OCR GCSE Biology - B2

Mitosis

The reproduction of cells by splitting to form 2 identical offspring. It is done so our body can grow and replace damaged cells.

The Period of Cell Growth and Replication:

Mitosis (M) - The cycle starts and begins here.
Gap Phase 1 (G₁) - Cell grows and new cell structures and proteins are made
Synthesis (S) - Cell replicates its DNA so that when it splits during mitosis the 2 new cells will contain identical DNA
Gap Phase 2 (G₂) - Cells keep growing and proteins needed for cell division are made

Stages of Mitosis:

Differentiation

Process in which cells become specialized in structure and function.

Examples of Specialised Cells: Sperm Cell

Function: To pass on the necessary biological information required to produce a new organism.
Structures within a Sperm Cell:
• Flagellum - Makes the sperm cell move (tail-like)
• Lots of Mitochondria - Provides energy (ATP) through respiration, needed to sustain sperm motility
• Acrosome - Contains digestive enzymes to break down the egg's membrane to allow the sperm's entry

Examples of Specialised Cells: Red Blood Cells

Function: To transport oxygen around the body.
Structures within a Red Blood Cell:
• Haemoglobin - Protein that binds to oxygen
• No Nucleus - More room to store haemoglobin
• Biconcave Disc Shape - Increases surface area to volume ratio to increase the rate of diffusion

Examples of Specialised Cells: Ciliated Cells

Function: To move mucus that contains bacteria and dirt.
Structures within a Ciliated Cell:
• Goblet Cells - Make mucus to trap dirt and bacteria. It is then swept away to the back of the throat by the cilia.

Examples of Specialised Cells: Palisade Cells

Function: Site of photosynthesis.
Structures within a Palisade Cell:
• Lots of Chloroplasts - To perform its function of photosynthesis
• Regular Shape - To allow it to be packed into the leaf of the cell
• Found Near the Surface of a Leaf

Stem Cells

Unspecialized cells that retain the ability to become a wide variety of specialized cells.

The 2 Types of Stem Cells:

Embryonic Stem Cells
Adult Stem Cells

Embryonic Stem Cells

- Found in early human embryos
- Divide by mitosis
- Can differentiate into any specialised cells

Adult Stem Cells

- Found in tissues: Brain, Bone Marrow, Skin and Liver
- Can only differentiate into some cell types
- Used to repair damage

Meristems

- Meristems produce unspecialised cells
- Found in shoot tips, root tips, and buds
- The only plant cells that divide by mitosis are found here

Diffusion

The passive, net movement of particles from an area of higher concentration to an area of lower concentration. Moving DOWN a concentration gradient.

Active Transport

The movement of particles across a membrane AGAINST a concentration gradient, from an area of lower concentration to an area of higher concentration, using ATP released during respiration.

Osmosis

The net movement of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential. Moving DOWN a concentration gradient.

Water Potential

The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure.

Turgid vs. Flaccid Cell

Effects of too Much or too Little Water on a Cell:

Investigating Osmosis:

1. Cut equal-sized pieces of potato
2. Blot with tissue paper and weigh
3. Put pieces into different concentrations of sucrose solution for a few hours
4. Remove, blot with tissue paper, and reweigh
5. Calculate % Change in mass and then plot a graph with your results

3 Factors that Affect the Movement of Substances

- Surface Area to Volume Ratio
- Temperature
- Concentration Gradient

Factors that Affect the Movement of Substances: Surface Area to Volume Ratio

The rate of diffusion, osmosis and active transport is higher in cells with a larger surface area to volume ratio.

Factors that Affect the Movement of Substances: Temperature

As the particles in a substance get warmer they have more energy - so they move faster. This means as temperature increases, substances move in and out of cells faster.

Factors that Affect the Movement of Substances: Concentration Gradient

Substances 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.

Transport in Multicellular Organisms

They have a small surface area to volume ratio so the rate of diffusion would be slow and therefore not fast enough to transport substances around, hence why they have specialised exchange organs.

Exchange Organs

An organ (e.g. the lungs) specialised to exchange substances. They are usually very thin, have a large surface area and in animals, have lots of blood vessels.

Gas Exchange in the Lungs

The alveoli are specialised to maximise diffusion of oxygen and carbon dioxide.
- O₂ concentration is higher in the lungs than in the blood, so O₂ diffuses into blood
- CO₂ concentration in the blood s higher than in the lungs, so CO₂ diffuses out of blood

Gas Exchange in Leaves

When plants respire they use up oxygen and produce carbon dioxide. They keep their stomata open just enough to allow photosynthesis to take place, but not to much so they don't lose excessive water

Active Transport in Root Hair Cells

The root hair cells have carrier proteins in their cell membranes. These pick up the mineral ions and move them across the membrane into the cell against the concentration gradient. Because active transport moves ions against the concentration gradient into the root hair cells, energy is needed.

The Double Circulatory System

1. In the first one, the heart pumps deoxygenated blood to the gas exchange surfaces in the lungs to take in oxygen. The oxygenated blood then returns to the heart.
2. In the second one, the heart pumps oxygenated blood around all the other organs of the body. The blood gives up its oxygen at the body cells and the deoxygenated blood returns to the heart to be pumped out to the lungs again.

Blood Flow

Blood flows in through the veins and out through the arteries.
1. Blood flows in through the vena cava and the pulmonary vein.
2. The blood goes into the aorta and the aorta contracts allowing the blood to flow into the ventricles.
3. The ventricles then contract to push the blood out of the pulmonary artery and the aorta

Arteries

Carry blood away from the heart.
- The heart pumps blood out at high pressure so the artery walls are strong and elastic
- Thick walls
- Contain thick layers of muscle to make them strong and elastic fibers to allow them to stretch
- Arteries branch into arterioles

Capillaries

A microscopic vessel through which exchanges take place between the blood and cells of the body.
- Arterioles branch into capillaries
- Very tiny (only 1 cell thick)
- They have permeable walls, so substances can diffuse in and out
- They supply food and oxygen and take away waste like CO₂
- Capillaries branch into venules

Veins

Blood vessels that carry blood back to the heart.
- Venules join up to form veins
- The blood is at a lower pressure in the veins so the walls are not s thick as artery walls
- Large lumen to help blood flow despite low pressure
- Have valves to help the blood flow in the right direction

Blood

A connective tissue with a fluid matrix called plasma in which red blood cells, white blood cells, and cell fragments called platelets are suspended.

Plasma

Liquid part of blood, made mostly of water, in which oxygen, nutrients, and minerals are dissolved.

Platelets

Platelets are colorless blood cells that help blood clot. Platelets stop bleeding by clumping and forming plugs in blood vessel injuries.

Red Blood Cell

Blood cells that carry oxygen from the lungs to the body cells.

White Blood Cells

Blood cells that perform the function of destroying disease-causing microorganisms.

Contents of Plasma:

- 92% water
- Plasma Proteins
- Gases
- Nutrients
- Salts
- Hormones
- Waste Materials

Phloem Tubes

- Made of columns of living cells
- Transport food substances (mainly dissolved sugars)
- Made in leaves to the rest of the plant for immediate use or storage
- Transport goes in both directions
- This is the process of translocation

Xylem Tubes

- Made of dead cells joining end to end
- Thick side walls made of cellulose
- Carry water and mineral ions from the roots to the stem and leaves
- This is called the transpiration stream

Investigating Transpiration:

1. Cut a shoot underwater to prevent air from entering the xylem. Cut it at an angle to increase the surface area available for water uptake
2. Assemble the potometer in water and insert the shoot under water, so no air can enter
3. Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water.
4. Check the apparatus is waterproof and airtight
5. Dry the leaves, allow time for the shoot to acclimatise and then shut the tap
6. Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water
7. Record the starting position of the air bubble
8. Start a stopwatch and record the distance moved by the bubble per hour. Calculate the speed of the air bubble movement to give an estimate for the rate of transpiration.
9. Keep all other conditions constant. e.g. Humidity, Temperature

Transpiration

Transpiration is the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves. Carbon dioxide enters, while water and oxygen exit through a leaf's stomata. Transpiration also involves osmosis, where water moves from the xylem to the mesophyll cells.

Transpiration Stream

The transpiration stream, which is the movement of water up the stem, enables processes such as photosynthesis, growth and elongation as it supplies the plant with water which is necessary for all these processes. Apart from this, the transpiration stream supplies the plant with the required minerals, whilst enabling it to control its temperature via evaporation of water.

Factors Affecting the Rate of Transpiration:

1. Temperature (INCREASE)
2. Humidity (DECREASE)
3. Light intensity (INCREASE)
4. Air Movement (INCREASE)

Factors Affecting the Rate of Transpiration - Temperature

The warmer it is, the faster transpiration happens. When it's warm the water particles have more energy to evaporate and diffuse out of the stomata.

Factors Affecting the Rate of Transpiration - Humidity

Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.

Factors Affecting the Rate of Transpiration - Light Intensity

Increasing light intensity will cause more stomata to open in order to facilitate photosynthetic gas exchange.

Factors Affecting the Rate of Transpiration - Air Movement

Wind will move the air around, with the result that the more saturated air close to the leaf is replaced by drier air around the leaf, increasing the concentration gradient.

Stomata

Stomata are small pores on the surfaces of leaves and stems, bounded by a pair of guard cells, that control the exchange of gases - most importantly water vapour and CO2 - between the interior of the leaf and the atmosphere.

Gas Exchange in Plants: Stoma

The stomata control gas exchange in the leaf. Each stoma can be open or closed, depending on how turgid its guard cells are. In the light, the guard cells absorb water by osmosis, become turgid and the stoma opens. In the dark, the guard cells lose water, become flaccid and the stoma closes.