Biology Chapter on Diffusion

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Questions and Answers

What is a key characteristic of simple diffusion?

Utilizes membrane proteins for transport

Requires energy to move substances

Occurs down a concentration gradient (correct)

Moves substances against a concentration gradient

Which type of membrane protein is always open for the transport of substances?

Carrier proteins

Aquaporins

Gated channel proteins

Un-gated (leak) channels (correct)

What role do carrier proteins play in facilitated diffusion?

They only transport ions across the membrane.

They require energy for conformational changes.

They act like tunnels for passive transport.

They allow the passage of large uncharged molecules. (correct)

Why is water able to cross cell membranes more quickly than some other polar molecules?

Water is facilitated by specific channel proteins. (D) Signup and view all the answers

What distinguishes facilitated diffusion from simple diffusion?

Facilitated diffusion involves passive transport via channel or carrier proteins. (A) Signup and view all the answers

What does osmosis primarily refer to?

The diffusion of water across a semi-permeable membrane. (C) Signup and view all the answers

Which of the following statements about simple diffusion is true?

It continues until equilibrium is reached. (B) Signup and view all the answers

What is the main factor that affects the rate of simple diffusion?

The size and polarity of molecules. (A) Signup and view all the answers

What occurs when an animal cell is placed in a hypotonic solution?

The cell expands and may burst. (B) Signup and view all the answers

Which term describes the movement of water towards an area of higher solute concentration?

Osmosis (A) Signup and view all the answers

In plant cells, what is the term for the pressure created by water entering the cell without causing rupture?

Turgor pressure (B) Signup and view all the answers

What is the result of placing a plant cell in a hypertonic solution?

The cell membrane and cytoplasm shrink away from the cell wall. (A) Signup and view all the answers

Which process requires energy for the movement of molecules against their concentration gradient?

Active transport (B) Signup and view all the answers

What characterizes receptor-mediated endocytosis?

Binding of specific ligands to receptors. (B) Signup and view all the answers

What happens during exocytosis?

Materials are released from the cell using vesicles. (D) Signup and view all the answers

What is typically found in regions of the membrane specialized for receptor-mediated endocytosis?

Coated pits that cluster receptor proteins. (B) Signup and view all the answers

During osmosis, how does water move in relation to solute concentration?

Towards high solute concentration. (B) Signup and view all the answers

What is the role of the sodium-potassium pump?

It moves sodium ions out and potassium ions into the cell. (A) Signup and view all the answers

What term describes the shrinking of the plasma membrane away from the cell wall in plant cells?

Plasmolysis (D) Signup and view all the answers

Which of the following best describes hypotonic solutions?

Lower solute concentration outside than inside the cell. (D) Signup and view all the answers

How does bulk membrane transport differ from osmosis?

Bulk transport occurs with large quantities of molecules. (D) Signup and view all the answers

Study Notes

Simple Diffusion

Small, non-polar substances move across membranes without assistance.
Movement is down a concentration gradient (high to low).
No energy required.
Continues until equilibrium is reached.
Also known as simple transport.
Very small, non-polar molecules (like O₂ and CO₂) move easily.
Non-polar steroid hormones and drugs also cross readily.
Water and glycerol, though polar, can move relatively quickly due to their small size.
Large molecules and ions have very slow movement across membranes.

Facilitated Diffusion: Channel and Carrier Proteins

Similar to simple diffusion, it's passive transport across a semi-permeable membrane until equilibrium is reached.
A key difference: it involves transport proteins.

Channel Proteins

Act like tunnels.
Can be ungated (always open) or gated (can open/close).
Moves small, dissolved charged molecules (like ions).
Charged particles need help crossing the hydrophobic core.
Aquaporins are a specific type of channel protein that facilitate water movement.

Carrier Proteins

Act like revolving doors.
Undergo conformational changes to move molecules.
Move a variety of substances from charged particles to large, uncharged ones (like glucose).

Osmosis

Water movement across a semi-permeable membrane.
In cells, aquaporins facilitate water movement.
Moves from high water concentration to low water concentration (or to dilute solutes).
Water moves in the opposite direction of solute movement.
Tonicity refers to the osmotic pressure due to solute concentration differences across a membrane, influenced by non-penetrating solutes. It refers to the concentration of solutes in the environment surrounding the cell (external).

Osmosis in Animal Cells

Isotonic: Equal water movement in and out. Cells are in equilibrium.
Hypotonic: Lower solute concentration outside the cell. Water moves into the cell, causing it to swell and potentially burst (osmotic lysis).
Hypertonic: Higher solute concentration outside the cell. Water moves out of the cell, causing it to shrink (crenation).

Osmosis in Plant Cells

Plant cells have cell walls, affecting osmotic effects.
Isotonic: Cells are flaccid (lacking firmness). Water movement is in equilibrium but does not provide the pressure necessary for support.
Hypotonic: Cell wall prevents bursting, resulting in turgor pressure and turgidity.
Hypertonic: Cell membrane and cytoplasm shrink (plasmolysis) away from the cell wall, but the cell wall retains shape.

Active Transport

Moves molecules against their concentration gradient (low to high).
Requires energy in the form of ATP.
Uses transport proteins called pumps.
ATP changes the pump's conformation, allowing specific molecules to enter/exit against their gradient.

Types of Pumps

Often function similarly to carrier proteins but move molecules against the concentration gradient, requiring energy.
Can be coupled to move two molecules against their concentration gradient.

Sodium-Potassium Pump (Na-K Pump)

Located in the plasma membrane.
Simultaneously moves 3 Na+ ions out and 2 K+ ions into the cell.
The pump oscillates between two conformational states to perform this.

Bulk Membrane Transport

Transports molecules in large quantities.
Used for molecules that are too large or polar to pass through the membrane.
Involves folding of the cell membrane to create vesicles.
Includes endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis.

Endocytosis

Phagocytosis: "Cellular eating"; ingestion of cells or large particles. Used for feeding in simpler forms of life and defense in higher organisms.
Pinocytosis: "Cellular drinking"; ingestion of dissolved materials (unspecific).
Receptor-mediated endocytosis: Specific intake of molecules that bind to receptors on the cell surface. Ligands bind to receptors, membrane pinches to form a vesicle, ligands detach from receptors, vesicle pinches into two parts – free ligand and empty receptor, ligands fuse with lysosomes, and receptors are returned to the cell surface

Exocytosis

Movement of materials from inside the cell to the cell surface within membrane-bound vesicles.
Vesicles form from the Golgi or endocytosis.
Reverse of endocytosis.
Functions in secretion, recycling membrane proteins, and restoring the cell membrane.

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Description

This quiz covers the concepts of simple and facilitated diffusion as well as the role of channel and carrier proteins in transport across membranes. Understand the mechanisms behind the movement of small, non-polar substances and how proteins assist in passive transport.