Module 1: Cells

• Light passes from a bulb under the stage, through a condenser lens, then through the specimen.
• This beam of light is focused through the objective lens, then through the eyepiece lens.
• To view specimens at different magnifications, light microscopes have a number of objective lenses that can be rotated into position.
• Usually four objective lenses are present: x4, x10, x40 and x100.
• The eyepiece lens then magnifies the image again. This is usually x10.

Magnification: The degree to which the size of an image is larger than the object itself. Most light microscopes are capable of magnification up to a maximum of x1500.

Resolution: The degree to which it is possible to distinguish between two objects that are very close together. Resolution of a light microscope is 200 nm.

Staining: Need to stain specimen in order to see it. Coloured stains are chemicals that bind to chemicals on or in the specimen. 

Sectioning: Specimens are embedded in wax when viewed under a light microscope. Thin sections are then cut without distorting the structure of the specimen. Light microscopes can only view dead specimen.

A microscope eyepiece can be fitted with a graticule, which is transparent with a small ruler etched onto it.

How to callibrate a microscope:

• Line up stage micrometer scale to eyepiece graticule scale.
• Work out how many large eyepiece graticule units on the eyepiece graticule there are in the stage micrometer, e.g. 3 large e.p.us to 4mm.
• Work out the value of 1 large e.p.u
• µm = no. of mm x 1000
• mm = no. of µm ÷ 1000

Actual size = Image Size ÷ Magnification

Magnification = Image Size ÷ Actual Size

Image Size = Actual Size x Magnification

• Electron microscopes generate a beam of electrons.
• A beam of electrons has a wavelength of 0.004nm, which is 100000 times shorter than a light wavelength.
• Electron microscopes can distinguish between objects 0.2nm apart.
• These microscopes use magnets instead of lenses to focuse the beam of electrons onto a prepared specimen.
• Electrons are not visible to the human eye. The image produced from the electron beam is projected onto a screem or onto photographic paper to make a black and white image. 
• These images are called electron micrographs.
• The resolution of an electron microscope is about 500000 times greater than that of the human eye.

How to prepare specimen for electron microscopes (liver tissue is the example used):

• Fix specimen in glutaraldehyde to make tissue firm.
• Dehydrate it to replace water with ethanol.
• Embed dehydrated tissue in solid resin.
• Cut very thin slices using a diamond knife.
• Stain it using lead salts to scatter electrons differently - gives contrast.
• Mount onto copper grid.
• Place the specimen on the grid in a vacuum in the microscope.

There are two types of electron microscope:

Transmission Electron Microscope (TEM):

• The electron beam passes through a very thin prepared sample.
• Electrons pass through the denser parts of the sample less easily, so giving some contrast.
• The final image produced is two-dimensional.
• The magnification possible with a TEM is x500000.

Scanning Electron Microscope (SEM):

• The electron beam is directed onto a sample. The electrons don't pass through the specimen.
• Electrons are 'bounced off' the sample.
• The final image produced is a 3D view of the surface of the sample.
• The magnification possible with an SEM is about x100000

Advantages:

• The resolution is 0.2nm
• This means the electron microscope can be used to produce detailed images of the structures and organelles inside cells.
• The SEM produces 3D images that can reveal the detail of contours and cellular or tissue arrangements - this is not possible using light microscopes.

Limitations:

• Electron beams are deflected by the molecules in air, so samples have to be placed in a vacuum.
• Electron microscopes are expensive to buy and to run.
• Preparing samples and using an electron microscope both require a high degree of skill and training.

7 processes carried out by all living things:

• Movement
• Respiration
• Sensitivity
• Nutrition
• Excretion
• Reproduction
• Growth

Ultrastructure: The detail of the inside of cells, i.e. the organelles. This can be revealed only by using an electron microscope.

Division of labour: When each organelle has a specific job within the cell, and they work together.

Cytoskeleton: The network of fibres made of protein that keep the cell stable by providing an internal framework. Responsible for movement around cells.

Actin Filaments: Like fibres found in muscle cells. Cause movement in white blood cells and move some organelles around inside cells.

Microtubules: Cylinders about 25nm in diameter. Made of a protein called tubulin. Used to move a microorganism through a liquid, or to waft a liquid past the cell.

Microtubule motors: Move chromosomes during mitosis, and move vesicles from endoplasmic reticulum to Golgi apparatus.

ATP (adenosine triposphate): The universal energy carrier. All activities that need energy in the cell are driven by energy released from ATP.

Undulipodia (flagella): Hair-like extensions that stick out from the surface of cells. Made up of a cylinder that contains nine microtubules arranged in a circle, with two microtubules in a central bundle. Longer than cilia and occur in ones of twos. 

Cilia: Hair-like extensions that stick out from the surface of cells. Also made up of a cylinder that contains nine microtubules arranged in a circle. Shorter than flagella, and occur in large numbers. Sweeping movement of cilia moves mucus over the cells.

Vesicles: Membrane-bound sacs found in cells. Used to carry many different substances around cells.

Vacuoles: Found in plant cells. Filled with water and solutes so that it pushes the cytoplasm against the cell wall, making the cell turgid, which supports the plant.

Plant cell walls: Found on outside of plant cell plasma membranes, made of cellulose.

The Nucleus

Structure: The largest organelle. When stained, it shows darkened patches known as chromatin. It is surrounded by a nuclear envelope, which is a structure made of two membranes with fluid between them. A lot of holes, called nuclear pores, go right through the envelope. These holes are large enough for relatively large molecuales to pass through. There is a dense, spherical structure, called the nucleolus, inside the nucleus.

Function: 

• Houses nearly all the cell's genetic material.
• Chromatin consists of DNA and proteins.
• It has the instructions for making proteins. Some of these proteins regulate the cell's activities.
• When cells divide, chromatin devides into visible chromosomes.
• The nucleolus makes RNA and ribosomes. These pass into the cytoplasm and proteins are assembled at them.

Endoplasmic Reticulum (ER)

Structure: ER consists of a series of flattened, membrane-bound sacs called cisternae. They are continuous with the outer nuclear membrane. Rough endoplasmic reticulum is studded with ribosomes. Smooth endoplasmic reticulum does not have ribosomes.

Function: Rough ER transports proteins that were made on the attached ribosomes. Some of these proteins may be secreted from the cell. Some will be placed on the cell surface membrane. Smooth ER is involved in making the lipids that the cell needs.

Golgi apparatus

Structure: A stack of membrane-bound, flattened sacs (looks like a pile of pitta bread).

Function: Golgi apparatus receives proteins from the ER and modifies them. It may add sugar molecules to them. The Golgi apparatus then packages the modified proteins into vesicles that can be transported. Some modified proteins may go to the surface of the cell so that they may be secreted.

Mitochondria

Structure: These may be spherical or sausage-shaped. They have two membranes separated by a fluid-filled space. The inner membrane is highly folded to form cristae. The central part of the mitochonrdion is called the matrix.

Function: Mitochondria produce most of the ATP during respiration.

Chloroplasts

Structure: Found only in plant cells and the cells of some protocists. Have two membranes separated by a fluid-filled space.

Function: Chloroplasts are the site of photosynthesis in plant cells. Light energy is used to drive the reactions of photosynthesis, in which carbohydrate molecules are made from carbon dioxide and water.

Lysosomes

Structure: Spherical sacs surrounded by a single membrane.

Function: Lysosomes contain powerful digestive enzymes. Their role is to break down materials.

Ribosomes

Structure: Tiny organelles. Some are free in the cytoplasm and some are bound to ER. Each ribosome consists of two subunits.

Function: Ribosomes are the site of protein synthesis. They act as an assembly line where coded information (mRNA) from the nucleus is used to assemble proteins from amino acids.

Centrioles

Structure: Small tubes of protein fibres (microtubules).

Function: Centrioles take part in cell division. Form fibres, known as the spindle, which move chromosomes during nuclear division.

Division of labour: Any system where different parts perform specialised functions, each contributing to the functioning of the whole

A good example of division of labour is the synthesis of a protein:

• The instructions to make the hormone are in the DNA, in the nucleus.
• The specific instruction to make the hormone is known as the gene for that hormone. The gene is on a chromosome.
• The nucleus copies the instructions in the DNA into a molecule called mRNA.
• The mRNA molecule leaves the nucleus through a nuclear pore and attaches to a ribosome. In this case, the ribosome is attached to rough endoplasmic reticulum (RER).
• The ribosome reads the instructions and uses the codes to assemble the hormone (protein).
• The assembled protein inside the rough ER is pinched off in a vesicle and transported to the Golgi apparatus.
• The Golgi apparatus packages the protein and may also modify it so that it is ready for release. 
• The protein is now packaged into a vesicle and moved to the cell surface membrane, where it is secreted outside (exocytosis).

Eukaryotes (having a true nucleus): These are cells which contain organelles, some of which are bound by membranes. This gives these cells a complicated internal structuer, where each organelle performs a specific role. 

Prokaryotes: These are bacteria and are much smaller than eukaryotes (1-5 nanometers). There features include:

• The only membrane they have is the cell surface membrane. They do not contain any membrane-bound organelles.
• They are surrounded by a cell wall made of peptidoglycan, not cellulose.
• Outside the cell wall is often a slippery protective layer called the capsule.
• They contain ribosomes that are smaller than eukaryotic ribosomes.
• They have a single loop of DNA in the cytoplasm, rather than the linear chromosomes of eukaryotes. Many prokaryotic cells also contain very small loops of DNA called plasmids.
• The DNA is not surrounded by a membrane. The general area in which the DNA lies is called the nucleoid.
• ATP production takes place in specialised infolded regions of the cell surface membrane called memosomes.
• Some prokaryotic cells have flagella, and many have hair-like appendages known as pili.

Membranes are found surrounding all cells, and around many organelles in eukaryotic cells. The basic structure of all cell surface membranes is the same. The major roles of membranes include:

• Separating cell contents from the outside environment
• Separating cell components from cytoplasm
• Cell recognition and signalling
• Holding the components of some metabolic pathways in place
• Regulating the transport of materials into or out of cells

The nature of phospholipids

• The phosphate head is hydrophilic - water-loving, while the two fatty acid tails are hydrophobic - water-hating. These properties come from the way charges are distributed across the molecule.
• Molecules with charges that are evenly distributed around the molecule do not easily dissolve or mix with water, and in fact repel water molecules. 
• If phospholipid molecules are mixed with water, they form a layer at the water surface. The phosphate heads stick into the water, while the fatty acid tails stick up out of the water.

• If phospholipid molecules are completely surrounded by water, a bilayer can form. Phosphate heads on each side of the bilayer stick into the water, while the hydrophobic fatty acid tails point towards each other in a sort of mirror image.
• The phospholipid bilayer is the basic structural component of all biological membranes. The hydrophobic layer creates a barrier to many molecules and separates the cell contents from the outside world.
• The membranes are about 7-10nm thick.

Other components are needed in order to make a functioning biological membrane.The number and type of these other components varies according to the function of the particular membrane. This specialisation of cell membranes is diffrentiation. Some examples are:

• The plasma membrane (cell surface membranes) of the cells in a growing shoot contain receptors that allow them to detect the molecules that regulate growth.
• Muscle cell membranes contain a large number of the channels that allow rapid uptake of glucose to provide energy for muscle contraction.
• The internal membranes of chloroplasts contain chlorophyll and other molecules needed for photosynthesis.
• The plasma membranes of white blood cells contain special proteins that enable the cells to recognise foreign cells and particles.

• Where phospholipid molecules have a carbohydrate part attached they are called glycolipids. Can be involved in cells signalling that they are 'self', to allow recognition by the immune system.
• Where protein molecules have a carbohydrate part attached they are called glycoproteins. Do the same as glycolipids, plus can also bind cells together in tissues.
• Cholesterol gives membranes of some eukaryotic cells mechanical stability.
• Channel proteins allow molecules and ions that are too large and hydrophilic to pass through the phospholipid bilayer.
• Carrier proteins actively move substances across the membrane, for example magnesium ions are actively pumped into the root hair cells from the surrounding soil.
• Receptor sites allow hormones to bind with the cell so that a cell 'response' can be carried out, and they are important in allowing drugs to bind, and so affect cell metabolism.
• Enzymes and coenzymes can be bound to the membranes of mitochondria; where some stages of respiration take plase.
• Increasing temperature gives molecules more kinetic energy, so they move faster. However, this increased movement of phospholipids makes membranes leaky e.g. beetroot.

One of the characteristics of living organisms is that they can sense what is present in their environment and respond appropriately to chaanges in that environment.

In order to detect signals, cells must have on their surface 'sensors' capable of receiving signals. These sensors are known as receptors.

Hormone receptors

• Hormones are chemical messengers, produced in specific tissues and then released into the organism. Any cell with a receptor for the hormone molecules is called a target cell.
• A hormone molecule binds to a receptor on a target cell surface membrane because the two have complementary shapes. Binding of the hormone and receptor causes the target cell to respond in a certain way.

The insulin receptor

• Insuling is a protein molecule that attaches to the insulin receptors on the plasma membranes of many cells, inclusing muscle cells and liver cells.
• When it attaches to its receptor, it triggers internal responses in muscle cells that lead to more glucose channels being present in the plasma membrane. This allows the cell to take up more glucose from the blood, so reducing the blood glucose level.

Diffusion - molecules even out

• In a gas or a liquid, the molecules (or ions) move around, They possess kinetic energy that keeps them moving. Processes such as diffusion that depend only on this energy are known as passive processes.
• Molecules in a gas or liquid move from an area of high concentration to an area of low concentration.
• This tendency to spread out is called diffusion.

Diffusion and net movement

• When diffusion has taken place, molecules are distributed evenly.
• This doesn't mean their movement stops; the molecules continue to move around, but there is no overall movement of molecules in one direction (no net movement), and this is known as equilibrium.

The rate of diffusion is affected by a number of factors:

• Temperature: Increasing temperature gives molecules more kinetic energy, so the rate of diffusion increases.
• Concentration gradient: Having more molecules on one side of a membrane increases the concentration gradient, and increases the rate of diffusion.
• Stirring/movement: Stirring a liquid, or the movement of air currents in a gas, increases the movement of molecules and thus the rate of diffusion.
• Surface area: Diffusion across membranes occurs more rapidly if there is a greater surface area to diffuse across. Cells are adapted to increase the surface area for diffusion. Red blood cells are biconcave. Epithelial cells in the small intestine have folds called microvilli. Alveoli increase the surface area of the lungs.
• Distance/thickness: Diffusion is slowed down by thick membranes. There is a greater distance for molecules to travel.
• Size of molecule: Smaller molecules or ions diffuse more quickly than larger ones.

• Lipid based molecules: As the membrane is made of phospholipids, fat-soluble molecules can simply pass through the bilayer.
• Very small molecules and ions: Carbon dioxide and oxygen molecules are small enough to pass through the bilayer between phospholipid molecules.
• Larger or charged molecules need to be carried across: Small, charged particles such as sodium ions, or larger molecules such as glucose, cannot pass through the lipid bilayer. Two types of protein molecule are involved in allowing such substances to pass through membranes. Because these proteins allow substances to pass through the membrane, the diffusion of these molecules and ions is known as facilitated diffusion.
• Channel proteins: Basically form pores in the membrane, which are often shaped to allow only one type of ion through. Many are also 'gated' meaning they can be opened or closed.
• Carrier proteins: Shaped so that a specific molecule can fir into them at the membrane surface. When the molecule fits, the protein changes shape to allow the molecule through to the other side of the membrane.

Active transport: 'pumping' molecules across membranes

• Some of the carrier proteins found in membranes act as pumps. These proteins are similar to the protein carriers used for facilitated diffusion.
• They are shaped in a way that is complementary to the molecule they carry. They carry larger or charged molecules and ions through membranes. These molecules and ions cannot pass through the bilayer by diffusion.

These protein pumps differ significantly from the proteins used in facilitated diffusion:

• They carry specific molecules one way across the membrane.
• In carrying molecules across the membrane, they use metabolic energy in the form of ATP.
• They can carry molecules in the opposite direction to the concentration gradient.
• They can carry molecules at a much faster rate than by diffusion.
• Molecules can be accumulated either inside cells or organelles, or outside cells.

Ensuring one-way flow

• The energy used in pumping molecules across membranes by active transport changes the shape of the carrier protein.
• The shape change means that the specific molecule to be transported - or pumped - fits into the carrier protein on one side of the membrane only.

Moving large amounts - bulk transport

• Some cells need to move large quantities of material either in or out.
• Endocytosis: when it involves bringing materials into the cell.
• Exocytosis: when it involves moving materal out of the cell.
• This bulk transport is possible because membranes can easily fuse, separate and 'pinch off'.
• Bulk transport requires energy in the form of ATP, and the energy is used to move the membranes around to form the vesicles thta are needed, and to move the vesicles around the cell.

Some examples of bulk transport:

• Hormones - pancreatic cells make insulin in large quantities. The insulin is processed and packaged into vesicles in the Golgi apparatus. These vesicles fuse with the outer membrane to release insulin into the blood.
• In plant cells, materials required to build the cell wall are carried outside in vesicles.
• Some white blood cells engulf invading microorganisms by forming a vesicle around them. This vesicle then fuses with lysosomes so that the enzymes from the lysosomes can digest the microorganism. These cells are called phagocytes.

Different names given to the movements of materials in bulk transport:

• endo - inwards
• exo - outwards
• phago - solid material
• pino - liquid material

• A substance that can dissolve is called a solute.
• The liquid it dissolves in is called a solvent.
• The two together form a solution.

Water potential is a measure of the tendency of water molecules to diffuse from one place to another. Water always moves from a region of high water potential to a region of lower water potential.

• Osmosis is the movement of water molecules by diffusion across a partially permeable membrane.
• Water potential is a measure of the concentration of water molecules that are able to diffuse.
• Net movement of molecules occurs until the concentrations are evened out; osmosis will occur until the water potential is the same on both sides of the membrane.
• The water potential of cells is lower than that of pure water, because of the sugars, salts and other substances dissolved in the cytoplasm.
• Water potential is measured in kiloPascals (kPa). 
• Pure water has the highest water potential - 0kPa.

Cells in solutions of high water potential:

• Placing plant or animal cells in pure water (or any solution with a water potential higher than the cell contents) means there is a water potential gradient from outside to inside the cells.
• Water molecules will move down the water potential gradient into the cells by osmosis.
• In animal cells, the cell will swell, and the membrane will eventually burst open.
• In plant cells, the swelling cytoplasm and vacuole will push the membrane against the cell wall. The cell will not burst, but will become turgid.

Cells in solutions of low water potential:

• Placing plant or animal cells in a concentrated salt or sugar solution (with a water potential lower than the cell contents) means there is a water potential gradient from inside to outside the cells.
• Water molecules will move out of the cell by osmosis.
• In animal cells, the cell contents will shrink and the membrane will wrinkle up; crenation.
• In plant cells, the cytoplasm and vacuole will shrink, and the cell surface membrane will pull away from the cell wall; plasmolysis.

• Chromosomes are in the nucleus of eukaryotic cells.
• Each chromosome contains one molecule of DNA, which includes specific lengths of DNA called genes. 
• Chromosomes hold vital instructions for making new cells.
• In eukaryotes, the molecules of DNA that make up each chromosome are wrapped around proteins called histones.
• The DNA and the histone proteins together are called chromatin.
• Before a cell can divide to produce two new daughter cells, the DNA of each chromosome must be replicated.
• Each replica (2) is an exact copy of the original, and they remain held together at a point called the centromere.
• At this stage, you can't see the chromosomes under a light microscope. Each chromosome now consists of two replica DNA strands. These replicas are called a pair of sister chromatids.
• The chromatin is supercoiled to form visible chromosomes. Chromatin threads are about 30nm thick, but after supercoiling a chromosome is about 500nm thick.
• Supercoiled chromosomes can't perform their normal functions in the cell, so the length of time they spend coiled up needs to be as short as possible.

• As chromosomes are being replicated, proof-reading enzymes move along the new DNA strands and check that the copying has been done properly. If the genes are not copied precisely, the resulting mutations may mean the new cells fail to function.

The cell cycle is divided into stages:

• Interphase:
• - G1: biosynthesis (proteins made, organelles replicate)
• - S phase: DNA replicates
• - G2: growth
• Mitosis: the nucleus divides and chromatids separate
• Cytokinesis: the cytoplasm divides or cleaves

All organisms need to produce genetically identical daughter cells.

• Asexual reproduction: single-celled organisms divide to produce two daughter cells that are separate organisms. Some multicellular organisms produce offspring from parts of the parent.
• Growth: Multicellular organisms grow by producing new extra cells. Each new cell is genetically identical to the parent cells and so can perform the same functions.
• Repair: Damaged cells need to be replaced by new ones thatperform the same functions and so need to be identical.
• Replacement: Red blood cells and skin cells are replaced by new ones.

Mitosis refers to the process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus. This is divided into four stages:

• Prophase: replicated chromosomes supercoil (shorten and thicken)
• Metaphase: replicated chromosomes line up and down the middle of the cell
• Anaphase: the replicas of each chromosome are pulled apart from each other towards opposite poles of the cell
• Telophase: two new nuclei are formed.

• In prophase, the nuclear envelope breaks down and disappears.
• An organelle called a centriole divides into two, and each daughter centriole moves the the poles of the cell to form the spindle.
• In metaphase, the chromosomes move to the central region of the spindle (the equator) and each becomes attached to a spindle thread by its centromere
• In anaphase, the replicated sister chromatids that make up the chromosome are separated from each other when the centromere that holds them together splits. Each sister becomes an individual chromosome.
• Spindle fibres shorten, pulling the sister chromatids further and further away from each other towards the poles.
• In telophase, the sister chromatids reach the poles, and a nuclear envelope forms around each set.
• The spindle breaks down and disappears. The chromosomes uncoil, so you can no longer see them under a light microscope.
• In cytokinesis, the whole cell splits to form two new cells.

• Genetically identical cells are called clones
• All the bacteria in a single colony have been produced by one cell dividing (by binary fission), and they are also clones.
• Many plants undergo asexual reproduction using specialised parts of the plant that are derived from adult plant cells. These specialised parts can produce many new individual organisms that are genetically identical to the original parent. This form of asexual reproduction is known as vegetative propagation.
• Bacteria are prokaryotes. They have a single, naked stand of DNA that is in the cytoplasm, not in a nucleus. They may also have small plasmids of DNA. These may have genes for antibiotic resistance.
• Because bacteria can swap plasmids, they are used in genetic engineering.
• Bacteria divide by binary fission, not by mitosis. (Mitosis is only cell division involving chromosomes).
• Stem cells have the capability to divide and to develop into any of several different cell types.

• In animals, many cells are capable of mitosis and cytokinesis. This means organisms can repair damage to most organs by producing new ccells to replace damaged ones.
• In plants, only the cells of special growing regions - meristem cells - are able to divide in this way. Meristems are located at the root and shoot tips, and in a ring of tissues in the stem or trunk.
• In animal cells, cytokinesis starts from the outside - 'nipping in' the cell membrane and cytoplasm along what is termed a cleavage furrow.
• In plants cells, cytokineses starts with the formation of a cell plate where the spindle equator was. The cell then lays down new membrane and cell wall material along this plate. 
• Cells of yeast undergo cytokinesis by producing a small 'bud' that nips off the cell, in a process called budding.
• Meiosis produces two daughter cells that are not genetically identical.
• Meiosis happens in sex cells, and each gamete contains half the number of chromosomes.
• When two gametes fuse, it produces a zygote, which can then divide by mitosis to grow into a new individual organism.

• When organisms consist of many cells, some cells will be different from others. In this way, some cells perform one role very well. They become specialised in that role, while other cells are specialised for other roles.
• We refer to cells becoming specialised to carry out a particular role or function as differentiation.

Cells can differentiate in a number of ways, with changes to:

• the number of a particular organelle
• the shape of the cell
• some of the contents of the cell

Erythrocytes (red blood cells) and neutrophils (a type of white blood cell) play different roles. Both are human cells and each began with the same set of chromosomes, so each is potentially capable of carrying out the same functions.

All blood cells are produced from undifferentiated stem cells in the bone marrow.

The cells destined to become erythrocytes lose their nucleus, mitochondria, Golgi apparatus and RER. They are packed full of haemoglobin. They become biconcave.

Cells destined to become neutrophils keep their nucleus. Their cytoplasm appears granular because enormous numbers of lysosomes are produced. The role of neutrophils in the blood is to ingest invading microorganisms - so all those potent enzymes in the lysosomes enable the neutrophils to be specialised for killing microorganisms.

Sperm cells are specialised in a number of ways:

• Energy for movement of the undulipodium is generated by the many mitochondria present within the cell.
• The sperm head contains a specialised lysosome - an acrosome - that releases enzymes onto the outside of the egg so that the sperm can penetrate the egg.
• They are very small, long and thin to help in easing their movement.
• The single long undulipodium helps to propel the cell up the uterine tract towards the egg.
• The sperm cell nucleus contains half the chromosomes half the number of chromosomes of an adult cell in order to fulfil its role as a gamete.

• Xylem and phloem come from dividing meristem cells such as cambium. Meristem cells undergo differentiation to form the different kinds of cells in the transport tissues.

Animal tissues are grouped into four main categories:

• epithelial tissues - lauers and linings
• connective tissues - hold structure together and provide support 
• muscle tissue - cells specialised to contract and move parts of the body
• nervous tissue - cells that can convert stimuli to electrical impulses and conduct those impulses.

How the leaf is adapted for photosynthesis:

• A transparent upper surface layer; the upper epidermis, lets light through.
• A layer of cylindrical palisade cells beneath the epidermis, packed with chloroplasts.
• A loosely packed spongy mesophyll layer has air spaces to allow circulation.
• A lower epidermis layer has stomata, which allow gases to be exchanged between the leaf and the outside air.
• A leaf vein system containing xylem and phloem