Physiology of Cold Exposure and Cellular Function

Hypothermia

It impairs the sinoatrial node, reducing heart rate and cardiac output.

Cold air and lungs

Because air is warmed progressively through the respiratory tract before reaching the lungs.

Cold exposure on respiration

It decreases both respiratory rate and volume.

Frostbite

Prolonged skin vasoconstriction leads to reduced blood flow, oxygen, and nutrients, causing tissue death.

Asthma-like symptoms in cold

Cold air is dry, which can trigger exercise-induced asthma during high-intensity activity.

First line of defense against cold

Peripheral vasoconstriction, which reduces heat transfer from skin to air.

Body's insulating shell

Superficial skin, subcutaneous fat, and underlying muscle.

Increasing body insulation

Through increased vasoconstriction, thicker subcutaneous fat, and more inactive muscle mass in limbs.

Nonshivering thermogenesis

Metabolic heat production via SNS activation and hormones, without muscle activity.

Shivering thermogenesis

It increases metabolic heat production through rapid involuntary muscle contractions.

Cold adaptation patterns

Cold habituation, metabolic acclimation, and insulative acclimation.

Body size and heat loss

A higher surface area-to-mass ratio and lower fat/muscle mass increase heat loss.

Women in cold environments

More subcutaneous fat provides better insulation, although smaller muscle mass limits heat production.

Windchill

The increased cooling effect caused by wind, expressed as an equivalent temperature.

Cold water immersion

It dramatically increases convective heat loss; exercise may partially offset this.

Cooled muscles

Force production decreases and fatigue occurs more quickly.

Prolonged cold exercise and hypothermia risk

As fatigue sets in, intensity drops, lowering metabolic heat output.

Catecholamines in cold exercise

They increase free fatty acid (FFA) mobilization and usage for energy.

Fat use for fuel in cold

Vasoconstriction limits blood flow to fat stores, impairing FFA availability.

Metabolism during cold exposure

The body reduces carbohydrate use in favor of fat to conserve glycogen stores.

Cell discovery

Robert Hooke looked at a thin slice of cork through his two-lens microscope, discovering hollow structures & naming them cells.

Cell theory by Theodor Schwann

1. All organisms are made up of one or more cells 2. The cell is the basic unit of life; it is the smallest unit that shows all the characteristics of life 3. All cells come from pre-existing cells; i.e. process of cell division.

Prokaryote

Lacks a distinct nucleus bounded by a membrane and lacks membrane-bound organelles such as mitochondria & chloroplasts.

Eukaryote

Has a true nucleus that is separated from cytoplasm by a nuclear envelope; cytoplasm contains membrane-bound organelles.

Human cell estimate

Humans contain est. 37.2 trillion cells, each different type intricately suited to its function.

Osteoblasts

Cells that produce collagen.

Skeletal myocytes

Cells that contain contractile proteins.

Pancreatic beta cells

Cells that produce insulin.

Neurons

Cells that transmit neurotransmitters.

Organelle

A specialised subunit within a cell that has a specific function.

Mitochondria

Folds of mitochondria contain most essential protein for cell function.

Cytosol

Jelly-like material in which organelles float, if organelles are removed, the cytosol would be left (soluble). Consists mostly of water with dissolved substances (e.g. AA). Keeps cell alive; glycolysis occurs here, where the pyruvates are then moved to the mitochondria.

Nucleus

Contains DNA, condensed & organised with proteins (e.g. chromatin), surrounded by a nuclear envelope on the exterior; a double membrane 50nm apart continuous with the interior RER. Contains ~3000 nuclear pores (which allow mRNA to travel out & be processed by the endoplasmic reticulum), regulated by a protein structure, the nuclear pore complex. Small molecules (mw<20kDa - protein 33 - 55kDa) can pass through but larger molecules are strongly regulated.

Nucleolus

Spherical body of nucleus of most eukaryotes, most enlarged during protein synthesis, containing DNA templates for ribosomal RNA (rRNA) transcribed by RNA polymerase I.

Genetic code

The genetic code of the body (DNA) is identical, but the same content is not produced every day - central dogma. Humans have around 25,000 genes & each cell has the full set of the human genome but only makes specific proteins.

Genes

Encode for proteins, the most diverse & complex macromolecules in the cell. Used for structure, function & information, they are made of linearly arranged AA residues, folded up with active regions.

Protein types

1) Enzymes - catalyses covalent bond breakage or formation. 2) Structural - collagen, elastin, keratin, etc. (intra/extracellular spaces). 3) Motility - actin, myosin, tubulin, etc. 4) Regulatory - bind to DNA to switch genes on or off. 5) Storage - ovalbumin, casein, etc. 6) Hormonal - insulin, nerve growth factor (NGF), etc. (slow-response). 7) Receptors - hormone and neurotransmitter receptors. 8) Transport - carries small molecules or ions. 9) Special purpose proteins - green fluorescent protein, etc.

Mitochondria

Organelles that generate ATP through oxidative phosphorylation, which occurs in a membrane bound electron transport system. The process by which the enzymatic oxidation of cell metabolites is converted into ATP (resynthesis of ADP to ATP, using ATP synthase). Makes >80% cellular ATP.

Endosymbiotic Theory

The theory that mitochondria are primitive bacterial cells that have formed a mutually beneficial relationship with eukaryotes over millions of years, allowing eukaryotes to utilise oxygen more efficiently.

Mitochondrial DNA

Mitochondria have their own circular DNA, which combines with nuclear DNA in mitochondrial biogenesis. Zygote derives mtDNA from the ovum (fertilised egg of maternal side), encodes for approximately 37 genes involved in oxidative phosphorylation, 2 mtRibosome & 22 mtRNAs.

Endoplasmic reticulum

Forms an interconnected network of tubules, vesicles & cisternae within cells. It is the site of protein synthesis & packaging of cell chemicals into transport vesicles.

Smooth ER

Takes part in the synthesis of membrane lipids and steroids. The SER represents only a small portion of the ER in most cells, serving as transport vesicles for protein to the Golgi apparatus. More prominent in cells of the adrenal cortex and hepatocytes.

Rough ER

Rough ER is smooth ER + ribosomes (ribosomes sit on the external layer of the ER and translate the mRNA into protein). These proteins are then folded into the ER membrane or in the lumen between the two membranes of the ER if destined for export.

Golgi apparatus

Stacks of membrane-bounded cisternae located between the ER and cell surface, mainly devoted to processing the proteins synthesised in the RER (packaging and shipping of a cell).

mRNA translation

The process by which ribosomes synthesize proteins by clamping over the tRNAs and mRNAs.

Ribosomes

Complexes made of protein and rRNA that facilitate the translation of mRNA into proteins.

A site

The site on the ribosome that receives new tRNA.

P site

The site on the ribosome that receives peptide-bearing tRNA after peptide bond formation.

E site

The site on the ribosome where tRNAs exit.

Lysosomes

Organelles responsible for 'bulk' protein breakdown, created by the addition of hydrolytic enzymes to early endosomes from the Golgi apparatus.

Hydrolytic enzymes

Enzymes that work best at low pH (~5) and are involved in the breakdown of proteins in lysosomes.

ATP-dependent ubiquitin proteasome

A system for protein breakdown that targets 'tagged' cellular proteins essential for normal cell turnover.

Calpains

Proteins activated by increased intracellular Ca+ levels, contributing to muscle remodelling by cleaving cytoskeletal and regulatory proteins.

Caspases

Proteins activated by intrinsic or extrinsic pathways that lead to programmed cell death (apoptosis) by systematically degrading cellular components.

Peroxisomes

Membrane-bound enzyme sacs where oxidation reactions produce H2O2, which is converted to H2O by catalase.

Cytoskeleton

An extensive network of protein fibres providing mechanical strength, locomotion, remodelling, chromosome separation, intracellular transport, and cellular signalling.

Microfilaments

Linear polymers of actin subunits that are flexible and strong, resisting buckling and fracture.

Microtubules

Conveyor belts inside cells that move vesicles, granules, organelles, and chromosomes via special attachment proteins.

Intermediate filaments

Components of the cytoskeleton that withstand mechanical stress.

Focal adhesions

Attachment complexes that anchor contractile filaments to the cell membrane, comprising at least 150 protein components.

Extracellular matrix

A connective tissue consisting of high quantities of fibrous proteins (e.g., collagen) exocytosed into the extracellular space.

Endocytosis

The process where the cell membrane invaginates, pinches in, and creates a vesicle enclosing contents.

Exocytosis

The process where a membrane vesicle fuses with the cell membrane, releasing enclosed material to the extracellular space.

Fluid mosaic model

A model describing the arrangement of amphipathic lipid molecules forming a lipid bilayer in the cell membrane.

Hydrophilic head groups

Separate the hydrophobic tails from the aqueous cytosolic and extracellular environments.

Hydrophilic molecules

Also known as polar molecules.

Hydrophobic molecules

Also known as non-polar molecules.

Cell membrane

Selectively permeable.

Osmosis

The diffusion of water across a membrane that is semi-permeable, permeable to water but not to all solutes.

Solvent

The liquid which contains the solute.

Solute

Substance that is put into the solvent.

Hypertonic fluid

Has a greater concentration of solutes than does the fluid in the cell; water moves from inside to outside the cell.

Isotonic fluid

Has the same concentration of solutes as the fluid in the cell; immersion in it causes no net movement of water.

Hypotonic fluid

Has a lower concentration of solutes than does the fluid in the cell; water moves from outside to inside the cell.

Passive transport

Can occur all the time; includes simple diffusion and osmosis.

Simple diffusion

Occurs without channels and moves along the concentration gradient until equilibrium is reached.

Facilitated diffusion

Polar molecules and charged ions use integral membrane proteins to pass across the membrane without energy.

GLUTs

Transporters that facilitate the entry of glucose into cells.

Active transport

Requires energy to move solutes from a region of lower concentration to a region of higher concentration.

Secondary active transport

One ion moves down an electrochemical gradient but against the concentration gradient, energy supplied by the gradient.

Membrane potential

Generated by the net charge between the inside and outside of the cell, regulated by selective permeability.

Na+/K+ pump

An example of active transport that uses ATP hydrolysis to transport Na+ out of the cell and K+ into the cell.

Na+/K+ ATPase

An enzyme located in the plasma membrane that maintains cell resting membrane potential.

Signal transduction

The process by which a cell acts on a chemical signal through a receptor.

Responses to signals

May include activation of genetic material, production of proteins, or suppression of protein production.

Homeostasis

The internal environment remains relatively constant despite changes in the external environment.

Endocrine system

A collection of glands that secrete hormones into circulation to be carried to distant organs.

Hormones

Chemical messengers synthesized and released by glands of the endocrine system.

Autocrine

A type of hormone signaling where a cell targets itself.

Paracrine

A type of hormone signaling where a cell targets nearby cells.

Juxtacrine

A type of hormone signaling that requires direct contact between cells.

Target cells

Cells that possess specific receptors for a hormone and respond to it.

Lock+key theory

A model explaining how hormones interact with specific receptors, similar to enzymes.

Hormonal effect magnitude

Dependent on the number of target receptors, hormone concentration, receptor affinity, and influence of other hormones.

T2 diabetes

A condition that can develop from poor management of insulin receptors, leading to down-regulated hormones.

Steroid hormones

Lipid-soluble hormones synthesized from cholesterol that circulate in blood bound to carrier proteins.

Non-steroid hormones

Water-soluble hormones including protein hormones and catecholamines that cannot diffuse across cell membranes.

First messenger

The hormone that binds to a receptor to initiate a signal transduction cascade.

Negative-feedback systems

Regulatory systems where hormone secretion reduces the original stimulus.

Positive-feedback systems

Regulatory systems where the rate of a process increases as the concentration of the product increases.