Systematic Homeostasis

- Maintain a constant internal environment within human body for cells and tissues

- Some examples of homeostasis: 

• Water regulation (Osmoregulation)
• Temperature regulation (Thermoregulation)
• Glucose regulation

Feedback Loops

• Self-regulatory systems (automatic) in which
• It will feedback to the input a part of a system's output (Positive / Negative feedback)
• So as to reverse or enhance the direction of change
• 2 types of feedback loops
• Negative feedback loops (common, e.g. in regulation, the 3 types as stated above: Body temperature (thermoregulation), Amount of water in the body (osmoregulation), Glucose concentration in the blood
• The process can reduce / increase the output of a system in order to re-establish steady state / set point
• Positive feedback loops (rare, not meant to maintain homeostasis, but a mechanism we can see in our body)
• The process increases the output of the system, further enhancing deviation from steady state / set point
• Deviation from set points leads to enhancement of amplification of input
• Examples include: Oxytocin (Giving birth), Histamine (Rashes: the more you scratch, the mort histamine released, due to damaged cells, which makes it swell and itchy), Blood clotting
• Mosquito itch (Anti-coagulant), prevents blood from clotting, recognized as foreign substance by body -> itchy skin

What are the main components in a feedback loop?
1. Stimulus (Produces change in variable)
2. Change detected by receptor -> Receptor (Sensor)
3. Input: Information sent along afferent pathway to -> Control Center
4. Output: Information sent along efferent pathway to -> Effector
5. Response of effector feedsback to influence magnitude of stimulus and returns variable to homeostasis

Cell Membrane (4) - Thinking Exercise

Thinking exercise: Compare channel & carrier proteins

Question: How could regulation of substances in and out of cells result in cellular homeostasis? Give an example to illustrate.

Answer: Homeostasis is the regulation of processes to maintain a constant, stable, living internal environment despite other external conditions. Hence, regulation of substances in and out of cells will result in cellular homeostasis. Example: The cell membrane regulates substances in and out of cells. When the cell is put in a substance with a much higher water potential, osmosis will take place. This is when the molecules of the cells with a higher water potential move towards the cells with a lower water potential, to maintain an equilibrium between the substances. An equilibrium would have been reached when the concentrations of both substances are the same.

Question: Think of a cell within our body that can do this and what is the transport mechanism that is happening that is helping.
Answer: Villi Cell (Small Intestine). The villi cells have a rich blood supply, which has a lower concentration of food molecules as compared to the surroundings. Hence, diffusion of the food molecules will take place and homeostasis will be achieved.

Cell Membrane (3)

Transport of Substances in & out of Cells across Membrane
Passive Transport

• Does not require ATP
• Driven by concentration gradient (concentration difference) – moving down its concentration 
• Occurs in both living and non-living systems
• Mechanism
• Simple Diffusion / Diffusion (Oxygen, CO₂, minerals in soil etc)
• Types of substances that undergo diffusion are:
• Small, hydrophobic
• Lipid, soluble
• Small, hydrophilic (slower)
• Diffusion
• Spread of particles through random motion from regions of higher concentration to regions of lower concentration
• Simple Diffusion
• Hydrophilic molecules diffuse into cells very slowly, because of repulsion faced in the hydrophobic interior core of the lipid bilayer
•       Osmosis (water only)
•        Net movement of water molecules across a selectively permeable membrane from an area of less negative (high) water potential, to an area of more negative (low) water potential
•       Facilitated Diffusion (carrier proteins and pores which increase diffusion rate)
•        Type 1: Transmembrane proteins (Changes shape to facilitate entry and exit of some nutrients, e.g. fructose) create a water-filled pore through which ions and small hydrophilic molecules can pass by diffusion. Channel protein can open and close like gates
  Some molecules are too large for ion channel (e.g. amino acid, glucose etc)
•        Type 2: Some molecules are too large for ion channel (e.g. amino acids, glucose). Solution = carriers. Carrier protein has a specific binding site. Once substance binds-> conformation change of protein occurs. Move substance across. Release and protein returns back to original conformation

Active Transport

• Requires the use of energy (ATP) (E.g. Proton pump)
• Substances are transported against concentration gradient by specific proteins
• Occurs only in living systems
• Transport is specific-- carrier can only recognize certain molecules / ions
• Glucose and amino acids taken in by active transport to the villi cells of small intestine where there is lower concentration of these molecules in the lumen of the small intestines than the blood capillaries 

Endocytosis (Bulk Transport)

• Phagocytosis
• Formation of pseudopodia observed
• Pinocytosis
• Invagination of cell membrane

Exocytosis (Bulk Transport)

• Where secretory vesicles fuse with cell surface membrane
• E.g. Secretion of extracellular enzymes, hormones and antibodies, removal of waste products of digestion
• Are Endocytosis and Exocytosis a form of Active Transport?
• Yes. They require ATP and go against the concentration gradient.

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Cell Membrane (2)

Functions of Cell Membrane

1. To compartmentalize the cell as seen in different organelles. Different metabolic processes require different enzymes and conditions. Compartmentalization provides optimum conditions for the reaction to occur without interference from other enzymes and factors. It also prevents the autolysis of cells, e.g. Enzymes of pancreatic juice are found in vesicles prior to their secretion (Auto – self, Lysis – breakdown = Breakdown by itself)
2. Increases surface area for exchange of substances. E.g. Microvilli of intestinal cells making up the villi
3.  Cell recognition. Membranes contain certain protein / glycoprotein / glycolipids molecules which act as identity markers. E.g. Antigens on red blood cells
4.  Cell communication. Some proteins / glycolipids / glycoproteins embedded in the membrane act as receptors for hormones and neurotransmitter
5. Site of chemical reactions. E.g. light reactions of photosynthesis take place on thylakoid membranes found in chloroplasts
6.  Controls entry and exit of substances. Separates cytoplasm from external environment maintaining constant environment inside cell. Cellular homeostasis. 

      Extra

      Liver cells perform 500 different functions. A sign that the liver is not working: Yellow skin

Cell Membrane (1)

Water

- IMPORTANT because it serves as the universal solvent in biological systems (living organisms), and makes up 70% or more of the weight in cells

- Solutes (e.g. sugars, organic acids, and some of the amino acids) that have an affinity of water and therefore dissolve readily in it are called hydrophilic. [Hydrophilic = Hydro (Water) + “Philic” (Loving)]
- Molecules that are not very soluble in water, such as lipids and some proteins found in biological membrane, are termed hydrophobic
- Some biological macromolecules, notable proteins and phospholipids, have both hydrophobic and hydrophilic regions on the same molecule. Such molecules are known as amphipathic molecules.

Phospholipids

-  Lipids with phosphate groups (PO₄^3-) that are hydrophilic while hydrocarbon chairs are hydrophobic

-  When phospholipid molecules come into contact with water, they tend to line up polar heads in water and hydrocarbon tails away from water

Cell Surface Membrane Structure

Fluid Mosaic Model

Fluid- a bilayer that allows both phospholipids and proteins to move about laterally

Mosaic- collage of proteins randomly distributed in the bilayer

Model- a hypothesis proposal supported by evidence

- Fluid goes through transverse diffusion (flip-flop) or lateral diffusion (frequent)

Cell membrane (mosaic) Structure and Composition

Proteins

• Interspersed randomly among phospholipids
• 2 main types
• Peripheral / Extrinsic proteins
• Loosely attached at charged surface / hydrophilic part of phospholipid bilayer
• Integral / Intrinsic proteins
• Either partially penetrating the phospholipid bilayer or spanning the membrane entirely
• Proteins partially embedded in phospholipid bilayer:
• Contains both hydrophilic and hydrophobic regions to interact with charged heads and hydrocarbon tails of phospholipid bilayer respectively

Glycoproteins (Also a type of protein but slightly different)

• Interspersed among phospholipids
• Consists of carbohydrate chains bound to peripheral proteins and hydrophilic regions of integral proteins that occur on exterior surface of outer membrane
• Carbohydrate chains involved in recognition of same cell type or adhesion of cells to neighboring cells for immune response

Glycolipids

• Interspersed among phospholipids
• Consists of carbohydrate chains bound to two hydrophobic hydrocarbon tails
•              Involved in recognition of same cell type or cell signaling pathways

Cholesterol

• Interspersed among phospholipids
• Essential in maintaining membrane fluidity

Extra :)

Why do you not feel pain?

You will feel pain from your nerve cells, so as long as you don’t touch your nerve cells, it will be okay. Areas where you hardly feel pain: Elbow XD

Seafood has a lot of cholesterol. Why?

Cold temperature. Cholesterol helps you adapt to a cold environment, and prevents the membrane from becoming frozen.