Chapter 3 Movement of Substances across the Plasma Membrane3.1 Movement across the Plasma Membrane

Substances required by the cell are nutrients (glucose and minerals) and oxygen

Substances to be eliminated are metabolic wastes Why is this important?

To continue cellular life process, concentration of ions inside the cell must be different than outside the cell

Maintain a constant cellular environment (homeostasis) Structure

Composed of phospholipids and proteins Fluid-mosaic model

Phospholipid bilayer barrier which isolates two sides of membrane

Contains cholesterol stabilize and strengthen plasma membrane Pore protein forms channel/pore Carrier protein acts as carrier Glycoprotein protein with carbohydrate attached Fluidity of membrane cells are more flexible

The plasma membrane is semi-permeable/selectively permeable (only some substances can pass through)

Factors determining whether molecule can pass through size and polarity Molecules that can pass through

Lipid-soluble molecules (fatty acids and glycerol) Non-polar molecules (oxygen and carbon dioxide) Small molecules such as water ( Basically water is a polar molecule. However, its

small size enables it to slide between phospholipid bilayer) Pore proteins allow small water–soluble molecules and ions to pass through Carrier proteins have site that can bind to specific molecules (glucose

molecules) before transporting them to plasma membrane.

Passive transport (movement of substances across plasma membrane without input of energy) Example: gaseous exchange in alveolus and blood capillary

Simple diffusion

Movement of substances from a region of higher concentration to a region of lower concentration, thus, going down a concentration gradient until a dynamic equilibrium is reached

Osmosis: Diffusion of water Movement of water molecules from a dilute solution (water concentration high) to a

concentrated solution (water concentration low) through semi-permeable membrane Facilitated diffusion

Movement of substances across plasma membrane with the aid of carrier proteins and pores following the concentration gradient

Example: ions, nucleic acids, amino acids and glucose Carrier protein are specific (only can bind with certain molecules) Pore proteins form pore/channel

Active transport

Movement of solute/ion across plasma membrane against concentration gradient Requires energy and carrier protein Energy comes from ATP (adenosine triphosphate) generated during respiration in

mitochondria Carrier protein has an active site to bind with molecule and another active site to

bind with ATP. The carrier protein changes shape when phosphate group from ATP binds to it. Then, the solute is moved across the membrane.

Ex. Absorption of water and intake of ions in plants

Passive Transport Differences Active TransportFollows concentration

gradientConcentration gradient Opposes concentration

gradientDoes not need energy Cellular energy Consumes energy

Can take place in living cells or non-

living physical conditions

Condition Can only take place in living cells

3.2 Movement of Substances across the Plasma Membrane in Daily Life

Hypotonic- A solution with higher water potential than another solution Hypertonic- A solution with lower water potential than another solution Isotonic- A solution with same water potential with another solution Haemolysis- The bursting of red blood cells Crenation- The shrinking of red blood cells Plasmolysis- A shrinking of cytoplasm due to osmosis Deplasmolysis- A process of a cell gaining its turgidity back

Animal and Plant Cells in an Isotonic Solution

Water diffuses into and out of cell at equal rate. Hence, the cell retains its normal shape.Likewise with the plant cell.

Animal and Plant Cells in a Hypotonic Solution

In animal cell, water enters the cell and causes it to swell up and eventually to burst (red blood cell). This is because the plasma membrane is too thin towithstand the pressure. The bursting of red blood cells is known as haemolysis. In plant cell, water enters the large central vacuole of the cell, causing the vacuole to expand and swell up and the plasma membrane pushes against the cell wall. In this condition, the cell is said to be turgid. The cell does not burst because the rigid cell wall able to withstand the pressure. This condition creates turgor pressure. Turgidity is important to support, give shape, and causing the guard cell to swell so that the stomata remains open for photosynthesis.

Animal and Plant Cells in a Hypertonic Solution

In animal cell, there is a net movement of water from inside to outside of the cell. This causes the cell to shrink. In red blood cell, the cell shrivel and the plasma membrane crinkles up. The cell has undergone crenation. In plant cell, water diffuses out of vacuole through osmosis. Both vacuole and cytoplasm shrink and plasma membrane pulls away from cell wall (plasmolysis). The cell becomes flaccid. The flaccidity causes the plant to become limp and stem to drop (wilting). The cell can deplasmolysed by immersing it back to a hypotonic solution.

Wilting occurs in plants when too much fertilizers like potassium nitrate is given. Too much fertilizers cause the soil to turn hypertonic to the plant cell. As a result, water diffuses from the cell sap into the soil by osmosis and the cell is plasmolysed. Water shortage in soil also causes the plant to wilt.

Food such as mushrooms, fruits and fish can be preserved using natural preservatives (salt and sugar). The preservative makes the surroundings more hypertonic to the food and causes water to leave through osmosis. The food becomes dehydrated. Microbes loses water to the surrounding and dies.

3.3 Appreciating the Movement of Substances across the Plasma Membrane

The proper functioning of plasma membrane is important to: Maintain a suitable pH and ionic concentration inside the cell for enzymatic

activities To obtain certain food supplies for energy and raw materials

To remove toxic substances