Understanding the Cell Membrane and Cell Wall - Biology

The fluid-mosaic model for the cell membrane is a double layer of phospholipids with embedded proteins. The proteins do active transport in pumping molecules across the membrane. The phosphates are on the outside and the lipid chains are on the inside of the membrane.

The cell wall provides support and protection for the cell. Most are made from carbohydrate and protein. They are not made of phospholipids or lipids.

Bacteria and plant cells both have cell walls, although the cell walls are composed of different macromolecules in different cell types. Plants use the protein chitin, while bacteria use peptidoglycan. Bacteria are a certain class of prokaryotes.

Animal cells only have a plasma membrane, and do not have cell walls.

Diffusion and osmosis are both used in order to equalize the concentrations of solutes on both sides of a membrane. This act requires no energy to take place, as solutes will passively flow from regions of high concentration to areas of low concentration. Facilitated diffusion requires a channel protein to allow substances to cross the membrane, but also allows flow down a concentration gradient and does not require energy.

Active transport is needed in order to accumulate solutes on one side of a barrier against their concentration gradient. This requires ATP in order to take place, as the solutes will not flow in this direction naturally.

Two general concepts allow you to predict how easily a molecule is able to cross the plasma membrane.

1. The smaller the molecule, the more permeable the membrane is to it. Large molecules have a harder time crossing the membrane.

2. Polar and charged molecules have a very hard time crossing the membrane. Nonpolar molecules can cross the membrane much more easily.

As a result, small, nonpolar molecules are ideal for crossing the membrane easily. Larger molecules do not fit through the membrane gaps, and polar molecules are repelled by the hydrophobic interior of the membrane.

Peptidoglycan is found in the cell walls of bacteria. Plant cells have cell walls made of cellulose, and animal cells lack a cell wall entirely. Archaea are a class of prokaryote, but have cell walls that differ from those of bacteria. Archaea cell walls do not contain peptidoglycan.

Since salt is unable to pass the membrane, the animal cell will attempt to equalize the salt concentrations on both sides by expelling water into the solution. The concentration of salt outside the cell is higher than the concentration inside the cell. This means that water itself is more concentrated inside the cell than outside. The water will flow down its gradient from high solvent concentration (in the cell) to low solvent concentration (outside the cell) via the process of osmosis. As the water exits the cell, it will lose volume and shrivel.

The cell wall is a very tough structure that is able to help the cell withstand extracellular stressors. A plant cell or bacterium can survive hypotonic solutions better than an animal cell due to protection from the cell wall. As water flows into the cell, but the cell wall will keep the cell from bursting.

The cell wall does not protect well against hypertonic environments, however. As water exits the cell, the plasma membrane pulls away from the cell wall. The cell shrinks within the cell wall, which maintains its original size and does not prevent cellular damage.

The cell membrane is composed of a phospholipid bilayer. Polar regions of the membrane face outward and shield a hydrophobic interior. Only certain compounds can cross both regions of the membrane. Polar compounds and ions will be able to interact with the polar regions of the membrane, but be unable to cross the hydrophobic interior. Similarly, large compounds will be unable to fit between the phospholipids. Only small, nonpolar molecules can cross the membrane freely.

Protein channels are placed in the membrane to allow polar and large molecules to cross, further adding to the selective nature of the membrane. Harmful compounds can still enter the cell from time to time, but the selectivity of the membrane helps prevent the potency of these attacks.

The cell membrane controls what enters and leaves the cell. The cell membrane is a selectively permeable phospholipid bi-layer, which admits molecules by passive and active transport into the interior of the cell. The DNA is the genetic code found in the nucleus while cytoplasm is the liquid inside the cell.

All these answers are correct and can be used interchangeably, so it's good to be aware of all of them. All cells have a cell membrane, regardless of organism, to separate their internal environment from their external environment. The term plasma membrane refers to the cytoplasm within the cell that the membrane is separating from the external environment. Phospholipid bilayer describes the molecular makeup of the cell membrane as it is composed of a double layer of amphiphilic (something that is both hydrophobic/lipophilic and hydrophilic) phospholipids. Semipermeable membrane refers to the ability of the membrane to allow certain substances cross from one side to the other, while restricting the passage of other substances. Cytomembrane uses the Greek root "cyto", meaning cell, to create a compound word that in the end means the same thing as cell membrane.

Cell membranes are composed of a phospholipid bilayer. Phospholipids have hydrophilic heads and hydrophobic tails. The tails come together to form the hydrophobic inside of the membrane, whereas the hydrophilic heads face the outside of the cell and the inside aqueous cytoplasm.

Bacteria and plant cells both have cell walls, although the cell walls are composed of different macromolecules in different cell types. Plants use the protein chitin, while bacteria use peptidoglycan. Bacteria are a certain class of prokaryotes.

Animal cells only have a plasma membrane, and do not have cell walls.

Diffusion and osmosis are both used in order to equalize the concentrations of solutes on both sides of a membrane. This act requires no energy to take place, as solutes will passively flow from regions of high concentration to areas of low concentration. Facilitated diffusion requires a channel protein to allow substances to cross the membrane, but also allows flow down a concentration gradient and does not require energy.

Active transport is needed in order to accumulate solutes on one side of a barrier against their concentration gradient. This requires ATP in order to take place, as the solutes will not flow in this direction naturally.

Two general concepts allow you to predict how easily a molecule is able to cross the plasma membrane.

1. The smaller the molecule, the more permeable the membrane is to it. Large molecules have a harder time crossing the membrane.

2. Polar and charged molecules have a very hard time crossing the membrane. Nonpolar molecules can cross the membrane much more easily.

As a result, small, nonpolar molecules are ideal for crossing the membrane easily. Larger molecules do not fit through the membrane gaps, and polar molecules are repelled by the hydrophobic interior of the membrane.

Peptidoglycan is found in the cell walls of bacteria. Plant cells have cell walls made of cellulose, and animal cells lack a cell wall entirely. Archaea are a class of prokaryote, but have cell walls that differ from those of bacteria. Archaea cell walls do not contain peptidoglycan.

Since salt is unable to pass the membrane, the animal cell will attempt to equalize the salt concentrations on both sides by expelling water into the solution. The concentration of salt outside the cell is higher than the concentration inside the cell. This means that water itself is more concentrated inside the cell than outside. The water will flow down its gradient from high solvent concentration (in the cell) to low solvent concentration (outside the cell) via the process of osmosis. As the water exits the cell, it will lose volume and shrivel.

The cell wall is a very tough structure that is able to help the cell withstand extracellular stressors. A plant cell or bacterium can survive hypotonic solutions better than an animal cell due to protection from the cell wall. As water flows into the cell, but the cell wall will keep the cell from bursting.

The cell wall does not protect well against hypertonic environments, however. As water exits the cell, the plasma membrane pulls away from the cell wall. The cell shrinks within the cell wall, which maintains its original size and does not prevent cellular damage.

The cell membrane is composed of a phospholipid bilayer. Polar regions of the membrane face outward and shield a hydrophobic interior. Only certain compounds can cross both regions of the membrane. Polar compounds and ions will be able to interact with the polar regions of the membrane, but be unable to cross the hydrophobic interior. Similarly, large compounds will be unable to fit between the phospholipids. Only small, nonpolar molecules can cross the membrane freely.

Protein channels are placed in the membrane to allow polar and large molecules to cross, further adding to the selective nature of the membrane. Harmful compounds can still enter the cell from time to time, but the selectivity of the membrane helps prevent the potency of these attacks.