Cell Membranes & Solutions

​Homeostasis - the maintaining of a stable internal environment that is optimal for functioning for the organism.

​In a cell, homeostasis starts with the cell membrane.

​The cell membrane helps maintain homeostasis by controlling what passes into or out of the cell.

​All cells, whether prokaryotic or eukaryotic, have a cell membrane.

The cell membrane is selectively permeable, or semi-permeable. This means that only very small substances, like water, can pass through.

​Semi = partly

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​Permeable = allowing to pass through

The cell membrane is sometimes called the plasma membrane. It is made of a phospholipid bilayer.

Bi = two. Bilayer means there are two layers. One layer faces out, and one layer faces in.

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The phospholipid bilayer goes all the way around the cell.

​Each layer has polar heads that are hydrophilic (water-loving), and

​nonpolar tails that are hydrophobic (water-hating).

Multiple Choice

Which type of cells have a cell membrane?

• Eukaryotic only

• Prokaryotic only

• All cells have a cell membrane

• No cells have a cell membrane

Multiple Choice

Cell membranes are semi-permeable. What does semi-permeable mean?

• Anything can pass through

• Water and large molecules can pass through

• Water and very small molecules can pass through

• Nothing can pass through

Multiple Choice

Cell membranes are mostly made of lipids. The lipids form a phospholipid bilayer which can best be described as:

• One layer of lipids with the heads facing out

• Two layers of lipids with the heads facing both out and in

• One layer of proteins with the tails facing out

• Two layers of proteins with the tails facing both out and in

Multiple Choice

The cell membrane helps maintain homeostasis by:

• Controlling what comes into or goes out of the cell

• Using ribosomes to assemble proteins

• Using genetic material to pass instructions to the cell

• Preventing anything from ever entering or exiting the cell

​1. It controls what goes in or out of the cell (separates inside of cell from outside).

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​2. It is flexible to allow for movement. This flexibility is referred to as the fluid mosaic model.

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3. The lipid bilayer holds the cell together.

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​The lipid bilayer has 3 main functions:

​Notice how the red blood cells squeeze to get through the blood vessels. This is because their cell membranes are flexible.

​The cell membrane has proteins embedded in it. These proteins are mostly for transporting larger substances that cannot pass through the lipid bilayer.

There are three main types of proteins embedded in the lipid bilayer.

​1. Glycoproteins: attach carbohydrate chains to the cell membrane.

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​2. Integral Proteins: act as cell receptors.

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​3. Channel Proteins: transport substances that are impermeable across the lipid bilayer.

Finally, a cell membrane has carbohydrates and cholesterol.

Carbohydrates: function as recognition sites and identification markers.​

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Cholesterol:

helps keep the cell membrane flexible and semi-permeable.

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Three things to remember about the cell membrane:

​1. It is held together by a lipid bilayer.

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​2. The lipid bilayer is semi-permeable. This means water and very small substances can pass through.

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​3. Proteins embedded in the lipid bilayer can transport larger substances in and out.

Multiple Choice

Which of the following is NOT a function of the phospholipid bilayer?

• Keeping the cell flexible

• Holding the cell together

• Controlling what goes into or out of the cell

• Keeping the cell stiff and impermeable

• Separating the inside of the cell from the outside

Multiple Choice

Which biomolecule in a cell membrane transports substances into or out of the cell?

• Lipids 

• Proteins

• Carbohydrates

• Nucleic Acids

Multiple Choice

Which biomolecule in a cell membrane holds the cell membrane together and separates the inside of the cell from the outside?

• Lipids 

• Proteins

• Carbohydrates

• Nucleic Acids

Multiple Choice

The cell membrane can best be described as:

• Stiff and impermeable

• Flexible and semi-permeable

• Stiff and semi-permeable

• Flexible and impermeable

Multiple Choice

Which of the following best describes the role of proteins in a cell membrane?

• Transporting substances across the cell membrane

• Keeping the cell membrane flexible and semi-permeable

• Functioning as recognition sites and identification markers

• Holding the cell membrane together

Solution - a liquid mixture

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​Solvent- the liquid part of the solution (usually water)

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​Solute - The dissolved substance, such as salt or sugar

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​Example: sports drinks, like Gatorade, are solutions. The solvent is water, and the solutes are sugar, electrolytes, and other nutrients.

​Osmosis:

​Water moves through a semipermeable membrane to achieve homeostasis.

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​​Concentration: amount of solute or solvent.

​Water flows from a high

​concentration to a low concentration.

​A cell membrane is a semi-permeable membrane.

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​Water flows in and out (osmosis) to maintain homeostasis.

Multiple Choice

Which substance can flow easily across the cell membrane through osmosis?

• Solutes

• Large particles

• Water

• No substance can do this

Multiple Choice

You pour some sugar into water and stir until the sugar is dissolved.

Which best describes the solution you made?

• Water is the solvent and sugar is the solute

• Water is the solute and sugar is the solvent

• Water is the solution and sugar is the solvent

• Water is the solution and sugar is the solute

Multiple Choice

Which statement best describes the flow of water?

• Water flows from high concentration to low concentration.

• Water flows from low concentration to low concentration.

• Water flows from high concentration to high concentration.

• Water flows from low concentration to high concentration.

It is important for your cells to be surrounded by an isotonic solution.

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​Otherwise, they could either shrivel up, or swell and burst.

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​There are three types of solutions: hypertonic, isotonic, and hypotonic.

Description: ​The ratio of solutes to water is the same inside and outside of the cell.

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​What happens to the cell?

• Water flows equally in and out

  No changes in the cell. It is in homeostasis.

Isotonic Solution

​An example of an isotonic solution is saline.​

​In the hospital, patients with an IV are given saline because saline is isotonic with their blood cells.

Cells in an isotonic solution.

​Water flows in and out equally.

​The cells are in homeostasis.

Description: ​The concentration of solutes in the solution is greater than in the cell. ​

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What happens to the cell?​

• Water will move out of the cell.​

• The cell will shrivel as it loses water (dehydration).​

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Hypertonic Solution

​An example of a hypertonic solution is corn syrup.

​If exposed to corn syrup, your blood cells would shrivel.

​Cells in a hypertonic solution.

​Water flows out of the cells.

​The cells shrivel.

​Description:

There is a lower concentration of solutes in the solution than inside the cell. ​

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​What happens to the cell?

• Water will move into the cell.

  The animal cell will swell and could burst.​

• The plant cell vacuole will swell, but should not burst. The plant will be in a normal turgid state. 

​Hyp-O - the cell gets big like an O

Hypotonic Solution

​An example of a hypotonic solution is pure distilled water. ​

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​If exposed to pure water, your blood cells would swell and might burst.

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​However, plant cells are different. Plant cells have a large vacuole that fills with water. The plant cell swells, but does not burst. The plant cells become turgid, which is their normal, healthy state.

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Hypotonic Solution

​Cells in a hypotonic solution.

​Water flows into the cells.

​The cells swell and may burst.

Multiple Choice

When a patient is in the hospital with an IV, what type of solution should they be given so their blood cells stay healthy?

• An isotonic solution, like saline.

• A hypotonic solution, like pure water.

• A hypertonic solution, like corn syrup.

• No solution is safe.

Multiple Choice

What could happen to animal cells in a hypotonic solution, like pure water?

• Water flows in. Their vacuole swells and they become turgid (healthy).

• Water flows in. They swell and could burst.

• Water flows out.

  They shrivel.

• Water flows equally in and out.

  They are in homeostasis.

Multiple Choice

What could happen to plant cells in a hypotonic solution, like pure water?

• Water flows in. Their vacuole swells and they become turgid (healthy).

• Water flows in. They swell and could burst.

• Water flows out.

  They shrivel.

• Water flows equally in and out.

  They are in homeostasis.

Multiple Choice

What could happen to cells in a hypertonic solution, like corn syrup?

• Water flows in. Their vacuole swells and they become turgid (healthy).

• Water flows in. They swell and could burst.

• Water flows out.

  They shrivel.

• Water flows equally in and out.

  They are in homeostasis.

Multiple Choice

What could happen to cells in an isotonic solution, like saline?

• Water flows in. Their vacuole swells and they become turgid (healthy).

• Water flows in. They swell and could burst.

• Water flows out.

  They shrivel.

• Water flows equally in and out.

  They are in homeostasis.