Module 1 Section 2

what is the plasma membrane

• separates cell internal + externals environment

• controls what moves in and out of the cell

• joins cells together to form tissues and organ

what are the 3 main functions of the plasmam membrane

• ensures cells survival

• maintains homeostasis

• functions cooperatively + in coordination w nearby cells 

What forms the basic structure of the membrane?

• phospholipids

• cholsterol

• membrane proteins

• ion channels

• carbohydrates

what do phospholipids do

phospholipids — they create a lipid bilayer with polar heads and non-polar tails.

What is the role of cholesterol in the membrane?

embedded in the bilayer to prevent it from freezing and maintain fluidity.

What are membrane proteins responsible for?

Facilitate signalling, maintain structure, and function as transporters.

What do ion channels do?

Permit only certain ions to cross the membrane.

What do carbohydrate chains contribute to the membrane?

Form glycolipids and glycoproteins that stabilize the membrane and act as receptors or transporters.

What makes the membrane fluid?

Lipids and proteins can move freely within the layer; phospholipids are not rigidly fixed.

What makes the membrane mosaic?

It contains many embedded molecules and proteins arranged like a mosaic.

How does size affect membrane permeability?

Small substances (like ions) can pass through channels; large ones need glycoprotein transporters.

How does solubility affect permeability?

Lipophilic (fat-loving) molecules can pass easily; lipophobic (water-loving) molecules cannot.

What are the types of passive transport?

Diffusion and facilitated diffusion (carrier- or channel-mediated).

What are the types of active transport?

Carrier-mediated and vesicular transport (uses ATP).

How does diffusion work?

Molecules move from high to low concentration down the gradient until dynamic equilibrium is reached.

What happens when a membrane is permeable to a substance?

t diffuses passively until concentrations equalize.

What happens when a membrane is impermeable?

Diffusion cannot occur; the concentration gradient remains.

What is osmosis?

Movement of water across a semipermeable membrane from low solute to high solute concentration.

Through what structure does water move during osmosis?

Aquaporins.

What happens if solutes can’t cross the membrane?

Only water moves until concentration equalizes.

What happens when water moves to one side but solute can’t move?

Concentration equilibrium cannot be achieved — osmotic pressure results.

How are cells held together?

• Extracellular matrix

• cell adhesion molecules

• cell junctions

What does collagen do in the ECM?

Forms flexible connective fibres.

What does elastin do?

Allows tissues to stretch and recoil after being stretched.

What is the role of proteoglycans?

Form a gel that binds cells and tissues together

What do cell adhesion molecules (CAMs) do?

Transmembrane proteins that link the cytoskeleton of one cell to another.

what are the 3 types of cell junctions

• desmosomes

• tight junctions

• gap junctions

What are desmosomes?

Anchoring junctions connecting adjacent cells via cadherin proteins that are not in contact

What are tight junctions?

Seal gaps between cells, preventing molecule movement between them.

What are gap junctions?

Channels formed by connexons allowing small, water-soluble molecules to pass between cells.

What is osmotic pressure?

The underlying force that moves water down its concentration gradient; a larger gradient creates greater pressure.

What is hydrostatic pressure?

Pressure caused by water volume; can build up until equilibrium is reached.

What is carrier-mediated transport?

Transport using proteins to move molecules across the membrane — includes both facilitated diffusion and active transport.

What happens in facilitated diffusion?

No energy required; carrier proteins act as channels to move solutes down their concentration gradient.

What happens in active transport?

Carrier proteins move solutes against their concentration gradient and require ATP.

How does facilitated diffusion work?

1. Carrier protein binds to solute on the side of higher concentration.

2. Protein changes shape, carrying solute to the other side.

3. Solute is released where concentration is lower.

4. Protein returns to original shape.

Characteristics of Carrier-Mediated Transport

• Specificity: Each carrier protein recognizes and transports specific substances.

• Saturation: There’s a limit (Tm) to how much can be transported — once all carriers are in use, rate can’t increase.

• Competition: Different substances may compete for the same carrier proteins.

What is endocytosis?

The process of taking large molecules into the cell using ATP and forming vesicles.

what are the forms of endocytosis

• pinocytosis

• receptor-mediated endocytosis

• phagocytosis

What is pinocytosis?

“Cell drinking” — the membrane engulfs extracellular fluid, forming vesicles through dynamic pinching.

What is receptor-mediated endocytosis?

Molecules attach to membrane receptors; the membrane folds inward to form a pocket that pinches off.

What is phagocytosis?

Cell eating” — pseudopods surround large particles, engulfing them into the cell where lysosomes digest them.

What is exocytosis?

The reverse of endocytosis — membrane-enclosed vesicles fuse with the membrane to release contents outside the cell.

What kinds of molecules are released by exocytosis?

Large or polar molecules that cannot cross the membrane otherwise (e.g., proteins).

What are the steps of exocytosis?

1. Secretory vesicle formation:

2. Budding from the Golgi:

3. Uncoating:

4. Docking at plasma membrane:

5. Exocytosis:

what is the secretory vesicle formation step in exocytosis 

1. Secretory vesicle formation: Recognition markers in the Golgi membrane bind sorting signals on proteins to be secreted → membrane coated in clathrin → forms bud.

what is the budding from the golgi step in exocytosis

Budding from the Golgi: Membrane closes beneath the bud and pinches off to form a secretory vesicle.

what is the uncoating stage in exocytosis

1. Uncoating: Vesicle loses its clathrin coating → exposes v-SNARE docking markers on vesicle surface.

what is the docking at plasma stage of exocytosis 

Docking at plasma membrane: v-SNAREs bind to t-SNARE docking markers on the target plasma membrane.

what is the final stage in exocytosis

Exocytosis: Vesicle fuses with plasma membrane → contents released outside the cell.