[OCR] GCSE Biology: B2

What is the cell cycle?

It is when cells in the body divide to produce more cells, so your body can grow and replace damaged cells and cells grow and divide over and over again.

What are the three separate growth stages called?

G, S and G2

What is 'gap phase one'?

At this phase, the cell grows and new cell structures and proteins are made.

What is the 'S' phase?

At this phase, cells replicate its DNA, so that when it splits during mitosis and the two new cells will contain identical DNA.

What is 'gap phase two'?

At this phase, cells keep growing and proteins needed for cell division are made.

Where does the cell cycle start and end?

With mitosis.

Define mitosis.

Mitosis is when a cell reproduces itself by splitting to form two identical offspring.

What is the first stage of mitosis?

The cell has two copies of its DNA all spread out in long strings.

What is the second stage of mitosis?

The DNA forms X-shaped chromosomes. Each 'arm' of a chromosome is an exact copy of the other. This happens before the cell divides.

What is the third stage of mitosis?

The chromosomes then line up at the center of the cell and cell fibers pull them apart. The two arms of each chromosome go to opposite ends of the cell.

What is the fourth stage of mitosis?

Membranes from around each of the sets of chromosomes. these become nuclei of the two new cells.

What is the fifth and final stage of mitosis?

The cytoplasm divides; two new cells containing exactly the same DNA as they're genetically identical to each other and to the parent cell.

What is differentiation?

Differentiation is the process by which a cell changes to become specialised for its job.

What is an example of differentiation in plants?

Palisade leaf cells carry out photosynthesis as they contain chloroplasts. Their tall shape means that they have a lot of surface area exposed down the side for absorbing carbon dioxide from the air in the leaf, and their thin shape means that you can fit loads of them in at the top of a leaf, so they're nearer the light.

What is an example of differentiation in animals and humans?

A sperm's function is to get the male DNA to the female DNA during reproduction. Sperm have long tails and streamlined heads to help them swim, they contain lots of mitochondria to provide them with energy and they have enzymes in their heads to digest through the egg's cell membrane.

What do specialised cells in multicellular organisms do in order to carry out their function?

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.

What is an example of an undifferentiated cell?

Stem cells are undifferentiated but depending on what instructions they're given, they can divide by mitosis to become new cells, which then differentiate.

Why are embryonic stem cells so important for growth and repair?

They have the potential to become any kind of cell.

Where can you find stem cells in the body?

In adults, stem cells can be found in bone marrow. You can also find them in the umbilical cord.

What is the main disadvantage of stem cells?

They cannot turn into any type of cell.

What are the only cells in plants that divide by mitosis?

Meristems- they are found in plant tissues.

Where is meristem tissue found in the plant?

Anywhere in the plant thats growing- e.g the roots and shoots.

What type of cells do meristems produce?

They produce unspecialised cells that are able to divide into any cell type in the plant and they act like embryonic stem cells.

What can the unspecialised cells in the plant become?

They can become specialised and form tissues like xylem and phloem.

What is diffusion?

Diffusion is the net (overall) movement of particles from an area of higher concentration to an area of lower concentration.

In what states can diffusion occur?

Liquids and gases as the particles are free to move around.

What type of molecules can diffuse through the cell membrane?

Very small molecules like glucose, amino acids, water and oxygen.

What is the first step of diffusion?

Particles move through the cell membrane from where there is a higher concentration to where there is a lower concentration and they are moving in random motion.

What is active transport?

Active transport is 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.

How does active transport work in the digestive system?

When there is a higher concentration of nutrients in the gut they diffuse into the blood but sometimes there is a lower concentration of nutrients in the gut than in the blood. Active transport allows nutrients to be taken into the blood, despite the concentration gradient being the wrong way. This stops us from starving.

What does active transport need in order to take place?

ATP from respiration.

What is osmosis?

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

What is a partially permeable membrane?

A partially permeable membrane is a membrane with very small holes in it. Only tiny molecules, like water, can pass through them. A cell membrane is a partially permeable membrane.

What direction do water molecules pass through the membrane? Why does this happen?

They pass both ways through the membrane during osmosis because water molecules move randomly all the time.

What is water potential?

Water potential is the likelihood of water molecules to diffuse out of or into a solution. If a solution has a high water potential, it has a high concentration of water molecules and vice versa.

What does it mean if a plant is turgid?

Plump and swollen due to the increased water potential in the soil when you water a plant.

What is turgor pressure?

When the contents of the cell push against the cell wall. this helps support the plant tissues.

What does it mean if a plant is flaccid?

The plant will wilt as it starts to lose water. however, the plant does not lose its shape because the inelastic cell wall keeps things in position.

What happens to animal cells when they are surrounded by a solution with a high water potential?

The cells can burst as they do not have a cell wall.

What factors effect the movement of substances?

What happens to proteins in the body, as they cannot be stored?

Any excess amino acids are converted in the liver into fats and carbohydrates, which can be stored.

What is the waste product formed from converting excess amino acids?

Urea- it is poisonous and is removed from the blood by the kidneys and removed from the body in the urine.

How do single-celled organisms exchange substances?

They are only one cell big, so substances can diffuse straight into and out of single-celled organisms across the cell membrane. The substances only have to travel a short distance and they have a large surface area to volume ratio.

Why is it difficult for multicellular organisms to exchange substances?

Some cells are deep inside the organism- it's a long way for the substances to travel and they have a low surface area to volume ratio which means their outer surface is small.

How are the exchange surfaces in specialised exchange organs adapted to maximise effectiveness?

>They are thin, sos substances have a short distance to travel.
>They have a large surface area, so lots of substance can move at once.
>Exchange surfaces in animals have lots of blood vessels to get things into and out of the blood quickly.
>Gas exchange surfaces in animals are often ventilated (air moves in and out).

Where does gas exchange happen in the lungs?

The lungs contain millions of little air sacs called alveoli where gas exchange takes place.

How are alveoli specialised to maximise diffusion?

>An enormous surface area.
>Thin walls.
>A moist lining for dissolving gasses.
A good blood supply.

What happens to the blood passing next to the alveoli?

It returns to the lungs from the rest of the body via the hear so it contains lots of carbon dioxide and very little oxygen.

What happens to carbon dioxide during gas exchange in the lungs?

Carbon dioxide diffuses out of the blood (with a higher concentration) into the alveolus (a lower concentration) to be breathed out.

What happens to oxygen during gas exchange in the lungs?

Oxygen diffuses out of the alveolus (higher concentration) into the blood (lower concentration).

Where are dissolved food molecules absorbed out of the digestive system?

The small intestine is where dissolved food molecules are absorbed out of the digestive system and into the blood.

What is the small intestine covered with?

Tiny little projections called villi.

What is the role of 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 and a very good blood supply to assist quick absorption.

How are leaves specialised to maximise the diffusion of oxygen and carbon dioxide?

>Leaves are broad, so there is a large surface area.
>They are also thin, which means gases only have to travel a short distance.
>There are air spaces inside the leave which lets gases move easily between cells.
>The lower surface is full of little holes called stomata. They let the gases diffuse in and out and they also allow water escape (transpiration).

How are root hairs important for absorbing water and mineral ions?

The cells on a plant roots grow into long 'hairs' which stick out into the soil and each branch of a root will be covered in millions of microscopic hairs. This gives the plant a big surface area for absorbing water and mineral ions.

What happens when root hairs take in water and mineral ions?

There is 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 the soil.

What is the double circulatory system made up of?

The heart, blood vessels and blood. Because it is a double circulatory system, there are two circuits joined together.

What happens in the first circulatory circuit?

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.

What happens in the second circulatory circuit?

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.

What is an example of an animal that does not have a double circulatory system?

Fish.

What are the advantages of having a double circulatory system?

>Returning the blood to the heart after it's picked up oxygen at the lungs means it can be pumped around the body at a much higher pressure.
>Increases the rate of blood flow to the tissues- pumped around the body much faster, so more oxygen can be delivered to the cells.
>Important to mammals as they use up a lot of oxygen maintaining their body temperature.

What is the purpose of the valves in the heart?

They make sure that blood flows in the right direction. When the ventricles contract, the valves to the atria close and the valves to the blood vessels open. This prevents back flow.

What are the four chambers of the heart?

>Left atria.
>Right atria.
>Left ventricle.
>Right ventricle.

Where does blood flow to in the heart?

Blood flows into the two atria from the vena cava and the pulmonary vein.

What happens when blood enters the atria?

It contracts, pushing the blood into the ventricles.

What happens when the ventricles contract?

It forces the blood into the pulmonary artery and the aorta and out of the heart.

Where does the blood flow when it leaves the heart?

It flows to the organs through arteries and returns through the veins.

What does the heart muscle 'cells' contain?

The heart is made up of a cardiac muscle. These muscle cells contains lots of mitochondria to provide cells with ATP. They also need their own blood supply to deliver the nutrients and oxygen needed to keep the heart beating.

How is blood supplied to the heart?

By two coronary arteries, which branch from the base of the aorta.

What are the three main types of blood vessel?

Arteries, capillaries and veins.

What is the function of the arteries?

They carry blood away from the heart.

What is the function of the capillaries?

They are involved in the exchange of materials at the tissues.

What is the function of the veins?

They carry blood to the heart.

How are arteries designed to carry blood under pressure?

The walls are strong and elastic, the walls are thick compared to the size of the lumen. They contain thick layers of muscle to make them strong and elastic fibres to allow them to stretch and spring back.

What do arteries branch into?

Arterioles.

How are capillaries designed to exchange substances?

They are really tiny and there are networks of capillaries (capillary beds). They carry the blood really close to every cell in the body to exchange substances with them.

>They have permeable walls, so substances can diffuse in and out.
>They supply food and oxygen as well as taking away waste (e.g carbon dioxide).
>The walls are only one cell thick as it increases the rate of diffusion by decreasing the distance over which it occurs.

What do capillaries branch into?

Venules.

How are veins designed to help blood flow?

>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 than arteries to help the blood flow.
>They have valves to help keep blood flow in the right direction.

Where do veins take the blood?

Back to the heart.

What does the blood consist of?

Plasma, platelets, red blood cells and white blood cells.

What is plasma?

Plasma is a pale yellow liquid which carries just about everything that needs transporting around the body.

What does plasma transport around the body?

>Red blood cells.
>Water.
>Digested food products (e.g glucose and amino acids) from the gut to all body cells.
>Carbon dioxide from the body cells to the lungs.
>Urea from the liver to the kidneys.
>Hormones- these act like chemical messengers.
>Antibodies- these are proteins involved in the body's immune response.

What do red blood cells transport to the body and lungs?

Oxygen.

How is a red blood cell adapted to its function?

>They are small and have a biconcave disc shape to give a large surface area to volume ratio for absorbing and releasing oxygen.
>They contain haemoglobin which gives blood its colour- it contains a lot of iron.
>They do not have a nucleus which frees up space for more haemoglobin, so they can carry more oxygen.
>very flexible so they can easily pass through the tiny capillaries.

What happens when haemoglobin bonds with oxygen in the lungs?

It creates a product called oxyhemoglobin. In body tissues the reverse happens to release oxygen to the cells.

What do phloem tubes transport?

Food substances (mainly sugars) both up and down the stem to growing and storage tissues.

What is translocation?

The movement of food substances around a plant.

What are phloem tubes composed of?

Living cells called sieve tube elements. These have perforated end-plates to allow things to flow through.

What is the structure of sieve tubes?

They have no nucleus- meaning that they cannot survive on their own, so each sieve tube element has a companion cell.

What do companion cells do for sieve tubes?

They carry out living functions for both themselves and their sieve cells.

What do xylem tubes do?

They carry water and minerals from the roots up the shoot to the leaves in the transpiration stream.

What are xylem tubes composed of?

They are made of dead cells joined end to end with no end walls between them and a hole (lumen) down the middle.

What is the structure of xylem tubes?

They have thick side walls made of cellulose which are strong and stiff, to give the plant support and they are also strengthened with a material called lignin.

What is transpiration caused by?

The evaporation and diffusion of water from a plants surface- most transpiration happens at the leaves.

What is a consequence of transpiration?

The evaporation and diffusion creates a slight shortage of water in the leaf, so more water is drawn up from the rest of the plan through the xylem vessels to replace it. This results in a constant transpiration stream of water through the plant.

What is transpiration?

It is just a side-effect of the way leaves are adapted for photosynthesis. They have to have stomata int hem so gases can be exchanged easily because there is more water inside the plant than in the air outside.

How does the water escape from the leaves?

Through the stomata by diffusion.

What are the benefits of the transpiration stream to the plant?

>Keeps the plant cool.
>Provides the plant with a constant supply of water for photosynthesis.
>The water creates turgor pressure, which helps support the plant and stops it from wilting.
>Minerals needed by the plant can be brought in from the soil along with the water.

How does light intensity effect transpiration?

The brighter the light, the greater the transpiration rate because more light increases the rate of photosynthesis, causing the stomata to open to let carbon dioxide in.

How does an increase in temperature effect transpiration?

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