Biology Quiz 2

What bonds hold together the membrane?

hydrophobic interactions

How is the membrane so fluid?

Kinks in the fatty acid tails of phospholipids reduce tight packing, allowing lateral movement; if it is rich in phospholipids and unsaturated tails, it will remain fluid even in low temperatures

Factors that affect membrane fluidity

1. Unsaturated Tails

   1. kinked tails prevent packing leading to enhanced membrane fluidity

   2. more likely for organisms in cold temperatures

2. Saturated Tails

   1. pack closely together, increasing membrane viscosity

   2. more likely for organisms in hotter temperatures

3. Cholesterol

   1. reduce membrane fluidity at room temp; hinder solidification when temperatures drop

integral protein

• penetrate the hydrophobic interior of the lipid bilayer

• majority are transmembrane proteins

peripheral proteins

• not embedded in lipid bilayer; loosely bound to surface of membrane

membrane proteins location and functions

On the cytoplasmic side, they are held in place by attachment to the cytoskeleton; on the extracellular side, they may attach to materials outside cell

1. transport- provide hydrophilic channels or change shape to shuttle

2. enzymatic activity-

3. signal transduction- relays messages from outside to inside the cell by changing shape

4. cell-cell recognition- glycoproteins with ID tags; done by binding with carbohydrates on extracellular surface

5. intercellular joining- link the adjacent cells through gap junctions or tight junctions

6. attachment to the cytoskeleton and ECM- stabilize cell shape and location of certain proteins through noncovalent binding of microfilaments

Membrane Carbohydrates

• glycolipids

  • They are formed in the Golgi apparatus, where lipids acquire carbohydrates, transforming into glycolipids. This process is part of the broader synthesis and modification of membrane components, which also includes glycoproteins and secretory proteins.

• glycoproteins

  • proteins with carb groups bound

  • secretory proteins made in ER are added to transmembrane proteins (making them glycoproteins)

  • In golgi apparatus, they undergo further carb mods

  • glycoproteins, glycolipids, and secretory proteins are transported to the plasma membrane thru vesicles

  • vesicles fuse with plasma mmembrane and become continuous with cytoplamic face; this releases secretory proteins (exocytosis) and positions glycoproteins and glycolipids on extracellular face

What molecules pass through the membrane easily?

nonpolar (hydrophobic) and small molecules, no charge

channel proteins

transport proteins with a hydrophilic channel that help with passing of polar molecules; transport proteins are specific for the substance it translocates

aquaporins

passage of water molecules through membrane

carrier proteins

hold passengers and change shape to shuttle them across

traffic direction

determined by concentration gradient and membrane protein selectivity

General diffusion rule

In absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated (diffused down its concentration gradient); diffusion is spontaneous and each substance diffuses down its own gradient

Osmosis

Water diffuses across the membrane from the region of higher free water concentration to that of lower free water concentration (high solute concentration)

tonicity

the ability of a surrounding solution to cause a cell to gain or lose water; depends on nonpenetrating solutes

isotonic

no net movement because the concentrations are the same, water moves in both directions at same rate

animal: normal

plant: flaccid

hypotonic solution

animal: too much water moves in, lysing the cell

plant: cell wall pushes back down on the growing uptake of water (healthy)- turgid pressure to stop cell from taking in more water

hypertonic solution

animal: cell shrivels up

plant: plasmolyzed

osmoregulation

the control of solute concentrations and water balance; for organisms that lack a rigid cell wall

ion channels

• transport ions

• gated channels

  • open or close due to a stimulus

    • electrical or chemical

    • for chemical, it would be a specific solution that is not being transported binding to the cheannel

Sodium-potassium pump

• terminal phosphate group is transferred directly to the transport protein (change shape)

• High Na/ low K outside cell

• Low Na/ high K in cell

• When enough Na binds (three) phosphate group put on by ATP and Na are carried against gradient to outside

• two K ions come and diffuse inside cell, which causes loss of phosphate group=back to original shape

• antitransporter

membrane potential

• separation of opposite charges

• usually cytoplasm is more negative compared to extracellular matrix

  • usually favors passive transport of anions out and cations in

electrochemical gradient

driving force of diffusion across membrane with chemical force (ion’s concentration gradient) and electrical force (membrane potential of ion’s movement)

electrogenic pump

transport protein that generates voltage across a membrane; Na-K pump is the major electrogenic pump in animals; in plants: proton pump (transporting H+ ions out the cell); help store energy that can be tapped for cellular work

cotransport

• The coupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient.

• diffusion of one molecule/ ion helps with the uptake of a molecule usually not able to diffuse just by itself

  • plant: H+/sucrose: H+ diffuses into cell along with sucrose, proton pumps use energy to keep concentrations high on the outside of cell

  • animal: Na+/glucose: Na moving down its electrochemical gradient along with glucose, while Na/K pump maintains concentration on the outside

exocytosis

• cell secretes certain biological molecules by the fusion of vesicles with plasma membrane

• transport vesicle from GA move along microtubules to the membrane

• when vesicle and plasma membrane touch, they fuse together

• contents of vesicle then spill to the outside of the cell

endocytosis

• cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane

• reverse of exocytosis (vesicle already in membrane sinks inside taking in materials and pinching it shut)

• phagocytosis

  • cell engulfs particle and packing it within food particle to digest

• pinocytosis

  • gulps droplets of extracellular fluid into vesicles through infolding of plasma membrane

  • any particles taken into cell through this method are nonspecific

• receptor-mediated endocytosis

  • there are receptor sites exposed to extracellular fluid

  • only the solutes it wants will be inside the coated pits

  • used to take in cholesterol for membrane synthesis and synthesis of other steroids

  • hypercholesterolemia is high cholesterol in blood because LDL cannot enter cells due to receptor defects

paracrine signaling

• signaling cell acts on nearby target cells by secreting molecules of a local regulator

• growth factors in animals: stim nearby target cells to grow and divide

synaptic signaling

• nerve cell releases neurotransmitter molecules into synapses, stimulating target cell

• occurs in animal nervous system

• electrical signal move along a nerve cells to trigger secretion of neurotransmitter molecules carrying a chemical signal

endocrine signaling

• plant and animals using hormones that travel via the circulatory system to other parts of the body

epinephrine

• made by body to elicit fight or flight (mobilize fuel reserves)

• breakdown of polysaccharide glycogen within liver and skeletal muscle cells due to activation of cytosolic enzyme

reception

• target cell’s detection of a signaling molecule (chemical) coming from outside the cell

• can bind to receptor outside of cell or inside the cell

• signaling molecule is complementary in shape to the specific site on the receptor

  • ligand are what bind to receptors

• signaling receptors are plasma membrane proteins

  • ligands are water-soluble

GPCR

• cell-surface transmembrane receptor that works with the help of G-protein that binds the energy-rich molecule GTP

• activated G-protein leaves receptor and diffuses to any enzyme

  • changes enzyme’s shape and cellular response

• binding to GPCR is reversible

ligand-gated ion channel

• membrane receptor that acts as a gate for ions

• ligand binds to receptor, allowing the channel to open and let in specific ions

• when ligand dissociates, receptor closes

Intracellular Receptors

• found in cytoplasm or nucleus of target cells

• signaling molecule must pass through membrane to do this

  • can be a hydrophobic molecule

  • aldosterone

    • passes through membrane and binds to a receptor protein that can enter the nucleus and bind to a gene

    • activates transcription of gene into mRNA that is translated into a specific protein

      • transcription factors control which genes are turned on and off

transduction

• series of steps that convert that signal to a form that brings about a specific cellular response

• signal transduction pathway

  • relay molecules

    • something activates something which activates another etc

• steps could include activation of proteins by the addition or removal of phosphate groups through protein kinase or protein phosphatases

  • addition= phosphorylation which changes shape of protein

  • dephosphorylation= reverse of phosphorylation; phosphatases make it possible for protein kinases to be available for reuse

  • amplifies the signal

• Second messengers

  • small, non-protein, water-soluble molecules or ions

  • can spread readily throughout the cell by diffusion

  • cAMP and Ca2+

    • cAMP: adenylyl cyclase converts ATP to cAMP which activates another protein kinase A

      • activation caused by activated G-protein binding to it

      • binding of epinephrine increases cytosolic cAMP

response

• cellular activity finally done

  • cytoskeleton remodeling

  • activation of specific genes

    • transcription factors that synthesize mRNA

  • regulate activity of proteins