Membranes and Transport First, the Phospholipid Challenge

Membrane Structure The membrane is made of many different pieces that retain the ability to move around; called a FLUID MOSAIC Main component is the phospholipid; proteins and carbs work as special structures with jobs (tunnels, ID badges, etc) The membrane is selectively permeable, meaning only certain things can come in/out

Phospholipids Phospholipids consist of a hydrophilic phosphate head and hydrophobic lipid tails. Since the heads love water and the tails hate water, they form a double layer with the heads facing out towards the water and the tails facing in away from the water.

Membrane Proteins Proteins in the membrane act as doors and filters, allowing good things to cross the membrane barrier, but keeping bad things out. Some proteins are always open, but others must be opened. These proteins help the membrane control transport. Other proteins/lipids/carbs perform other jobs like identification and communication

WHY? Examples of maintaining homeostasis Membranes help the cell maintain homeostasis (a constant internal balance). If a cell does not maintain homeostasis, then it will die. Examples of maintaining homeostasis Constant body temperature Right amount of food/water Keeping out diseases

Types of Transport There are two types of transport. One that requires the cell to do work/use energy (ACTIVE) and one that does not require the cell to work (PASSIVE). - Passive Transport: -Uses No Cell Energy, molecules move themselves -Only moves in one direction, from high concentration to low concentration (spread out) - Active Transport: Requires Cell Energy/Work (ATP “batteries”) Used to move things to a specific location OR to move from low to high concentration

Types of Passive Transport Diffusion = movement of ANY molecule from high concentration to low (downhill/down the gradient)

Types of Passive Transport Osmosis = diffusion of WATER (still from high to low); water crosses the membrane through proteins called Aquaporins; how the water moves depends on the concentration of water inside and outside the cell. Since water is ALWAYS allowed in or out of the cell, having a lot of stuff (solvent) dissolved INTO the water will affect how the water moves. To determine whether water will enter or leave a cell, we look at the tonicity (or concentration) of the cell’s surroundings.

Red Blood Cells in hypo/hyper/isotonic conditions Red Blood Cells in hypo/hyper/isotonic conditions. (Note that the isotonic cells are “normal”) Hypo/hyper/isotonic concentrations always refer to what is OUTSIDE the cell, never the inside.

HYPOTONIC: More dissolved “stuff” inside the cell than outside, meaning more WATER outside; the stuff can’t move to equalize the gradient, so the water has to move instead. Water will move IN causing the cell to swell Animal cells could burst Plant cells are protected by their cell wall (called Turgor Pressure) “Hypo, Oh No! There’s too much H2O outside my cell, I’m going to swell.”

HYPERTONIC: More dissolved stuff outside, meaning most of the water is inside the cell. Water leaves the cell to try and even things out. Cell will shrink or shrivel (like a raisin) Plants wilt because they lose the turgor pressure (called plasmolysis) “Hyper Sucks”

ISOTONIC: Concentration of water/solutes is the SAME in and out Cell does not gain or lose water “iso means same”

Osmosis Practice – Identify the following as hypo, hyper, or iso (Hint: find the WATER)

Types of Passive Transport Facilitated Diffusion = diffusion through protein tunnels to help larger molecules (like sugar) cross the membrane. Facilitated means “assisted”. Still passive. No energy. These are the protein “doors that are always “open”

What if you want to move from low to high concentration? ACTIVE TRANSPORT

Types of Active Transport Anytime a molecule wants to move against the concentration gradient (from low to high), it must be moved through ACTIVE transport. This requires the cell to use its own energy with ATP “batteries”. Protein pumps use ATP energy to move molecules against the gradient from low to high Example - Sodium-Potassium Pump: Protein that actively transports sodium and potassium ions into or out of the cell. Necessary for your nervous system.

Protein Pumps

Types of Active Transport The Golgi uses ATP to move material through endo/exocytosis Endocytosis = moving particles into the cell using vesicles; opposite of exocytosis Pinocytosis = drinking action, used for small particles or water Phagocytosis = devouring action, used for large particles like food Exocytosis = removing particles from the cell using vesicles; opposite of endocytosis

Endo/Exocytosis

Transport Video Demos Passive transport (high to low, no cell energy) Diffusion Facilitated Diffusion Osmosis Active transport (low to high, needs cell energy) Protein Pumps Endo/Exocytosis Amoeba using endocytosis to eat

So why are cells so small? Cells are small because it makes transport easier. Larger cells have difficulty bringing in materials through diffusion. It takes too long. The smaller a cell, the faster it can absorb material. This is due to the surface area to volume ratio. As any object gets larger, the volume (inside) gets bigger faster than the surface area (outside). The more surface area you have AS COMPARED TO your volume, the easier transport becomes. Cells can also be slightly larger than other cells, but they need to be flat to increase their surface area as much as possible.

Surface Area to Volume Ratio As the cubes get larger, the ratio gets smaller. Having a large surface:volume RATIO is what is most important to cell transport. Therefore, in general smaller is better.

Surface Area to Volume Ratio To be larger than microscopic, cells form multicellular groups. This means they can be a large organism together, but individually they are all still small. Many small cells have easier transport than one giant cell.