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Flashcards in Cell membranes Deck (21)
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What are membranes for?

1. To separate the cytoplasm from the outside contents.
2. To separate organelles from the cytoplasm.
3. To control the movement of chemicals in and out of the cell.
4. Where chemicals that contribute towards self recognition are held. Preventing autoimmunity and helping with development of organism.
5. Houses many receptors essential for the sensitivity of the cell.
6. Site of some chemical reactions. Enzymes may attach to the surface membrane.


What is the basic chemical structure of a membrane?

The main structure of any membrane consists of mainly phospholipid molecules. These molecules consist of hydrophilic phosphate heads bonded to hydrophobic lipid tails. Charge is evenly distributed within the molecule so the phospholipid doesn't dissolve in water easily. When it's surrounded by water, phospholipids form a bilayer where two layers of hydrophilic phosphate groups sandwich hydrophobic lipid tails between them. There are usually other components to a membrane as well, like glycolipids, cholesterol and other proteins.


What is the fluid mosaic model of membranes?

This is currently the widely accepted model for describing the structure of membranes. This model states that the main structure of a membrane is composed of a phospholipid bilayer with other chemicals like proteins, cholesterol etc. embedded into it.
The fluid part refers to how the phospholipid molecules in the bilayer aren't held together by any bonds so they and all other components in the membrane are free to move about the bilayer plain,
The mosaic part refers to all the proteins and other chemicals embedded into the phospholipid bilayer, like a mosaic.


What is the structure of phospholipids?

The phospholipid consists of a hydrophilic head composed of glycerol and a phosphate group bonded to a hydrophobic tail made out of two fatty acid groups.A molecule with both hydrophobic and hydrophilic elements are called amphipathic.


Why are membranes considered 'fluids'?

The basic phospholipid structure of a membrane is not bonded together by proper bonds. This means that phospholipids are free to move in the plain of one monolayer. However, it is possible for a phospholipid to travel across to the other side of the bilayer. However, the hydrophobic center repels the hydrophilic head which offers the membrane some form of stability.


What is differentiation?

A simple phospholipid bilayer is not enough to be the membrane. It is too weak and can break easily, as well as not being very selective about which molecules can pass through them. A membrane requires other components in order to function properly but those components depend on the function of the membrane. This is the process of differentiation.


Examples of differentiation.

1. Plasma membrane of cell in growing shoot contains receptors which regulate growth.
2. Muscle cell surface membranes contain many channels that allow glucose to diffuse through.
3. Thykaloid in chloroplast are membranes that contain chlorophyll which is essential to photosynthesis.
4. Plasma membrane of white blood cells contain proteins that are able to detect foreign pathogens.


What is the permeability of membranes?

All membranes are permeable to water but some more than others. These contain channel proteins that allow water to diffuse through them. Most membranes are partially permeable. They allow water and some other molecules through them but not others.


What types of proteins are found in the membrane?

Proteins that are partially embedded on either the inside or outside of the phospholipid bilayer are called extrinsic and proteins that span the whole membrane are called intrinsic. Some proteins float freely, some are bonded to other components and some are bonded to components inside the cell.


What are glycolipids?

Similar in structure to phospholipids. Instead of a phosphate group bonded to glycerol; making the hydrophilic head, a carbohydrate group is bonded to glycerol. Thses carbohydrate groups are also hydrophilic and thus stick out from the outer surface of the membrane and are very important in cell recognition and receptors.


What is cholesterol in membranes for?

Helps bring mechanical stability to cell membrane. Fits in between fatty acid tails and makes it harder for molecules to pass through hydrophobic center of the bilayer.


How are the transport functions of cell membranes achieved?

Proteins with cavities or channels in the middle that are intrinsic and create pores in the cell to allow large or charged molecules to diffuse into the cell are channel proteins.
Proteins that move molecule or ions across membranes are called carrier proteins. They use energy to transport certain ions across a membrane, especially against the concentration gradient.


How do cell membranes aid cell signalling?

Many receptors are embedded in the phospholipid bilayer. These receptors allow hormones to bind to them and trigger certain metabolic actions within the cell. These receptors are also essential in the action of certain drugs.
Glycolipids and glycoproteins have hydrophilic carbohydrate groups that stick out of the cell surface membrane. These can act as receptors to allow the cell to be recognised as 'self' in the immune system. They also help stick cells together in a tissue.


What roles do the membranes play in metabolism?

Sometimes, enzymes are found within cell membranes and the membranes become the site of the metabolic reaction the particular enzyme catalyses. This is the case with cristae in mitochondria and thykaloid in chloroplast.


How does temperature affect the permeability of cell membranes?

As the temperature increases, molecules in the cell membrane such as the phospholipids gain more kinetic energy and vibrate. This vibration widens the gaps in the membranes and thus makes the membranes more permeable and allows substances that wouldn't normally diffuse across the membrane to do so.
This can be proven by an experiment with beetroot. If you put beetroot chunks into hot water, the red pigment begins to diffuse into the water. This is because the high temperature makes the plasma membrane and vacuole membrane of beetroot cells more permeable, allowing the red pigment to diffuse out of the cells into water.


Why is sensitivity important to a cell?

A cell needs to detect changes in its environment and respond to them appropriately. A cell also needs to have the ability to communicate to other cells if it requires them to do something. This is vital in growth, development, movement and excretion. The mechanisms used in this process are very complex.


How do cells detect changes in their environment?

Cells must have sensors on their surface capable of detecting signals from other cells. These are called receptors and are usually located on the outer surface of the cell and are usually proteins or modified proteins.


What are hormones?

Hormones are chemicals produced by cells in order to communicate to other cells. They are excreted into the bloodstream and carried to the target cells (cells with receptors to that particular hormone). Receptors are specific to hormones because their shapes are complementary to each other. The hormone binding onto the receptor brings about a change in the cell.


How does the insulin response work?

Insulin is a hormone made in beta cells in islets of Langerhans in the pancreas. It attaches to insulin receptors on the surface of many cells (including liver and muscle cells) and causes more glucose channel proteins to be present in the plasma membranes. This allows more glucose to diffuse into the cell and thus reduces blood sugar levels.


How do some drugs work?

Some chemicals have been made that mimic the shape of certain hormones. They bind onto receptors but do not trigger a change, thus inhibiting the receptor from its hormone. This is done in people with conditions where the changes caused by the hormone can have adverse effects. For example, beta-blockers attach to certain heart muscle cell receptors which are responsible for receiving hormones that increase heart rate. Other drugs do trigger the change and are used when the individual cannot produce/ produce enough of the hormone.


How can cells be negatively affected?

Some viruses bind onto receptors of certain cells and enter the cell. HIV viruses bind onto receptors on the surface of T-helper cells and enter. Inside, they reproduce and eventually destroy the cell.
Poisons can also bind onto receptors on cells and prevent them from detecting hormones. Clostridium botulinum binds onto receptors in muscle cells and cause paralysis.