Biology exam revision - UNIT 1

The cell

• The cell is a basic structural and functional unit of an organism.

• Organisms can be simple and unicellular (one-celled) or more complex multicellular (made up of many

            cells) 

THE CELL THEORY

• An organism is composed of one or more cells

• Every cell come from a pre-existing cell

• The cell is the basic unit of life

UNICELLULAR ORGANISMS

Made of one cell

MULTICELLULAR ORGANISMS

• More than one cell

• Each specialised cell performs a specific function

• Advantages and disadvantages of having specialised cells, advantages outweigh disadvantages

ADVANTAGES AND DISADVANTAGES

ADVANTAGES	DISADVANTAGES
Energy efficient	More cells need more energy
Longer Life spans	Cannot function independently
Sexual reproduction increases complexity, compared to asexual reproduction	Energy required for reproduction, finding mates, other plant to reproduce
Can grow large, unicellular need to be small to obtain nutrients	Take much longer to evolve and adapt
Increased size of limbs - more mobile, more efficient - resources/predators
Perform more functions

CELL TYPES

• prokaryotes

• eukaryotes

Prokaryotes

• Are single-celled organisms that do not contain membrane bound organelles

• E.g Bacteria and cheese

Eukaryotes

• Can be single celled or multicellular. They contain membrane-bound organelles

• E.g animals, plants, fungai, protists

Features of prokaryotic cells

• Lack membrane bound organelles 

• Have single circular chromosomes which is free floating within the cytoplasm

• Most have either flagella or cilia they use to move around.

• Have a plasma membrane which surrounded by an outer cell wall

Features of Eukaryotic cells

• DNA which is located within the nucleus and is in linear chromosomes

• Membrane-bound organelles which have specific functions in the cell

• Have a distinct double layered nuclear membrane

Generalised cell

Cells generally have the same basic parts and many functions in common. Hence, we can talk of a generalised cell, although not all cells are the same.

Cell specialisation

• A cells shape reflects its function

• Structure is how it is built or what it looks like.

• Function is what it does or how it works; its physiology

Cytoplasm

 is the cellular material inside the cell membrane and outside the nucleus.

Cytosol

 is the fluid component of the cytoplasm. It consists largely of water, with dissolved salts, sugars etc.

Organelles

• which are specialised structures with a specific function

• All cells contain genetic material in the form of DNA

Plasma membrane

• All cells have an outer plasma membrane (cell membrane), which separates its contents from the environment - bilayer of phospholipids molecules

• Transport of substances into or out of the cell

Cell compartmentalisation

• Internal membranes that form specialised membrane-bound compartments within the cell. 

• Known as organelles

• Each has different function, therefore different internal composition

• Membrane controles in/ot movements b/w organelle and cytosol

• Benefits: 

- Allows for efficient functioning

- allows for different functions to occur at the same time

- makes cell less vulnerable to changes in external environment

NUCLEUS

• The nucleus is the control centre of the cell; responsible for transmitting genetic information and providing instructions for protein synthesis.

• It is surrounded by a nuclear membrane (double membrane). Large nuclear pores are present in it 

All cells contain genetic material in the form of DNA

NUCELOLI

• Nucleoli are dark staining spherical bodies within the nucleus.

Ribosome parts are assembled here; chromatin (DNA material)

MITOCHONDRIA

• are the site of energy production. Energy is known as ATP. (adenosine triphosphate)

CELLULAR RESPIRATION

• is the name given to this method of producing ATP 

• The overall equation for cellular respiration is:

                                           C₆H₁₂O₆+ 6O₂ → 6CO₂ + 6H₂O 30/32 ATP

                                      glucose + oxygen → carbon dioxide + water + 30/32 ATP

RIBOSOMES

• are small and dark staining

• They are the site of protein synthesis (production)

ENDOPLASMIC RETICULUM

• is a system of interconnected membranes

• Rough ER has ribosomes attached. Makes proteins that are secreted from the cell etc.

• Smooth ER has no ribosomes attached. It is a continuation of the RER. SIte of lipid synthesis, detoxification of drugs storage of calcium in the muscle. No role in the protein synthesis (abundant in testis)

GOLGI APPARATUS

•  consists of a stack of flattened membrane sacs and secretory vesicles

• It packages, modifies and secrets proteins for secretion from the cell. (post office)

LYSOSOMES

•  contain digestive enzymes eg. to breakdown bacteria, toxins, worn-out organelles etc.

CENTRIOLES

• are small bodies that are involved in cell division

• They contain microtubules that determine the overall shape of cell

• They also form the bases of cilia and flagella

NOT FOUND IN PLANT CELLS

• Cilia are short, cell surface projections that move, creating a current that can move substances across the cell surface

• Flagella are longer than cilia and can propel the cell itself, often with a whip-like action eg. sperm cell

CHLOROPLAST

• A green organelle in which photosynthesis takes place.

• Chlorophyll is the green pigment responsible for absorbing light energy.

SURFACE AREA

• Surface area (SA) is the outside area of an object 

•  For a cube you measure L x H x number of sides

VOLUME

• Volume (V) is the amount of space inside an object

• For a cube, you measure L x H x W

THE SA:V RATIO

• would be 54:27, simplified to 2:1

• Meaning for every 2 units of surface area covering the outside of the cube, there is one unit of volume inside the cube. 

WHAT INCREASES SA;V RATIO

• Long, thin shape

• Folding of the membrane ie. Villi

• Flattened shape

WHY IS SA:V RATIO IMPORTANT

• Cells need to be small because they rely on diffusion to get substances into and out of their cells

• When a cell is very small, it has a large surface area to volume ratio, meaning it is more efficient ie. unicellular organisms 

• Larger cells have a smaller surface area to volume ratio

PLASMA MEMBRANE

The plasma membrane is a phospholipid bilayer embedded with proteins, carbohydrates and cholesterol.

• It is a selectively permeable or semipermeable membrane, allowing only some substances to pass across it.

PHOSPHOLIPID BILAYER

• The main layer of the plasma membrane is phospholipids. Phospholipids have a phosphate head and two fatty acid tails

HYDROPHILIC

• heads (water loving) are polar made up of glycerol and phosphate group

HYDROPHOBIC

• tails (water fearing) are non-polar, made of long chains of C & H, uncharged molecules

THE FLUID MOSAIC MODEL OF THE PLASMA MEMBRANE

• thin , but stable structure 

• Fluid-phospholipids continually move laterally (side to side) in the plasma membrane

• Mosaic-looks like a mosaic, due to the proteins and carbohydrates embedded in the plasma membrane

TRANSPORT PROTEINS

• Transport proteins such as integral or transmembrane proteins - channels or pumps for specific substances to pass across the membrane in either direction.

PERIPHERAL PROTEINS

• adhere only temporarily to the membrane and regulate ion channels and act as receptors

CHOLESTEROL MOLECULES

•  help to keep the membrane stable. It regulates the fluidity. Embedded between fatty acid tails

GLYCOPROTEINS

• cell recognition (antigens/defence) and cell-cell communication, signalling, adhesion

TEMPERATURE CAN AFFECT MEMBRANE FLUIDITY

• At higher temperatures, the cholesterol keeps the phospholipids bound together

• At lower temperatures, the cholesterol disrupts the fatty acid tails, drifting apart.

DIFFUSION

• Diffusion is the net movement of a substance, from a region of high concentration to a region of low concentration (concentration gradient)

FACILITATED DIFFUSION

• Sometimes a carrier molecule is required to move molecules down through a protein channel, binding to a protein pump or causing a change to the protein pump

• Movement of substances by facilitated diffusion mainly involves substances that cannot diffuse across the plasma membrane by dissolving in the phospholipid bilayer of the membrane

• No energy is required (passive)

PROTEIN CHANNELS

• Carrier molecule envelops and then releases transported molecule

• It is highly selective i.e. the carrier for glucose only binds with glucose

OSMOSIS

• Osmosis is the net movement of water across as SPM from an area of high concentration of water (low solute) to low concentration of water (high solute)

MOVEMENTOF WATER MOLECULES IN PLANT CELLS

• Water molecules move by osmosis into the plant vacuole

PLASMOLYSIS IN PLANT CELLS

• Causing the cell to shrink. This process is called plasmolysis

• If water moves into the cell, the cell wall prevents the cell from bursting

RED BLOOD CELLS

• When a red blood cell is placed in freshwater, water enters the cell by osmosis, causing the cell to swell until it bursts

• When a red blood cell is placed in a solution more concentrated than the cytoplasm, water leaves the cell by osmosis causing the cell to shrink (crenation)

TONICITY

The ability of an extracellular solution to make water move into or out of a cell by osmosis is known as its tonicity

HYPERTONIC

• If a cell is placed in a hypertonic solution there will be a net flow of water OUT of the cell and the cell will lose volume. A solution will be hypertonic to a cell if its solute concentration is higher than that inside the cell, and the solutes cannot cross the membrane

HYPOTONIC

• If a cell is placed in hypotonic solution, there will be a net flow of water INTO the cell and the cell will gain volume. If the solute concentration outside the cell is lower than inside the cell, and the solutes cannot cross the membrane, then that solution is hypotonic to the cell.

ISOTONIC

• If a cell is placed in an isotonic solution, there will be no net flow of water into or out of the cell’s volume will remain stable. If the solute concentration outside the cell is the same as inside the cell, and the solutes cannot cross the membrane, then that solution is isotonic to the cell. 

ACTIVE TRANSPORT

• From a region of low concentration to a region of high concentration

TRANSPORT REQUIRNING ENERGY

• Active energy is the net movement of dissolved substances into or out of the cells against a concentration gradient

BULK TRANSPORT ENDOCYTOSIS

• Is the process where substances are brought into a cell by a folding of the plasma membrane and the creation of a vesicle

EXOCYTOSIS

• Is the process where by a vesicle fuses with the plasma membrane to allow the release of its contents from the cell

CELL DIVISON

Division occurs when a single parent cell divides into two genetically identical cells

Eukaryotic cells is referd to as mitosis

Prokaryotic cells use a much simpler method known as binary fission.

BINARY FISSION

• The purpose of binary fission is for a bacteria cell to reproduce. This is an asexual process as only one parent cell is required to produce many identical offspring in a short period of time

THE PROCESS OF BINARY FISSION

• Step 1: Before binary fission, the genetic material of the prokaryote (chromosomes) is present in a region of the cell in the cytoplasm as a nucleoid.

• Step 2: The circular chromosome is uncoiled and the DNA is replicated. Plasmids also replicate.

• Step 3: The cell elongates and DNA moves to opposite ends

• Step 4: The cell starts to undergo cytokinesis (division of cytoplasm) The membrane pinches inwards forming a septum (new cell wall)

• Step 5: The septum divides itself to form two independent cells

• Step 6: Two genetically identical cells are formed

EUKARYOTIC CELL DIVISION - THE CELL CYCLE

• Growth and repair

• One parent cell produces two genetically identical daughter cells

• There are three stages: interphase, mitosis and cytokinesis

INTERPHASE 

The cell spends most of its time in interphase

Interphase is broken into stages called the G1, S, G2 and in some cells, G0 phases of the 

cell cycle

G1 (first gap) phase:

• Cell grows larger (almost doubles in size of cytosol)

• Organelles are replicated (so that they can be passed down into the daughter cells )

• Synthesise proteins for DNA replication

S (synthesis) phase: 

• DNA in nucleus is replicated

• Chromosomes are not present in this stage of the cycle it is easier to explain by

Imagining that they are. During DNA replication, single chromosomes become double

Chromosomes consisting of two sister chromatids joined by a centromere

G2 (second gap) phase:

• Cells continues to grow

• Proteins needed for mitosis are synthesised

G0 phase:

• Not all cells progress through the whole vell cycles. Some cells exit the cycle in G1 and enter a resting state in the G0 phase. For some cells this resting state is not permanent and they eventually re-enter the cycle and divide normally. For other cells, such as neurons, this state of rest is permanent, which explains why damage to the Nervous system is very difficult to treat.

MITOSIS - DIVISION OF THE NUCLEUS 

PMAT

• Prophase

• Metaphase 

• Anaphase

• Telophase

PROPHASE

• Chromosomes visible

• Centrioles move to opposite poles

• Spindle forms

• Nuclear membrane break down

• Nucleolus disappears

METAPHASE

• Chromosomes line up along the equator of the cell

• Spindle attach to centromere

ANAPHASE

• Chromosomes move to opposite poles

TELOPHASE

• Nuclear membrane formed

• Spindle disappears

• Chromatin

CYTOKINESIS

• The final stage of the cell cycle

• Division of the cytoplasm

CHECKPOINTS

G1	G2	M
Correct size Proteins for DNA Replication DNA damage Nutrients and oxygen	DNA replicated properly Cell size Resources for mitosis	Spindle fibres Chromosomes lined up correctly

APOPTOSIS - PROGRAMMED CELL DEATH

Apoptosis is the controlled death of a cell

Its purpose:

• To ensure unhealthy, damaged cells, cells with damaged DNA do not divide and pass this defect on

• Has a role in development - E.G webbing digits

• Old cell/cells not required removed

Major steps of apoptosis

• Cell shrinks

• Chromatin (DNA) condenses

• Enzymes called caspases break down mitochondria, releasing cytochrome C

• Caspases breakdown of nucleus and organelles

• Cytoskeleton collapses

• Blebs form on plasma membrane

• Apoptotic bodies form (contain organelles and cytoplasm)

• Phagocytosis of apoptotic bodies

Pathways of apoptosis

A signal initiates apoptosis - either from inside the cell or

Intrinsic - mitochondrial pathway 

• Damage to the cell from radiation, viruses, toxins, damaged DNA, stress

• Internal components such as DNA are damaged, mitochondria detect this and release cytochrome C into cytosol which binds with proteins to form an apoptosome with activities caspase enzymes, initiating apoptosis

Extrinsic - death receptor pathway

• These death signalling molecules are often released by immune cells (attack viral infected cells)

• Death signaling molecules bind to death receptors (transmembrane proteins) activating caspases initiating apoptosis

NECROSIS CELL DEATH

• Cell death by trauma or injury

• Cell swells and bursts

• Causes inflammation and damage in nearby cells

CELL CYCLE MALFUNCTION AND APOPTOSIS

• If errors are detected at any of the checkpoints (G1, G2, M), the cell will repair itself or undergo apoptosis

• Sometimes cells may no longer express functional death receptors, hence have a reduced rate or apoptosis leading to cell growth

• Cancer is uncontrolled division of abnormal cells

• This can result in tumors. Tumors are cells with mutations that accumulate into a mass

Different types of malfunctions

• Benign tumors - slow growing and in a capsule

• Malignant tumors-cells of some benign tumors mutate and invade other body tissues via blood, lymph (metastasis)

• Angiogenesis: the formation of new blood vessels supplying the cancer cells with nutrients and oxygen

MALFUNCTIONS IN APOPTOSIS

• Too much apoptosis:

• Alzheimer's - shrinkage of the brain (loss of neurons)

• Plaques (abnormal clusters of) contain beta - amyloid protein

• Caspases degrade abnormal tau protein present in the cytoskeleton forming tangles

• Parkinson’s disease

STEM CELLS

A stem cell is an unspecialized or undifferentiated cell that is capable of giving rise to any type of specialised cell with a particular function

Each cell begins as a stem cell.

Stem cells are capable of self-renewal, meaning they can replicate themselves

There are two types of stem cells:

• Embryonic stem cells

• Adult stem cells

EMBRYONIC STEM CELLS

Embryonic stem cells are usually sourced from extra embryos around 3-5 days old that arise from IVF programs.

• embryonic stem cells are found in the early stages of a developing embryo

• 0-5 days: zygote to blastocyst stage before they implant in the uterus:

• Undifferentiated cells of an embryo

• Obtained at day 3-5 embryos from IVF programs

• Can become many types of cells and replicate indefinitely

• Prior to implantation inner cell mass are capable of developing into all cell types (except placenta)

GERM LAYERS

• The 3 germ layers are formed in the earliest stages of embryonic development, consisting of the endoderm (inner layer) the ectoderm (outer layer) and the mesoderm (middle layer).

• The germ layers form during the process of gastrulation, when the hollow ball of cells that constitutes the blastula begins to differentiate into more specialised cells that become layered across the developing embryo.

• Each germ layer eventually gives rise to certain tissue types in the body

• The endoderm is so called because it is the innermost of the three germ layers

• The mesoderm lies between the endoderm and the ectoderm, the outer layer.

ADULT STEM CELLS

• Adult stem cells are undifferentiated cells found in certain tissues in the human body (e.g. hair follicles, bone marrow, spinal cord, brain, skin, liver flow, blood vessels, heart)

• Produces a limited range of cells (e.g. haematopoietic stem cells in bone marrow only give rise to blood cells

• Their purpose is repair and regeneration of damage or old cells 

• Can't replicate indefinitely 

INDUCED PLURIPOTENT STEM CELLS

• An IPSC is an adult cell that has been genetically programmed to revert back to an embryonic stem cell state

• Reprogram specialised somatic cells such as skin cells into pluripotent cells

• They may revolutionize regenerative medicine which involves replaces, regenerates or engineers human cells, tissues or organs to restore normal functions

• Low success rate, tendency to form tumours 

TISSUES

A tissue is a group of specialised cells working together to peform a specific function.

ORGANS

• When two or more types of tissues act together to perform one (or more) specific functions, an organ is formed

SYSTEM

• A system is a group of organs working together to perform a complex task vital to an organism’s survival.

ORGANISM

• In complex multicellular organisms, such as mammals and vascular plants, many systems work together to ensure to ensure the organism has everything needed to thrive in its environment 

Non-vascular plants

• non-vascular plants lack vascular (transport) tissue

• They simply use osmosis to transfer water from cell to cell, diffusion for the movement of minerals/nutrients between cells. This is due to their high SA;V

• They are small in size and lack a root system

• Eg. mosses, liverworts, hornworts

VASCULAR PLANTS

• They range from grasses to flowering plants, ferns to pine trees, shrubs to fruit trees.

• A vascular plant transports water and nutrients through a specialised vascular tissues: xylem (water & minerals) and phloem (sugars such as sucrose & other substances such as hormones, aminoacids)

• Vascular tissue is arranged in vascular bundles through the centre of each root, up into the stems and leaves (veins) of flowering plants called dicots.

THE ROOT SYSTEM

• Absorbing water and minerals from the soil

• Providing anchorage, stability and support for the plant

• Storing food and nutrients

Lining the surface of every root tip are thousands of root hairs, which increase the surface area of the root to absorb water and minerals from the soil. 

PATHWAYS OF WATER AND MINERAL ABSORPTION

1. Extracellular pathway

• Water diffuses into the root in the gaps between cells. The casparian strip is impermeable to water which forces water to move through cells plasma membranes to reach the xylem

2. Cytoplasmic Pathway

• Mineral ions passively diffuse into cytoplasm or are actively transported in root hair cells. 

LEAF STRUCTURE

• Photosynthesis occurs primarily in the leaf. The sugar (usually sucrose) is then transported to cells all over the plant via the phloem

TRANSPIRATION

• Water passes from the soil into the root through the process of osmosis. This creates root pressure, which pushes water up the root. Once inside the root. Water continues to move osmotically through and between adjacent cells until it reaches the vascular bundle at the centre of the root. Here it is drawn into the xylem and transported up towards the leaves of the plant.

• Once in the vascular tissue, water and dissolved ions are transported upward into the rest of the plant and evaporates at the leaves. This movement of water is called transpiration. As water evaporate from the leaf, it draws more water up the xylem

• The adhesion and cohesion properties of water enable transpiration to pull a continuous stream of water from the roots up to the highest shoots. Capillary action is caused by the adhesive properties of water, pulling the water up.

XYLEM

Xylem transports water and dissolved minerals from the roots to the leaves. Xylem tissue also provides the plant with structural support, assisting it to remain upright,

Xylem tissue consists of two types of specialised celled: vessel elements and tracheids

LIGNIN

• A carbohydrate found in the vascular tissue of plants. It is located in the cell wall, providing strength to the wall and contributing to the structural support of the plant, preventing it from collapsing as water is pulled through the xylem