Eukaryotic Cells, Organelles and Membranes

Learning Objectives

• Describe the structure of eukaryotic plant and animal cells.
• State the role of the plasma membrane.
• Summarize the functions of the major cell organelles.
• Describe the cytoskeleton and extracellular matrix.

Eukaryotic Cells

• Eukaryotic cells have a more complex structure than prokaryotic cells.
• Organelles allow for various functions to occur in the cell simultaneously.

Plasma Membrane

• Eukaryotic cells have a plasma membrane, similar to prokaryotes.
• The plasma membrane is a phospholipid bilayer with embedded proteins.
• A phospholipid consists of two fatty acid chains, a glycerol backbone, and a phosphate group.
• The plasma membrane regulates the passage of substances like organic molecules, ions, and water.
• It prevents the passage of some substances to maintain internal conditions.
• It actively brings in or removes other substances.
• Some compounds move passively across the membrane.
• The plasma membrane also contains cholesterol and carbohydrates.
• Cells specialized in absorption have microvilli, which are fingerlike projections that increase surface area.
• Example: Cells lining the small intestine have microvilli to absorb nutrients from digested food.
• Celiac disease: An immune response to gluten damages microvilli, leading to malnutrition, cramping, and diarrhea. Patients must follow a gluten-free diet.

Cytoplasm

• The cytoplasm consists of the contents of a cell between the plasma membrane and the nuclear envelope.
• It is made up of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals.
• The cytoplasm consists of 70-80% water, giving it a semi-solid consistency due to the proteins within it.
• The cytoplasm contains glucose, simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and glycerol derivatives.
• Ions of sodium, potassium, calcium, and other elements are dissolved in the cytoplasm.
• Many metabolic reactions, including protein synthesis, take place in the cytoplasm.

Cytoskeleton

• The cytoskeleton is a network of protein fibers within the cytoplasm.
• It maintains the shape of the cell.
• It secures certain organelles in specific positions.
• It allows cytoplasm and vesicles to move within the cell.
• It enables unicellular organisms to move independently.
• There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules.

Microfilaments

• Also known as actin filaments.
• They are the thinnest of the cytoskeletal fibers.
• They function in moving cellular components, for example, during cell division.
• They maintain the structure of microvilli.
• They are common in muscle cells and are responsible for muscle cell contraction.

Intermediate Filaments

• They are of intermediate diameter.
• They have structural functions, such as maintaining the shape of the cell and anchoring organelles.
• Keratin, which strengthens hair and nails, forms one type of intermediate filament.

Microtubules

• They are the thickest of the cytoskeletal fibers.
• They are hollow tubes that can dissolve and reform quickly.
• They guide organelle movement.
• They pull chromosomes to their poles during cell division.
• They are the structural components of flagella and cilia.
• In cilia and flagella, the microtubules are organized as a circle of nine double microtubules on the outside and two microtubules in the center.

Centrosome

• The centrosome is a region near the nucleus of animal cells that functions as a microtubule-organizing center.
• It contains a pair of centrioles, two structures that lie perpendicular to each other.
• Each centriole is a cylinder of nine triplets of microtubules.
• The centrosome replicates itself before a cell divides.
• Centrioles play a role in pulling duplicated chromosomes to opposite ends of the dividing cell.
• The exact function of centrioles in cell division is not clear, as cells without centrioles can still divide, and plant cells lack centrioles.

Flagella and Cilia

• Flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane.
• They are used to move an entire cell (e.g., sperm, Euglena).
• A cell has just one flagellum or a few flagella.
• Cilia (singular = cilium) are many in number and extend along the entire surface of the plasma membrane.
• They are short, hair-like structures used to move entire cells (e.g., paramecium) or move substances along the outer surface of the cell.
• Examples: Cilia in fallopian tubes move the ovum toward the uterus; cilia in the respiratory tract move particulate matter toward the throat.

Endomembrane System

• The endomembrane system (endo = within) is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.
• It includes the nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, and Golgi apparatus.
• The plasma membrane interacts with the other endomembranous organelles, so it is included in the endomembrane system.

Nucleus

• The nucleus is typically the most prominent organelle in a cell.
• It houses the cell’s DNA in the form of chromatin.
• It directs the synthesis of ribosomes and proteins.

Nuclear Envelope

• The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus.
• Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.
• The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm.

Chromosomes and Chromatin

• A chromosome is a structure within the nucleus made up of DNA (the hereditary material) and proteins.
• The combination of DNA and proteins is called chromatin.
• In eukaryotes, chromosomes are linear structures.
• Every species has a specific number of chromosomes in the nucleus of its body cells.
• For example, in humans, the chromosome number is 46, whereas in fruit flies, the chromosome number is 8.
• Chromosomes are visible and distinguishable from one another when the cell is getting ready to divide.
• During growth and maintenance phases, chromosomes resemble an unwound, jumbled bunch of threads.

Nucleolus

• Some chromosomes have sections of DNA that encode ribosomal RNA.
• The nucleolus (plural = nucleoli) is a darkly staining area within the nucleus.
• It aggregates ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported through the nuclear pores into the cytoplasm.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is a series of interconnected membranous tubules that collectively modify proteins and synthesize lipids.
• These functions are performed in separate areas: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).
• The hollow portion of the ER tubules is called the lumen or cisternal space.
• The membrane of the ER is a phospholipid bilayer embedded with proteins and is continuous with the nuclear envelope.

Rough Endoplasmic Reticulum (RER)

• The RER is named so because ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope.
• The ribosomes synthesize proteins while attached to the ER, resulting in transfer of their newly synthesized proteins into the lumen of the RER.
• Inside the lumen, proteins undergo modifications such as folding or addition of sugars.
• The RER also makes phospholipids for cell membranes.
• If the phospholipids or modified proteins are not destined to stay in the RER, they will be packaged within vesicles and transported from the RER by budding from the membrane.
• The RER is abundant in cells that secrete proteins, such as the liver.

Smooth Endoplasmic Reticulum (SER)

• The SER is continuous with the RER but has few or no ribosomes on its cytoplasmic surface.
• The SER’s functions include synthesis of carbohydrates, lipids (including phospholipids), and steroid hormones; detoxification of medications and poisons; alcohol metabolism; and storage of calcium ions.

Golgi Apparatus

• Vesicles that bud from the ER need to be sorted, packaged, and tagged so that they wind up in the right place.
• The sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs.
• The Golgi apparatus has a receiving face near the endoplasmic reticulum and a releasing face on the side away from the ER, toward the cell membrane.
• Transport vesicles from the ER travel to the receiving 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.
• The most frequent modification is the addition of short chains of sugar molecules.
• The newly modified proteins and lipids are then tagged with small molecular groups to enable them to be routed to their proper destinations.
• Finally, the modified and tagged proteins are packaged into vesicles that bud from the opposite face of the Golgi.
• Some vesicles (transport vesicles) deposit their contents into other parts of the cell where they will be used, while others (secretory vesicles) fuse with the plasma membrane and release their contents outside the cell.
• 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 abundant number of Golgi.
• In plant cells, the Golgi has an 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

• In animal cells, the lysosomes are the cell’s “garbage disposal.”
• Digestive enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and worn-out organelles.
• In single-celled eukaryotes, lysosomes are important for digestion of the food they ingest and the recycling of organelles.
• Lysosomal enzymes are active at a much lower pH (more acidic) than those located in the cytoplasm.
• Compartmentalizing the eukaryotic cell into organelles provides an advantage, since many reactions that take place in the cytoplasm could not occur at a low pH.
• Lysosomes also use their hydrolytic enzymes to destroy disease-causing organisms that might enter the cell.
• Example: Macrophages, a type of white blood cell, use phagocytosis to engulf 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, and the lysosome’s hydrolytic enzymes destroy the pathogen.

Vesicles and Vacuoles

• Vesicles and vacuoles are membrane-bound sacs that function in storage and transport.
• Vacuoles are somewhat larger than vesicles.
• The membrane of a vacuole does not fuse with the membranes of other cellular components.
• Vesicles can fuse with other membranes within the cell system.
• Enzymes within plant vacuoles can break down macromolecules.

Ribosomes

• Ribosomes are the cellular structures responsible for protein synthesis.
• When viewed through an electron microscope, free ribosomes appear as either clusters or single tiny dots floating freely in the cytoplasm.
• Ribosomes may be attached to either the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum.
• Electron microscopy has shown that ribosomes consist of large and small subunits.
• Ribosomes are enzyme complexes that are responsible for protein synthesis.
• Because protein synthesis is essential for all cells, ribosomes are found in practically every cell, although they are smaller in prokaryotic cells.
• They are particularly abundant in immature red blood cells for the synthesis of hemoglobin, which functions in the transport of oxygen throughout the body.

Mitochondria

• Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell.
• They are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule.
• The formation of ATP from the breakdown of glucose is known as cellular respiration.
• Mitochondria are oval-shaped, double-membrane organelles that have their own ribosomes and DNA.
• Each membrane is a phospholipid bilayer embedded with proteins.
• The inner layer has folds called cristae, which increase the surface area of the inner membrane.
• The area surrounded by the folds is called the mitochondrial matrix.
• The cristae and the matrix have different roles in cellular respiration.
• Muscle cells have a very high concentration of mitochondria because muscle cells need a lot of energy to contract.

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.
• Alcohol is detoxified by peroxisomes in liver cells.
• A byproduct of these oxidation reactions is hydrogen peroxide, H2O2, which is contained within the peroxisomes to prevent damage to cellular components.
• Hydrogen peroxide is safely broken down by peroxisomal enzymes into water and oxygen.

Animal Cells versus Plant Cells

• Animal cells have centrioles, centrosomes, and lysosomes, whereas plant cells do not.
• Plant cells have a cell wall, chloroplasts, plasmodesmata, and plastids used for storage, and a large central vacuole, whereas animal cells do not.

Cell Wall

• The cell wall is a rigid covering external to the plasma membrane that protects the cell, provides structural support, and gives shape to the cell.
• Fungal and protist cells also have cell walls.
• The chief component of prokaryotic cell walls is peptidoglycan, while the major organic molecule in the plant cell wall is cellulose, a polysaccharide made up of long, straight chains of glucose units.
• Dietary fiber refers to the cellulose content of food.

Chloroplasts

• Like mitochondria, chloroplasts also have their own DNA and ribosomes.
• Chloroplasts function in photosynthesis and can be found in eukaryotic cells such as plants and algae.
• In photosynthesis, carbon dioxide, water, and light energy are used to make glucose and oxygen.
• Plants (autotrophs) are able to make their own food, like glucose, whereas animals (heterotrophs) must rely on other organisms for their organic compounds or food source.
• 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 called thylakoids.
• Each stack of thylakoids is called a granum (plural = grana).
• The fluid enclosed by the inner membrane and surrounding the grana is called the stroma.
• The chloroplasts contain a green pigment called chlorophyll, which captures the energy of sunlight for photosynthesis.
• Like plant cells, photosynthetic protists also have chloroplasts.
• Some bacteria also perform photosynthesis, but they do not have chloroplasts; their photosynthetic pigments are located in the thylakoid membrane within the cell itself.

Endosymbiosis

• Both mitochondria and chloroplasts contain DNA and ribosomes, explained by endosymbiosis.
• Symbiosis is a relationship in which organisms from two separate species live in close association and typically exhibit specific adaptations to each other.
• Endosymbiosis (endo-= within) is a relationship in which one organism lives inside the other.
• Microbes that produce vitamin K live inside the human gut.
• Bacteria, mitochondria, and chloroplasts are similar in size and have similar features.
• Mitochondria and chloroplasts have their own DNA and ribosomes, just as bacteria do.
• Lynn Margulis proposed endosymbiotic theory, which indicated that these organelles originated from separate organisms.
• Host cells and bacteria formed a mutually beneficial endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them.
• Through evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the photosynthetic bacteria becoming chloroplasts.

Central Vacuole

• Plant cells each have a large, central vacuole that occupies most of the cell.
• The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions.
• In plant cells, the liquid inside the central vacuole provides turgor pressure, which is the outward pressure caused by the fluid inside the cell.
• When the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm and into the soil.
• As the central vacuole shrinks, it leaves the cell wall unsupported, resulting in a wilted appearance.
• The central vacuole fluid has a very bitter taste, which discourages consumption by insects and animals.
• The central vacuole also functions to store proteins in developing seed cells.

Extracellular Matrix of Animal Cells

• Most animal cells release materials into the extracellular space.
• The primary components are glycoproteins and the protein collagen.
• Collectively, these materials are called the extracellular matrix.
• The extracellular matrix holds the cells together to form a tissue and allows the cells within the tissue to communicate with each other.
• Example: Blood clotting; when the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. Tissue factor binds with another factor in the extracellular matrix, causing platelets to adhere to the wall of the damaged blood vessel, stimulates adjacent smooth muscle cells in the blood vessel to contract (constricting the blood vessel), and initiates a series of steps that stimulate the platelets to produce clotting factors.

Intercellular Junctions

• Cells can communicate with each other by direct contact, referred to as intercellular junctions.
• Plasmodesmata (singular = plasmodesma) are junctions between plant cells.
• Animal cell contacts include tight and gap junctions, and desmosomes.
• Plasmodesmata are numerous channels that pass between the cell walls of adjacent plant cells, connecting their cytoplasm and enabling signal molecules and nutrients to be transported from cell to cell.

Tight Junctions

• A tight junction is a watertight seal between two adjacent animal cells.
• Proteins hold the cells tightly against each other.
• This tight adhesion prevents materials from leaking between the cells.
• Tight junctions are typically found in the epithelial tissue that lines internal organs and cavities and composes most of the skin.
• Example: The tight junctions of the epithelial cells lining the urinary bladder prevent urine from leaking into the extracellular space.

Desmosomes

• Found only in animal cells, desmosomes act like spot welds between adjacent epithelial cells.
• They keep cells together in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.

Gap Junctions

• Gap junctions in animal cells are like plasmodesmata in plant cells.
• They are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate.
• Gap junctions and plasmodesmata differ structurally.