IMS1 TBL 4: Cell Signalling and Movement

Approximately what percentage of the human body is water?

60%

What is maintaining the distribution of water essential for?

Homeostasis

Homeostasis

The tendency towards a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes

What are biological membranes primarily made of (describe aswell)

Lipid bilayers about 5 nm thick composed of phosphatidylcholine, sterols and various proteins

Membrane proteins…

Cane be embedded within or attached to the bilayer and perform signalling and transport functions

What do saturated lipids form?

rigid membranes

What do unsaturated lipids increase?

fluidity

Hydrophobic nature of membrane

The lipid core repels water and charged molecules

Permeability

Small nonpolar molecules diffuse easily

Water diffuses more slowly

Ions require channels or pumps

Brownian Motion

When molecules in fluid move randomly leading to diffusion which over time makes the solutes reach an equilibrium

Osmosis

The diffusion of water through a semi-permeable membrane driven by concentration gradients and brownian motion (no energy)

Osmolality

Osmoles per kilogram of solvent

Is Osmolality temperature and pressure independent or dependent?

Independent

Is Osmolarity temperature and pressure independent or dependent?

Dependent

Osmolarity

Osmoles per litre of solvent

Clinically Osmolarity equals (equation)

2[Na⁺] + [glucose]/18 + [BUN]/2.8

what does tonicity do?

compare osmotic pressure between two solutions

Hypotonic plasma

Water flows into cells (swelling or bursting)

Hypertonic plasma

Water flows out of cells (shrinking)

Distribution of solutes in extracellular fluid

High in sodium, chlorine and calcium (low in potassium)

Distribution of solutes in intracellular fluid

High in potassium, phosphate and protein (lower in sodium and chlorine)

What charge are proteins?

Negative

What does the negatively charged nature of proteins allow them to contribute to?

electrical gradients, but not significantly to osmolarity

What does the movement of solutes and water across membranes depend on?

Osmotic gradients and permeability

Does water move slowly or fast across membranes

water moves slowly (while larger molecules cant cross at all)

What maintains ionic imbalance?

The sodium potassium pump by moving 3 sodium out and 2 potassium in preventing swelling

What is the membrane potential?

the electrical difference between the inside and outside of a cell (typically -60 to -90 mV) resulting from unequal ion distribution and active pumping

What does the Nernst equation quantify?

The potential difference created by ionic gradients across a membrane

Intracellular Fluid (distribution of body fluids in a 70kg adult)

23L

Interstitial Fluid (distribution of body fluids in a 70kg adult)

15L

Plasma (distribution of body fluids in a 70kg adult)

3L

Transcellular Fluid (distribution of body fluids in a 70kg adult)

1L

Total body water (distribution of body fluids in a 70kg adult)

42L (60%) (women typically have a slightly higher fat content and body water amounts decrease with age.

What is dehydration indicated by?

Elevated plasma sodium (larger than 145 mmol/L) or high urine osmolarity (larger than 700mOsm/kg)

What can dehydration lead to?

Kidney stones, infections, seizures, organ failure

Possible causes of dehydration

Diarrhoea, vomiting, sweating, diabetes

Lipid-soluble substances (capillary exchange and oncotic pressure)

diffuse through endothelial cells

Small water-soluble molecules (capillary exchange and oncotic pressure)

pass through small pores (5nm)

Proteins (capillary exchange and oncotic pressure)

largely retained, maintaining oncotic pressure in capillary beds

Arterial end (capillary exchange and oncotic pressure)

High hydrostatic pressure and high oncotic pressure draw water back in

Venous end (capillary exchange and oncotic pressure)

Lower hydrostatic pressure and high oncotic pressure draw water back in. this balance maintains blood volume and prevents swelling

Oncotic pressure

the osmotic pressure exerted by plasma proteins that draws water back into blood capillaries from the surrounding intersitial fluid

Interstitial

situated between or in the small spaces of tissues, cells, or organs

What do capillary walls allow for in terms of molecule movement?

selective movement of molecules

Although skeletal muscles vary greatly in shape and size, what are the common structural features they share?

a tendon of origin, a muscle belly, a tendon of insertion

Biceps brachii

A fusiform-shaped muscle belly with two heads (long and short) that flexes the elbow

Tensor fasciae latae (thigh region)

A short tendon of origin with a long tendon of insertion that merges with the iliotibial tract

Sartorius (thigh region)

A long strap like muscle running diagonally across the thigh

Iliopsoas (thigh region)

originates from the ilium and spine, inserting on the femur, acting as a hip flexor

Adductor longus (thigh region)

uses flat sheet-like tendon to adduct the thigh

Vastus medialis (thigh region)

part of the quadriceps, with fibres attaching obliquely to a central tendon leading to the patellar ligament

Latissimus dorsi (lats)

A triangular muscle with a broad origin along the spine and a concentrated insertion on the humerus

when activated by the CNS, skeletal muscles generate what type of forces?

contractile (tensile) forces, pulling equally on both the tendon of origin and tendon of insertion

What does the overall movement of a muscle depend on

the net force at each end

Epimysium

Connective tissue that surrounds the whole muscle

groups of what form the entire muscle?

groups of fasicles

What are fasicles

several muscle fibres together

perimysium

fine connective tissue layer that surrounds each fascicle

endomysium

fine connective tissue layer that envelopes each muscle fibre cell

each muscle fibre contains multiple …

myofibrils

sarcomeres

repeating contractile units that make up myofibrils

myofibrils

chain of sarcomeres

myofilaments

fine strands of protein composed primarily of actin and myosin which are within the myofibrils

What does skeletal muscle fibres being multinucleated allow for?

greater DNA content and increased capacity for protein synthesis

multinucleated

a cell or organism that has more than one nucleus

what else do multinucleated cells contain multiple of?

mitochondria, to support high energy demands during contraction

What do the properties of a multinucleated cell together enable the muscle cells to do?

generate, sustain and adapt to force production

What do skeletal muscle fibres display under the light microscope

a regular banding pattern of alternating light and dark regions

A bands (microscopic appearance of striated muscle)

Darker regions

I bands (microscopic appearance of striated muscle)

lighter regions

Nuclei (microscopic appearance of striated muscle)

flattened and positioned near the periphery of the cell

I band (sarcomere structure and bands)

Light region with thin (actin) filaments only

A band (sarcomere structure and bands)

Dark region containing overlapping thick (myosin) and thin (actin) filaments

H zone (sarcomere structure and bands)

Central lighter region within the A band where thick filaments are present without overlap

M line (sarcomere structure and bands)

Dark line in the centre of the H zone representing proteins linking adjacent myosin filaments

Z lines (sarcomere structure and bands)

serve as anchoring points for actin filaments forming repeating boundaries that create the banded appearance of striated muscle

Myosin (contractile proteins)

A thick filament with a long coiled tail, a hinge region, and two globular heads that project outward to form cross-bridges with actin

Actin (contractile proteins) (gentle helix in structure)

A thin filament composed of helical chains of actin molecules providing binding sites for myosin heads

Troponin complex (regulatory proteins) (gentle helix in structure)

a protein that binds calcium and regulates the position of tropomyosin

Tropomyosin (regulatory protein) (gentle helix in structure)

a filamentous protein that covers myosin-binding sites on actin in the resting state, preventing contraction

Nebulin (structural proteins)

anchored at the Z disc, runs alongside actin filaments, stabilising and aligning them

Titin (structural proteins)

extends from the Z disc to the M line, providing elasticity and maintaining myosin alignment after stretching or contraction

What do the structural proteins maintain for the sarcomere to ensure efficient contraction?

integrity and alignment

Z lines in relaxed sarcomere

define the sarcomere boundaries

Actin (thin) filaments in relaxed sarcomeres

extend inward from Z discs toward the sarcomere’s midline

Myosin (thick) filaments in relaxed sarcomeres

lie centrally with projecting heads that can form cross-bridges

Titin in relaxed sarcomeres

connects Z discs to myosin contributing to elasticity

A bands (darker regions) in relaxed sarcomeres

contain thick filaments

I bands (light regions) in relaxed sarcomeres

correspond to actin only areas

What changes in the Z discs between relaxed and contracted sarcomeres?

Distance between z discs changes (decreases as it contracts and vice versa)

What changes in the I band thickness between relaxed and contracted sarcomeres?

I band thickness changes (gets smaller as contraction occurs and vice versa)

What changes to the A band thickness between relaxed and contracted sarcomeres?

thickness remains unchanged

Myosin heads relevance in the principles of the sliding filament theory

Myosin heads bind to actin hence forming cross bridges

ATP hydrolysis relevance in the principles of the sliding filament theory

ATP hydrolysis drives conformational changes in myosin heads

Repeated cycles relevance in the principles of the sliding filament theory

repeated cycles of attachment, shape change, pulling and detachment progressively shorten sarcomere

filaments relevance in the principles of the sliding filament theory

thick and thin filaments remain the same length but their degree of overlap changes hence producing contraction

What does the Cross-bridge cycle do?

couples ATP hydrolysis to mechanical contraction in four main stages

1 Actin Binding (The cross bridge cycle)

The myosin head (containing ADP+Pi) attaches to actin

2 Power stroke (The cross bridge cycle)

the release of Pi triggers a change in myosin head orientation from vertical to bent, pulling actin toward the sarcomere centre

3 Detachment (The cross bridge cycle)

ATP binds to myosin, causing it to release actin