Final Exam Questions Flashcards
What biological processes are membranes critical to?
Membranes form a barrier between the cells/organelles and the outside environment. This barrier can control movement of molecules into or out of that space.
This movement is facilitated by either passive or active transport.
Membranes also contain receptor proteins, which sit on the outer edge of the membrane and interact with signaling molecules to form a reaction inside the cell. This allows the cell to react to any environmental changes.
Membranes, particularly within organelles provide surface area for a reaction to take place. The more membrane, the larger the more products the reaction can produce.
Describe the structure and chemical composition of a membrane.
The basic structure of a cell membrane is a lipid bilayer containing hydrophilic heads and hydrophobic tails. Intermixed in the inner membrane is cholesterol, which provides fluidity.
The membrane is full of proteins:
-Integral: embedded within the lipid bilayer, function as channels
-Peripheral: on the surface interact with integral proteins
-Glycoproteins: on the cell surface attached to carbohydrate chains, involved in cell signaling
What mathematical equation best describes the chemical potential for an uncharged and a charged solute to cross the membrane by diffusion alone? Why do charged molecules diffuse differently to non-charged molecules?
The Nernst equation.
Charged molecules diffuse differently because they are not just affected by the physical concentration gradient, but by an electrochemical one as well. A charge difference across a membrane can influence the movement on charged particles.
What is passive and active transport?
Passive: movement down the electrochemical gradient from a more positive μj to a more negative μj
Active: movement against the electrochemical gradient from a more negative μj to a more positive μj
Distinguish between protein channels, passive carriers, and pumps. Describe the structure and the function of each.
Protein channels: Integral membrane proteins that allow the passive transport of specific molecules to pass through based on their properties.
Passive carriers: Undergo conformational changes in response to solute binding, facilitating the passive transport of molecules down their concentration gradients.
Pumps: Actively transport ions or molecules against their concentration gradients, requiring energy.
How is membrane voltage generated, and how is it used to transport substances across the membrane?
Membrane voltage is generated by the difference in charge across a membrane. Active transport pumps move ions across the membrane, creating a charge gradient.
The charge difference can encourage passive transport of charged ions. Ion channels also change position depending on the charge gradient.
Which membranes have transporters? Are they all in the same orientation?
What are the 3 types?
Both the outer membranes around a cell and the membrane surrounding organelles have transporters. One of the main purposes of a membrane is to transport molecules across.
The types of transporters include:
Uniporters: facilitate the movement of a single type of solute in one direction across the membrane.
Symporters: move two different types of solutes in the same direction across the membrane.
Antiporters: move two different types of solutes in opposite directions across the membrane.
What are the three main steps in plants for nitrogen assimilation?
NO₃⁻ -> NO₂⁻ -> NH₄⁺ -> amino acids
Nitrate Reduction: Plants take up nitrate (NO₃⁻) from the soil. Nitrate (NO₃⁻) is reduced to nitrite (NO₂⁻) by the enzyme nitrate reductase.
Nitrite Reduction: Nitrite (NO₂⁻) is further reduced to ammonium (NH₄⁺) by the enzyme nitrite reductase.
Ammonium Incorporation into Amino Acids: Ammonium produced through nitrite reduction can be incorporated into organic compounds, particularly amino acids.
Describe the structure and chemical reactions of nitrate and nitrite reductase.
What are the chemical formulas? Where do the reactions take place?
Nitrate Reduction:
NO3- + NAD(P)H + H+ 2e- ->NO2- + NAD(P)+ + H2O
cytoplasm
Nitrite Reduction:
NO2- +6FDred + 8H+ +6e- ->NH4+ +6FDox + 2H2O
chloroplast
NO3 -> NO2 -> NH4+ -> amino acids
Draw the two pathways for assimilation of ammonia into the amino acids glutamate and glutamine. Why would a plant choose one pathway over another?
draw picture
When lots of ammonia is present, the pathway prefers to produce more glutaimine, when there is less amonia present, glutamate is prefered.
How are other amino acids produced from assimilated nitrogen?
Once plants assimilate nitrogen in the form of ammonia into amino acids like glutamate and glutamine these serve as precursors for the creation of a number of other amino acids through various biochemical processes.
What forms of nitrogen are found in in the plant phloem? Why are some nitrogen compounds better than others for transport?
Some forms are better than other based on factors like solubility, stability, and toxcicity.
Water soluble forms are better transported over long distances. The same can be said for stable, non-reactive compounds. Nitrite and ammonia are toxic, while nitrate is not.
Describe the enzyme that fixes nitrogen from N2, where is it found?
Nitrogenase is typically found in certain nitrogen-fixing bacteria and archaea, as well as in the symbiotic nodules of leguminous plants.
It contains a MoFe protein, which is responsible for the actual reduction of N₂ to ammonia (NH₃).
How has nitrogen fixation by plants through symbiosis evolved? Compare this to the evolution of mycorrhizal fungi symbiosis? Why do plants have so many symbiotic relationships?
The bacteria provide usable nitrogen while the plant provides safety and sugars.
To facilitate this process, NOD factors are used. NOD factors act as a unique chemical password used to communicate between the plants and the bacteria. During the process of nodulation a plant root hair grows out towards a bacterial grouping and circles around it. Then, the plant breaks down its own cell wall, leaving only the cell membrane to let the bacteria inside. The bacteria move through the hair until they are transported in golgi vesicles and are fused with the cell membrane. From here they can continue through the cell layers.
For fungi, the mycorrhiza is sent into the plant cortex by breaking down portions of the cell wall (the plant helps with this). The fungi can sense nutrient availability across long distances and move it towards the host plant. In return, the plant provides sugars from photosynthesis to the fungi. This relationship is ancient and shows up in all plant groups.
Draw a picture of a cell, including primary, secondary walls and middle lamella
insert picture
What is the chemical and physical structure of a cellulose microfibril, and how is it made?
Can be moved away from each other vertically, but not slid horizontally. They sit on top of one another and are bonded together using hydrogen bonds. Trying to slide them apart horizontally creates a series of bonds breaking and reforming, whereas separating them vertically (pulling the top one off of the bottom) only has to break the bonds once.
They are made in the plasma membrane by large protien complexes called cellulose synthesis complexes.
What are hemicelluloses and pectins and how are they made and delivered to the cell wall?
Hemicellulose: a branched polysaccharide that is a major component of the cell wall
Pectin: a monosaccharide derived from glucose
Both are synthesized in the golgi apparatus. After synthesis, vesicles transport them to the cell membrane and release their contents into the cell wall matrix where the hemicellulose and pectin become integrated into the structure.
How are hemicelluloses different to pectins?
What are they made of? Where are they located?
Hemicellulose:
-Polysaccharides
-Composed of various sugar molecules
-Found all over the cell wall
-Bind to cellulose microfibrils in the cell wall to help with flexibility
Pectin:
-Monosaccharides
-Made of only glucose
-Mostly in the middle lamela
-Form a gel-like matrix that holds water & helps with cell adhesion
What kinds of sugars and other molecules make up hemicelluloses and pectins?
Hemicellulose can be composed of a variety of sugars. Some common ones are xylose, mannose, arabinose, and glucose.
Pectins are made almost entirely of glucose, but may have side chains of various sugars.
How is cellulose synthesis different from hemicellulose and pectin?
Location: Hemicelluloses and pectins are synthesized in the Golgi apparatus, while cellulose is synthesized in the plasma membrane.
Branching: Hemicellulose is branching, pectin has various structures, and cellulose is linear.
How does the cellulose microfibril orientation influence cell growth, shape, and function?
What else are microfibrils known for?
Cellulose microfibrils act like a scaffolding for cell expansion and growth. The direction of the microfibrils indicate to the cell which way to expand. The tensile strength of the microfibrils within the cell walls determines the direction, shape, and extent of the expansion.
Microfibrils are also involved in cell division, turgor pressure, thigmotropism, and gravitropism.
Describe the acid growth hypothesis and the role of expansins.
The acid growth hypothesis proposes that the expansion of plant cell walls during elongation is driven by changes in pH.
Proton pumps actively transport protons into the cell wall, leading to its acidification. This lowered pH activates expansin enzymes, which disrupt bonds between cellulose microfibrils and other cell wall components, resulting in cell wall loosening. As the cell wall becomes more flexible, the cell can take up water through osmosis, increasing turgor pressure. The combined effects of cell wall loosening and water uptake lead to controlled and reversible cell elongation.
How are secondary cell walls different to primary cell walls?
Primary cell walls:
-Formed during cell growth and expansion
-Consist of cellulose microfibrils embedded in a matrix of hemicelluloses and pectins.
-Thin, flexible, and contribute to cell shape.
Secondary cell walls:
-Develop in specialized cells during later stages of maturation
-Higher proportion of organized cellulose, and lignin
-Provide increased rigidity and strength, serving a structural role, offering enhanced mechanical support and protection to cells.
Define photomorphogenesis, and describe light grown vs dark grown seedlings. What is the role of phytochrome in photomorphogenesis?
Photomorphogenesis: refers to the developmental processes and morphological changes in plants that are influenced by light.
When seedlings are grown in a light environment, they typically have a short hypocotyl, open cotyledons, and greening of vegetative structures. Seedlings grown in the dark may have closed cotyledons, an extended hypocotyl, and pale coloration.
Phytochrome is a photoreceptor protein that plays a central role in mediating light responses in plants. There are two forms, Pr and Pfr.
Red light response:
When phytochrome absorbs red light (660nm) Pr is converted to Pfr. This activates a light grown response.
Far-red light response:
When phytochrome absorbs far-red light (730nm) Pfr converts to Pr. This is the inactive form and is associated with a dark growth response.
