape. In addition, the cytoskeleton provides a conduit for the transport of materials around the cell. There are three components of the cytoskeleton: microfilaments, microtubules, and intermediate filaments.

Figure 1.4. Cytoskeletal Elements The rounded shape near the center in each of these photographs is the nucleus.

Microfilaments

Microfilaments are made up of solid polymerized rods of actin. The actin filaments are organized into bundles and networks and are resistant to both compression and fracture, providing protection for the cell. Actin filaments can also use ATP to generate force for movement by interacting with myosin, such as in muscle contraction.

Microfilaments also play a role in cytokinesis, or the division of materials between daughter cells. During mitosis, the cleavage furrow is formed from microfilaments, which organize as a ring at the site of division between the two new daughter cells. As the actin filaments within this ring contract, the ring becomes smaller, eventually pinching off the connection between the two daughter cells.

Microtubules

Unlike microfilaments, microtubules are hollow polymers of tubulin proteins. Microtubules radiate throughout the cell, providing the primary pathways along which motor proteins like kinesin and dynein carry vesicles.

Motor proteins like kinesin and dynein are classic examples of nonenzymatic proteins, along with binding proteins, cell adhesion molecules, immunoglobulins, and ion channels. Motor proteins often travel along cytoskeletal structures to accomplish their functions. Nonenzymatic proteins are discussed in Chapter 3 of MCAT Biochemistry Review.

Cilia and flagella are motile structures composed of microtubules. Cilia are projections from a cell that are primarily involved in movement of materials along the surface of the cell for example, cilia line the respiratory tract and are involved in movement of mucus. Flagella are structures involved in movement of the cell itself, such as the movement of sperm cells through the reproductive tract. Cilia and flagella share the same structure, composed of nine pairs of microtubules forming an outer ring, with two microtubules in the center, as shown in Figure 1.5. This is known as a 9 + 2 structure and is seen only in eukaryotic organelles of motility. Bacterial flagella have a different structure with a different chemical composition.

Figure 1.5. Cilium and Flagellum Structure Microtubules are organized into a ring of 9 doublets with 2 central microtubules.

Centrioles are found in a region of the cell called the centrosome. They are the organizing centers for microtubules and are structured as nine triplets of microtubules with a hollow center. During mitosis, the centrioles migrate to opposite poles of the dividing cell and organize the mitotic spindle. The microtubules emanating from the centrioles attach to the chromosomes via complexes called kinetochores and can exert force on the sister chromatids, pulling them apart.

Intermediate Filaments

Intermediate filaments are a diverse group of filamentous proteins, including keratin and desmin. Many intermediate filaments are involved in cell–cell adhesion or maintenance of the overall integrity of the cytoskeleton. Intermediate filaments are able to withstand a tremendous amount of tension, making the cell structure more rigid. In addition, intermediate filaments help anchor other organelles, including the nucleus. The identity of the intermediate filament proteins within a cell is specific to the cell and tissue type.

One of the unique characteristics of eukaryotic cells is the formation of tissues with division of labor, as different cells in a tissue may carry out different functions. For example, in the heart, some cells participate in the conduction pathways while others cause contraction still others serve a supportive role, maintaining structural integrity of the organ. There are four tissue types: epithelial tissue, connective tissue, muscle, and nervous tissue. While muscle and nervous tissue are considered more extensively in subsequent chapters, we explore epithelial and connective tissues below.

Epithelial Tissue

Epithelial tissues cover the body and line its cavities, providing a means for protection against pathogen invasion and desiccation. In certain organs, epithelial cells are involved in absorption, secretion, and sensation. To remain one cohesive unit, epithelial cells are tightly joined to each other and to an underlying layer of connective tissue known as the basement membrane. Epithelial cells are highly diverse and serve numerous functions depending on the identity of the organ in which they are found in most organs, epithelial cells constitute the parenchyma, or the functional parts of the organ. For example, nephrons in the kidney, hepatocytes in the liver, and acid-producing cells of the stomach are all composed of epithelial cells.

Epithelial cells are often polarized, meaning that one side faces a lumen (the hollow inside of an organ or tube) or the outside world, while the other side interacts with blood vessels and structural cells. For example, in the small intestine, one side of the cell will be involved in absorption of nutrients from the lumen, while the other side will be involved in releasing those nutrients into circulation for use in the rest of the body.

We can classify different epithelia according to the number of layers they have and the shape of their cells. Simple epithelia have one layer of cells stratified epithelia have multiple layers and pseudostratified epithelia appear to have multiple layers due to differences in cell height, but are, in reality, only one layer. Turning to shape, cells may be classified as cuboidal, columnar, or squamous. As their names imply, cuboidal cells are cube-shaped and columnar cells are long and thin. Squamous cells are flat and scalelike.

Connective Tissue

Connective tissue supports the body and provides a framework for the epithelial cells to carry out their functions. Whereas epithelial cells contribute to the parenchyma of an organ, connective tissues are the main contributors to the stroma or support structure. Bone, cartilage, tendons, ligaments, adipose tissue, and blood are all examples of connective tissues. Most cells in connective tissues produce and secrete materials such as collagen and elastin to form the extracellular matrix.

MCAT Concept Check 1.2:

Before you move on, assess your understanding of the material with these questions.

1. Briefly describe the functions of each of the organelles listed below:

·&emspRough endoplasmic reticulum:

·&emspSmooth endoplasmic reticulum:

2. A child is diagnosed with an enzyme deficiency that prevents the production of hydrogen peroxide. What would the likely outcome be of such a deficiency?

3. What are the predominant proteins in each cytoskeletal element?

4. How do the cytoskeletal structures of centrioles and flagella differ?

5. Classify each of the following cells as epithelial cells or connective tissue:

·&emspFibroblasts, which produce collagen in a number of organs:

·&emspEndothelial cells, which line blood vessels:

·&emsp&alpha-cells, which produce glucagon in the pancreas:

·&emspOsteoblasts, which produce osteoid, the material that hardens into bone:

·&emspChondroblasts, which produce cartilage:

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Nucleus

Master control of cellular functions via its genetic material (DNA)

  1. Nuclear membrane: Double membrane controlling the movement of materials between the nucleus and Cytoplasm contains pores that communicate with the ER
  2. Chromatin: Nudcoprotcin component of chromosomes (seen clearly only during nuclear division when the chromatin is highly condensed) only the DNA component is hereditary material.
  3. Nudeolus: Site(s) on chromatin where ribosomal RNA (rRNA) is synthesized disappears
    from light microscope during cellular replication.
  4. Nucleoplasm: Nonchromatin components of the nucleus containing materials for building DNA and messenger RNA (mRNA molecules serve as intermediates between nucleus and cytoplasm).

Functions of Cell Membrane of Eukaryotic Cell

Some of the major functions of cell membrane of eukaryotic cell are as follows:

1. Compartmentalization 2. Selective Permeability 3. Cellular recognition and adhesion 4. Cellular movements 5. Vital functions 6. Receptors 7. Enzymes 8. Disease.

1. Compartmentalization:

The cell membrane encloses the protoplasm and maintains the individuality of the cell. Prokaryotic cell is a uni-compartment system while eukaryotic cell has multi-compartment system in which the internal membrane bound compartments are organelles containing different set of chemicals.

2. Selective Permeability:

The cell membrane serves as selective permeability barrier allowing the entry or exists of some ions and molecules through it. The membrane proteins (carriers and channels) provide sites at which molecules cross the membrane either actively or passively.

3. Cellular recognition and adhesion:

Glycoproteins and glycolipids of the cell membrane act as cell surface markers and help in recognizing self from non-self. The RBCs have surface antigens that determine the various blood group systems. The HLA antigens (Human Leucocyte Antigens) on cell membrane are recognized by immune system.

4. Cellular movements:

The cell membrane from pseudopodia, cilia and flagella which help in cellular movements.

5. Vital functions:

(i) The membranes are the location of vital processes like respiration, photosynthesis, synthesis of cell wall constituents, lipids, transmission of nerve impulses etc.

(ii) The fluid nature of the cell membrane is helpful various functions like cell division, cell growth, secretin, endocytosis and formation of intercellular junctions.

Cell membranes have receptors for certain hormones.

Cell membranes possess enzymes for performing certain reactions on their surface, e.g., ATPase (for ATP synthesis and release of energy from ATP), phosphatases, esterase’s etc..

Defects in the organisation of cell membrane may cause certain disease, e.g., Bernard Soulier syndrome, a type of bleeding disorder in human. The term membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and molecules through biological membranes. As few molecules are able to diffuse through a lipid bilayer the majority of the transport processes involve transport proteins.

Another Question

Selective transport of substances through cell membrane occurs by three methods:

(I) Passive transport. (Downhill transport):

In this type of transport, substances cross the membrane without any energy expenditure. The driving force for passive transport requires the concentration gradient. Passive transport is of two types: Simple diffusion and Facilitated diffusion.

(a) Simple diffusion:

It occurs through phospholipid bilayers of the membranes, where no membrane proteins involved. Diffusion is a slow passive transport of molecules that occurs from higher concentration to lower concentration due to their own kinetic energy until equilibrium is reached. The rate of diffusion of substances across a membrane depends upon the concentration gradient, temperature, pressure and, size and lipid solubility of substances.

The lipid soluble substances (O2, N2, H2, CH4, NH3, benzene) and small uncharged polar molecules (CO2, urea and glycerol) diffuse through the membranes by dissolving in lipid matrix. The diffusion of solutes rather than solvents through the semi permeable membrane is called dialysis.

(b) Facilitated diffusion:

It involves the use of membrane proteins (Channels and Carriers) to facilitate the movement of molecules in either direction across a membrane. In some cases, molecules pass through channels within the protein. In other cases, the protein changes shape, allowing molecules to pass through.

Channel proteins form open pores through the membranes, allowing the transport of any molecules of the appropriate size. For example, ion channels allow specific type of ions (cations and anions) to diffuse through the membrane. Aquaporin’s are water channels in biological membranes for passive transport of water.

In contrast to channel proteins, Carrier proteins transport ions as well as solutes like sugar and amino acids across the membranes by physically binding to them and then undergo conformational change to release the same to the other side of the membrane.

Some carrier proteins allow transport only if two types of molecules transport together. This is called cotransport which is of two types i.e. symport and antiport. In symport, two molecules move together in the same direction. In antiport, two molecules move in opposite direction. When a carrier protein transports a single molecule across a membrane in one direction, the process is called uniport. The transport by carrier proteins can be either active or passive, whereas transport by channel proteins is always passive.

(II) Active transport (Uphill transport):

In this case, the substances are transported against their concentration gradient i.e. from lower to higher concentration. This form of transport requires energy and carriers. In primary active transport, the energy obtained by ATP hydrolysis used directly for transport, e.g. Na + -K + pump, Cat+pump. In secondary active transport, indirect energy source is required, e.g. transport of glucose and amino acids is coupled to active transport of Na+.

(III) Bulk transport or vesicular transport:

Active transport of materials in large quantity (bulk) through vesicles is called bulk or vesicular transport. It is very common in secretory and excretory cells. Bulk transport occurs by two processes i.e. endocytosis and exocytosis.

It is the bulk import of materials into the cells by vesicles. Vesicle (bleb) formation or blobbing occurs by in-folding of the cell membrane. It does not occur in plant cells due to rigid cell wall and internal turgor.

Endocytosis is of three types:

(iii) Recep-tormediated endocytosis.

(i) Phagocytosis [“Cell eating”):

It involves the ingestion of relatively large, solid particles, such as bacteria or cellular debris, via large vesicles pinched off from plasma membrane. These vesicles are called phagosomes. The phagosome fuses with lysosome to form a digestive vacuole. The solid food is digested. The digested food diffuses into the cytoplasm. The vacuole containing the indigestible vacuole is called residual vacuole. The undigested food particles are thrown out by the process of exocytosis (Fig. 3.12).

Many one-celled organisms, such as amoebas, feed in this way, as do plasmodial slime molds and cellular slime molds. In mammals, macrophages & neutrophils are phagocytic.

(ii) Pinocytosis (“Cell drinking”):

It involves taking in of bulk amount of fluid and substances dissolved in it by cells across the cell membrane by forming small detachable vesicles called pinosome. The pinosome migrates towards the interior where it liberates the fluid either in the cytoplasm or a vacuole. Lysosomes are required if digestion of solutes is involved (Fig. 3.13).

Unlike phagocytosis, which is carried out only by certain specialized cells, pinocytosis is believed to occur in all eukaryotic cells, as the cells continuously and indiscriminately “sip” small amounts of fluid from the surrounding medium.

(iii) Receptor-mediated endocytosis (RME):

The cells that undergo RME have coated pits, where specific receptors are localized. These coated pits are depressions of the plasma membrane coated with protein clathrin. The substance being transported attaches to the receptors in the coated pit. Shortly thereafter the coated pit invaginates and pinches off to form a coated vesicle. Within the cell, the coated vesicles shed their coats and then fuse with some other membrane-bound structure (e.g., Golgi bodies or small vacuoles), releasing their contents in the process. For exam pie, transport of iron & cholesterol into the cells by RME.

It is the reverse of endocytosis by which bulk materials exit the cells with the help of vesicles. The vesicles are formed internally from Golgi apparatus and moved by cytoskeleton to the cell surface where they fuse to expel their contents. This is called ephagy, cell vomiting or emeiocytosis. It occurs during cell secretion, excretion, removal of undigested remains from food vacuoles, release of neurotransmitters from nerve cells etc.


Biology 171

By the end of this section, you will be able to do the following:

  • Describe the structure of eukaryotic cells
  • Compare animal cells with plant cells
  • State the role of the plasma membrane
  • Summarize the functions of the major cell organelles

Have you ever heard the phrase “form follows function?” It’s a philosophy that many industries follow. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should include several elevator banks. A hospital should have its emergency room easily accessible.

Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells ( (Figure)). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others and 3) several, rod-shaped chromosomes. Because a membrane surrounds eukaryotic cell’s nucleus, it has a “true nucleus.” The word “organelle” means “little organ,” and, as we already mentioned, organelles have specialized cellular functions, just as your body’s organs have specialized functions.

At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm.



If the nucleolus were not able to carry out its function, what other cellular organelles would be affected?

The Plasma Membrane

Like prokaryotes, eukaryotic cells have a plasma membrane ((Figure)), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane 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 plasma membrane.


The plasma membranes of cells that specialize in absorption fold into fingerlike projections that we call microvilli (singular = microvillus) ((Figure)). Such cells typically line the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function.
People with celiac disease have an immune response to gluten, which is a protein in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.


The Cytoplasm

The cytoplasm is the cell’s entire region between the plasma membrane and the nuclear envelope (a structure we will discuss shortly). It is comprised of organelles suspended in the gel-like cytosol , the cytoskeleton, and various chemicals ((Figure)). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are also there. Ions of sodium, potassium, calcium, and many other elements also dissolve in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.

The Nucleus

Typically, the nucleus is the most prominent organelle in a cell ((Figure)). The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail ((Figure)).


The Nuclear Envelope

The nuclear envelope is a double-membrane structure that constitutes the nucleus’ outermost portion ((Figure)). Both the nuclear envelope’s inner and outer membranes are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.

Chromatin and Chromosomes

To understand chromatin, it is helpful to first explore chromosomes , structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight.
Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins attach to chromosomes, and they resemble an unwound, jumbled bunch of threads. We call these unwound protein-chromosome complexes chromatin ((Figure)). Chromatin describes the material that makes up the chromosomes both when condensed and decondensed.



The Nucleolus

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.

Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. When we view them through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the plasma membrane’s cytoplasmic side or the endoplasmic reticulum’s cytoplasmic side and the nuclear envelope’s outer membrane ((Figure)). Electron microscopy shows us that ribosomes, which are large protein and RNA complexes, consist of two subunits, large and small ((Figure)). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA transcribes into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.


Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), there are ribosomes in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.

Mitochondria

Scientists often call mitochondria (singular = mitochondrion) the cell’s “powerhouses” or “energy factories” because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the cell’s short-term stored energy. Cellular respiration is the process of making ATP using the chemical energy 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 byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need considerable energy to keep your body moving. When your cells don’t get enough oxygen, they do not make much ATP. Instead, producing lactic acid accompanies the small amount of ATP they make in the absence of oxygen.

Mitochondria are oval-shaped, double membrane organelles ((Figure)) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. We call the area surrounded by the folds the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.


Peroxisomes

Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water.) For example, peroxisomes in liver cells detoxify alcohol. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Plant cells contain many different types of peroxisomes that play a role in metabolism, pathogene defense, and stress response, to mention a few.

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them. Vesicle membranes can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The vacuole’s membrane does not fuse with the membranes of other cellular components.

Animal Cells versus Plant Cells

At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles, but there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells also have centrioles associated with the MTOC: a complex we call the centrosome. Animal cells each have a centrosome and lysosomes whereas, most plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole whereas, animal cells do not.

The Centrosome

The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other ((Figure)). Each centriole is a cylinder of nine triplets of microtubules.


The centrosome (the organelle where all microtubules originate) replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the centriole’s exact function in cell division isn’t clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division.

Lysosomes

Animal cells have another set of organelles that most plant cells do not: lysosomes. The lysosomes are the cell’s “garbage disposal.” In plant cells, the digestive processes take place in vacuoles. Enzymes within the lysosomes aid in breaking down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than the cytoplasm’s. Therefore, the pH within lysosomes is more acidic than the cytoplasm’s pH. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

The Cell Wall

If you examine (Figure), the plant cell diagram, you will see a structure external to the plasma membrane. This is the cell wall , a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and some protistan cells also have cell walls. While the prokaryotic cell walls’ chief component is peptidoglycan, the major organic molecule in the plant (and some protists’) cell wall is cellulose ((Figure)), a polysaccharide comprised of glucose units. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That’s because you are tearing the celery cells’ rigid cell walls with your teeth.


Chloroplasts

Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals. Plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food.

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs we call thylakoids ((Figure)). Each thylakoid stack is a granum (plural = grana). We call the fluid enclosed by the inner membrane that surrounds the grana the stroma.


The chloroplasts contain a green pigment, chlorophyll , which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle.

Endosymbiosis We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.

Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. We have already mentioned that microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just like mitochondria and chloroplasts. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you look at (Figure)b, you will see that plant cells each have a large central vacuole that occupies most of the cell’s area. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the plant’s cell walls results in the wilted appearance.

The central vacuole also supports the cell’s expansion. When the central vacuole holds more water, the cell becomes larger without having to invest considerable energy in synthesizing new cytoplasm.

Section Summary

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning a membrane surrounds its DNA), and has other membrane-bound organelles that allow for compartmentalizing functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleus’s nucleolus is the site of ribosome assembly. We find ribosomes either in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration. They are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.

Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells.

Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives the cell shape. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm.

Art Connections

(Figure) If the nucleolus were not able to carry out its function, what other cellular organelles would be affected?

(Figure) Free ribosomes and rough endoplasmic reticulum (which contains ribosomes) would not be able to form.

Free Response

You already know that ribosomes are abundant in red blood cells. In what other cells of the body would you find them in great abundance? Why?

Ribosomes are abundant in muscle cells as well because muscle cells are constructed of the proteins made by the ribosomes.

What are the structural and functional similarities and differences between mitochondria and chloroplasts?

Both are similar in that they are enveloped in a double membrane, both have an intermembrane space, and both make ATP. Both mitochondria and chloroplasts have DNA, and mitochondria have inner folds called cristae and a matrix, while chloroplasts have chlorophyll and accessory pigments in the thylakoids that form stacks (grana) and a stroma.

Why are plasma membranes arranged as a bilayer rather than a monolayer?

The plasma membrane is a bilayer because the phospholipids that create it are amphiphilic (hydrophilic head, hydrophobic tail). If the plasma membrane was a monolayer, the hydrophobic tails of the phospholipids would be in direct contact with the inside of the cell. Since the cytoplasm is largely made of water, this interaction would not be stable, and would disrupt the plasma membrane of the cell as the tails were repulsed by the cytoplasm (in water, phospholipids spontaneously form spherical droplets with the hydrophilic heads facing outward to isolate the hydrophobic tails from the water). By having a bilayer, the hydrophilic heads are exposed to the aqueous cytoplasm and extracellular space, while the hydrophobic tails interact with each other in the middle of the membrane.

Glossary



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