Bio1 Wed 10/14 Lecture: Cell Structure and Membranes

Describe Cell Theory.

Cells are the fundamental units of life.

All living organisms are composed of cells.

All cells come from preexisting cells.

Are viruses considered cells? Are viruses considered living? Explain.

Viruses are not considered cells, so many scientists say that viruses are not alive. One reason is that they cannot replicate independently.

The two main types of cells are…

There are two main types of cells: eukaryotic and prokaryotic.

Compare prokaryotic and eukaryotic cells.

They both have ribosomes, DNA, cytoplasm, and a plasma membrane.

Eukaryotes have a nucleus and membrane-bound organelles; prokaryotes do not.

The DNA is in different places: in eukaryotes, DNA is contained in the nucleus. In prokaryotes, DNA is floating freely in the cytoplasm.

What does the cell membrane (plasma membrane) do?

separates the interior of the cell from its environment

Compare Cytoplasm vs Cytosol

The cytoplasm is everything in a cell, except the nucleus.

The cytosol is just the liquid contents of a cell (doesn’t include the nucleus and other organelles, etc.)

What is the best model that describes the cell membrane? Describe it.

In the fluid mosaic model, the cell membrane is made of two layers of phospholipids called the phospholipid bilayer. The bilayer is flexible, not rigid, and has proteins and lipids moving freely (sideways) in it.

Explain how (and why) phospholipids are arranged/oriented in the phospholipid bilayer.

Hint available.

The extracellular fluid and the cytoplasm inside the cell are mostly made of water.

The hydrophilic heads of the outer layer are oriented towards the extracellular fluid (fluid outside the cell) and the hydrophilic heads of the inner layer are oriented towards the cytoplasm.

The hydrophobic tails are oriented towards each other (towards the inside of the bilayer).

What are proteins and lipids with carbohydrates attached called?

Proteins with carbohydrates attached are called glycoproteins.

Lipids with carbohydrates attached are called glycolipids.

Describe 4 types of proteins in the phospholipid bilayer (based on the extent to which they are embedded).

• Peripheral membrane proteins don’t penetrate the bilayer at all.

• Anchored membrane proteins are attached to a lipid or fatty acid in the membrane.

• Integral proteins penetrate the bilayer either partially or fully.

  • The integral proteins that cross the entire phospholipid bilayer (penetrate fully) are called transmembrane proteins.

One type of protein completely penetrates the lipid bilayer. Describe how amino acids in this type of protein are oriented according to their side chains.

Amino acids with polar side chains tend to be in the region of the protein outside the membrane.

Amino acids with nonpolar acids with nonpolar side chains tend to be in the region of the protein that is inside the membrane.

Cytoskeleton: What does “cyto” mean?

‘cyto’ means ‘cell’

What is membrane fluidity?

the extent of lateral movement of lipids and proteins in the membrane

What three things affect membrane fluidity? Explain how for each.

Temperature

• Higher temperatures increase membrane fluidity.

• Lower temperatures decrease membrane fluidity.

Phospholipids

• Phospholipids with unsaturated fatty acid chains increase fluidity since unsaturated fatty acid chains cannot pack as tightly.

• Phospholipids with saturated fatty acids decrease fluidity by packing tightly together to prevent the membrane from becoming overly leaky or falling apart.

Cholesterol

• It increases the fluidity of the membrane by disrupting the packing of fatty acid chains.

• It decreases the fluidity by holding the phospholipids together. Cholesterol has a polar part (though the rest is very hydrophobic) that attracts the hydrophilic heads of the phospholipids.

Are triglycerides major components of cell membranes?

No.

Phospholipids and cholesterol are.

What are two ways in which phospholipids and triglycerides differ?

Phospholipids have a larger, more polar hydrophilic head due to being bonded to a phosphate group. Triglycerides do not have hydrophilic heads; they have some polar bonds but the overall molecules are nonpolar.

Phospholipids have two fatty acid chains whereas triglycerides have three.

What is Fluorescence Recovery After Photobleaching (FRAP) used for?

How does it work (simply)?

an experimental technique that allows scientists to measure membrane fluidity

Fluorescently-labeled lipids or membrane proteins are “bleached” with a laser in a small area of the membrane, and the time needed for fluorescence recovery is measured. The faster the recovery, the more fluid the membrane.

What are the two primary mechanisms through which molecules move across the cell membrane?

Passive Transport and Active Transport

What is passive transport?

The movement of molecules across a cell membrane without requiring energy input from the cell

In which direction do molecules move during passive transport?

From an area of high concentration to low concentration, down their concentration gradient, until equilibrium is reached.

What happens at equilibrium during passive transport?

Molecules continue crossing the membrane in both directions, but equal amounts move in each direction, so there is no net movement.

What is facilitated diffusion? What molecules need it and which don’t?

A form of passive transport in which molecules cross the membrane using channel proteins or carrier proteins, without requiring cellular energy

Small, uncharged molecules, such as water, don’t need it and can cross the membrane without transport proteins.

Molecules that are too large to cross the membrane easily, such as glucose, and charged molecules, such as ions, require facilitated diffusion and cannot cross the membrane without proteins.

Describe how channel proteins work vs carrier proteins.

Channel proteins form open tunnels/pores that open or close in response to specific triggers (can be chemical or electric), allowing target molecules to pass through.

Carrier proteins have specific binding sites for target molecules and undergo conformation (shape change) to move the molecules across the membrane.

Does facilitated diffusion require energy?

No.

It is passive transport, so molecules move down their concentration gradient without energy input from the cell.