Red blood cell structure and function

Question 1

describe the structure of RBCs

Answer 1

→Mammalian RBCs are biconcave disc-shaped.

→ have flexible membranes with a high surface-to-volume ratio.

→They are 8 microns in diameter but are able to deform and pass through capillaries or RES without fragmentation.

→The RBC is unique amongst eukaryotic cells as it lacks a nucleus, mitochondria and ER, and its cytoplasm is essentially rich in haemoglobin.

Question 2

what is the function of red blood cells and what are the requirements for this ?

Answer 2

→ transport of respiratory gases to and from the tissues.

→ RBC should be capable of transversing the microvascular system without mechanical damage, and retain its shape

→the red cell membrane should be extremely tough, yet highly flexible

Question 3

describe the structure of the red blood cell membrane and what it consists of

Answer 3

→semipermeable lipid bilayer with proteins scattered throughout.

It consists of
→an outer hydrophilic portion composed of glycolipids, glycoproteins and proteins

→ a central hydrophobic layer containing proteins, cholesterol and phospholipids

→an inner hydrophobic layer of mesh-like cytoskeletal proteins to support the lipid bilayer

Question 4

how much by % do RBC membrane lipids make up?

and describe the distribution of RBC membrane lipids

Answer 4

→RBC membrane lipids make up about 40% of the membrane.

→There is an asymmetrical phospholipid distribution throughout the RBC membrane.

→ unesterified free cholesterol between the phospholipids

Question 5

what are the types of phospholipids in the RBC membrane and where are they found?

Answer 5

UNCHARGED PHOSPHOLIPIDS IN THE OUTER LAYER →phosphatidylcholine (PC)
→sphingomyelin (SM)

CHARGED PHOSPHOLIPIDS IN THE INNER LAYER →phosphatidyl ethanolamine (PE)
→ phosphatidyl serine (PS)

Question 6

how does membrane cholesterol exist within a RBC?

Answer 6

→Membrane cholesterol exists in free equilibrium with plasma cholesterol.

Question 7

what does an increase in free plasma cholesterol mean?

Answer 7

→An increase in free plasma cholesterol results in an accumulation of cholesterol in the RBC membrane.

Question 8

what % of the membrane do RBC membrane proteins make up?

Answer 8

50%

Question 9

what are the two categories of RBC membrane proteins?

Answer 9

→integral membrane proteins and peripheral membrane proteins

Question 10

describe integral membrane proteins and give examples

Answer 10

→extend from the outer surface and traverse the entire membrane to the inner surface.

→Two major integral membrane proteins are Glycophorins (types we’ve identified are A, B and C) and Band 3 (an anion transporter).

→other integral membrane proteins:
→ Na+/K+ ATPase
→Aquaporin 1
→surface receptors (eg. TfR)

Question 11

describe peripheral membrane proteins and give examples

Answer 11

→limited to the cytoplasmic surface of the membrane and form the RBC cytoskeleton.

Major peripheral proteins include: 
→ spectrin 
→ankyrin 
→protein 4.1 
→actin

Question 12

what are the functions and properties of spectrin?

Answer 12

→Spectrin is the most abundant peripheral protein.

→composed of α and β chains

→ important in RBC membrane integrity as it binds with other peripheral proteins to form the cytoskeletal network of microfilaments.

→It controls the biconcave shape and deformability of the cell.

Question 13

what is the function of ankyrin?

Answer 13

→anchors the lipid bilayer to the membrane skeleton via interaction between spectrin and Band 3.

Question 14

what is the function of protein 4.1?

Answer 14

→links the cytoskeleton to the membrane by means of its associations with glycophorin.

→It also stabilizes the interaction of spectrin with actin.

Question 15

what is the function of actin?

Answer 15

→responsible for the contraction and relaxation of the membrane.

Question 16

what are the functions of the RBC membrane?

Answer 16

→shape : provides the optimum surface area to volume ratio for respiratory exchange

→provides deformability and elasticity : allows for passage through microvessels (capillaries)

→regulates intracellular cation concentration

Question 17

what are red cell metabolic pathways for?

Answer 17

Metabolism provides energy required for:
→maintenance of cation pumps
→maintenance of Hb in its reduced state
→maintenance of reduced sulfhydryl groups in Hb and other proteins
→maintenance of RBC integrity and deformability

Question 18

what are the key metabolic pathways in RBCs?

Answer 18

→Glycolytic or Embden-Meyerhof Pathway
→ Pentose Phosphate Pathway
→Methaemoglobin Reductase Pathway
→ Luebering-Rapoport Shunt

Question 19

what does the glycolytic or Embden-Meyerhof pathway do?

Answer 19

→It generates 90-95% of the energy needed by RBCs.
→In it, glucose is metabolised and generates two molecules of ATP.
→It functions in the maintenance of the RBC shape, flexibility and cation pumps.

Question 20

what does the pentose phosphate pathway do?

Answer 20

→RBCs need GSH to protect them from oxidative damage.

→The pentose phosphate Pathway provides the reducing power, NADPH.

→NADPH maintains glutathione in its reduced form (GSH).

Question 21

what is the methaemoglobin reductase pathway for?

Answer 21

→It maintains ion in its ferrous state (Fe2+).

→In the absence of this enzyme, methaemoglobin accumulates and cannot carry oxygen.

Question 22

what does the Luebering-Rapoport shunt do?

Answer 22

→permits for the accumulation of 2,3-DPG, which is essential for maintaining normal oxygen tension, regulating haemoglobin affinity.

Question 23

what are the methods RBCs use to avoid structural deterioration?

Answer 23

→geometry of cell & surface area to volume ratio facilitates deformation whilst maintaining constant surface area

→membrane deformability
spectrin molecules undergo reversible changes in conformation, some are uncoiled and extended, whilst others are compressed and folded

→cytoplasmic viscosity determined by MCHC :
as MCHC rises, viscosity rises exponentially

Question 24

what determines the strength and flexibility of RBCs?

Answer 24

→ interactions between the membrane & cytoskeletal proteins

Question 25

what two effects does an increase in cholesterol have on RBCs?

Answer 25

→RBCs with increased cholesterol levels appear distorted resulting in acanthocytosis.

→ cause of target cells

Question 26

what maintains surface area in the RBC?

Answer 26

→ strong cohesion between bilayer and membrane skeleton

Question 27

what is the structure of haemoglobin?

Answer 27

→ globular haemoprotein
→ contain haem as a tightly bound prosthetic group
→ complex of protoporphyrin IX and ferrous iron (Fe 2+)
→ iron held at the center of the haem molecule by bonds to the 4 nitrogen of porphyrin
→ 2 alpha and 2 beta globin chains