Cellular Homeostasis

You want to maintain certain levels for the cell to function normally. Homeostasis is the used to describe the overall health and condition of the cell. You want to maintain certain levels for the cell to function normally. you need to maintain a certain level of oxygen, food, water, etc… to function properly…well a cell needs to maintain things too.

A cell membrane helps organisms maintain homeostasis by controlling what comes in and out of a cell. Small molecules can come through the plasma (cell) membrane. This is called the Permeability of the membrane.

This movement of substances across a cell membrane without any energy used is called PASSIVE TRANSPORT

TYPES OF PASSIVE TRANSPORT

1. DIFFUSION – the movement of molecules from an area of high concentration to an area of low concentration Low High The distance it needs to travel we will call the Gradient.

So we are talking about concentrations, and the distance they diffuse across (the membrane)… so this is known as CONCENTRATION GRADIENT

So molecules that move through the lipid layer from high to low concentration is an example of DIFFUSION Low High What happens when you have the same numbers of molecules on both sides of the cell membrane?

There’s no concentration gradient…both sides are equal. When you have the same number of molecules on both sides of the cell membrane you have something that is called EQUILIBRIUM There’s no concentration gradient…both sides are equal.

50% 50% 70% 30% 40% 60% 49% 51% 54% 46%

“SEMI-PERMIABLE MEMBRANE” Diffusion will only occur with small molecules, molecules that are large will not be able to go through the cell membrane easily. Only select small molecules go through the membrane…So the cell membrane is then called a “SEMI-PERMIABLE MEMBRANE”

2. Facilitated Diffusion Special Proteins in the cell membrane are designed to diffuse one exact type of substance through the protein without any energy being used by the cell Blurry MOVIE!!! Non-Blurry Movie

3. OSMOSIS Osmosis is almost the same as diffusion…except we are talking about water and nothing else. Osmosis – is the movement of water molecules from a high concentration to an area of low concentration

However there are 3 types of Osmosis… ISOTONIC – is a state of equilibrium. Where water molecule levels are equal on both sides of the cell membrane. - water can freely flow in or out of the cell 50% 50% 50% 50% Animal Cell Plant Cell

so if water rushes in, the cell starts to swell. . HYPOTONIC this is where there is High concentration of pure water OUTSIDE the cell and a Low concentration of water INSIDE the cell. so if water rushes in, the cell starts to swell. In Animal cells, if too much water comes in, the cell expands and bursts open. This is called LYSED or Haemolysis in blood cells. 70% 30%

Hypotonic In Plants… - water rushes into the cell, but the cell does not explode ! Why? plant cells have a tough double cell wall, so they hold the cell shape even when water rushes in so it doesn’t explode! 70% 30% The pressure of the water pushing against the cell walls is called TURGOR PRESSURE

HYPERTONIC is where you have a higher concentration of water INSIDE the cell, than outside the cell So water moves out of the cell ! In animal cells all the water leaves the cell, so it shrivels up and can possibly die In blood cells this is known as Crenation 30% 70%

Hypertonic In Plants water is leaving the cell but the plant cell wall stays in place. All the insides start to shrivel up. This action is called PLASMOLYSIS 30% 70% Diffusion movie Movie on RBC

Endocytosis & Exocytosis Active Transport Endocytosis & Exocytosis

We know that the concentration gradient means moving from a area of high concentration to an area of low concentration Low High Occasionally this needs to be moving from low to high gradient” High Low To go against the normal concentration gradient it will require energy to be spent.

Moving from Low to High on the concentration gradient, using energy is a process known as ACTIVE TRANSPORT Molecule to be carried Molecule being carried Energy It requires special membrane proteins (carrier proteins) that are often called “pumps” because they pump against the normal concentration gradient The energy that is added causes the protein to change shape, allowing the molecule to be carried in.

Active transport in the cells is normally used for large molecules Molecule to be carried Molecule being carried Energy However, concentrations of sodium, Calcium, and Potassium are also pumped through for different cell functions.

The most common active transport pump we talk about in Biology is the Sodium-Potassium pump. 1. The protein in the membrane attracts sodium ions into special receptors. 2. ATP give up 1 of its “Phosphorous ions” to start the pump. This is known as Phosphorylation.

3. The protein now begins to change shape & move the Na+ against the concentration gradient. Releasing it on the other side of the membrane. 4. The protein now attracts Potassium ions (K+) into specially designed receptacles.

5. The binding of the Potassium causes “De-phosphoryliation” as the phosphate group falls off. 6. The “De-phosphoryliation” causes the protein to change shape and move the K+ ions across the concentration gradient. Ready to revieve new NA+ ions.

Otherwise, you would not be able to constantly make muscles contract. Active transport (sodium / potassium pump) is very important because it helps return the muscle cells to normal homeostasis levels after muscle contraction. Otherwise, you would not be able to constantly make muscles contract. MOVIE 1 MOVIE 2

So how do cells eat? They actually have a way of bringing in material into the cell to digest. It is called ENDOCYTOSIS (Endo = towards) There are 2 types of Endocytosis: PINOCYTOSIS – “cell drinking” where the cell engulfs solutes or fluids PHAGOCYTOSIS – “cell eating” where whole particles or large compounds are brought in

STEPS OF ENDOCYTOSIS The cell membrane forms a pouch on the cell surface and brings in the material The cell membrane’s pouch then surrounds the material Eventually the pouch closes completely capturing the material inside The pouch pinches off from the cell membrane to become a vesicle 1 2 3 4

Cells also have a way to get rid of waste or proteins in a process called EXOCYTOSIS (exo = exit) It’s practically the reverse of endocytosis. The Golgi body forms a vesicle that is packaged for leaving the cell

The vesicle approaches the cell membrane The vesicle fuses with the cell membrane The pouch now opens up and expels it’s material The material is released outside of the cell MOVIE!! 4 3 2 1

VOCABULARY! Passive Transport Diffusion Concentration gradient Equilibrium Semi-permeable membrane Osmosis Isotonic Hypotonic Lysed Turgor Pressure Hypertonic Plasmolysis Facilitated diffusion Active Transport Carrier proteins Endocytosis Pinocytosis Phagocytosis Exocytosis