Chapter 3 Glossary, Summary, and Practice Questions
KEY TERMS
active transport the method of transporting material that requires energy
cell wall a rigid cell covering made of cellulose in plants, peptidoglycan in bacteria, non-peptidoglycan compounds in Archaea, and chitin in fungi that protects the cell, provides structural support, and gives shape to the cell
central vacuole a large plant cell organelle that acts as a storage compartment, water reservoir, and site of macromolecule degradation
chloroplast a plant cell organelle that carries out photosynthesis
cilium (plural: cilia) a short, hair-like structure that extends from the plasma membrane in large numbers and is used to move an entire cell or move substances along the outer surface of the cell
concentration gradient an area of high concentration across from an area of low concentration
cytoplasm the entire region between the plasma membrane and the nuclear envelope, consisting of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals
cytoskeleton the network of protein fibers that collectively maintains the shape of the cell, secures some organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move
cytosol the gel-like material of the cytoplasm in which cell structures are suspended
desmosome a linkage between adjacent epithelial cells that forms when cadherins in the plasma membrane attach to intermediate filaments
diffusion a passive process of transport of low-molecular weight material down its concentration gradient
electrochemical gradient a gradient produced by the combined forces of the electrical gradient and the chemical gradient
endocytosis a type of active transport that moves substances, including fluids and particles, into a cell
endomembrane system the group of organelles and membranes in eukaryotic cells that work together to modify, package, and transport lipids and proteins
endoplasmic reticulum (ER) a series of interconnected membranous structures within eukaryotic cells that collectively modify proteins and synthesize lipids
eukaryotic cell a cell that has a membrane-bound nucleus and several other membrane-bound compartments or sacs
exocytosis a process of passing material out of a cell
extracellular matrix the material, primarily collagen, glycoproteins, and proteoglycans, secreted from animal cells that holds cells together as a tissue, allows cells to communicate with each other, and provides mechanical protection and anchoring for cells in the tissue
facilitated transport a process by which material moves down a concentration gradient (from high to low concentration) using integral membrane proteins
flagellum (plural: flagella) the long, hair-like structure that extends from the plasma membrane and is used to move the cell
fluid mosaic model a model of the structure of the plasma membrane as a mosaic of components, including phospholipids, cholesterol, proteins, and glycolipids, resulting in a fluid rather than static character
gap junction a channel between two adjacent animal cells that allows ions, nutrients, and other low-molecular weight substances to pass between the cells, enabling the cells to communicate
Golgi apparatus a eukaryotic organelle made up of a series of stacked membranes that sorts, tags, and packages lipids and proteins for distribution
hypertonic describes a solution in which extracellular fluid has higher osmolarity than the fluid inside the cell hypotonic describes a solution in which extracellular fluid has lower osmolarity than the fluid inside the cell isotonic describes a solution in which the extracellular fluid has the same osmolarity as the fluid inside the cell
lysosome an organelle in an animal cell that functions as the cell’s digestive component; it breaks down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles
microscope the instrument that magnifies an object
mitochondria (singular: mitochondrion) the cellular organelles responsible for carrying out cellular respiration, resulting in the production of ATP, the cell’s main energy-carrying molecule
nuclear envelope the double-membrane structure that constitutes the outermost portion of the nucleus nucleolus the darkly staining body within the nucleus that is responsible for assembling ribosomal subunits nucleus the cell organelle that houses the cell’s DNA and directs the synthesis of ribosomes and proteins organelle a membrane-bound compartment or sac within a cell
osmolarity the total amount of substances dissolved in a specific amount of solution
osmosis the transport of water through a semipermeable membrane from an area of high water concentration to an area of low water concentration across a membrane
passive transport a method of transporting material that does not require energy
peroxisome a small, round organelle that contains hydrogen peroxide, oxidizes fatty acids and amino acids, and detoxifies many poisons
phagocytosis a process that takes macromolecules that the cell needs from the extracellular fluid; a variation of endocytosis
pinocytosis a process that takes solutes that the cell needs from the extracellular fluid; a variation of endocytosis
plasma membrane a phospholipid bilayer with embedded (integral) or attached (peripheral) proteins that separates the internal contents of the cell from its surrounding environment
plasmodesma (plural: plasmodesmata) a channel that passes between the cell walls of adjacent plant cells, connects their cytoplasm, and allows materials to be transported from cell to cell
prokaryotic cell a unicellular organism that lacks a nucleus or any other membrane-bound organelle
receptor-mediated endocytosis a variant of endocytosis that involves the use of specific binding proteins in the plasma membrane for specific molecules or particles
ribosome a cellular structure that carries out protein synthesis
rough endoplasmic reticulum (RER) the region of the endoplasmic reticulum that is studded with ribosomes and engages in protein modification
selectively permeable the characteristic of a membrane that allows some substances through but not others
smooth endoplasmic reticulum (SER) the region of the endoplasmic reticulum that has few or no ribosomes on its cytoplasmic surface and synthesizes carbohydrates, lipids, and steroid hormones; detoxifies chemicals like pesticides, preservatives, medications, and environmental pollutants, and stores calcium ions
solute a substance dissolved in another to form a solution
tight junction a firm seal between two adjacent animal cells created by protein adherence
tonicity the amount of solute in a solution.
unified cell theory the biological concept that states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells
vacuole a membrane-bound sac, somewhat larger than a vesicle, that functions in cellular storage and transport
vesicle a small, membrane-bound sac that functions in cellular storage and transport; its membrane is capable of fusing with the plasma membrane and the membranes of the endoplasmic reticulum and Golgi apparatus
CHAPTER SUMMARY
How Cells Are Studied
A cell is the smallest unit of life. Most cells are so small that they cannot be viewed with the naked eye. Therefore, scientists must use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution, and more detail than light microscopes. The unified cell theory states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.
Comparing Prokaryotic and Eukaryotic Cells
Prokaryotes are predominantly single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, a cell wall, DNA, and lack membrane-bound organelles. Many also have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1–5.0 µm.
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. Eukaryotic cells tend to be 10 to 100 times the size of prokaryotic cells.
Eukaryotic Cells
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.
The Cell Membrane
The modern understanding of the plasma membrane is referred to as the fluid mosaic model. The plasma membrane is composed of a bilayer of phospholipids, with their hydrophobic, fatty acid tails in contact with each other. The landscape of the membrane is studded with proteins, some of which span the membrane. Some of these proteins serve to transport materials into or out of the cell. Carbohydrates are attached to some of the proteins and lipids on the outward-facing surface of the membrane. These form complexes that function to identify the cell to other cells. The fluid nature of the membrane owes itself to the configuration of the fatty acid tails, the presence of cholesterol embedded in the membrane (in animal cells), and the mosaic nature of the proteins and protein-carbohydrate complexes, which are not firmly fixed in place. Plasma membranes enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux.
Passive Transport
The passive forms of transport, diffusion and osmosis, move material of small molecular weight. Substances diffuse from areas of high concentration to areas of low concentration, and this process continues until the substance is evenly distributed in a system. In solutions of more than one substance, each type of molecule diffuses according to its own concentration gradient. Many factors can affect the rate of diffusion, including concentration gradient, the sizes of the particles that are diffusing, and the temperature of the system.
In living systems, diffusion of substances into and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane, but others are hindered, and their passage is only made possible by protein channels and carriers. The chemistry of living things occurs in aqueous solutions, and balancing the concentrations of those solutions is an ongoing problem. In living systems, diffusion of some substances would be slow or difficult without membrane proteins.
Active Transport
The combined gradient that affects an ion includes its concentration gradient and its electrical gradient. Living cells need certain substances in concentrations greater than they exist in the extracellular space. Moving substances up their electrochemical gradients requires energy from the cell. Active transport uses energy stored in ATP to fuel the transport. Active transport of small molecular-size material uses integral proteins in the cell membrane to move the material—these proteins are analogous to pumps. Some pumps, which carry out primary active transport, couple directly with ATP to drive their action. In secondary transport, energy from primary transport can be used to move another substance into the cell and up its concentration gradient.
Endocytosis methods require the direct use of ATP to fuel the transport of large particles such as macromolecules; parts of cells or whole cells can be engulfed by other cells in a process called phagocytosis. In phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and leaving the particle wholly enclosed by an envelope of plasma membrane. Vacuoles are broken down by the cell, with the particles used as food or dispatched in some other way. Pinocytosis is a similar process on a smaller scale. The cell expels waste and other particles through the reverse process, exocytosis. Wastes are moved outside the cell, pushing a membranous vesicle to the plasma membrane, allowing the vesicle to fuse with the membrane and incorporating itself into the membrane structure, releasing its contents to the exterior of the cell.
ART CONNECTION QUESTIONS
- Figure 3.7 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?
- Figure 3.13 Why does the cis face of the Golgi not face the plasma membrane?
REVIEW QUESTIONS
- When viewing a specimen through a light microscope, scientists use to distinguish the individual components of cells.
- a beam of electrons
- radioactive isotopes
Figure 3.22 A doctor injects a patient with what he thinks is isotonic saline solution. The patient dies, and autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?
- special stains
- high temperatures
- The is the basic unit of life.
- organism
- cell
- tissue
- organ
- Which of these do all prokaryotes and eukaryotes share?
- nuclear envelope
- cell walls
- organelles
- plasma membrane
- A typical prokaryotic cell
compared to a eukaryotic cell.
- is smaller in size by a factor of 100
- is similar in size
- is smaller in size by a factor of one million
- is larger in size by a factor of 10
- Which of the following is found both in eukaryotic and prokaryotic cells?
- nucleus
- mitochondrion
- vacuole
- ribosome
- Which of the following is not a component of the endomembrane system?
- mitochondrion
- Golgi apparatus
- endoplasmic reticulum
- lysosome
- Which plasma membrane component can be either found on its surface or embedded in the membrane structure?
- protein
CRITICAL THINKING QUESTIONS
- What are the advantages and disadvantages of light, transmission, and scanning electron microscopes?
- Describe the structures that are characteristic of a prokaryote cell.
- In the context of cell biology, what do we mean by form follows function? What are at least two examples of this concept?
cholesterol- carbohydrate
- phospholipid
- The tails of the phospholipids of the plasma membrane are composed of and are ?
- phosphate groups; hydrophobic
- fatty acid groups; hydrophilic
- phosphate groups; hydrophilic
- fatty acid groups; hydrophobic
- Water moves via osmosis .
- throughout the cytoplasm
- from an area with a high concentration of other solutes to a lower one
- from an area with a low concentration of solutes to an area with a higher one
- from an area with a low concentration of water to one of higher concentration
- The principal force driving movement in diffusion is
.
- temperature
- particle size
- concentration gradient
- membrane surface area
- Active transport must function continuously because
.
- plasma membranes wear out
- cells must be in constant motion
- facilitated transport opposes active transport
- diffusion is constantly moving the solutes in the other direction
- Why is it advantageous for the cell membrane to be fluid in nature?
- Why does osmosis occur?
- Where does the cell get energy for active transport processes?
