The Cell Membrane The Cell Membrane Double layered

The Cell Membrane

The Cell Membrane • Double layered barrier • Protects the inside of the cell.

• It helps the cell to maintain homeostasis – the maintenance of stable internal conditions in spite of changes in the external environment. • Made of phospholipids (fatty acid + phosphate)

Phospholipid Bilayer: the phospholipids form two layers that are like a fatty acid oreo – the phosphates are the chocolate cookie layer, and the fatty acids are the cream inside

Remember: Lipids are non-polar molecules and water are polar molecules. Non-polar and polar molecules do not mix. Lipids and water do not mix.

Phospholipid Bilayer When placed in water, the phospholipids form a double layered membrane around the cell – Phosphates are on the outside of the oreo because they are attracted to their watery environment. – Fatty acids are repelled by water, so they face in towards each other

This bilayer construction acts like a wall which keeps things from entering and exiting the cell through the membrane

Proteins stuck within the membrane act like a door that lets particles pass into and out of the cell. Proteins

Selective Permeability The cell membrane is selectively permeable (permeable means that things can pass through). It lets some things pass through (small particles, gases, water) but not others.

Solutions • Solutions are a mixture of water and solute (stuff) • Solute can be sugar, salt, or any substance • In the picture below, which has a higher concentration of solute?

• When solutions are put together, then solute, stuff, tries to move to the area that has the least amount of solute. Solute hates a crowd. • When solutions are put together, water tries to move to the area that has the most solute (stuff) – water follows the solute (stuff). • Just remember – (1) solute hates a crowd, and (2) water follows the solute (stuff).

• When solutions are placed next to one another with only a selectively permeable membrane between them, only some things will be able to move from one side of the membrane to the other. Usually, only water can move and the stuff is left behind. In this case, water wants to follow the stuff, so it goes to the side with the most solute.

Which has the higher concentration of solute?

1. Which bottle has the highest solute concentration? 2. Which has the lowest? 3. Which two have equal solute concentrations?

Passive Transport Osmosis & Diffusion • Passive transport - movement through the cell membrane that doesn’t require energy • Dropping a weight from the roof is passive – no energy is required • In passive transport, water/solute go where they want to – solute goes away from the crowd, and water follows the stuff

• Two examples of passive transport through the cell membrane: osmosis and diffusion

Diffusion: • movement of solute (particles) from areas of high solute concentration to areas of low solute concentration until they are evenly distributed – solute is trying to spread out – reaching equilibrium, condition in which the concentration of a substance is equal throughout a space.

» Examples: • Perfume spreads out when sprayed into the air • Chocolate particles spread out in milk • Some gases and small particles diffuse through the cell membrane until the inside and outside of the cell are the same.

Diffusion

Osmosis: - the diffusion of water through a selectively permeable membrane. The water moves from low solute concentrations to high solute concentrations – it follows the stuff! • The water moves into the area with the most solute

– Osmosis Through the Cell Membrane • The cell membrane is permeable to water but not large particles • Water moves from low solute concentration to high (water follows the stuff)

3 Types of Solutions: 1) Isotonic Solution Øwhen there are equal amounts of water on both sides of the cell

2) Hypotonic Solution ØWhen water enters the cell, it gets larger ØTurgor pressure will increase (pressure inside the cell) ØCell may lyse (burst)

3) Hypertonic Solution ØWhen water leaves the cell, the cell shrinks

Which is the following?

• Channel Proteins: Some proteins act as doors in the cell membrane. Particles diffuse from areas of high to low concentrations by passing through these proteins.

Facilitated diffusion • – another example of passive transport in which it moves substances down their concentration gradient without using energy. It transports these substances, like amino acids and sugars, by using carrier proteins. Carrier proteins can bind to a specific substance on one side of the cell membrane and then release it on the other side.

Active Transport Energy Required! • Active Transport - movement through the cell membrane against a concentration gradient (from low to high) (Carrying a weight to the roof is active because it requires energy from the cell) • Any transport that requires ATP energy is active transport

Active Transport Movement of the solute from the area of low concentration to the area of high concentration requires ATP energy

– Movement of the solute from the area of low concentration to the area of high concentration requires ATP energy – in other words, it takes energy to push water away from the solute (stuff), or to force solute to go into a crowd – One type of active transport uses carrier proteins to carry unwilling particles across the membrane

Movement in Vesicles • Many substances are too large to be transported by carrier proteins. These substances, such as proteins and polysaccharides, are moved across the cell membrane by vesicles.

• Endocytosis = the movement into a cell by a vesicle. Ø Ø Ex. Think of cells needing to eat and drink Pinocytosis – cell drinking, taking in a liquid Phagocytosis – cell eating, taking in a solid • Exocytosis = the movement of a substance out of a cell by a vesicle.

Regulating Water in Protists • Many protists, such as paramecium and euglena, constantly absorb water by osmosis. They get rid of excess water by using a contractile vacuole, which is a saclike organelle that expands, collecting excess water, and then contracts, squeezing water out of the cell.