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3.2: Membranes - Biology

3.2: Membranes - Biology


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Membrane Lipids

The cell membrane is a dynamic structure composed of lipids, proteins, and carbohydrates. We will first investigate the anatomy of the cell membrane and then continue on to study the physiology of membrane transport.

The phospholipid bilayer is the main fabric of the membrane. The bilayer’s structure causes the membrane to be semi-permeable. Remember that phospholipid molecules are amphiphilic, which means that they contain both a nonpolar and polar region. Phospholipids have a polar head (it contains a charged phosphate group) with two nonpolar hydrophobic fatty acid tails. The tails of the phospholipids face each other in the core of the membrane while each polar head lies on the outside and inside of the cell. Having the polar heads oriented toward the external and internal sides of the membrane attracts other polar molecules to the cell membrane. The hydrophobic core blocks the diffusion of hydrophilic ions and polar molecules. Small hydrophobic molecules and gases, which can dissolve in the membrane’s core, cross it with ease.

Other molecules require proteins to transport them across the membrane. Proteins determine most of the membrane’s specific functions. The plasma membrane and the membranes of the various organelles each have unique collections of proteins. For example, to date more than 50 kinds of proteins have been found in the plasma membrane of red blood cells.

Importance of Phospholipid Membrane Structure

What is important about the structure of a phospholipid membrane? First, it is fluid. This allows cells to change shape, permitting growth and movement. The fluidity of the membrane is regulated by the types of phospholipids and the presence of cholesterol. Second, the phospholipid membrane is selectively permeable.

The ability of a molecule to pass through the membrane depends on its polarity and to some extent its size. Many non-polar molecules such as oxygen, carbon dioxide, and small hydrocarbons can flow easily through cell membranes. This feature of membranes is very important because hemoglobin, the protein that carries oxygen in our blood, is contained within red blood cells. Oxygen must be able to freely cross the membrane so that hemoglobin can get fully loaded with oxygen in our lungs, and deliver it effectively to our tissues. Most polar substances are stopped by a cell membrane, except perhaps for small polar compounds like the one carbon alcohol, methanol. Glucose is too large to pass through the membrane unassisted and a special transporter protein ferries it across. One type of diabetes is caused by misregulation of the glucose transporter. This decreases the ability of glucose to enter the cell and results in high blood glucose levels. Charged ions, such as sodium (Na+) or potassium (K+) ions seldom go through a membrane, consequently they also need special transporter molecules to pass through the membrane. The inability of Na+ and K+ to pass through the membrane allows the cell to regulate the concentrations of these ions on the inside or outside of the cell. The conduction of electrical signals in your neurons is based on the ability of cells to control Na+ and K+ levels.

Selectively permeable membranes allow cells to keep the chemistry of the cytoplasm different from that of the external environment. It also allows them to maintain chemically unique conditions inside their organelles.

Fluidity of Cell Membranes

The cell membrane is not a static structure. It is a dynamic structure that allows the movement of phospholipids and proteins. Fluidity is a term used to describe the ease of movement of molecules in the membrane and is an important characteristic for cell function. Fluidity is dependent on the temperature (increased temperatures it more fluid and decreased temperatures make it more solid), saturated fatty acids and unsaturated fatty acids. Saturated fatty acids make the membrane less fluid while unsaturated fatty acids make it more fluid. The correct ratio of saturated to unsaturated fatty acids keeps the membrane fluid at any temperature conducive to life. For example, winter wheat responds to decreasing temperatures by increasing the amount of unsaturated fatty acids in cell membranes to prevent the cell membrane from becoming too solid in the cold. In animal cells, cholesterol helps to prevent the packing of fatty acid tails and thus lowers the requirement of unsaturated fatty acids. This helps maintain the fluid nature of the cell membrane without it becoming too liquid at body temperature.


Biological membrane

A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates cell from the external environment or creates intracellular compartments. Biological membranes, in the form of eukaryotic cell membranes, consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions. The bulk of lipid in a cell membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to surface of integral membrane proteins. The cell membranes are different from the isolating tissues formed by layers of cells, such as mucous membranes, basement membranes, and serous membranes.


Wilting refers to the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells. The process of wilting modifies the leaf angle distribution of the plant (or canopy) towards more erectophile conditions.

Lower water availability may result from:

    conditions, where the soil moisture drops below conditions most favorable for plant functioning
  • the temperature falls to the point where the plants vascular system can not function.
  • high salinity, which causes water to diffuse from the plant cells and induce shrinkage
  • saturated soil conditions, where roots are unable to obtain sufficient oxygen for cellular respiration, and so are unable to transport water into the plant or or fungi that clog the plant’s vascular system.

Wilting diminishes the plant’s ability to transpire and grow. Permanent wilting leads to plant death. Symptoms of wilting and blights resemble one another.

In woody plants, reduced water availability leads to cavitation of the xylem.

Wilting occurs in plants such as Balsam and tulasi.

7 reasons why plants wilting and how to fix them

  1. Overwatering – this is a common mistake with growing indoor plants. We often water them the same as we would those growing outdoors but forget that evaporation is much lower inside. So plants end up sitting in very moist soil and their roots begin to struggle.
    Overwatering is also a common mistake early in Spring as gardeners adjust to their plant’s requirements. We’re eager to see them grow that we begin watering a little too early.
  2. Lack of water – the flip side of the first problem is not watering them enough. If your plants are wilting because the soil has become too dry then the obvious solution is to begin watering them and keep this constant until the plant picks up again.
    Container plants have a knack of drying out quicker than those growing in the ground. So, the best way to resuscitate your pot plants is to plunge them into a bucket of water and hold until all the air bubbles have subsided. Note: this is only for extreme cases.
  3. Too much sun – plant wilt often happens when you’re growing them in the wrong position or if indoors, the plant is too close to a window. Too much sun for a shade loving plant is like too much social activity for an introvert.
    If outdoors, try moving your plant to another garden bed where it is less likely to be scorched by the sun’s rays. Indoor plants may need to be moved away from the window but still where it can receive some indirect sunlight.
  4. Not enough sun – and this ties in with the overwatering idea. Plants wilt sometimes because they’re not receiving enough sunlight. Picture an extrovert confined to a cubicle office space every day and you’ll understand the problem. The answer, again, is to move them.
  5. Rootbound plants – often plants can outgrow their containers if they’re not transplanted very year or two. Once a plant gets too large for its pot it struggles to draw nutrients and moisture from the soil – if there is any left, that is.
    The answer is to repot your plant into a larger container and use some quality potting mix as its growing medium.
  6. Too much fertiliser – overzealous gardeners can cause plant wilt just by feeding it too much. When adding fertiliser to a plant’s growing medium, whether it be soil or potting mix, take into account the size of the plant and when you last fed it. Plants don’t usually become obese, they just die.
    Try using slow release fertilisers where possible and usually they should only be added at the start of the growing season and again during flowering times.
  7. Disease – plants can often wilt as a result of an infection as well. There are a few main types of plant wilt related to disease, namely – Fusarium wilt which is a fungal disease common to cotton, tomatoes and palms. This type of wilt can be controlled via a fungicide which should be used as per the directions. Other forms are Bacterial wilt and Verticillium wilt.

So, finding your favourite plant wilt doesn’t mean it’s the end of it. There are some things you can do to try and save all your effort and hopefully turn your plant around so that it blooms another day.


Ehrenfest&rsquos Definition of Phase Transition

The Austrian physicist Paul Ehrenfest provided a definition for phase transitions, where phase behavior is described in terms of free energy ( &Delta G ) as a function of different thermodynamic variables. The &ldquoorder&rdquo is the derivative of &Delta G (as a function of some physical or chemical variable) and the point at which it is discontinuous is the point of phase change [ 4 ] .

Physical state phase changes, such as the transition between a solid, liquid, gas (or liquid to gel state of a lipid bilayer) are first order phase transitions because they are discontinuous in density at their Tm, which is the first derivative of &Delta G with respect to chemical potential.

In a second order phase transition, the first derivative of &Delta G is continuous but the second derivative is discontinuous for the second derivative of &Delta G . Magnetization is an example of a second order phase transition process.
The phase transition of gel to liquid is a first order process and involves a latent heat, in which the thermodynamic system (membrane) absorbs or emits a fixed amount of enthalpy &DeltaS. During phase transition, the temperature stays constant while heat is added to the system [ 4 ] .


A simplified diagram of phase change

The heat flow required for a first order phase change can be modeled as T&DeltaS. Plotting heat flow versus temperature shows intuitively the energy required for phase transition, with an integral of the curved area representing the free energy of the process:



Hysteresis is the phenomenon where there is a difference in Tm depending on which direction the phase transition is going (a difference between Tliquid&rarrgeland Tgel&rarrliquid).This phenomenon occurs only in first and not second order phase transitions [ 5 ] .


The Nitrogen Cycle

Getting nitrogen into living organisms is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere (where it exists as tightly bonded, triple covalent N2) even though this molecule comprises approximately 78 percent of the atmosphere. Nitrogen enters the living world through free-living and symbiotic bacteria, which incorporate nitrogen into their organic molecules through specialized biochemical processes. Certain species of bacteria are able to perform nitrogen fixation, the process of converting nitrogen gas into ammonia (NH3), which spontaneously becomes ammonium (NH4 + ). Ammonium is converted by bacteria into nitrites (NO2 − ) and then nitrates (NO3 − ). At this point, the nitrogen-containing molecules are used by plants and other producers to make organic molecules such as DNA and proteins. This nitrogen is now available to consumers.

Organic nitrogen is especially important to the study of ecosystem dynamics because many ecosystem processes, such as primary production, are limited by the available supply of nitrogen. As shown in Figure 4 below, the nitrogen that enters living systems is eventually converted from organic nitrogen back into nitrogen gas by bacteria (Figure 4). The process of denitrification is when bacteria convert the nitrates into nitrogen gas, thus allowing it to re-enter the atmosphere.

Figure 4. Nitrogen enters the living world from the atmosphere via nitrogen-fixing bacteria. This nitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply terrestrial food webs with the organic nitrogen they need. (credit: “Nitrogen cycle” by Johann Dréo & Raeky is licensed under CC BY-SA 3.0)

Human activity can alter the nitrogen cycle by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and by the use of artificial fertilizers (which contain nitrogen and phosphorus compounds) in agriculture, which are then washed into lakes, streams, and rivers by surface runoff. Atmospheric nitrogen (other than N2) is associated with several effects on Earth’s ecosystems including the production of acid rain (as nitric acid, HNO3) and greenhouse gas effects (as nitrous oxide, N2O), potentially causing climate change. A major effect from fertilizer runoff is saltwater and freshwater eutrophication, a process whereby nutrient runoff causes the overgrowth of algae, the depletion of oxygen, and death of aquatic fauna.

In marine ecosystems, nitrogen compounds created by bacteria, or through decomposition, collects in ocean floor sediments. It can then be moved to land in geologic time by uplift of Earth’s crust and thereby incorporated into terrestrial rock. Although the movement of nitrogen from rock directly into living systems has been traditionally seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process may indeed be significant and should be included in any study of the global nitrogen cycle.


Protein translocation mutants defective in the insertion of integral membrane proteins into the endoplasmic reticulum.

Yeast mutants defective in the translocation of soluble secretory proteins into the lumen of the endoplasmic reticulum (sec61, sec62, sec63) are not impaired in the assembly and glycosylation of the type II membrane protein dipeptidylaminopeptidase B (DPAPB) or of a chimeric membrane protein consisting of the multiple membrane-spanning domain of yeast hydroxymethylglutaryl CoA reductase (HMG1) fused to yeast histidinol dehydrogenase (HIS4C). This chimera is assembled in wild-type or mutant cells such that the His4c protein is oriented to the ER lumen and thus is not available for conversion of cytosolic histidinol to histidine. Cells harboring the chimera have been used to select new translocation defective sec mutants. Temperature-sensitive lethal mutations defining two complementation groups have been isolated: a new allele of sec61 and a single isolate of a new gene sec65. The new isolates are defective in the assembly of DPAPB, as well as the secretory protein alpha-factor precursor. Thus, the chimeric membrane protein allows the selection of more restrictive sec mutations rather than defining genes that are required only for membrane protein assembly. The SEC61 gene was cloned, sequenced, and used to raise polyclonal antiserum that detected the Sec61 protein. The gene encodes a 53-kDa protein with five to eight potential membrane-spanning domains, and Sec61p antiserum detects an integral protein localized to the endoplasmic reticulum membrane. Sec61p appears to play a crucial role in the insertion of secretory and membrane polypeptides into the endoplasmic reticulum.


  • All life on Earth exists as cells. These have basic features in common. Differences between cells are due to the addition of extra features. This provides indirect evidence for evolution.
  • All cells arise from other cells, by binary fission in prokaryotic cells and by mitosis and meiosis in eukaryotic cells.
  • All cells have a cell-surface membrane and, in addition, eukaryotic cells have internal membranes.
  • The basic structure of these plasma membranes is the same and enables control of the passage of substances across exchange surfaces by passive or active transport.
  • Cell-surface membranes contain embedded proteins. Some of these are involved in cell signalling – communication between cells. Others act as antigens, allowing recognition of ‘self’ and ‘foreign’ cells by the immune system. Interactions between different types of cell are involved in disease, recovery from disease and prevention of symptoms occurring at a later date if exposed to the same antigen, or antigen-bearing pathogen.

Included in this download

ModuleTitle
3.2.1Cell structure
3.2.2All cells arise from other cells
3.2.3Transport across cell membranes
3.2.4Cell recognition and the immune system

3.2.4 State one function of glucose, lactose and glycogen in animals, and of fructose, sucrose and cellulose in plants.

In animals, glucose is used as an energy source for the body and lactose is the sugar found in milk which provides energy to new borns until they are weaned. Finally, glycogen is used as an energy source (short term only) and is stored in muscles and the liver.

In plants, fructose is what makes fruits taste sweet which attracts animals and these then eat the fruits and disperse the seeds found in the fruits. Sucrose is used as an energy source for the plant whereas cellulose fibers is what makes the plant cell wall strong.


Accredited Post-Secondary Programs - RPBio

New academic standard requirements for the Biologist in Training (BIT) and Registered Professional Biologist registrant categories came into effect January 1, 2020. In spring of 2021 accredited programs underwent a review to ensure they met to new requirements.

Current College Accredited Institutions and Programs

Institutions and their associated programs in table 1.0 have been accredited by the College. Applicants graduating from these programs:

  • Meet the academic requirements for entry into the College under Stream 1 as a BIT or RPBio if you graduated from the program with a Bachelors degree during the program’s accredited time period and
  • Do not need to submit course descriptions.

Applicants that graduated from programs prior to the year of accreditation or after the program was no longer accredited (January 1, 2022) must apply through stream 2 and include course descriptions with their applications.

Note: Some programs are still undergoing a review and the institution and the College are working together on the accreditation process for the program and are marked [!] below. Applicants from these accredited programs need to apply under Stream 2. We expect to resolve this issue next week.

Table 1.0 Current College accredited institutions and programs for Stream 1 BIT and RPBio applications.

Institution

BC Institute of Technology (BCIT)

Ecological Restoration (BSc) + Fish, Wildlife & Recreation

Accredited (2019) (must complete FWR diploma pathway)

Simon Fraser University (SFU)

Applied Biology Concentration

Simon Fraser University (SFU)

Ecology, Evolution and Conservation stream in Biological Sciences

Accredited (2017) [!] Under Review

For applicants who have completed their degree through SFU in this program from 2017 to 2021 please apply as a BIT or RPBio through Stream 1. SFU and the College are working on the accreditation process for the program.

Thompson Rivers University (TRU)

Natural Resources Sciences

Accredited (2014) [!] Under Review

For applicants who have completed their degree through TRU from 2014 to 2021 please apply as a BIT or RPBio through Stream 1. TRU and the College are working on the accreditation process for the program.

University of Northern BC (UNBC)

Accredited (2014) - Renewed March, 2021

University of Northern BC (UNBC)

Accredited (2014) - Renewed March, 2021

Past Accredited Institutions and Programs

The following programs below in table 2.0 are no longer accredited by the College. At this time applicants from these past accredited programs need to apply under Stream 2 for a Biologist in Training or Registered Professional Biologist. We expect to resolve this issue next week.

Table 2.0 Past College accredited institution and programs for Stream 1 BIT and RPBio applications.

Accredited RPBio/BIT Institution Programs

Accredited Time Frame

Conditional required course(s) the below course(s) must have been taken by an applicant in the program to apply under Stream 1

Major Ecosystem Management (Faculty of Environmental Sciences/Ecosystem Management)

  • Mathematics and
  • 1 of the following 2 nd year or higher courses:
    • Organismal Biology 1 OR
    • Cellular 2

    University of Victoria (UVic)

    Major Biology (Faculty of Science, Department of Biology)

    • Applied Biology 3 2 nd year or higher (BIOL 370 or BIOL 461)
    • Communications 4 and
    • 1 of the following 2 nd year or higher courses:
      • Systematics or Classication 5 (BIOL 324L BIOL 307 BIOL 329 BIOL 312 BIOL 449 or BIOL 355) OR
      • Organismal Biology 1 (BIOL 365 or BIOL 366)

      Vancouver Island University

      University of British Columbia, Okanagan (UBCO)

      Biology Program, Evolutionary Biology Major

      Biology Program, Microbiology Major

      Biology Program, Zoology Major

      Biology Program, Biology Major

      University of British Columbia, Vancouver (UBC)

      Department of Forest and Conservation Sciences, Natural Resources Conservation Program, Science and Management Major

      1 A course in Organismal Biology will focus on the relationship between the structure and functioning of individual organisms relative to the environments they occupy. The majority of course content (i.e., >80%) should consider the physiology of the organism in the context of environmental responses and/or the interaction between anatomical structure and life history including adaptive behaviour. The course can include the study of a single taxonomic group or a broader perspective across a number of Kingdoms. Courses focused exclusively on animal behaviour, systematics, anatomy or ecosystem biology are considered in the context of other Education categories and do not meet the content requirements for Organismal Biology. Typically, a course in physiology meets the requirements for Organismal Biology. Definitions for this suite of courses includes: Cell physiology: Topics should include the cytoskeleton, the cell membrane, cell dynamics, and regulation of cellular activities. Animal physiology: The principles of homeostasis - the regulation of a constant internal state - and the systems involved in maintaining this constant internal environment: cardiovascular, respiratory, osmoregulatory, endocrine, and excitable membranes of nerve and muscle. Plant physiology: Mechanisms and regulation of functional processes contributing to the assimilation, transport and utilization of water, mineral nutrients and carbon by plants. Introduction to the processes involved in growth and development: cell division, tissue culture, meristems, differentiation, the action of major growth regulators, and photomorphogenesis."

      2 A Cellular course will include cellular chemistry, bioenergetics, enzyme production and function, membranes and cell signaling, membrane transport processes, signal transduction mechanisms, extracellular structures (adhesions, junctions, etc.), chemotrophic energy metabolism, intracellular compartments, phototrophic metabolism, structural basis of cellular information, sexual reproduction, gene expression – transcription and protein synthesis, regulation of gene expression, cytoskeletal systems, as well as motility and contractility. Courses which would meet this requirement include genetics, molecular biology, cell biology and, in some cases, biochemistry.

      3 A course in Applied Biology will focus on the application of biological, ecological or socioeconomic principles, including law and governance, to the management or conservation of biological resources, elements or systems. The course can focus on a specific group of organisms or consider broader ecosystem-level issues, however, the majority of the course content (i.e., >80%) must consider biological resources, elements or systems not topics related to the management or conservation of abiotic resources or the more general idea of environmental sustainability. Courses that typically meet the requirements for this subject category include Conservation Biology, Environmental Biology, Wildlife Management, Fisheries Management, Range Management, Natural Resource Policy, or Landscape Ecology.

      4 A course in Communications is focused on communication skills such as English composition, technical writing, journalism, public speaking or use of mass media.


      3.2: Membranes - Biology

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      3.2: Membranes - Biology

      Both prokaryotic and eukaryotic cells possess the basic features of a plasma membrane and cytoplasm. The plasma membrane is the outermost surface of the cell which separates the cell from the environment. The cytoplasm is the aqueous content within the plasma membrane.

      Plasma membrane : It is like any other membrane in the cell but it plays a very important function. It forms the border of a cell, so it is also called the cell membrane. It is primarily composed of proteins and phosphalipid. The phospholipids occur in two layers referred to as a bilayer . Protein is embedded within the lipid layer, or attached to the surface of it. The plasma membrane is elastic and very fluid because of protein and lipid. Normally the function of the plasma membrane is that of a gate-keeper. It allows certain important substances to enter and exit the cell.

      Cytoplasm and organelles : The cytoplasm is a semi-solid substance which is present in the cell and which gives structure, size, shape and foundation to the cell. It is enclosed by the plasma membrane. Within the cytoplasm are a number of microscopic bodies called organelles that perform various functions essential for the survival of the cell.

      Figure 3.2 Endoplasmic reticulum with the nucleus and the Golgi complex

      Endoplasmic reticulum (ER) : is one of the important organelles present in the cytoplasm. Endoplasmic reticulum is a series of membranes which extend throughout the cytoplasm in eukaryotic cells. In certain cases in ER there are submicroscopic bodies called ribosomes which are involved in production of protiens.

      Rough ER : In this kind of ER the ribosomes are presenton the surface.The endoplasmic reticulum is responsible for protein synthesis in a cell. Ribosomes are suborganelles in which the amino acids are actually bound together to form proteins. There are spaces within the folds of ER membrane are known as cisternae.

      Smooth ER : This type of ER does not have ribosomes.

      Another organelle is the Golgi body or Golgi apparatus (G.A.) . The Golgi body is a series of flattened sacs usually curled at the edges. Proteins which have formed in ER are processed in G.A. After processing, the final product is discharged form the G.A. At this time the G.A. bulges and breaks away to form a dropline vesicle known as secretory vesicles. The vesicles move butward to the cell membrane and either insert their protien contents in the membrane, or release their contents outside the cell.

      There is another organelle which is related to the Golgi apparatus called the lysosome . The lysosome is derived from the Golgi body. It is a sac of enzymes in the cytoplasm, used for digestion within the cell. These enzymes break down particles of food taken into the cell and make the food product available for use. There are also cytoplasmic organelles called peroxisomes in the cell which produces the enzymes to degrade fatmolecules.


      Watch the video: Chapter Plasma Membrane BIO201 (February 2023).