Biology: Unit F211: Cells, Exchange and Transport …revisionnotes.weebly.com/uploads/8/6/5/6/8656153/cell... · Web viewBiology: Unit F211: Cells, Exchange and Transport Module 1:

O2 Na++

Water outside cell

Water inside cell

Hydrophobic Region

Biology: Unit F211: Cells, Exchange and Transport Module 1: Cells

1.1.2 – Cell Membranes

Membranes surround all cells, separating the cell contents from the outside world. Eukaryotic cells also have organelles which are bound by membranes, such as the Golgi body, the endoplasmic reticulum and the mitochondria.

Functions of Membranes

1. Selectively Permeable - keeping certain things in and out of the cell, for example, red blood cells must keep haemoglobin inside the cell.

2. Identify the cell and communicate with others – identifying cells as your own, and knowing which are invading cells.

3. Containing enzymes for chemical reactions – like respiration which is carried out using enzymes in the mitochondria.

All membranes have the same basic structure – Phospholipids

These form a Phospholipid Bilayer:The phospholipids naturally arrange themselves into a Bilayer. The heads are hydrophilic, and so want to be next to water. There is water both within the cell and outside of the cell, and so this means that heads must be on the outer edge of either side of the membrane. Two layers of phospholipids line up, so that the heads are

Hydrophilic Phosphate Head – loves water. It is made from a molecule that is covered in small charges, making it polar. This means that it can interact with water.

Hydrophobic fatty acid tail – hates water. Large molecule with no charges and so moves away from water.

GlycoproteinGlycolipid

Channel Protein

Carrier Protein

Intrinsic Protein

Extrinsic Protein

Cholesterol

Phospholipid Bilayer

on the outside, close to the water, and the tails, which are hydrophobic, are away from the water.

However, slats, amino acids and glucose are all essential for our cells, and so there must be a mechanism of getting these molecules into the cell, because they can’t travel through the Phospholipid Bilayer. The diagram below shows the fluid mosaic model of the membrane.

The Phospholipid Bilayer is 7nm thick and is selectively permeable. It prevents large, charged molecules from passing through.

Channel Proteins form a hydrophilic channel through the Bilayer to allow specific large and charged molecules to pass through.

Carrier Proteins – some of the intrinsic proteins act as carrier proteins are able to actively move large and charged substances across the membrane. This process uses ATP from respiration. The incoming molecule binds to a site causing a change in shape in the carrier protein, which then deposits the molecule in the cell cytoplasm.

H20

CO2

O2

Glucose

Protein

Na+

Small, uncharged molecules like water, carbon dioxide and oxygen can pass through the Phospholipid Bilayer freely.

Large charged particles cannot get through, like glucose, sodium and proteins.

Glycoproteins are a combination of carbohydrate and protein which help in cell signalling and cell interaction. They identify the cell as your own to your immune system. They also are hormone receptors, responding to hormones in the blood stream, as well as

drugs. Glycolipids are involved in cell adhesion. Glycolipids are a glucose joined to a Phospholipid molecule. They stick cells together forming tissues and organs.

Intrinsic Proteins pass all the way through the Bilayer

Extrinsic Proteins are exposed on only one side of the membrane. They act as enzymes in the membrane, carrying out reactions like respiration on the cristae of the mitochondria, and photosynthesis on the thylakoids in the chloroplasts.

Cholesterol acts like a staple on the inside layer of the membrane to hold together the phospholipids. It fits between the fatty acid tails, maintaining the membrane’s stability and fluidity.

Communication and Cell Signalling

Cell Signalling is how cells communicate with each other. In multicellular organisms, each cell has its individual role to play, and so cells must be able to detect the various internal and external signals used to coordinate and carry out the processes involved in growth, development, movement and excretion. Cells must then be able to carry out reactions or functions in response to the signals. In order to detect these signals, cells have receptors which are capable of receiving signals. Receptors are often protein molecules or modified protein molecules.

Hormone Receptors – Hormones are chemical messengers produced in specific tissues and then released into the organism. Any cell with a receptor for the hormone molecule is a target cell. A hormone molecule will bind to the receptor on a target cell plasma membrane because

they have complementary shapes, like a jigsaw. Binding of the hormone and receptor causes the target cell to respond in a certain way.

Hello!

Insulin is a protein molecule hormone which is released from the beta-islet cells of the pancreas in response to increased blood sugar levels. It attaches to the insulin receptors on the plasma membranes of muscle and liver cells. This triggers internal responses in the cell which allow more glucose to be taken up from the blood, so reducing blood sugar levels.

Medicinal drugs have been developed that are complementary to the shape and type of a receptor molecule. Beta-blockers prevent heart muscle from increasing heart rate. Drugs used to treat schizophrenia mimic a natural neurotransmitter that some individuals cannot produce themselves.

Hijacking receptors – viruses enter cells by bonding to receptors on the plasma membrane which would usually bind to signalling molecules. The HIV virus fits onto the receptors of important immune cells like T-lymhocytes. The virus may then reproduce inside the cell and destroy it. Toxins can bind to receptors, like the toxin used in BOTOX to paralyse small muscles in the face and reduce the wrinkling in the skin.

Crossing Membranes

In order to survive, cells need nutrient molecules. They may need oxygen for aerobic respiration. Waste products generated by the cell metabolism must be removed. Any molecules that need to enter or leave a cell will usually have to cross a membrane to do so.

Passive Processes – A) Diffusion Diffusion is the net movement of particles from an area of high concentration to low concentration down a concentration gradient. Particles tend to even out, so that there are equal concentrations on each side of the membrane. When there are equal concentrations, the net movement of particles has stopped. Particles are still moving, but overall there is no change. This is a state of dynamic equilibrium.

The rate of diffusion is affected by:

1. Temperature – increasing the temperature gives the particles more kinetic energy, so the rate of random movement increases, so rate of diffusion increases.

2. Concentration Gradient – Increased concentration gradient (a greater difference in concentration gradient between the two sides) increases the rate of diffusion.

3. Movement – stirring a liquid increases the movement of molecules and so the rate of diffusion.

4. Distance – if the membrane is thinner, rate of diffusion is quicker because there is a smaller distance for the molecules to travel.

5. Size of molecule – smaller molecules diffuse more quickly than larger ones.

B) Facilitated Diffusion

Facilitated diffusion is when protein molecules are involved in the movement of large or charged particles.

Facilitated Diffusion with Carrier Proteins moves larger molecules like glucose and amino acids. They are a specific shape allowing one molecule to fit, and the protein then changes shape to release the molecule. Molecules can be carried in either direction according to the concentration gradient.

Facilitated Diffusion with Protein Channels moves charged molecules like sodium and calcium ions. They are effectively pores in the membrane. Some are gated: at a synapse, a transmitter substance binds to a receptor protein, which will then open the channel protein.

Active Processes – C) Active Transport

Active transport is the movement of molecules from an area of low concentration to an area of a higher concentration up a concentration gradient.

Sometimes, a cell cannot meet its needs by diffusion. A plant cell may need more magnesium ions for photosynthesis. It needs to be able to move these ions into the cell against a concentration gradient.

Some of the carrier proteins found in plasma membranes acts as pumps. Their shape is complementary to the molecule they carry. These protein pumps can only carry molecules one way across the plasma membrane, and they use metabolic energy in the form of ATP to change their shape. By changing the shape using ATP, the active transport protein ensures that the molecule can only go one way.

Bulk Transport – Endocytosis and Exocytosis

Endocytosis and Exocytosis are processes which bring large amounts of materials in (endo) or out (exo) of the cell. This bulk transport is made possible because plasma membranes can easily fuse, separate and pinch off vesicles. Lots of energy from ATP is required to form the vesicles that are needed and to move the vesicles around the cell.

Calcium Ion movement in muscles – muscle fibres can only contract if calcium ions are present. When a muscle is stimulated to contract, calcium ions are released from specialised endoplasmic reticulum, where they are in high concentration. When the muscle needs to relax again, the calcium ions are pumped back into the stores by the calcium ion pumps in the plasma membranes of the specialised endoplasmic reticulum.

Hormones – Pancreatic cells make insulin in large quantities. The insulin is processed and packaged into vesicle by the Golgi apparatus, and these vesicles then fuse with them membrane to release insulin into the blood.

White blood Cells – engulf invading microorganisms by forming a vesicle around them. This vesicle then fuses with the lysosome so that the enzymes in there can digest the microorganism. These cells are called Phagocytes.

In this solution, each water molecule has a solute molecule. The ratio between water and solute is 1:1. This means that the water does not want to leave the solution, and so water potential is low.In this solution, each water molecule does not have a solute molecule. The ratio between water and solute is 3:1. This means that the water does want to leave the solution, and so water potential is high. There are lots of water molecules which are not attracted to solute molecules, and so can leave the solution.

D) Osmosis - Osmosis is the diffusion of water from an area of high water potential to an area of low water potential.

Water potential is the desire of water to move out of a solution. It is measured in units of pressure –

Kpa (kilopascals). Its symbol is the Greek letter Psi, ψ. It could be thought of as the concentration of free water molecules – the more free water, the higher the water potential.

Osmosis takes place until there is no longer a water potential gradient. If both have equal water potentials, there is no overall movement. Only water can move in osmosis, never the solute. The movement of water is the thing which changes the volume of the cell. The cell can increase or decrease in size according to the direction of water movement.

Random movement of molecules directly through the Phospholipid Bilayer from an area of high water potential to an area of low water potential.

Pure Water – no solute dissolvedHighest Water Potential – 0kPA

Dilute Solution – a little solute dissolvedLower Water Potential – -10kPA

Concentrated Solution – lots of solute dissolvedLower Water Potential – -500kPA

In pure water or a hypotonic solution with a very high water potential, water moves into the cell by osmosis down a water potential gradient. Plant cell membrane will be pushed against the wall – the cell will be turgid. An animal cell will burst open - it is haemolysed.

In a very concentrated solution with a very low water potential, water will move out of the cell by osmosis down a water potential gradient. A plant cell’s plasma membrane will pull away from the wall –the cell will be plasmolysed. An animal cell will shrink and wrinkle – it will be crenated.

In a hypertonic solution, there is more solute, making it more concentrated. This means there is less free water, and so there is lower water potential. Water moves from an area of high water potential inside the cell to the low water potential outside of the cell, meaning that the cell volume decreases.

In a hypotonic solution, there is less solute, making it dilute (less concentrated). This means there is more free water, and so there is higher water potential. Water moves from an area of high water potential outside the cell to the low water potential inside of the cell, meaning that the cell volume increases.

An isotonic solution will have the same water potential as the cell, and so no net movement occurs.

The plasma membrane forms a selectively permeable barrier around every cell. Signals must first negotiate the plasma membrane before they can communicate with the nucleus. Chemicals which change a cell’s behaviour are drugs and hormones. These signal molecules can either be made form Lipids or Proteins.

There are 2 ways in which signals can be sent across membranes:

1. Receptors acting as ion channels:

Neurotransmitters such as acetylcholine attach to the receptors on the channel proteins. These channel proteins only open when they are bonded to a hormone signal molecule. When a signal molecule arrives at a receptor, the channel opens, allowing things to pass into the cell.

This is how nerve signals make muscles contract. Vesicles containing a neurotransmitter are released from the nerve cell by Exocytosis. These vesicles move to the plasma membrane of the muscle cell. The neurotransmitter binds to a particular channel protein in the muscle membrane, causing the channel to open. Ca+ ions can then rush into the muscle, causing it to become positive and contract.

2. Receptors acting as enzymes:

Intrinsic proteins have receptor sites for drugs and hormones, with inactive enzymes attached to one side. When a protein hormone or drug attaches to the receptor site, the proteins are dragged closer together (dimerisation) and the enzymes switch on.

This is how insulin reduces blood glucose: the insulin protein hormone binds to its glycoprotein receptor. This causes the enzyme attached to the glycoprotein on the inside of the membrane to activate. This starts other reactions inside the cell. It opens another channel protein which allows

Lipids are hydrophobic and so can diffuse freely through the Phospholipid Bilayer. Their target is inside the cytoplasm, not on the membrane.

Proteins are large molecules which cannot pass through the Phospholipid Bilayer, so their target is on the outside of the membrane. Insulin is an example.

glucose into the muscle, thereby reducing blood glucose, and converts glucose into fat and glycogen for storage.

Temperature and the Permeability of Membranes

Any passive process (diffusion, facilitated diffusion, osmosis) is speeded up by an increased temperature, because particles have a greater kinetic energy.

At lower temperatures, the membrane’s phospholipids do not have very much kinetic energy. This means that there are no gaps that large and charged molecules can fit through.

At higher temperatures, the membrane’s phospholipids have a greater amount of kinetic energy. This means that larger temporary gaps are created between them, increasing the chance that large and charged molecules can pass through the Phospholipid Bilayer.

If an organism lives in a warm environment, their membranes will have more cholesterol between the phospholipids. This gives stability to the membrane, and stops as many gaps being created.