GCSE Biology: Cell Biology

Describe eukaryotic cells (plant and animal cells)

Cells that contain a nucleus and other organelles.

What are eukaryotes?

Organisms that are made up of eukaryotic cells e.g. plants or animals.

What is a prokaryote?

A single-celled organism which doesn’t store genetic information in a nucleus e.g. bacteria

What does an animal cell consist of?

• Nucleus

• Mitochondria

• Cell membrane

• Cytoplasm

• Ribosomes

What does a plant cell consist of?

• Nucleus

• Mitochondria

• Cell membrane

• Cytoplasm

• Ribosomes

• Rigid cell wall

• Chloroplasts

• Permanent vacuole

What does a bacterial cell consist of?

• cell membrane

• cell wall

• cytoplasm

• circular strand of DNA

• plasmids

Function of nucleus

Contains genetic material that controls the activities of the cell

Function of mitochondria

These are where most of the reactions for aerobic respiration take place.

Respiration transfers energy that the cells need to work.

Function of cytoplasm

Gel-like substance where most of the chemical reaction happen.

It contains enzymes that control these chemical reactions.

Function of cell membrane

Holds the cell together and controls what goes in and out

Function of ribosomes

These are where proteins are made in cells.

Describe the cell wall in plants and it’s function

• Made of cellulose

• Gives the plant cell it’s shape and strengthens it.

Describe the permanent vacuole and it’s function

• Contains cell sap ( a weak solution of sugars and salts) which helps keep cell turgid

Describe the chloroplasts and their function

• Contain chlorophyll which is the site of photosynthesis

• Also contains enzymes needed for photosynthesis

Eukaryotic cells are bigger than prokaryotic cells

True

Centimetres

1/100th of a metre

1 × 10^-2m

Millimetres

1/1000th of a metre

1 × 10^-3 m

Micrometres (μm)

1/1 000 000th of a metre

1 × 10^-6 m

Nanometres

1/1 000 000 000th of a metre

1 × 10^-9 m

Animal cell specialisation

• Most animals cells are specialised.

• This means that they have adaptations which help them to carry out their particular function.

• When cells become specialised, scientists call that differentiation.

How are sperm cells specialised to carry out their function?

Function: To join with an ovum (egg cell) in a process called fertilisation. During fertilisation the genetic information of the ovum and the sperm cell combine.

Adaptations:

1. Contain only half the genetic information of a normal adult cell.

2. Sperm cells have a long tail which allows them to swim to the ovum.

3. Streamlined head to help it swim to the ovum.

4. Sperm cells are packed full of mitochondria which transfer the energy needed for the tail to do work

5. The acrosome stores digestive enzymes that can break down the outer layer of the ovum.

How are nerve cells specialised to carry out their function?

Function: To send electrical impulses around the body.

Adaptations:

1. Long axon, which carries the electrical impulses from one part of the body to another, to cover more distance.

2. The axon is covered in myelin, which insulates the axon and speeds up the transmission of nerve impulses.

3. The end of the axon has synapses, which are junctions which allow the impulse to pass from one nerve cell to another.

4. There are dendrites, which increase the surface area so that other nerve cells can connect more easily.

How are muscle cells specialised to carry out their function?

Function: To contract (get shorter) quickly

Adaptations:

1. They contain special proteins that slide over each other making the fibres contract.

2. Lots of mitochondria to transfer energy needed for contraction.

3. They can store glycogen( a chemical that can be broken down into glucose and used in cellular respiration).

Plant cell specialisation

• Most plant cells are specialised.

• They have special adaptations which help them to carry out their particular function.

• When cells become specialised, scientists call that process differentiation.

Describe how a root hair cell is specialised to carry out it’s particular function.

Function: Absorbing water and minerals

Adaptations:

1. Root hair cells do not contain chloroplasts because they’re underground.

2. They have root hairs which increase surface area available for water to move into the cell.

3. They have a large permanent vacuole that speeds up the movement of water by osmosis.

4. They have many mitochondria that transfer the energy needed for the active transport.

How are xylem cells specialised to carry out their particular function?

Function: To carry water and dissolved minerals ions from the roots to the leaves.

Adaptations:

1. The end walls in xylem cells are broken down forming a long tube so water and dissolved minerals can easily flow through.

2. Xylem cells have no organelles so that water and dissolved minerals can flow through more easily.

3. Contains lignin which makes xylem cells very strong and help support the plant

How are phloem cells specialised to carry out their particular function?

Functions: To carry dissolved sugars from the leaves to other parts of the plant

Adaptations:

1. Phloem vessel cells very few subcellular structures to aid the flow of substances.

2. but they are supported by companion cells which provide energy to transport substances in the phloem.

3. the cell walls between the cells break down to form special sieve plates that allow water carrying dissolved food to move up and down freely

Disadvantages of light microscopes

• They have limited magnification

• Limited resolution (image is blurred)

Resolution

• The shortest distance between two points on an object, that can still be distinguished as two separate entities.

• A measure of how detailed the image is.

Advantages of electron microscope

• Greater magnification (than light microscopes)

• Greater resolution (than light microscopes)

Equation for magnification

magnification= size of image/ size of real object

What is binary fission?

• The splitting of a bacterial cell into two bacterial cells.

How often can bacteria carry out binary fission?

• Every twenty minutes as long as they have enough nutrients and the temperature is suitable.

Formula for binary fission

no of bacteria= 2^n

• n means the number of rounds of division.

Where do we find chromosomes?

In the nucleus

What does an agar gel plate contain?

• Nutrient broth- which contains all the nutrients that the bacteria need to grow and divide.

• The nutrient broth has been set into a jelly using a chemical called agar.

• This is then poured into a petri dish and allowed to set.

How to avoid contamination when making agar gel plates?

• First, sterilise all Petri dishes, bacterial nutrient broth and agar.

• This kills any unwanted microorganism and it prevents contamination.

• Sterilise the inoculating loop by passing it through a Bunsen burner flame.

  • An inoculating loop is what we use to transfer the bacteria into the culture.

• Once we’ve transferred bacteria onto the dish, we attach the lid of the Petri dish using adhesive tape.

• This stops the lid from falling off and unwanted microorganisms entering.

• We then place the agar plate upside down into an incubator.

• This stops moisture from dripping down onto the bacteria and disrupting the colonies.

• In school laboratories, we usually incubate at 25 degrees C- this reduces the chances that harmful bacteria will grow.

Required Practical: Effect of antibiotics on bacterial growth

• Clean the bench with disinfectant solution in order to kill microorganisms that could contaminate the culture.

• Next sterilise an inoculating loop by passing it through a Bunsen burner flame.

• Open a sterile agar plate near a Bunsen burner flame- the flame kills bacteria in the air.

• Now use the loop to spread the chosen bacteria evenly over the plate

• Place sterile paper discs containing antibiotic onto the plate.

• Incubate the plate at 25 degrees C- this reduces the chance of harmful bacteria growing.

• Calculate the area of the zone of inhibition to see which antibiotic is the most effective using A= pi r²

What is the zone of inhibition?

• The region around the bacteria discs where the bacteria have not grown.

• We can measure the effect of an antibiotic by calculating the area of the zone of inhibition.

What are chromosomes made out of?

The molecule DNA?

How many of each chromosome do body cells contain?

Two-they’re paired

• except gametes

How many pairs of chromosomes do human body cells contain?

23

Which certain cell has chromosomes which are not paired?

Gametes- They have 23 single chromosomes

What do chromosomes carry?

Genes- which determine many of our features

Cell division by mitosis

1. The DNA replicates to form two copies of each chromosome. The cell also grows and copies it’s internal structure, such as mitochondria and ribosomes.

2. Then (mitosis takes place.) One set of chromosomes is pulled to each end of the cell. The nucleus also divides.

3. The cytoplasm and cell membrane divide to form two identical daughter cells.

What is the cell cycle?

When a cell divides in a series of stages.

Functions of mitosis

• Mitosis is essential for growth and development of multicellular organisms (e.g. plant and animals).

• Mitosis takes place when an organism repairs itself (e.g. when a broken bone heals).

• Mitosis happens during asexual reproduction.

What is a stem cell?

A stem cell is an undifferentiated cell of an organism which can give rise to more cells of the same type and can differentiate to form other types of cells.

What are embryonic stem cells?

Stem cells in early human embryos.

They’re capable of turning into any type of body cell.

Where can adult stem cells be found?

Bone marrow

Why is bone marrow different to embryonic stem cells?

Adult stem cells cannot differentiate into any type of cell-only certain types, such as cells found in blood: red blood cells, white blood cells and platelets.

What is leukaemia?

A cancer of the bone marrow.

How does medicine use bone marrow?

Stem cells transferred from the bone marrow of a healthy person can replace faulty blood cells in the patient who receives them.

Problems with bone marrow transplant

• Donor has to be compatible with the patient otherwise the white blood cells produced by the donated bone marrow could attack the patient’s body.

• Viruses can be passed from the donor to the patient.

• Procedure can be painful

Two ways stem cells can be used in medicine

• Bone marrow transplant

• Therapeutic cloning

What is produced in therapeutic cloning?

An embryo with the same genes as the patient.

This means that stem cells from the embryo can be transplanted into the patient without being rejected by the patient’s immune system.

Therapeutic cloning

In therapeutic cloning, an embryo is produced with the same genes as the patient.

Stem cells from the embryo can be transplanted into the patient without being rejected by the patient’s immune system.

Once inside the patient, the stem cells can then differentiate to replace cells which have stopped working correctly.

What can therapeutic cloning be used for?

A whole range of medical conditions such as diabetes or paralysis

• diabetes- check key cards

Disadvantages of therapeutic cloning

Some people have ethical objections to this procedure as embryos are destroyed in process and people view embryos as potential life.

What can meristem tissue in plants do?

Differentiate into any type of plant tissue, at any point in the life of the plant.

What can meristem tissue be used for?

Stem cells from meristems in plants can be used to produce clones of plants quickly and economically.

• Rare species can be cloned to protect them from extinction.

• Stem cells can also be used to grow crops of identical plants that have desired features for farmers, e.g. disease resistance

What is diffusion?

Diffusion is the spreading out of particles resulting in a net movement from an area of high concentration to an area of low concentration.

Diffusion of O2 and CO2

Cells need oxygen for respiration, which is carried out by the mitochondria.

Cells are surrounded by a high concentration of oxygen as oxygen is transported into the bloodstream from the lungs.

The oxygen molecules move into the cell by diffusion from an area of high concentration to an area of low concentration.

The oxygen is used to generate energy in respiration and this produces the waste gas carbon dioxide.

That means there’s a higher concentration of carbon dioxide inside the cell than outside so the carbon dioxide moves out of the cell by diffusion.

What is urea?

A waste products produced inside cells.

Diffusion of urea

It diffuses out of the liver cells (high concentration) into the blood plasma (low concentration) and is excreted by the kidneys.

How does the concentration gradient affect the rate of diffusion?

The greater then concentration gradient, the higher the rate of diffusion.

How does temperature affect the rate of diffusion?

The higher the temperature, the greater the rate of diffusion, as the particles have more kinetic energy and are moving faster.

How does surface area affect the rate of diffusion?

The larger the surface area (of the cell membrane), the greater the rate of diffusion.

Surface area to volume ratio of amoeba

Amoeba are single-celled organisms. A single-celled organism has a relatively large surface area to volume ratio.

This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism

As organisms get larger, what happens to the SA:V ratio

It decreases (this is a problem for multi-cellular organisms)

Problem of having a low SA:V ratio on multicellular organisms.

• Their surface area is not large enough for their volume.

• Cells on the surface can get enough oxygen simply by diffusion however, not enough oxygen can diffuse into the centre of the organisms as they’re too far away from the surface.

• Due to this, multicellular organisms have exchange surfaces and a transport system to solve this problem.

Describe how the gills in fish are adapted for exchanging materials.

Gills are covered in a very large number of fine filaments- where gases pass in and out of the blood. (Deoxygenated blood passes into the filament. Then oxygen from the water diffuses into the blood. Oxygenated blood is then returned to the body).

• Adaptations of filaments for efficient diffusion:

  • Give the gills a massive surface area

  • Thin membrane to provide a short diffusion pathway.

  • The filaments have an efficient blood supply to take the oxygenated blood away. This ensures that the concentration gradient is always high.

What is osmosis?

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

What’s a partially permeable membrane?

A membrane than only allows some molecules to pass through.

Osmosis of animal cells

• The cytoplasm is a relatively concentrated solution so if an animals cell is placed into water, then osmosis will take place.

• Water will move by osmosis from outside the cell to inside the cell.

• This causes the cell to expand or possibly burst.

• If an animal cell is placed into a very concentrated solution, then water will move out of the cell by osmosis and the cell will shrink.

Osmosis of plant cell

• The cytoplasm is a relatively concentrated solution so if an plant cell is placed into water, then osmosis will take place.

• Water will move into the cell by osmosis and the cell will expand.

• Unlike an animal cell, plant cells have a cell wall which prevents the cell from bursting so the cell becomes turgid after water moves into it by osmosis.

• If a plant cell is placed into a concentrated solution, then water moves out of the plant cell by osmosis, causing the cell to shrink/become flaccid.

Describe how to use microscope to view a specimen

• Place slide onto stage and use the clips to hold the slide in place.

• Select the lowest power objective lens.

• Slowly turn the coarse focusing dial to focus the image, looking from the side not the eyepiece to prevent damage.

• Then look through the eyepiece and slowly turn the coarse focusing dial but this time increase the distance between the objective lens and the slide. Do this until the cells come into focus.

• Then use the fine focusing dial to bring the cells into clear focus.

• Change the objective lens to a higher power to observe image with higher magnification.

• Use a sharp pencil to make a sketch, and label all features and draw the features in proportion and include a scale bar.

How to calculate total magnification (with microscopes)?

Multiply the magnification of the eyepiece lens by the magnification of the objective lens.

How to prepare a slide for onion cells

1. Peel off epidermal layer using forceps

2. Place sample on drop of water on microscope slide

3. Add drops of iodine (methylene blue for cheek cells) solution to the sample.

4. Lower cover slip onto sample using mounting needles to ensure there aren’t air bubbles

What is active transport?

A process that moves substances from an area of low to high concentration (against the concentration gradient), using energy from respiration.

Effect of osmosis on plant tissue

1. Peel the potato, because the potato skin can affect osmosis.

2. Use a cork borer to produce three cylinders of potato. Using a cork borer makes all of the cylinders the same diameter.

3. Use a scalpel to trim the cylinders to the same length (around 3cm).

4. Measure the length and mass of each cylinder (using a ruler and balance)

5. Now add each cylinder unto a test tube. Add 10cm³ of 0.5 molar sugar solution into the first test tube.

6. Add 10cm³ of 0.25 molar sugar solution in the second test tube.

7. Add distilled water into the third test tube. (distilled water contains no dissolved substances which could affect the rate of osmosis)

8. Leave the potatoes overnight and allow osmosis to take place.

9. Next remove the potato cylinders and roll them on a paper towel to remove any surface moisture (which could affect mass. Also don’t press too hard so that you don’t force water out of cells).

10. Measure the lengths and the mass of the cylinders again and calculate the % change using the formula.

percentage change formula

% change- change in value/original value x 100

What does it mean when potato has no change in mass

Concentration of water inside and outside the cell was the same so no overall osmosis took place.