Microscopes and Magnification

Magnification & Resolution
Magnification = Image Size / Actual Size. (How much bigger the image is compared to the specimen.
Resolution = The point at which a microscope can distinguish between two points that are close together.
Increasing the magnification, will not make the image clearer.

Light Microscopes

• It uses light (duh)
• Maximum resolution: 0.2 micrometres.
• Usually used to look at whole cells or tissues
• Maximum magnification: 1500X

Transmission Electron Microscopes

• Uses electrons instead of light.
• Produces more detailed images
• Uses electromagnets to focus a beam of electrons which are transmitted through the specimen.
• Denser parts of the specimen absorb more electrons, so they appear darker on the image produced.
• they are often used to look at organelles.
• they can only be used on thin specimens.

Scanning Electron Microscopes

• Scans a beam of electrons across the specimen.
•  This knocks electrons off the specimen, which are collected in a cathode ray tube to form an image. 
• The image produced shows the surface of the specimen and can be a 3D image. 
• However, they give a lower resolution that TEMs.

	Light Microscope	TEM	SEM
Maximum Resolution	0.2µm	0.0002 µm	0.002 µm
Maximum Magnification	X1,500	X1,000,000 +	X500,000

Prokaryotic Cells

PRO KARY NO CARRY!

Prokaryotes	Eukaryotes
Small (2 µm)	Larger (10-100 µm)
Circular DNA	Linear (usually double helix)
No Nucleus (DNA is free in cytoplasm)	Nucleus present with DNA inside
Cell wall made of polysaccharides, no cellulose or chitin	No cell wall (in animals). Cellulose cell wall (in plants) Chitin cell wall (in fungi)
No membrane bound organelles	Many organelles
Flagella (when present) is made of flagellin, arranged in a helix	Flagella (when present) is made of microtubule proteins arranged in 9+2 formation
Small ribosomes	Larger ribosomes
Example: E.coli bacterium	Example: Human Liver Cell

Bacterial Cells

Cytoskeleton

The Cytoskeleton is a Network of Protein Threads running through the Cytoplasm. In Eukaryotic cells, the protein threads are arranged as Microfilaments (small solid strands)  and Microtubules (tiny protein cylinders. The best way I can think of to remember this is microTUBUles are shaped like tubes!

The cytoskeleton has 4 main functions:

1. Microtubules and microfilaments Support the Organelles,  keeping them in place.
2. They Strengthen the cell and Maintain its Shape.
3. They're responsible for the Movement of Materials within the cell. i.e. during cell division, microtubules contract the Spindle Fibres to separate the chromosomes.
4. The proteins in the cytoskeleton cause the cell to Move. For example, the movement of Flagella or Cilia is caused by the cytoskeletal proteins. so in the case of sperm cells, the cytoskeleton propels the whole cell.

Protein Synthesis

• Proteins are made at ribosomes.
• The ribosomes on the RER make proteins that are attached to the cell membrane.
• The free ribosomes in the cytoplasm make proteins that stay in the cytoplasm.
• New proteins produced at the RER are folded and processed in the RER.
• They are then transported from the RER to the Golgi apparatus in vesicles.
• At the Golgi Apparatus, the proteins are processed again (sugar chains are trimmed or more are added. 
• The proteins enter more vesicles to be transported around the cell. For example, glycoproteins move to the cell surface and are secreted from the cell.

Cell Structure and Organelles

Prokaryotic - no nucleus - single celled - much smaller and simpler. e.g. bacteria

Eukaryotic - has a nucleus- multicellular organisms - plant and animal cells

Organelles

Round in shape, surrounded by a membrane, with no clear internal structure,

ORGANELLE	DIAGRAM	DESCRIPTION	FUNCTION
Plasma Membrane		Found on the surface of animal cells, and just inside plant cell walls and prokaryotic cells. It is made of proteins and lipids.	Regulates the movement of substances entering and exiting the cell. It also has receptor molecules so it can respond to chemicals such as hormones.
Cell Wall		Rigid structure that surrounds plant cells and mostly consists of cellulose.	Mostly for support.
Nucleus		A large organelle surrounded by a nuclear envelope, which  contains some pores. The nucleus contains chromatin (DNA and Proteins) and also contains a structure called a nucleolus	Controls what the cell does. The pores allow substances (RNA) to move between the nucleus and the cytoplasm. The nucleolus makes ribosomes.
Lysosome		Contains digestive enzymes, which are kept separate from the cytoplasm ---------------> They can be used to digest invading cells, or to break down worn-out components of the cell.
Ribosome		Very small, and floats free in the cytoplasm, or can be found attached to the RER. It's made up of proteins and RNA. It has no membrane.	The site of protein production.
Rough Endoplasmic Reticulum (RER)		A system of membranes enclosing a fluid-filled space. The surface is covered with ribosomes.	Folds  and processes proteins that have been made at the ribosomes.
Smooth Endoplasmic Reticulum (SER)		Similar to RER, but with no ribosomes.	Synthesises and processes lipids.
Vesicle		A small fluid-filled sac found in the cytoplasm, surrounded by a membrane	Transports substances in and out of the cell (across the cell membrane), and between organelles. Some are formed by the Golgi Apparatus or the ER while others are formed at the surface
Golgi Apparatus		A group of fluid filled, membrane bound, flattened sacs. Vesicles are often seen at the edges of the sacs.	Processes and packages new lipids and proteins. It also makes lysosomes
Mitochondrion		Usually oval-shaped, they have  a double membrane. The inner membrane is folded to form structures called cristae, inside is the matrix, which contains enzymes involved in respiration.	The site of aerobic respiration where ATP is produced. They're found in large numbers in the cells that are very active and require a lot of energy.
Chloroplast		A small, flattened structure found only in plant cells. It has a double membrane and has membranes inside called thylakoid membranes. These stack up in some parts of chloroplasts to form grana. Grana are linked together by lamellae - thin, flat pieces of the thylakoid membrane.	Where photosynthesis takes place. Some parts of photosynthesis take place in the grana, other parts take place in the stroma (thick fluid found in chloroplasts).
Centriole		Small, hollow cylinders made of microtubules (tiny protein cylinders). They are found in most animal cells, but only some plant cells.	Involved in the separation of chromosomes during cell division.
Cilia		Small, hair-like structures found on the surface membrane of some animal cells. In cross section, they have an outer ring made of pairs of protein microtubules, with two microtubules in the middle. (9+2 formation)	The microtubules allow the cilia to move. This movement is used by the cell to move substances along the cell surface.
Flagellum		Flagella on eukaryotic cells are like cilia, but longer and found in fewer numbers. They stick out from the cell surface and are surrounded by the cell membrane. Inside, they are similar to cilia. They are also in the 9+2 formation.	The microtubules contract to make the flagellum move. The flagellum is used like outboard motors to propel cells forward (Sperm cells).

Tissues, Organs and Systems

Multicellular organisms are made of lots of different cell types, organised to work together. Cells that carry out the Same Function are organised into Tissues (i.e. epithelial), and Different Tissues are organised into Organs, and organs Work Together as Organ Systems.

Tissues

Tissue - A Group of Cells specialised to Work Together to carry out a Particular Function. A tissue can contain more than one cell type.

Examples:

Tissue Type	Explanation
Squamous Epithelium	Single Layer of Flat Cells lining a surface. It's found in many places, including the Alveoli and the Lungs
Ciliated Epithelium	A layer of cells covered in Cilia. It's found in places where things need to be   i.e. the trachea, to Waft Mucus.
Muscle Tissue	Bundles of Elongated cells (Muscle Fibres). There are 3 different types: Smooth, Cardiac, and Skeletal. They're all Slightly Different in structure.
Cartilage	A type of Connective Tissue found in the joints. It also Shapes and Supports the Nose and Windpipe. It is found where cells (chondroblasts) Secrete a Jelly-like substance containing Protein Fibres which become trapped inside.
Xylem Tissue	Two jobs - Transports Water, and Supports the plant. It contains hollow xylem vessel cells (dead) and parenchyma cells (living)
Phloem Tissue	Transports Sugars around the plant. It's Arranged in Tubes and is made up of Sieve Cells, Companion Cells and Regular Plant Cells.

Organs and Organ Systems

Organ - a Group of Different Tissues that Work Together to perform a Particular Function.

LUNGS - Contain squamous epithelial tissue (alveoli), Ciliated epithelial tissue (bronchi), and elastic connective tissue and vascular tissue (blood vessels).

LEAVES - Contain palisade tissue for photosynthesis, epidermal tissue to prevent water loss from the leaf, and xylem and phloem tissues in the veins. 

Each system has a particular function. The respiratory system is made up of the lungs, trachea, larynx, nose, mouth and diaphragm.

The circulatory system consists of the heart, arteries, veins and capillaries.

Stem Cells

Stem Cells & Differentiation
Multicellular organisms are made up of many different cell types which are Adapted for their Function. i.e. blood cells, muscle cells e.t.c. All of these originated from Stem Cells. Stem cells are Unspecialised, but can develop into different types of cell. All multicellular organisms have stem cells in some form. In Humans, they are found in early Embryos and in a few places in adults. Stem cells in embryos can develop into Any Type of cell. However, Adult stem cells can only develop into a Limited Range of cells

The process where stem cells become specialised for their function is called Differentiation. In adults, stem cells can be used to Replace Damaged Cells, i.e. skin cells or blood cells. Plants are always growing, so stem cells are needed to Produce New Shoots and Roots. Stem cells in plants can also differentiate into Xylem and Phloem Cells.
Stem cells can also divide to produce more Unspecialised Stem Cells.

BONE MARROW
Bones are living organs. They contain Nerves and Blood Vessels. The main bones of the body have Bone Marrow in the Centre. It is here that adult stem cells can differentiate to Replace Worn Out Blood Cells - Erythrocytes (Red) and Neutrophils (White).

MERISTEMS
Meristems are the part of the plant where Growth takes place. In the Root and the Stem of the plant, stem cells of the Vascular Cambium Divide and Differentiate into Xylem Vessels and Phloem Sieve Tubes.

STEM CELL TREATMENT
As stem cells can differentiate into different types of cell, Scientists think we can Replace Damaged Tissue Cells to cure diseases, mostly neurological disorders like Alzheimer's and Parkinson's.

Alzheimer's	Parkinson's
Alzheimer's is where Nerve Cells in the Brain Die in increasing numbers.	People with Parkinson's suffer from Uncontrollable Tremors
This results in Severe Memory Loss.	The disease causes the loss of a particular type of nerve cell located in the brain.
In this case, researchers want to Replace Damaged Nerve Cells with re-grown nerve cells that have been differentiated from stem cells.	These cells release a chemical called Dopamine, which is needed to Control Movement
	It is thought that transplanted stem cells will help to Regenerate the dopamine-producing cells.

Stem cells are also used for Developmental Biology (studying how cells grow and develop). This can help us understand developmental disorders and cancer.

DIFFERENTIATION

Once cells differentiate, they have a Specific Function. Their structure is then adapted to perform that function.

Below is a table of cell types that are specialised for their specific function.

Cell Type	Function	Adaptations
Neutrophil	Defends the body against disease	Flexible shape to engulf foreign particles or pathogens More lysosomes contain digestive enzymes to break down engulfed particles
Erythrocytes	Carry oxygen in the blood	Biconcave shape provides large surface area for gas exchange No Nucleus to allow more haemoglobin
Epithelial Cells	Cover the surfaces of organs	Ciliated epithelia have cilia that beat to move particles away Squamous epithelia are very thin to allow  efficient diffusion of gases
Sperm Cells	Fertilisation of ovum cells	Flagellum so they can swim to the ovum Lots of mitochondria to provide energy to swim The acrosome contains digestive enzymes to enable the sperm to penetrate the surface of the egg.
Palisade Cells	Photosynthesis	Lots of chloroplasts to absorb more sunlight Thin walls so CO2 can easily diffuse into the cell.
Guard Cells	Open and close the stomata	Thin outer walls and thickened inner walls force them to bend outwards opening the stomata, allowing the leaf to exchange gases for photosynthesis
Root Hair Cells	Absorb water and mineral ions from the soil	Large surface area for absorption Thin permeable cell wall for entry of water and mineral ions More mitochondria to provide energy needed for active  transport

Cell Division

The Cell Cycle
The cell cycle is the process that all body cells in multicellular organisms use to grow and divide.

1. The cell cycle starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells
2. The cell cycle consists of a period called Cell Growth and DNA Replication which is collectively called Interphase. Next is Cell Division, which is known as M Phase. M Phase involves Mitosis and Cytokinesis.
3. Interphase is divided into 3 separate Growth Stages, G1, S, and G2
4. The cell cycle is regulated by checkpoints, these checkpoints let the cell know it is okay to continue.

Mitosis

Mitosis is needed for Growth of multicellular organisms and to Repair Damaged Tissues. It is also a method of asexual reproduction in some animals, plants and fungi. Mitosis is really just one continuous process but is described as a series of stages, Prophase, Metaphase, Anaphase and Telophase (PMAT)

INTERPHASE

Interphase actually comes Before Mitosis and is essentially the Preparation Phase, where cells Grow and Replicate their DNA ready for division. All while carrying out its Normal Functions.

1.  INTERPHASE: The cells DNA is Unravelled and Replicated, to double its genetic content. Organelles are also Replicated so it has spare ones and its ATP content is Increased (ATP is the energy needed for the cell to divide.
2. PROPHASE: The CchromosomesCcondense, getting Shorter and Fatter. Tiny bundles of protein called Ccentrioles start moving to Opposite Ends of the cell, forming a network of protein fibres across called a Spindle. The Nuclear Envelope Breaks Down and the Chromosomes lie Free in the Cytoplasm. 
3. METAPHASE: The chromosomes, each with two chromatids line up along the middle of the cell and become attached to the spindle by their centromere. At the metaphase checkpoint, the cell checks that all chromosomes are attached to the spindle before mitosis can continue.
4. ANAPHASE: The centromeres divide, separating each pair of sister chromatids to opposite ends of the cell, centromere first.
5. TELOPHASE: The Chromatids reach Opposite Poles on the spindle. They Uncoil and become long and thin again. They're now called chromosomes again. A Nuclear Envelope Forms around each group of chromosomes, so there are now Two Nuclei.
6. CYTOKINESIS: The Cytoplasm Divides. In animal cells, a Cleavage Furrow Forms to divide the cell membrane. There are now two daughter cells genetically identical to the original cell and to each other. Cytokinesis usually starts in anaphase and ends in telophase. It is a Separate Process to mitosis.

Sexual Reproduction and Meiosis

In sexual reproduction, two Gametes (sex cells) join together at fertilisation to form a Zygote. The zygote then develops into a new organism. Meiosis is the process that takes place in the reproductive organs to produce gametes. Meiosis involves a Reduction Division. Cells have a full number of chromosomes to start with, but after meiosis, they only have half. The product cells are called Haploid Cells. Cells formed  by meiosis are all Genetically Different because each new cell has a different set of chromosomes.

• Meiosis involves two separate parts, Meiosis I, and Meiosis II.
• Meiosis I is the Reduction Division, where the number of chromosomes are Halved.
• Both Meiosis parts are each split into Prophase, Metaphase, Anaphase and Telophase (PMAT) as with Mitosis.
• The whole of meiosis begins with Interphase, where the DNA Unravels and Replicates itself to produce double armed chromosomes called sister chromatids.

MEIOSIS I

• Prophase I - The Chromosomes Condense, getting shorter and fatter and they arrange themselves into Homologous Pairs (Chromosomes from Mum and Dad) and crossing over occurs. Centrioles move to Opposite Ends forming Spindle Fibres and the Nuclear Envelope Breaks Down.
• Metaphase I - The homologous pairs Line Up across the Centre  of the cell and Attach to the Spindle Fibres by their Centromere.
• Anaphase I - The Spindles Contract separating the homologous pairs, one chromosome to each end of the cell
• Telophase I - A Nuclear Envelope Forms around each group of chromosomes.
• Cytokinesis - The Division of the Cytoplasm and two haploid daughter cells are produced.

MEIOSIS II

In Meiosis II, the two daughter cells go through the same process, Prophase II, Metaphase II, Anaphase II and Telophase II. In anaphase II, each of the sister chromatids are separated, so each daughter cell has one chromatid from each chromosome, so four genetically different haploid daughter cells are produced. These are the Gametes.

Transport in Plants

Xylem	Phloem
Transports Water and Mineral  Ions	Transports Solutes (dissolved substances) which are mostly sugars such as sucrose around plants to where they need to be.
Substances move UP	Substances move UP AND DOWN
Also used for SUPPORT	Phloem is  formed from cells arranged in tubes.
Long and Tube-like made from cells (VESSEL ELEMENTS) joined end to end.	All tissue is purely transport tissue, no structural tissue present.
There are no end-walls on the cells to form one long continuous tube.	Contains Phloem fibres, Phloem Parenchyma, Sieve tube elements and companion cells.
These cells are actually dead, so contain no cytoplasm	Sieve tube elements are the most important cells for transport in the phloem.
Cell Walls are thickened with LIGNIN (a woody substance) which helps support the Xylem vessels and stops them collapsing inwards	Sieve Tube Elements Companion Cells The LIVING cells that form the tube for transporting solutes through the plant. The lack of a nucleus and other organelles meant that they can't survive on their own. This means there is a 1:1 ratio of Companion cells to sieve tube elements. They are joined end-to-end to form Sieve Tubes. Companion cells carry out the living functions for  both themselves and their sieve cells. They provide the energy for ACTIVE TRANSPORT of solutes The 'Sieve' part are the end walls, which have lots of holes in them to allow solutes to pass through. While alive, they usually have no nucleus, a thin layer of cytoplasm and very few organelles. The cytoplasm of adjacent cells is connected through the holes in the sieve plates.
Lignin can be supported in different ways - spiral, or distinct rings
The amount of lignin increases as the cells get older
Water and Mineral Ions move in and out of the vessels through small pits in the walls where lignin doesn't occur.

Water enters a plant through its ROOT HAIR CELLS. Water has to go through Soil --> Root --> Xylem to be transported around the plant.

Root Hair Cells --> Root Cortex + Endodermis -->Xylem.

Water is drawn into the roots through osmosis. It travels Down a WATER POTENTIAL GRADIENT.

• Water travels from areas of High water potential gradient to areas of LOW WPG. This means it travels down a water potential gradient.
• The soil usually has a high water potential (there's lots of water present).
• Leaves have a Low water potential because water always evaporates from them.
• This creates a water potential gradient that keeps moving water from roots (high) to leaves (low) constantly.

The Symplast Pathway	The Apoplast Pathway
Goes through the LIVING parts of cells - the cytoplasm.	Goes through the NON-LIVING parts of cells - The cell walls.
The cytoplasm of neighbouring cells connects through plasmodesmata (small channels in the cell walls).	The cell walls are very absorbent and water can diffuse through them as well as pass through  spaces between them.
Water moves through the Symplast pathway using osmosis.	The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. (i.e. along a pressure gradient). This is an example of mass flow.

In the apoplast pathway, when the water gets to the endodermis cells in the root, its path is blocked by a waxy strip in cell walls called the CASPARIAN STRIP. The water now has to take the symplast pathway. This is useful because the water has to go through a cell membrane, which controls what goes in and out of a cell, so it controls which substances in the water get through. Cell Membranes are PARTIALLY PERMEABLE. The water then moves into the XYLEM.

Both Pathways are used, but APOPLAST is the main one because there is less resistance.

• Xylem vessels transport water all around the plant.
• At the leaves, water leaves the Xylem and moves into the cells through the apoplast pathway.
• Water evaporates from the cell walls into the spaces between the cells in the leaf.
• When the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the leaf into the surrounding air.
• The loss of water from a plant's surface is called TRANSPIRATION.
• Water moves up a plant AGAINST THE FORCE OF GRAVITY.
• The movement from roots to leaves is called the TRANSPIRATION STREAM.
• The mechanisms that move the water include COHESION, TENSION and ADHESION

COHESION AND TENSION	ADHESION
Helps water move up plants against the force of gravity.	As well as being attracted to itself, water is attracted to the walls of the xylem vessels
Water evaporates from the leaves at the top of the xylem (transpiration)	This helps water rise up through the xylem vessels.
This creates tension (suction) which pulls more water into the leaf.
Water molecules are cohesive (they stick together) so when some are pulled into the leaf, others follow.
This means the whole column of water in the xylem moves up the plant.
Water enters the stem through the root cortex cell.

Cohesion and Tension allow the mass flow of water over long distances up the stem.

Transpiration

• Transpiration is a consequence of gas exchange.
• Transpiration is defined as The Evaporation of water from a plant's surface, specifically, the leaves.

1. A plant opens its stomata to let in CO2 so it can produce glucose by photosynthesis.
2. This also lets water out because there is a higher concentration of water inside the leaf than the air outside.
3. Transpiration is technically a side effect of gas exchange, needed for photosynthesis.

What factors effect TRANSPIRATION RATE?

• Light - More Light, Faster Rate - this is because the stomata open when it is light. when it's dark, the stomata close, so there is less transpiration happening.
• Temperature - Higher Temperature, Faster Rate - warmer water molecules have more energy, so they evaporate from cells faster, this increases water potential gradient, increasing the rate of transpiration.
• Humidity - Lower Humidity, Faster Rate. - if the air around the plant is dry, water potential gradient increases, so they transpire at a faster rate.
• Wind - More Wind, Faster Rate - Lots of air movement blows away water molecules around the stomata, increasing water potential gradient Blah Blah Blah…..

Potometer

A potometer is a special piece of apparatus which ESTIMATES TRANSPIRATION RATES.

It measures water uptake by the plant, and it is ASSUMED that water uptake is directly related to water loss by the leaves.

Xerophytic Plants

Xerophytic plants are those like cacti and marram grass, They're adapted to live in dry climates. These adaptations prevent them losing too much water during transpiration.

• Marram grass has its stomata sunk in pits so they're sheltered from the wind, slowing transpiration
• It also ha a layer of hairs on the epidermis, this traps moist air reducing the water potential gradient between the leaf and the air.
• In hot or windy conditions, marram grass rolls up their leaves trapping moist air and reducing the exposed surface area and protects the stomata from the wind.
• Both marram grass and cacti have a thick waxy layer on the epidermis - reducing water loss by evaporation because this layer is waterproof.
• Cacti have spines instead of leaves reducing the surface area for water loss.
• Cacti also close their stomata at the hottest times of the day when transpiration rates are highest.

Hydrophilic Plants

Plants that are adapted to survive in water, such as water lilies, live in aquatic habitats. 

As they grow in water, they don't need adaptations to reduce water loss. However, they do need adaptations to help them cope with low oxygen levels...

• Air spaces in the tissues help the plants to float and can act as a store of oxygen for use in respiration. Water lilies have air spaces in the leaves to allow them to float on the surface of the water, increasing the amount of light they receive. Air spaces in the stem and roots allow oxygen to move from the floating leaves down to parts of the plant that are underwater.
• Stomata are usually only present on he upper surface of floating leaves. This maximises gas exchange.
• Hydrophytes have flexible leaves and stems. The plants are supported by the water around them, so they don't need rigid stems for support. Flexibility helps prevent damage by the currents of the water.

Translocation.

Translocation is the movement of dissolved substances (sugars like sucrose, and amino acids) to where it's needed in a plant. Dissolved substances are sometimes called ASSIMILATES.

• It's an energy-requiring process that happens in the phloem.
• Translocation moves substances from SOURCES to SINKS. Sources are where the substances are made, sinks are where substances are used up.
• Some parts of plants can be both a sink and a source.
• Enzymes maintain a concentration gradient by changing dissolved substances at the sink by breaking them down or making them into something else. This means there's always a lower concentration at the sink than at the source.

The Mass Flow Hypothesis

No-one actually knows exactly how dissolved substances are transported from source to sink. This is the best they can come up with...

1. Active transport is used to actively load the solutes into the sieve tubes of the phloem at the source. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.  this creates high pressure at the source end of the phloem.
2. At the sink end of the phloem, solutes are removed from the sieve tubes to be used up. this increases the water potential inside the sieve tubes, so water leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes.
3. The reduces the pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes to where they're needed.

Active Loading.

• Active loading is used to move substances into the companion cells from surrounding tissues, and from companion cells into the  sieve tubes, against a concentration gradient.
• The concentration of sucrose is usually higher in the companion cells than in the surrounding tissue cells, and higher in the sieve tube cells than in the companion cells.
• So, sucrose is moved to where it needs to be using active transport and cotransporter proteins. This is how it works:

1. In the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into surrounding tissue cells. This creates a concentration gradient of Hydrogen ions. There's more in the surrounding tissue than in the companion cells.
2. An H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell down the concentration gradient.
3. At the same time,  a sucrose molecule binds to the cotransport protein. the movement of H+ ions is used to movethe sucrose back into the cell against its concentration gradient.
4. Sucrose molecules are then transported out of the companion cells and into sieve tubes bythe same process.

ATP is a product of respiration. The breakdown of ATP supplies the initial energy needed for active transport of the H + ions.