Transport Across Membranes

What is the description (or structure) of the Cell Membrane?

A thin layer surrounding the cytoplasm.

What is the primary function of the Cell Membrane?

It is selectively permeable and controls the entry and exit of materials from the cell.

What types of organisms have a Cell Membrane?

Animal cells, plant cells, fungi, and bacteria. (Basically, all cells have one!)

What are the two main molecules the Cell Membrane is made from?

Phospholipids and Proteins.

What term describes the double-layered structure formed by phospholipids in the membrane?

The Phospholipid Bilayer.

In the fluid mosaic model, where are the proteins located?

They are embedded in or attached to the phospholipid bilayer.

What does it mean when the Cell Membrane is described as "Selectively Permeable"?

It means the membrane only allows certain things to pass through. Specifically, it lets small and soluble molecules through, but blocks large and insoluble molecules.

Name three molecules that can easily move across the cell membrane.

Oxygen, Carbon Dioxide, and Water (or Glucose, or Amino Acids).

Name two large or complex molecules that CANNOT move across the cell membrane without special help.

Protein and Starch

Name the three main transport methods used by cells to move molecules in and out.

1. Diffusion 2. Osmosis 3. Active transport

What is a Concentration Gradient?

It is the difference in the concentration of a substance between two areas (like inside a cell and outside a cell)

Define Diffusion.

The movement of molecules down a concentration gradient (from a higher concentration to a lower concentration).

Is Diffusion an active or passive process, and what does that mean?

It is a passive process, which means it does not require energy (ATP).

Name two everyday examples of Diffusion in Gasses.

1. Being able to smell something (like food or perfume) from a distance.

2. The spread of Ammonia gas.

Name three examples of Diffusion in Liquids

1. The spreading of potassium permanganate crystals (or ink) in water.

2. Brewing Tea or Coffee.

3. Diluting Juice (the color and flavor spread).

How is smelling perfume from across a room an example of diffusion?

The gas molecules of the perfume move from the high concentration area (the bottle/person) to the low concentration area (across the room).

Why is diffusion considered essential to all living things?

It is the primary way that small, necessary substances (like gases and nutrients) move in and out of cells to maintain life processes (e.g., respiration).

Name three substances that move in or out of cells via diffusion.

In: Oxygen (for respiration) and Glucose (for energy).

Out: Carbon Dioxide (waste product of respiration).

Define Osmosis.

The movement of water molecules from a higher water concentration to a lower water concentration through a selectively permeable membrane.

Is Osmosis an active or passive process, and what does that mean for energy?

It is a passive process and doesn't require energy (ATP).

In an osmosis experiment, why is Visking tubing a good model for a cell membrane?

Because Visking tubing has microscopic pores that make it selectively permeable, allowing small molecules like water to pass through, but blocking larger molecules like starch or sucrose.

In the Visking tubing experiment, what role does the Visking tubing itself play?

It acts as the selectively permeable membrane.

Why do animal cells burst (lyse) when they take in too much water via osmosis, but plant cells don't?

Plant cells have a strong Cell Wall that provides structural support and prevents the cell membrane from bursting when water rushes in. Animal cells only have a flexible cell membrane, so they rupture.

Define Active Transport.

The movement of molecules from a low concentration to a high concentration (against the concentration gradient).

Why is Active Transport called 'Active,' and what molecule supplies the energy?

It is an active process because it requires energy, which is supplied by the molecule ATP.

What specific cell structure is required to move molecules during Active Transport?

Proteins within the cell membrane (often called carrier proteins).

What is the function of the Sodium-Potassium Pump in cells?

It actively pumps sodium ions (Na+) out of the cell and potassium ions (K+) into the cell (this action helps generate nerve impulses)

How does the accumulation of Iodine in Seaweed demonstrate active transport?

Seaweed concentrates iodine (up to 0.4% of its dry weight) from the surrounding seawater, where iodine concentration is very low. Moving a substance against its concentration gradient requires active transport

What happens to a plant cell placed in a dilute solution (where water moves into the cell)?

The cell swells (the vacuole pushes against the cell wall) and is called TURGID.

What happens to a plant cell when the surrounding solution is concentrated (where water moves out of the cell)?

The vacuole shrinks, the cell membrane pulls away from the cell wall, and the cell becomes PLASMOLYSED.

What term describes a plant cell when the rate of water movement into and out of the cell is equal?

The Cell Stays the Same (it is in an isotonic solution).

What happens to an animal cell placed in a dilute solution (where water moves into the cell)?

The cell will swell and burst (a process called lysis) because it lacks a cell wall.

What happens to an animal cell when water moves out of the cell (in a concentrated solution)?

The cell will shrivel and shrink (a process called crenation).

How is an animal cell affected by an isotonic solution (where water moves in both directions equally)?

The cell stays the same and is in its normal, healthy state.