1.1 Cell structure Flashcards
Eukaryotic cells
Eukaryotic cells are found in plants, animals, fungi and protists (single-celled organisms that don’t fit other categories).
They are 10 - 100 micrometres in size.
A eukaryote is an organism made up of eukaryotic cells.
Prokaryotic cells
Prokaryotic cells are 0.1 - 5.0 micrometres in size.
A prokaryote is an organism made up of prokaryotic cells.
Bacteria are prokaryotes.
Prokaryotic cells do not have a nucleus (where DNA is stored).
Instead, most of their genetic material is stored in a single DNA loop in the cytoplasm (watery jelly that fills the cell).
Prokaryotic cells do NOT contain mitochondria (where respiration takes place) or chloroplasts (where photosynthesis takes place).
Some prokaryotic cells contain small rings of DNA called plasmids.
These plasmids can replicate and move between cells so that genetic information can be shared.
Cell membrane
The cell membrane holds the cell together, separating the inside of the cell from the environment outside, controlling what can and cannot enter and exit the cell. It is selectively permeable.
Cytoplasm
Cytoplasm is a solvent in which chemical reactions take place and surrounds the sub-cellular structures. It also supports internal cell structures.
Nucleus
It contains the genetic material (DNA) which control the activities of the cell.
Ribosomes
Ribosomes are found in the cytoplasm and are the site of protein synthesis.
Mitochondria
Mitochondria is the site of aerobic respiration.
Cells with high rates of metabolism (carrying out many different cell reactions) have significantly higher numbers of mitochondria than cells with fewer reactions taking place.
Vacuole
A permanent vacuole is a fluid-filled sac that stores water.
It is enclosed in a membrane (a wall that substances can pass through).
It can make up as much as 90% of a plant cell’s volume.
Chloroplast
Chloroplast is an organelle that contains green pigment called chlorophyll, which absorbs light energy for photosynthesis.
Cellulose cell wall
Cellulose cell wall is a tough outer coating made of cellulose that helps support the plant and gives the cell a rigid structure.
Scale and size of cells
Cells are very small and require a microscope to be seen.
Scientists measure the size of a cell to be approximately 0.003mm in diameter.
Practical 1 (using a light microscope)
Aim: To use a light microscope to observe, draw and label a selection of plant and animal cells, including a magnification scale.
Preparing a slide - Specimens must be prepared on a microscope slide to be observed under a light microscope
This must be done carefully to avoid damaging any biological specimen.
The most common specimens to observe under a light microscope are cheek cells (animal cells) and onion cells (plant cells).
Stains are used to highlight structures within cells – methylene blue is used to stain cheek cells, iodine for onion cells
Using a microscope - Understanding the main features of a light microscope is essential if you are to use it correctly.
Always hold the microscope by the arm when moving it around the lab, and always start your observation with the lowest-powered objective lens.
Specialised cells
A specialised cell is a cell that has a structure that aids its specific function.
This could relate to cell shape, or the combination of cellular structures present within the cell.
Cells specialise by undergoing a process known as differentiation
Fat cells
Function – acts as an energy store
Contains a fat store which can be broken down to release energy.
Can increase in size too store more fat when needed.
Contains very little cytoplasm to allow more space for fat storage.
Excess fat is stored in lipocytes, which expand in size until fat is used for fuel.
Muscle cells
Function - Contraction for movement
Muscle cells have many mitochondria to release energy for contraction.
All muscle cells contain protein filaments that can slide over each other to allow muscle contraction.
Ciliated epithelial cell
Function – line the airways to protect from pathogens
Mucus is made by goblet cells. Mucus trap pathogens.
Ciliated epithelial cells contain cilia (like tiny hairs) that waft mucus.
Contain many mitochondria as energy is needed for cilia to waft mucus.
Sperm cells
Function - Transfer of genetic material to an egg cell for fertilisation
The mid-piece is packed with mitochondria to release energy (via respiration) for the tail.
The tail rotates, propelling the sperm cell forward and allowing it to move.
The acrosome in the head contains digestive enzymes that can break down the outer layer of an egg cell so that the haploid nucleus can enter to fuse with the egg’s nucleus.
The head contains a nucleus with half the normal number of chromosomes, allowing the sperm cell to fuse with an egg cell to restore the normal chromosome number.
Egg cells
Function – it is a gamete used in sexual reproduction to pass on the mothers genes
Contains a nucleus which contains half the normal number of chromosomes. Therefore, when the egg and the sperm fuse, the embryo will have the full amount.
Cell membrane changes after fertilisation so only one sperm can enter.
Cytoplasm contains lots of nutrients needed for growth of an early embryo.
Nerve cell
Function - Conduction of electrical impulses
Nerve cells are long, meaning that they can conduct nerve impulses between different areas of the body
Extensions of the cytoplasm known as dendrites allowing nerve cells to communicate with other nerve cells, muscles and glands
The axon is covered with a fatty sheath which speeds up nerve impulse transmission
Cone cell
Function – allows colour vision
Contains visual pigment to allow colour vision.
One end usually links up to a nerve cell so our body can respond to visual stimuli.
Contains lots of mitochondria to release energy.
Differentiation
Structural differences between different types of cells enables them to perform specific functions within the organism.
Cell differentiation is an important process by which a cell changes to become specialised.
Cells that have not differentiated are therefore unspecialised. As an organism develops, cells differentiate to form different types of cells.
Almost all of the cells in a multicellular organism will contain the same genetic information (the same genes or alleles), but depending on what role one particular cell needs to have, only some of the total sum of genes in a particular cell are used to control its development.
When a cell differentiates, it develops a structure and composition of subcellular structures which enables it to carry out a certain function.
Differentiation and development
As a multicellular organism develops, its cells differentiate to form specialised cells.
In an animal, most cells differentiate at an early stage of its development. Cell division is mainly restricted to repair and replacement in mature animals.
Animal cells therefore lose their ability to differentiate after they have become specialised early in the life of the animal.
Some cells in various locations throughout the body of an animal retain the ability to differentiate throughout the life of the animal.
These cells are called adult stem cells and are mainly involved in replacing and repairing cells (such as blood or skin cells).
Plants differ from animals in that many types of plant cell retain the ability to fully differentiate throughout the life of a plant, not just in the early stages of development.
Light microscopes
Advantages
They are relatively cheap from £200 – £2000
You can see colour
They don’t require special training to use
They are portable
You can see living specimens
Disadvantages
They have low magnification meaning you can’t see organelles such as mitochondria
They have a low resolution
Electron microscopes
There are two types of electron microscopes
Transmission electron microscope (TEM)
Scanning electron microscope (SEM)
Advantages
The SEM has a large depth of field which allows more of a specimen to be in focus at one time
Has 1,000,000 to 2,000,000 times magnification
Disadvantages
They are very expensive £2,000,000 +
They are not portable
They can’t see live organisms
They don’t show image in colour
Magnification
Magnification = image size / actual size
Binary fission
Bacteria multiply by a type of simple cell division known as binary fission
In the right conditions, a bacterial cell prepares to divide by replicating its. genetic material before it increases in size.
A copy of each piece of circular DNA moves to each end of the cell before the cytoplasm divides, and new cell walls form around each daughter cell.
Growing bacterial cultures
The effect of disinfectants and antibiotics on microorganisms can be investigated using cultures of bacteria grown in the lab.
In the right conditions, some species of bacteria (such as coli) can multiply as much as once every 20 minutes. This is ideal as large cultures of bacteria for study can be grown in relatively short periods of time.
To multiply this quickly, bacteria require an adequate supply of nutrients (carbohydrates, proteins, minerals and vitamins) and an appropriate temperature (which varies depending on the species being grown).
Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate.
Uncontaminated cultures
It is vital that uncontaminated cultures of microorganisms are grown in the lab.
The presence of competing species can affect the growth of cultures, as well as the validity of any study performed on them.
Calculating inhibition zone area
Inhibition zone is an area which no bacterial growth has occurred. The larger the zone the more effective the substance tested is against the bacteria.
The effectiveness of different antibiotics, antiseptics or disinfectants can be determined by calculating the area of an inhibition zone around a disc of the substance being tested.
Calculating bacteria in a population
The average amount of time it takes for a bacter cell in a population to divide is the mean division time.
The number of times a cell has divided and how many cells it produces can be determined if you know the mean division time and how long division has been occurring.
Practical 2 (microbiology)
Aim: To investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition.
Procedure - Use an aseptic technique to place filter paper discs soaked in different antiseptics/antibiotics onto uncontaminated agar plates containing bacteria.
Measure the zone of inhibition around the growing colonies of bacteria to compare the effect of different antiseptics/antibiotics.
Calculate the area of each zone.
Clear zones of inhibition are not always perfectly circular, so the diameter of each zone should be measured twice (at 90° angles to each other) and a mean diameter and area calculated for each clear zone.
