Lipid Fractions Research Articles

Effects of Breed and Lactation-stage of Goat on the Composition and Lipid Fraction of Milk in Algeria

Background: Fatty acid profile plays an important role in human health and sensory quality of dairy products. This study focused on the influence of breed and lactation-stage, on the composition and lipid fraction of goat milk in Algeria. Methods: Milk composition of a herd comprising a total of 50 dairy goats of various breeds was monitored during the 7 months of lactation. Result: Results showed that Alpine breed milk is richer in fat, protein and casein as compared to Saanen and local breeds. Same changes were observed depending on the lactation stage in favour of the Late Lactation stage. Similarly, the fatty acid profile varies depending on the breed and lactation stage. This study highlights the potential benefits of selecting dairy goats by combining the hardiness of the local breed with the milk quality of imported breeds, which appear to have a healthier profile than the milk of the local breed.

Nutritional and Industrial Insights into Hemp Seed Oil: A Value-Added Product of Cannabis sativa L.

Industrial hemp is mainly cultivated for its fibers aimed at the production of textiles, paper, and cordage; the inflorescences for medicinal purposes; and the seeds are used by the food industry due to their high nutritional and functional matrix of protein, fiber, lipids, and microelements. Hemp seed oil (HsO) is a unique source of polyunsaturated fatty acids, with a phenomenal ω6:ω3 ratio of 2.5–3.0, significantly enhancing human health when consumed daily. HsO is mostly obtained through cold pressing due to minimal thermal treatment, and although of lower yield compared to solvent extraction, it presents higher quality lipid fractions and organoleptic characteristics such as color, taste, flavor, and density. Although HsO is a powerful source of polyunsaturated fatty acids, antioxidants, and phytosterols, its production lacks standardized quality control parameters, except for THC, which is subject to EU legislation. Therefore, it is essential to build up a quality protocol system for standardizing seed conservation, oil extraction methods, and quality parameters. This review aims to display an overall nutritional framework of the HsO and encourage further research into its use in the food value chain.

α-Synuclein interaction with POPC/POPS vesicles.

We have investigated the adsorption of the amyloid-forming protein α-Synuclein (αSyn) onto small unilamellar vesicles composed of a mixture of zwitterionic POPC and anionic POPS lipids. αSyn monomers adsorb onto the anionic lipid vesicles where they adopt an α-helical secondary structure. The degree of adsorption depends on the fraction of anionic lipid in the mixed lipid membrane, but one needs to consider the electrostatic shift of the serine pKa with increasing fraction of POPS. The vesicles with adsorbed αSyn monomers are kinetically stable. However, after fibrils have been formed, here triggered by the addition of a small concentration of pre-formed fibrils (seeds), we observed that the average vesicle size increased by approximately a factor of two. This increase in the vesicle size can be explained by vesicle fusion taking place during the fibril formation process.

Valorization of Oil Cakes in Two-Pot Lactone Biosynthesis Process.

Oil cakes are biomass wastes created by pressing oil from oilseeds. Their chemical composition (including high fat or protein content, a favorable fatty acid profile, and a high proportion of unsaturated acids) makes them valuable raw materials not only in animal feeding but are increasingly gaining popularity in biotechnological processes. This article examines the possibility of valorizing oil cakes using the lipid fraction extracted from them or their raw form in a two-pot biosynthesis process of GDDL-a cyclic ester with a creamy-peach aroma. This study tested five types of oil cakes (hemp seeds, rapeseed, safflower, camelina, and flax), analyzing their physicochemical composition and the fatty acid profile of their lipid fraction. Due to the high content of oleic acid (over 62% lipid fraction) and the wide availability, rapeseed cake was used in the biotransformation process. The synthesis of GDDL involved a three-step process: hydrolysis of triacylglycerols, hydration of oleic acid (via lactic acid bacteria in anaerobic conditions), and β-oxidation (via Yarrowia yeast, aerobic process). The analysis showed that it is possible to produce because of the two-pot biotransformation of approximately 1.7 g of GDDL/dm3. These results highlight the process's potential and justify the feasibility of waste valorization. The proposed biotransformation requires optimization and is a good example of the application of the circular economy in food processing and waste management.

Separation of polar and neutral lipids from mammalian cell lines by high-performance thin-layer chromatography.

The introduction of high-performance TLC (HPTLC) instrumentation that allows precise control of critical parameters has transformed the technique into an efficient and rapid tool for analyzing various metabolites, namely lipids. Although mass spectrometry (MS) has largely replaced lipid analysis techniques over recent decades due to its comprehensive lipidome profiling capabilities, it typically lacks the rapidity and simplicity of TLC. HPTLC remains advantageous due to its ease of use, simpler data interpretation, and compatibility with complementary techniques. In this study, we established a HPTLC protocol to fractionate both polar and non-polar lipids on a single normal phase plate. Twenty lipid standards were fractionated and the method was successfully applied to whole extracts from six mammalian cell lines. Standards and extracted lipids were applied with an automated sampler, and polar lipids were first fractionated in a 5-step automated gradient elution, followed by the fractionation of neutral lipids in a twin-trough chamber with three different elutions. Plates were automatically sprayed with a modified copper sulfate solution and charred to reveal lipids and obtain the respective chromatograms. LC-MS was used to identify ambiguous bands, thus ensuring the accuracy of lipid identification.

Effect of a monounsaturated fatty acid-enriched fat infusion on fatty acid composition in various lipid fractions of plasma and follicular fluid of 46-day versus 67-day postpartum dairy cows

Effect of a monounsaturated fatty acid-enriched fat infusion on fatty acid composition in various lipid fractions of plasma and follicular fluid of 46-day versus 67-day postpartum dairy cows

Sex differences in the association between lipids and cognitive function in older adults

Background: It is well known that cognitive function is associated with gender differences. However, the effect of sex differences on the relationship between lipids fractions and cognitive function in older adults has been contentious. Methods: 2,170 participants from the U.S. National Health and Nutrition Examination Surveys (1999-2002 and 2011-2014) were included. Total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglycerides (TG) were measured. Cognitive function was assessed using the Digit Symbol Substitution Test (DSST). Multiple linear regression models and restricted cubic spline curve fitting were used. Results: Overall, low HDL-C levels were negatively associated with DSST scores in every group. The levels of TC, TG, and LDL-C were not significantly associated with DSST scores, not only in the total population but also in the males. In the females, after adjustment for potential confounding factors, high TC levels were negatively related to DSST scores (OR = −3.590, 95% CI: – 6.343 to – 0.837), and high TG levels were found positively associated with DSST scores (OR = 2.323, 95% CI: 0.159–4.488). Conclusion: Low plasma HDL is associated with cognitive dysfunction in older adults. In older women, high TC levels are positively associated with cognitive decline while high TG levels may protect cognitive function.

Clinical Study of Lipid Profile in Intracerebral Haemorrhage in Tertiary care Center

Background: Stroke caused by Intracerebral haemorrhage (ICH) has high mortality rate. Among various risk factors for ICH, hypertension is the most important factor. Certain population studies have reported a paradox of inverse association between serum cholesterol and risk of ICH. Aims and objectives: The study aim was to evaluate the serum lipid profile total cholesterol (TC), triglycerides (TGL), high density cholesterol (HDL-C), very low density cholesterol (VLDL-C) and low density cholesterol (LDL-C) in intracerebral haemorrhage patients and look for correlation. Methods: 50 patients with ICH admitted in Chalmeda Anand Rao Institute of Medical Sciences, Karimnagar who fulfilled the inclusion and exclusion criteria were selected. The study design was a hospital based observational clinical study. History, clinical examination and investigations (CT & MRI Brain and basic blood biochemistry with serum lipid profile) were collected and the data were analysed statistically. Results: Majority of the ICH patients in our study were >55 years and were males. The total serum cholesterol was < 200mg/dl in 72 % of our patients, with a mean of 168.09 ±43.74mg%. Serum triglyceride level was patients, with mean of 108.26±43.31mg%. HDL-C was < 40 mg/dl in 76% of patients and VLDL-C was < 30mg/dl in 72% patients. The various lipid fractions observed were found to be low in majority of our ICH patients (p value < 0.05), suggesting a negative association between the two. The results obtained were comparable to other similar studies. Conclusion: Majority of intracerebral haemorrhage patients in our study had lower levels of total cholesterol, triglyceride, HDL-C, LDL-C and VLDL-C. Whether the inverse association between serum lipid levels and ICH is a true causal association or only by chance due to other common confounding factors needs to be evaluated with large scale studies.

The Potential of Probiotics from West Sumatra, Indonesia on Low-Density Lipoprotein and Interleukin-6 Levels with Dyslipidemia

Dyslipidemia is a lipid metabolism disorder characterized by abnormal elevations or reductions in one or more lipid fractions within the plasma. This condition is a critical component of both metabolic syndrome and cardiovascular disease (CVD). This study aims to see the potential of curd administration on Low-Density Lipoprotein (LDL) and Interleukin-6 (IL-6) levels in dyslipidemia patients. This study is an experimental study with a pre-post test control group design consisting of a control group and a treatment group totaling 42 people taken randomly at the DR Drs M Hatta Bukittinggi Brain Hospital in June and July 2024. In the three groups, LDL and IL-6 examinations were carried out and measured before and after curd for 14 days. Treatment group 1 was given curd as much as 150 grams, treatment group 2 was given curd as much as 200 grams, given 1x a day, and the control group was not given curd. Data analysis was performed using the Wilcoxon statistical test and paired t-test. Results: There was no significant difference in LDL (p=0.360) and IL-6 (p=0.932) levels after giving curd for 14 days. However, there was a decrease in LDL and IL-6 levels after giving curd for 14 days. Conclusion: The results showed that giving curd for 14 days in the treatment group 1 (150gram) and treatment 2 (200gram) can reduce LDL and IL-6 levels. Consuming probiotics for 14 days can reduce LDL and IL-6 levels.

Essential and nonessential fatty acid composition and use in overwintering monarch butterflies.

Monarch butterflies (Danaus plexippus) undertake one of the most remarkable long-distance insect migrations, travelling thousands of kilometres to overwinter in the central trans-volcanic belt of Mexico. This study explored how monarch butterflies use essential fatty acids (EFA) and nonessential fatty acids (NFA) during overwintering. We collected 150 (male/female) butterflies from the Sierra Chincua wintering colony from the time of arrival (December 2022) to before departure (February 2023) and analysed their lipid content. Our findings revealed that although females have a higher mass fraction of lipids, male and female monarch butterflies depleted their lipids similarly over time, resulting in low abdominal lipid mass fractions by late February. NFA, including oleic and palmitic acid, were predominantly used for energy during overwintering by male and female butterflies. In contrast, the EFA alpha-linolenic and linoleic acids, critical for reproductive success and cellular functions, were conserved in both sexes. Males began the overwintering period with a higher mass fraction of EFA in the polar components of the head and thorax, which may impact the degree of cold acclimation of these tissues during this period. Strategic lipid utilisation, prioritising the preservation of EFA over NFA and optimizing overwintering survival probably enhance readiness for spring remigration and reproduction. This differential fatty acid use underscores the delicate balance monarch butterflies maintain to survive overwintering and highlights the potential impacts of environmental changes on their lipid dynamics and survival.

Dependence of brain-tissue R2 on MRI field strength.

To quantify T2 relaxation in the brain at 3 T and 7 T to study its field dependence and correlation with iron content, and to investigate whether iron can be separated from other sources of T2 relaxation based on this field dependence. Nine subjects were scanned at both field strengths with the same acquisition technique, which used multiple gradient-echo sampling of a spin echo. This allowed for separation of T2 relaxation from static dephasing by B0 field inhomogeneities and the effects of radiofrequency refocusing imperfections. The average relaxation rates (R2 = 1/T2) in multiple regions of interest in the brain were fitted with a model linear in B0 and correlated with literature iron values. The relationship between the R2 values at the two field strengths appeared to be linear over all regions of interest. The R2 values (in s-1) in the regions of interest for which both an iron and a lipid mass fraction have been documented in the literature were fitted as , where and indicate the putative mass fractions of iron and lipid. The R2 relaxation rate is well described by a constant plus a term linear in B0, with both iron and lipid content contributing to the slope. This indicates that the contributions of lipid and iron to R2 cannot be separated based solely on the field dependence of R2 in the field range of 3-7 T.

Estimating and visualising multivariable Mendelian randomization analyses within a radial framework.

Mendelian randomization (MR) is a statistical approach using genetic variants as instrumental variables to estimate causal effects of a single exposure on an outcome. Multivariable MR (MVMR) extends this to estimate the direct effect of multiple exposures simulatiously. MR and MVMR can be biased by the presence of pleiotropic genetic variants in the set used as instrumental variables, violating one of the core IV assumptions. Genetic variants that give outlying estimates are often considered to be potentially pleiotropic variants. Radial plots can be used in MR to help identify these variants. Analogous plots for MVMR have so far been unavailable due to the multidimensional nature of the analysis. We propose a radial formulation of MVMR, and an adapted Galbraith radial plot, which allows for the estimated effect of each exposure within an MVMR analysis to be visualised. Radial MVMR additionally includes an option for removal of outlying SNPs which may violate one or more assumptions of MVMR. A RMVMR R package is presented as accompanying software for implementing the methods described. We demonstrate the effectiveness of the radial MVMR approach through simulations and applied analyses. We highlight how outliers with respect to all exposures can be visualised and removed through Radial MVMR. We present simulations that illustrate how outlier removal decreases the bias in estimated effects under various forms of pleiotropy. We apply Radial MVMR to estimate the effect of lipid fractions on coronary heart disease (CHD). In combination with simulated examples, we highlight how important features of MVMR analyses can be explored using a range of tools incorporated within the RMVMR R package. Radial MVMR effectively visualises causal effect estimates, and provides valuable diagnostic information with respect to the underlying assumptions of MVMR.

Acrylamide Contamination, Shelf-Life and Sensory Properties of Puffed Potato Starch Chips Deep-Fried in Rapeseed Oil-Based Oleogels

The utilization of oleogels as a frying medium has been studied in recent years. In this research, rapeseed oil structured by rice bran wax (RW) and beeswax (BW) was used as a frying medium for the production of puffed potato starch chips. Chips and their lipid fractions were analyzed for their changes in quality during 9 weeks of storage. It was observed that the addition of beeswax contributed to the increase in the acid value of the puffed chips’ lipid fraction. The level of unsaturated fatty acids decreased in all fried products; however, the lowest changes were observed for products fried in RW oleogel. The presence of waxes slowed down oxidation which was indicated by the lowering of the peroxide value from 67.84 (for rapeseed oil) to 52.62 meq O2/kg (BW oleogel) at the end of storage. A similar situation was observed in the case of changes in the hardness and water activity of puffed chips. The addition of waxes to the frying medium contributed to a significant reduction in oil uptake in the puffed chips, where products fried in oil with RW fat were 17.7% lower than the control sample. The addition of BW 5 g/1 L of frying oil can increase the acrylamide content from 57 μg/kg for rapeseed oil to 97 μg/kg. Sensory evaluation of the puffed chips showed no statistically significant differences between products fried in oil with or without the addition of waxes.

Very broad distribution of β sheet registries of the HIV gp41 fusion peptide supports mutational robustness for fusion and infection

HIV, like other membrane-enveloped viruses, has protein spikes that include a fusion peptide (Fp) segment that binds the host cell membrane and plays a critical role in fusion (joining) viral and cell membranes. The HIV Fp is the ~23 N-terminal residues of the gp41 spike protein. Fp adopts intermolecular antiparallel β sheet structure when lipid fraction cholesterol ≈0.3, which is comparable to host cells. Rotational-echo double-resonance NMR was applied to probe the registries (alignments) of adjacent Fp molecules in membrane-bound sheets. The data were fitted to determine quantitative populations, f(t)'s, of individual antiparallel registries indexed by t, the number of residues in the registry. Both wild-type (WT) and fusion-defective V2E Fp sheets have broad but very different registry distributions, each with at least eight populated registries with f(t) > 0.02, and [Formula: see text] and ⟨t⟩V2E = 18.5. The broad WT distribution likely improves mutational robustness for HIV, as Fp is a neutralization epitope of the immune system, and Fp mutations are required for immune evasion during chronic HIV infection. V2E fusion is reduced because longer Fp sheets increase separation between initially apposed membranes. The f(t)WT were well-fitted to free energies that were sums of contributions from sheet length, aligned leucines, and sidechain membrane insertion. The f(t)V2E's were similarly well-fitted except there wasn't the insertion contribution. Relative to V2E, WT fusion is enhanced by deeper membrane insertion of Fp with accompanying greater dislocation of neighboring lipids. This study provides a rare quantitative determination of broad molecular structural distributions by experiment.

Age-dependent self- and individual chemosensory discrimination in male Psammodromus algirus lizards is based on lipids and proteins from their scent

Self- and individual discrimination are needed to maintain social relationships. However the extent of precision of these abilities may depend on the social strategy of each individual, which may vary with ontogeny. Many animals use chemical cues to discriminate conspecifics. For example, many lizards use femoral gland secretions, which are composed of lipids and proteins, but it is not well known the role of each of these chemical classes in conspecific discrimination. We examined whether and how differences in age-dependent social strategies of male lizards Psammodromus algirus affect self- and individual chemosensory discrimination, and whether these are mediated by the lipid and/or protein fraction of femoral secretions. In this lizard species, there are two age categories of males that adopt alternative reproductive strategies; dominant and territorial “old” males and “young”, but reproductively mature, satellite-sneaker males. We examined chemosensory responses to separated lipid and protein fractions of femoral secretions. Both young and old males discriminated both their own lipids and proteins from those of conspecific males of their same age category. With respect to individual discrimination, only old males distinguished different unfamiliar old males based on their lipids alone, but not on proteins, while they do not discriminate between different individual young males. In contrast, young males do not show individual discrimination in any case. Therefore, our results show age-related differences in chemical social discrimination abilities in male P. algirus lizards that could be explained by their different age-dependent social strategies.

Circulating Fetuin-A concentrations in rheumatic diseases: asystematic review and meta-analysis.

Rheumatic diseases (RDs) include a broad group of disabling conditions with different phenotypes, from autoimmune to autoinflammatory, degenerative, metabolic or mixed manifestations. With the continuous efforts to identify therapeutic targets for new biologic drugs to treat overt clinical manifestations, research is also focusing on the discovery of new biomarkers to diagnose and manage early disease stages. In this context, we conducted a systematic review and meta-analysis of Fetuin-A (FtA), a glycoprotein synthesized by the liver that participates in several biological processes and has been proposed as a biomarker for several disorders, including rheumatoid arthritis. A systematic search in PubMed, Scopus and Web of Science, from inception to the 24th of August 2024, led to the identification of 13 manuscripts from 219 records; six additional studies were identified through reference hand-search, for a total of 19 studies. There was a significant decrease in FtA concentrations in RD patients (standardized mean difference, SMD = -.91; 95% CI -1.43 to -.39, p = .001), with no substantial contribution from any individual study nor publication bias. The effect size was significantly associated with erythrocyte sedimentation rate, various lipid fractions, geographical area of study conduction, study design and specific type of RD. In conclusion, our study identified significant reductions in FtA concentrations in RD patients versus healthy controls. These alterations were significantly associated with specific study and patient characteristics. Further research is required to identify the exact pathophysiological mechanisms underlying these alterations and the possible utility of measuring FtA for the diagnosis and management of RDs.

Enrichment optimization of Milk Fat Globule Membrane extracts from buttermilk using Response Surface Methodology

This study investigates the effect of rennet-induced coagulation on the composition of Milk Fat Globule Membrane (MFGM) extracts from buttermilk using Response Surface Methodology. Optimizing coagulation conditions, such as rennet concentration, incubation time and temperature, significantly affects the protein and lipid fractions of the resulting MFGM extracts. Certain treatments yielded promising results in terms of MFGM concentration and casein removal. Principal Component Analysis showed different compositional patterns in the samples, shedding light on the intricate interaction of factors throughout the coagulation process. Buttermilk wheys (BMWs) characterized by high levels of both lipid and protein MFGM components, are of interest for practical applications. Although the extract with the highest MFGM protein (43.60 g/100 mL) and a medium content of polar lipids (12.98 g/100 g of fat) was found in sample C35min40°C, other BMW, such as those from the treatments C60min30°C, E35min30°C or C35min20°C, were also sufficiently enriched in MFGM associated compounds. In conclusion, the study highlights the importance of tailored coagulation conditions for optimizing the nutritional and functional properties of MFGM extracts, offering valuable insights for further nutritional research and potential applications in large-scale industrial processes.

Alcoholic Fermentation Activators: Bee Pollen Extracts as a New Alternative

Searching for natural alternatives to synthetic fermentation activators has led to the study of bee pollen as a natural alcoholic fermentation activator. This study evaluated the potential of different bee pollen extracts (0.25 g/L) as activators in a Palomino Fino grape must. By analysing the composition of each extract, it was possible to identify the specific bee pollen fractions with the highest efficacy for activating alcoholic fermentation. Four extracts were obtained through sequential extraction using various organic solvents of increasing polarity (hexane, acetone, ethanol, and water), and their compositions were characterised. The effect of each extract was evaluated by monitoring the viable yeast populations and fermentation kinetics throughout the alcoholic fermentation process, along with the physicochemical and colour characterisation of the white wines obtained. The bee pollen fraction extracted with hexane, which was rich in long-chain fatty acids, significantly increased the maximum yeast populations and improved the fermentation kinetics. However, the extracts rich in polyphenolic compounds exhibited slower fermentation rates. Based on the obtained results, the lipid fraction of bee pollen extracted with hexane may be responsible for its ability to activate alcoholic fermentation.

Understanding the Impact of Lipids on the Solubilizing Capacity of Human Intestinal Fluids.

Lipids in human intestinal fluids (HIF) form various structures, resulting in phase separation in the form of a lipid fraction and a micellar aqueous fraction. Currently used fed state simulated intestinal fluids (SIF) lack phase separation, highlighting the need for a deeper understanding of the effect of these fractions on intestinal drug solubilization in HIF to improve simulation accuracy. In this study, duodenal fluids aspirated from 21 healthy volunteers in fasted, early fed, and late fed states were used to generate 7 HIF pools for each prandial state. The apparent solubility of seven lipophilic model drugs was measured across these HIF pools, differentiating between the micellar fraction and the total sample (including both micellar and lipid fractions). The solubilizing capacities of these fluids were analyzed in relation to their composition, including total lipids, bile salts, phospholipids, total cholesterol, pH, and total protein. The solubility data generated in this work demonstrated that current fed state SIF effectively predicted the average solubility in the micellar fraction of HIF but failed to discern the considerable variability between HIF pools. Furthermore, the inclusion of a lipid fraction significantly enhanced the solubility of fed state HIF pools, resulting on average in a 13.9-fold increase in solubilizing capacity across the seven model compounds. Although the average composition of the fluids was consistent with previous studies, substantial variability was observed in micellar lipid concentrations, despite relatively stable total lipid concentrations. This variability is critical, as evidenced by the strong correlations between the solubilizing capacity of the micellar fraction and its micellar lipid concentrations. Additionally, this study identified that fluctuations in bile salt concentrations and pH contributed to the observed variability in micellar lipid concentration. In summary, the influence of the lipid fraction on solubility was 2-fold: it enhanced the solubility of lipophilic drugs in the total fluid, and contributed to the variability in the solubilizing capacity of the micellar fraction.

Hydrogen peroxide diffusion across the red blood cell membrane occurs mainly by simple diffusion through the lipid fraction

Hydrogen peroxide (H2O2) is an oxidant produced endogenously by several enzymatic pathways. While it can cause molecular damage, H2O2 also plays a role in regulating cell proliferation and survival through redox signaling pathways. In the vascular system, red blood cells (RBCs) are notably efficient at metabolizing H2O2. In addition to a robust antioxidant defense, we recently determined that human RBCs also have a high membrane permeability to H2O2 that is independent of aquaporin 1 or aquaporin 3. In this work, we sought to further investigate the permeation mechanism of H2O2 through the membrane of human RBCs. First, we explored the role of other erythrocytic membrane proteins in H2O2 transport, including urea transporter B and ammonia transporter Rh proteins. However, no differences were found in H2O2 permeability in RBCs lacking these proteins compared to control RBCs. We then focused on the hypothesis that H2O2 diffuses through the lipid bilayer. To test this, we studied H2O2 permeability in RBCs from patients with Gaucher disease (GD), which accumulate sphingolipids in the membrane, affecting RBC morphology and deformability. We found that RBCs from GD patients exhibited lower H₂O₂ membrane permeability. In another approach, we treated normal RBCs with hexanol, which fluidizes the lipid fraction of the RBC membrane, and observed an increase in the permeability to H2O2. In contrast, hexanol had no effect on the rate of water efflux by aquaporin 1. Together, these results support the hypothesis that H2O2 diffusion through the RBC membrane occurs primarily through the lipid fraction.