What is the cell theory?
The cell theory states that all organisms are made up of cells, that the cell is the fundamental unit of life, and that cells come from preexisting cells. There is no life without cells, and the cell is the smallest unit of life.
T/F: Cells are defined by membranes
True. Membranes physically separate cells from their external environment. Also, membranes define spaces within many cells that allow them to carry out diverse functions.
What are the main components of cell membranes?
Lipids are the MAIN component of cell membranes. Proteins are also embedded in or associated with the membrane. Carbohydrates can also be found in cell membranes, usually attached to lipids (glycolipids) and proteins (glycoproteins)
Who first observed cells?
Robert Hooke used a simple microscope to observe small chambers in a sample of cork tissue that he described as cells
T/F: Cell membranes are composed of two layers of lipids
True.
What are the major types of lipid found in cell membranes?
Phospholipids. Phospholipids have both hydrophobic and hydrophilic regions in a single molecule. The phosphate head group is hydrophilic because it is polar, enabling it to form hydrogen bonds with water. IN CONTRAST, the two long fatty acid tails are hyd
What are molecules with both hydrophilic and hydrophobic regions referred to as?
Amphipathic.
Are phospholipids amphipathic?
Yes
Explain how amphipathic molecules behave in an aqueous environment
The molecules spontaneously arrange themselves into various structures that have the polar head groups on the outside interacting with water and the nonpolar tail groups grouped together on the inside away from water.
What are micelles?
The shape of the structure is determined by the bulkiness of the head group relative to that of the hydrophobic tails. Lipids with bulky heads and a single hydrophobic fatty acid tail are wedge-shaped and pack into spherical structures called micelles.
What is a bilayer?
Lipids with less bulky head groups and two hydrophobic tails forms a bilayer. A lipid bilayer is a 2 layered structure organized in such a way that the hydrophilic portion of the lipid faces out toward the aqueous environment and the hydrophobic portion f
Do bilayers form closed structures?
Yes. Bilayers form closed structures with an inner space since free edges would expose the hydrophobic chains to the aqueous environment. This organization partly explains why bilayers are effective cell membranes. It also explains why membranes are self
Explain why membranes can be called self healing
Small tears in a membrane are rapidly sealed by a spontaneous rearrangement of the lipids surrounding the damaged region, due to the tendency of water to exclude nonpolar molecules.
Explain the importance of pH to phospholipids
The phospholipid bilayer only forms spontaneously as long as the concentration of free phospholipids is high enough and the pH of the solution is similar to that of the cell. The pH is important because it ensures that the head groups are in their ionized
What happens when phospholipids are added to a test tube of water at neutral pH?
They spontaneously form enclosed bilayer structures called liposomes. An enzyme is not required to catalyze any chemical reaction.
Does the membrane enclosure require an enzyme?
No, an enzyme is not required to catalyze any chemical reaction. The membrane enclosure forms by a process of self-assembly that depends only on the chemical properties of the lipids. As the liposomes form, they may capture macromolecules present to solut
T/F: Experiments show that liposomes can form and re-form in environments like tidal flats where wetting and drying occur repeatedly
True. Liposomes can even grow, incorporating more and more lipids from the environment. Also, liposomes can incorporate nucleic acids and other molecules into their interiors.
T/F: Depending on their chemical composition, early membranes might have been either leaky or almost impervious to the molecules of life
True. Over time, they evolved to allow at least limited molecular traffic between the environment and cell interior.
T/F: Lipids freely associate with one another because of extensive van der Waals forces between their fatty acid tails
True. These weak interactions are easily broken and re-formed, so lipid molecules are able to move within the plane of the membrane.
T/F: A single phospjolipid can move across the entire length of a bacterial cell in less than a second
True. Lipids can also rapidly rotate around their vertical axis, and individual fatty acid chains are able to flex of bend. As a result, membranes are dynamic, forming and re-forming continually during the lifetime of a cell
Why are membranes said to be fluid?
Because membrane lipids are able to move in the plane of the membrane. The degree of membrane fluidity depends on which types of lipid make up the membrane.
T/F: In a single layer of the lipid bilayer, most of the van der Waals and hydrophobic interactions occur between the hydrophobic tails of lipids
True. The strength of these interactions depends on
1) the length of the fatty acid tails
2) the presence of double bonds between neighboring carbon atoms
T/F: longer fatty acid tails tend to reduce lipid mobility
True. The longer the fatty acid tails, the more surface is available to participate in van der Waals interactions. The tighter packing that results tends to reduce lipid mobilitiy.
T/F: Saturated fatty acid tails, which have no double bonds, are straight and tightly packed, reducing mobility
True
T/F: Unsaturated fatty acids introduce kinks in the fatty acid tails, reducing the tightness of packing and enhancing lipid mobility in the membrane
True
T/F: In addition to phosphlipids, cell membranes often contain other types of lipid, and these can also influence membrane fluidity
True. For example, cholesterol is 30% by mass of the membrane lipids.
T/F: Like phospholipids, cholesterol is amphipathic, with both hydrophilic and hydrophobic groups in the same molecule
True. In cholesterol the hydrophoilic region is a hydroxyl group (OH) and the hydrophobic region consists of 4 interconnected carbon rings with an attached hydrocarbon chain. This structure allows cholesterol to insert into the lipid bilayer.
T/F: The hydrophilic head group of cholesterol interacts with the hydrophilic head group of phospholipids, while the ring structure participates in van der Waals interactions with the fatty acid chains
True
T/F: Cholesterol increases or decreases membrane fluidity depending on temperature
1. At normal temperature the interaction of the rigid ring structure of cholesterol with the phospholipid fatty acid tails reduces the mobility of the phospholipids and hence the fluidity of the membrane
2. At low temperatures, cholesterol prevents phosph
T/F: Cholesterol helps maintain a consistent state of membrane fluidity by preventing dramatic transitions from a fluid to a solid state
True
T/F: Recent studies show that specific types of lipid sometimes assemble into defined patches called lipid rafts
True. i.e. Sphingolipids cluster in discrete patches in the membrane.
T/F: Although lipids are free to move in the plane of the membrane, the spontaneous transfer of a lipid between layers of the bilayer, known as flip flop is rare
True. The flip-flop would require the hydrophilic head group to pass through the hydrophobic interior of the membrane. As a result there is little exchange of components between the two layers of the membrane, which allows the two layers to differ in comp
T/F: In many membranes, different types of lipid are present primarily in one layer or the other
True
T/F: Most membranes contain proteins as well as lipids.
True.
Name some functions of membrane proteins
1. Some act as transporters, moving ions or other molecules across the membrane. Transporters include channels that allow movement of molecules through them, and carriers that facilitate movement.
2. Membrane proteins act as receptors that allow the cell
List the two groups membrane proteins can be classified as depending on how they associate with the membrane.
1. Integral membrane proteins are permanently associated with cell membranes and cannot be separated from the membrane experimentally without destroying the membrane itself
2. Peripheral membrane proteins are temporarily associated with the lipid bilayer
T/F: Most integral membrane proteins are transmembrane proteins that span the entire lipid bilayer.
True. These proteins are composed of 3 regions:
2 hydrophilic regions, one protuding from each face of the membrane AND a connecting hydrophobic region that spans the membrane.
T/F: The hydrophilic region on the internal side of the membrane often interacts with other proteins in the cytoplasm of the cell, whereas the hydrophilic region on the external side can interact with signaling molecules
True
T/F: Peripheral membrane proteins may be associated with either the internal or external side of the membrane
True. These proteins interact either with the polar heads of lipids or with integral membrane proteins via weak noncovalent interactions such as hydrogen bonds.
T/F: Peripheral membrane proteins are only transiently associated with the membrane and can play a role in transmitting information received from external signals.
True. Other peripheral membrane proteins limit the ability of transmembrane proteins to move within the membrane and assist proteins to cluster in lipid rafts
How do we know that proteins, like lipids, are free to move in the membrane?
Through FRAP - fluorescence recovery after photobleaching.
T/F: The idea that lipids and proteins co-exist in the membrane, and that both are able to move in the plane of the membrane led Singer and Nicolson to propose the fluid mosaic model
True. The model says the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane, and is a mosaic (mixture) or 2 types of molecules, lipids and proteins
What is the plasma membrane? (also known as cell membrane)
It is the boundary that defines the space of the cell, separating its internal contents from the surrounding environment. The plasma membrane also actively maintains intracellular conditions compatible with life (homeostasis)
What is the cell wall?
The cells of many different groups of organisms have a cell wall external to the plasma membrane. The cell wall plays important role in maintaining the shape and internal composition of these cells.
T/F: The plasma membrane maintains homeostasis
True. The active maintenance of a constant environment is known as homeostasis. i.e. Chemical reaction and protein folding only takes place in an narrow range of conditions.
What is the selective barrier?
It allows plasma membrane to let molecules in and out freely. It lets them in and out only under certain conditions and prevents some molecules from passing through at all
T/F: The membrane's ability to act as a selective barrier is result of combination of lipids and embedded proteins that make it up
True.
1. The hydrophobic lipid bilayer prevents ions as well as charged or polar molecules from diffusing freely across the plasma membrane.
2. Many macromolecules such as proteins and polysaccharides are too large to cross plasma membrane on their own
3.
What is the simplest form of movement into and out of cells?
Passive transport, which works by diffusion (the random movement of molecules)
Define diffusion
The random motion of individual molecules, with net movement occurring where there are areas of higher and lower concentration
1. Movement from higher solute concentration to lower solute concentration.
T/F: Diffusion occurs even in the absence of concentration differences, due to the random motion of molecules
True, BUT in this case there is no net movement in the substance.
What is facilitated diffusion?
Diffusion through a membrane protein, bypassing the lipid layer. In the case of facilitated diffusion, the molecule moves through a membrane protein channel or carrier (vs. simple diffusion where molecule moves directly through lipid bilayer.
T/F: Water moves into and out of cells by passive transport
True. Although the plasma membrane is hydrophobic, water molecules are small enough to move passively through the membrane to a limited extent by simple diffusion.
What are aquaporins?
They allow water to flow through the plasma membrane more readily by facilitated diffusion.
What is osmosis?
The diffusion of water through a semipermeable membrane is known as osmosis. Water moves from regions of higher water concentration to regions of lower water concentration OR you could say water moves from regions of lower solute concentrations to higher
Define active transport:
The movement of substances from low concentration to high concentration (going uphill against the concentration gradient) across the cell membranes that requires energy (ATP). 1/3 of energy made by cell is used for active transport. Uses a protein carrier
T/F: During active transport, cells move substances through transport proteins embedded in the cell membrane
True. Some of these proteins act as pumps, using energy directly to move a substance into or out of a cell. i.e. the sodium-potassium pump (moves sodium out of cell and potassium into cell against concentration gradient)
What is primary active transport?
Active transport that uses the energy of ATP directly. When protein transporters move ions in different directions (i.e. sodium and potassium) they are called antiporters. When moved in same direction called cotransporters.
What are electrochemical gradients?
A gradient that combines the charge gradient and the chemical gradient of protons and other ions.
What is secondary active transport?
Secondary active transport uses the energy of an electrochemical gradient to drive the movement of molecules (contrasted to primary active transport uses the energy of ATP directly.
T/F: The use of an electrochemical gradient as a temporary energy source is a common cellular strategy
True. i.e. cells use the sodium electrochemical gradient generated by the sodium-potassium pump to transport glucose and amino acids into cells.
Define hypertonic solution
Hypertonic solutions have a higher solute concentration. This causes the water inside a cell to use osmosis and get out of the cell. In this case the cell flattens or shrinks known as plasmolosis.
In a hypotonic solution there is a higher solute concentra
What is a contractile vacuole?
A type of cellular compartment that takes up excess water and waste products from inside the cell and expels them into the external environment. Helps the cell to not burst from too much water
The contractile vacuole of some organisms takes in water throu
How do organisms without contractile vacuole's maintain their cell shape?
The cell wall, a rigid structure that surrounds the plasma membrane, plays a critical role in the maintenance of cell shape and volume.
How does the cell wall provide structural support and protection to the cell?
Because the cell wall is fairly rigid and resists expansion, it allows pressure to build up in a cell when it absorbs water.
What is turgor pressure?
The force exerted by water pressing against an object results in hydrostatic pressure aka turgor pressure. The pressure exerted by water provides structural support for many organisms that is similar to the skeletons of shells or animals
What is the vacuole?
Many plant cells have vacuole's that also absorb water and contribute to turgor pressure.
Why do plants wilt when dehydrated?
The loss of water from the vacuoles reduces turgor pressure and the tissue can no longer maintain rigidity
What is the cell wall made of?
Many different components, including carbohydrates and proteins. The plant cell wall is composed of polysaccharides (such as cellulose). Fungi have cell walls made of chitin. In bacteria, the cell wall is made of peptidoglycan.
T/F: Animal cells do not have plant walls
True. Only plants
T/F: All cells have a plasma membrane and contain genetic material
True. Only eukaryotes have a nucleus (where the genetic material is housed)
List some differences between prokaryotes and eukaryotes
1. No nucleus or extensive internal compartmentalization in prokaryotes. Prokaryotes use different transcription process. Most prokaryotes do not synthesize sterols, but some instead synthesize hopanoids. (5 ringed structures similar to cholesterol)
Where is the genetic material in a prokaryote concentrated?
In a discrete region of the cell interior known as the necleoid.
What are plasmids?
Bacteria often contain additional small circular molecules of DNA known as plasmids that carry a small number of genes. Plasmids are commonly transferred between bacteria through threadlike, hollow structures known as pilli which extend from one cell to a
T/F: Prokaryotes are small compared to eukaryotes
True. Eukaryotic cells are usually 10 times larger in diameter and 1000 times larger in volume.
T/F: Prokaryotes have a large amount of membrane surface area for absorption relative to the volum of the cell that it serves
True. this makes sense for an organism that absorbs nutrients through its environment
T/F: The nuclear membrane allows for more complex regulation of gene expression than is possible in prokaryotic cells
True.
What are organelles?
Little organs" - Small structures found floating in the cytoplasm along with the nucleus.
What is the endoplasmic reticulum?
Organelle involved in the synthesis of proteins and lipids. The Endoplasmic Reticulum is a system of membrane channels that are a continuation of the nuclear envelope.
What is the Golgi apparatus?
Modifies proteins and lipids produced by the ER and acts as a sorting station as they move to their final destination in or out of the cell
What are lyosomes?
They contain enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates
What are Mitochondria?
Specialized organelles that harness energy for the cell. Produces most of the ATP that serves as the energy currency of the cell.
What is the cytoskeleton?
- Helps cells maintain their shape and serves as a network of tracks for the movement of substances within cells.
- Some cells even move with the help of the cytoskeleton.
- Is a network of protein filaments and other associated proteins that provide the
What are chloroplasts?
Convert energy of sunlight into chemical energy.
What is the cytoplasm?
The entire contents of a cell other than the nucleus make up the cytoplasm.
What is the cytosol?
The region of the cell inside the plasma membrane but outside the organelles is referred to as the cytosol. This is the jelly-like internal environment surrounding the organelles.
T/F: In eukaryotes, the total surface area of intracellular membranes is about tenfold greater than that of the plasma membrane
True. This high ratio shows the significant degree to which a eukaryotic cell is divided into internal compartments.
T/F: Many of the organelles inside cells are not distinct, isolated entities, but instead communicate with one another
True. The membranes of these organelles are either physically connected by membrane bridges or communicate by the budding off and fusing of vesicles (small membrane enclosed sacs that transport substances).
What is the endomembrane system?
Includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, the plasma membrane, and the vesicles that move between them.
T/F: Because of the selective permeability of cell membranes, the endomembrane divides the interior of a cell into two distinct worlds, one inside the spaces defined by these membranes and one outside these spaces
True. Molelcules in the cytosol are in a different physical space, separated by membranes of the endomembrane system (i.e. a molecule within the ER can stay in the ER or end up within the Golgie apparatus etc.)
What is exocytosis?
When waste particles are released from cell as vesicles
Fuses with membrane and is then released.
Provides a way for a vesicle to empty its contents to the extracellular space
What is endocytosis?
When large molecules (such as food) move into the cell
Membrane engulfs it and moves it in
A vesicle can bud off from the plasma membrane, bringing material from outside the cell into a vesicle, which can then fuse with other organelles.
T/F: Together, exocytosis and endocytosis provide a way to move material into and out of cells without passing through the cell membrane
True
What is the nuclear envelope?
Defines the boundary of the nucleus. It consists of two membranes and each is a lipid bilayer with associated proteins.
1. Inner membrane
2. Outer membrane
What are nuclear pores?
These pores act as gateways that allow molecules to move into and out of the nucleus, and are essential for the nucleus to communicate with the rest of the cell. RNA molecules leave out of the nucleus (and control of how and when this info is expressed de
What is the endoplasmic reticulum?
It produces and transports many of the lipids and proteins found inside and outside the cell. It is the site of production of most of the lipids that make up the various cell membranes. In addition, transmembrane proteins and proteins destined for the Gol
What does the ER consist of?
A complex network of interconnected tubules and flattened sacs. The interior of the ER is continuous throughout and is called the lumen. Has a mazelike appearance.
Why is the ER membrane extensively convoluted?
Allows a large amount of membrane surface area to fit within the cell.
T/F: The amount of ER membrane in a cell varies among cells of different functions
True. In cells that do not secrete large quantities of protein, the ER can be quite small.
What is the rough endoplasmic reticulum
The majority of ER membranes have small, rounded particles associated with them that are exposed to the cytosol. The particles bound to the cytosolic face of the RER are ribosomes. Ribosomes are the site of protein synthesis.
What is the smooth endoplasmic reticulum
In most cells there is a small amount of ER membrane that lacks ribosomes and is consequently called smooth endoplasmic reticulum. The smooth ER connects the rough Endoplasmic Reticulum to the Golgi Apparatus. The smooth ER has diverse metabolic functions
T/F: Smooth ER dominates in cells specialized for the production of lipids
True
What does the Golgi apparatus do?
Here, Lipids and proteins are successively modified and delivered to their final destinations. The Golgi apparatus has 3 roles
1) It further modifies proteins and lipids produced by the ER
2) It acts as a sorting station as they move to their final destin
What are cisternae?
Under the microscope, Golgi apparatus looks like a series of flattened membrane sacs, which are called cisternae. The cisternae are stacked and surrounded by many small vesicles. These vesicles transport proteins from the ER to the Golgi apparatus, betwee
T/F: Vesicles are the primary means by which proteins and lipids move through the Golgi apparatus to their final destinations
True
T/F: Enzymes within the Golgi apparatus chemically modify proteins and lipids as they pass through it
True. These modifications are sequential since each region of the Golgi apparatus contains a different set of enyzmes that catalyze specific reactions.
What are lysosomes?
Lysosomes are specialized vesicles derived from the Golgi apparatus that degrade damaged or unneeded macromolecules. They contain a variety of enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates.
T/F: Macromolecules destined for degradation are packaged by the Golgi apparatus into vesicles. The vesicles then fuse with lysosomes, delivering their contents to the lysosome interior.
True. This illustrates the ability of the Golgi apparatus to sort key proteins.
T/F: The function of lysosomes underscores the importance of having separate compartments within the cell bounded by selectively permeable membranes
True. Many of a cell's enzymes and proteins would unfold and degrade if the entire cell were at a ph of 5, and lysosomal enzymes cannot function in the normal cellular environment, which has a pH of about 7. By restricting the activity of these enzymes to
What is protein sorting?
The process by which proteins end up where they need to be to perform their function.
What are signal sequences?
Proteins are often directed to their proper cellular compartments by means of particular amino acid sequences called signal sequences. Proteins with no signal sequence remain in the cytosol. Most proteins with a signal sequence at their amino ends are tar
T/F: Proteins targeted to the nucleus usually have signal sequences located internally
True
What is a nuclear localization signal?
The signal sequence for the nucleus, which enables proteins to move through pores in the nuclear envelope.
What is the SRP?
Signal Recognition Particle that binds to both the signal sequence and the ribosome and brings about a pause in translation.
T/F: The membranes of two organelles, mitochondria and chloroplasts are not part of the endomembrane system
True. Both of these organelles are specialized to harness energy for the cell. They are both semi-autonomous organelles taht grow and multiply independently of the other membrane-bound compartments, and they contain their own genomes.
What is the thylakoid?
a membrane sack, sight of light absorption, and contains chlorophyll
What is chlorophyll?
A green pigment that captures the energy from the sunlight. Aids in the photosynthesis process.
How are micelle's different from a liposome?
1. They are composed of a single layer of phospholipids
2. Micelle phospholipids have bulky head groups
3. Micelle phospholipids have a single fatty acid tail
What is turgor pressure the result of?
Water accumulation within a cel
What factors are required for facilitated diffusion to occur?
- A plasma membrane
- A concentration gradient
- A transport protein
A cell's plasma membrane contributes to homeostasis by:
Acting as a selective barrier
A phospholipid molecule in a membrane can:
both spin (rotate around its vertical axis) and move side-to-side (lateral movement).
Some lipid rafts are characterized by an accumulation of cholesterol. What does this mean for the fluidity of the raft domain?
These lipid rafts are less fluid than the surrounding membrane at normal temperatures.
If cells had monolayer membranes reminiscent of micelles, how would the structures of transmembrane proteins be affected?
Transmembrane proteins would possess a hydrophobic region in the cell interior and a hydrophilic region in the extracellular space.
How is a carrier protein different from a channel protein?
Carriers undergo a conformation change.
Which of the following cellular processes would tend to reduce entropy?
both transcription in the nucleus and translation in the endoplasmic reticulum
Which of the following molecules would not easily diffuse across a plasma membrane?
large polar molecules
During osmosis, water moves from a region of _________ to a region of _________.
high solvent concentration; low solvent concentration" AND "low solute concentration; high solute concentration" are both correct answers.
According to the textbook, prokaryotes absorb their nutrients from the environment; however, prokaryotes also have cell walls. What can you conclude from these statements?
Cell walls are permeable, allowing nutrients to pass through.
How is the energy stored in a molecule of ATP used by the sodium-potassium pump protein?
It is used to alter the conformation of the pump protein.
What is secondary active transport?
the use of the electrochemical gradient of one molecule to move a second molecule
A researcher is very surprised to find a prokaryotic cell that is greater than 100 ?m in diameter. How could this be possible?
There may be a large vacuole in the cell, which allows nutrients to still reach all areas of the cytoplasm in the cell.
Which of the following lipid compositions would yield the greatest membrane fluidity?
Phospholipids with short-chain, unsaturated fatty acids, plus cholesterol at low temperature
What would be the outcome if the pump protein shown in Fig. 5.13a instead acted as a carrier that allowed protons to move from the extracellular fluid into the cytoplasm?
Two answer options are true: ATP could be synthesized from ADP + Pi AND the number of protons on either side of the membrane would equalize.
A researcher is using a modified version of FRAP. He has labeled all peripheral membrane proteins with a fluorescent tag, and has bleached a specific area of the cell membrane. After 24 hours, he notices that fluorescence has returned to the bleached area
Peripheral membrane proteins may not be permanently associated with the cell membrane.
The process of diffusion requires:
the random movement of molecules.
Predict how phospholipids would arrange themselves if they were placed into a nonpolar solution.
The phospholipid tails would orient toward the solution.