AP Bio - Cell Size, Plasma Membrane, Passive Transport

Quiz #2

What does cell size affect?

Cellular metabolism

How does the surface area-to-volume ratio affect the cell?

A high ratio optimizes the exchange of material through the plasma membrane

What is the plasma membrane composed of?

Phospholipids

Describe the heads and tails of phospholipids

• Polar hydrophilic heads (phosphate)

• Nonpolar hydrophobic tails (fatty acid)

What makes the membrane selectively permeable?

Hydrophobic interior

Why is the plasma membrane called a fluid mosaic model?

• Fluid: membrane is held together by weak hydrophobic interactions, allowing it to move and shift

• Mosaic: comprised of many macromolecules

Temperature affects fluidity. What can help maintain fluidity?

Unsaturated hydrocarbon tails and cholesterol

Describe unsaturated hydrocarbon tails’ effect on fluidity.

• Helps maintain fluidity at low temperatures

• Kinked tails prevent tight packing of phospholipids

Describe cholesterol’s effect on fluidity.

• Helps maintain fluidity and both high and low temperatures

• High temp: reduces movement

• Low temp: reduces tight packing of phospholipids

Integral proteins

• Embedded into the lipid bilayer

• Hydrophilic interior or channel of the protein

  • Hydrophobic surface of the protein

Peripheral proteins

• Not embedded into the lipid bilayer

• Loosely bonded to the surface

What are the membrane carbohydrates?

Glycolipids and glycoproteins

What is the main purpose of membrane carbohydrates?

They are important for cell-to-cell recognition

What are glycolipids and glycoproteins?

• Glycolipids - carbohydrates bonded to lipids

  • Glycoproteins - carbohydrates bonded to proteins

Functions of membrane carbohydrates

• cell signaling

• immune response

• bonds with hormones

Which molecules have easy passage across the membrane?

Small, nonpolar, hydrophobic molecules

• Ex: hydrocarbons, gases

What molecules have difficult or protein-assisted passage across the membrane?

Large polar hydrophilic molecules, charged ions

What is the function of cell walls?

• Provide structural boundary

• Permeability barrier for some substances

• Protection from osmotic lysis (cell bursting when excess water goes into the cell)

What is the cell wall composed of in plants?

Cellulose and plasmodesmata (hole-like structures in the cell wall filled with cytosol)

Why do phospholipids form a bilayer?

They are amphipathic, meaning the heads and tails join, forming a bilayer.

What would happen to transport across the membrane if the protein did not fold correctly?

It could alter the protein’s ability to facilitate transport across the membrane, causing reduced function or loss of function.

What is passive transport?

The transport of a molecule that does not require energy from the cell because the solute is moving with/down the concentration gradient

What is diffusion?

Spontaneous process resulting from the constant motion of molecules where substances move from a high to low concentration

What factors affect rates of diffusion?

Molecular size, weight, surface area, etc.

Osmosis

the diffusion of water down its concentration gradient across the membrane (water goes where there is less water)

Facilitated diffusion

Diffusion of molecules through the membrane via channel or carrier proteins (does not require energy)

Channel proteins

• Form an open pore/channel in the membrane

• Moves charged ions

• Most of them are not permanently open (gates open and close in response to stimuli)

• Direction of flow is determined by electrochemical gradient

Aquaporins

Specialized channel proteins for water

Carrier proteins

• Alternate between 2 conformations/shapes

• Moves large polar molecules (like glucose, sugars, etc.)