Home » Physiology » Transport across cell membrane

Transport across cell membrane

►Concentration of substances is different in ECF and ICF

►This difference is essential for survival

►Cell membrane ensures concentration difference between ECF & ICF

►Cell membrane is a selectively permeable barrier between ICF and ECF

Modes of transport

►Passive transport (diffusion)

►Active transport

►Vesicular transport

Passive transport (diffusion)

►Transport of molecules from high to low concentration due to their kinetic movement without using energy

Types

►Simple diffusion

►Facilitated diffusion

►Simple diffusion -net diffusion of molecules through cell membrane without using carrier proteins

►Through interstices of lipid bilayer

►Lipid soluble substances (non polar)

►O2 , CO2 , N2 , Fatty acids, steroid hormones

►Through protein channels

►Water & water soluble substances (polar)

►Urea, ions (Na+, K+, Ca++)

►Rate of diffusion increases in direct proportion with the concentration difference

Attributes of protein channels

►Selective permeability

►Size of the channel

►Charge on the channel

►Gating

►Voltage gating

►Chemical gating (ligand gating)

►Facilitated diffusion -diffusion of molecules through cell membrane with help of carrier proteins (carrier mediated diffusion)

►Follows maximum rate of diffusion (Vmax)

►Due to dependence upon carrier protein

Attributes of carrier proteins

►Specificity

►Presence of receptors

►Saturation

►Vmax

►Facilitated diffusion

►Glucose

Insulin increases number of carrier proteins (transporters) for glucose

►Amino acids

►Factors affecting net diffusion

►Concentration difference

►Temperature

►Surface area

►Mass of diffusing substance

►Diffusion distance

►Pressure difference

►Membrane electrical potential

Osmosis

►Net simple diffusion of water across a semi-permeable membrane

►Occurs at

►Cell membranes

►Capillary membranes

Osmotic pressure

►Pressure required to stop osmosis

►More concentrated a solution the more osmotic pressure it will exert

Osmoles

-number of osmotically active particles in a solution

►One osmole is equal to one mole of a non dissociated molecule

►1 mole of glucose=1 osmole

►1 mole of NaCl =2 osmoles

►1 mole of Na2SO4= 3 osmoles

Osmolarity

►Osmoles per liter of solution

►ECF osmolarity (mainly due to Na+)

►281 mOsm/L

Why to regulate ECF osmolarity?

►To maintain osmotic equilibrium between ICF & ECF

►Important for cell survival

►Hyperosmolar coma (diabetes mellitus)

Active transport

►Movement of substances againstconcentration gradient using energy with the help of carrier proteins

Types (on the basis how energy is derived)

1) Primary active transport

►Energy derived directly by cleaving ATP

2) Secondary active transport

►Energy derived by concentration gradient

Primary active transport

►Active transport of a molecule by directly cleaving ATP for the use of energy

►Carrier proteins involved

►Called pumps

►Have ATPase activity

►Can break ATP to release energy

►Examples

►Na+, K+, Ca++, H+, I

Sodium-potassium ATPase pump

►A carrier protein

►Present in all the cells

►A complex of 2 globular proteins

►Possesses ATPase activity on inside

►3 receptors for Na+on inside

►2 receptors for K+on outside

►Pumps 3 Na+outwards for 2 K+inwards

Functions:

►To establish Na+& K+concentration gradients between ICF and ECF

►Development of electrical potential across cell membrane

►Cell volume regulation

►Ca++ATPase pump

►Pumps Ca++out

Located at

►Cell membrane

►Intracellular membranes (sarcoplasmic reticulum of muscle cells)

►H+ATPase pump

►Pumps H+out of the cells

Located at

►Cell membranes of

►Tubular cells (intercalated cells) of late distal tubules of kidney

►Parietal cells of stomach

Secondary active transport

►Active transport of a molecule by using energy stored in the form of concentration gradient of some other molecule

►Mediated by carrier proteins

►Carrier proteins do not have ATPase activity

►ATP can not be cleaved to liberate energy

►Na+is generally utilized for secondary active transport of other molecules

►Na+concentration gradient is established in first place by primary active transport

►Na+while moving downhill into the cell moves other molecules uphill

►Types

1)Co-transport (symport)

►Na+ and molecule to be transported move in the same direction (into the cell)

2)Counter transport (antiport)

►Na+and molecule to be transported move in opposite direction (Na+always inwards)

►Co-transport

►Na+ -glucose co-transport

►Renal proximal tubular cells

►Na+ -amino acids co-transport

►Renal proximal tubular cells

►Counter transport

►Na+ -Ca++counter transport

►In all the cells

►Na+ -H+counter transport

►Renal proximal tubular cells

Vesicular transport

►Movements of substances across cell membrane in vesicles using energy

►Large molecules are transported

►Types

►Endocytosis

►Phagocytosis

►Pinocytosis

►Exocytosis

►Transcytosis

►Pinocytosis

►‘Cell drinking’

►Macromolecules, ECF fluid and solutes

►Cell membrane invaginates pinocytic vesicle

►Phagocytosis

►‘Cell eating’

►Large particles, bacteria, dead cells

►Function of tissue macrophages and white blood cells

►Cell membrane evaginates by pseudopodia → phagocytic vesicle

►Exocytosis

►Reversal of endocytosis

Transcytosis

►Movement of materials from one side of the cell to other using vesicular transport

►No contact of vesicle with lysosomes

►First endocytosis and then exocytosis

►Transfer of proteins through capillary endothelial cells

Check Also

Hormones- Mechanism of Action, Regulation and Clearance

Hormones are chemical messengers, directly secreted into the blood or extracellular fluid, which bind specific …

7 comments

  1. Love the blog here. Nice colors. I am definitely staying tuned to this one. Hope to see more.

  2. The content on this submit is really 1 of the most beneficial material that I’ve ever occur across. I love your submit, I’ll are available back to examine for new posts.

  3. Wonderful article post on the blog bro. This valuable is just a totally nicely structured post, just the information and facts I was searching just for. Thanks

  4. We simply could not depart your website prior to stating that I truly enjoyed the standard information you provide for your visitors? Will be back again soon in order to check up on new articles

  5. I am grateful to you for this good content. You truly did make my day :

  6. Nice piece of facts that you’ve got in this weblog submit. Hope I can get some much more of this stuff on your website. I’ll appear back.

  7. Wow! This really is one particular of the most beneficial blogs I’ve actually arrive throughout on this subject. Basically Amazing

Leave a Reply to Auto Traffic Hijack Cancel reply

Your email address will not be published. Required fields are marked *