B2.1 Membranes & Membrane Transport

What does the phospholipid bilayer form? What’s it made up of?

The phospholipid bilayer forms the structure of cell membranes; It is made up of phospholipids with a hydrophobic fatty acid tail (non-polar), and a hydrophilic head (polar) made up of a phosphate group and a glycerol molecule

What makes the lipid bilayer selectively permeable?

The fact that it is made up of a hydrophilic outer region that can interact with water in the cytoplasm; and a hydrophobic core that repels polar molecules which are soluble in water, controlling wht can pass through the membrane.

What are the two categories of membrane proteins? What are some of their jobs? What about physical properties?

Integral proteins = Permanently embedded inside the plasma membrane. They can be channels, carriers, receptors, or enzymes. They are amphipathic, the region embedded in the hydrophobic core of the phospholipid bilayer is hydrophobic, the exposed portions are hydrophilic.

Peripheral proteins = Attached to the surface of the membrane or attached to integral proteins through hydrocarbon chains. They can help maintain the cytoskeleton of the cell and can be receptors for signaling molecules (hormones and neurotransmitters). Are hydrophilic, because they are exposed to the outside of the cell.

Which are the two types of integral proteins? Describe their basic function

Integral membrane proteins are the ones embedded within the phospholipid bilayer and can either span one or two layers.

1. Transport Proteins (this includes channel proteins and carrier proteins): They carry molecules across the membrane

• Channel proteins - form pores that allow molecules to pass freely

• Carrier proteins = change shape to actively transport molecules, example: protein pumps
  

2. Receptor proteins = Bind to hormones, neurotransmitters, and antibodies to trigger intracellular responses (ex. Glycoproteins)

What is a key distinction between Glycoproteins and Glycolipids?

Glycoproteins are typically Integral proteins, meaning unlike Glycolipids, they are actually proteins that are directly embedded into the phospholipid bilayer that also have a carbohydrate attached.

On the other hand, glycolipids are just loosely attached to the cell membrane, and are NOT Integral proteins; but they are also NOT peripheral proteins, because they are not proteins at all. They’re just another thing attached to the cell membrane.

What are some other molecules that you can find in the phospholipid bilayer that are not necessarily just proteins?

1. Glycoproteins and Glycolipids

   • Glycoproteins = proteins attached to carbohydrate chains
     (antigens on red blood cells/involved in cell-cell recognition)

   • Glycolipids = lipids attached to carbohydrate chains that help with cell recognition and immune signaling
     

2. Cholesterol = A lipid, not a protein, embedded in the cell membrane

   • interacts with the hydrophobic tails, stabilizing the cell membrane and making it more rigid & less permeable to small molecules
     

3. Immobilized Enzymes = Enzymes that have been made immobile (are stuck) in the cell membrane, with active sites that are exposed to allow reactions to take place
   

4. Cell Adhesion Proteins (CAMs) = Help cells attach to other cells to form tissues

What are the categories of membrane transport? Name all the different types of transport:

PASSIVE TRANSPORT (no energy required)

Simple Diffusion = Not Selective, movement from high to low concentration

Facilitated Diffusion = Not Selective, movement through channel & carrier proteins

Osmosis = Not Selective, movement of water through aquaporins

ACTIVE TRANSPORT (energy required, selective)

Primary Active Transport = Selective, movement of molecules against concentration gradient using ATP

Secondary Active Transport = Selective, movement of two molecules at the same time, one moving down the concentration gradient, other going against concentration gradient (usually an ion). No ATP used, indirectly powered by Primary Active Transport

BULK TRANSPORT (energy required, selective)
Endocytosis (engulfing particles)

• Pinocytosis = engulfing liquids (think of a pina colada)

  (Pinocytosis is a type of endocytosis)
  

Exocytosis = vesicles (already containing waste) fusing with the plasma membrane and releasing waste to the outside of the cell

What is simple diffusion? What kind of transport is this?

Simple diffusion is the net movement of molecules from a high to a low concentration because of random kinetic energy.
It does not require energy or transport proteins, molecules can just pass directly through the phospholipid bilayer;

Can you give an example of simple diffusion in the human body?

• Oxygen diffuses into cells from capillaries (used in respiration, creating a gradient).

• Carbon dioxide diffuses out of cells (produced in respiration, creating a gradient).

What are some of the factors that impact diffusion rate in Simple Diffusion? Is simple diffusion selective?

Simple diffusion allows and small polar molecules to pass through, and diffusion is dictated by the amphipathic nature of the phospholipid bilayer

Steepness of the concentration gradient = A higher concentration gradient means a faster rate of diffusion

Temperature = Higher temperature increases kinetic energy = faster diffusion.

Surface Area = Larger membrane surface = increased diffusion rate

Size & Properties of Molecule = Small, non-polar molecules diffuse faster, while large or charged molecules diffuse slower.

Simple diffusion is not selective, it allows any molecule that can dissolve in the phospholipid bilayer to cross as long as its small enough to fit

What is osmosis? Does water even require help getting through the membrane? Does the process require energy? How does water move in osmosis?

Osmosis is the diffusion of water across a semi permeable membrane, moving from an area of low solute concentration to high solute concentration. Water potential determines the movement in osmosis;

It moves from an area of higher water potential (low solute concentration) to an area of lower water potential (higher solute concentration)

Key Features of Osmosis:

• Water can move directly through the bilayer on its own but it is helped by aquaporin channels for faster movement

• Water is unusual because, despite being polar, it can pass through the membrane more easily than most polar molecules

• No energy is required, water moves passively through the channel

What is facilitated diffusion? What are some key features of facilitated diffusion?

Facilitated diffusion is SELECTIVE; it is the movement of large/polar/charged molecules through the cell membrane through carrier and channel proteins

Key Features of Facilitated Diffusion:

• It requires no energy and moves from an area of high concentration to low concentration
  

• Uses 2 transport proteins:

  #1. Channel Proteins = Pores for specific ions or molecules to travel through; Some Channel proteins are gated.
  

  #2. Carrier Proteins = Undergo a conformational change to transport substances across the membrane; No energy needed

What is active transport? What are some of the key features of active transport?

Active transport is the movement of molecules or ions against their concentration gradient (so from low to high concentration) using ATP

Key Features of Active Transport:

• It requires energy, ATP produced during cellular respiration is used

• It uses protein pumps, a Carrier Protein that uses ATP to move molecules against their concentration gradient

• No Channel Proteins Involved

What’s the difference between Channel Proteins and Carrier Proteins?

Channel Proteins are pores that allow specific molecules to passively pass through

Carrier proteins bind to the molecule they are transporting, undergoing a conformational change to move it across

What distinguishes Carrier proteins from Channel proteins?

Carrier proteins can change shape to transport molecules; The molecule binds to the binding site of the protein, which causes it to change shape and pass through

How does the membrane itself control transport?

Through selective permeability, only allowing small, non-polar molecules to diffuse through without the help of proteins. This is because the fatty tails facing inwards of the phospholipid bilayer are hydrophobic, and non-polar themselves

Describe the structure of a glycoprotein vs a glycolipid?

A glycoprotein is made up of a protein and a short carbohydrate chain (an oligosaccharide)

A glycolipid is made up of a lipid and a short carbohydrate chain

Where can you find glycolipids and glycoproteins? What is their function in general?

Found attached to the outer surface of the membrane.

Cell Signaling = Act as receptor molecules, binding to hormones or neurotransmitters to trigger responses inside the cell

Cell Recognition = Help the immune system recognize itself vs foreign cells, and can act as antigens on the surface of red blood cells

Cell adhesion = Help cells stick together and form tissues

Endocytosis Receptors = Helps engulf molecules into vesicles

What does the fluid mosaic model state?

• Fluid – It states that phospholipids and proteins can move freely, making the membrane flexible.

• Mosaic – It states that membrane proteins appear scattered across the bilayer, forming a patchwork pattern.

What are the key components of the fluid mosaic model:

Phospholipids – Form the bilayer, creating hydrophobic and hydrophilic regions.

Cholesterol – Controls the fluidity of the cell membrane; The more cholesterol there is, the less fluid the cell membrane is. This prevents it from being either too rigid or too loose.

Glycoproteins & glycolipids – Helps with recognition, adhesion, and signaling.

Membrane proteins – Classified as integral (embedded) or peripheral (attached externally).

What influences the fluidity of the cell membrane?

#1. Fatty Acids

Saturated fatty acids (C-C single bonds) create straight, highly packed chains that are stable at high temperatures

Unsaturated fatty acids (C=C bonds) create kinks, preventing tight packing and increasing the fluidity and flexibility of the cell membrane at low temperatures

#2. Cholesterol

High amounts of cholesterol makes phospholipids pack together, causing membranes to be more stable and rigid at high temperatures

Less cholesterol prevents phospholipids from tightly packing together, increasing the cell membrane fluidity at low temperatures

What are some mechanisms that specimens have for regulating the fluidity of the cell membrane?

In cold temperatures, some bacteria produce “fatty acid desaturases”, enzymes that introduce double bonds (C=C) into fatty acids, making the membranes more fluid in the cold (since they tend to get stiff)

Deep-sea organisms (piezophiles) can adjust their fatty acid saturation (single or double bonds) based on water temperature

In response to high temperatures, some plants can modify their fatty acids

What is bulk transport? Describe the different types of bulk transport

Bulk transport is moving large amounts of material IN OR OUT of the cell through vesicles.

#1. Endocytosis - The plasma membrane engulfs materials, enclosing them in vesicles to be transported into the cell

• Phagocytosis: Solid materials are engulfed by the vesicles, bringing them into the cell (e.g., bacteria getting engulfed by immune cells).

• Pinocytosis (“pina colada” “cell drinking”): Liquids & Dissolved Substances are engulfed by the vesicles
  

#2. Exocytosis = Vesicles fuse with the plasma membrane to release waste substances (like secreting hormones or enzymes)

What are vesicles?

Vesicles are small sacs that secrete, uptake and transport materials in the plasma membrane

What are gated ion channels? Can you give an example of a gated ion channel?

Gated Ion Channels = Channel proteins that open in response to chemical (neurotransmitter) or electrical signals

Ex. Nicotinic Acetylcholine Receptors
Opens when acetylcholine or nicotine binds to it. When acetylcholine binds, it allows Na⁺ and Ca²⁺ ions to flow into muscle cells, triggering muscle contraction. When nicotine binds, it allows Na⁺ and Ca²⁺ ions to flow, leading to neural activation that can impact brain function, mood, and addiction pathways.

What are sodium potassium pumps?

Sodium Potassium Pumps are Carrier Proteins used during Active Transport to pump Na+ ions out, and K+ ions into the cell. It requires ATP.

How do sodium potassium pumps work, go through the steps; Why are sodium potassium pumps important?

The process:

Three Na+ (sodium) ions (already on the inside of the pump) attach to the 3 binding sites inside the Sodium Potassium Pump.

ATP is hydrolyzed (broken down by water), releasing a single phosphate group that attaches to the pump, causing the pump to change shape (conformational change).

The conformational change causes it to release the 3 Na+ ions to the outside of the cell, and creates an opening for 2 K+ ions from the outside to bind.

Two K+ ions from outside attach to the binding sites on the “extracellular side” of the sodium potassium pump.

After the K+ ions bind, the pump dephosphorylates, releasing the phosphate group, and changing it back to its original shape. The two K+ ions can now be released into the cell.

And the process repeats!

When are Sodium Potassium Pumps even used? What is their purpose?

Sodium potassium pumps help maintain a higher concentration of Na+ ions outside of the cell, and a higher concentration of K+ ions inside the cell.

They are used all the time, and are important for:

• Restoring ion gradients after action potentials in nerve signaling

• Muscle contractions

• Regulating the size of the cell, preventing it from swelling or shrinking

What cells are sodium potassium pumps famous for playing a role in?

The Sodium Potassium Pump is known for playing a big role in electrical gradients in nerve & muscle cells to prepare neurons for nerve signaling.

By moving more Na+ out than K+ in, it keeps the inside of the neuron (cell) negatively charged until a signal arrives. When the sodium channel opens to let Na+ back in, it reverses the charge in a process called depolarization, which initiates a nerve impulse.

What are the two types of Active Transport? What’s the difference between them?

Primary & Secondary Active Transport

Primary Active Transport = A pump uses ATP to actively move ions against their concentration gradient

Secondary Active Transport = Uses Protein pumps, but does not use ATP directly. It relies on the energy from the gradient created by primary active transport. The gradient created by Primary Active Transport allows one molecule to move against its concentration gradient, while simultaneously allowing an ion to move down its concentration gradient in Secondary Active Transport.

In secondary Active Transport, only molecules are transported against their concentration gradient, while ions are transported down their concentration gradient (from high to low concentration)

They both use carrier proteins not channel proteins

What is another name for Secondary Active Transport? Why is it called this?

Secondary Active transport is also called Cotransport!!

It is also called Cotransport because it is the coupled/simultaneous movement of two molecules through the cell membrane; One molecule is moved against its concentration gradient, and this allows an ion to be transported down the concentration gradient.

Give an example of Cotransport/Secondary Active Transport & Primary Active Transport

Primary Active Transport: ex. Sodium Potassium Pump

Secondary Active Transport: ex. Glucose Cotransport

Sodium Ions (Na+) move down their concentration gradient, which in turn, pulls Glucose into the cytoplasm of the cell, against its concentration gradient.

Where can the process of Glucose Cotransport happen? What does it rely on in order to occur?

Glucose Cotransport happens in cells that need to absorb glucose, including: intestinal cells, kidney cells, and some neurons.

• In the Small Intestine = Helps absorb nutrients from digested food into the bloodstream

• In the Kidneys = helps reabsorb valuable glucose that would otherwise be lost in Urine

• In Neurons = helps make sure there is a steady supply of glucose for the energy, especially in brain cells

It relies on the Na+ gradient created by the Sodium Potassium Pump in Primary Active Transport. Secondary Active Transport uses this gradient, moving Na+ down its gradient, which provides the energy to move Glucose against its concentration gradient into cells.

Describe What happens in Glucose Co-Transport?

Primary Active Transport using the Sodium Potassium Pump happens first (outlined on a previous card); where 3 Na+ were pumped out of the cell, into the blood, and 2 K+ were pumped into the cytoplasm of the cell using ATP.

Primary Active Transport created a gradient where there was a higher concentration of Na+ outside of the cell compared to inside of the cell; As a result, Na+ wants to move back down its concentration gradient into the cell.

Secondary Active Transport uses this gradient, moving Na+ down its concentration gradient, generating enough energy to move Glucose against its concentration gradient, without the use of ATP.

What is cell adhesion? Why is cell adhesion important?

Cell adhesion is the process of cells binding together to form tissues and organs.

It helps with growth/development, sealing wounds, and immune response

What are cell adhesion molecules?

Cell adhesion molecules (aka CAMs) are Proteins found on the cell membrane that bind cells to other cells or to the extracellular matrix.

Can you draw a flow chart summarizing all the different types of membrane transport?

Can you draw the fluid mosaic model?

Be sure to include:

The phospholipid bilayer, making it clear which part is the phosphate head and which parts are the hydrocarbon tails

Integral proteins, e.g. channel/carrier proteins

Peripheral proteins that do not extend into the hydrophobic region

Glycoproteins and Glycoproteins

What are cell-cell junctions? How is it different from cell-cell adhesion?

Cell-cell adhesion is just the general ability for cells to stick together, forming tissues and organs using Cell Adhesion Molecules (CAMs)

Cell-cell junctions are the structural connections between adjacent cells that form different types of junctions (meeting spots).

What are the different types of cell-cell junctions (don’t need to describe each one, just name a few)? What is the purpose of them?

Tight Junctions

Adherens Junctions

Desmosomes

Gap Junctions

These all structures that create sealed barriers, physical support, and communication between cells.

4. A cell, X, was placed into a solution containing a dye. After two hours the concentration of the dye inside cell X was higher than in the solution. This was repeated with an identical cell, Y,  in the presence of a substance that inhibits ATP. What would be the expected outcome two hours later in cell Y?

• A. The concentration of dye inside cell Y was the same as cell X

• B. No dye entered the cell Y

• C. The concentration of dye inside the cell Y was greater than the concentration in cell X

• D. The dye outside cell Y became more concentrated

✅ The dye entered cell X, even though its concentration was higher inside the cell than outside. ✅ This suggests active transport was involved, because passive diffusion would not normally move substances against their concentration gradient.

What Changed in Cell Y?

✅ Cell Y was placed in the same solution, but this time ATP was inhibited (meaning active transport cannot occur). ✅ Since active transport requires ATP, the dye cannot be actively moved into the cell. ✅ This means only passive diffusion can happen—meaning the dye would move into the cell only if it followed its natural concentration gradient.

Correct Answer: B no dye entered cell Y, because active transport was blocked

In which organisms is cholesterol found in? What does it do?

Cholesterol is found in all animals, but not in plants or fungi.

At lower temperatures, cholesterol prevents the membrane from becoming too stiff and rigid by stopping the phospholipid tails from tightly packing together; But at higher temperatures, cholesterol stabilizes the membrane, preventing it from becoming too fluid and permeable;
Think of something becoming more stiff and rigid in the winter (colder temps) and more fluid/melting in the summer (warmer temps); cholesterol prevents this from happening