Unit 2 - Scaling Up

• Mitosis

  • Mitosis is when a cell reproduces itself by splitting to form two identical offspring's.
  • The cell has two copies of its DNA spread out in long strings.
  • Before the cell divides the DNA forms X-shaped chromosomes. Each 'arm' of a chromosome is an extra copy of the other.
  • The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two 'arms' go to opposite ends.
  • Membrane form around each of the sets of chromosomes. These become the nuclei.
  • Lastly the cell divides.
  • ou have two new cells containing exactly the same DNA.
• Specialised cells

  • Differentiation is the process by which a cell changes to become specialised for its job.
  • In most animal cells, the ability to differentiate is lost at an early stage, but lots of plants never lose this ability.
  • Having specialised cells is important - it allows organisms to work more efficiently.
  • Most cells are specialised to carry out a particular job. For example: Palisade leaf cells and Sperm
• Palisade leaf cells

  • Palisade leaf cells do most of the photosynthesis in plants, so they are packed with chloroplast. Their tall shape means they have a lot of surface area exposed down the side for absorbing CO2 from the air in the leaf, and their thin shape means you can fit loads of them in at the top of the leaf, so they're nearer the light.
• Sperm

  • The function of sperm is basically to get the male DNA to the female DNA during reproduction. Sperm have long tails and streamlined heads to help them swim. They contain lost of mitochondria to provide the with energy, and they have enzymes in their heads to digest through the egg cell membrane.
• Multicellular organisms

  • In multicellular organisms, specialised cells are grouped together to form tissues - groups of cells working together to perform a particular function. Different tissues work together to form organs. Different organs make up an organ system.
• Stem cells

  • Stem cells are undifferentiated. Depending on what instructions they're given, they can divide by mitosis to become new cells, which then differentiate.
• Embryonic stem cells

  • These are found in early human embryos. They have the potential to turn into any kind of cell cell at all. This means stem cells are really important for the growth and development of organisms.
  • Adults also have stem cells, but they're only found in certain places, like bone marrow. These aren't as versatile as embryonic stem cells - they can't turn into any cell type, only certain ones.
• Plant stem cells

  • In plants, the only cells that divide by mitosis are found in plant tissues called meristems.
  • Meristem tissue is found in the areas of a plant that are growing - such as roots and shoots.
  • Meristems produce unspecialised cells that are able to divide and form any cell type in the plant - they act like embryonic stem cells. But unlike human stem cells, these cells can divide to generate any type of cell for as long as the plant lives.
  • The unspecialised cell can become specialised and form tissue like xylem and phloem.
• Diffusion

  • Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
  • Diffusion happens in both liquids and gases - that's because the particles in these substances are free to move about randomly.
• Cell membrane

  • They hold the cell together, but also let stuff in and out.
  • Substances can move in and out of cells by diffusion, active transport and osmosis.
  • Only very small molecules can diffuse through cell membranes through things like glucose, amino acids, water and oxygen.
• Active transport

  • Active transport is the movement of particles cross a membrane from an area of lower concentration to an area of higher concentration using ATP released during respiration.
• The digestive system

  • This is an example of active transport.
  • When there's a higher concentration of nutrients in the gut they diffuse naturally into the blood.
  • But sometimes there's a lower concentration of nutrients in the gut than in the blood.
  • Active transport allows nutrients to be taken into the blood, despite the fact that the concentration gradient is the wrong way. This is essential to stop us starving.
• Osmosis

  • Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher concentration to an area of lower concentration.
• Partially permeable membrane

  A membrane with very small holes in it, where only tiny molecules (like water) can pass through and bigger molecules (sucrose) can't
• Osmosis

  1. Water molecules pass both ways through the membrane

  2. There is a steady net flow of water into the region with fewer water molecules
• More water molecules on one side than the other

  The sucrose solution gets more dilute
• Water potential

  • You can talk about osmosis in terms of water potential.
  • Water potential is the likelihood of water molecules to diffuse out of or into a solution.
  • If a solution has high water potential, then it has a high concentration of water molecules. If it has a low water potential, then it has a low concentration of water molecules.
• Turgid cells

  • Watering a plant increases the water potential of soil around it. This means that all the plant cells draw water in by osmosis until they become turgid (plump and swollen).
  • The contents of the cell push against the cell wall - this is called turgor pressure, which helps support the plant tissue.
  • If there's no water in the soil, a plant starts to wilt. This is because the cell loses water. The plant doesn't totally lose its shape though, because the inelastic cell walls keeps things in position.
• Surface area to volume ratio

  • The rate of diffusion, osmosis and active transport is higher in cells with larger surface area to volume ratio
• As temperature increases


  Particles in a substrate have more energy and move faster, so substances move in and out of cells faster
• Concentration gradient

  Substrates move in and out of a cell faster if there's a big difference in concentration between the inside and outside of the cell
• If there are lots more particles on one side, there are more there to move across
• Exchanging substances in multicellular organisms

  • As on organism needs to supply all its cells with the substances it needs. It also needs to get rid of the waste.
  • It is difficult to exchange substances. Diffusion across the outer membrane is to slow because; some cells are deep inside the organism. Larger organisms have a low surface area to volume ratio.
  • Multicellular organisms have specialised exchange organs each with a specialised exchange surface.
  • They also need transport systems to carry materials from the exchange organs to the body cells and to remove waste products.
• Exchanging substances in single-celled organisms

  • Single-celled organisms exchange substances differently to multicellular organisms. As they're only one cell big, substances can diffuse straight into and out of single-celled organisms across the cell membrane.
  • Diffusion is quick because; substances only have to travel a short distance. Single-celled organisms have a relatively large surface area to volume ratio.
• Specialised exchange organs

  The exchange surfaces in specialised exchange organs are adapted to maximise effectiveness:

  • They are thin, so substances only have a short distance to travel.
  • They have a large surface area, so lots of a substance can move at once.
  • Exchange surfaces in animals have lots of blood vessels, to get stuff into and out the blood quickly.
  • Gas exchange surfaces in animals are often ventilated too.
• Alveoli

  The alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide. They have:

  • An enormous surface area.
  • Very thin walls.
  • A moist lining for dissolving gasses.
  • A good blood supply.
• Gas exchange

  • The job of the lungs is to transfer oxygen to the blood and to remove waste carbon from it.
  • To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place.
  • The blood passing next to the alveoli has just returned to the lungs from the rest of the body, so it contains a lot of CO2 and very little oxygen.
  • CO2 diffuses out of the blood into the alveolus to breath out.
  • Oxygen diffuses out of the alveolus into the blood.
• Villi

  • The small intestine is where dissolved food molecules are absorbed out of the digestive system and into the blood.
  • the inside of the small intestine is covered in millions and millions of tiny little projections called villi.
  • They increase the surface area in a big way so that dissolved food molecules are absorbed much more quickly into the blood.
  • they have; a single layer of surface cells. A very good blood supply to assist quick absorption.
• Gas exchange in leaves

  • When plants photosynthesise they use u CO2 from the atmosphere and produce oxygen as a waste product.
  • When plants respire they use up oxygen and produce CO2 as a waste product.
• Diffusion in plants

  Leaves are specialised to maximise the diffusion of O2 and CO2.

  • Leaves are broad, so there is a large surface area for diffusion.
  • They're also thin, which means gases only have to travel a short distance.
  • There are air spaces inside the leaf. This lets gases move easily between cells. It also increases the surface area for gas exchange.
  • The lower surface is full of little holes called stomata. They're there to let gases diffuse in and out. They also allow water to escape, which is known as transpiration.
• Root hairs

  • the cells on plants roots frow into long 'hairs', which stick out into soil.
  • Each branch of a root will be covered in millions of these microscopic hairs.
  • This gives the plant a big surface area for absorbing water and mineral ions from the soil.
  • There's usually a higher concentration of water in the soil than there is inside the plant - so the water is drawn into the root hair cell by osmosis.
  • Mineral ions move in by active transport, since the concentration of mineral ions in the root hair cells is usually higher than in the soil.
• Double circulatory system

  The circulatory system is made up of the heart, blood vessels and blood. Humans have a double circulatory system.

  • In the first one, the heart pumps deoxygenated blood to the gas exchange surfaces in the lungs to take in oxygen. The oxygenated blood returns to the heart.
  • In the second one, the heart pumps oxygenated blood around all the other organs in the body. The blood gives up its oxygen at the body cells and the deoxygenated blood returns to the heart to pumped to the lungs again.
• Advantages of a double circulatory system in mammals

  • Returning the blood to the heart after it's picked up oxygen at the lungs means it can be pumped out around the body at a much higher pressure.
  • This increases the rate of blood flow to the tissues, so more oxygen can be delivered to the cells.
  • This is important for mammals because they use up a lot of oxygen maintaining their body temperature.
• The heart

  • The heart is a pumping organ that keeps the blood flowing around the body.
  • The heart has valves to make sure that blood flows in the right direction. When the ventricles contract, the tricuspid and bicuspid valve close and the semi-lunar valves open. This prevents backflow.
  • The heart is made up of cardiac muscles. these muscles cells contain loads of mitochondria to provide cells ATP.
  • Blood is supplied to the heart by two coronary arteries, which branch from the base of the aorta.
• The heart structure

  A) Pulmonary vein

  B) aorta

  C) bicuspid valve

  D) pulmonary artery

  E) vena cava

  F) tricuspid valve

  
• How the heart pumps blood around the body

  The heart uses its four chambers to pumps blood around:

  • Blood flows into the two atria from the vena cava and the pulmonary vein.
  • The atria contract, pushing the blood into the ventricles.
  • The ventricles contract, forcing the blood into the pulmonary artery and the aorta and out of the heart.
  • The blood then flows to the organs through arteries and returns through the veins.
  • The atria fill again and the whole cycle starts again.
• Blood vessels

  There are three main types of blood vessels:

  • Arteries - these carry the blood away from the heart.
  • Capillaries - these are involved in the exchange of materials at the tissues.
  • Veins - these carry blood to the heart.
• Arteries

  • The heart pumps the blood out at high pressure so the artery walls are strong and elastic.
  • The walls are thick compared to the size of their lumen.
  • They contain thick layers of muscle to make them strong and elsastic fibres to allow them to stretch and spring back. Arteries branch into arterioles.
• Capillaries

  • Arterioles branch into capillaries.
  • Capillaries are really tiny - to small to see.
  • Networks of capillaries in tissues are called capillary beds.
  • Capillaries carry the blood really close to every cell in the body to exchange substances with them.
  • They have permeable walls, so substrates can diffuse in and out.
  • They supply food and oxygen, and take away waste like CO2.
  • Their walls are usually only one cell thick. This increases the rate of diffusion by decreasing the distance it occurs.
  • Capillaries branch into venules.
• Veins

  • Venules eventually join up to make veins.
  • The blood is at a lower pressure in the veins, so the walls don't need to be as thick as artery walls.
  • They have a bigger lumen then arteries to help the blood flow despite the low pressure.
  • They also have valves to help keep the blood flowing in the right direction.