2.1 Cells, Tissues, Membranes, and the Immune and Lymphatic Systems
Key Takeaways
Mitochondria produce most ATP through the citric acid cycle and electron transport chain, while ribosomes and rough endoplasmic reticulum synthesize proteins and smooth endoplasmic reticulum makes lipids and detoxifies drugs.
The sodium-potassium pump uses one ATP to move three sodium ions out of the cell and two potassium ions in, creating the gradient that powers secondary active transport such as SGLT1 glucose uptake.
Epithelial tissues such as the intestinal lining renew every few days, so they are among the first to suffer when protein, energy, zinc, folate, or vitamin A intake is inadequate.
Lacteals in the intestinal villi carry chylomicrons into the lymphatic system, which delivers absorbed long-chain fats to the blood through the thoracic duct.
Protein-energy malnutrition shrinks the thymus and depresses cell-mediated immunity, lowering lymphocyte counts and increasing the severity of infections.
The anatomy and physiology area carries 13% of the Nutritional Biochemistry and Clinical Dietetics paper. The table of specifications asks examinees to explain the make-up of cells, tissues, and membranes and how their integrity supports normal function. This section links cell structure to nutrient needs.
Cell Structure and Organelle Functions
| Organelle | Main function | Nutrition link |
|---|---|---|
| Plasma membrane | Phospholipid bilayer with cholesterol and proteins (fluid mosaic model); controls entry and exit | Needs essential fatty acids, choline, and vitamin E to protect it from peroxidation |
| Nucleus | Holds DNA; directs protein synthesis | Folate and vitamin B12 are needed for DNA synthesis |
| Mitochondria | Citric acid cycle, beta-oxidation, electron transport chain, most ATP | Thiamin, riboflavin, niacin, pantothenic acid, iron, and copper act as cofactors |
| Ribosomes and rough ER | Protein synthesis | Need essential amino acids and energy |
| Smooth ER | Lipid and steroid synthesis; drug detoxification in the liver | Basis of many drug-nutrient interactions |
| Golgi apparatus | Modifies, packages, and secretes proteins | Packages digestive enzymes and mucus |
| Lysosomes | Break down worn-out organelles and engulfed material | Active in tissue wasting during starvation |
| Peroxisomes | Oxidize very-long-chain fatty acids; break down hydrogen peroxide | Catalase protects cells from oxidative damage |
| Cytosol | Glycolysis, fatty acid synthesis, pentose phosphate pathway | Insulin activates many cytosolic synthetic pathways |
Membrane Transport
Nutrients cross membranes by several mechanisms:
- Simple diffusion: movement down a concentration gradient without a carrier, as with oxygen, carbon dioxide, and small fat-soluble molecules.
- Facilitated diffusion: a carrier speeds movement down the gradient without using energy, as with fructose through GLUT5 and glucose through GLUT2.
- Primary active transport: the carrier uses ATP directly. The sodium-potassium pump (Na+/K+-ATPase) moves 3 sodium ions out and 2 potassium ions in per ATP. This pump accounts for a large share of resting energy expenditure.
- Secondary active transport: a carrier uses the sodium gradient made by the pump, as with SGLT1, which carries glucose or galactose with sodium. Oral rehydration solution works because glucose-sodium cotransport pulls water into the cells even during diarrhea.
- Osmosis: water moves toward the side with more solute. Solutions are isotonic (0.9% sodium chloride), hypotonic (cells swell), or hypertonic (cells shrink).
- Endocytosis and exocytosis: bulk transport in vesicles, as when cells take up LDL through the LDL receptor or secrete hormones.
- Pinocytosis in newborns: the newborn gut can absorb some intact proteins, including antibodies from colostrum.
The Four Tissue Types
| Tissue | Examples | Key nutrition point |
|---|---|---|
| Epithelial | Skin, intestinal lining (enterocytes), lining of airways and urinary tract | Enterocytes are replaced every 3-5 days; vitamin A keeps mucus-secreting epithelium healthy; zinc, folate, and protein support rapid cell division |
| Connective | Bone, cartilage, tendons, adipose tissue, blood | Collagen formation needs vitamin C, copper, and protein; bone needs calcium, phosphorus, vitamin D, and vitamin K |
| Muscle | Skeletal (voluntary), cardiac, smooth (gut, blood vessels) | Skeletal muscle is the body's main protein reserve and is broken down in stress and starvation |
| Nervous | Brain, spinal cord, nerves | Uses glucose as its main fuel; needs thiamin, vitamin B12, and omega-3 fatty acids |
Tissues with the fastest turnover, such as the gut lining, bone marrow, skin, and hair, show nutrient deficiencies first. That is why malnutrition produces glossitis, diarrhea, anemia, poor wound healing, and hair changes.
The Immune System
Innate (nonspecific) immunity responds immediately:
- Physical and chemical barriers: skin, mucous membranes, gastric acid, lysozyme in tears and saliva.
- Cells: neutrophils, macrophages, natural killer cells.
- Proteins: complement and acute-phase proteins such as C-reactive protein.
Adaptive (specific) immunity develops over days and creates memory:
- Cell-mediated immunity: T lymphocytes mature in the thymus. Helper T cells coordinate responses, and cytotoxic T cells kill infected cells.
- Humoral immunity: B lymphocytes become plasma cells that make antibodies (immunoglobulins). Secretory IgA protects mucosal surfaces, and IgG crosses the placenta to protect the newborn.
The Lymphatic System
Lymphatic vessels return excess tissue fluid to the blood and carry immune cells. Lymphoid organs include the lymph nodes, spleen, thymus, tonsils, and the gut-associated lymphoid tissue (GALT), which includes Peyer's patches. In each intestinal villus, a central lymph vessel called the lacteal absorbs chylomicrons, which carry long-chain fats and fat-soluble vitamins. Lymph drains through the thoracic duct into the left subclavian vein. Medium-chain fats bypass this route and go directly to the liver through the portal vein.
Nutrition and Immunity
Malnutrition and infection form a vicious cycle: undernutrition weakens immunity, and infection reduces appetite, increases losses, and raises needs.
| Nutrient problem | Immune effect |
|---|---|
| Protein-energy malnutrition | Thymic atrophy, low total lymphocyte count, impaired cell-mediated immunity, weak skin test reactions |
| Vitamin A deficiency | Damaged mucosal barriers, higher risk and severity of measles and diarrhea |
| Zinc deficiency | Impaired T-cell function and wound healing; zinc is given with oral rehydration solution in childhood diarrhea |
| Iron deficiency | Reduced lymphocyte and neutrophil function |
| Vitamin D, selenium, vitamin C, vitamin E | Support immune cell function and antioxidant defense |
Breastfeeding strengthens infant immunity through secretory IgA, lactoferrin, lysozyme, live white cells, and human milk oligosaccharides that feed protective gut bacteria.
Glucose absorption in the small intestine depends indirectly on ATP even though the SGLT1 carrier does not split ATP itself. Which mechanism explains this?
Glucose is absorbed by simple diffusion through the lipid bilayer
The Na+/K+-ATPase keeps cell sodium low, and SGLT1 uses that sodium gradient to carry glucose in
Glucose enters by pinocytosis using vesicles that need ATP
Glucose moves through GLUT5 by facilitated diffusion, which needs ATP to change the carrier's shape
A child with severe protein-energy malnutrition develops diarrhea, glossitis, and poor wound healing early in the course. Which feature of these tissues explains why they are affected first?
They store most of the body's glycogen
They contain no blood vessels, so they depend entirely on slow diffusion of nutrients from nearby tissue
They are made mostly of fat and lose mass quickly
They renew rapidly, so cell division slows when protein, energy, and micronutrients are lacking
Which structure in the intestinal villus absorbs chylomicrons, and where does this route deliver them?
The lacteal, which drains through the lymphatic system and thoracic duct into the blood
The portal vein capillaries, which carry chylomicrons directly to the liver
Peyer's patches, which store chylomicrons for later release
The crypts of Lieberkuhn, which secrete chylomicrons into the colon
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