Cell Membrane and Transport

Integral proteins are embedded in the cell membrane.
Peripheral proteins are attached to the membrane, either extracellularly or intracellularly, but are not an intimate part of it.

Channel proteins:
- Have an open pore.
- Allow specific molecules to pass through.
- Multiple specific molecules can bind to the protein.
Carrier proteins:
- Bind to a very specific molecule.
- Escort that molecule across the membrane (either from outside to inside or vice versa).
- Highly specific for one type of molecule.

Docking:
- Loose, temporary attachment (like tying a boat to a dock).
- Serves as an anchor.
- Performed by docking-type proteins, which are anchoring proteins.
Binding:
- Firm attachment, like a lock and key.
- Highly specialized.
- Stronger and longer attachment.
- Typically involves chemical messengers (hormones).
- Results in action; an actionable interaction.

Receptors are large proteins that transmit a signal from a messenger to the cell, causing a cellular response.
This process is called signal transduction.
A receptor has part of its protein exposed to the extracellular environment and part to the intracellular environment.
When a messenger binds to the receptor, it initiates a signaling cascade, a sequence of activation of other molecules inside the cell for the cell to respond.
- Analogy: domino effect.

Docking proteins are common in the digestive tract, serving as anchoring sites for enzymes.
Example: lipase, which cuts up lipids.
Peripheral proteins can be enzymes attached to docking proteins on the surface of columnar cells in the small intestine.

Cells need the tiniest parts of molecules to absorb them.
Proteins must be broken down into amino acids.
Lipids must be separated into fatty acids.
Digestion starts in the mouth and continues in the stomach, turning food into chyme, where nutrients are separated (proteins, carbohydrates, lipids, vitamins, and minerals).
Enzymes in the digestive tract break down nutrients into single molecules for cellular uptake and transport to the blood, then to the liver, and finally to the rest of the body.
Example: Blood drawn after eating a fatty meal can appear milky due to the presence of broken down lipids in the bloodstream.
Enzymes like lipases break down fats quickly, leading to their uptake into the bloodstream.

Receptors are not channels or carriers; they have parts sticking outside and inside the cell.
They transmit a signal when bound to a chemical messenger.
Peripheral proteins inside the cell can be part of the transduction cascade.
Docking proteins are inserted in the membrane and serve as anchors for enzymes to grab nutrients.

Cells are held together to perform specialized functions in specific tissues (e.g., heart, liver, pancreas).
Large proteins serve as adhesion points, and some facilitate communication.

Colostrum is nutrient- and immune-dense milk made in the first 24 hours.
Newborns need colostrum because their guts are open, meaning they lack sufficient tight junctions.
This allows immune cells, antibodies, and nutrients to slip between cells.
After 24 hours, tight junctions are in place (gut closure), preventing passage between cells.
Bacteria can slip between cells in newborns, necessitating the immune components from colostrum.

Large proteins forming a channel or pore between cells.
Allow transfer of cytosol (nutrients) from one cell to another.
Example: Gametes (oocytes and sperm cells) lack a full set of chromosomes and are vulnerable to immune rejection if they come into contact with blood.
Supporting cells around gametes take up nutrients from the blood and transfer them to the gametes via gap junctions.
We will revisit the concept of gap junctions when discussing cardiac muscle tissue.

Cells have large proteins that identify them by species, gender, and tissue type, which is important for the immune system to recognize and eliminate foreign or misplaced cells.
Examples:
- Dog oocytes cannot be placed in a cow because the cow's immune system will recognize and eliminate them.
- A liver cell in the brain would be attacked and eliminated.
- Goat A's liver cell versus Goat B's liver cell have slight variations that make the individual unique.

Some large proteins, especially those associated with binding receptor-type proteins, have enzymatic activity to accelerate signal transduction.
Enzymes speed up chemical reactions.

Two types of transportation across the cell membrane:
- Passive transport (no energy used).
- Active transport (energy used, e.g., ATP).

Passive movement of solutes (molecules dissolvable in a liquid medium, usually water).
Factors affecting the rate of transportation:
- Concentration gradient: The greater the difference in solute concentration on either side of the membrane, the faster the diffusion.
- Size of the molecule: Smaller solutes move faster than larger ones.
- Charge: Opposite charges attract; like charges repel. The inside of a cell is typically negatively charged, while the outside is positive. Example: sodium being drawn into a the negatively charged cell.
- Temperature: Higher temperatures speed up reactions and diffusion. Molecularly speaking, molecules move faster and collide more frequently at those higher temperatures.
- Membrane lipid solubility: Lipid-soluble solutes (e.g., steroids) can easily pass through the lipid layer of the membrane. Because steroids are lipid, they can enter any cell and be stored there. Steroids must be observed to make sure that they do is below the safe amount.

Requires the help of a carrier protein but does not consume energy.
Example: glucose transporter (GLUT).
Intestine cells are heavily populated with these large protein systems to bring nutrients into the bloodstream.
Analogy of how this is a cheaper more efficient process: it would be inefficient to have to use ATP only to create ATP, as that's like using the fuel in your vehicle to fuel the fuel in your vehicle. Therefore, if possible, cells will use facilitated diffusion to carry nutrients in.
Glut = Capital G, L, U, capital T. = glucose transporter. Integral protein with extracellular and intracellular portion. Binds > Closes top > Opens at bottom.

GLUT 1
- Found in red blood cells. The job of red blood cells is to transport O2 for CO2 transfer. The red blood cells are swimming in glucose the whole time, so it does not have to work hard to get the Glucose, hence the:
- Low affinity for glucose.
GLUT 2
- Found in the liver which metabolizes, packages, and dispersed tissues.
- High affinity.
- Bidirectional (can bring glucose in and out of the cell). Has to have the ability to get glucose out in the event of something like a shortage. When we lack nutrients, the liver can perform gluconeogenesis and make it for us.
GLUT 3
- Found in nervous tissue (neurons, neuroglia cells).
- High affinity.
GLUT 4
- Muscle, bone, fat, all cells have these transporters, are big sinks for insulin-dependent glucose.
- Hormone-regulated (insulin).
- Activity induces hormone independence. Running or activity causes muscle to say it needs more glucose. In the case of someone suffering from elevated glucose count, activity can help lower it down to a more normal range.