cell membrane & transport

Flashcards covering the fundamentals of medical physiology, including cell membrane structure, communication, transport mechanisms, and homeostasis.

internal environment

identified as the extracellular fluid (ECF) of the body

homeostasis

maintenance of a steady state of the internal environment (ECF) of the body

• all organ systems of human body function to achieve this state

• depends on the functioning of a number of biological control systems

control system

a group of organs that work together to keep a biological variable at its normal value in the ECF

• component

  • sensor

  • afferent pathway

  • integrating center

  • efferent pathway

  • effectors

sensor

a component of a control system that detects the difference between a physiological variable and its set point value

afferent pathway

the pathway that carries information from the sensor to the integrating center

integrating center

the component that receives information, decides how to deal with the situation, and directs the effectors

efferent pathway

the pathway that carries information from the integrating center to the effector organs

effectors

organs that function to bring a physiological variable back to its set point value

temperature control system

normal (set point) temperature - 37°C or 98.6°F

• stimulus - exposure to heat/cold

  • body temperature starts changing

• sensor - thermoreceptors in skin

• afferent pathway - somatic nerves in skin

• integrating center - hypothalamus

• efferent pathway - skeletal muscles, sweat glands, blood vessels

• reponse - shivering, sweating, vasodilation/vasoconstriction

gap junction communication

cell-to-cell communication where molecules and electrical signals move between cells through connexons (2 - 3 nm in width)

• substances with molecular weight < 1000 daltons

• present in neurons, cardiac, & smooth muscles

paracrine communication

cell-to-cell communication where molecules from one cell act on another cell that is close by

• e.g. enterochromaffin-like cells (ECL) of stomach secretes histamine that acts on neighboring parietal cell & causes it secrete HCl

autocrine communication

cell-to-cell communication in which molecules act on the same cell that secreted them

• rely on positive/negative feedback mechanism

endocrine communication

cell-to-cell communication in which hormones are secreted into the bloodstream and act on distant target cells

neuronal communication

communication seen at the neuromuscular junction between a neuron and a muscle, gland, or another neuron

juxtacrine communication

growth control and intercellular communication involving specific cell-to-cell contacts established by membrane-bound growth factors and receptors on adjacent cells

• e.g. immune cells in the body (white blood cells)

plasma membrane

a lipid bilayer (hydrophilic heads & hydrophobic tails) containing proteins that acts as a functional barrier for the cell

• fluid mosaic model - flexible layer made of lipids and proteins

• semi-permeable

  • lipid-soluble substances dissolve in hydrophobic lipid bilayer & therefore can cross cell membrane

    • O₂, CO₂, steroid hormones

  • water-soluble substances cannot dissolve in lipid bilayer but can cross cell membrane through water-filled channels or pores, or may be transported by carrier proteins

    • Na⁺, Cl^-, glucose, H₂O

lipid-soluble substances

substances that can dissolve in hydrophobic lipid bilayer & therefore can cross cell membrane

• O₂, CO₂, steroid hormones

water-soluble substances

substances that cannot dissolve in lipid bilayer but can cross cell membrane through water-filled channels or pores, or may be transported by carrier proteins

• Na⁺, Cl^-, glucose, H₂O

plasma membrane proteins

proteins embedded in the plasma membrane that facilitate transport (transporters & channels); act as receptors enzymes, or antigens; and provide structural support for the cell

• two types

  • integral / intrinsic / transmembrane

  • peripheral / extrinsic

integral (intrinsic) proteins

transmembrane proteins that span the entire cell membrane or are embedded within it

• ion channels, transport proteins

peripheral (extrinsic) proteins

membrane proteins that are not embedded but located on either the intracellular or extracellular side of the membrane

• ankyrin in red blood cells

passive transport

aka downhill transport ; the movement of a substance along an electrical, chemical, or pressure gradient requiring no ATP

• involves processes like simple diffusion, facilitated diffusion, and osmosis

active transport

aka uphill transport ; the movement of a substance against a gradient requiring ATP investment

• involves processes like primary & secondary transport

carrier-mediated transport

transport processes that require a specific carrier protein

• includes facilitated diffusion, primary, and secondary active transport

• 3 common features

  • chemical specificity

  • competition

  • saturation

carrier-mediated transport - stereospecificity

a characteristic of carrier proteins where binding sites are specific for certain isomers

• e.g. renal proximal tubule is specific for D-glucose & cannot transport L-glucose

carrier-mediated transport - competition

a characteristic of carrier proteins where structurally and chemically similar solutes compete for binding sites on the same carrier molecule

• galactose = competitive inhibitor of glucose transport in small intestine

carrier-mediated transport - saturation

a characteristic of carrier proteins where the rate of transport is dependent on the concentration of solute as carrier proteins have a limited number of binding sites

• low [solute] - many binding sites available = ∝ (↑ [ ] , ↑ T_rate)

• high [solute] - binding sites become limited & occupied =T_rateplateau

transport maximum (T_m)

the maximum rate at which a substance can be reabsorbed or transported

• point of saturation = all binding sites on carrier proteins are occupied + rate of transport reaches a plateau

• 1° active transport, 2° active transport, facilitated diffusion

1° active transport, 2° active transport, facilitated diffusion

blocking ATP doesn’t shift this graph

facilitated diffusion

A 45‑year‑old man presents with polyuria, polydipsia, and polyphagia. Laboratory testing shows markedly elevated blood glucose, and urinalysis reveals glucose in the urine. He is diagnosed with diabetes mellitus and started on insulin. At follow‑up, his urine glucose is no longer detectable.

Which of the following best explains the presence of glucose in his urine before treatment?

saturation of proximal tubular glucose transporters

A 45‑year‑old man presents with polyuria, polydipsia, and polyphagia. Laboratory testing shows markedly elevated blood glucose, and urinalysis reveals glucose in the urine. He is diagnosed with diabetes mellitus and started on insulin. At follow‑up, his urine glucose is no longer detectable.

Which of the following best explains the absence of glucose in the urine after insulin administration?

decreased filtered load of glucose below the T_m

A 45‑year‑old man presents with polyuria, polydipsia, and polyphagia. Laboratory testing shows markedly elevated blood glucose, and urinalysis reveals glucose in the urine. He is diagnosed with diabetes mellitus and started on insulin. At follow‑up, his urine glucose is no longer detectable.

Why is glucose normally absent from the urine in healthy individuals?

glucose is completely reabsorbed in the proximal tubule below its T_m

A 45‑year‑old man presents with polyuria, polydipsia, and polyphagia. Laboratory testing shows markedly elevated blood glucose, and urinalysis reveals glucose in the urine. He is diagnosed with diabetes mellitus and started on insulin. At follow‑up, his urine glucose is no longer detectable. When blood glucose reaches approximately 300 mg/dL, the filtered load exceeds the tubular maximum (T_m) for glucose.

Which of the following best describes what occurs at this point?

glucose begins to appear in the urine

A 45‑year‑old man presents with polyuria, polydipsia, and polyphagia. Laboratory testing shows markedly elevated blood glucose, and urinalysis reveals glucose in the urine. He is diagnosed with diabetes mellitus and started on insulin. At follow‑up, his urine glucose is no longer detectable.

Which of the following renal processes is restored to normal?

complete proximal tubular reabsorption of filtered glucose

simple diffusion

the movement of a substance downhill along its gradient directly through the lipid bilayer or through pores until concentrations are equal on both sides

• depends on

  • concentration gradient

  • membrane permeability

  • surface area

• example: lipid soluble substances such as blood gasses or steroids

Fick’s Law of diffusion

states that the rate of diffusion is proportional to the concentration gradient between two regions, the surface area available for diffusion, and the permeability of the membrane

• rate of diffusion (D) ∝ surface area (SA) x concentration gradient (Δ C or Δ P) x solubility

permeability factors

diffusion rate is

• directly proportional (∝) to lipid solubility

• inversely proportional ( $\frac{1}{\varpropto}$ ) to molecular size & membrane thickness

simple diffusion

facilitated diffusion

passive transport of a substance along its gradient with the help of a carrier protein

• rate of transport is faster at [↓] due to presence of carrier protein

• rate of transport reaches saturation at [↑]

• follows all general characteristic features of carrier-mediated transport (chemical specificity, competition, saturation)

• example: transport of glucose, galactose, and fructose via GLUT

GLUT-4 transporter

type of insulin-regulated glucose transporter that mediates the uptake of glucose from blood into cells via facilitated diffusion

• particularly in skeletal muscle and adipose tissues

GLUT-5 transporter

type of transporter that facilitates the absorption of fructose from the intestinal lumen into cell via facilitated diffusion

• primarily found in the small intestine (apical brush border membrane), kidney (proximal tubule cells), and testis

GLUT-2

transporter that facilitates the transport of glucose, galactose, fructose from intestinal cells into the bloodstream via facilitated diffusion

• predominantly expressed in the liver (basolateral membrane of enterocytes), pancreas (β-cells), kidneys, and small intestine

primary (1°) active transport

the process in which energy from ATP is used directly to drive the uphill (low → high) transport of substances against their concentration gradient

• carrier has ATP-splitting (ATPase) activity

• follows all general characteristic features of carrier-mediated transport (chemical specificity, competition, saturation)

• examples

  • sodium-potassium pump (Na⁺/K⁺ ATPase)

  • calcium pump (Ca²⁺ ATPase)

  • hydrogen-potassium pump (H⁺/K⁺ ATPase)

sodium-potassium ATPase (Na⁺− K⁺ pump)

a primary active transporter that uses ATP to pump 3 Na⁺ out of & 2 K⁺ into the cell against their electrochemical gradient creating an electrical potential difference across the cell membrane

• electrogenic pump - maintains low intracellular [Na⁺] & high intracellular [K⁺]

  • ↑ + charges out of cell → ↑ negative charge within the cell & ↑ positive charge outside cell

• inhibited by cardiac glycosides (Digitalis, Ouabain)

digitalis

a cardiac glycoside derived from the foxglove plant used to treat heart conditions by inhibiting sodium-potassium ATPase

• increases intracellular calcium levels and enhances cardiac contractility

calcium ATPase (Ca2⁺ pump)

a primary active transporter in the cell membrane, sarcoplasmic reticulum, and endoplasmic reticulum that uses ATP to pump Ca²⁺ out of the cytosol against its electrochemical gradient

• ↑ Ca²⁺ uptake + calsequestrin = muscle relaxation

• ↓ Ca²⁺ uptake − calsequestion = muscle contraction

hydrogen-potassium ATPase (proton pump)

a primary active transporter in gastric parietal cells that uses ATP to pump H⁺ into the stomach lumen against its electrochemical gradient

• acidifies gastric contents → responsible for gastric acid secretion

• inhibited by Omeprazole (PPI)

omeprazole

a proton pump inhibitor (PPI) that reduces gastric acid secretion by irreversibly inhibiting the hydrogen-potassium ATPase in gastric parietal cells

secondary (2°) active transport

active, carrier-mediated transport of solutes against a gradient coupled with sodium where energy is derived indirectly from the sodium concentration gradient

• driving force for carrier = [Na⁺] gradient created by 1 active transport

  • Na⁺ moves downhill which provides energy to move solutes uphill

  • no [Na+] gradient = transport stops

• carrier may transport 1 or more solutes

• examples: sodium-glucose cotransport (SGLT), sodium-calcium exchange

sodium-glucose transport (SGLT)

sodium-calcium exchange

SGLT-1

The sodium-glucose cotransporter used in the small intestine for secondary active transport.

uniport

the transport of a single substance across the cell membrane by a carrier protein

symport / cotransport

the transport of more than one substance in the same direction at the same time by the same mechanism

• examples

  • SGLT-1

  • Na+ amino acids

antiport / countertransport / exchange

the transport of more than one substance in opposite directions across the cell membrane

• examples: Na+-Ca2+ exchange, Na+-H+ exchange

vesicular transport

the transpor of macromolecules in & out of the cell in the form of vesicles that fuse to the membrane

• 2 types

  • endocytosis

  • exocytosis

endocytosis

vesicular transport where the plasma membrane invaginates to move macromolecules from outside to inside the cell

• energy-dependent, active process

• example

  • phagocytosis

  • receptor-mediated

phagocytosis

form of endocytosis often called 'cell eating,' primarily used for solids like bacteria or dead tissue.

receptor-mediated endocytosis

the entry of viruses, toxins, or hormones into cells via the formation of clathrin-coated vesicles.

1. cell membrane invaginates making pocket containing macromolecules

2. pocket begins to pinch off

3. membrane closes around the material to form a vesicle

4. vesicle separates from cell membrane carrying material into cell cytoplasm

endocytosis

exocytosis

the process by which macromolecules in secretory vesicles fuse with the plasma membrane and are released outside the cell

• requiring Ca2+ and energy

• examples

  • release of water-soluble hormones

  • release of neurotransmitters at NMJ

1. vesicle moves towards cell membrane

2. fuses with membrane

3. membrane ruptures at the fusion point

4. release of contents

5. vesicle memrbane becomes part of cell membrane

exocytosis

osmosis

the movement of water across a membrane from an area of low solute concentration to high solute concentration

• [solute] difference creates pressure that makes H2O move

• H2O moves until the [solutes] are equal on both sides

• example

  • movement of H2O from intestinal lumen → blood when solutes move in the same direction

  • H2O transport in kidneys

osmotic pressure

colloid oncotic pressure

the osmotic pressure at the capillary level exerted specifically by proteins like albumin.

osmolarity

the concentration of osmotically active solutes

• calculated as C x g

  • C = [solute] in mmol/L

  • g = # of particles of solute in solution

• expressed as milliosmoles/L (mOsm/L)

Osmolarity Calculation (g)

The number of particles a solute dissociates into, where $g$ is 1 for glucose, 2 for $NaCl$, and 3 for $CaCl_2$.

isosmotic solution

two solutions having the same calculated osmolarity, such as $300\,mOsm/L$.

hyperosmotic solution

a solution having a higher osmolarity compared to another solution

hyposmotic solution

a solution having a lower osmolarity compared to another solution

isosmotic

isotonic saline

A $0.9\% \, NaCl$ solution used for intravenous rehydration because it causes no change in cell volume.

endosmosis

the entry of water into red blood cells when they are suspended in hypotonic saline

• RBCs bulge or rupture

exosmosis

the exit of water from red blood cells when suspended in hypertonic saline

• RBC shrink and become crenated